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
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
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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
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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.
05 V 015 V
Feedback is put
right after
current booster
2N3055
current booster
BC107 + 0.22 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.
2
=1+
1
Figure 6. Non inverting amplifier circuit
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Introduction to Control System
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.
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.
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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
3
4
Input (V)
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
C1
= 1 1
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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