modelling and simulation of closed loop bldc motor control

i | Closed Loop Control of BLDC Motor

A project on

Modelling and Simulation

of

Closed Loop BLDC Motor Control

Submitted in the partial fulfilment of the requirements of the Bachelor in Technology Degree in

Electrical Engineering

Under

Prof. Madhusudan Singh

By

Anurag Arya (2K11/EE/016)

Anurag Mittal (2K11/EE/017)

Avinash Kumar (2K11/EE/025)

Bhotik Singh (2k11/EE/030)

ii | Closed Loop Control of BLDC Motor

Certificate

This is to certify that the following is the bona fide work of Anurag Arya (2K11/EE/016) ,

Anurag Mittal (2K11/EE/017), Avinash Kumar (2K11/EE/025) and Bhotik Singh

(2K11/EE/030) in partial fulfilment of the requirements of the degree in Bachelor of

Technology in Electrical Engineering. They have worked under my supervision on the project

titled Modelling and Simulation of Closed Loop BLDC Motor Control during the Odd

Semester 2013-14.

Dr. Madhusudan Singh

Professor and Head

Department of Electrical Engineering

Delhi Technological University

ii | Closed Loop Control of BLDC Motor

Acknowledgements

We are highly grateful to our mentor Dr. Madhusudan Singh for his guidance and support.

Special thanks to our seniors Bharat Garg and Rohit Gupta for advising us in times of

difficulties throughout the project.

iii | Closed Loop Control of BLDC Motor

Abstract

Brushless Direct Current Motors or BLDC Motors are currently growing in popularity owing

to the advent of power electronic switching circuits and further improvements in sensing

technologies. As such, now, a lot of fields employ this machine for varied purposes primary

among them being motion control, positioning and actuation systems. The industrial

engineering industry is shifting to BLDC use due to its high power density, good speed

torque characteristics, high efficiency and wide-speed ranges. Also being brushless, they

require lesser maintenance than their brushed counterparts.

The following report presents the Modelling and Simulation of Closed Loop BLDC Motor

Control along with its construction, mechanism of working, applications and advantages.

Basically, control of BLDC motor consists of supplying the gating pulses for the inverter

bridge configuration in order to excite the phases in a particular sequence. Hall sensor input is

fed back to the control circuit which then determines which phase to excite next in order to

move the motor in a particular direction. This control can then be applied in a closed loop

scheme, where a reference input can be used to adjust the output of the control circuit to get

the desired gating pulses. This reference input is usually taken as the speed for fixed speed

operation or to provide variable speed operation to the user independent of the load. For this,

the speed is also taken as an output from the motor normally by using a shaft encoder. It is

then fed to the control circuitry after suitable conversion.

The aim of our minor project is to create such a closed loop control for the brushless DC

motor which will provide constant speed operation independent of the shaft torque.

