Testing; Choosing the right sensor for RPM 1

[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 lazer 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 is 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

 

[3DDave]

You are correct - I extrapolated from "lawn mower" to include a motor. So, substitute "lawn mower" for "motor" in the objection because "lawn mower" adds no clarifying information. It could be a push mower or a 50 foot wide tractor pulled gang mower.

There is still an infinitely large solution space. One could use a tachogenerator or a magnetoinductance tachometer or just two pickup wires that are shorted by a paperclip taped to something that moves attached to a timer or a centrifugal switch via slip ring or ................

What still impresses me is that your stated goal is measuring coast down time, which is a matter of losing kinetic energy stored in the machine (can be calculated) to various friction losses (some can be measured, others estimated) eliminating the need for testing.

Alternatively, the whole thing can be estimated by running the machine at various RPMs and measuring the power required to drive it at those speeds and then integrating to get an energy-loss curve. The area under the curve to any point represents the kinetic energy that will be absorbed when the power is cut off. This can be integrated over RPM to create a direct reading chart. Since the mechanical energy storage can be calculated for any RPM it's just a matter of looking at the chart.

Or you could just use a camera and record a video. Depending on the camera you can easily get the time to within 1/50th or 1/60th of a second or less. Or a sound recorder and run the result through Audacity or something similar.

 

[MacGyverS2000]

240fps on an iPhone... surely a buddy has one if you don't.


Dan - Owner

http://www.Hi-TecDesigns.com

 

[Zarathuztra]

I think based on everything I've looked through and feedback received. CWB1's suggestion is the one I will be moving forward with. Does anyone have a suggestion for optical sensors or brands they have found to be reliable?

 

[Strong]

I have used several of the Monarch Instruments sensors as indicated by Tmoose above. You have three choices for optical sensor: visible light beam (most common), laser beam (gaining popularity) and infrared beam (IR has advantages depending on application). The laser and IR sensors can possibly detect passing shaft key or keyway, so reflective tape may not be needed. Contrasting light-dark paint may also give a signal without needing reflective tape. You cannot measure down to Zero speed with most 1-pulse per revolution sensors. We would need much more details about your application to be of help.

Walt

 

[Zarathuztra]

@Strong, sorry details are a little limited, it is a test I will be doing in the future, so unfortunately I don't yet have the test materials/equipment on hand.. I figured the concept is simple enough and common enough to get started on it now, before I get the equipment. Once the machine is on hand I wont have time to wait for shipping and such, so need to prep ahead of time.

 

[Strong]

"I figured the concept is simple enough and common enough to get started on it now, before I get the equipment."
This is a good idea. Having measured many machine startups and coast-downs, I can say that transient measurememnts of speed, vibrations, sound, ultrasound, motor current, torque, and torsional vibrations presents greater challenges than steady-state measurements.

Walt

 

[Tmoose]

Walt Strong may have some examples of the practical resolution limitations on a rapid coast down and a 0nce-per-rev tach signal.
Monarch could probably give you some idea as well. I would trust them to shoot straight.

 

[Strong]

OP: "So I'm trying to find out how long it takes for it to come to a rest when I remove power."

If you truly need to detect when Zero speed (at rest) occurs, then you should look at encoders and not 1xSS sensors, of any type. There are some hollow shaft encoders, but most mount on the end of a shaft. If you can do your test down to some defined "minimum speed", for example 5-rpm, then you could probably use a 1xSS or multi-pulse tachometer. I suggest additional study on what you want/need and what can be done with various speed measurement methods and sensors. Consider getting a free demonstration or renting before purchasing stuff that does not work to your expectations.

Walt