Detecting crests on threads

>Date: Fri, 16 Jul 1999 16:42:58 -0700
>To: cad_cam_edm_dr0@onelist.com
>From: Ted Robbins <rtr@...>
>Subject: Detecting crests on threads
>
>The detection of threads or gear teeth or metal for limits is usually done

with a proximity switch. They are available often in the surplus market,
though my own surplus source is now this list. You can wind them if you
want. Just some low reluctance iron for a core (Read this as "soft iron
nails".) and a winding around the nails. Sometimes you can find a hollow
core inductor or relay coil or reed relay coil and slip the nails into it.
Feed it with AC, any voltage which can find enough inductive reactance to
not get hot. Put a resistor is series with it chosen to get the widest
voltage swing as you move it toward and away from the iron you are trying
to detect. Then rectify it, integrate it across a capacitor with a bleeder
resistor across it to set the right time sensitivity, and connect it to
switching op amp set up with adaquate hysteresis. This debounces pretty
good. If you want to know direction you can set up two amplifiers and
sensors to get quadrature, or two switchpoints if you are setting up a
limit and want a slowdown signal.

>
>Another sensor you should know about which gives you good resolution over

a short distance is a LVDT (Linear Voltage Differential Transformer.
Unfortunately it requires contact with the surface you are measuring. Hall
effect devices allow you to sense without contact under certain
circumstances, and capacitive probes allow the same thing. Depends on what
you need and the luck of the surplus draw.