iv | Closed Loop Control of BLDC Motor

Table of Contents

Chapter Title Page Number

Title Page..i

Abstract....ii

Table of Contents........iii

List of Figures.iv

List of Tablesv

List of Abbreviations...vi

Chapter1Introduction.1

1.1 BLDC Motor Construction.1

1.2 BLDC Motor Operation and Characteristics..2

1.3 Comparison of BLDC motor and Brushed Motor..4

1.4 Comparison of BLDC and PMSM.4

Chapter 2 Control Schemes....6

2.1 Types of Control.....6

2.2 Open Loop Control.....6

2.3 Closed Loop Control..6

Chapter 3 Modelling in SIMULINK.9

3.1 Machine Specifications..9

3.2 MATLAB model....9

3.3 Generation of Error Signal.......10

3.4 Generation of Pulses.....11

3.5 Results of Manual Tuning12

Chapter 4 Hardware Implementation...17

4.1 Suggested Hardware Model.17

4.2 Bipolar power Supply...18

Chapter 5 Conclusion...20

v | Closed Loop Control of BLDC Motor

5.1 Conclusion20

References.21

vi | Closed Loop Control of BLDC Motor

List of Figures

1. Constructional view of a BLDC Motor..1

2. Waveforms in two phase ON operation and the torque ripple2

3. Hall sensor waveforms and corresponding phase voltage waveforms...3

4. Control strategy 1 : Speed feedback...6

5. Control strategy 2 : No speed/ current feedback.7

6. Control strategy 3 : Speed and current feedback7

7. Comparison of torque outputs for sinusoidal and trapezoidal BEMFs...8

8. Simulink Model of the system..10

9. Generation of the error signal...11

10. Generation of pulses for excitation of gates.11

11. Current regulator for generation of pulses12

12. Circuit showing connection between inverter and machine.12

13. Wr vs. Wref for no load13

14. Fluctuations in speed at steady state conditions...13

15. Current waveform of calculated current I*abc.13

16. Current waveform of output at motor...13

17. Waveform of torque vs. reference torque.14

18. Wr vs. Wref for 2.5 N-m...14

19. Fluctuations in speed at steady state conditions for 2.5 N-m...14

20. Waveforms for I*abc for 2.5 N-m15

21. Waveforms for I abc for 2.5 N-m.15

22. Torque vs. reference torque for 2.5 N-m..15

23. Wr vs. Wref for 5 N-m..................................15

24. Fluctuations in speed at steady state conditions for 5 N-m..16

25. I*abc waveforms for 5 N-m..16

26. I abc waveforms for 5 N-m...16

27. Torque vs. reference torque for 5 N-m.16

28. Wr vs. Wref for 6.5 N-m load...17

29. Selected set of values from the iterations.17

30. Circuit diagram for bipolar power supply.18

31. Circuit implementation of the bipolar power supply19

vii | Closed Loop Control of BLDC Motor

List of Tables

1. Comparison between brushed DC and brushless DC motors4

2. Comparison between BLDC and PMSM...5

3. Machine specifications for the model used9

4. Table of excitation sequence....12

viii | Closed Loop Control of BLDC Motor

List of Abbreviations

1. BLDC Brushless Direct Current

2. PMSM Permanent Magnet Synchronous Motor

3. BEMF Back EMF

4. PI Proportional Integral

5. PID Proportional Integral and Derivative

2 | C l o s e d L o o p C o n t r o l o f B L D C m o t o r

Chapter 1

Introduction

Conventional dc motors are highly efficient and their characteristics make them suitable for use as

servomotors. However, their only drawback is that they need a commutator and brushes which are

subject to wear and require maintenance. When the functions of commutator and brushes were

implemented by solid-state switches, maintenance-free motors were realised. These motors are now

known as brushless dc motors.

1.1 BLDC Motor Construction

The construction of modern brushless motors is very similar to the ac motor, known as the permanent

magnet synchronous motor (PMSM). Fig.1 illustrates the structure of a typical brushless dc motor.

The stator windings are similar to those in a polyphase ac motor, and the rotor is composed of one or

more permanent magnets. Brushless dc motors are different from ac synchronous motors in that the

former incorporates some means to detect the rotor position (or magnetic poles) to produce signals to

control the electronic switches. The most common position/pole sensor is the Hall element, but some

motors use optical sensors.

Figure 1: Constructional view of a BLDC Motor [1]


1.2 BLDC Motor Operation and Characteristics

The biggest advantage that a BLDC motor has over conventional brushed DC motors is that it

requires electronic commutation. This is achieved with the help of input from the motor in the form of

hall sensor output and electronic control circuitry that controls the firing pulses for the inverter

configuration.

The principle of the BLDC motor is, at all times, to energize the phase pair, which can produce the

highest torque. To optimize this effect the back EMF shape is trapezoidal. The combination of a DC

current with a trapezoidal back EMF makes it theoretically possible to produce a constant torque. In

practice, the current cannot be established instantaneously in a motor phase; as a consequence the

torque ripple is present at each 60 phase commutation.

