design project
DESCRIPTION
PI DC motor controlTRANSCRIPT
THE UNIVERSITY OF THE WEST INDIES
ST. AUGUSTINE, TRINIDAD & TOBAGO, WEST INDIES
FACULTY OF ENGINEERING
Department of Electrical & Computer Engineering
BSc. in Electrical & Computer Engineering
ECNG 2005
Lab and Project Design III
Individual Design Project
PI Control of a DC Motor Using a
Peripheral Interface Controller
Dale Persad
810000263
Group E
Course Lecturer: Mr. Marcel Byron
Date Submitted: April 19th, 2013
Abstract Outlined in this project are the design and construction details of a DC motor controller which
utilises a PI control strategy. This design was done taking into consideration, the design
requirements which were outlined in the project outline (Engineering 2013) such as current limiting,
current sensing, and voltage regulation. Other general requirements which were taken into
consideration are, standards, laboratory protocol ad industrial standards. This design was also done,
in a safe and environmentally conscience manner, taking into consideration, the users and
stakeholders involved.
ii
Table of ContentsAbstract................................................................................................................................................. ii
Table of Figures..................................................................................................................................... iv
List of Tables..........................................................................................................................................v
1 Introduction...................................................................................................................................1
2 Background Theory........................................................................................................................2
2.1 DC motor Operation..............................................................................................................2
2.2 Existing Control Schemes for a DC motor and their Advantages and Disadvantages.............4
2.3 Existing Motor Protection and Current Sensing Technology................................................11
2.4 Existing Approaches to Implementation of Control Algorithm............................................14
2.5 Existing Approaches for the Measurement of Motor Speed................................................15
3 Design Approach and Methodology............................................................................................16
3.1 Design Brief..........................................................................................................................16
3.2 Product Design Specification...............................................................................................19
3.3 Concept design and Evaluation............................................................................................21
3.4 Motor & Controller design...................................................................................................25
3.5 Implementation of PI Controller and Data Processing.........................................................35
3.6 Other Design Considerations...............................................................................................39
4 Results and Analysis.....................................................................................................................48
4.1 Presentation of Results........................................................................................................48
4.2 Discussion............................................................................................................................51
4.3 Conclusion...........................................................................................................................52
5 References...................................................................................................................................53
iii
Table of FiguresFigure 1 Components of a DC motor.....................................................................................................2
Figure 2 Fleming's Left Hand Rule (TeacherTube 2010).........................................................................3
Figure 3 Block Diagram showing Proportional Control..........................................................................7
Figure 4 Block Diagram showing Integral Control..................................................................................8
Figure 5 Block Diagram showing Derivative Control..............................................................................9
Figure 6 Gantt chart.............................................................................................................................18
Figure 7 Block diagram of System........................................................................................................23
Figure 8 DC motor equivalent Circuit..................................................................................................25
Figure 9 DC Motor Signal Flow............................................................................................................29
Figure 10 Simplified DC Motor block diagram.....................................................................................30
Figure 11 Characteristic Graphs of DC Motor (Jameco n.d.)................................................................31
Figure 12 Block diagram of system......................................................................................................32
Figure 13 Complete block diagram......................................................................................................33
Figure 14 Step response of system transfer function..........................................................................34
Figure 15 Step response with compensator........................................................................................34
Figure 16 Simple Flow Diagram of System...........................................................................................35
Figure 17 PI control Flow Chart...........................................................................................................35
Figure 18 Sixteen bit addition Routine................................................................................................36
Figure 19 Sixteen by Eight Division Routine.........................................................................................36
Figure 20 Display Output Flowchart....................................................................................................39
Figure 21 Phototransistor optical interrupter switch..........................................................................40
Figure 22 Display wiring (Best-Micocontroller-Projects 2013).............................................................41
Figure 23 External hardware design....................................................................................................42
Figure 24 Risk Assessment Key............................................................................................................45
Figure 25 Step Response of closed Loop control System.....................................................................48
Figure 26 Step Response with Compensator.......................................................................................49
iv
List of TablesTable 1 Advantages and Disadvantages of Open Loop Control.............................................................5
Table 2 Advantages and Disadvantages of closed loop control.............................................................5
Table 3 Summary of Control Schemes...................................................................................................6
Table 4 Advantages and Disadvantages of Proportional Control...........................................................7
Table 5 Advantages and Disadvantages of Integral Control..................................................................8
Table 6 Advantages and disadvantages of derivative control................................................................9
Table 7 Methods used for DC motor protection..................................................................................12
Table 8 Summary of current sensing techniques.................................................................................13
Table 9 Control Algorithm Implementation.........................................................................................14
Table 10 Task description....................................................................................................................16
Table 11 Motor Parameters.................................................................................................................25
Table 12 Model parameters................................................................................................................25
Table 13 Conversion of Parameters to Laplace Domain......................................................................27
Table 14 Calculation of Ki and Kp..........................................................................................................32
Table 15 Microprocessor Peripherals..................................................................................................37
Table 16 Hardware Components.........................................................................................................37
Table 17 Different Oscillator modes....................................................................................................41
Table 18 Risk Assessment....................................................................................................................44
Table 19 Summary of Key Parameters.................................................................................................48
Table 20 Results of display testing.......................................................................................................49
v
1 IntroductionMotor control is very important in industry especially for the automation of processes. For this
design project it is required that one design and build a DC motor controller using PI control. This
simple circuit can be implemented in industry of example on a conveyor (ACS 2013), where it is
required for the conveyor to move at a constant speed irrespective of how much product is on top of
it. Therefore the circuit/conveyor should be able to monitor/measure its speed. If the speed drops or
increases above a threshold value, parameters must be adjusted so that the speed of the conveyor/
dc motor returns to its set speed. Finally, protection systems must be implemented to prevent
expensive damage to the circuit in the event of:
Over voltage and high currents for example during short circuit conditions.
High temperature conditions
1
2 Background Theory
2.1 DC motor OperationThis project is centred on the DC motor. More specifically a 6V brushed DC motor. In this section the
operation of this motor shall be explored by disassembling a DC motor from a Team Orion Racing RC
car.
Figure 1 Components of a DC motor
From the above diagram we can see two distinct parts, the stator and the rotor. The stator
comprises of two magnets, positioned opposite in polarity, whilst the rotor is metallic structure upon
which enamel coated copper wire is wound. The DC motor operates on the principle of
electromagnetism and is based on the following laws:
Lenz Law
This law states that an induced electromotive force or voltage in a conductor will produce a current
such that its direction will oppose the change which causes it. (McGraw-Hill 2010) From this law,
Flemings Left Hand rule was developed.
Fleming Left Hand Rule
His rule states that, the force which is produced due the Lenz’s law and the associated magnetic
field, will be at right angles to each other (Fleming 1902) as can be seen from the diagram below:
2
Stator Rotor
Brushes
Commutator
Armature Winding
DC MotorShaft
Permanent Magnets
Figure 2 Fleming's Left Hand Rule (TeacherTube 2010)
Therefore when a voltage is applied to the rotor this sets up a magnetic field. This magnetic field will
interact with the fixed magnets located on the stator. This reaction is such that the forces produced
will result in rotational motion.
Design features such as the DC motor’s brushes, along with split ring commutators feeding individual
armatures around the rotor which are shown below, allow for the switching of the direction current
flow so as to ensure the magnetic force which is produced is always opposite to the magnetic force
of the fixed magnets located in the stator. This therefore produces smooth rotational motion along
with increased torque.
