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I know that the frequency and rotational speed of a spinning shaft or wheel is measured using a device that senses a piece of reflective tape on one of the rotating components. I am thinking a laser device might be best suited but haven't been able to find the correct device. What is this device called?

This is for a fairly high rotational speed (3600 RPM).

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  • $\begingroup$ Lasers don't sense. They illuminate. $\endgroup$ Jul 28 '16 at 12:08
  • $\begingroup$ @CarlWitthoft but a laser sensing device will both illuminate and sense xD $\endgroup$ Jul 29 '16 at 17:30
  • $\begingroup$ No, there's no such thing. You can buy a combo laser and photodiode, but it's not a "laser sensing device" $\endgroup$ Jul 29 '16 at 18:28
  • $\begingroup$ 3600RPM is a common shaft speed for pumps and other rotating equipment. If you were trying to measure in the 50-100k RPM, that would be more challenging. $\endgroup$
    – DLS3141
    Aug 1 '16 at 14:15
  • $\begingroup$ Are you looking to build the device, or to buy one? $\endgroup$
    – DLS3141
    Aug 1 '16 at 14:15
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The "laser device" you're looking for is called a Laser Tachometer, though other illumination sources will work. I have had good success with devices from Monarch Instruments. Specifically the ROS-P line of optical sensors (I see they have a laser version ) and the PLT200 Pocket Laser Portable Tachometer. The laser devices do work very well in more challenging environments.

In my typical application using the PLT200, there's a small piece of reflective tape attached to the rotating shaft and the device is aimed at the target. The shaft speed is displayed on the screen, which gives you a number to check against what's expected. For longer term testing, I've also set these up on a fixed mount and used their data port which will output a TTL pulse train with one pulse/tape mark/rev to the data acquisition system of my choosing. While the device has its own frequency to speed conversion for the display, the output is just the pulsetrain so you'll need to do that on your own.

I am in no way associated with Monarch and I'm sure there are other manufacturers, but I have always had good success with these.

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Hall effect sensor - put a small magnet on the wheel and the sensor mounts alongside - your counting circuit will receive a "pulse" each time the magnet goes past.

Optical sensor - you arrange an infra red light emitting diode (or similar light source) beside the wheel, and at just one point on the wheel a hole is drilled through to let the light pass through the wheel itself. Then a photodiode or some other light sensor is on the other side. Again, it gives a signal each time the hole pops round.

(There are other types of sensor I guess, but these are the most common ones you'll find schematics for.)

Then you need some sort of electronic counter to count pulses, or maybe measure the time interval between each pulse, and convert that to a "speed" measurement. You don't say anything about a counter - but I'd guess one of the cheaper Arduino devices will do it. (They have a lot of circuits for this sort of thing if you dig through their website.) Here's an example to show the principle.

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    $\begingroup$ Putting a magnet on the wheel is a bad idea if the balancing of the wheel is important - which it might be, at 3600 RPM. Also, the centripetal loads on the magnet will be large. Depending on the radius, it might have radial accelerations of the order of hundreds of g (i.e. a small 10-gram magnet might "weigh" several kilograms when it is rotating), so make sure it is fixed securely! $\endgroup$
    – alephzero
    Jul 28 '16 at 23:28
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    $\begingroup$ yes optical sensor is preferred due to the speed. looked at using an attached encoder but there is not a good spot to attach it $\endgroup$ Jul 29 '16 at 17:33
  • $\begingroup$ A standard Hall effect sensor is not mounted on the wheel you are measuring, instead the shaft gears into a smaller shaft which mounts to an off-the-shelf Hall effect sensor (this is how it was done on the Dodge Vehicle I drove / worked on in college at least). This in turn helps signal the spark plugs to ignite. $\endgroup$
    – Mark
    Aug 1 '16 at 14:56
  • $\begingroup$ @Mark I didn't say the Hall sensor mounted on the wheel - the cables would twist :) They can be mounted in lots of ways though - you mention a geared version, and I've seen them on DC electric motors mounted right beside the motor shaft, where a directly-driven magnet triggers the sensor. (Easily capable of handling at least a few thousand pulses per second.) $\endgroup$
    – Andy
    Aug 1 '16 at 15:08
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    $\begingroup$ There aren't really any gaps. The wheel is attached directly to the brakes whose bearings sit on a spindle. The only thing that rotates is the wheel/brake assembly. The sensor would have sense something on the wheel or rim of negligible mass (so as to avoid unbalanced rotation). Sorry I don't have a picture on hand right now but the other answer is on point. basically a photo emitter/sensor system is what I need. Thank you for the help $\endgroup$ Aug 2 '16 at 7:57

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