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From: bkoball@cup.portal.com (Bruce R Koball)
Newsgroups: sci.electronics
Subject: Re: velocity sensing for robotic joints
Message-ID: <32643@cup.portal.com>
Date: 10 Aug 90 03:33:05 GMT
References: <1990Aug7.205751.21206@ecf.utoronto.ca>
  <9781@rice-chex.ai.mit.edu>
Organization: The Portal System (TM)
Lines: 50

John Purbrick's suggestion of differentiating sinusoidal signals
from a quadrature shaft encoder is precisely what the chip I described
in my last posting does. The L290 Tachometer Converter is part of a 
three-chip set that implements a complete hybrid position/velocity
servo system for small DC motors with a direct interface to a micro-
processor controller.

The L290 requires a shaft encoder with pseudo-sinusoidal outputs (in 
quadrature of course) of +-0.5V and is remarkably tolerant of phase jitter 
and amplitude ripple. The sinusoidal signals don't have to be very clean, 
in fact, most of the encoders I've seen that were designed for
use with these chips produce a waveform closer to a triangle wave. The
L290 implements the following function:

                (  dVb     CA  )     (  dVa      CB  )
      Vtach  =  ( ----- * ---- )  -  ( ----- *  ---- )
                (  dt     |CA| )     (  dt      |CB| )

where CA and CB are the two channels from the shaft encoder and Va and
Vb are amplified versions of the same signals which are fed into an
external RC differentiation network and then returned to the chip, multiplied
by the sign of the opposite channel and differenced to produce the Vtach
output. Some ripple is produced by this method but it is the 4th harmonic
of the fundamental (input) frequency so it is relatively easy to filter
out.

The encoder signals are also fed into schmitt triggers with open collector
outputs to square them up for interface to external logic (e.g. counters
or microcontroller).

The system does remarkably well even a low speeds and small angular
displacements.

The other two chips are the L291 DAC/Amplifier which provides a 5-bit DAC 
plus sign for velocity control, error amplifier and mode strobe input to 
switch between position and velocity mode. All six of these signals (5-bit 
DAC, sign, mode strobe) are TTL compatible and so can be driven directly
from a microcontroller output port.

The final chip is the L292 Switch mode Driver which provides an H-bridge
PWM power driver (2A, 36VDC max) with appropriate level shifting, loop
gain adj., and current sensing to drive a small motor from a single-ended
supply.

Bruce Koball
Motion West
2210 Sixth Street
Berkeley, CA 94710
415-540-7503
bkoball@cup.portal.com