Mathematical Analysis of DC motor
The DC motor operation can be represented using two main equations the first being:
F=BILsinθ Equation 1
Where:
F : Force
B : Magnetic Field Strength
I : Current
L : Length of Conductor
θ : Angle of conductor with respect to magnetic field
3
From the above equation, we can see that for the DC motor the Force is proportional to the,
magnetic field strength and by extension the current in the armature. In addition the length and
position of the conductor influences the magnitude of the force. From the above equation it can be
clearly seen that maximum force will be experienced when the conductor is positioned at right
angles to the magnetic field.
The second equation which can be used to investigate the DC motor is:
T=KT I Aϕ Equation 2
Where:
T : Torque
KT : Motor constant
IA : Armature current
ϕ : magnetic Flux
From Equation 2 we can infer that the motor of the DC motor is directly proportional to the current
and magnetic flux produced by the armature windings.
2.2 Existing Control Schemes for a DC motor and their Advantages and Disadvantages
Control schemes can be divided into two basic categories, open loop and closed loop systems. (Kuo
1991) These first two systems shall be examined using real life existing examples which utilised DC
motors.
Open Loop Control – The Electric Toothbrush
The electric toothbrush is a device which is used to brush one’s teeth and it utilises a DC motor in its
operation along with control circuitry. The function of the control circuitry within the tooth brush is
to decrease the speed of the bristles to provide a gentler brushing experience. (Oral-B 2013) It must
be noted that is control system does not use a feedback to determine if its output has being
achieved. Therefore if excessive force is applied to the toothbrush bristles causing decrease of
speed, control system within the tooth brush does not compensate for this by increasing the torque
of the DC motor.
From this example we can clearly see that with an open loop controller, output errors cannot be
corrected, therefore the system cannot compensate for disturbances in the system. This control
scheme is only used to simplify and reduce the cost of the toothbrush.
4
Table 1 Advantages and Disadvantages of Open Loop Control
Advantages Disadvantages
Control Circuitry only affects/filters the input.
Therefore simpler circuitry
Output is not monitored.
Low cost easy to implement circuit. Input cannot be adjusted based on output
No set point control
(Bucknell n.d.)
Closed Loop Control – Laptop Central Processing Unit Fan
The function is this system is cool the central processing unit of the laptop computer, CPU so as to
prevent overheating. (Erik Steel 2009) With closed loop systems, the output of the system, the
temperature of the CPU is continuously monitored so as to maintain a set point temperature. If at
any instant of time, the temperature increases above the threshold value, the output will be feed to
the input via a comparator which will increase the speed of the DC motor/fan until the temperature
returns to its set point value.
From the above example we can see that with closed loop control, verification of the output result is
achieved and maintained irrespective of disturbances. The table below outlines the advantages and
disadvantages of closed loop control. (Kuo 1991)
Table 2 Advantages and Disadvantages of closed loop control
Advantages Disadvantages
Maintenance of steady output can be achieved Complex to design
Careful tuning of output will result in an output
which can be easily predicted
More expensive to implement than open loop
control systems
Modelling and errors due to external sources
can be reduced
(Bucknell n.d.)
5
Open and Closed Loop can be further integrated with the following Control Schemes:
Table 3 Summary of Control Schemes
(Copeland 2013)
Proportional, Integral, Derivative Control and a combination of each are most commonly used in
industry especially for the control of the DC motor. As a result in the each of these control schemes
shall be investigated mathematically and their advantages and disadvantages outlined in the below.
6
For example Water pump using pressure switchWhen water pressure falls below a set point value electric motor is swtiched on
On/Off Control
Used in control systems in industry where it may be required for the DC motor to be activated based on some external logic input.
Logic Control/Fuzzy Logic Control
Used in electric conveyor systems where a constant speed is required. The torque and speed of the DC motor is adjusted based on the offset from the setpont value.This adjustment is made proportionally to the error.
Proportional Control
Used with DC motor systems with variable loads.The sum of the erros is fed back to the comparator.Produces zero steady state errorSlower responce time
Integral Control
Uses extrapolation to predict error.this error is then feed back to comparartorFaster than intergral Control
Derivative Control
Proportional Control
This form of control can be represented mathematically:
Pout=K pE (t) Equation 3
Where:
Pout = Proportional Term of Output
Kp = Proportion Gain
E(t) = Error: Output – Set Point
Laplace Domain transformation of Equation 3:
Gp(s) = Kp Equation 4
The above can be further represented using the block diagram below:
Figure 3 Block Diagram showing Proportional Control
From the above diagram it can be seen that the output of the system, Y(s) is fed back to the input,
U(s), where the difference between the set point and the output is calculated and multiplied by the
proportional gain, Kp which is then transmitted to the system to adjust the output.
If the value of Kp is large, system instability may occur due to the fact that a large error has to be
corrected. If the value of Kp is very small, then the system may take a long period of time to stabilise.
(Copeland 2013) These observations, along with others are presented in the table below:
Table 4 Advantages and Disadvantages of Proportional Control
Advantages Disadvantages
Accurate Analytical model need not be
designed so as to control system.
Does not reduce eliminated state error.
Simple to implement Oscillation of system may be experienced
Suitable for systems which do not require exact
overshoot, peak response and settling time.
Not suitable for systems requiring a set
overshoot and/or peak response and/or settling
time.
(Copeland 2013)
7
Integral Control
This form of control can be represented mathematically by:
I out=K i∫0
t
E (τ )dτ Equation 5
Where:
Iout = Integral of output
Ki = Tuning parameter of Integral Gain
E(t) = Error: Output – Set Point
Laplace Domain transformation of Equation 5:
Gi (s )=K i
s Equation 6
The above can be further represented using the block diagram below:
Figure 4 Block Diagram showing Integral Control
From the above block diagram we it can be seen that the integral or sum of the error over a time
interval is fed back to the input, where it is multiplied to the integral gain. After which it is fed back
into the system. If implement to a system which utilises a DC motor where a set point speed needs
to be maintained, the speed shall be adjusted based on how much time has passed and the
magnitude of the difference between the set point and output value. The table below outlines
further features of this system. (Copeland 2013)
Table 5 Advantages and Disadvantages of Integral Control
Advantages Disadvantages
Capable of reducing steady state error to zero Slow response time
Due to slow response time, large variation of
output may occur at the instant at which the
error occurs
(Bucknell n.d.) (Copeland 2013)
8
9
Derivative Control
This form of control can be represented mathematically by:
Dout=KD
dE( t)dt
Equation 7
Where:
Dout = Derivative of the Outputs
KD = Derivative gain of System
E(t) == Error: Output – Set Point
Laplace Domain transformation of Equation 7:
GP(s) = KD s Equation 8
The above can be further represented using the block diagram below:
Figure 5 Block Diagram showing Derivative Control
From the above diagram the characteristics of derivative control can be observed. This control
scheme finds the product of the rate of change of error over a period of time and multiples this error
by the KD or the derivative gain. (Smuts 2010) Advantages and disadvantages of this scheme shall be
explored in the table below:
Table 6 Advantages and disadvantages of derivative control
Advantages Disadvantages
Capable of reducing steady state error to zero Slow response time
Due to slow response time, large variation of
output may occur at the instant at which the
error occurs
(Smuts 2010)
10
Proportional Integral Control
This control scheme can be described as a mixture of both the proportional and integral control
systems as a result when implement with industrial DC motor systems, steady state error will be
reduced and set point control is also improved due to the introduction of a pole and zero into the
open loop of the transfer function. It must be noted however that PI control should be used in
situations where it is required that the system tolerates significant overshoot. (Wang 2001)
Proportional Integral Derivative Control
This control scheme incorporates all the features of Proportional Integral Control mentioned above,
with the addition of a faster response time. As previously mentioned one of the main disadvantages
of Integral Control is its slow response time. (Copeland 2013) It must be noted however that the
addition of derivative control not only increases the response time but also introduces inaccuracies
in the system due to the its tendency to magnify noise within the system. (Smuts 2010)
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2.3 Existing Motor Protection and Current Sensing TechnologyIndustry DC motors are vital to many operations, as a result their failure will is significant downtime
and losses. The windings of the DC motor are most susceptible to damage as a result of:
Mechanical abuse – caused mechanical vibrations, lack of lubrication and lack of
maintenance
High temperature – this will result in the degradation of insulation leading to burn outs. The
temperature of a DC motor may rise dude to:
o High operational temperature
o High amperage in armature – may be due to over load of DC motor, a locked rotor or
short circuit condition along line.
