ECE4330 – Embedded Systems Design
Lab 6 – PID Motor Controller
Lab 6 – PID Motor Controller
In most engineering applications, electric motors are controlled with some kind of
feedback controller. In this lab, you will demonstrate why a feedback controller is better
than an open loop controller. You will also build two kinds of feedback controllers to
demonstrate why the PID variety is superior.
Pre-Lab Assignments
1. Review PID without a PhD at
http://www.embedded.com/2000/0010/0010feat3.htm.
2. Complete the Pre-Lab Questions at the end of the lab instructions.
Open Loop Controller
As you learned in lab 5, there are two kinds of control systems: open loop and closed
loop. Your task in part 1 of this lab is to build an open loop DC motor controller.
The ‘plant’ to be controlled in this lab is a DC motor. The speed of the motor is
controlled by a power FET. The signal to the gate of the FET is the PWM output of the
microprocessor (μP). The greater the duty cycle of the PWM output, the faster the motor
turns. The circuit is shown in figure 1 below.
VCC
5V
A
PWM_Output
Q1
Figure 1 - DC Motor Speed Control with Power FET
The RPM of the DC motor is sensed with the NTE3100 optical sensor that was used in
lab 3. The DC motor spins a disk or paddle that interrupts the IR beam between the LED
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ECE4330 – Embedded Systems Design
Lab 6 – PID Motor Controller
and the sensor in the NTE3100. The NTE3100 is then connected as the clock source of a
counter in the μP. Each rotation of the DC motor thus registers as a count. By reading
the count periodically, the RPM of the motor is determined. The circuit for the RPM
sensor is shown in figure 2. Note that the circuit uses the CD4050 buffer as discussed in
lab 3.
VCC
5V
VCC
5V
U1
R1
E
+
2.2kOhm
+ S
NTE3100
To_AVR
R2
1kOhm
Figure 2 - DC Motor RPM Sensor
In-lab Task 1 of 3
1. Build an open loop controller based on the circuits in figures 1 and 2. Use the
various VTG pins for Vcc.
2. Use the switches on the STK500 board the vary the duty cycle of the PWM output
from 0% to 100%. Record the data and use Excel to figure out a linear
relationship between the number loaded into the PWM output compare register
(OCR) and the RPM of the motor.
3. Program the switches on the STK500 to control the RPM of the DC motor. Make
SW0 set the RPM to the low end of the range e.g. 2,000 RPM. Make SW7 set the
RPM to the high end of the range e.g. 14,000 RPM. Have the switches between
SW0 and SW7 set the RPM to increasing values that are integer multiples of 500
RPM e.g. SW1 sets the RPM to 2,500, SW2 sets the RPM to 4,000, SW3 sets the
RPM to 5,000 and so forth. Note that the switches now command RPM, not a
certain PWM duty cycle.
4. Use HyperTerminal to record actual RPM versus commanded RPM for various
step changes in commanded RPM.

HyperTerminal is the serial port available on all Windows PC’s. Open
HyperTerminal by clicking on Start, then All Programs, Accessories,
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ECE4330 – Embedded Systems Design
Lab 6 – PID Motor Controller
Communications, HyperTerminal. Configure HyperTerminal to the same
settings as the UART in the P (probably 9600 baud, 8-N-1).

Commanded RPM is the RPM set by the switch.

Actual RPM is the RPM detected by the optical sensor.

Output RPM data to HyperTerminal using the printf statement. A good
period for data output is once a second. Record the data sent to
HyperTerminal using the Capture Text function. In HyperTerminal click
on Transfer, then Capture Text … Browse to a convenient directory and
create a file for your test data.

Run several step changes while recording data via HyperTerminal, then
close out the data file by clicking on Transfer, then Capture Text …, then
Stop.
5. Use Excel to graph the commanded RPM versus actual RPM data.

Import the data into Excel by clicking on Data, then Get External Data,
then Import Text File. Browse to the data file created by HyperTerminal
and open it. In the Text Import Wizard - Step 1 of 3, select Delimited then
Next >. In Step 2 of 3, select Space then Finish. Click on OK in the final
screen and the data put out by the printf statement will be transferred to
Excel.

