EMCH 367
Fundamentals of Microcontrollers
Lab 4
LAB 4 – DC MOTOR TACHOMETER
OBJECTIVE
The objectives of this laboratory are:
(a) To understand the functioning of a DC motor
(b) To test the DC motor tachometer program developed in Hmwk6.
PREREQUISITES
Floppy disk with the asm codes for the program:

LASTNAME_Firstname_RPM.asm
Hard copy (printout) of Hmwk6 – Digital tachometer. When printing, use the 'pages per sheet'
option in the lower right corner of the print dialog-box with settings of 4 or 2 (depending on your
eyesight) to save paper. You may want to experiment a little with this before printing the full
document.
PROCEDURE
The students will utilize the asm code developed with the THRSim11 simulator for Hmwk6. The
students will go through the printout of Hmwk6 step by step and will verify that the MCU responds to
instructions as expected.
The lab is divided into sections. After completing each section, the student will ask the TA to check the
student’s work and make a check mark on that section.
The asm code is activated into the MCU following the standard procedure learned in Lab 1.
EXPERIMENTAL SETUP
The experimental setup for this experiment consists of a DC electric motor, a speed-control
potentiometer and its electronics, an emitter-detector sensor, a disk with an aperture (hole) attached to
the DC motor shaft, and a pair of 7-LED displays connected through appropriate electronics to a 8-pin
Port B connector. The aperture in the disk spins through the infrared emitter-detector sensor. The
emitter-detector sensor sends a High (5V) signal when the aperture in the disk allows the beam of light
to pass through. The emitter-detector sensor output wire is attached to the input capture pin IC1 on Port
A.
The speed of the motor is controlled by a potentiometer and an electronic timer circuit. The
potentiometer is a variable resistor. By varying the angular position of the potentiometer (turning the
knob), one varies the timer’s capacitor’s charge and discharge path resistance, which, in turn, varies
the duty cycle supplied to the DC motor. As the duty cycle to the DC motor is varied, the power
delivered and speed also vary.
Dr. Victor Giurgiutiu
Page 1
2/17/2016
EMCH 367
Fundamentals of Microcontrollers
Lab 4
WIRING DIAGRAM
Wire
Red wire
Black wire
Yellow wire
Connection
+5 V
0 V (Ground)
Signal wire to input capture pin IC1
MOSFET
DC Motor
Transmissive
Photomicrosensor
(Emitter/Detector)
7-Segment
Display
Port B
Connection
Lamp Test
555 Timer
Speed-Controlling
Potentiometer
Figure 1 DC Motor tachometer experiment to which the present software example is relevant.
CIRCUIT DIAGRAM
Dr. Victor Giurgiutiu
Page 2
2/17/2016
EMCH 367
Fundamentals of Microcontrollers
Lab 4
PRE-TEST PROCEDURE
Before starting your test, perform the following pre-test procedure to verify that your experimental setup is performing correctly:
1)
Check the correct wiring of the DC motor set up:
Wire
Red wire
Black wire
Yellow wire
2)
Connection
+5 V
0 V (Ground)
Signal wire to the input capture pins IC1
Check mark
Press the small black “Lamp Test” button on the DC motor board. Do all the LEDs light up,
displaying the number 88? Y ___ N ____. Connect port B to the 2-digit 7-LED display. Send $99
through port B. Does the display indicate correctly? Y ___ N ____. Repeat with $66. Try other
base-10 numbers. Does the display correctly indicate all these numbers? If not, contact your TA.
TA checkmark ________
PART I –DC MOTOR SPEED-VOLTAGE CHARACTERISTIC (30%)
3)
Disconnected the motor from the circuit and measure its internal resistance, R = _________ 
(You may want to borrow a digital multimeter from your TA for this measurement).
4)
Connect the circuit to the power supply terminals. Attach the voltmeter probes to the DC motor
terminals.
5)
Connect the oscilloscope Ch. 1 probe to the sensor output wire (yellow) of the circuit board. Set
the oscilloscope trigger mode to Auto, source to Ch. 1, and coupling to DC.
6)
By rotating the knob of the potentiometer, modify the duty cycle of voltage delivered to the DC
motor to increase and decrease the motor speed. Find the voltage value for the fastest speed
(full-speed). Enter this value in the table. Then find other values until you have matched all these
descriptors: full-speed, ¾-speed, ½-speed, ¼-speed, very slow, and stopped.
7)
Run the DC motor at full speed. Examine the signal on screen. Modify the sec/div setting until the
signal best fits the screen and can be measured. Measure the period of rotation,  and enter your
result in Table 1. Repeat for the other speed settings in the table.
8)
Calculate the angular frequency, , motor constant, K, and the measured rotation speed based
on the measured period of rotation. Enter the values in Table 1.
Dr. Victor Giurgiutiu
Page 3
2/17/2016
EMCH 367
Fundamentals of Microcontrollers
Lab 4
Table 1
Speed Voltage
description (V)
Period of
rotation, 
(ms)
Rotation
speed, 
(rad/s)
RPM
Motor
constant, K
Full-speed
¾-speed
½-speed
¼-speed
Very slow
Stopped
Average motor
constant, Kav
PART II – DC MOTOR TACHOMETER (70%)
The program LASTNAME_Firstname_RPM.asm developed in Hmwk6 will be used to measure and
display the rotation speed using the microcontroller input capture function IC1. Recall from Hmwk6 that
T1 is the first time when a falling edge transition is encountered on pin IC1, while T2 is the second time
when a falling edge transition is encountered on the same pin IC1. This corresponds to the time
between two consecutive passages trough the emitter-detector sensor of the hole in the rotating disk.
The difference between T2 and T1, plus the time taken by the overflows, will represent the period of
rotation of the disk. Hence, one can calculate the rotation speed and display it. Since the experimental
setup has only two 7-LED displays, the rotation speed will be displayed in 100s RPM.
1)
Activate the program LASTNAME_Firstname_RPM.asm and let it run
TA checkmark ________
2)
Read the displayed rotational speed and enter the values in the table.
3)
Calculate the error between the displayed value and the calculated value rotation speed value. In
this calculation, take the rotation speed resulting from oscilloscope measurements as the
standard.
4)
Calculate the average motor constant and enter its value in the table.
TA checkmark ________
5)
Repeat for the other voltage settings.
TA checkmark ________
Dr. Victor Giurgiutiu
Page 4
2/17/2016
EMCH 367
Fundamentals of Microcontrollers
Lab 4
Error in rotation speed
measurement (%)
Displayed speed
(x100 RPM)
Measured speed
(RPM)
Period of rotation, 
(ms)
Voltage (V)
Speed description
Table 2
Full-speed
¾-speed
½-speed
¼-speed
Very slow
Stopped
Dr. Victor Giurgiutiu
Page 5
2/17/2016
EMCH 367
Fundamentals of Microcontrollers
Lab 4
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Dr. Victor Giurgiutiu
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EMCH 367
Dr. Victor Giurgiutiu
Fundamentals of Microcontrollers
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Lab 4
2/17/2016
EMCH 367
Dr. Victor Giurgiutiu
Fundamentals of Microcontrollers
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Lab 4
2/17/2016