ENG H193 Hands-on Lab
Lab 2: Motor Performance
INTRODUCTION
Background
Most likely your robot will need to use several DC motors to perform
different tasks (such as moving or lifting things). In order to select the
appropriate motors for your application, it is important to understand their
operating characteristics and their dynamic performance.
Purpose
The purpose of this lab is to familiarize you with the dynamic performance of
a range of DC motors.
Basic Principles
In this lab write-up, we will cover some basic principles behind:
1)
2)
3)
4)
Lab Experience
DC Motors,
Pulse Width Modulation,
Stall Current, and
Torque Vs Speed.
The lab experience will encompass:
1)
Measuring the armature resistance, and
2)
Collecting data using a shaft encoder.
BASIC PRINCIPLES
DC Motors
A DC motor has a two wire connection. All drive power is supplied over
these two wires -- think of a light bulb. When you turn on a DC motor, it just
starts spinning round and round. Most DC motors are pretty fast, about 5000
RPM (revolutions per minute).
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Pulse Width
Modulation
With the DC motor, its speed (or more accurately, its power level) is
controlled using a technique named pulse width modulation, or simply PWM.
This idea of controlling the motor's power level is achieved by turning the
power on and off. The key concept here is duty cycle -- the percentage of "on
time" versus "off time." If the power is on only 1/2 of the time, the motor runs
with 1/2 the power of its full-on operation.
If you switch the power on and off fast enough, then it just seems like the
motor is running weaker -- there's no stuttering. This is what PWM means
when referring to DC motors. The Handy Board's DC motor power drive
circuits simply switch on and off, and the motor runs more slowly because it's
only receiving power for 25%, 50%, or some other fractional percentage of
the time.
Handy Board’s
Current and
Voltage draw
The Handy Board is designed for motors that will operate at 9.6 volts and
draw up to about 1 ampere of current maximum. This includes all of the
motors sold by the LEGO Group (the 9v standard motor, 9v gear motor, and
9v micro motor) and a variety of hobby motors that will be used in today’s
laboratory exercise.
Stall Current
A motor draws its maximum amount of current when it is stalled (this is
sometimes referred to as the "stall current"). A good way to get an estimate of
how much current this will be at the Handy Board's rated 9.6v battery supply
level is to measure the motor's resistance using a standard VOM (volt-ohm
meter), and apply the Ohm's Law formula: current = voltage divided by
resistance. Set the meter to its most sensitive resistance scale (typically, 0 to
200 ohms), apply the probes to the motor terminals, and gently rotate the
motor shaft by hand until you obtain the smallest possible reading. The idea is
to find the static position of the shaft that gives you the lowest resistance and
take that as the measurement.
If this reading is nine ohms or more, then the motor should be nicely
compatible with the Handy Board. (At 9 volts, a 9 ohm reading would mean a
current draw of one amp, the maximum the Handy Board can deliver.) If the
reading is less than four ohms, the motor will probably draw too much current
and won't work.
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Motor
Incompatibility
with the Handy
Board
Please be aware that the Handy Board is not directly compatible with 3v to
4.5v motors found in many toy cars. These inexpensive motors are extremely
noisy from an electrical standpoint. Also, they typically will draw several
amperes of current, thereby overloading the Handy Board's motor drivers and
causing the board to reset.
The Handy Board also is not compatible with motors used in high-end radio
control cars. These motors are designed to draw huge amounts of power—25,
50, or more amperes of starting current.
Torque Vs.
Speed
Important characteristics of a DC motor are usually given as a plot of one of
the motor’s output quantities versus another. The output quantities of primary
interest for a DC motor are shaft torque and speed. Thus, an important
characteristic of a DC motor is a plot of its output torque versus speed. The
types of DC motors that you will use generally exhibit an operating
characteristic called negative speed regulation. This simply means that the
motor’s speed decreases as the load connected to its shaft increases, and an
example of this characteristic is shown in a simplified motor performance plot
shown below.
The maximum speed with no load attached is called "no-load speed" (nr). The
load that stalls the motor is called the "stall" torque (or sometimes the starting
torque)
The Torque vs. Speed characteristic of a DC motor can be obtained using a
shaft encoder connected to the Handy Board. The shaft encoder is normally
used along with an 8-hole bushwheel (one of the erector set components).
Every time a hole passes through the shaft encoder two counts are sent
through the port of the controller. These signals can be measured in terms of
the number per unit time. Knowing this information can give the rotational
speed and can ultimately be used to calculate a speed for the robot.
LAB EXPERIENCE
Make sketches of equipment used in class; include them in your lab write-up.
