Question
Asked September 4 2017

Community Question

Having received a JD Humanoid Robot (in U.K, via RobotShop, Paris, France) in August 2017 I have had two complete servo failures and I am suspicious of the design of other servos in this JD.

Both the Left Ankle and the Left Knee servo failed, by "jittering" backwards & forward uncontrollably. Obviously JD could not stand up with this problem.

While driven by a steady mid-range servo pulse (approximating to 90 degrees, or straight leg) The servos jittered, with occasional excursions to fully forward (which would have been commanded with the minimum pulse width of approx 0.55mS, except the pulse was stable and not calling for an extreme position).

The servo drive pulse was stable and constant at about 1.3 mS, and it was a very clean signal with no "noise".

I have examined a couple of other servos from JD which do not have this problem, but they both show features which I have never seen on other servo makes.

When being driven, by a slowly varying pulse train, from limit to limit, both servos performed ok, moving smoothly, but I noticed two "odd" things. When being driven both servos took about 0.25 ~ 0.45 Amps (at 5V) while moving, which dropped to Zero Amps when the commanded position was reached - as normal & expected.

However, when either servo was driven to the Maximum Pulse width Extreme limit, i.e. nominally 180 degrees (approx 2.4 mS pulse width) the current suddenly rose to 1.2 Amps (overloading my PSU) and stayed there as long as this extreme position was held. Needless to say I did not hold this "very unhealthy" position for long !

Also, quite often, as the servo reached this (max-pulse-width) extreme position it started to "jitter" back to the other extreme position (min pulse width) even though this was un-commanded. The servo control pulse width stayed constant and "clean" so the servo should just have stayed at the max-pulse-width extreme position as was commanded.

I have never had any of this behaviour from any other servos and, in fact, over several years, I have never had a servo fail me.

I have now arranged with RobotShop to return this JD Robot and they will send a completely new model - I hope this works O.K.
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Synthiam
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September 22 2017
I wonder if it is really the Pots.

When the FIRST JD played up I replaced a "dud" HDD servo with a similar ModelCraft servo which I had spare. This worked fine, until another HDD servo elsewhere in JD died and I hadn't any more compatible spares..

I then dismantled one of the "dud" HDD servo and isolated the pot, because I thought that was the problem. I found that the pot seemed OK, with progressive resistance throughout it's range, and no obvious "noise".

On re-assembly the servo was still "dud", in that the "90 degree" signal still sent it to full "90 degree" travel and the current went through the roof.

Inside the servo was an IC, maybe two, I can't remember and I wonder if this had died rather than the pot.

Even the "good" servos worked differently from other servos I have used - in that when approaching 180 degrees they often started to jitter all the way back to 0 degrees, and if held at full traverse they suddenly pulled a high current. You can see this "jittering" on the videos I sent to RobotShop Paris (France)

It is all a great shame since I think JD and his software is superb - but not reliable. He's going back to Gay Paree.
Jeremie
Synthiam
September 22 2017
Hi @RogerR,

I was part of the HDD servo investigation and I can definitively say that the potentiometer is the cause of the issue that you are seeing. You likely will not see this issue by simply doing an resistance measurement as you sweep the servo, it is a bit more difficult to isolate then that. As @DJ mentioned we are working very hard with our manufacturer to remedy this quality issue that affects certain servos in our last batch of HDD servos.
September 22 2017
@Jeremie... Oh Lord... I bought 20 HDD servos a month or so ago from Robotshop.ca.... I haven't tested all of them but here's hoping they weren't part of your last batch... *eek*
September 22 2017
I have purchased the upgrade kits (HDD servos) for my JD and Six.
I only tested the 2 NEW HDD JD Shoulder servos, one jitters, and one is fine.
Will try to test the others this coming week.
DJ Sures
Synthiam
September 22 2017
Contact us and we'll make sure your jittery or self moving servos are taken care of