o Lack of maintenance – Blockage of ventilation and lack of lubrication will increase
frictional forces within DC motor
Electrical Faults – this will result in a high current flow in windings resulting in burning of
insulation. Several factors can produce electrical faults:
o Internal Faults – this is causes by improper wiring and failure of insulation with in
the DC motor.
o Under voltage – this will result is high currents in the windings, which will produce
high temperatures due to I2R causing degradation of insulation leading to fault
conditions.
o Improper Operation – Over-speed, high duty cycle, and continue disconnection and
reconnection to voltage source produce overcurrent and overload conditions which
result is high temperatures, degradation of insulation and fault conditions.
(Blackburn 1998)
From the above we can see that it is imperative that systems be put in place to prevent overcurrent
and over-temperature conditions with the DC Motor. The table below lists some methods used in
industry for DC motor protection and Current and Temperature Sensing:
12
Table 7 Methods used for DC motor protection
(Miller and Miller 2008)
From the above we can clearly see that most of these protection systems rely on its ability to
measure/sense current. In this section, exactly how this is done in industry shall be explored.
13
Bi-Metal Switches - this deform as temperature increase. If set point is exceeded, a switch is tripped, diconnecting motor.Thermistors - incoperated with current sensing circuit. As temperature increases, resistance decrease, therefore current rises to a set point which will disconnect DC motor using a relay
Thermal Overlad Relays
In the event of a short circuit condition and rating of fuse is exceeded, filament with file will burn out resulting is isolation of DC motor
Fuses
In the event of the loss of a winding, dangerous overspeed conditions may occure. Relays incoperated with current sensing circuitry will isolated DC motor.
Loss of Field Relays
Locked rotor conditions which can result in excessive heating and over current conitions are prevented using this device.When power is applied the control system aticipates a responce from the zero speed switch. If the the switch does not swtich state within a set time, the DC motor is isolated.
Zero Speed Switch
This protects the DC Motor from surges, harmonics and transients from source.This is achieved by simply using an isolation transformer along with filters/line conditioning circuitry.
Source Protection
Current Sensing Techniques
Table 8 Summary of current sensing techniques
(Microchip 2010) (NKT n.d.)
14
This a comprised of a calibrated resistor place in the current path. It produces a voltage drop which is proportional to the current flow according to Ohm's Law. This voltage drop is then amplified by an operation amplifier for easy measuring or operation of cut off componentsThis circuit can be configured for either Unidirectional Low Side current sensing or High Side current sensing.
Resistive Shunt
This non contact form of current sensing is based on the prinicple of induction, where for a given current flow, a proprtional magnetic filed will be produced.This magnetic field produces an EMF due to Faraday's law, which is amplifed and measured.This design and safer and more accurate due to its non contact design, no insertion impedance is introduced.
Hall Effect Sensor
This system is made up of a wire which is wound around the current carrying conductor.Due to Faraday's law a voltage is inducted in the the wire.Conditioning circuitry along with an operational amplifier work together to create an accurate and isolated form of current sensing.
Inductive Sensors
2.4 Existing Approaches to Implementation of Control AlgorithmThe table below outlines some approaches used for the implementation of control algorithms:
Table 9 Control Algorithm Implementation
(KronoTech n.d.) (Xilinx 2012) (Microchip 2010)
15
Algorithm can be implemented using C based code or assembly language.Uses Pluse Width Modulation, PWM which sets a fixed frequency and variable duty cycle.Normally integrated with electronics such as Transistors, Thyristors and H-Bridges to provide the required motor driving wattage.H-Bridges utilises Sign Mgnitude bits whose primary function is to not only vary the power but to switch the rotation of the motor.Locked Anitphase is another version motor control implemented through a H-Bridge and Microcontroller set up. Its function is very similar to that of the Sign Magnitude however only one line is needed to completely control the H-Bridge.
Microcontrollers
Used to execute complex motor control algorithms with increased efficiency.Algorithm implemented using code such as VHSIC hardware description language, VHDL.Capable of implementing algorithms which common micorprocessors can't due to its capability to be implemented with peripherals such as Ethernet, PowerLink and PCI Express.Similar DC motor integration to the Microcontroller. That is utilising solid state electronics.
Field Programmable Gate Array
This is most commonly used in industry as it is the most flexible. It is also very easy to program, therefore control algorithms can be implemented using logic symbols and easy to use graphical software supplied by the manufacturer.
Programmable Logic Control
2.5 Existing Approaches for the Measurement of Motor SpeedThe table below lists the different methods used for the measurement of DC Motor Speed in
industry:
(Opto22 2013) (Fairchild 2001)
16
These are physically mounted onto the shaft. The rotational motion of the DC motor generates pulses. Encodes has a high no of pulses per revolution, this therefore results in a better resolution. Since the enoder is physically attached to the shaft, it introduces errors in the measurement.
Encoders
Normally mounted close to a gear or bolt on the shaft of the DC motor.As the motor rotates, the promity will vary generating pulses, as the bolt/gear teeth move back and forth from the sensor.Contact less in design, therefore does not introduce errors in the measurement.Number of pulses per revolution is dependant on number of gear teeth/bolts. Normally resolution is very low.
Proximity Sensor
Reflective target is placed on DC motor shaft. When light is shone on this reflective taget, the reflection is captured using an optical transitor which generates pulses.This method is contactless, and number of pulses per revolution is dependant on the number of reflective targets placed onto the shaft.Resolution is therefore low, however no resistance is added to the shaft.
Photoelectric Sensors
This utilises a Phototransistor Interrupter Switch along with an Interrupter Disc.The Interrupter Disc is attached to the shaft, as the DC motor spins, light from the emitter end of the Phototransistor Interrupter Switch is interrupted by the holes found the on the surfance of the Interrupter Disc.Therefore, light reaches the base of the optical transistor in pluses, thus creating a plused signal.The resolution id dependant on the number of holes in the interrupter disc. In addition the Interrupter disc adds a load and by extension an error to the measured speed.