Figure 2: Waveforms in two phase ON operation and the torque ripple [2]

The key to effective torque and speed control of a BLDC motor is based on relatively simple torque

and back EMF equations, which are similar to those of the DC motor. The back EMF magnitude can

be written as [3]:

E = 2NIRBw

and the torque term as:

T =

i2

-

B

2

+

Brli

Where,

N = number of winding turns per phase,

l = length of the rotor,

r = internal radius of the rotor,

B = rotor magnet flux density,

w = motors angular velocity,


i = phase current,

L = phase inductance,

= rotor position,

R = phase resistance.

The first two terms in the torque expression are parasitic reluctance torque components. The third

term produces mutual torque, which is the torque production mechanism used in the case of BLDC

motors. To sum up, the back EMF is directly proportional to the motor speed and the torque

production is almost directly proportional to the phase current.

The BLDC motor is characterized by a two phase ON operation to control the inverter. In this control

scheme, torque production follows the principle that current should flow in only two of the three

phases at a time and that there should be no torque production in the region of the back EMF zero

crossings.[4]

The easiest way to know the correct moment to commutate the winding currents is by means of a

position sensor. Many BLDC motor manufacturers supply motors with a three-element Hall Effect

position sensor. Each sensor element outputs a digital high level for 180 electrical degrees of

electrical rotation, and a low level for the other 180 electrical degrees. The three sensors are offset

from each other by 60 electrical degrees so that each sensor output is in alignment with one of the

electromagnetic circuits.

Figure 3: Hall sensor waveforms and corresponding phase voltage waveforms

Another precaution against both drivers being active at the same time is called dead time control.

When an output transitions from the high drive state to the low drive state, the proper amount of time


for the high side driver to turn off must be allowed to elapse before the low side driver is activated.

Drivers take more time to turn off than to turn on, so extra time must be allowed to elapse so that both

drivers are not conducting at the same time. The high drive period and low drive period of each output

is separated by a floating drive phase period. This dead time is inherent to the three phase BLDC drive

scenario, so special timing for dead time control is not necessary. The BLDC commutation sequence

will never switch the high-side device and the low-side device in a phase, at the same time.

1.3 Comparison of BLDC Motor and Conventional DC motors

Although it is said that brushless dc motors and conventional dc motors are similar in their static

characteristics, they actually have remarkable differences in some aspects. When we compare both

motors in terms of present-day technology, a discussion of their differences rather than their

similarities can be more helpful in understanding their proper applications.

When we discuss the functions of electrical motors, we should not forget the significance of windings

and commutation. Commutation refers to the process which converts the input direct current to

alternating current and properly distributes it to each winding in the armature. In a conventional dc

motor, commutation is undertaken by brushes and commutator; in contrast, in a brushless dc motor it

is done by using semiconductor devices such as power transistors, thyristors, power MOSFETs etc.

A tabulated study is shown between the distinctive properties of both the motors below.

S No. Property Conventional Motors BLDC Motors

1. Mechanical Structure Field magnets/ windings

on stator

Field magnets on the

rotor.

2. Distinctive feature Quick response and

excellent controllability

Long lasting and easy

maintenance

3. Commutation method Mechanical contact

between brushes and

commutator

Electronic switching

using transistors

4. Detecting method of rotor position Automatically detected

by brushes

Hall element, optical

encoder

Table 1 : Comparison between brushed DC and brushless DC motors

1.4 Comparison of BLDC and PMSM motors

Both BLDC and PMSM motors have permanent magnets on the rotor, but differ in the flux

distributions and back-EMF profiles. While one is excited by DC supply (BLDC) the other is supplied

through a three phase AC supply.


S.No. BLDC PMSM

1. Synchronous Machine Synchronous Machine

2. Fed with direct currents Fed with sinusoidal currents

3. Trapezoidal Back EMF Sinusoidal Back EMF

4. Stator Flux position commutation each

60

Continuous stator flux position variation

5. Only two phases ON at the same time Possible to have three phases ON at the same

time

6. Torque ripple at commutations No torque ripple at commutations

7. Low order current harmonics in the

audible range

Less harmonics due to sinusoidal excitation

8. Higher core losses due to harmonic

content

Lower core loss

9. Less switching losses Higher switching losses at the same switching

frequency

Table 2 : Comparison between BLDC and PMSM[5]

Thus, from the above comparison it can be seen that even though both PMSM and BLDC are similar

in construction the use depends on the kind of control strategy employed and the power source used to

drive the motor.


