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Wiggling PowerHD LW-20MG digital servo

Old 02-21-2020, 04:13 AM
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Mariners_PL
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Default Wiggling PowerHD LW-20MG digital servo

Hello everyone. I got a problem with a Power HD LW-20MG Digital Servo. When I connect it to the power supply set to either 4.8V or 6.6V, the servo wiggles back and forth in place. PWM is supplied from STM32 Nucleo Board. Its frequency is set to 333Hz and the width of the signal is set from 800us to 2200us where 1500us is the middle position. However the servo wiggles despite the position in which it is set. Amplitude of PWM is 3.3V as itís supplied from STM32, but we tried connecting it by optoisolator so that PWM is also 4.8V or 6.6V, but it didnít change a thing. What could be the problem? Thank you for your help in advance.
Old 02-21-2020, 06:07 AM
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AndyKunz
 
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Jitter in the output would be one thing. What happens if you stop sending the PWM pulse? If the wiggling stops, that tells you the problem is with the signal.

Have you put a scope on it?

Another possibility is that the servo isn't seeing the signal clearly. What's the amplitude of the PWM? How square is it? If you insert a comparator inline to boost the signal so it matches the bus, does this change the behavior (a simple way to do this is using the Spektrum SPMCP).

You might need to filter the power a bit - try putting a big electrolytic right at the servo connector to clean up the power bus. Digital servos are notorious for sending crap back to the supply, since they are usually designed around the supposition that a battery (a big filter) is on the other end.

Andy
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Old 02-21-2020, 08:41 AM
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Mariners_PL
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When I stop sending PWM it stops. I have a video which I cannot upload to the forum yet as I haven't created 10 posts yet, and I also cannot upload a photo...

The signal is quite square, but wiggling servo causes some weird shapes which occur periodically. In the video we use 3V3 PWM signal from STM32 Nucleo and 6.6V from the power supply. We also tried to step-up the PWM by using optoisolator hooked up to 6.6V and a control signal from PWM which resulted in clear 6.6V amplitude PWM. However it was also periodically distorted by servo. Maybe using that electrolytic will help with something as it is making some noise?


Old 02-21-2020, 08:44 AM
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AndyKunz
 
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Put the video on YouTube and link it here. You can do that.

Can you power off a battery and see if that changes things?
Old 02-21-2020, 12:23 PM
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Mariners_PL
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Here's the link: youtu.be/XkuJESPR5eI I will hook it up to the battery and test it.
Old 02-21-2020, 12:29 PM
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You have a grounding or power rail issue. The mass on the servo is just a contributing factor.

Put another probe on the supply line.

Andy
Old 02-22-2020, 04:52 AM
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Mariners_PL
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Okay so I've connected servo first to the servo tester which was hooked up to the battery. Everything worked like charm. After that I connected it to the power supply with 3300uF capacitor in parallel. And it still behaves like that. Scope on the power rail seems okay as it is rather flat line of 5V.
Old 02-22-2020, 07:40 AM
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Can you use your code with a battery? We need to see 1 change at a time. Going to battery & servo tester is two changes.

Andy
Old 02-23-2020, 08:22 AM
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Mariners_PL
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Yeah. We also used only power from battery and signal from PWM and the servo response was nearly perfect.
Old 02-23-2020, 10:08 AM
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Originally Posted by Mariners_PL View Post
Yeah. We also used only power from battery and signal from PWM and the servo response was nearly perfect.
Sounds like the power supply is the problem. Are you near the current limit? Some PS just don't like the high frequency switching loads that some digital servos create.

Andy

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