Thread: Manipulator designs
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Unread 01-11-2004, 11:56
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Ken Patton Ken Patton is offline
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Re: Manipulator designs
Some comments/advice:

If you can figure out a way to use springs (I suggest surgical tubing - its lightweight, durable, cheap, easy to install, fits a lot of designs) to counterbalance the mechanism, then the motor has a lot less work to do and you can gear it for the speed/accuracy tradeoff that you need. Think about springing the arm so that its weight is compensated for by the spring.

Friction is your enemy. Make sure you try to eliminate any sliding contacts and that you use bearings where possible or properly lubed bushings if you must.

When the robot tips over or runs into something, sometimes an arm might take extreme loads. Think about what is going to bend/break in that sort of situation, and make some spares. Lots of times the shaft on which the arm pivots is going to be the weak link.

There's nothing wrong with some deflections of the arm as it is being operated, but make sure the deflections don't screw up gear alignment or sprocket alignment.

We have used both gears and chain+sprockets as arm drive mechanisms, and both have worked satisfactorily. When selecting the motor and ratio, make sure to consider the counterbalancing.

Think about the need to hold position under situations where power is on and power is off. Do you need the arm to hold position or is it acceptable for the arm to sag? Some motors may be able to hold position by themselves, some may not. Some motors (van door and Globe come to mind) may seem "tight" at first but after use they are back-driveable and will probably not hold position under load.

Think about the need for very precise positioning of the arm - do you need to add a potentiometer or encoder to provide feedback to the controller? Can you write a control loop to take this feedback signal and convert it into a motor command? (search for threads on arm positioning and I think you'll find some good info on people's preferred methods)

Hopefully some of the experienced designers/builders of multi-segment arms might have some comments for us mere mortals....

hth,
Ken