How to fix Servos

[amorti]

Sometimes Lego Servos just quit returning to centre. There are lots of videos of this phenomenon on YouTube, and I know that plenty of us have had it happen.

When it does happen, your first action should be to ask Lego for a new one. I've asked twice, and they've twice sent me a new one with no questions asked, and didn't require the old ones back.

Now, when Lego sent you a new one (or if you didn't like to ask, or can't wait for the new one, or when the new ones run out) you'll be sitting looking at a servo which does something like this, or which doesn't return from one side or the other.

First thing you need to do is use something sharp to remove two of the clips which hold it together. Even then, it's going to take some force to get this open.

Keep in mind the servo will never hold itself together against pulling forces quite the same again, but generally it shouldn't be a problem.

Inside you'll find it looks like this:

The positioning circuit works by dragging the two stainless/spring steel contacts along the brass (copper?) tracks connecting various points in a circuit, then a chip figures out where the motor's head is pointing. However that's not a brilliant design, because the tracks are soft, and the contacts are hard. 

Here (click for high resolution), you can see the smaller tracks near the middle show clear signs of wear.

I don't know whether it's happening by "smearing" the metal or whether it's by arcing between two blocks, but what happens is that two neighbouring blocks end up with continuity between them, where none should exist (click here to open it bigger so you can see where I'm testing). At this point, the logic circuit can't figure out where the motor is, and strangeness ensues.

The fix is to use a small sharp blade to clear a gap between the tracks.

If it's due to friction, grease should help (even then you're still dragging steel along a much softer metal) and if it's due to arcing, a dielectric (non-conductive) grease will help. I have this stuff and confirm it's safe for plastic. I would prefer something thicker, though.

When you're done, you'll have a fully functioning servo again

Edited September 5, 2020 by amorti

[2GodBDGlory]

Huh. I have worked on a few servos, with mixed results, but I never tried that. Also, as a side note, it appears that newer servos have some grease on the contacts from the factory, but they are still breaking.

[amorti]

@2GodBDGloryI had two fail which I'd bought new just this year. They both had some white grease in them, it hadn't saved them though.

In the end you're dragging what seems to be spring steel against copper/brass. It's a fight which only has one winner.

I really don't understand why they had to reinvent the wheel here. RC servos are cheap and reliable and use a totally different technology, they should have copied that. I still have a dream that the port on top of a buwizz could power an RC type servo.

[Mechbuilds]

I wish they would sell an RC type servo that was converted to work with buwizz using those lego connectors. The servo could have a frame of their own around the servo so it's easy to connect to technic. 
The lego servo is garbage. 

[amorti]

On 9/6/2020 at 11:14 PM, Mechbuilds said:

I wish they would sell an RC type servo that was converted to work with buwizz using those lego connectors. The servo could have a frame of their own around the servo so it's easy to connect to technic. 
The lego servo is garbage. 

RC servos use a potentiometer to sense position, whereas Lego servos use this 15-step sensing system.

They're pretty much not compatible at all, so I think the 4 pin on top of the buwizz is the one you'd have to use. If it can give that output

Edited May 20, 2021 by amorti

[Rezvonflux]

Hey guys I started a nee topic as I hadn't seen this one, but....

Can anyone spot why this one isn't centering and sticking in one direction? 

Good news is after my first attempt and reassemble (which left me with a wiper mother going from full left to full right) I'm back to the initial problem.

https://drive.google.com/file/d/17UGanyWMhV5P6DSHFweZOniU17wsdTS8/view?usp=drivesdk

https://drive.google.com/file/d/184lY4QGUmLbc8P-OZskQb3KwM2WVy-lX/view?usp=drivesdk

 

https://drive.google.com/file/d/189KuGY6cZzh_me8FXzxQW15lsm8kYOca/view?usp=drivesdk

[Andman]

10 minutes ago, Rezvonflux said:

Hey guys I started a nee topic as I hadn't seen this one, but....

Can anyone spot why this one isn't centering and sticking in one direction? 

Good news is after my first attempt and reassemble (which left me with a wiper mother going from full left to full right) I'm back to the initial problem.

These copper conductor tracks are not looking good. Is that recess? Or is that only superficial?

[2GodBDGlory]

Yeah, problems with those motors, in my experience, are usually based on one of two things:

1. Poor connection on that copper track. I've typically been able to fend this off by cleaning the tracks. (Some recommend using an eraser for this) and perhaps bending the tabs a little bit to add tension against the track.

2. Worn insulation where the cable enters the motor or the plug (more often the motor)

This can be solved by opening up the motor, cutting out the part of the cable that was rubbing at that joint, and resoldering the shortened wire back to the motor. (This is significantly harder on Servos than other PF motors, because all four wires have to be soldered, and close together, unlike the simpler motors)

[Rezvonflux]

40 minutes ago, Andman said:

These copper conductor tracks are not looking good. Is that recess? Or is that only superficial?

I may have to open a second working one to honestly see the difference. Wasn't sure if that was normal or actual wearing.

38 minutes ago, 2GodBDGlory said:

Yeah, problems with those motors, in my experience, are usually based on one of two things:

1. Poor connection on that copper track. I've typically been able to fend this off by cleaning the tracks. (Some recommend using an eraser for this) and perhaps bending the tabs a little bit to add tension against the track.

2. Worn insulation where the cable enters the motor or the plug (more often the motor)

This can be solved by opening up the motor, cutting out the part of the cable that was rubbing at that joint, and resoldering the shortened wire back to the motor. (This is significantly harder on Servos than other PF motors, because all four wires have to be soldered, and close together, unlike the simpler motors)

I'm thinking the copper track as well but they seem worn equally on both sides

Raised the points on the metal ring as well

Both tracks and contacts were cleaned using an ersed head too.

I'm at a loss,  I even tried correcting the center while it was apart but continued to do the same once it was put together again