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From: whit@milton.u.washington.edu (John Whitmore)
Newsgroups: comp.robotics,comp.lsi,sci.electronics
Subject: Re: velocity sensing for robotic joints
Summary: Linear Variable Differential Transformer
Keywords: LVDT
Message-ID: <6273@milton.u.washington.edu>
Date: 14 Aug 90 01:40:48 GMT
References: <19481@well.sf.ca.us> <140483@sun.Eng.Sun.COM> <8441@ccncsu.ColoState.EDU>
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Organization: University of Washington, Seattle
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In article <8441@ccncsu.ColoState.EDU> jn163051@longs.LANCE.ColoState.Edu (Joel Nevison) writes:
>
>I have seen this implemented on a GCA stepper as velocity feedback to 
>prevent the auto-focus from oscillating.  They called it an LVTD which
>I believe stands for linear velocity difference transducer? Correct me
>someone.   I don't believe the coil was energized, I think they had a 
>magnet moving inside.  

	There is a position-measuring device called LVDT (Linear Variable
Differential Transformer) which is used to measure distance displacement.
Our machine shop has a gage using this sort of cell that gives about
one microinch resolution.
	For velocity measurement, we use a Hewlett-Packard gizmo
that looks similar, but the bead in the center is a permanent magnet;
it is called an LV-Syn, or sometimes LVT (Linear Velocity Transducer).  
Our machine does Mossbauer measurements, requiring control of 
velocity to one part in 1000 of 1 mm/sec velocities.  It works fine.
	I have seen similar linear velocity transducers (basically
it's just a linear generator) in large hard disk drives, in the
head assembly, also presumably to control seek speeds.  Unfortunately,
this sort of device fails at LOW velocities because of its inductive
output impedance; the signal/noise ratio can be guaranteed worse than
any given value for some sufficiently small velocity.  Creep of the
system becomes a problem (so the disk drives had ANOTHER feedback
mechanism to control absolute positions).

	Since the original question was about controlling a rotational
velocity (like in an arm joint), I wonder why no one has suggested
looking at the motor's back EMF; it gives the same information as
a separate electric generator would, and it's ALREADY built in.

		John Whitmore