ERIC ED216877: Minicomputers In The Teaching Laboratory An Example From Physics

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Farr, John E.; van den Berg, Willem H.

Minicomputers in the Teaching Laboratory - An Example
from Physics.

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•College Science; ‘Computer Oriented Programs;
Engineering Education; Higher Education;
•Instrumentation; ‘Microcomputers; ‘Physics; Science
Activities; Science Education; ‘Science Experiments;
‘Science Instruction
Microprocessors

ABSTRACT

Microcomputers are commonly interfaced to external
devices in scientific, industrial, and consumer settings for data
acquisition and for control. The general problem under consideration
is the task of taking measurements of some continuous phenomenon,
transforming them into digital form, and storing the data in the
microcomputer for later use. First, the physical variable to be
measured must be changed to a voltage (or resistance) by means of
some transducing device; for example, light intensity can be
transduced to a voltage using a photocell. Then, too-large or
too-small voltages need to be amplified. Next, the continuous voltage
is converted to a digital representation in eight bits. Finally, the
analog-digital converter is connected to the data address, and
control buses of the microcomputer. Microcomputers such as Apple,
TRS-80, Atari, Compucolor, and others, have a "game paddle" which is
used to accomplish all of these steps while another method involves
using a thermocouple. Once the system is ready, such experiments as
those involving pendulums can be easily accomplished, a typical
program recording the position of a swinging pendulum, displayed the
motion on the monitor, and displaying graphs of variables examined
during the experiment. Most students prefer using the computer as it
swiftly and accurately performs the experiment’s busywork.

(Author/JN)

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MINICOMPUTERS IN THE TEACHING LABORATORY
-AN EXAMPLE FROM PHYSICS
John E. Farr and Willem H. van den Berg
DuBois Campus, The Pennsylvania State University

"PERMISSION TO REPRODUCE THIS
MATERIAL HAS BEEN GRANTED BY

TO THE EDUCATIONAL RESOURCES
INFORMATION CENTER (ERIC)."

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Introduetio n

Minicomputers or microprocessors are commonly interfaced to external
devices in scientific, industrial, and consumer settings. They are used
for data acquisition and for control. We think it is important for not
only our science and engineering students but for everyone in our physics
courses to begin to understand how computer interfacing can be used.

Using a minicomputer as a data acquisition device can remove the
tedium of acquisition and analysis. The process is so much speeded up
that the student-experimenter can devote more time and energy to the
phenomenon itself. The TV monitor is a friendly environment which most
students relate to easily. They are moire likely to be comfortable and
interested, especially if the computer program incorporates graphics.

The cost of an experiment can be minimal (excluding the purchase of
the minicomputer). In fact, it may be cost effective in setting up new
laboratories to use minicomputers and substantially fewer meters, oscillo¬
scopes, timers, etc.

Interfacing

The general problem under consideration is the task of taking mea¬
surements of some continuous phenomenon, transforming them into digital
form, and storing the data in the minicomputer. Later the stored data can
be manipulated. This process can be easy, as will be shown, so the novice
should not be deterred by the potential difficulties. Some measurements
can be done with minimal electronics and many more difficult situations
can be handled with commercially available components at reasonable cost.

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First, the physical variable to be measured must be changed to a
voltage (or resistance) by means of some "transducing" device. For in¬
stance, angular position can be transduced to a resistance using a poten¬
tiometer (pot.), or a temperature can be changed to a voltage using a
thermocouple. Light intensity can be transduced to a voltage using a
photocell.

Second, in the electronic "signal conditioning" phase, Loo-large or
voltages may need to be amplified. Additionally there may be
a need to isolate the transducer or match inipedences. The goal is to
make the voltage variations compatable with the input requirements of
the next stage.

The third step converts the continuous voltage (frequently 0 —> 5v
or -5->5v) to a digital representation in eight bits. (An analog-to-
digital (A/D) converter could create more or fewer bits, but many micro¬
computers have an eight bit data bus so having more bits creates program¬
ming complexities while having fewer bits reduces precision.) Eight bits
resolves a voltage swing of 0 5v into 256 steps of .0195v or 0.4% incre¬
ments. This gives adequate precision for most applications.

Finally, the A/D converter is connected to the data bus, address
bus, and control bus of the minicomputer. This step requires a lot of
knowledge about the signal timing and organization of the minicomputer
and A/D converter. Many users both new and experienced will wish to buy
a commercial unit to short cut the complications.

Once the connections are made properly, the A/D converter can be
viewed as several memory locations with specific addresses. Therefore
the commands normally used in Basic to read from and write to memory

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(PEEK and POKE) can be used to read and control the A/D converter. For
some purposes programming in machine language is desirable and LDA and
STA commands would be used.

