Counting number of turns

[DjScribbles]

It depends on what your purpose is:

As others have suggested, the standard approach in many cases is encoders; there are other types available as well, there were some magnetic encoders in First choice this year, and I found a few of these in our electronics boxes (we didn't order them that I know of, so they may have come in the kit this year or last), which if you can get them sourced from the EU may be much easier to get.

If you are measuring wheel speed, you can also build a hall effect sensor quite easily with some magnets, a few discrete electical components and one latching hall effect sensor like this (see the datasheet on sparkfun and build the typical 3-wire circuit). These work well in high velocity measurements like a shooter wheel, but not for high precision stuff like drive trains or arms where a single revolution is a big movement.

If you need to control the position of an arm, a simple potentiometer (ideally one with 10Kohms of resistance, definitely more than 1Kohm), the poteniometer can tell you the exact position of an object (you need to convert the voltage to degrees manually though) but they aren't typically built to withstand high speeds (even continuous potentiometers would not be a good idea for anything that spins over a 120rpm or so)

[Tem1514 Mentor]

If you have been around since Logomotion the line tracking sensors will work to give you a simple rotation feedback for a 'slow moving' shaft.

You could also try using the encoders inside an the older style 'ball' mouse. But this will take a little bit of electronic work on your part to interface it to the digital sidecar.

[tr6scott]

Originally Posted by Tem1514 Mentor

If you have been around since Logomotion the line tracking sensors will work to give you a simple rotation feedback for a 'slow moving' shaft.

I guess that depends on your definition of slow... Our frisbee shooter wheel ran at 3600 RPM direct coupled to CIM, with essentially the same photoeye from Banner as the AB line trackers. We also had three reflectors per revolution, to increase the "encoder" ticks. The cRio handles these speeds without issue.

http://www.chiefdelphi.com/forums/sh...d.php?t=105965

Edit: found the line tracker part number was/is 42EF-D1MNAK-A2 looking up the data sheet from AB shows a 1ms response time. For round numbers, say you only want to push it to 1 Rev per 5ms that still gets you to 12,000 RPM range.
We accurately recorded 5000rpm with 3 pulses per rev last year using the encoder class, last year with the banner sensor.