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I currently have an inspection robot deployed in a remote part of Africa that has a failing Turnigy TGY-6114MD servo. The issue started happening during normal operation and was working fine previously. Now the servo won't respond to commands unless given a slight push or nudge and then it will move to it's commanded position. We thought there may be some binding in the gearbox. After removing the gearbox and the gears we found that the motor shaft itself needs a slight push to start spinning. So with no load on the motor at all, it still won't start unless given a slight push.

Does anyone know what may be causing this behavior? We are unfortunately unable to get a replacement part in a reasonable amount of time so any repair ideas are welcome!

EDIT: Update. We measured the voltage across the motor electrical connections during operation. When the motor is turning (with no physical load) the voltage is 4.98V. When the motor stops, the voltage drops to 1.4V. On an identical working servo on the robot, the voltage is 0V when the motor stops. What would cause the 1.4V at the motor when stopped?

Cross posted here: https://robotics.stackexchange.com/questions/18424/tgy-6114md-servo-motor-needs-push-to-start

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  • \$\begingroup\$ Perhaps bad brushes or a loose potentiometer wiper. But such a question is off topic here. \$\endgroup\$ – Chris Stratton Mar 16 '19 at 1:53
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    \$\begingroup\$ you already asked this question .... robotics.stackexchange.com/questions/18424/… ..... if you are going to cross-post, then provide links in the question \$\endgroup\$ – jsotola Mar 16 '19 at 1:57
  • \$\begingroup\$ @jsotola Sorry for not adding the link I'm totally new and figuring things out. I'll add the link to the main question. \$\endgroup\$ – WhiskeyTango Mar 16 '19 at 20:56
  • \$\begingroup\$ @ChrisStratton Thanks for the suggestions. Is there a better place to ask a question like this? I understand it's not a design question, but I assumed failure modes would fall under electrical engineering. \$\endgroup\$ – WhiskeyTango Mar 16 '19 at 20:58
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Most likely the brushes have worn down and are intermittently shorting out the commutator. This is a very common failure mode in servos which use 'precious metal' brushed motors, especially if the servo is being worked hard. The usual way of dealing with this issue is to replace the servo well before reaching the end of its lifespan, and keep a spare on hand in case it fails earlier.

Repair would involve replacing the motor with an equivalent unit. Without specs getting the right motor may be tricky, and the pinion gear might be hard to move onto a new motor without damaging it. For a temporary repair you may be able to get a suitable motor from another servo of similar size but different model, or (if you are desperate) perhaps from a small motorized appliance. If you have another motor with identical brush assembly you could try exchanging it, but the brushes are very delicate and easy to damage.

For the future, replace servos on a regular preventative maintenance schedule, and don't buy from dodgy Chinese manufacturers!

Here's an example 'precious metal' brush assembly from a disassembled Mabuchi RF-020 6VDC motor. One of the brush fingers is already bent out of line.

enter image description here

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