Tips for Reducing Servo Failure


#1

Properly taken care of, a Hexy can easily last 30+ hours before any gears need replacement. We’ve done this at plenty of trade shows, where we’ve had it run for 10+ hours for 2 days without issue, taking the same robot to multiple events. In the beginning, I had a fair number of servo failures myself. However, at this point, I’ve run Hexys for probably 1000+ hours total. We’ve also done a lot of torture testing to discover what it is exactly that causes the failures. We’ve run servos under load for hours at a time with IR thermometers, and probably broken 50+ servos solely in the name of science of breakage.

In the process of all this, we’ve narrowed down pretty specifically the cause of failures, and tips to help. Given that servos seem to be the #1 issue people are having, here are some tips:

1.) Calibrate the servos
With all moves, walking, resetting and getting up especially, it is important that all legs meet the ground equally. Even when being careful to center the legs in the build process, it is likely that the legs will still be off-center because of how the gear horns and servos are made. PoMoCo has a feature that lets you make software-based offset adjustments that will result in the legs being much more equal. You can see this as the +/- signs next to each servo. You can even save these offsets to the same folder as PoMoCo.py, and it will automatically load these settings when PoMoCo starts.

2.) Turn the servos off between uses.
This is a big one. The primary cause of failure is the breakage of the small gear next to the output shaft. During normal operation, unless jerked, it tends not to fail. Heat, however, worsens this problem. The motors inside the servos are coreless motors, also known as ‘poor man’s brushless’. The advantage of coreless motors is that they have very high power output for their size/weight. The downside is that they tend to generate a lot of heat. Being inclosed in a plastic box, unless given time to rest, they can raise the temperature inside the box enough not to melt the gears, but enough to soften them significantly to the point where failures can occur. Even with a servo “not being used”, but being on (like say, holding postion, whatever the load), the servo can still vibrate back and forth significantly just holding position. This may not be visible at the output, but before the 4 stages of gearing, the motor is still actively going back and forth, generating heat. A good way to deal with this without compromising a move is to selectively turn motors off that aren’t needed. Another option is to just turn Hexy off in between uses.

3.) Use the lowest voltage needed
Higher voltage means more current, and thus higher power. So why not just run them at 6v+ all the time? It also means more heat, and more vibrations. Because the servos use a cheap analog feedback circuit to maintain position, they can ‘overshoot’ their intended postion, try to correct, and ‘overshoot’ again, going into an oscillatory feedback loop.This feedback loop generates heat, and heat generates failures. This effect is most pronounced with the hip servo, because it has the most moment-arm inertia (the most weight swinging back and forth), acting like a pendulum. The more power into each overshoot, the farther it will overshoot, the more it overcompensates. the wider the feedback. Lower voltage will mean less power, yes, but it will also mean less overshooting and jittering. It may seem counter-intuitive, but using less batteries in series will also make hexy last longer. Using 6v increases the current drawn vs 4.8v (Current = Voltage/Resistance), and thus shortens the battery life.

4.) Dropping/hitting
Any sudden jarring movements can result in ‘spraining’ Hexy, just like falling on your ankle can sprain it. The servos do fail under quite low levels of abuse. Suddenly moving a leg, even when not powered, can be too much for the gears and cause them to snap. This can happen as easily as someone walking by a leg sticking out from a table, catching a leg going past and snapping it back. Dropping Hexy is also obviously a no-no.

Common questions about the servos:
1.) Why didn’t you use metal servos? Can we get metal ones? What about XYZ servo?
The simple answer is that we couldn’t find one that worked properly at a decent enough price that we could source at quantity. Remember, you can’t just walk into your local hobby shop and order 10,000 servos. Even if you could, would it be at a price that would survive also cutting the acrylic, designing and manufacturing the electronics, sourcing screws and cables, and running a company, also considering you have to sell them at a discount for retailers?
The few we did find at a price range that were reasonable had issues with freezing at the extremes (+/-90 degrees), as in they would just straight lock up and not move. We thought this was pretty unsatisfactory, so we decided to ship the plastic ones. We’re still looking for a proper metal gear servo, and if we do find one, we’ll be sure to offer a metal-gear upgrade kit, along with the acrylic pieces that would fit it.

2.) So what about the failures?
Servos that are dead on arrival we’ll replace free of charge, just email replacements@arcbotics.com, no questions asked. We’ll rely on you guys to be honest about which ones broke on arrival, and which ones are broken getting the kit working. Servos that aren’t DoA are 95% because of the first full gear before the output shaft. This can almost always be fixed with a gear replacement. We’ve talked to the manufacturer, and should be having servo gear replacement sets (10 gear sets for $5), available in the store. No servos last forever, and these ones aren’t any different. However, we do want to make it as cheap and easy as possible to get them back up and running. Based on the advice and feedback we’ve been getting from the forum, as soon as we get the gears in, we’re going to start including them with new outgoing kits as well. We could include metal gear servos like a Hitec HS-82MG, the servos would last 2-3x as long, but we’d be selling $800 hexapod kits. The tradeoff is the cheapest hexapod kit by far, it just needs dirt-cheap servo gears replaced every once in a while.


Battery Life
#2

you might want to try to centralize things a bit.

The build instructions http://makeprojects.com/Project/Building+an+ArcBotics+Hexy/2661/#.UIi6Iml24vg don’t link to the wiki, then you have this post in the forum.

The http://arcbotics.com/products/hexy/start/ actually points first to the make build instructions while people should first update the firmware and do the software part setup to test the servotor32 board en center servo’s with PoMoCo

Might be good to choose one “entry” point like the wiki and then go from there and ping back to that “central” page


#3

Agreed that new users need to be funneled through one authoritative collection of documentation resources, we’ll leave it to Joe to decide the best approach.

In the meantime, I took the liberty of posting an edited version of Joe’s servo post to the Wiki.


#4

so is the Hitec HS-82MG servo a drop-in upgrade for those of us with money/ time inbalance?
//when I have money, I have little time…


#5

As of this time there is no known drop-in replacement besides the SG90/HXT900. We do not currently know of a metal gear drop-in replacement.


#6

I’m a little confused about the proper approach here.

You have documentation that says it’s best to use the Powerex 2700 batteries in the 5 battery holder - which is 6 volts. Yet, you say here that 6 volts is really too high because the servers over throw. Which is it? I just spent over $100 on a battery charger and batteries and Hexy works worse than on alkalines. I have 4 servos that jitter so much there’s no way I can even tell where the legs are to set the offsets properly.

Is there a definitive answer here? I’m very disappointed - waited a long time for the kit, and now that it’s together it really doesn’t work.

Any input is appreciated!

Also, before anyone asks, I’m running the 2.0 Servotor code, and can control the Hexy both by USB or Bluetooth (although via USB, my Mac sometimes reboots itself!).


#7

I usually go for the 4 pack to start, and only go to the 5pack if it really needs it.


#8

I have found the robot to be MUCH more stable with only 4 of the Powerex batteries. It seems to me that 5 fully charged batteries just puts out too much voltage.


#9

I’ve had really nice success with a drop in replacement using Hi-Tech HS-65MG … granted, these are $20 US each, still they are nice servos. It did require a little bit of filing to make the thigh / hip portions uniform in the opening dimensions. On some legs they dropped right in, on others the opening simply needed a bit of filing.


#10

FYI, we’re doing our own servos:


#11

wowsers, 9 buck huh … I got a deal on the Hi-Tech HS-65MG’s but still paid $20 each …