Testing: Choosing the right sensor for RPM

[Zarathuztra]

I have no previous experience collecting RPM data so would like advice from those with previous experience. I need to collect RPM data from my machine and record it's behavior as I apply and remove power. I'll be engaging and disengaging a load. So I'm trying to find out how long it takes for it to come to a rest when I remove power. There are a few pulleys and spindles I could have a sensor observe to collect the data from.

My question, what is the optimal sensor for this job? I've come across information to suggest a hall effect sensor or an optical laser sensor could fit the bill. There will be a fair amount of vibration so I'm not sure if that creates a big disadvantage for either sensor.

*max RPM will be under 4,000
*time for the output to come to a rest should be about 2~10 seconds.
*sample rate only needs to be about 100 Hz. (would like to reliably know within at least 0.1 s of output stopping)

Anyone familiar with this type of test have some recommendations??

Thanks

 

[Dr.D]

You don't mention the shaft speed involved, and that is relevant.

If you have an exposed gear,a simple magnetic pickup will often do the trick.

 

[Zarathuztra]

You don't mention the shaft speed involved, and that is relevant.

If you have an exposed gear,a simple magnetic pickup will often do the trick.

Thanks, I have added that and some more information to the OP. Unfortunately I do not have an exposed gear =(

 

[berkeman]

What are you going to be using for data acquisition from this sensor? A PC? A Raspberry Pi? A cellphone?

What data acquisition software will you be using? LabWindows? LabView? MATLAB? Home-Brew C code and a PC serial port?

 

[jrmichler]

The sensor is only half of the problem. The other half is what to do with the sensor output. Do you need real time readout, or a data acquisition system that stores data for post processing? Do you just need to measure coastdown time, or do you need speed plots?

If the you are looking for a zero speed switch, here is one: http://www.redlion.net/product/speed-switch-ac-powered. The sensor can be a proximity sensor or an encoder. Or maybe you could use a frequency to voltage converter: http://www.ti.com/lit/ds/symlink/vfc32.pdf. The sensor can be a proximity sensor or an encoder. In either case, the response time requirement requires input with more than 1 or 4 pulses per revolution. That means a prox sensor sensing gear teeth, or an encoder. Sometimes you have to add a stub shaft to hold a gear or drive an encoder.

 

[Zarathuztra]

@berkeman @jrmichler

For the data acquisition.. In an effect to keep this inexpensive, I plan to use DAQExpress on a laptop, with cDAQ-9174. If anyone recommends something different, please let me know.

Knowing the speed of the rotation is not entirely necessary, just the coast-down timing. [Begin == power off (load coast); End == load is at rest]

I should also note, that I might have to do this for multiple units, so an easier to setup sensor would be preferable.

 

[berkeman]

Does the cDAQ-9174 provide a 5V or 12V output voltage that you can use to power a sensor? If so, I'd just look for an optical pickup that uses a reflective sensor and a stick-on optical pattern...

http://www.citizen-micro.com/tec/items/images/en2.jpg

 

[Zarathuztra]

@berkman, I would not have the luxury of being able to install such a disc. I'll need a way that doesn't require me to add anything to the rotating spindle. ( I think reflective tape might be OK though, since it would not have an effect on the results.)

 

[berkeman]

Can you post a picture of the setup?

 

[Zarathuztra]

@berkman, this is for a future setup so there isn't anything to photo just yet. But to give you an idea, I'll be reading the speed from a spindle on a lawnmower deck.

 

[berkeman]

Push mower or riding mower? Can you use Google Images to give us a better idea?

https://i.ytimg.com/vi/ufGzlocZpBk/maxresdefault.jpg

 

[Zarathuztra]

I circled a pulley as an example. I will need to track it's RPM.

 

[berkeman]

So just print out one of these patterns, cut a hole in the middle, and glue it to the top of the pulley. Then mount a reflective optical sensor right over a portion of the pattern, and you're mostly done!

http://heliosoph.mit-links.info/wp-content/uploads/2013/06/rotary_encoder_index_disc.jpg

 

[Zarathuztra]

hmm... that seems too easy to be true... I would have to get information on the machine to make sure it has clearance to lay on the encoder and to mount the optical sensor. Is there an optical option that would be able to read RPM from viewing the pulley/shaft from a radial perspective? In case vertical clearance is an issue.

 

[berkeman]

If you could put marks on the outside of the belt, you could bounce the LED source off of the belt and into the pickup device...

 

[jrmichler]

Put a mirror above the pulley at a 45 degree angle, then mount the sensor off to the side.

 

[Tom.G]

Put a spot of paint, White or metallic, on the rim of the pulley to optically sense with a reflective sensor.

If that is a photo of the real item, I notice that there is a pulley at the left side that is raised above the deck. Perhaps you could sense the underside of that pulley. It would be easier to get a solid, vibration free, mount down closer to the deck there.

Overall though you will be much better off with a magnetic sensor in an environment that dirty. Fortunately the pulleys are stamped/assembled with holes and rivet heads in them, ideal for magnetic sensing.

Cheers,
Tom