High RPM Rotary Encoder?

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mesh

High RPM Rotary Encoder?

Postby mesh » Sat Jul 17, 2010 1:22 am

Can anyone recommend a high RPM Rotary encoder that works with phidgets?

I need one that can support up to 50,000 RPM.

The one listed on the phidget site:

http://www.phidgets.com/products.php?ca ... ct_id=3530

only supports 3000 RPM.

thanks...

mike

fraser
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Re: High RPM Rotary Encoder?

Postby fraser » Sun Jul 18, 2010 4:29 pm

i dont know if you wil have much luck finding one that will support 50000 RPM, most max out around 10000. If you look hard enough you might be able to find one but i was not able to. It will likely not be cheap if you do find one.

If the 10000 will work then check out www.USDigital.com they have a decent selection.

mesh

Re: High RPM Rotary Encoder?

Postby mesh » Sun Jul 18, 2010 11:06 pm

Thanks. I found one that could do up to 15,000 RPM, but I am thinking I may be approaching it wrong anyways.

I want to measure the RPM of a motor, and translate that into a speed. Im thinking I could use some gears, so my sensor wouldnt have to be able to measure so high, and then in my calculations I could adjust for the gears.

This is all new stuff to me though, so Im not 100% sure I am going down the right track.

mike

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Re: High RPM Rotary Encoder?

Postby richfiddler11 » Mon Jul 19, 2010 8:56 am

You need a tachometer. The analog variety are like a small motor/generator attached to the main shaft that produce a voltage proportional to speed.

Alternately, If your encoder has an index pulse (i.e. fires once per rev) maybe you could somehow count index pulses only -- 50,000 RPM is about 833 index pulses/second.

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Re: High RPM Rotary Encoder?

Postby fraser » Mon Jul 19, 2010 9:14 am

Alternately, If your encoder has an index pulse (i.e. fires once per rev) maybe you could somehow count index pulses only -- 50,000 RPM is about 833 index


A lot of encoders have limited RPM due to their physical construction as well as their ability to output signals, so i doubt the encoder could handle the 50000 even though you are only counting index. And 50000RPM would generate 50000 index per minute, which still isn't a problem.

Gearing it down is not a bad idea and will probably be your best alternative. Tachometers are good if you don't need high resolution. What are you using that requires a 50000 RPM motor? The only thing i can think of is a turbine?

Robert

Re: High RPM Rotary Encoder?

Postby Robert » Mon Jul 19, 2010 10:56 am

Even gearing it down might be tricky. at 50K rpm, you need some pretty well balanced gears. An encoder to measure 50K rpm is not normally going to be mechanically coupled to the device it's reading, they're usually optical or hall-effect encoders. Take a look at automotive engines, and how they measure RPM. They usually use a toothed wheel with a VR sensor.

I bet it's some sort of turbine he's trying to measure too. I've always wanted to measure the rotational speed of the turbocharger in my car, which spins at 100-130K rpm. The best way I could think of doing it would be to paint one of the fins on the compressor wheel with a reflective mark, and use a light sensor that can react fast enough to watch for it, then count the pulses per second. You need to make sure that your light sensor can react fast enough though. In reality, a hall-effect sensor is the best (or a VR sensor), but it can sometimes be tricky to set something like that up.

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Re: High RPM Rotary Encoder?

Postby richfiddler11 » Mon Jul 19, 2010 11:43 am

In reality, a hall-effect sensor is the best (or a VR sensor), but it can sometimes be tricky to set something like that up.


My college roomate installed a JC Whitney (aftermarket) cruise control unit in his car (back in teh 80's) and had to glue some magnets on a drive shaft for the tachometer, so it was definitely using a hall-effect switch. Worked OK, but I'm sure the RPMs weren't that high.

For a speed reduction approach, I'd be more inclined to use small pulleys and belts than gears. Years ago I used to have a catalog with toothed micro-timing belts and matching pulleys.

Custom gearing is really tricky, but I suppose off-the-shelf reduction boxes are available.

mesh

Re: High RPM Rotary Encoder?

Postby mesh » Mon Jul 19, 2010 1:15 pm

Im looking at ways to measure speed in an RC Car (top speeds can be up to 80 mph).

The two approaches I have been considering are:

Optical Encoder connected to engine gears.

Magnet on the wheel, and then putting a sensor that detects the magnet.

http://www.phidgets.com/products.php?ca ... ct_id=3562

My concern with the magnet, was that I was afraid that the sensor would not be able to keep up with how fast the magnet would be going around.

Thoughts?

mike

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Re: High RPM Rotary Encoder?

Postby fraser » Mon Jul 19, 2010 1:42 pm

Oh yes i forgot nitro engines can reach up to 40-50K RPM. The magnet sensor is intended for the digital inputs of our 1018 which will only allow up to about 7500 RPM. Although, just estimating, i would think that the wheels only spin at around 10000 RPM at 80mph depending on wheel size; which is still too fast for the 1018. What about a 15Krpm encoder on the wheel axle?

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Re: High RPM Rotary Encoder?

Postby richfiddler11 » Mon Jul 19, 2010 3:04 pm

how about an onboard GPS module? :)


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