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shobley
03-21-2011, 12:30 PM
I work for an ISO responsible for monitoring the national power grid and we have a lot of digitial readouts of frequency, phase relationship etc...

I thought it would be cool to create an "analog" representation of the information using an old box 'o' servos I have at home and an Arduino.

It occured to me that if I can control the power to the servo, then in theory I can control gobs of servos with just a shift register and a single PWM pin.

The idea is to only power up the servo when it needs to move - via the shift reg and mosfets to do the power switching.

When I do an update I just power each servo in turn and enable the (single) PWM pin to move the servo to it's new position, after a few seconds I just power the servo down again. In theory the servo will hold its position - and if I enable the PWM before I power up it shouldn't jiggle around too much when it has to move again.

Anyone done this?

Is it feasible?

Steve

Quantum
03-21-2011, 03:20 PM
Yes using shift reg's to control servo's will work.

Lynxmotion's SSC-32 board works using 74hc595 shift reg's. They run with 4 reg's controlling 8 servos on each.

But when you turn the power off to the servo it will lose its position. It needs to constantly have power and told its position every few uS.

Schematic at the bottom of the pdf.
http://www.lynxmotion.com/images/data/ssc-32.pdf

shobley
03-22-2011, 08:33 AM
It's the losing position thing that interests me...

Since the servos will just be moving very lightweight pointers, if I power them down I don't see them moving, as there is minimal load.

Once I power them up again I'll be sending the new position signal, and if I understand how servo's work (reference voltage from potentiometer), it should know which way to turn to assume the new position.

I think... :)

djsures
03-22-2011, 10:24 PM
It's the losing position thing that interests me...

Since the servos will just be moving very lightweight pointers, if I power them down I don't see them moving, as there is minimal load.

Once I power them up again I'll be sending the new position signal, and if I understand how servo's work (reference voltage from potentiometer), it should know which way to turn to assume the new position.

I think... :)


http://www.ez-robot.com/Shop/View.aspx?id=1

can do 20 servos, holding position. and its bluetooth so u don't need to run a wire. use the ez-sdk and voila :)

lnxfergy
03-22-2011, 11:21 PM
It's the losing position thing that interests me...

Since the servos will just be moving very lightweight pointers, if I power them down I don't see them moving, as there is minimal load.

Once I power them up again I'll be sending the new position signal, and if I understand how servo's work (reference voltage from potentiometer), it should know which way to turn to assume the new position.

I think... :)

On some of our robots in the lab, we stop sending the PWM at times to avoid twitching -- for things like eyebrow servos (which aren't under load) they hold position.

I think you want to do it in that order -- stop sending PWM, then power down. The opposite order (power down while PWM signal is still active) could lead to unpredictable behavior, but I can't say I've ever done that way.

-Fergs