Need a help on absolute position command

Dear All,
I am using a Pololu usb 16 servo controller to build a pan/tilt system, but I found my servo motor can only response to position between [2000, 4000], rather than [500, 5500] from the Pololu manual, (if I gave a postion larger than 4000 or less than 2000, my servo motors do not move) the driver I am using is from this thread USB 16-servo Controller linux, my servo motors are Hitex hs 7955tg and HeliDirect DS928BB.

Thank you very much.

That could be completely normal behavior for servo, the range certainly sounds reasonable.

When you send it commands less than 2000 or greater than 4000 is it straining against its mechanical limits? For example, if you disconnect power from your servo and move it by hand, are you able to move it farther than you can by commanding the position 2000 or 4000?

The standard servo control pulse range is between 1ms and 2ms. Since servos can damage themselves over time by hitting or straining against their limits, most servo manufacturers use a generous safety margin, so the 1ms and 2ms positions of the servo aren’t even close to the mechanical limits of motion. The Pololu servo controllers let you generate control signals well outside the standard range (absolute position 500=0.25ms, absolute position 5500=2.75ms) so you can take advantage of the entire range of motion of your servo, but you then need to determine which position values correspond its physical limits (and keep from slamming into them regularly).


Hi Adam, thank you for you kind reply, based on your suggestion, I tried to move my servo by hand further (larger than 4000), I can still move about 10 degrees.

Sorry, I didn’t notice this earlier, but both of your servos are digital. That still sounds like completely normal behavior.

Hitec digital servos have programmable motion ranges (along with some other motion characteristics) and come pre-programmed to stop within 5-10 degrees of their mechanical limits. I’m not familiar with the HeliDirect brand, but I would assume they do something similar.

Hitec sells programmers (like these) for configuring their digital servos, but I’ve never actually bothered to pick one up, so I’m not even sure they would let you move the endpoints towards the mechanical stops.


Dear Adam,

Thank you very much indeed.