> From:         Phil[SMTP:phil@SPIDERWEB.COM.AU]
> Sent:         Friday, September 12, 2003 12:33 AM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      [EE]:Shaft position indicator

> Hello All,

> I have a need to know the position of a shaft, within one degree accuracy,
> that rotates at a speed of one revolution in two minutes. The shaft can only
> make one turn.

> I have been experimenting with a Hall Effect device (UGN 3503U) trying to
> count the teeth on the sprocket that turns the shaft. Although promising, I
> only get a one millivolt difference between the tooth and the gap and the
> Hall Effect device has to almost tough the sprocket. To make matters worse,
> there is a lot of grease in the area.

> While searching for inspiration I came across a linear pot that uses a Hall
> Effect device and so contacted the distributer. Unfortunately, they didn't
> respond.

> Does anyone know of any information that will show me how to construct such a
> linear pot?

> I could use a photo optic device to count the sprocket teeth and if I don't
> get any further with the Hall Effect device that's what I'll do. However a
> direct read out device such as a very precise linear pot would make the job
> so much easier.

You could try using a 10 turn precision pot coupled to the shaft via a steel cable
wrapped around a wheel on the shaft being measured. If this wheel has 10 times
the diameter of a wheel on the shaft of the pot, the pot will make 10 revolutions
for one turn of the measured shaft. A typical 10 turn pot (Helipot) can resolve
1 part in 500 (at least that is what the 10 turn counting dials would have you
believe). Steel control cable used for model airplanes is available at model shops.
You will have to deal with the issue of slack and maintenance of tension, as well
as hysteresis, especially if the wheel has to be measured in both directions.

John Power

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