1.1 Dc motor control  (Page 2/3)

I/f circuit

This circuit was designed as part of the demo that will be described in detail in chapter .

Measurement equipment

1. Signal Generator
2. Voltmeter
3. Oscilloscope.
4. Stroboscope

Related files

• Simulink Model for Simulation - DCMotorControl.mdl
• MATLAB script for Real-Time - speedControlDCLoop.m
• Simulink Model for Real-Time - DCMotorControlc2812.mdl

System identification

The transfer function of a DC motor can be approximated by a first order model with unknown constants. These constants can be identified experimentally.

The tachometer provides the feedback signal for speed control systems. A schematic diagram of the tachometer is given in Figure 6.

In this section we will identify the constants in the mathematical models of a DC motor and the tachometer experimentally. The process consists of consists of two parts:

1. Measurement of the tachometer coefficient and the motor constant
2. Measurement of the motor time constant.

Measurement equipment

• Signal Generator
• Voltmeter
• Oscilloscope.
• Stroboscope

Measurement of tachometer coefficient and motor constant

The tachometer coefficient and motor constant can be measured by generating a control voltage, using the PWM block of the DSP, and measuring:

1. The motor speed using a stroboscope
2. The tachometer output using a voltmeter

The setup is shown in Figure 7.

Figure 8 shows the model that will be used to generate the control voltage.

The PWM block will be configured to generate a PWM signal with a period of 4096 clock cycles of 75 MHz (please refer to Figure 9).

Measurement procedure:

1. Connect the circuit shown in Figure 7.
2. Generate a 0 V input to the DC Motor Kit (Compile Build and Run the model with Duty Cycle=0)
3. Measure the motor speed using the stroboscope and the tachometer voltage using a voltmeter.
4. Repeat step 3 for input voltages in the range 0-15 V. Change the Duty Cycle from 0 to 100% in steps of 10 %.
5. Obtain a plot of tachometer voltage vs. servo amplifier input voltage.

The ADC input voltage of the DSP should be in the range 0-3V, the therefore the tachometer output voltage must be inverted and scaled down. This is done with the circuit shown in Figure 19. The gain of this circuit is G = - 100/270 = - 0.37, i.e., inverting and attenuating approximately 1/3. The results obtained are shown in Figure 10.

Measurement of the motor time constant

The measurement of the time constant will be obtained by generating a square pulse and measuring the motor response to this stimulus. The principle of operation is shown in Figure 11; the experimental setup is shown in Figure 12.

The Function Generator generates pulses, as shown in Figure 13.

Measurement procedure

1. Connect the circuit shown in Figure 12.
2. Generate the pulse shown in Figure 13.
3. Measure the rising and falling times of the motor response.

The DC motor was modeled as a first-order linear system where ${\tau }_{\text{rise}}={\tau }_{\text{fall}}$ ; however, the actual motor is not linear (due to friction, for example), therefore the measured values were: