EASYDYNO Dynamometer Project
ME153, Spring 2020
Xinghao Huang
Problem
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Measuring the rotational torque on a dc motor usually requires an expensive torque sensor, but EasyDyno can be an alternative with low price and high precision. EasyDyno uses eddy current force to achieve contactless torque measuring. When the motor rotates the metal disc that is exposed to a magnetic field, by Lenz law, the changing field induces a current that opposes the field and the motion. The reaction force on the magnet is then measured by a load cell. The device consists of commonly used materials, and can be easily assembled.
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CAD Design and Assembly
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Materials:
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Circuit Design and Data Aquisition
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Constant voltage for motor (6V)
Components:
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Some important debugs:
1. The hall-effect sensor can be disturbed by strong magnetic field. It should be placed as far away from the magnet as possible.
2. The interrupt pin used to calculate the RPM needs a pull down resistor. This is very important because any touch or vibration can possibly trigger an interrupt and thus ruin the RPM.
3. The motor and Arduino share a common ground. When motor is at high speed, the high frequency noise created by the brush/commutator can propagate through the ground to the Arduino, which will disturb the calculation and damage the board. The motor must have a capacitor (usually 0.1uF) soldered across its terminals.
4. Copper is good conductor comparing to aluminum. Using copper disc can greatly increase the eddy current force.
1. The hall-effect sensor can be disturbed by strong magnetic field. It should be placed as far away from the magnet as possible.
2. The interrupt pin used to calculate the RPM needs a pull down resistor. This is very important because any touch or vibration can possibly trigger an interrupt and thus ruin the RPM.
3. The motor and Arduino share a common ground. When motor is at high speed, the high frequency noise created by the brush/commutator can propagate through the ground to the Arduino, which will disturb the calculation and damage the board. The motor must have a capacitor (usually 0.1uF) soldered across its terminals.
4. Copper is good conductor comparing to aluminum. Using copper disc can greatly increase the eddy current force.
Results
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A typical DC motor should yield data like the plot shown on the right. In Prototype V2, the test results show the torque vs rpm curve align closely to the spec data, which is a linear line between the stall torque and no-load rpm. However, the experiment result deviate from the spec data at high torque. This was probably due to the load was beyond the load cell's limit (100g). The mechanical power was product of torque and rotation speed in radian per second. The result curve is a well symmetric parabola. The efficiency is the ratio of mechanical power over electrical power, which is the product of current and voltage. The voltage is kept at 6V, and the current is measured by the current sensor. The result was a curve that has a maximum efficiency located near high rpm (1105 rpm).
One disadvantage of this dynamometer is that it cannot reach a stall torque. It's not really possible because eddy current force requires speed, and low speed needs a magnet with very large magnetic field or superconductor as the disc. Another solution can be adding a frictional stop mechanism, but that will defeat the purpose of contactless torque measurement. |
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In Prototype V3, a new load cell with 500g limit was installed. The result torque vs rpm curve moves closer to the spec data at higher torque.
A new testing approach was developed. The magnet would first approach to the disc at a certain speed until they are 1mm apart (called forward stroke), and then disengage at the similar speed (called backward stroke). The torque vs rpm curves for forward and backward stroke are different, as the backward stroke gave a less steep slope. The backward stroke also gave lower peak power and efficiency, which are shown on the right. After researching a little bit, I found there is a phenomenon called magnetic hysteresis. When DC motor is experiencing a change of torque load, the magnetic field inside the motor is also changing. To decrease the magnetic field or completely reverse the field, the action needs certain level of current in the opposite direction to effectively demagnetize the material. This lack of reversibility is called hysteresis; it actually implies some energy loss during the magnetize-demagnetize cycle [1]. The magnetic hysteresis can contribute to motor inefficiency. Thus, measuring the magnetic hysteresis loss can be an extra function for EasyDyno.
A new testing approach was developed. The magnet would first approach to the disc at a certain speed until they are 1mm apart (called forward stroke), and then disengage at the similar speed (called backward stroke). The torque vs rpm curves for forward and backward stroke are different, as the backward stroke gave a less steep slope. The backward stroke also gave lower peak power and efficiency, which are shown on the right. After researching a little bit, I found there is a phenomenon called magnetic hysteresis. When DC motor is experiencing a change of torque load, the magnetic field inside the motor is also changing. To decrease the magnetic field or completely reverse the field, the action needs certain level of current in the opposite direction to effectively demagnetize the material. This lack of reversibility is called hysteresis; it actually implies some energy loss during the magnetize-demagnetize cycle [1]. The magnetic hysteresis can contribute to motor inefficiency. Thus, measuring the magnetic hysteresis loss can be an extra function for EasyDyno.
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Final Presentation Poster Slide
