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Servo Motor Acceleration

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neoandrewson

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Dear All,

It appears i have another problem with the basics. I am designing an articulated robotic arm. I have stuck with calculating acceleration profile for servo motors. Datasheets refer to velocites ( sec/60 deg) but not with acceleration. I am not able to link with basic formulae ( velocity/s) either. Can anyone help me with this? Thanks in advance. :)
 
Without tapping into the feedback loop of the servo, there's no way for you to know or control the acceleration. It's just one of those things that isn't given. You've probably noticed servo motors specs tend to be sparse and vague. You could always measure it experimentally under different loads and input commands.
 
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Without tapping into the feedback loop of the servo, there's no way for you to know or control the acceleration.
...

That's not entirely true. If you reduce the PSU voltage (or increase the PSU source impedance by adding a series resistor) you get much lower accelerations. I added an adjustable voltage regulator before the RC servo on one project to give a good control of its acceleration (and deceleration) because it was too severe and breaking into oscillation at the end of it's travel. That can be a big problem with a high moment load like an arm.
 
Adjusting voltage to a servo will reduce its torque. Another way to control the speed of servo movement and hence apparent acceleration is to electronically modify the pulse to the servo. That way, the servo power can remain unchanged.

Jomar makes a gear door sequencer or servo sequencer that does that:

Electronic Model Systems / JOMAR Products - Product Catalog

Other manufacturers do as well. Their literature is scanty on the way in which it is done.

For DIY, this Seattle Robotics link describes one way to do it:



It is interesting that the "off" time of the servo PWM position signal is used to do that.

In theory, but I could not find an example, one could also control the rate at which a change in the pulse width that controls a final position is reached.

John
 
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