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1. Use an ohmmeter to measure the armature's resistance of the DC
motor you were given. Calculate the "stall" current for the motor's
rated voltage.
2. Connect both the shaft encoder and the DC motor to the appropriate
ports on the Handy Board.
3. Download the appropriate libraries (use "f" encoder icb routine) onto
the Handy Board for the shaft encoder. Use the provided IC code to
run the experiment. Modify the code so that the port numbers match
with the ports you are using. The code will allow you to determine
the speed of the motor while a specific torque is being applied.
4. Run the program to determine the no load speed.
5. Different weights being lifted at a constant speed can be used to
determine points on the Torque vs. speed curve. Work to obtain three
data points including the no-load speed and extrapolate to get the stall
torque. Express torque in units of "ounce-inches" or oz.-in. Do not
stall the motor since it may cause problems to both motor and
controller due to high current demand.
6. Fill out the given table indicating the type of DC motor tested and its
calculated "stall" current.
7. Print copies of your table and your Torque vs. speed graph and turn
them in at the end of the period. (One set of copies per team).
Procedure
LAB REPORT
Format



General
Guidelines



Lab reports must be done Individually.
Follow given lab report format.
Nominally 4-5 pages (including figures and tables)
Cover page: title, name, section, team number, date, and professor.
Include a sketch of the motor test stand.
Show the "stall" current your measured and include sample calculation of
torque.
 Plot the Torque versus Speed characteristic, with Torque expressed in units
of Inch-ounces. Extrapolate to obtain the stall (or starting torque). Use
labels accordingly.
 Obtain an equation that allows you to compute speed for a given torque (use
appropriate units).
 Discuss briefly the operating characteristics of the motor you tested.
Indicate your opinion of this motor’s suitability for use in a robot drive
system or robot arm mechanism, or other robot applications.
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APPENDIX: SHAFT ENCODER ROUTINES
Introduction
There are 2 sets of seven software routines available within the IC libraries to
use up to seven shaft encoders, one connect to each of the Handy Board’s
analog inputs. Those routines work with simple break-beam or reflective
encoders (shaft encoders), and provide a count of encoder counts or "clicks"
as well as velocity. They count upward regardless of which direction the
encoder is turning.
The appropriate drivers must be loaded into IC for downloading to the Handy
Board in order for the shaft encoder(s) to work. The routines are named either
"sencdr#.icb" or "fencdr#.icb", where the "#" is replaced by a numeral from 0
to 6 (corresponding to an interrupt-driven service routine for analog inputs 0
to 6).
The "s" in the "sencdr#.icb" routine name stands for "slow" while the "f" in
the "fencdr#.icb" routine name stands for "fast". The "s" versions update at
250 times per second (called Hertz or Hz) while the "f" versions update at a
rate of 1000 Hz. It is usually recommended to use the "s" versions unless you
are losing counts due to a high shaft rotational speed because the "s" versions
take up less CPU time.
Installing
Software
To install the software driver for a shaft encoder connected to analog port 1,
load either "sencdr1.icb" or "fencdr1.icb". In IC, first click on "Load", and
then click on "Download File". Next select the "libs" sub directory in the
"IC" directory on drive C (that is, C:\IC\libs\ ). Pull down the list of "file of
type" name extensions and select "*.icb", which is a designation for an
"Interactive C binary" file type. Now select the appropriately named encoder
routine and then click on "Open" to load the file into IC.
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Encoder
Routines
For any given encoder routine, there are four global variables defined within
the IC routine. The following example assumes that either "sencdr1.icb" or
"fencdr1.icb" has been loaded. Note that there is no difference in the four
global definitions between the "s" and "f" versions of the encoder routines.
encoder1_counts -- A running count of the encoder clicks. To reset to zero,
simply use an assignment statement; e.g., encoder1_ counts= 0;
encoder1_velocity -- A successive differences velocity measure, calculated
every 64 milliseconds. It is simply the number of "clicks" which have been
counted during the last 64 millisecond time period.
encoder1_low_threshold -- The lower limit below which the encoder count is
increased. The default value is 50.
encoder1_high_threshold -- The upper limit above which the encoder count is
increased. The default value is 200.
Depending on the performance of your shaft encoder sensor, the encoder click
detection thresholds may need to be changed. To change them, simply assign
them a new value. For example, to increase the lower threshold for an
encoder connected to analog port 1:
encoder1_low_threshold = 75 ;
The default values are restored on system reset, so any encoder threshold
assignment statements need to be executed each time the board is restarted.
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Shaft Encoder
Wiring
Schematics
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