Optical Isolator
3 Design Approach and Methodology
3.1 Design Brief
Project Objectives
The main objective of this project is to design and build a DC motor controller, which utilises a PI
control strategy. This objective must be undertaken with the following considerations:
Development of a design which incorporates:
o Industrial Standards for an inherently safer and efficient design
o Business Principles and Practise for efficient time and resource management
This design must be implemented observing:
o Laboratory and Safety Protocols though conducting Risk Analysis
o Engineering Code of Ethics
Project Plan
These objectives where divided into 12 tasks and undertaken over a 7 week period as showing in the
table below:
Table 10 Task description
Task Duration
(days)
1 Background Research of DC motor control systems’ implementation and design 7
2 Development of conceptual hardware design of Display based on Industrial
Standards and Best Business Practise
1
3 Development of algorithm for multiplexing of Display 1
4 Mathematical Modelling of Motor 2
5 Development of PI control algorithm 3
6 Development of conceptual code of PI control algorithm 3
7 Development of conceptual motor driving and protection circuitry taking into
consideration Industrial Standards and Best Business Practise
2
8 Familiarise myself with lab protocols. Conduct risk analysis. Apply appropriated
safety measures.
1
9 Phased Circuit Construction and Code Development. Circuit will be built in
modules. Each module will be tested before another module is built and added
10
17
to the design:
Module 1 – 5V Regulator
Module 2 – PIC16F877 support circuitry
Module 3 – Multiplexing of 4 Seven Segment Displays
Module 4 – Current Limiter and Overcurrent Isolation
Module 5 – Temperature Monitoring and Isolation of Voltage Regulator
Module 6 – Temperature Monitoring and isolation of DC motor.
Module 7 – DC motor driver
Module 8 – Integration of Optical Isolator
10 Testing of system accuracy, and modifications 3
11 Recording of results and observations. 3
12 Compilation of final report 13
18
The Gantt chart below graphically represents the above data, along with the start and end dates.
Task 1
Task 2
Task 3
Task 4
Task 5
Task 6
Task 7
Task 8
Task 9
Task 10
Task 11
Task 12
3/1/
2013
3/8/
2013
3/15
/201
3
3/22
/201
3
3/29
/201
3
4/5/
2013
4/12
/201
3
4/19
/201
3
Figure 6 Gantt chart
19
3.2 Product Design SpecificationThe specific design specifications of this project bearing in mind performance, environment and
product life cycle are detailed below. This table will refer to the different modules for which the
project was divided. Industrial standards by which each module adheres to will be included.
Module 1 – 5V Regulator
Performance Factors - This regulator must be able to accept a voltage range between 6V to
12V, and output a constant 5V. In addition the output must be free of noise and ripples.
Operational Environment Considerations – This regulator will be used to power the PIC
16F877. Apart from the output being a steady DC at a constant 5V, this circuit will be
handled by students. Accommodations must be made.
Product Life Cycle – Excessive heat has the potential to destroy and diminish the life span of
electronics. Regulators tend to get hot especially at high input voltages. Accommodations
must be made.
Module 2 – PIC16F877 Microcontroller and Circuitry
Performance Factors – Requires a voltage of nominally 5V for operation. Exceeding 5.5V will
cause damage to the device. PI control algorithm will be implemented on this
microcontroller. (Microchip, PIC16F87X Data Sheet 2001)
Operational Environmental Considerations – apart from insulation and proper circuit
design. This device will be integrated with many other modules. Design must effectively
utilises PIC16F877 pins.
Product Life Cycle – Destroyed by excessive voltage and current conditions.
Module 3 – Multiplexing of 4 Seven Segment Displays
Performance Factors – This device is required to display the speed of the DC motor clearly.
They are common cathode displays, with seven control lines each. Each segment of the
display operates at 2.0V each. (Jameco n.d.)
Operational Environmental Considerations – These displays will be operated in bright
laboratory environments. Display must be bright enough to be read. These displays will be
interconnected and powered by the PIC 16F877. They should not disrupt the operation or
the PIC 16F877.
Product Life Cycle – Easily destroyed by excessive voltage and current conditions.
Module 4 – Current Limiter and Current Isolation
20
Performance Factors – The current limiter will limited the current to the motor. The current
isolation will protect the entire circuit especially the PIC16F877 from short circuit conditions.
Immediate and reliable reaction is required of this circuit.
Operational Environmental Considerations – this circuit will be supplied with 12V. There is
also be a high power demand.
Product Life Cycle – Lifespan will decrease as a result of high temperatures.
Accommodations must be made.
Module 7 – DC Motor Driver
Performance Factors – This driver circuit is required to be integrated with the PIC16F877,
where it is required to be able to vary the supply voltage to the DC motor from 3V to 12V
whilst not exceeding the maximum rated current of the DC motor of 0.57 A (NICHIBO 2002)
Operational Environmental Considerations – this circuit must not negatively affect the
operation of the PIC16F877.
Product Life Cycle – Lifespan is negatively affected by, current overload in the windings as
well as excessive heat. Necessary accommodations must be made.
Module 8 – Optical Isolator
Performance Factors – must be able to accurately measure the DC motor speed in
revolutions per minute in real time. And must communicate this information effectively with
the PIC16F877. Emitter voltage must not exceed 1.7V whilst sensor’s emitter to collector
voltage must not exceed 4.5V (Fairchild 2001)
Operational Environmental Considerations – this circuit must not negatively affect the
operation of the PIC16F877.
Product Life Cycle – negatively affected by high voltages and currents.
21
3.3 Concept design and EvaluationHaving outlined the design specifications for each module, the process of conceptualising the design
can now begin. Each module has an associated hardware requirement, whilst some modules require
both hardware and software design.
The conceptualisation of each module will all exhibit a similar theme, which is one of Simple
Intelligent Design.
Module 1 – 5V Regulator
Performance Factors – Design will utilise L7805CV regulator, which gives a 5V output and
accepts a voltage range of 6V to 16V. Output will be conditioned using appropriate
capacitors.
Operational Environment Considerations – Appropriated colour codes to distinguish, 5V
and 12V output, positive and negative. LED will be used to indicate that circuit has power. A
switch will also be installed.
Product Life Cycle – Heat sinks and over-temperature circuitry shall be implemented on
regulator.
Module 2 – PIC16F877 Microcontroller and Circuitry
Performance Factors – Both power sources of PIC16F877 to be connected to ensure reliable
power supply. Oscillator output wire to PIC16F877 will be kept short as possible to prevent
noise from distorting the signal.
Operational Environmental Considerations – wiring will be done again following colour
codes.
Product Life Cycle – All wiring was done flat, cut to exact lengths to avoid loss of connection
and short circuits.
Module 3 – Multiplexing of 4 Seven Segment Displays
Performance Factors – Multiplexing of signal lines to optimise the use of PIC16F877 pins.
Due to rapid switching using transistors, power drawn from pins was reduced.
Operational Environmental Considerations – Grey automotive tint was applied to the
displays to improve visibility. The grey tint will improve the contrast of the display.
Product Life Cycle – Resistor pack was chosen so as allow maximum illumination without
exceeding rated current. Resistors were also implement on the base of the NPN transistors.
Module 4 – Current Limiter and Current Isolation
22
Performance Factors – Ensures that current supplied to the DC motor will not exceed rating
stipulated in datasheet. A relay will be used along with a current sensing circuit to isolated
entire circuit in the event of a short circuit.
Operational Environmental Considerations – in the event of a short circuit condition, apart
from the relay tripping, a red LED will be triggered along with an audible alarm, the form of a
piezo buzzer.
Product Life Cycle – a LM350T will be used in this design. This component doesn’t generate
much heat, however a heat sink will be installed to prolong lifespan.
Module 7 – DC Motor Driver
Performance Factors – TIP31C configured in a Darlington Pair will be used for the control
and driving of the DC motor.
Operational Environmental Considerations – Due to the fact that they high powered
transistors and their configuration, the switching/control of the DC motor can be
accomplished without drawing too much power from the PIC15F877.
Product Life Cycle – two TIP31C will be used in this design. This component doesn’t
generate much heat, however a heat sink will be installed to prolong lifespan.
Module 8 – Optical Isolator
Performance Factors – Resistors will be installed in series with the emitter and the senor to
achieve optimal working conditions outlined in the datasheet.
Operational Environmental Considerations – will be integrated with PIC16F877, resistor will
limit sinking current to less than 25mA.
It must be noted that all power supplied to each module will be regulated and protected by
overcurrent isolation circuitry.
The above can be represented using a block diagram which can be used to distinguish the skill
sets required for the production of the entire system.
23
Figure 7 Block diagram of System
From the above diagram one can see that the two min skill sets need in this design project is control
systems and microprocessor programming. All of the modules previously mentioned will work
together to achieve the project’s objectives, a detailed explanation of this process will listed below.
Operation Detail
Set point seed will be determined by a potentiometer which will be interfaced with the PIC16F877.
The output of the potentiometer will be analogue voltage, which will be fed into the PIC16F877,
where it will be converted to a digital 10 bit binary format. This conversion process will ignore the
two lowest bits, whilst the remaining 8 bits will be used to as the set point speed of the DC motor.
The PI control algorithm employs a feedback loop. This therefore requires the output measured
speed to be fed back to the PIC16F877. The speed measuring process will be achieved via, Module 8
along with an interrupter disc. The hardware design of this process was introduced previously. When
properly interfaced with the PIC16F877, the phototransistor along with the interrupter disc, will
convert the rotational motion the DC motor, to a series of pulses which are fed to the PIC16F877,
where it will be counted for a specific period of time.
The counted value, is then stored in a register with in the PIC16F877, this 8 bit value, is then
compared to the set point value, set by the potentiometer. From this comparison an error value is
determined, which will be integrated and scaled so that the output voltage can be adjusted to the
set point value if need be. Therefore under no load conditions the error value will be minimal, little
or no adjustments will have to be made to the output.
24
However if a load was added to the DC motor, its RPM will decease below the set point value.
Module 8 will measure this drop in speed, transfer this information to the PIC16F877, where it will
be processed, stored and compared to the set point. The error value will be determined.
Proportional and integral operations will be performed on this value, which will then be used to
adjust the PWM output to Module 7 – DC motor driver.
25
3.4 Motor & Controller design In this section mathematical modelling along with the determination of parameters and
performance characteristics of design will be undertaken.
Mathematical model of the Motor and Load
The following table outlines the DC motor parameters which will be used:
Table 11 Motor Parameters
Product Number Pc-280-16210
Voltage Range 3VDC – 12VDC
Nominal Voltage 6VDC
Current 0.28A
Stall Current 0.98A
Torque 18gcm
Speed 4260RPM
Efficiency 47.6%
Length of Shaft 11 mm
Diameter of Shaft 2 mm
From these parameters we can develop an equivalent circuit, which is shown below:
Figure 8 DC motor equivalent Circuit
Other Parameters which will be used, not shown in the diagram above, are listed in the table below:
Table 12 Model parameters
Parameter Description Units
Jm Motor moment of Inertia kgm2
Ji Load moment of Inertia kgm2
Jeq Inertia of Motor and Load Equivalent Moment kgm2
26
bm Motor Friction Co-efficient kgm2
beq Motor and Load equivalent friction co-efficient Nm/A
bl Load Friction Co-efficient kgm2
Ka Motor Torque Constant Nm/A
Kb Back EMF constant Vs
Having outlined all the parameters of the system we can now convert the equivalent circuit to the
Laplace domain. From examination of the equivalent circuit we can infer the following:
When a voltage, Ea(t) is applied to the armature of the DC motor, a current Ia(t) will follow
due to the resistive, Ra and inductive Ia components.
Due to Lenz’s Law, and the permanent magnets in the stator, the current in the armature
will produce a torque in the motor, which will produce a turning motion.
This motion will result in the creation of a back EMF.
The table below summarizes the conversion of parameters to the Laplace Domain:
27
Table 13 Conversion of Parameters to Laplace Domain
Operation Process and Reasoning Result
Conversion of Voltage to
Current in Laplace Domain
Applying Kirchhoff’s Voltage Law to equivalent circuitEa( t)=La
d I a(t)dt
+Eb(t )+Ra I a( t)
Laplace Transformation of expression for Ea( t) Ea(s)=s La I a(s)+Eb(s)+Ra I a(s )
Ea ( s)−Eb(s)=Ra I a(s)+s La I a(s)
Making Current, I a (s ) the subject of the formula. From this expression
we can infer that the current can be found by dividing the difference of
between of the applied voltage and the back EMF.
I a (s )= 1La s+Ra
[−Eb ( s)+Ea ( s ) ]
Therefore block diagram should include 1La s+Ra
Conversion of Current to
Torque in Laplace Domain
A force is experienced in the armature, due to the application of a
voltage, thus inducing a current. This force can be represented by:
F=BI a l
This force also generates a torque, where r is the distance between the
axis of rotation and the winding.
T=Fr
Substituting the expression for Force, F into the expression for Torque,
T gives
T=LrB I a
Therefore the Armature Torque Constant, Ka can be represented Ka=LrB
Substituting for Ka reveals that a Ka block may be used to represent the
system Torque
T=Ka I a
Conversion of Torque to Moment of inertia of motor with load Jeq=1
n2J L+Jm
28
Speed in Laplace Domain Co-efficient of motor with load beq=1
n2bL+bm
Since no gears were used in this design n = 1. Appling Newton’s Laws.
Where:
θ is the Angular Acceleration
θ is the Angular Velocity
Jeq θ=−beqθ+T
Laplace transform of T=Ka I a gives θ ( s)=T (s) 1
sbeq+s2 J eq
Substituting for ω ( s )=sθ (s ) gives ω ( s )=T (s) 1beq+s Jeq
Therefore the following block is needed 1beq+s J eq
Conversion of Speed to Back
E.M.F in Laplace Domain
From Lenz’s Law, where:
ϕ is Magnetic FluxEb=
dϕdt
Total/Net Magnetic Flux, where:
A is Area passed through by the armature winding.