Chapter 2

Control Schemes

2.1 Types of Control

Control of BLDC Motor can be carried out both in :

i. Open Loop control

ii. Closed Loop control

2.2 Open Loop Control

Open loop control of BLDC motor does not require any speed comparison for negative feedback. The

hall sensor input is fed to the PWM control unit which decides the sequence of pulses based on the

output of the hall sensor.

2.3 Closed Loop Control

Closed Loop control of BLDC can be done by any one of the following standard methodologies:

1.

Figure 4: Control strategy 1 : Speed feedback [4]

In the above model, speed is computed from the VI measurement at the terminals of the BLDC motor.

The hall sensor output is used for synchronization and PWM control, while the speed that is

calculated from the BLDC motor is fed back as negative feedback for calculating the error in the

desired speed. The error is fed back to the PI controller which is again used in PWM generation. The

above mentioned control system is advantageous as VI measurement is easy at the terminals of the

output.

2.


Figure 5 : Control strategy 2 : No speed/ current feedback[4]

The second type of control strategy as described in the above block diagram employs a current control

loop configuration. In this only a reference current is fed to the PID controller. VI measurement is

again done at the terminals of the BLDC motor. The current is used to generate the gate pulses for the

inverter by comparing it to the terminal output.

3.

Figure 6 : Control strategy 3 : Speed and current feedback [4]

This method of BLDC motor closed loop speed control has similar feedback as seen in the first

scheme. However, the feedback is fed at different points of the control path. The speed is first

compared to the reference speed. The error is passed through a PI controller and reference current is

calculated. The current is then compared to the current at the terminal of the output and this is again

fed through a PID controller for further tuning. By far the most complex strategy, the scheme requires

two stages of tuning and more circuitry. The output of the PID controller is then fed to the PWM

control unit which generates the logic pulses for the PWM control.

The BLDC motor is characterized by a two phase ON operation to control the inverter. In this control

scheme, torque production follows the principle that current should flow in only two of the three

phases at a time and that there should be no torque production in the region of the back EMF zero

crossings.

This control structure has several advantages:

Only one current at a time needs to be controlled

Only one current sensor is necessary (or none for speed loop only)


The principle of the BLDC motor is, at all times, to energize the phase pair, which can produce the

highest torque. To optimize this effect the back EMF shape is trapezoidal. The combination of a DC

current with a trapezoidal back EMF makes it theoretically possible to produce a constant torque. In

practice, the current cannot be established instantaneously in a motor phase; as a consequence the

torque ripple is present at each 60 phase commutation.[7]

If the motor used has a sinusoidal back EMF shape, this control can be applied but the produced

torque is not constant but made up from portions of a sine wave. This is due to its being the

combination of a trapezoidal current control strategy and of a sinusoidal back EMF. But a sinusoidal

back EMF shape motor controlled with a sine wave strategy (three phase ON) produces a constant

torque. Also, the torque value produced is weaker.

Comparison of Torques produced due to sinusoidal and trapezoidal back EMFs is shown below.

Figure 7 : Comparison of torque outputs for sinusoidal and trapezoidal BEMFs


Chapter 3

Modelling in SIMULINK

3.1 Machine Specifications:

Machine Type BLDC

Voltage 48 V

Maximum Torque 17.8 N-m

Rated Speed 500 rpm

Number of poles 4

Stator resistance per phase 0.1 ohm

Table 3 : Machine specifications for the model used

3.2 SIMULINK Model

The modelling of the closed loop BLDC motor control is done using Simulink. Following figure

shows the entire model as developed on Simulink using machine specifications as specified above:

Figure 8: Simulink Model of the system


The Simulink model can be divided into two parts above, one the driving circuitry which comprises of

the error detection from speed, the Discrete PI controller and the pulse generator. The other part is the

inverter and the BLDC motor.

