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Section 2 Lab Experiments

Section 2: Lab Experiments

Page 12 Oregon State University ME 101 Lab Book

Section Overview This set of labs is provided as a means of learning and applying mechanical engineering concepts as taught in the mechanical engineering orientation course at Oregon State University. In the first part of the procedure for each lab are the hardware setups required, mainly soldering motors, switches, and sensors. The rest of the procedure will outline the setup and details for the lab. The programs in this section will need to be written by the user. For reference and help, refer to the mech_lib website or the board experiments. Some labs may require extra hardware or parts that are not included in the Mechatronics kit such as weights and mechanical parts; this is left to the user to find and implement.


ME 101 Lab Book Oregon State University Page 13

Motors and Efficiency – Lab 1

Objective • Learn basic usage of the mx_ctlr board • Practice graphing in Excel • Importance of accurate measurements • Understand the relationship between RPM, torque, and efficiency

Prelab • Come to lab with knowledge of Microsoft Excel • Basic knowledge of the mx_ctlr board (Appendix A)

Procedure Attach wires to your motors and switches that will be used in this lab; refer to Section 3 for instructions. For this lab, the switches will need to be in the normally open position. Attach a motor to ‘motor output 1’ and a switch to ‘input 1.’

Setup 1 Setup the experiment as in Figure 2, in this part you will need to obtain data to plot a RPM vs. weight curve. Using various weights, measure the distance traveled in a certain amount of time to determine the RPM of the motor. For this first setup, you will manually press the limit switch to start and stop the motor. Write a program such that when you press a switch, the motor will start turning, and after the weight has traveled some distance, pressing the switch again will stop the motor. The distance measured will be used in calculating the RPM of the motor. Repeat the process as many times as necessary to obtain enough data to plot the RPM vs. weight curve using Excel.

What causes the ‘knee’ in your plot? Why can the motor maintain a stable revolutions per minute regardless of weight until a certain amount of weight? Is this a mechanical or electrical issue? Is it both?



Figure 2: Setup 1.

Table 1: Data for RPM vs. weight measurements

Weight Distance Time RPM



Setup 2 In the first setup, there was error in measurement due to human reaction time in pressing the switch and recording time. In this setup you will try to remove this human error to get better results. You will need to plot the RPM vs. weight curve again, but this time program the motor to turn on for a fixed length of time and measure the distance the load travels. Compare the two curves you produced of RPM vs. weight. Is one more accurate?

Excel is a powerful tool. Can you find a way of plotting a ‘best fit curve’ to your data? How about a ‘correlation coefficient’ to see how close your data fits the best fit curve?

Table 2: Data for weight vs. RPM w/o human error measurements

Weight Distance Time RPM

As an option, you can use the board to obtain the data directly and send it to the computer (this will require more programming on your part), which can then be plotted in Excel. In order to transfer data, you will need to hook up a male to female serial extension cable on the serial port of the board to either port com1 or com2 on the computer. Refer to Section 3 for an example of how to use the serial communication.



Write Up Turn in the graphs and the code used for Setups 1 & 2. On one typed page, explain how you were able to reduce the amount of human error and why it is important. Also identify the ‘sweet spot’ of the motor (the most RPM for the weight). What is the relationship between RPM, voltage, and loading in the motor? What makes a motor more or less efficient? (You may need to do a little research on the internet to find these answers.)



Power and Torque – Lab 2

Objective • Explore further function of the mechatronics board • Explore the concepts of power and torque in a motor

Prelab • Read over Lab 2 • Obtain materials to be used in construction of the lever arm

Procedure In this lab you will use a lever arm that can have various weights hung from it to examine the torque of a motor. The current of the motor will be measured across a 1Ω ½W resistor (brown, black, gold) soldered in series with the motor. Using a voltmeter, measure the voltage across the resistor. Since the resistor is 1Ω, the voltage is equal to the current.

When constructing the lever arm, take into account the weight of the material being used. They should be fairly strong but not too heavy. The internal gears of the motors are made of plastic; too much stress could cause permanent damage.

