CompactLogix Calculate Frequency From Discrete Input

Hi,

I have an application where I need to determine the frequency of a pulse train coming from an inductive prox sensing gaps in the structure of a flywheel. I'm estimating it to be somewhere between 60hz to 100hz when it is at speed. This is not critical to the process so much as a crude protection for the reversing motor starter and indication we are at at least 90% of flywheel speed to enable full power stroking. Normally I'd have a press control and could just use an output from that but this is a hot-work forging application where OSHA doesn't require a safe press control.

I have a prox with a 300hz switching speed running into a 1734-IB/8 PointI/O card. Is this possible given the hardware I have?

I've looked around the net for some sample code to get me started but my Google-Fu is coming up short here.

Any ideas?

Jim

That might be a little fast for a normal input card, but you can check the specs on update times and see how fast you can set the RPI. Also depends on your PLC's scan rate. Another option would be to use a HSC card and then count the pulses for one second.

I think the best and cheapest route would be to use a dedicated frequency counter device that has an adjustable 4-20mA output. Look those up, I know there's a bunch of options out there.

Check these threads:
http://www.plctalk.net/qanda/showthread.php?t=98399
http://www.plctalk.net/qanda/showthread.php?t=95153

Nice search work, Tarik !

I agree (with myself, from years ago) that it's probably better to get a 1734-IK module for this application (the -IJ is 5V, the -IK is 15-24V).

A 100 Hz square wave has a pulse that's ON for 5 milliseconds and OFF for 5 milliseconds.

You have to sample that twice per period to be sure you get each pulse, so now you're down to 2.5 milliseconds.

The POINT I/O regular DC input modules have a 0.5 ms filter on them, minimum, which gets you down to 2 milliseconds.

Which is the fastest I think you can run the RPI on a 1734-AENT POINT I/O adapter.

And that doesn't take into account the backplane speed of the POINT Bus.

I'd go with a dedicated-purpose speed relay or Speed/Analog signal conditioner or the 1734-IK counter module for this application.