The Better MouseTrap Servo Tester
I have decided to make my own advanced servo tester after finding it awkward to calibrate all 18 servos on my HexPi hexapod robot.
While blogging about HexPi, I decided to split out my servo tester as a separate article, as I figured it was significant enough a project on its own!
“The Better MouseTrap Servo Tester” is based on:
- RoboPi advanced controller board
- four thumb joystick modules
- R/C battery pack for providing power to the servos
- Styrene base to mount everything on
- assorted wires, mounting hardware, etc
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I have been taking photos during the development process, and I will keep uploading the best shots here.
May 30, 2014
TBMST has been working for a while now, but I was not happy with the jitter I was seeing with a Tower Pro MG-995 servo (even after I improved the PWM timing), so today I decided to get to the bottom of it.
I already figured out that the servo did not like being powered at 6.7V, so I used a 5V 2A wall ward. This did reduce the jitter a bit – most likely to the motor not providing as much torque at 5V – but I wanted to get to the bottom of it. After all, if a GWS MT-1 servo tester could control the servo without jitter, so must TBMST!
I decided to look at the servo pulse and the servo power at the same time:
(Click on the image to see a larger photo)
Look at that spike on Vservo! (the yellow line)
Something is fishy here… let’s add 1000uF between Vservo and GND…
(Click on the image to see a larger photo)
Ok, that looks better. But we don’t want any noticeable disturbances on Vservo…
Let’s make that 2000uF between Vservo and GND!
(Click on the image to see a larger photo)
Better. But not good enough!
Let’s try 3000uF.
(Click on the image to see a larger photo)
Now isn’t that much better?
And guess what – it cured the power supply jitter problem!
I’ll have to dissasemble and reverse engineer one of these servos. I suspect that the microcontroller is run from the motor power directly, and may even brown out at times due to insufficient filtering of its power. Frankly, I expected servos to filter their microcontrollers power supply better than this.
The $64,000 question is… For HexPi, will I need to add a 3300uF electrolytic capacitor to every MG-995 servo? Or can I get away with adding say 24000uF to 33000uF per group of eight servos?
Soon I can start wiggling HexPi’s legs, I just need to make a better servo power supply for him.
