Xref: utzoo comp.robotics:312 comp.lsi:1146 sci.electronics:13443
Path: utzoo!attcan!uunet!clyde.concordia.ca!mcgill-vision!snorkelwacker!ai-lab!jpexg
From: jpexg@wheaties.ai.mit.edu (John Purbrick)
Newsgroups: comp.robotics,comp.lsi,sci.electronics
Subject: Re: velocity sensing for robotic joints
Summary: Maybe sine-wave output encoders
Message-ID: <9781@rice-chex.ai.mit.edu>
Date: 9 Aug 90 04:56:42 GMT
References: <1990Aug7.205751.21206@ecf.utoronto.ca>
Reply-To: jpexg@rice-chex.ai.mit.edu (John Purbrick)
Organization: MIT Artificial Intelligence Laboratory
Lines: 39

In article <1990Aug7.205751.21206@ecf.utoronto.ca> apollo@ecf.utoronto.ca
 (Vince Pugliese) writes:
>
>assuming one is using optical encoders (with 2-phase output)
>to measure position of a (rotary) robotic joint, what should one use
>to then measure velocity??

2-phase encoders can work reasonably well (Galil motion control, based in 
California somewhere, has a range of PC-based controllers which run servos
using encoder-only feedback) though most companies sell encoder-plus-tachometer
systems.

One idea which I'd like to hear of someone trying is to use a 2-phase encoder
with sinusoidal output, which actually means a sine and cosine. Take one of
the phases and differentiate it using an op-amp circuit, then divide the 
result by the magnitude of the other phase. The result should be proportional
to velocity, because in the case of the differentiated phase, the voltage 
would be 

V = A sin(w t), so
d(A sin(w t))/dt = A w cos(w t)

and the second phase would be 

V = A cos(w t)

Hence dividing one by tother results in w.

[V = voltage out for either encoder phase
A = max voltage at peak of the sine/cosine wave
w = rotation rate
t = time (thus speed * time = present position)]

This only works if the sine waves are pure, equal in magnitude and properly
phased, which may not be true. Plus differentiating analog quantities is a 
famous way to get in trouble.

John Purbrick
jpexg@ai.mit.edu