Figure 3: 1Ω in series and measuring voltage.

When measuring voltage, place the black lead in the com port and the red lead in the V/Ω port.



Attach a weight to your lever arm some distance away from the center of your motor and measure the current of your motor as the arm rotates from vertical to a horizontal position, Figure 4. Record the largest current. Move the weight to a different distance from the center of the motor and repeat, filling in Table 3. Be sure to have some measurements where the motor cannot quite raise the arm all the way to horizontal. Plot the torque vs. current curve using Excel.

Table 3: Data for current vs. torque measurements

Weight Distance Current Torque



Figure 4: Lever arm apparatus.

Have a TA check off your lab after completion.

Write Up Turn in a current vs. torque graph from Excel. Explain in a short write-up if current and torque are related. What about torque and RPM? You should be able to explore the torque of the measurements from Lab 1 compared to weight and RPM. Convince your TA of your findings.



Simple Control System – Lab 3

Objective • Learn basic concepts of input/output systems • Thermal control

Prelab • This is a very tough lab. You should read ahead and start your design before coming to lab. If you don’t have a

soldering iron, work on the code as much as you can.

Procedure In this lab you will construct a simple cooling control system. The input for the system is from a thermistor that will be connected to a resistor across the motor terminals. A small computer fan will be connected to another motor terminal which will turn on once the resistor gets hot enough to try and cool it down.

Setup 1 Solder two 56Ω ½W (green, blue, black) resistors in parallel and attach to motor terminal 1. Then attach a small fan across motor terminal 2 (Note: if the fan is DC, the wires can only be inserted one way: the positive lead is on the forward side of the terminal, the side closer to the power terminal).

Figure 5: Attaching a resistor and fan to the motor outputs.

Assemble a thermistor sensor. Solder a thermistor in series with a 4.7KΩ 1/8W (yellow, violet, red) resistor, Figure 6. Then attach three wires as shown in Figure 7; these wires will need to be approximately 8-10 inches long.



Figure 6: A thermistor sensor.

Connect the power lead on the resistor to Vcc on terminal J10, the power lead on the thermistor to ground on terminal J11, and the signal lead to alternate input 1 on terminal J10. Using a small piece of tape, tape the thermistor to the resistor on the motor terminal.

Figure 7: Attaching the thermistor to the resistor.

Write a program that will continuously poll an input from the thermistor; then depending on the level given by the thermistor, decide when to turn the fan on or off to keep the resistor at a fairly cool temperature. In this program, motor output 1 is always on; this will heat up the resistor. As the resistor heats up, the number given by the ADC will go down. Write the program such that the resistor temperature will always be around 124. As a safety precaution, at 50 the resistor has become too hot; program the board such that everything will be shut off.

You are building a simple control system, Figure 8, that monitors the heat of the resistor. There is a lot of math behind what you are doing that can make your program more efficient, but you will have to wait to learn more



about it.

Figure 8: Simple control system.

Feedback is often needed to determine if a product is functioning correctly. Send the output from the ADC to Hyper Terminal, and refer to Section 3 for help. Sending all the data is impractical; however, in your program, find a way to only display critical values or only when prompted by the user. When reading from the ADC, use the ‘read_adc’ function.

Setup 2 Most systems that require heat dissipation use a combination of a heat sink and a fan. Hook up the thermistor, Figure 9. The two resistors and thermistor will need to be soldered onto the heat sink. Run the program again and note the differences.

Figure 9: Heat sink and thermistor.



The new system you have built is slightly different than the first one. You have added a new ‘gain stage’ to your control system, Figure 10. The heat sink acts to amplify the cooling from the fan. Is your fan on more or less often with the heat sink attached?

Figure 10: New control system.

Write up Turn in the program you wrote for this lab. In this lab, you used two ways to reduce temperatures in a system; describe at least two other ways that temperature in systems can be controlled. How do these methods differ; is one better or cheaper? Discuss these and other aspects in your analysis. Discuss what could happen if the heat sink was infinitely large (over-dampened) or infinitely small (under-dampened).

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