RPM Sensing on Lovejoy couplings

I have come to the conclusion that in my training efforts I cannot rely on just one motor alone and will have to have several. I would like to sense and display RPM to class. Display has to be visible from 10' away. I am not too concerned with tolerance 5% should be OK. Mostly what I will be showing is change in speed - possibly very quick which elimiates digital readout.

The tach readout I have is 14 Volt and 98 mA. I have tach generator which will require me to make up a voltage divider.

I would like to see if a prox type pickup could be used as a sensor. In theory this would be adaptible to any motor and driven load setup.

I like Lovejoy couplings and think they will give me a lot of versatility. These are matched 3 jaw units so in theory I have 6 gaps to measure with. The question comes up with the spider (plastic) will it "add" more gaps ie give me a total of 12??

Will my idea of sensing off lovejoy work?

Dan Bentler

I would think yes. If you mount an inductive sensor centered on the coupling, it will turn on each time one of the 6 jaws passed, so you will have 6 pulses each revolution. If you not only look at when the sensor turns on (leading edge), but also when it turns OFF (trailing edge), you will indeed get 12 transitions.

The space that the plastic spider creates will determine the maximum RPM that you can measure. In other words, the response time of the sensor (plus input response time, plus scan time, etc.) vs. the time it takes for the spider leg (and/or jaw) to pass. At some point, the program will just see a steady ON (or OFF).

Of course, I'm talking about using a standard PLC input. A high-speed counter card, or something like a Red Lion display has a much faster response time than a regular 'ol PLC input.



-Eric

Dan if you just see the steady state on and can't handle that. I would look at using a button head screw where the set screw would go. Then you can just use 1 pulse/ Rev.
Use a button head and grind it so it doesn't get caught on some students finger going where it shouldn't be.

JeffKiper said:

Use a button head and put a guard over the shaft/coupling so some student can't stick his finger where it shouldn't be.

Click to expand...

Fixed it for you.

Yea it should have been "gaurd". I was thinking make it a low profile head and show them. At least they can see it.

Being able to seeing it would be nice in a learning environment. A button head screw under a polycarbonate shield with the prox mounted in the shield would provide visual instruction on a common method but keep student fingers safe at the same time.

Thank you all
Button head screw - did not think of that.

You all refer to transparency in demos. That is a key issue. As example for DC motor starting using light bulbs for starting resistors - each bulb shorted out with a knife switch. Dynamic brake resistor is another light bulb.

Student safety is utmost priority. In early stages it will be low voltage ie <50 (which all my reading says is acceptible) but power supplies are 24 VDC (or AC in some cases) or less anyway.

So far my biggest accomplishment is I am starting a training program for future service techs on Seattle monorail - 750 VDC is drive voltage all resistive type motor control (no solid state) been in operation since 62. Same motors with couple overhauls of course. Shows what DC can do when properly operated and maintained.

Maybe I could have gotten on this first rung by myself but it sure has been easier with "the help of my friends".

Thank you all for the help support wisdom and brighter ideas.

Dan Bentler

Dan I toss out monkey wrenches when I can. Don't know that I can really help with anything.

I know my input is late, but a magnetic pick up that generates mV would probally work better than a prox switch that has to change state. The amount of mV would directly relate to the metal portions of the coupling passing by it. Thats my thought on this matter. :nodi:

I is a noob said:

I know my input is late, but a magnetic pick up that generates mV would probally work better than a prox switch that has to change state. The amount of mV would directly relate to the metal portions of the coupling passing by it. Thats my thought on this matter. :nodi:

Click to expand...

That is close to what I was thinking. I recall using something like linked below on a lovejoy...along with a (very responsive) digital meter:

https://store.redlion.net/store/comersus_viewItem.asp?idProduct=1493
Or one of these:
http://store.redlion.net/store/mp.html

And:
http://store.redlion.net/store/comersus_viewItem.asp?idProduct=1299

Paul

OkiePC said:

This is what I was thinking, along with a (very responsive) digital meter:

Click to expand...

All scaling has to be done manually i think...
Or i should say you have to know the top end at least

I is a noob said:

All scaling has to be done manually i think...

Click to expand...

With the CUB5, you have all sorts of scaling options. We even used one for batch counting many moons ago.

Good to know!