DETERMINATION OF DC MOTOR PARAMETERS, GAIN OF PREAMPLIFIER,

SERVO-AMPLIFIER AND TACHOMETER

EXPERIMENT # 2

Course: MECH 371

Participants:
1. Denis Mamudovski
2. Eugene To 9611630
3. Nelson Bi 9607978

Date of the experiment: Thursday, February 10th 2011

Due date: Thursday, March 3rd 2011

OBJECTIVE:

The objective of this experiment is to get familiar with the DC Motor Module,

amplifiers and tachometer, which will aid in the calibration of the gain of the pre-amplifier,

servo-amplifier and tachometer.

INTRODUCTION

The DC motor module consists of a preamplifier, servoamplifier, and DC motor &

reduction gear tachometer.

The operation of every DC motor has a deadband. This is due to friction between the

commutator brushes and in the bearings. To overcome this friction a preamplifier is used to

provide a deadband compensation voltage, which provides a fixed forward-path gain. The signal

is sent to the servoamplifier, which drives the DC motor.

The motor velocity is measured by the tachometer. The tachometer has a digital display

screen that can display the motor rpm or a voltage output. This equipment can be wired to an

oscilloscope and be used to evaluate the necessary parameters.

PROCEDURES:

Exp #1

1. Balance the preamplifier and determine the preamplifier bias by connection a common

signal to both inputs.

2. Adjust the zero set knob so that both outputs have the same voltage.

3. Make the connections as shown in Figure 2-7.

4. Set the top pot to 0.5 Volt.

5. Disconnect the common signal and make the connection shown in Figure 2-8.

6. Connect input #1 to CH1 and output #3 to CH2 by leaving input #2 and output #4

unconnected.

7. Vary the input signal by turning the bottom knob from 0 to 0.5 Volt and record data.

8. Disconnect +15V and connect a -15V to socket 3.

9. Connect input #1 and output #4 to the PA150C and leave input #2 and output #3

unconnected and record data by varying the signal from 0 to -0.5V.

Exp #2

1. Set up the connection as shown in Figure 2-9.

2. Apply a voltage of 0 to 3 V by using the top attenuator #3 connect to +15V, #2 to input

#1 of PreAmp.

3. Connect input #1 of ServoA-amp to CH1.

 4. Gradually increase the voltage until an input voltage of 1.5V is obtained and record

values.

5. Continue to increase until an input voltage of 2.5V is obtained and record values again.

6. Apply voltage from 0 to -3V to the input voltage of terminal #1 of the PreAmp by

switching the top attenuator #3 to -15V, and by connecting the terminal #2 of Servo-amp

to CH1.

7. Repeat step 4 and 5 and record data.

Exp #3

1. Make the circuit shown in the Figure 2-10.

2. Scope the terminal #1 of the Servo-Amp to CH1.

3. Turn on the power of the motor and fine adjust offset so that the motor run in one

direction and makes a full stop periodically.

4. Adjust the Oscilloscope’s settings if needed.

5. Determine the time constant by using the 63.2% of the final value method. This has to be

done using the time and voltage cursors.

6. Record the time constant and the values for both CH1 and CH2.

7. Connect CH1 to terminal #2 of Servo-Amp and adjust the Function Generator Offset

down to -0.1 to 0V to make the motor run in reverse direction and to make a periodical

full stop.

8. Repeat steps 4 and 5 and fill out the table.

9. Switch CH1 to #1 of Servo-Amp by repeating steps 2 and 3. Increase AM PL of the

Function Generator to 0 to 0.2V and adjust offset to make the motor run and stop

periodically. Record data.

10. Repeat step 7 and record values.

RESULTS

Experiment 1:

Input Output
Input Output
Voltage Voltage
Terminal # Terminal #
CH1 Volt CH2 Volt

1 -0.046 3 0.963 V

1 0.101 3 2.260 V

1 0.202 3 3.180 V

1 0.301 3 4.280 V

1 0.405 3 5.310 V

1 0.482 3 6.090 V

1 -0.034 4 1.060 V

1 -0.102 4 1.790 V

1 -0.201 4 2.780 V

1 -0.300 4 3.730 V

1 -0.400 4 4.850 V

1 -0.502 4 5.840 V

The bias was determined to be 1.01 V.

