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Monday, November 30, 2009

It was only a matter of time...

It was inevitable, really.

While chasing a guinea pig under a table, I managed to cause enough vibrations for the robot to topple off and land face-first on the ground. Although this may sound both comical and adorable, I was not amused.

At first, everything seemed OK, but closer inspection revealed that one of the spanner shaped connectors had snapped, and more thorough testing revealed that one of the servos was making a grinding noise.

Disappointed, but not discouraged, I voided my warranty and opened it up to see what the problem was.


As it turns out, I had stripped some teeth of a plastic gear, which meant that the motor was spinning uselessly as the feedback loop was broken. Fortunatley - in one of the more pleasant customer experiences I have had - it turns out that the robosavvy people had included a bag of spare servo gears.

So fixing it was just a matter of unscrewing the case and feasting on the delicious goo inside. If a screwdriver was included in your kit, don't use it to open the servo cases! It's just a shade too big, and I ruined the tip by trying to get the screws out. Just one size smaller should do it, but be sure to push down hard against the screw so that you don't strip the head off.

Included in my kit was an instruction sheet (the backdrop of these photos), which explained how the wCK modules can be re-assembled. I don't know if this is because the expect them to need servicing at some time, or just because they had the CAD models lying around. Either way, there isn't much to the gear assembly, so don't be afraid to open one up if you are curious. After replacing the damaged gear, I simply slid the bearing back on to the drive shaft and put the case back on.
At this stage, I was worried that I might have not oriented the output shaft correctly, resulting in the servo position being slightly out of phase with the desired angle. However, the wCKs must use optical encoding or something to establish their position, so after putting the case back on it just worked straight away. [UPDATE - The servos actually use a potentiometer, but the shaft is designed to only fit correctly].

So there you go - a potentially disastrous situation was averted thanks to the inclusion of some spare parts. Since nothing lasts forever, I would recommend you order from robosavvy - their service so far has been exemplary.

[UPDATE - the official servo repair instructions are available]

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