Finding the PPR

I am trying to use a switching proximity sensor to read the head of a bolt on a motor shaft to read the RPM's of the motor. In the set up of a Red lion IMI device which will in turn send an alarm output to my PLC 5 if low RPM's occur for a particular amount of time. In the scaling set-up I already know that I want to the Rate counter (Red Lion) to read up to 1800 RPM's max. In order to complete the scaling set up, it's asking me to find the Hz using formula:
(Hz) = Display x (pulses per unit / time)
? = 1800 x ( ? / 60sec)

The example in the manual is: Display is to indicate 1500 revolutions per minute (RPM) Input pulses are 39.45 pulses per revolution (PPR).
(Hz) = 1500 RPM x (39.45 PPR /60)
(Hz) = 986.25
My overall question is How would I find the Input pulses (PPR) for my application. Where is the number derived from? Is it "1", because the proxy will read the bolt only 1 time per revolution?

To answer your question, the number of pulses per revolution is indeed 1. The proximity switch generates the pulse. The pulse occurs each time the proximity switch "sees" the bolt head which happens once per revolution. Thus, one pulse per revolution.

Plugging that into the formula gives you 30 Hz.

The larger question is to determine if the setup can actually work. 1800 revolutions per minute is 30 revolutions per second. That gives you one revolution in one 30th of a second, 33.3 milliseconds. The size of the bolt head dictates what portion of one revolution the signal from the proximity switch will be on. To make the arithmetic easier, let's assume the bolt head encompasses an angle of 36 degrees. That's one tenth of a revolution, so the prox signal will be on for one tenth of 33.3 milliseconds, 3.3 milliseconds. The device that the prox switch is connected to needs to be able to detect that signal in that amount of time. If the bolt head is actually smaller, then you have less time to detect it.

Work it backwards

If my motor is doing 1800 RPMs how many times is the prox making per second? Your prox is giving one pulse per revolution.

1800 Revolutions Per Minute * 1 pulse per revolution = 1800 pulses per minute. (This part is pretty obvious but is necessary for the units to work out. If the ratio weren't 1:1 then it would make more sense.)

1800 pulses per minute / 60 seconds per minute = 30 pulses per second.

So 30 Hz is your rate at 1800 RPM.

Re: Steve's note - I know the Red Lion displays are really fast. This input setup will probably work. Give it a try.

I've used the Red Lion setup successfully, but there are a couple of cautions. The posters above assumed one bolt on the shaft. However, if you are picking up the bolts on a coupling or such you probably have six or more pulses per revolution. That needs to be factored into your equation. Also, there is a max Hz the Red Lion will handle, so if you have multiple bolt heads you need to check that as well as minimum response time for the prox and the Red Lion.

bernie_carlton said:

I know the Red Lion displays are really fast.

Click to expand...

Especially the IMI that showshocka is using... :nodi:

Max. Input Frequency: 50 KHz, 50% Duty Cycle



-Eric

OK. Thanks All! If I used 6 bolts would not that be something like 1800RPM *6 /60sec? for something like 180hz for my rate. the meter only reads in tenths so now i will have to multiply that by 10 and plug in 1800hz. (as the manual instructs) Pg. 47 http://www.redlion.net/products/groups/archive/imi/docs/imi_manual.pdf
I will also have to "PROGRAM LOW/HIGH UPDATE TIMES" (Pg. 9) which is another factor, the book says it will not affect the scaling but I don't see how not..

I'm noticing the faster the motor goes the proxy stop reading the bolt, I've never seen that, so now it reads only up to a certain revolution and then quits Should I get a faster switching proxy, maybe an NPN or PNP output. I've never seen this before..

The input freq of IMI is 50KHz
You may use 1 pulse per rev
Scale unit in PRO 2
decimal point (0), round (1), lo-update (1.0), hi-update (5.0), scale (yes), display 1 (60), rate 1 (1.0), segments (1)
The hi-update time of 5.0 seconds would allow the unit to display down to 12 rpm before it would go to zero. If you need to go lower, raise the hi-update a little.
The display 1 and rate 1 define the slope of the input curve.
You may use the 60 and 1.0 I listed or any numbers with the same ratio. You could have used rate display of 1800 and rate input of 30.0.
The three dip switches on the side of the unit will set-up the input circuitry for the type of sensor output.Page 40 in manual defines differnt sensor outputs and DP configuration.
The sensor will also determine how low the system will function.
Magnetic pickups drop out at a higher RPM then prox sensors.

Steve - Red Lion [email protected]

A 50Khz input implies a minimum of 20 microseconds ON time. Evaluate your system. Let's guess that the prox (assuming infinitely fast prox) would be ON for the duration that the bolt head is in front of it. Think of the bolt head as part of the circumference of a circle. What fraction of the circle is it? That part of the time to make one revolution is the max theoretical on time of the prox. Now subtract the response time of the prox (from its data sheet). Are you seeing the limitations?

Our machine shop made a two-lobed ring that fit over the shaft such that the prox would be ON for 1/4 of the circumference then off for the next 1/4. That allowed us to measure a faster rate than with a small target like a bolt head.

Use a mag pick up - the only problem is they generally only generate about 5 volts - you may need to boost the voltage.
These things are used on generator fly wheels all the time to pick up the teeth on the flywheel. Usually about 170 odd teeth and running at 1500/1800 RPM.

EDIT: I think I posted this on the wrong thread, you aren't using a normal prox, right?

You can also weld a half collar on a b-hub sprocket.

Now, if the motor shaft can't handle that imbalance of weight, (2" diameter?) drill out the heavy side of your flag to help balance it, leaving the outer ring intact for ferrous sensing.

yes. the mechanics had already install a regular prox to read, before i took on the job. i looked it up and it only switches at 20hz max so i odered this one- http://datasheet.octopart.com/PSAC0000-Red-Lion-Controls-datasheet-93663.pdf
You guys are GREAT! I TAKE IT AS BROTHERLY LOVE, TRUTH AND RELIEF!...

I've had the same experience - bolt heads went by too fast for the prox to pick up and switch consistently. (Hence my caution above about minimum response time.)

I solved my problem by going to a photo-switch, putting a piece of reflective tape on the shaft that covered about half the circumference, and using the photo-switch as input to the Red Lion. Worked like a charm. I believe I used a Turk photo-switch.