RPM monitoring w/ Proximity Sensor

Hello All,

I'm working on retrofitting a PLC (SLC 5/04) into a machine and need to monitor two shaft speeds. Both shafts are monitored with Proximity sensors;

Shaft #1 has a nub that sticks is sensed by the prox switch once per revolution. (This shaft will spin between 900 and 909 RPM) during machine operation)

This should yield about 15hz, correct? (900 RPM/60 Seconds=15 pulses per second)

On shaft #2 the prox counts the heads of the 12 bolts located on a flange at one end of the shaft. (This shaft will have a maximum speed of about 45 RPM)

The Signal from the Prox switch on Shaft #2 should be something in the range of 9hz during operation

(45RPM x 12 Pulses per revolution= 540/ 60 seconds = 9)

Are these frequencies withing the capability of standard input card and processor or will a HSCE card be required? The machine will already have an HSCE card to process the signal from an Anemometer but I believe the card I have (1746-HSCE, Not 1746-HSCE2) is only capable of monitoring two high speed inputs. Or can "Z" be used as a third input for rate measurement?

--Dave

It depends.

Both the on duration and the off duration of the pulse minus any delay at the input card need to be at least as long as the maximum PLC scan time.

I usually try to ensure that the scan time is at least twice as fast as the shortest pulse.

15 pulses per second should be do-able, but if the on time of the pulse is only a small fraction of the revolution then you may have trouble.

You can modify the prox flag so that the on and off times are nearly equal, or use a hardwired off-delay timer as a pulse stretcher to resolve this issue. Some prox switches offer a built in off-delay timer.

Calculate the minumum on and off times and look up the input card specs to learn about its latency for a precise answer.