 Output vs Input Voltage
7.0
y = -10.204x + 0.7249 y = 9.7998x + 1.3204
6.0
5.0
Output Voltage

4.0 Terminal 3
3.0 Terminal 4

2.0 Linear (Terminal 3)

1.0 Linear (Terminal 4)

0.0
-0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5
Input Voltage

Figure 1 - Output Voltage vs Input Voltage seen

As in Figure 1 the preamplifier was well balanced. Also from the equation of the trend lines, Kp

is noted to be roughly 10.

Experiment 2:

Servo-Amp Input Tachometer Output

Vm n Vt

Terminal # (CH1) (rpm) ω (rads/sec) (CH2) Kt (V.s/rad)

#1 1.5 V 750 78.54 2.14 V 0.0272

#1 2.5 V 1960 205.25 5.58 V 0.0272

#2 -1.5 V -680 -71.21 -1.96 V 0.0275

#2 -2.5 V -1940 -203.16 -5.43 V 0.0267

ω vs Vm
250
y = 126.71x - 111.53
200
150
ω (rads/sec)

100
Input 1
50
0 Input 2
-3.0 V -2.0 V -1.0 V -500.0 V 1.0 V 2.0 V 3.0 V Linear (Input 1)
-100
Linear (Input 2)
-150
-200
-250
y = 131.95x + 126.71
Vm (volts)
 The anglular velocity can be calculated from
From plot:
the experimental RPM by:
Input 1 @y=0, x= 0.877223581

Input 2 @y=0, x= -0.960287988

2𝜋 𝑟𝑒𝑣 𝑟𝑎𝑑
𝜔=𝑛∗ = 750 ( ) ∗ 2𝜋
60 𝑚𝑖𝑛 𝑟𝑒𝑣

1 𝑚𝑖𝑛 𝒓𝒂𝒅
∗ = 𝟕𝟖. 𝟓𝟒
60 𝑠𝑒𝑐 𝒔𝒆𝒄

Therefore the deadband is 1.84 V.

The tachometer voltage constant Kt is determined by:

𝑉𝑡 2.14 𝑽 ∗ 𝒔𝒆𝒄
𝐾𝑡 = = =. 𝟎𝟐𝟕𝟐
𝜔 78.54 𝒓𝒂𝒅
Experiment 3:

Vm Tach. (Vt

V Func. Gen. (CH1) CH2) Time Constant τm Km

0~0.1V 1.04 V 3.76 V 20 ms 133.90

-0.1V~0V 1.06 V 3.92 V 20 ms 136.97

Vm Tach. (Vt

V Func. Gen. (CH1) CH2) Time Constant τm Km

0~0.2V 2.08 V 7.12 V 20 ms 126.78

-0.2V~0V 4.16 V 7.52 V 20 ms 66.95

Using the value of Kt = 0.027 obtained above, Km is tabulated below for each voltage range.

Km is determined by:

𝑉𝑡 3.76 𝒓𝒂𝒅
𝐾𝑚 = = = 𝟏𝟑𝟑. 𝟗𝟎
𝑉𝑚 ∗ 𝐾𝑡 1.04 ∗ 0.027 𝒔∗𝑽
CONCLUSION

In this laboratory, the preamplifer and the servoamplifier were both introduced into the

circuits. Using these components, three main experiments were conducted.

In the first one, the preamplifier bias was determined before connecting the rest of the

circuit. Then, a graph of the recorded output values was plotted in which the slope was

determined to be the preamplifer gain Kp.

For the second experiment, the servoamplifier, the DC motor along with the tachometer

were added to the circuit. Using different input voltages for terminals 1 and 2, the output voltage

and the rpm were recorded from the tachometer. Subsequently, a graph was plotted where Kt

was found as the slope. The servomotor deadband was also found.

Lastly, a function generator was connected instead of the attenuator in experiment 3.

Input and tachometer voltages were recorded for different function generator amplitudes. Using

the 63.2% method, the time constant was determined.

Finally, experimental results were compared with ones simulated in Matlab Simulink

from obtained parameters in this laboratory.