Use Excel’s charting functions to graph commanded RPM and actual
RPM versus time for several step changes. Your graph should look
something like figure 3.

If actual RPM data is noisy, apply a single pole filter to the data. This can
be done using code like the following:
displayRPM = 0.1 * measuredRPM + 0.9 * displayRPM;
where displayRPM is the data sent out in the printf statement and
measuredRPM is the RPM measured in the last control cycle.
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ECE4330 – Embedded Systems Design
Lab 6 – PID Motor Controller
RPM Response - Open Loop Controller
18000
Commanded RPM
16000
Actual RPM
14000
12000
RPM
10000
8000
6000
4000
2000
0
0
10
20
30
40
50
60
Time (s)
Figure 3 - Example Graph of Commanded RPM versus Actual RPM for an Open Loop
Controller
Simple Feedback Controller
With a feedback controller, plant output is sampled and used to correct control signals to
the plant so that plant output adjusts to the desired output value. In this case, the plant
output is DC motor RPM, the output is sampled by measuring RPM with the NTE3100
optical sensor and the control signal is the duty cycle of the PWM signal to the FET.
In the second part of this lab, you will build a simple feedback controller that samples
RPM and adjusts the PWM so that the motor turns at the commanded RPM. This
controller is a simple feedback controller because it does not do any math to figure out
the fastest way to achieve the commanded RPM. The simple controller just senses if
actual RPM is too high or too low and then nudges the PWM by one increment in the
correct direction.
In-lab Task 2 of 3
1. Modify the software from part 1 so that the actual RPM is used to nudge the
PWM output in the necessary direction.

If measured RPM is too low, increment the PWM OCR value by one.

If the measured RPM is too high, decrease the PWM OCR value by one.
2. An important consideration is how often to sample the RPM and correct the PWM
OCR. This is the control frequency. A control frequency of 10 Hz will work well
for this lab.
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ECE4330 – Embedded Systems Design
Lab 6 – PID Motor Controller
3. Using HyperTerminal, collect commanded RPM versus actual RPM data for
several step changes in commanded RPM. Plot the data as in part 1. When
compared to the open loop controller in part 1, the actual RPM should be closer to
the commanded value.
Proportional Feedback Controller
In the simple feedback controller built in part 2, PWM output is modified as required to
make actual RPM conform to commanded RPM. The result should have been a more
accurate motor RPM. However, the response time is probably very long. That is, the
actual RPM adjusts to the commanded RPM very slowly. The simple feedback controller
is more accurate than the open loop controller, but its response time is very slow.
In part 3, you will add proportionality to the controller to improve its response time. Like
the simple feedback controller, the actual RPM will be very close to the commanded
RPM. Unlike the simple controller, the response time should be very short. Commanded
changes to RPM should very quickly be seen in actual RPM.
In-lab Task 3 of 3
1. Modify the software from part 1 so that the amount by which the PWM OCR is
adjusted depends on the size of the error between actual and commanded RPM.
2. Using HyperTerminal, collect commanded RPM versus actual RPM data for
several step changes in commanded RPM. Plot the data as in part 1. Controller
response time should be improved compared to the simple feedback controller in
part 2.
Parts List
1.
2.
3.
4.
5.
DC motor, Radio Shack p/n 273-258
Power FET IRF510
Optical sensor, NTE3100
Buffer, CD4050BC
Resistors
Deliverables
1. Demonstrate the proportional controller of part 3 to the lab instructor.
2. In your lab report, include graphs of controller performance for all three kinds of
controller. Compare their performance.
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ECE4330 – Embedded Systems Design
Lab 6 – PID Motor Controller
Pre-Lab Questions for Lab 6
Name: ______________________________
10 points total, 4 points for question 1 and 6 points for question 2.
1. According to the author of “PID without a PhD,” one of the three kinds of control
is the most problematic. Which one is it and what makes it difficult to
implement?
2. For Timer2, calculate two combinations of clock prescaler values (called Clock
Value in CodeWizardAVR) and Timer2 values that will result in a Timer2
interrupt every 10 ms.
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