It is seen from the Simulink model that the following variables /parameters are taken as output (or

sensed) from the machine:

Hall sensor output

Phase currents

Speed

Torque

Thus, any or all of these parameters shall be employed in the feedback for control scheme to be used

in the system. All outputs are used somewhere or the other as inputs to eliminate errors in the running

scheme of the machine.

3.3 Generation of Error Signal

With the speed as feedback parameter, it is passed through a low pass filter (discrete) and then

subtracted from reference speed to get error value as shown in the given block:

Figure 9: Generation of the error signal

The error signal is fed to a PI controller which is to be tuned to generate appropriate closed loop

response. The output of the PI controller is fed to a system block that generates the pulses required to

switch ON the gate signals of the three phase inverter bridge.

3.4 Generation of Pulses

The signal from the Discrete PI Controller is fed through a multiplier that calculates the value of

current from the value of torque using the given input and set relation between current and torque.

The hall input is then used to find the next state of excitation. This is done by passing it through a

suitable logic circuitry designed for this specific use. The pulses are then multiplied to the calculated

value of current. This gives the value of current in pulsed form. Finally, this output is compared to

current at the terminals of the motor. The logic is explained with the help of diagrams below.


Figure 10: Generation of pulses for excitation of gates

For generation of pulses, the Te* input from the PI controller is used to compute the current Iabc*

which is multiplied to the output of the hall sensors. The hall sensor output is passed through a logic

circuit to correspond to a specific switching sequence. The truth table is given below:

Ha Hb Hc Emf_a Emf_b Emf_c

0 0 0 0 0 0

0 0 1 0 -1 +1

0 1 0 -1 +1 0

0 1 1 -1 0 +1

1 0 0 +1 0 -1

1 0 1 +1 -1 0

1 1 0 0 +1 -1

1 1 1 0 0 0

Table 4: Table of excitation sequence

The Iabc output is then used as a feedback and compared with the calculated values of Iabc* which is

then passed through a current regulator, which converts the difference between the calculated and

measured currents to give pulsed output.

Figure 11: Current regulator for generation of pulses


The pulses are then fed to the three phase inverter to drive the gates of the MOSFETs.

Figure 12: Circuit showing connection between inverter and machine

3.5 Results of Manual Tuning

The above model has been manually tuned with the following results:

1. For No- load at Kp = 2.7 and Ki = 100

Figure 13: Wr vs. Wref for no load

Figure 14: Fluctuations in speed at steady state conditions


Figure 15: Current waveform of calculated current I*abc

Figure 16: Current waveform of output at motor

Figure 17: Waveform of torque vs. reference torque

2. For a load of 2.5 N-m the outputs are as shown below :

Figure 18: Wr vs. Wref for 2.5 N-m


Figure 19: Fluctuations in speed at steady state conditions for 2.5 N-m

Figure 20: Waveforms for I*abc for 2.5 N-m

Figure 21: Waveforms for I abc for 2.5 N-m

Figure 22: Torque vs. reference torque for 2.5 N-m

3. For a load of 5 N-m, the outputs are as shown below


Figure 23: Wr vs. Wref for 5 N-m

Figure 24: Fluctuations in speed at steady state conditions for 5 N-m

Figure 25 : I*abc waveforms for 5 N-m

Figure 26: I abc waveforms for 5 N-m


Figure 27: Torque vs. reference torque for 5 N-m

4. For a load of 6.5 N-m, an over damped response is obtained

Figure 28: Wr vs. Wref for 6.5 N-m load

Thus, no response is obtained for a torque of 6.5 N-m and above. However , if the output limits in the

PI controller are increased then a suitable response may be obtained

Figure 29: Selected set of values from the iterations

A certain set of values have been taken from the tabulated results obtained during the process of

tuning. It is seen here that in the highlighted value, for 5 N-m the values of system response i.e. rise

time, peak time, peak overshoot and steady state error are obtained and well within reasonable limits.

