EMCH 367
Fundamentals of Microcontrollers
Lab 4.doc
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. (We 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 to an electronic circuit. The potentiometer
is a variable resistor. By varying the resistance of the potentiometer (turning the knob), one varies the
base current to a power transistor, which, in turns, varies the voltage supplied to the DC motor. (This
action is similar to that of the volume control knob on your audio player.) As the supply voltage to the
DC motor is varied, its speed also varies.
Dr. Victor Giurgiutiu
Page 1
12/19/2001
EMCH 367
Fundamentals of Microcontrollers
Lab 4.doc
WIRING DIAGRAM
Wire
Green wire:
Red wire
Black wire
Yellow wire
Connection
+15 V
+5 V
0 V (Ground)
Signal wire to the input capture pins IC1
Speed-control
Potentiometer
7-LED Display Wheel with aperture
Emitter-Detector
sensor
Port B
connection
Electric Motor
Emitter-Detector
sensor output wire
Figure 1 DC Motor tachometer experiment to which the present software example is relevant.
CIRCUIT DIAGRAM
+15V
TIP120
10KΩ
Potentiometer
B C
E
+5V
4.7KΩ
TIP120
E
10Ω
+5V
B
C
To IC
10Ω
Emitter
Detector
DC
M otor
Dr. Victor Giurgiutiu
Page 2
12/19/2001
EMCH 367
Fundamentals of Microcontrollers
Lab 4.doc
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
Green wire:
Red wire
Black wire
Yellow wire
2)
Connection
+15 V
+5 V
0 V (Ground)
Signal wire to the input capture pins IC1
Check mark
Connect port B to the 2-digit 7-LED display. Send $99 through port B. Does the display indicate
correctly? Y ___ N ____. Repeat with $00, then $11, $22, $33, $44, $55, $66, $77, and $88.
Does the display indicate correctly all these numbers? If N, contact your TA.
TA checkmark ________
PART I –DC MOTOR SPEED-VOLTAGE CHARACTERISTIC (30%)
3)
With the DC motor disconnected from the potentiometer circuit, measure its internal resistance, R
= _________ Ω. (You may want to borrow a digital multimeter from your TA for this
measurement.)
4)
Connect the DC motor to the potentiometer circuit. Connect the potentiometer circuit to power
supply terminals. Attach the voltmeter probes to the DC motor terminals.
5)
By rotating the knob of the potentiometer, modify the voltage across the DC motor terminals 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.
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.
6)
Activate the program LASTNAME_Firstname_RPM.asm and let it run
7)
Connect the oscilloscope Ch. 1 probe to the sensor output wire (yellow) of the circuit board. Set
the oscilloscope trigger mode to Normal, source to Ch. 1, and coupling to DC.
8)
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, τ and enter your
result in the table.
TA checkmark ________
9)
Read the displayed rotational speed and enter the values in the table.
Dr. Victor Giurgiutiu
Page 3
12/19/2001
EMCH 367
Fundamentals of Microcontrollers
Lab 4.doc
10)
Calculate the circular frequency, ω, motor constant, K, and the measured rotation speed base on
measured period of rotation. Enter the values in the table.
11)
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.
12)
Calculate the average motor constant and enter its value in the table.
TA checkmark ________
13)
Repeat for the other voltage settings.
Motor constant, K
(mV·s/rad)
Circular frequency, ω
(rad/s)
Error in rotation speed
measurement (%)
Displayed rotation
speed (x100 rpm)
Measured rotation
speed (rpm)
Period of rotation, τ
(ms)
Voltage (V)
Speed description
TA checkmark ________
Full-speed
¾-speed
½-speed
¼-speed
Very slow
Stopped
Average motor
constant
Dr. Victor Giurgiutiu
Page 4
12/19/2001