Digital servo, modified for continuous rotation

Hi all,

hope you had a nice Christmass !

I use an Arduino FIO board to drive a servo (RC). I use the servo library and all is well, hum I should say almost all is well.

If I connect a standard (0 - 180 deg) analog servo, all is fine. If I connect a consinuous analog servo, again all is well.

But we have to use a digital servo, because we need more torque. Since we have a “standard” digital one on hand, we modified it according to instructions found on Internet: to sum up, cut the tab that prevent full rotation, and have the pot mechanically out of the way.

The problem is that the servo does not operate as expected. I “claibrate” it by sending an 1500us pulse and moving the pot arm until the motor stops. I should then be able to set the speed according to the pulse width, around ± 900us from 1500us. The problem is the maximum speed is reached around 100us on either side of 1500us: that means no way of setting the speed :o. Usually, the range is about 900us around 1500us.

The instructions on Internet are for modifying analog servos, and I was wondering if with digital servos the procedure could be different.

Thanks and Happy New Year to you !

Gilles Plante

A spec of the servo in question would be useful.

Hum, you're right :-/.

The servo is a JR Racing z9100t. You can find it http://www.jrradios.com/Products/Default.aspx?ProdID=JRPSZ9100T.

Thanks

Gilles Plante

The problem is the maximum speed is reached around 100us on either side of 1500us: that means no way of setting the speed . Usually, the range is about 900us around 1500us.

The ±100us band around your netural position of 1500us is standard behavior with the continous rotation hobby servos. The porportional speed control band can be widened with some very involved mods. The way your servo is currently modified you will probably have to live with in the ±100us band. In my testing you can get a discrete speed change for every ~5us change from the neutral value.

zoomkat,

with a Parallax servo (continuous) I achieve +-400us around 1500us. And with some others I found on Internet, +-400us seems to be standard.

Gilles Plante

Servos are not designed for continuous rotation and speed control; they are designed to reach a position as quickly and accurately as possible.

with a Parallax servo (continuous) I achieve +-400us around 1500us. And with some others I found on Internet, +-400us seems to be standard.

I've modified several standard analog hobby servos and the +-100us seems typical with them. The servos made for continous rotation may have slightly different electronics to broaden the porportional speed range. Below is info on DIY mods for a standard servo to provide for a wider speed range. Botton line, your servo probably "works the way it is wired", so +-100us may be it without additional mods.

http://www.seattlerobotics.org/encoder/200009/S3003C.html