∆ ϕ=B ∆ A
This Area can be represented by, where:
∆ l is the distance passed through by winding
∆θ is the corresponding angle traversed
∆ A=L∆ l=Lr ∆θ
Substituting the expression for Area traversed, ∆ A into the expression
for Lenz’s Law, Eb
Eb=LrωB
29
Taking Kb=LrB and substituting into Eb Ka=Kb=LrB
Therefore a Kb block may be used to relate Eb(s) to w(s)
Based on these calculations the block diagram for the signal flow of the DC motor used in this design project is shown below:
Figure 9 DC Motor Signal Flow
Transfer Function of Block diagram: T ω, Ea=Ka
J eq La s2+Rabeq+K aK b+ (J eq Ra+beq La ) s
Equation 9
30
Assuming:
Armature Coil inductance = 0
Therefore beq = 0
Block diagram can be simplified:
Figure 10 Simplified DC Motor block diagram
Simplified transfer function: T w , Ec=Ka
KaKb+J eq Ra s Equation 10
Having developed the simplified transfer function, determination of the key motor parameters must
be done.
Determination of Key DC Motor parameters
This analysis will be shown in the table below:
Operation Process and Reasoning Result
Value of Jeq The moment of inertia of the motor and load,
where: Jm=J load+J shaft
Jeq=J L+Jm
Equation for the moment of inertiaJ= r
2m2
Calculating moment of inertia of shaft, where:
r is the diameter of shaft = 2mm
m is mass of shaft ≈ 0.02 kg
¿0.02×(2mm)2
2
J=4×10−8 kgm2
Calculating moment of inertia of load/ rim, where:
r is the diameter of rim = 30mm
m is the mass of rim ≈ 0.005 kg
¿0.005×(30mm)2
2
J=2.25×10−6 kgm2
Therefore moment of inertia of DC motor Jm=J shaft+J disc
¿4×10−8+2.25×10−6
¿2.29×10−6 kgm2
31
The equivalent moment of inertia is found using,
where J L is the Load/Rim componentJeq=
1
n2J L+Jm
As a result, Jeq=Jm, therefore Jm=9.04×10−6 kgm2
Value of Ka Expressing Ka as a function of Torque and Armature
Current. From this equation, a linear characteristic
between Torque and Armature Current is indicated.
T=Ka I a
The gradient of the linear line in Characteristic
Curve for DC motor graph, see Figure 11 can be
used to determine Ka
m=T 2−T1I 2−I1
=Ka
Using the points (0.5, 40) & (0,0), Ka was
determined. Final answer was multiplied by
9.81gcm to take in consideration gravitational field
strength.
Ka=(40−0)10−5×9.81
0.5−0=7.85×10−3Nm/ A
Value of Kb Ka is equal to Kb due to the fact that they both have
the same base units. It must be noted however that
the units of Kb are Vs /rad
Ka=Kb
Kb=7.85×10−3Vs/rad
Value of Ra Armature resistance was measured using an
Inductance Capacitance meter.
Ra=20.01Ω
Figure 11 Characteristic Graphs of DC Motor (Jameco n.d.)
32
Having determined the key parameters of the motor, this values can be substituted into the transfer
function, Equation 10.
Simplified transfer function:
G(s)= 7.85×10−3NmA−1
7.85×10−3Nm A−1∗7.85×10−3Vsrad−1+¿2.29× 10−6kgm2∗20.01Ω¿
Equation 11
Further simplification:
G(s)= 171.31s+1.34 Equation 12
Determination of Suitable Performance Characteristics
Having determined the transfer function of the DC motor, the block diagram can be modified to suit:
Figure 12 Block diagram of system
The transfer function of the above block diagram and by extension the entire system:
G (s )=171.31(s K p+K i)
s2+(1.34+171.31K p ) s+171.31K i
Equation 13
General transfer function of system:
TF=ωn
2
s2+2ζ ωn s+ωn2 Equation 14
Comparing Equation 13 to Equation 14, so as to determine suitable values for K I and Kp. This process
will outlined in the table below:
Table 14 Calculation of Ki and Kp
Operation and Reasoning Eq.
noResult
33
Setting the s K p term to zero, to simply operations i 171.31K i
s2+ (1.34+171.31K p ) s+171.31K i
Let General representation equation be ii q ( s )=s2+2 ζ ωn s+ωn2
Comparing the denominator and simplifying Eq (i)
and Eq (ii)
iii 171.31K p+1.34=2ωn
171.31K i=ωn2
Using Peak Overshoot Mp = 0.05, Settling time, t2% =
0.15s
iv MP=e−πcotβ
Taking ln of both sides and simplifying for β vβ=tan−1( π
ln 0.05 )=46.36oωn expression for vi ωn=
4t sζ
Where ζ is equal to vii ζ=cos (β )
ζ=0.69
Substituting for ζand ts into Eq (vi) viii ωn=11.59rad /s
Substituting for ωnin Eq (ii) ix K p=15.99−1.34171.31
=0.0855
K i=134.33171.31
=0.7841
Having determined a Kp and Ki, they can be added to the block diagram:
Figure 13 Complete block diagram
Transfer function of entire system can be found by substituting into Equation 13:
14.647 s+134.324s2+15.987 s+134.324
Justification of Controller Choice
34
Testing using Equation 10, T w , Ec=Ka
KaKb+J eq Ra s was done using Matlab to prove that transfer
function, meets the system specifications, that is:
Peak Overshoot Mp = 0.05,
Settling time, t2% = 0.15
35
Figure 14 Step response of system transfer function
Figure 15 Step response with compensator
36
Transfer function for the entire system step response
was plotted in Matlab. The Overshoot was 18.1% whilst
the settling time was 0.424 seconds. These values were
very of, from the system specifications. As a result,
sisotool in Matlab was used to determine the
compensator.
Modification of the Transfer Function resulted, in an
Overshoot of 4.02% and a settling time of 0.122
seconds. Both of these values were within the
specifications of the system design. The compensator
used to achieve these values was 10. As a results the
new values of Ki and Kp are, 0.855 and 7.841
respectively
3.5 Implementation of PI Controller and Data Processing
Algorithm/Flow chart
The diagram below shows a basic flow chart of the system operation:
Figure 16 Simple Flow Diagram of System
In this section, focus shall be placed on the implementation of the PI controller. The following
diagram illustrates the flow process:
Figure 17 PI control Flow Chart
Implementation of Algorithm on PIC16F877
The operation which is required to be performed is Proportional and Integral control, from the
above block diagram we can see the error signal first need to multiplied by a proportional gain.
37
This is done using an 8 x 8 multiplication routine which is shown above. The integral operation of the
PI control, will be implemented, using summation, where the total of all errors will be added to a
particular register. This contents of this register will be added to the contents for the register,
containing the results of the proportional process. The diagram below outlines the 16 bit addition
routine:
Figure 18 Sixteen bit addition Routine
Considerations for Appropriate Scaling Values
The values of Ki = 0.9and Kp = 7.8 to one decimal place, is too small of a value to be interpreted by
the PIC16F877. As a result the 8 x 8 multiplication routine above had to be implemented to increase
their values, by a factor of 10, before they are implemented in the PI control routine.
The result from the PI controller routine will be a sixteen bit value. In order for this value to be
process by the PWM module, it must first be converted to an 8 bit value. This is done using a sixteen
by eight division routine which is shown below:
38
Figure 19 Sixteen by Eight Division Routine.
39
Choice of PIC16F877 Peripherals
Table 15 Microprocessor Peripherals
Peripheral Reason
RC2/CCP1 This pin was configured as a PWM output. This output voltage varies from 0-5VDC.
When integrated with the DC motor driver circuitry, provides speed variation.
TIMER1 This was configured in the count mode. When integrated with the
phototransistor/Speed measuring module, it is used to count the pulses
generated.