A NIFTY TRICK AND FIRST EXAMPLE

11 the proceeding explanation looks tedious and/or hopeless, take
heart; there is a wonderful short cut. Many minicomputers (Apple, TRS-80,
Atari, Compucolor, and others) have a "game paddle" which is used to ac¬
complish all these steps. The Apple uses a 150 kilohm pot. in its game
paddle. So any physical phenomenon that could be transduced into a 0 —$
150 kilohm resistance change could be read using the Basic command
PDL (0). For instance, a 150 kilohm thermistor could be used to measure
temperature.

In our teaching laboratory we used the shaft of a 1 megohm pot. as
the pivot of a pendulum. The 1 megohm pot. is wired to pin 1 and pin 6
of a 14 pin block. The game paddle block is removed and this block put
in its place. The pendulum is now connected to paddle port zero and
everything but very large swings will produce a 0-^150 kilohm change in
resistance. The angular position of the pendulum can be read and dis¬
played using: PRINT PDL (0).

Our program measures time by using a BASIC loop. This method is
the least accurate, but suitable for this case. For faster timing the
accuracy of a machine language routine is desirable. For some applica¬
tions an external clock that interrupts the computer would be needed.

A SECOND EXAMPLE - THE THERMOCOUPLE

This example is presented to show how the interfacing problem is
more generally solved.

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The transducer is a ccpper-constantan thermocouple with output of
approximately 0.2mV/°F. Over a temperature range of 32° to 102° F the
voltage change is about 1.5 mV.

The signal conditioner used is a scientific amplifier (Analog Devices

AD521). It is powered witli a 9 V battery and the gain resistors are

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adjusted (gain = 2x10 ) so that the voltage output varies from 1 to 5V
for temperature variation from 0° to 125° F. A voltage offset is re¬
quired to do this. The scientific amplifier is connected to the thermo¬
couple with the shortest possible leads to reduce pickup. The amplified
signal can be carried on fairly long wires to the A/D converter.

The A/D converter used is an Interactive Structures Inc. AI02 which
allows for sixteen 0—>5V inputs.

The total cost of the above system is less than $150.00. Each addi¬
tional thermocouple up to sixteen total costs less than $25.00. Even
though this is more expensive than necessary, the hookups and programming
considerations are minimal. Those that want to learn more and spend less
can purchase a cheaper scientific amplifier and build their own A/D con¬
verter for approximately $30.00. (See Hands On! , Winter 81-82, p. 14,
TERC, 8 Eliot Street, Cambridge, MA 02138.)

THE PENDULUM EXPERIMENT IN THE TEACHING LABORATORY

For the motion of a pendulum, four quantities are easy to measure
and correlate: mass (M) of the bob, length (L), amplitude (A), and peri¬
od (T). For an ideal simple pendulum,

T = 2tt V L/g~, (1)

(where g = acceleration due to gravity). T is independent of M and A,
as long as A is not large.

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Our program records the position of the swinging pendulum and dis¬
plays the motion on the monitor. It then displays T and asks the stu¬
dent for the values of M and L. After several such runs the computer
can display graphs of T vs. M, T vs. A, and T vs. L. These graphs clear¬
ly indicate the period's independence of mass or amplitude, but show a
nonlinear dependence on length.

The computer can then plot InT vs^. Ini,, and the result is a straight
line. Although the scales on this graph are arbitrary and unlabelled,
the program can calculate and display the slope, which is close to 0.5.
Thus it is shown that

InT = UlnL + constant, (2)

in agreement with the result obtained by taking the natural logarithm
of eq. (1) :

InT - hlnL + ln(2ir//g). (3)

Of the 54 students doing the lab, 49 said that they would prefer
using the computer this way over the traditional method, using a stop¬
watch and a mass on the end of a string. When asked how well they en¬
joyed the experiment as compared to other physics experiments without a
computer, 18 said "much more", 25 "somewhat more", 7 "no more", and 3
"somewhat less". Regarding their subjective impression of how much
they learned, the response was not as favorable, although generally
good: 6 "much more", 12 "somewhat more", 24 "the same", 8 "somewhat
less", and 4 "much less".

The most frequently cited reasons for preferring the computer
approach were: gaining firsthand experience with a microcomputer; and
letting the computer swiftly and accurately perform the busywork of the

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experiment. The most frequent complaints were: overcrowding (average
of 4.5 students/machine at a time); and inadequate background informa¬
tion provided by the instructor. A small minority of students complained
of a lack of sense of participation.

The students provided sensible suggestions for improving the lab;
providing more background information ahead of time; reducing crowding;
and making the program more foolproof and sophisticated, perhaps allow¬
ing greater student interaction with the machine ( e.g. , having the stu¬
dent measure the period, with the machine checking the result).

For those interested, our software is available on an unprotected
DOS 3.3 disk for $10.00.

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