Download Lab 6 - PSU MNE

ME 462 - Lab 6 - Real M.E. Stuff: Temperature, Motors, Noise
Goals
The objective of this lab is to investigate DC motor control techniques, the use of thermocouples for
temperature measurement, and techniques to avoid and minimize noise. These components are then used
for simple bang-bang and pulse width modulation (PWM) control of a permanent magnet (PM) DC
powered fan to cool and regulate the temperature of an electronic device.
Introduction
Thermocouples (TC) are accurate and inexpensive temperature measuring devices. They are not perfect
however. They produce output levels in the millivolt range, which makes them vulnerable to noise
pickup, and their output voltage is also a non-linear function of temperature. Some of these problems are
neatly solved by a single chip thermocouple amplifier produced by Analog Devices. The AD595 is
directly compatible with K type thermocouples, produces a linear output of 10 mV/°C, and comes in a 14
pin DIP. This chip also contains an internal temperature sensor which is used as an electronic ice point
reference to account for the junction temperature where the TC connects to the chip. The only
disadvantage of this device is its cost ($24 each). Please be careful with them. Data sheets are
available in the lab and on the class WWW page.
Fortunately, there are still some tasks which cannot be done solely by electrons or computers. As long as
physical objects need to be manufactured, there will be a need for motion, power transmission, and
hence, M.E.'s. The DC motor is still the preferred method of generating rotary motion, although the
stepper motor is also very popular. Permanent magnet (PM) DC motors have the desirable attribute of
linear speed versus torque. Under steady state conditions, torque T is linearly related to voltage V and
speed  where K is the torque constant and R is the electrical resistance of the windings, T = K (V/R ).
This characteristic makes speed control of the PM DC motor simple. One need only control the input
voltage. This is easily done by using an analog DC level (supplied for instance by the amplified output
of a D/A convertor), or by using pulse width modulation (PWM). PWM works on the principle that, if
the motor control voltage is toggled rapidly enough from 0 to full scale, the electrical and inertial
characteristics of the motor will smooth out the speed variations. A 0 to 5 V square wave with 50% duty
cycle will produce approximately the same motor response as a steady 2.5 V DC level. In this method,
the speed is controlled by varying the duty cycle - the width of the ON time relative to the OFF time. See
the "DC Motor Controls" article available in the lab for more information.
Noise problems can creep into any circuit if one is not careful. and sometimes even when one is.
Electrically transmitted or radio frequency induced noise is everywhere, caused by things like
fluorescent lights, local radio stations, motors, relays, digital circuits, CB radios in passing trucks, and
extra-terrestrial beings. Noise can be minimized by proper shielding, by providing separate isolated
power supplies, by separate ground lines, and by liberal use of bypass capacitors. See Chapter 7 in
Horowitz and Hill for further information.
ME462 Lab 6
Names:
Station:
Point values in parentheses.
Problem Statement
1) Build the thermocouple/amplifier circuit using a K type TC and the AD595 chip. Test its output
using the DVM. When you are sure it works properly, connect it to an ADC0831 and your Atom. Write
a program to read the voltage, convert it to temperature, and continuously display it on the screen.
2) Build the motor drive circuitry shown in Figure 1. Since the Atom outputs have very low current
capability and cannot drive the motor directly, we will employ a TIP122 Darlington NPN power
transistor. Before connecting to the Atom, test your motor circuit by applying +5 V to the input (motor
should turn fully on). Now add a routine to your program which will turn the motor on (5 V output) or
off (0 V output). Use digital scope software to check the TC output with motor off and motor on to
determine whether the motor is generating electrical noise which is corrupting the TC signal.
+5 VDC
PM DC
motor
with fan
1N4002
diode
M
1 K
TTL
1 K
NPN TIP 122
Darlington
diode
Figure 1. Uni-directional motor drive
Figure 2. Transistor heat source
3) As a heat source, build the transistor circuit shown in Figure 2. The 330 ohm resistor prevents a short
from +5 to the transistor base if the pot is turned fully one way. Control the transistor operation by
varying the potentiometer. Turn the transistor fully on (saturation V CE ~0V). How hot does it feel?
After VCE has stabilized, measure the temperature with the TC. Repeat this measurement with the
transistor midway in its linear range (VCE ~7.5V), and turned off (VCE ~15V). Which is the hottest?
What does this tell you about the power dissipation and efficiency of the various operating modes of a
transistor? (Q1: 4pts)
Do not attach the TC directly to the transistor's heat sink, since the heat sink is electrically connected
to the transistor's collector. If you do, the TC electrical signal you measure will have nothing to do with
temperature.
Why does VCE drift? What effect would this have if you were driving a motor instead of a resistor? (Q2:
4pts)
4) Attach a heat sink and measure the temperature again for the hottest operating condition from above.
Turn the fan on the heat sink and measure the new steady state temperatures. (Q3: 8pts)
Temp °C for
VCE ~0 V
Temp °C for
VCE ~7.5 V
Temp °C for
VCE ~15 V
no heat sink
fan off
heat sink
fan off
heat sink
fan on
5) Now close the control loop. Modify your program to regulate the transistor temperature to  5°C of
the midpoint between the temperatures with and without the fan and with the heat sink. This can be
accomplished by turning the fan on when the temperature reaches an upper limit, and off when it reaches
a lower limit (called bang-bang control). Empirically determine these limits for your system. If the
limits are set too close, the fan may chatter on and off. If too wide, the temperature will exceed the
desired limits.
TLOW = ________
THIGH = ________ (Q4: 1pt)
Demonstrate your system to the TA. (Demo1: 15pts)
6) Implement PWM speed control of the fan. Write a program to regulate the transistor temperature to
 5°C of the midpoint temperature by varying the duty cycle of the motor control signal.
What are the advantages of PWM? (Q5: 4pt)
Demonstrate your PWM control to the TA. (Demo2: 15pts)
7) Hand in these last 2 completed lab sheets, all program listings (20pts), all relevant plots (10pts) and a
diagram of the thermocouple circuit (10pts) (motor/heat source not req’d). Be sure to answer the
following questions:
Did you see any evidence of noise in your TC circuit? (Q6: 1pt)
What are the possible sources of noise in this application? Explain the steps one would take to
determine the actual source. (Q7: 4pts)
Describe the noise reduction measures you implemented (both successful and unsuccessful). (Q8:
4pts)