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. 1 of 5 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 š 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 š š 2 , and 0.0254 š, the motor’s radius. The speed of the motor in revolutions per minute was converted to radians per second by multiplying by 2π ššš ššš£ š and dividing by 60 ššš. 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 ššš š , when the torque is equal to zero, and the stall torque is 0.3916 š ā š, when the speed is equal to zero. 2 of 5 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, 3 of 5 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. 4 of 5 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. 5 of 5 Figures and Tables Table 1: The angle and switch sequence required for the Stepper Motor Angle 0ā° 45ā° 90ā° 135ā° 180ā° 225ā° 270ā° 315ā° 360ā° 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 1 of 3 Torque (Nām) 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 2 of 3 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 (Nām) 0.141231037 0.282462073 0.070615518 3 of 3 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. 1 of 1