So I decided to try calibrating my servos after seeing some odd behavior (Hexy is not assembled yet, I figured it would be easier to do with “naked” servos) when testing the software.
I used 4 fresh AA alkalines to give servo power, and both servos I tried were fine at 500 usec but stalled out and started jittering like crazy at about 2100-2200 usec, and looked like it was trying to go to about +110-degrees from zero. (At 500 usec, it was -90 deg.) (I used a partially straightened paperclip slipped over the servo rotor as a pointer to estimate the angles.)
Recalling what’s been said here about such behavior seen with too much servo voltage (eg 5 x AA), I measured the battery pack: 6.3 volts! I measured each cell (brand new Duracell alkalines) and got readings of 1.576 to 1.577 volts each. This is too high for the servos; the servo’s potentiometer can’t drop it low enough to null out at high pulse widths so the thing keeps trying to drive past its stop. This causes stalling, high current drain, and will probably eventually damage the servo either electronically or mechanically (broken gears), or both.
So then I tried 4xAA NiMH batteries (Engergizer 2500 mAh). Didn’t have brand new ones but did give them a fresh charge. Because they’ve already seen some varied use, the cell voltages I got were: 1.291, 1,356, 1.356 and 1.472, for a total of 5.47 V. (This dropped to 5.40 immediately after 5-10 minutes of testing the servos). The servos behaved much better. At the low end (500 us) I got some very slight chatter that mostly went away at 510 and completely at 520, with a 90-degree motion, maybe 95. At the high end it went all the way to 2500 with no problem at all, and again 90 degrees.
Conclusion: Don’t use alkaline batteries at all for the servos. Even 4 of them, when fresh, can (will) overdrive the servos and can damage them. This may also account for some jitter and vibration at extreme ranges of limb motion when the robot is assembled. NiMH works great. (Even 4 at the weirdly high voltage I saw on one would be within spec.)
Hope this helps some folks.
-Al