Further improvements can be done by online tuning once the hardware is set up.


Chapter 4

Hardware Implementation

4.1 Suggested Hardware Model

The hardware implementation to the above can be done using the following method:

The hall sensor inputs that are taken as feedback from the motor are fed back to the brushless motor

controller from Texas Instruments, UC2625. This IC is used to generate the requisite response to the

hall sensor output and this output is fed to the MOSFET driver also from Texas Instruments. This

MOSFET driver is to be connected to the three phase inverter circuit that is to be fed from the

rectified AC mains power supply.

Thus, in total, the following hardware has to be developed :

1. Circuit for UC2625 and pin connections for hall input and MOSFET driver output.

2. Connection of MOSFET driver output to three phase inverter

3. Single phase rectifier to convert AC mains to rectified supply

4. DC-DC step down converter to convert rectified output of rectifier to the range of input

voltage of motor i.e. around 48 V.

5. Three-phase inverter to convert DC supply to AC.

6. Current sensing circuit to sense phase current at the terminal of BLDC motor.

7. Shaft encoder circuit to capture speed of motor at the shaft.

4.2 Bipolar Power Supply

For the current sensing circuitry, the team is using a TELCON HTP25 current sensor. This requires a

bipolar supply of + 15 V. We have tested the circuit of the bipolar power supply for the same. The

circuit diagram and the circuit are as shown below:

Figure 30: Circuit diagram for bipolar power supply


The circuit execution for the same is as shown below:

Figure 31: Circuit implementation of the bipolar power supply

We shall now begin work on the implementation of the circuitry of the UC2625 and the rectifier

circuit.


Chapter 5

Conclusion

5.1 Conclusion

The above results show that even after much iteration in the tuning process only a passable accuracy

is achieved. More accuracy can be achieved by online tuning of the same. This can be achieved only

after hardware implementation is complete. We plan to completing the hardware during the inter-sem

period and then testing it in the next semester. The results achieved thus far show that the closed loop

BLDC control can be achieved pretty accurately by modelling it in the manner suggested above.

With the widespread use of power electronic devices and dedicated solutions from key players Closed

loop control of BLDC motor is now easily possible. The use of BLDC also is now growing with the

spurt in increase of electric vehicles on the roads. Efficiency and longevity of these machines have

proven to be a big advantage as compared to conventional machines. Also, since they pack more

power density than other types of the same rating they are preferred in applications where size is a

constraint. Thus, BLDC has a huge advantage to offer for several applications.

We look forward to executing the hardware for the above simulation and software model by the next

semester and obtain suitable results for our implementation.


References

[1] R. Kasim, K.A. Ismail, A. Jidin, N. Bahari Modelling and Simulation of Brushless DC Machines,

University of Technical Malaysia

[2] S. Lee Application of an Intelligent Motor Controller to the Three-Phase Brushless DC Motor

Drives, Penn State University

[3] Trapezoidal Control of BLDC Motors Using Hall Effect Sensors, Application Report Texas

Instruments

[4] 3-Phase BLDC Motor Control with Hall Sensors, Application Note, Freescale Semiconductors

[5] Power Electronics Circuits, Devices and Applications, M.H. Rashid 6th Ed.

[6] Control System Engineering, I.J. Nagrath and M. Gopal

[7] Brushless DC Motor Control, Microchip Appnote AN857

[8] I. Alphonse, S. HosiminThilager, F. Bright Singh Design of Solar Powered BLDC Motor Driven

Electric Vehicle

[9] Power Electronics, Dr. Ali Mohamed Eltamaly

[10] Analysis of Electric Machinery and Drive Systems, Paul C. Krause, Oleg Wasynczuk 2nd

edition.

[11] Modern Power Electronics and AC Drives, Bimal K. Bose

modelling and simulation of closed loop bldc motor control

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