dc motor speed control

Introduction to Control System

Do it yourself P a g e | 1

DC motor speed control In general, a closed loop control always consists of at least five

main blocks: desired voltage, controller, plant, actual voltage, and

feedback constant, as it is shown on figure 1. The implementation of

the theory is slightly different. Figure 2 shows the block diagram of

DC motor speed control. As it is shown on figure 2, the desired speed

firstly is converted to desired voltage by introducing the

sensitivity of speed sensor module. After calculating the error

between desired voltage and current voltage, the proposed control

action voltage is generated. This calculation is done in a computer.

The control action voltage which is still a digital data is then

converted into analog voltage by DAC (digital to analog) module before

it is amplified by a power amplifier which will drive the DC motor. A

speed sensor module (F2V) is sensing the speed of DC motor, and

converting this speed into voltage. After that, this analog voltage is

converted into digital data by an ADC (analog to digital) and the

computer can calculate the next control action. On this course, the

DAC and ADC is performed on a single hardware, namely NI USB 600X. It

is called as 600X since it could be 6001, 6002, 6008, and 6009.

Moreover, the power amplifier and F2V are placed in the single

hardware module.

Figure 1. General closed loop diagram block

Figure 2. DC motor speed closed-loop control

Desired

voltage Controller Plant

Actual

Voltage

K

-

+

F2V sensitivity

value

F2V Hardware

F2V : Frequency to voltage speed sensor module



DC motor speed control hardware module

The DC motor speed control hardware is developed as simple as possible

so that it can be made by student with basic electronic understanding.

There are three main circuits on this module:

1. Power supply circuit 2. Power amplifier circuit 3. And F2V circuit.

The photography of DC motor speed control hardware module is shown on

figure 3.

Figure 3. DC motor speed control hardware module

Power supply circuit

Power supply circuit aims to deliver a stable supply voltage to the

ICs employed by power amplifier and F2V. The power amplifier circuit

needs 18V supply to work while the F2V circuit needs 5V and 18 V

supply. Therefore, on this power supply circuit, two fixed voltage

regulator ICs is utilized: 7805 to supply 5V and 7818 to supply 18V.

The complete circuit of the power supply is shown on Figure 4.

Figure 4. DC Power Supply Circuit

Power

supply

Power

amplifier

F2V



Power amplifier circuit

The power amplifier circuit is applied to amplify a low power signal

(the output of NI USB 600X) so that it can drive the DC motor. The

full power amplifier circuit is shown on Figure 5. As it is shown, the

power amplifier is developed from three main circuits:

1. Non inverting amplifier 2. Emitter follower 3. Current limiting circuit

Figure 5. Power amplifier circuit

Non inverting amplifier

The input signal of non-inverting amplifier circuit is the voltage

generated by NI USB 600X. This arrangement makes the non-inverting

amplifier is the circuit interacts with the NI USB 600X. By having an

op-amp interact with the NI USB 600X, no current will be drawn from

voltage generator device. The second reason on using this simple

circuit is this circuit enabling us to multiply the input voltage by a

constant which is determined by R1 and R2.This amplification is an

advantage because many voltage generator devices are only able to

generate 0 up to 5 Volt.

Figure 6. Non inverting amplifier circuit

2N3055 current booster BC107 + 0.22 current limiter

Feedback is put

right after

current booster

Non inverting

amplifier, Gain = 3.

05 V 015 V

= 1 +21



Emitter follower feedback

Due to lack of current output, an op-amp couldnt drive a DC motor

directly. Therefore, the circuit needs a current booster. The most

common current booster device is transistor. On this module, a 2N3055

NPN transistor is applied. Furthermore, feedback line of op-amp is put

on the emitter side of NPN transistor. Meanwhile the output of op-amp

is driving the base of NPN transistor. By having this arrangement, the

voltage on emitter will always follow the input signal although there

is 0.7 V difference between B (Op amp output) and E. This circuit is

known as emitter follower.

B

C

E

18 V24 V

LoadLM 358

Input signal

2N3055

Figure 7. Emitter follower circuit

Current limiter circuit (Wikipedia)

A typical short-circuit/overload protection scheme is shown in the

Figure 8. The schematic is representative of a simple protection

mechanism employed in regulated DC supplies and class-AB power

amplifiers.

Figure 8. Current limiting circuit

Q1 is the pass or output transistor. Rsens is the load current sensing

device. Q2 is the protection transistor which turns on as soon as the

voltage across Rsens becomes about 0.65 V. This voltage is determined by

the value of Rsens and the load current through it (Iload).

When Q2 turns on, it removes base current from Q1 thereby reducing the

collector current of Q1. Neglecting the base currents of Q1 and Q2,

the collector current of Q1 is also the load current. Thus, Rsens fixes

the maximum current to a value given by 0.65/Rsens, for any given output

voltage and load resistance.



Calibration result

The calibration result of power amplifier is shown on figure 9. As it

shown below, the effect of current limiting circuit is clearly present

when the load is about 1.5 Amp (15V output voltage).

Figure 9. Power Amplifier Calibration Result

0

3

6

9

12

15

18

0 1 2 3 4 5 6

Ou

tpu

t (V

)

Input (V)

No Load

Load 10 Ohm



F2V circuit

F2V circuit works as a speed sensor of DC motor. This circuit employs

a single stage rotary encoder (shaft encoder) and photo interrupter as

the sensor. Furthermore, the signal conditioning of this sensor is

done by LM 2907 which is known as a frequency to voltage IC. The

complete circuit of F2V circuit is shown on figure 10.

Figure 10. F2V circuit

Rotary Encoder (http://www.robometricschool.com)

Rotary encoder or also called with shaft encoder used to change linear

movement or rotary to be digital signal 0 and 1. The above figure

shows that disk with hole (rotary encoder) rotate between opto-

interrupter which consists of a IR LED and photo transistor. The opto-

interrupter as rotary sensor will monitor the movement of disk with

infra-red light that transmitted from IR LED to the infra-red receiver

from photo photo-transistor. Digital signal will be get from infra-red

signal that allowed and not allowed in the disk hold. This system will

therefore generate a pulse-signal as the infra-red with frequency

following the shaft speed. On this module an H21A3 opto-interrupter is

employed under the following circuit, figure 11. On the figure, the

circuit utilizes a special transistor (2369 NPN transistor) which is

called as switching transistor. A switching transistor is a special

transistor which only needs a very short time to change from on to off

state and vice versa.

Sensitivity

adjuster

Output

stabilizer

Input signal

Comparator

reference 2.5V

Switching

transistor



Figure 11.a Rotary encoder

and opto-interrupter

Figure 11.b Speed sensor circuit

LM 2907 circuit

LM 2907 is a frequency to voltage IC. In general LM 2907 will

calculate how many times the input signal crossing the reference

voltage in a unit time. And therefore, the reference voltage is set to

one half of the opto interrupter source.

Figure 12. LM 2907 Circuit

DC Motor

DC Motor is manufactured by Canon. The maximum input of the DC motor

is about 24VDC. Meanwhile, the typical sensitivity of the motor is

about 150 rpm/V. This value varies between motors.

IR LED

Photo

transistor

Pul

Shaft

encoder

Sensitivity

adjuster Output

stabilizer

Output

impedance

Voltage

divider

Opto-interrupter

ref.z voltage

LM 2907 Circuit Calibration Result

C1 R1

= 1 1

Vcc

Comparator

reference 2.5V

dc motor speed control

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