RC1/T10SI This pin was configured as the input from the Speed measuring module to the
PIC16F877, and works along will TIMER1 to count the pulses generated.
RD0 – RD7 These were used as output pins for the display, done by configuring TRISD. PORTD
was chosen to perform this function mainly for ease of wiring.
RA1 – RA4 There were used as output pins for the switching of the display transistors to
enable multiplexing. These pins were chosen again, for ease of wiring.
RA0 This pin was configured to be used as the analogue input for the set point value.
CLKIN This pin was configured with the 4MHz oscillator. This examples the timing of the
circuit.
Hardware Justification
The follow table is a list of all the hardware components used in this design project along with their
purpose.
Table 16 Hardware Components
Component Justification
PIC16F877 X1 Flexible high performance RISC CPU, with Interrupt, PWM and Timer
features and peripherals
L780CV X1 5VDC regulator will 6-16VDC input range
10uF & 1uF X1 Filter 5V regulator output
TIP31C X2 High power transistor, capable to switching high loads
LM350T X1 Configured to be used as a current limiter for DC Motor
4MHz
Oscillator
X1 Used to enable PIC16F877 time keeping capabilities
H21A1 X1 Optical interrupted switch. Can be configured to give a digital output
40
which can be interpreted by PIC16F877
2N3904 X1 Cost effective, fast switching solution for multiplexing of display.
GNS-3011Cx X4 Operates with 2VDC, therefore can be easily integrated with PIC16F877,
without drawing too much power from its pins.
Diode X1 To prevent Back EMF
Relay X2 To isolate circuit
LM324AN X2 Current amplification for Cut off
LM35DZ X1 Temperature Sensing
LEDs X3 Used as warning and notification lights to indicate circuit is on, or
overcurrent relay has tripped
41
3.6 Other Design Considerations
DC Motor speed Output
The PWM feature of the PIC16F877 was used to vary the DC motor speed output and was
implemented by configuring the TIMER2 and CCP1 modules. A duty cycle of 500 Hz causes the motor
to spin, and this frequency is adjusted using the following equations:
PWM Fequency= Fosc4∗(PR2+1 )∗(TMR 2Prescalar)
Equation 15
PWM Period= 1PWM Fequency
Equation 16
If the T2CON register is configured to a prescalar of 1:16 and the TMR2 register is configured to zero
and the PR2 register is configured to 255, a PWM output of 500Hz would be generated. It must be
noted that the value of the duty cycle was varied by changing the value of the CCP1L register.
Display Output
As part of the design specification for this project, it is required that the RPM be displayed on the
four seven segment displays. As stated earlier, these displays will be connected to Port D, and since
they are common cathode, their part to ground will be controlled by four transistors each, which will
be connected to pins RA1 to RA4. This configuration allows for a process of multiplexing to occur.
Multiplexing allows multiple signals to be sent along the same line, in our case, four different digits
logics will be sent along the same set of lines periodically. Each display is switched on and the others
are switched off, exactly when that particular display information is being transmitted. This process
of switching each display on occurs at a rate of 20ms, which is too fast for the human eye to detect.
Benefits of this system are, a reduction of power, wiring and smarter utilization of the PIC16F877
resources.
42
Figure 20 Display Output Flowchart
43
Speed Input Routine
In order to determine the speed of the DC motor a phototransistor optical interrupter switch was
integrated with the PIC 16F877 as shown in the diagram below. When the interrupter disc, which is
attached to the shaft of the DC motor is spun, the phototransistor generates a pulsed output signal,
due the fact that the light from the emitter of the device is being continuously blocked and revealed
to the base of the transistor.
This output waveform is fed to pin RC1, which was configured as a TIMER1 peripheral in count
mode. The count will be interrupted every time the pulse transitions from high to low. The speed is
measured every 0.5 seconds where the measured value is stored in a register. This stored value to
output to the display and is used for comparison against the set point value for PI control.
Figure 21 Phototransistor optical interrupter switch
Improvement of Resolution of the Speed Sensor
This can be done, by increasing the number of holes or interrupts in the interrupter disc. In
computer mice, the scroll wheel utilises this technology, however, a greater resolution is required
due to precision required from this device. As a result the no of interrupts in the interrupter disc
found on a computer mouse is close to 200. Desktop printers require an extremely high resolution.
Thousands of minute opaque lines are drawn on a clear plastic to create, this particular high
resolution interrupter disc. (BiPOM 2002)
Design of circuitry to meet current and voltage requirements of the displays
The Seven Segment displays, require 2VDC at 150mA for optimum operation. (Jameco n.d.) However
the PIC16F877, sources 5VDC. In addition, LEDs have a very low resistance, therefore connecting
across a source without a load, can cause a dangerous high current flow due to ohms law.
44
In order for the seven segment displays to be safely integrated with the PIC16F877, they must be
connected in series with a resistor pack. This will not only reduce the voltage, to the display but will
also limited the circuit current. Calculation for the value of this resistor is shown below:
R=Vsupply−VledI
= 5−2150mA
=200Ω
Equation 17
Figure 22 Display wiring (Best-Micocontroller-Projects 2013)
Justification for Choice of Oscillator
An Oscillator is need so as to provide an accurate and stable periodic clock signal to the microcontroller. (Microchip, PIC16F87X Data Sheet 2001) There are different oscillator modes which are outlines in the table below:
Table 17 Different Oscillator modes
Mode Description HS High power consumptionXT Designed for use with Crystals and Resonator of 1 to 4 MHz, with moderate power
consumption, accurate and fast clock rate.LP Low power consumption. Slowest clock rateRC Used in applications where precision is not necessary
Examination of the above table reveals that the XT oscillator is the best choice for this design project, because it is the most accurate, energy efficient and has a fast clock rate.
45
Justification of Component values for Current Sensing, DC Motor Protection and Diver Circuit
Figure 23 External hardware design
46
5V regulator
Current Limiting Resistor for CKT on LED
Current Limiter Ckt
Current Sensing Ckt
Motor Driving Ckt
Diode to blk back EMF
Optical Transistor
Relay
Determination of R3:
V 0=1.25(1+ RaR3 )+ IaRa Equation 18
where Iais amature currnet∧Rais armature resistance
12=1.25 (1+20.01R3 )+5.60 R3=¿ 4.85Ω (PS the value in the diagram above was just included for simulation and is not
actual value used)
For complete hardware justification see Table 16.
Laboratory Protocol
General Rules
Students should not be allowed to enter the laboratory without wearing proper shoes and
clothing. Proper shoes can be described as close toed shoes whist proper clothes can be
describe as clothing which covers one’s body appropriately. Therefore knee high pants and
skirts are not allowed. Excessively loose clothing should not be worn.
Students should not wear long hanging jewellery nor should persons with long hair style
have it loose. This is to prevent personal injury as a result of items being caught in
apparatus.
Bags, food stuff or liquids should not be brought into the laboratory. Desk space is valuable.
Bags clutter equipment and can become a fire hazard. Liquids and food stuff have the
potential to damage equipment if they are spilled onto it. This may also result in personal
injury or loss of life.
Equipment Use
When using the soldering iron please switch off after use. Also switch on the vacuum fan
whist soldering. Long term inhalation of solder fumes can be harmful to one’s health.
When using voltage supply, do not cover ventilation holes. This can be a fire hazard.
