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Pan and Tilt?

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Ama

Sun Jul 12 2009, 11:43AM

Registered Member #1852 Joined: Mon Dec 08 2008, 09:54PM
Location:
Posts: 12

Iâ€™m facing a pan and tilt problem, I want to position an object (weights around 5Kg, centered at joints for weight balance) by panning & tilting, controlled by computer.

The object is centered at 2 points jointed to 2 ball bearings; Iâ€™ve tried many things but â€¦
I donâ€™t know how to do it, any ideas, and suggestions?

ADD:
Iâ€™ve tried with 2 strong SERVO motors (20 Kg.cm) positioned at exact center of ball bearings, coupled with 2 pots connected to ADC mcu (analogue to digital )connected to computer for position feed back, but when they move and reach the position they just donâ€™t lock on it and go up and down for while(I guess due to high momentum delivered to center of movement, motors shaft)

Havenâ€™t tried with jack (actually didnâ€™t find anything small, I guess, Iâ€™ve to build my own) specially for panning.

Sulaiman

Sun Jul 12 2009, 12:20PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Something like item 260370887871 on eBay ?

Hon1nbo

Sun Jul 12 2009, 04:39PM

Registered Member #902 Joined: Sun Jul 15 2007, 08:17PM
Location: North Texas
Posts: 1040

if you are using servos, make sure they are "Sailing" Servos (I think that might be an unofficial name for them) - they are designed so that once no more commands are sent, they retain their position rather than attempt to retract or stop applying torque

Steve Conner

Sun Jul 12 2009, 05:55PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

In order to do position feedback successfully, you need an understanding of control theory. You can't just connect the output back to the input and expect it to work: you need to use something like a PID controller and tune it to your plant.

Ama

Mon Jul 13 2009, 06:59PM

Registered Member #1852 Joined: Mon Dec 08 2008, 09:54PM
Location:
Posts: 12

Thank you all, I think I didnâ€™t explain right, my problem is with building the mechanical part, not with position feed back.

Yes Sulaiman, exactly like item 260370887871 on eBay, but something bigger and stronger in mind!

Well I asked for "Sailing" servos, servos that can lock, or even for motors that can lock! Wasn't able to find anything useful!

About using simple servo, the problem was after it reached position, and passed it, and reversed the torque in order to retain its position but after regaining the position, again same happens. After a many bounces which get smaller & smaller, it stops finally at exact position.

Bjørn

Mon Jul 13 2009, 07:57PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

About using simple servo, the problem was after it reached position, and passed it, and reversed the torque in order to retain its position but after regaining the position, again same happens. After a many bounces which get smaller & smaller, it stops finally at exact position.

This is what Steve is talking about. For it to work you need to slow it down before it hits the stop so it does not overshoot. For something that has a reasonable mass coming to an instant stop will require kilowatts of power and will be very expensive if it is going to last.

Either you don't understand the exact problem or you are not explaining it good enough.

GeordieBoy

Mon Jul 13 2009, 10:46PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

It is the good understanding of control theory that Steve Conner mentioned that allows you to get to the position you specify as quickly as possible with no overshoot and oscillation.

You need to compensate for the dynamics of the mechanical system like momentum and inertia by careful choice of component values in the electronics. Then you can arrive at a critically damped system and a stable control loop.

-Richie,

Proud Mary

Tue Jul 14 2009, 02:24AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

What about using servos with mechanical advantage - a twin axis chain drive made from bicycle parts perhaps. A bit clunky perhaps, but certainly up to ther load.

MinorityCarrier

Tue Jul 14 2009, 04:35AM

Registered Member #2123 Joined: Sat May 16 2009, 03:10AM
Location: Bend, Oregon
Posts: 312

The moments of inertia of your tilt and pan platforms would be less of an issue with digital positon control. A 10-bit or 12-bit angle encoder for each axis would be best, but would involve a lot of support circuitry or use of microcontrollers if you went with serial data output. Use of a two-bit quadrature encoder would be simpler to implement, but you would have to devise a strategy of slowing the servos as the final angle is approached, and quadrature encoders are typically only 200 counts/revolution/bit, so less positon accuracy unless you use reduction gearing, which introduces lash unless you go with a harmonic drive gearhead. (takes breath) With digital design, you can slow down on approach then lock on to the final position by using magnitude comparator functions in your uC, and then use electromechanical brakes to hold position.

Use of an analog PID controller would involve an analog position transducer like a precision potentiometer, and as you have already discovered, you'd have to have set a wide dead band to eliminate oscillation of the servo around the position set point. PID's are typically are tuned to go to set point as quickly and as stable as possible for their assigned load (I use them for heating chemical baths), and you can't easily alter their response (speed of change) curve and maintain stability and/or accuracy.

With digital positioning you can control speed to setpoint easily. Presumably, you are using servos that can handle the inertial loading.

All that said, some of the latest digital motorized telescope mounts are pretty good. If they're within your budget, You may want to look into modding one for what you want.

Sulaiman

Tue Jul 14 2009, 09:16AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

You could look at fully motorised Telescope mounts,
they are expensive but excellent .... eBay etc

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