Dc Motor Speed Control.pdf

Introduction to Control System

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.

Desired voltage

+

Controller

Plant

-

Actual Voltage

K

Figure 1. General closed loop diagram block

F2V sensitivity value F2V Hardware F2V : Frequency to voltage – speed sensor module

Figure 2. DC motor speed closed-loop control Do it yourself…

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Introduction to Control System

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.

Power amplifier

Power supply

F2V

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 Do it yourself…

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Introduction to Control System 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 Non inverting amplifier, Gain = 3. 0–5 V  0–15 V

Feedback is put right after current booster

2N3055  current booster BC107 + 0.22Ω  current limiter 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 R 1 and R2.This amplification is an advantage because many voltage generator devices are only able to generate 0 up to 5 Volt.

𝑉𝑜𝑢𝑡 𝑅2 =1+ 𝑉𝑖𝑛 𝑅1 Figure 6. Non inverting amplifier circuit Do it yourself…

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Introduction to Control System Emitter follower feedback Due to lack of current output, an op-amp couldn’t 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.

18 V

24 V C B

Input signal E

LM 358

2N3055 Load

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. R sens 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, R sens fixes the maximum current to a value given by 0.65/R sens, for any given output voltage and load resistance. Do it yourself…

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Introduction to Control System 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). 18

No Load Load 10 Ohm

Output (V)

15 12 9 6 3 0 0

1

2

3 4 Input (V)

5

6

Figure 9. Power Amplifier Calibration Result

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Introduction to Control System 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. Comparator reference – 2.5V

Switching transistor

Output stabilizer Input signal

Sensitivity adjuster

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 optointerrupter which consists of a IR LED and photo transistor. The optointerrupter 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.

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Introduction to Control System

Shaft encoder

Photo transistor Pul

IR LED 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. Opto-interrupter ref.z voltage

Comparator reference – 2.5V Vcc

Voltage divider

𝑉𝑜𝑢𝑡 C1

= 𝑉𝑐𝑐 × 𝑓𝑖𝑛 × 𝐶1 × 𝑅1 R1 Output impedance

LM 2907 Circuit Calibration Result

Sensitivity Output adjuster stabilizer

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.

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