Counting number of turns

[mstfldmr]

Hello,

how can we count the number of dc motor turns with the allowed parts?

[chi-town-biker]

Use an encoder.

[TheKeeg]

Something like this:

http://www.andymark.com/product-p/am-0180.htm

[mstfldmr]

we don't have them. and we don't have time to order them from US. any other suggestions?

[TheKeeg]

Originally Posted by mstfldmr

we don't have them. and we don't have time to order them from US. any other suggestions?

Do you have a string potentiometer?

[mstfldmr]

Originally Posted by TheKeeg

Do you have a string potentiometer?

is it different than a regular potentiometer?

[Chadfrom308]

What is turning? Wheels? Warm drive? Arm?

Try to get your hands on an encoder or gear tooth sensor (hall effect sensor) (Pretty much the same as an encoder)

Potentiometers normally go only 1-10 turns, encoders dont have a limit.

[Pranit T]

If you have a light sensor/photoreflective sensor, you can put a pulley in front of it, and paint it half white and half black. Connect the pulley to the motor. You can now track the number of times the motor rotates based on the number of times the light sensor counts it sees the white or black pass by.

You can fine tune this by adding more sections of black/white rather than just one of each colour. We did this on our 2009 bot to tune the PID for the shooting wheel.

[TheKeeg]

Originally Posted by mstfldmr

is it different than a regular potentiometer?

Yes, it measures distance by the unraveling of a string around a spool. I was going to suggest that you could wrap the string around whatever you are counting the rotations of and use the circumference of the "wheel" to figure out the number of rotations, but it would only work if the rotating object only has to rotate a small number of times like a winch and it cannot rotate very fast. I have never seen this on a robot, but it could work I suppose. Could you reveal what this would be used for?

http://www.andymark.com/product-p/am-2618.htm

[chi-town-biker]

Digikey in Turkey sells many types of encoders but not made by US Digital. Maybe someone can find a comparable model. I'm not sure I can match all the specs.

[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.

[BBray_T1296]

Originally Posted by TheKeeg

Yes, it measures distance by the unraveling of a string around a spool. I was going to suggest that you could wrap the string around whatever you are counting the rotations of and use the circumference of the "wheel" to figure out the number of rotations, but it would only work if the rotating object only has to rotate a small number of times like a winch and it cannot rotate very fast. I have never seen this on a robot, but it could work I suppose. Could you reveal what this would be used for?

http://www.andymark.com/product-p/am-2618.htm

I know Team 2468 used the Andymark String Pot last year to measure their shooter height. It was quite ingenious.

[Abhishek R]

Originally Posted by Pranit T

If you have a light sensor/photoreflective sensor, you can put a pulley in front of it, and paint it half white and half black. Connect the pulley to the motor. You can now track the number of times the motor rotates based on the number of times the light sensor counts it sees the white or black pass by.

You can fine tune this by adding more sections of black/white rather than just one of each colour. We did this on our 2009 bot to tune the PID for the shooting wheel.

This works quite well. We used it in both 2012 and 2013 for our shooter wheels.