EMCH 367 Fundamentals of Microcontrollers document.doc

LAB 4 – DC MOTOR TACHOMETER

OBJECTIVEThe 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.

PREREQUISITESFloppy 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.)

PROCEDUREThe 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 SETUPThe 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.

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EMCH 367 Fundamentals of Microcontrollers document.doc

WIRING DIAGRAM

Wire ConnectionGreen wire: +15 VRed wire +5 VBlack wire 0 V (Ground)Yellow wire Signal wire to the input capture pins IC1

Figure 1 DC Motor tachometer experiment to which the present software example is relevant.

CIRCUIT DIAGRAM

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EMCH 367 Fundamentals of Microcontrollers document.doc

PRE-TEST PROCEDUREBefore starting your test, perform the following pre-test procedure to verify that your experimental set-up is performing correctly:

1) Check the correct wiring of the DC motor set up:

Wire Connection Check markGreen wire: +15 VRed wire +5 VBlack wire 0 V (Ground)Yellow wire Signal wire to the input capture pins IC1

2) 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

= _________ . (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) 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.

6) 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.

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 circular frequency, , motor constant, K, and the measured rotation speed base on measured period of rotation. Enter the values in Table 1.

Table 1

Speed Voltage Period of Rotation Motor

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EMCH 367 Fundamentals of Microcontrollers document.doc

description (V) rotation, (ms) speed, constant, K

Full-speed

¾-speed

½-speed

¼-speed

Very slow

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.

9) Activate the program LASTNAME_Firstname_RPM.asm and let it run

TA checkmark ________

10) Read the displayed rotational speed and 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.

TA checkmark ________

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Average motor constant, Kav

EMCH 367 Fundamentals of Microcontrollers document.doc

Table 2S

peed

des

crip

tion

Vol

tage

(V)

Per

iod

of ro

tatio

n,

(ms)

Mea

sure

d ro

tatio

n sp

eed

(rpm

)

Dis

play

ed ro

tatio

n sp

eed

(x10

0 rp

m)

Err

or in

rota

tion

spee

d m

easu

rem

ent (

%)

Full-speed

¾-speed

½-speed

¼-speed

Very slow

Stopped

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EMCH 367 Fundamentals of Microcontrollers document.doc

(This page is left intentionally blank)

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