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DPRG: Re: servo hack

Subject: DPRG: Re: servo hack
From: Robert Allen endigo at metronet.com
Date: Mon Mar 15 19:43:30 CST 1999

If you can't get the servo controller board back in, then you are left with a DC
motor. That means you can use PWM for speed control and an H-bridge for direction
control. Also, hobby shops often carry the individual parts of the servo, I seen
cases, Gears, (Hmmm... ...can I change the gear proportion?) and probably even the
controller boards.

Jim Brown wrote:

> At 04:53 PM 3/15/99 EST, you wrote:
> >Hi Jim,
> >
> >I have a question. I have performed this hack numerous times. However, my
> hack
> >did not include the added resistors. I simply removed the stops on the gear
> >and pot. In fact, I threw away the circuit boards in the past. I simply put
> >4.8 volts directly to the motor. This created a constant speed gearmotor for
> >me.
> >
> >Now, I wish to have continuous turning of the servo, but with control of the
> >speed in which it turns. Can I plug a hacked servo (with the resistor
> >replacement) back into the receiver and have proportional continuous turning
> >of the servo? Does the replacement of the pot to a fixed resistance allow for
> >this? I have read several articles now that seem to take me to this same end.
> >The hack is made, but I feel you folks working with robotics do something
> else
> >from this point on (like some sort of programmable control). Could you
> >possibly shed some light on this for me? All I'm looking for is reversible
> >continuous turning but with proportional speed control.
> >
> >I am building a model towercrane. (www.clayt.com) and would like to use a
> >retract servo to allow the trolley to travel from one end of the cranes
> jib to
> >the other. I would need speed control and I hope to do this via the RC radio
> >transmitter that will be used to control the rest of the models functions.
> >Thanks, in advance, for anything that you could pass along to me.
> >
> >Sincerely,
> >
> >Clay Thoreson
>
> Yep, if you put the original board back it will give proportional
> and reversible control.  Please note that I have heard that not all
> servo controller boards do this, but the ones I have do.
> Basically, you can either use the resistors or use the pot if it
> is disconnected from the drive train (ie, not turning with the
> shaft).  If you just pull the stops from the pot and it turns,
> then that will _not_ work.  The reason is that when you tell the
> servo to go to a position, say all the way to the left, it
> tries to get there based on the pot's feedback.  If the feedback
> says it's sitting directly in the middle (which is what static resistors
> will say), then it trys to do a hard sweep to the left direction.
> If you tell it to move just off center, then it doesn't try
> very hard to get there.  Since the feedback will never get to
> where it thinks it should go, it will keep on trying to
> get there, and you'll have continuous proportional motion.
> So, if you only removed the stops from the pot, then the
> feedback will change, and you'll get weird results.  If you
> remove the pot from the drive, then it will be like static
> resistors and the hack will work.  Some servo controller boards
> have really great proportional control, and others only have
> two or three.  So, you'll have full, half, and off, control or then
> again, you might get one that has total gradient of full to off
> control. Sorry I  don't know which servos are the ones that have
> the best proportional control.
>
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>
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- --
- --...  ...--
Robert Allen
E-MAIL: Endigo at Metronet.com
Home Page: http://www.metronet.com/~endigo/
Radio License: N5IKD

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