Memorandum
To: Meagan Ita
From: Hank Rumpke (Seat 18), Margaret Wilson (Seat 3), Samuel Coles (Seat 30), Travis
Remlinger (Seat 11)
Date: 11/14/2012
Re: DC Motor Testing
1. Introduction
Employees of Reverse-engineering Or Testing Of Motors (MOTO ), Inc. are asked to test
three different types of motors: a stepper motor, a DC motor, and a servo motor. In addition
to the performance tests of the motors, a report should be included detailing the findings of
the engineers. Also, the engineers are to suggest a motor and a joint for the newest surgical
robot system based on the provided requirements.
Within this memo contains the results and description, discussion, and summary and
conclusion of the experiment. The Results and Description explain how data was collected
for each motor as well as properties of the pulse width vs. servo angle and torque vs. speed
plots. In the Discussion, real world applications of each motor are discussed and the sources
of error are stated. Lastly, the Summary and Conclusion restates the purpose of the lab and
makes a recommendation on which motor to use for the surgical robot.
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2. Results and Description
For the stepper motor, the data recorded was the angle the arrow on the permanent magnet
was pointing at for each combination of switches, which controlled 4 electromagnets. To see
which angle is paired with which switch combination, see Table 1 in the Figures and Tables
Attachment. If the switch sequence in Table 1 is followed repeatedly, the stepper motor will
continuously rotate.
With the DC motor, different weights applied torque at the radius of the motor to see the
effect on the motor’s angular speed. Weight was added in increments of 47.5 𝑔 and the speed
of the motor was read from the Arduino in revolutions per minute. The masses of the plates
were converted to torque by writing the mass in kilograms and multiplying the mass by
Earth’s gravitational acceleration constant, 9.8
𝑚
𝑠2
, and 0.0254 𝑚, the motor’s radius. The
speed of the motor in revolutions per minute was converted to radians per second by
multiplying by 2π
𝑟𝑎𝑑
𝑟𝑒𝑣
𝑠
and dividing by 60 𝑚𝑖𝑛. Table 2 in the Figures and Tables Attachment
shows the mass, speed, and torque for each test on the DC motor. Figure 1 in the Figures and
Tables Attachment shows the Angular Speed versus Torque plot. Using the trend line
application in Microsoft Excel on the points in the plot in Figure 1, a linear regression was
created and the equation is displayed on the plot in Figure 1. According to the equation
shown on the plot in Figure 1, the no-load speed of the motor is 8.4984
𝑟𝑎𝑑
𝑠
, when the torque
is equal to zero, and the stall torque is 0.3916 𝑁 ∙ 𝑚, when the speed is equal to zero.
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The data measured for the Servo Motor was angle of the motor and the pulse width of its
duty cycle. Data from the Servo Motor can be seen in Table 3 in the Figures and Tables
Attachment. Figure 2 in the Figures and Tables Attachment shows a Pulse Width versus
Angle plot of the Servo Motor. The equation, found using the trend line application in
Microsoft Excel, shown on the plot in Figure 2 is the empirical relationship between the
angle of this motor and its pulse width.
Another team in charge with the design and modeling of the robot listed requirements of max
speed and max torque, which may happen at the same time, for motors at the different joints
of the robot. The requirements, converted to metric units, can be seen in Table 4 in the
Figures and Tables Attachment [1].
3. Discussion
One application of a stepper motor would be a compact disk drive. In a compact disc drive,
stopping, starting, and continuing rotations are all needed in order for the drive to read the
disc and function properly. With a stepper motor, changing rotation or continuing rotation
can be achieved by flipping a couple of switches. With the functionality of a stepper motor
and the necessary properties of a compact disc drive so close together, a stepper would be
perfectly put to use in a compact disc drive.
Two applications of a servo motor would be headlights on a car and a radio. Headlights on a
car would be one application of servo motor because the lights need to be held at a constant
angle and servo motors hold things constant. Whether the lights are angled toward the road,
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angled straight ahead, or off, the state of the lights is always constant. A radio is another
application of a servo motor because a radio can be set to a constant frequency to listen to
music or a favorite talk show.
Two sources of error with the DC Motor testing would be slipping of the rope/string holding
the mass up and resetting the Arduino. If the string rapped around the motor holding up the
mass slips while it is pulling the mass upward, the speed on the Arduino would not be as
accurate and thus have an effect on the results. Not resetting and zeroing the Arduino before
starting a new test could cause the motor not to start from rest, according the measurements
of the Arduino, and thus creating precision error in the data.
4. Summary and Conclusions
Reverse-engineering Or Testing Of Motors (MOTO
), Inc. asked four of its employees to
test three different types of motors: a stepper motor, a servo motor, and a DC motor. Data
from each of these motors were found, recorded, and can be seen in the Figures and Tables
Attachment. Lastly, the employees were to determine the motor and joint for which one of
the three engines will be used in the surgical robot system.
According to Table 2, Table 4, and the plot in Figure 1, both of which are in the Figures and
Tables Attachment, the DC motor at Joint A would best serve the surgical robot system.
The torque and speed requirements in Joint A closely fit with the linear regression created
from the points on the plot in Figure 1. All of the other joints were nowhere near the linear
regression in Figure 1.
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For future work in this area, multiple DC motors should be tested to see the angular speed of
those motors with different applied torques. Maybe, some of the other DC motors would
match the torque and speed requirements of Joints B and C for the robot.
Attachment: Figures and Tables, 3.
Attachment: References, 1.
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Figures and Tables
Table 1: The angle and switch sequence required for the Stepper Motor
Angle
0⁰
45⁰
90⁰
135⁰
180⁰
225⁰
270⁰
315⁰
360⁰
Stepper Motor
Switch A
S2
S2
Off
S1
S1
S1
Off
S2
S2
Switch B
Off
S4
S4
S4
Off
S3
S3
S3
Off
Table 2: The masses, angular speeds, and torques on the DC Motor
Mass (kg)
0
0.0475
0.095
0.1425
0.19
0.2375
0.285
0.3325
0.38
0.4275
0.475
DC Motor
Speed (rad/s)
8.498447951
8.344206224
8.156454175
7.923316584
7.718966454
7.070353234
6.89449735
6.717594267
6.426714204
6.141635614
5.888643158
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Torque (N∙m)
0
0.0118237
0.0236474
0.0354711
0.0472948
0.0591185
0.0709422
0.0827659
0.0945896
0.1064133
0.118237
Figure 1: Scatter plot of the angular speeds and corresponding torques of the DC Motor with a linear regression
Table 3: The angle of the Servo Motor and its corresponding pulse width
A
B
C
D
E
Servo Motor
Angle (Degrees)
Pulse Width (ms)
20
0.7369
40
0.94615
85
1.35
125
1.76
165
2.18
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Figure 2: Scatter plot of the pulse width versus the angle of the motor for the Servo Motor with a linear regression
Table 4: Speed and torque requirements for the robot at different joints
Joint
A
B
C
Max Torque (N∙m)
0.141231037
0.282462073
0.070615518
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Max Speed (rad/s)
5.23598775
1.04719755
10.4719755
References
[1]
DC Motors Writeup w/ Procedure 11/7. 11/13/12. http://carmen.osu.edu.
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