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Servos
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:
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.
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 to soften the gears significantly to the point where failures can occur. Even having the servo in a "holding position", under little or no load, can cause oscillations that may result in damage. 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.
Higher voltage means more current, and thus higher power. Running them at 6v+ all the time creates more heat and more vibrations. Because the servos use a simple 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.
Just like spraining your ankle, any sudden jarring movements can result in "spraining" your Hexy. The servos can 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 and please keep away from kids or pets who might be tempted to attack your Hexy.
You can't just walk into your local hobby shop and order 10,000 servos, so the simple answer is that we couldn't find one that worked properly at a decent enough price that we could source at bulk quantities. Even if you could, the economics surrounding sourcing the other parts, manufacturing, and those of running a small business that sells both direct and to retailers pose additional challenges.
The few servos we found at a reasonable price range had issues with freezing at the extremes (+/-90 degrees), resulting in complete lock ups. We're actively 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 ensure a proper fit.
We could include metal gear servos like a Hitec HS-82MG, which would last 2-3x as long, but we'd be selling $800 hexapod kits. Remember, Hexy is the most affordable hexapod kit by far and using inexpensive replacement servos and gears dramatically lowers the cost of repair.
Servos that are dead on arrival (DoA) will be replaced free of charge, no questions asked. Please email replacements@arcbotics.com for a replacement. We rely on you, our customer, to be honest about which ones broke on arrival, and which ones break while getting the kit working. Servos that aren't DoA but break soon thereafter are usually caused by damage to first full gear before the output shaft. This can almost always be fixed with a gear replacement.
We've spoken to the manufacturer, and should have very affordably priced servo gear replacement sets available in the store soon. Please remember that no servos last forever, and these aren't any different.