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Testing the actuator

A project log for Wolfie

The second version of quadrupedal robot based on custom brushless actuators

Peter WasilewskiPeter Wasilewski 11/23/2020 at 07:170 Comments

Hi!

Recently I was involved in testing the actuator on my small torque test bench. My goal was to determine the maximum peak torque, maximum continuous torque, saturation current, torque constant and some thermal characteristics. The test  stand is equipped with a dynamic torque sensor, and one or two actuators - depending on the measurement goal. 

First I measured thermal responses when different current steps are commanded. The actuator was pushing against the table with a lever in order to keep the output shaft from rotating. This is the outcome: 

The temperature rise on vertical axis is measured from the initial 30*C. The ambient temperature was 25*C. As you can see I also tried predicting the temperature when a certain current is being applied, however this is not very accurate at very low and very high current setpoints due to the changing thermal resistance. The continuous torque that can be applied infinitely without overheating is about 0.875Nm (if we assume 65*C is the actuator maximum allowed temperature).  

Next I turned on the torque sensor and commanded currents from 1 to 30A in q axis. Near the 30-amp region the motor was heating up rapidly, so each test was really short (<8s). Here is the plot: 

In the linear region the torque constant is about 0.125Nm/A, however above 23A the characteristics starts to bend due to motor saturation. It can be accounted for in order to get a linear response over a wider currents range (just like Josh did: https://jpieper.com/2020/07/31/dealing-with-stator-magnetic-saturation/), however for now it is not my primary goal. 

The peak torque I achieved was about 3Nm at 30A phase current. I did not want to go further as the whole setup heated up very rapidly. There's still some room for improvement and increasing maximum current, but for now I decided this to be the safe area of operation.

If you are interested in more detailed description feel free to visit my blog: https://pwwprojects.blogspot.com/

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