When using multi-meter do not tug on leads. Neatly wrap leads after use and place back into
the storage compartment. The leads should never be cut or altered.
When using your bread board, please be aware that the maximum current rating of the
board is 0.5 A. Also one should not use resistors specified to be greater than 0.5 Watts.
47
Never alter, modify or move equipment from its original location. Ask the laboratory
assistant for assistance or permission. Never should network cables be unplugged or altered.
48
Risk Assessment
Table 18 Risk Assessment
Hazards Persons
Affected
Possible Effect of Hazard Likelihoo
d of the
Hazard
Severity
of the
Hazard
Level
of the
risk
Precautions
Burns
from
Soldering
Students
Teaching-
Assistants
Soldering involves the use of a soldering iron, which is
required to be at very high temperatures in order to
melt solder. If the soldering iron gets in contact with an
individual’s body this could lead to severe burns, but
rarely death. Damage to personal and university
equipment is also probable
5 5 E It is recommended that students
wear heat resistant soldering gloves
whist soldering. In addition if pays to
be careful and alert whilst performing
this task
Inhalation
of Solder
Fumes
Students
Teaching-
Assistants
Lab Staff
Whist soldering especially at high temperatures, lead
from the solder atomize, therefore becoming air borne.
Lead is poisonous. Prolonged exposure to solder fumes
can lead to the development of asthma.
2 5 E It is therefore recommend that
students switch on the vacuum fan
when soldering. This will prevent the
inhalation of the fumes.
Chafing
and Injury
to Hands
whist
stripping
wire
Students
Teaching-
Assistants
Due to improper use or poor quality of wire cutters and
strippers, students can damage their hands trying to
strip wire.
3 3 M In order to mitigate this risk, pre-cut
and pre stripped wire can be used.
Automatic wire stripping tools can
also be used. Wearing gloves can be
beneficial.
Burns due Students The transistors which are used in the voltage regulator 3 3 M Students should be aware of
49
to hot
compone
nts
Teaching-
Assistants
are capable of producing a significant amount of heat.
As a result, bodily contact with these particular
components can lead to personal injury.
components that have the potential
of reaching high temperatures. These
components should be identified and
labelled. Wearing gloves whilst
operation is also recommended
Electrical
shock
Students
Teaching-
Assistants
The voltage regulator being designed has a considerable
amount of voltage and current associated with the
design. If a student gets in contact with an un-insulated
conductor that is powered, he runs the risk of
experiencing an electric shock.
3 3 M Students show check circuit before
powering on for exposed conductors.
Teaching Assistants should also
double check the circuit s before
being powered.
Fire
Hazard
Students
Teaching-
Assistants
Lab Staff
Combustible objects may get into contact with hot
components or apparatus such as the soldering iron.
3 5 E Students are to place the soldering
iron back into its holder after use.
Also ventilation holes of equipment
should never be blocked.
Figure 24 Risk Assessment Key
50
Occupational Safety
In this section we will examine the Occupation Safety and Health Act of Trinidad and Tobago 2004 as
it relates to this design project
It is stated is the act in Section 22 that no young person should be allowed to operate
equipment unless he or she is fully trained to do so.
In Section 23 it is stated that persons working in an industrial environment, in our case a
laboratory environment, should wear protective clothing. Students are required to wear
clothing that is conducive to a laboratory environment as described previously. A
Subsection states that signs should be placed outside of the laboratory, warning laboratory
users of the rules and guides especially concerning protective wear. This can be found on the
door on the Electronics Lab.
Section 24 deals with the production of dust and fumes. Fumes which are produced in this
design are due to the soldering process. A vacuum fan can be found at the soldering station
hence complying with the act which states that the production of harmful dust and fumes
should be dealt with so as to prevent inhalation.
Code of Practise
The following is a summary of the codes of practice for engineers as stipulated by the Registered
Professional Engineers in Queensland. It is subdivided into three categories.
Obligations to Society
Engineers should be informed of the environmental, social and economic consequences of
their actions or decisions.
They should act honestly, with integrity and fairness.
They should always perform steps to protect the health and welfare of the community.
They should have a special regard to and try to reduce the effects on the environment.
Obligations to Clients or Employers
Engineers should be truthful and honest. Never should their intention be to mislead or
misrepresent their organisation.
They should always warn clients or users of the consequence of disregarding advice.
They must be open to their employers and disclose any conflict of interest.
They must always keep private information confidential.
General Professional Obligations
51
Engineers must bring their knowledge, expertise and care to any task.
They should not engage in fraudulent behaviour.
They should never promise, accept or receive bribes.
They should continue to develop their skill and expertise.
Industrial Standards
The following are Industrial Standards by which, this project abides:
Trinidad and Tobago bureau of standards wire colour code.
NFPA 70E – Standard for electrical Safety in the work place.
ANSI/NEMA ICS 61800-1 – standard for low voltage adjustable speed electric motors.
52
4 Results and Analysis
4.1 Presentation of Results
Table 19 Summary of Key Parameters
Parameter Calculated Value
Ra 20.01Ω
La -
Ka 7.85 x 10-3
Kb 7.85 x 10-3
Jeq 9.04 x 10-6 kgm2
Beq -
Kp 0.0855
Ki 0.7841
Tests Performed for Evaluation
Figure 25 Step Response of closed Loop control System
53
Figure 26 Step Response with Compensator
Accuracy of Speed Displayed
This test was performed using a hand held tachometer.
Table 20 Results of display testing
Measured Speed Displayed Speed
510 540
625 678
853 953
1401 2031
Protection Module Test
Current Cut off
The protection module was simulated using a 1kΩ pot across input the resistance was varied and
current measured so as to stimulate a short circuit condition. PIC16F877 and DC motor were
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removed from the circuit. At a current exceeding an average of three tests, 0.30 A, relay opened
disconnecting circuit.
Voltage Regulation
Input voltage was varied from 6VDC to 16VDC, output remained at steady 5.1VDC. When connected
to oscillator minimal ripple was observed.
Current Limiter
Again a short circuit condition was simulated using a pot. Current value never exceeded 0.30 A
System Response to difference Input Conditions
Difference loads was added to the system, whilst an oscilloscope was positioned across DC motor
input. The speed was monitored both by the display and the tachometer. Upon addition of the load
an increase in voltage was observed and an increase in speed when then averaged to the set point
value.
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4.2 DiscussionFrom the results in the previous section it was observed that the different modules operated as they
should, when compared to the calculated or measured values.
The display module
This was fairly accurate for small rpms. However there was large inaccuracies at larger values. It
must be noted however that a large about a vibration occurs at these higher rpms does due to the
imbalance of the interrupter disc. This may contribute to the error.
The Protection Module
This module was very successful. This was verified using measurements using the oscilloscope,
voltmeter and ammeter. In addition fault conditions was simulated to see the protection, the
response time in action.
The PI Controller
This module was also successful, however it was notices that the settling time that is for the system
to reach back to its set point after the over shoot was long, approximately 2 seconds. I believe this is
an error with my code, and will be correct before the oral presentation.
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4.3 ConclusionThis design project was indeed a rewarding experience as it may be aware if the importance of
control systems and microprocessors in the world today. There are countless example where this
simple project is used in our everyday lives and I believe that is paramount that especially electrical
and computer engineers understand this design project. In addition this project also made me aware
of the different protection systems, which can be applied not only to my circuit but to real life DC
motors in industry.
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