PLC Speed Sensing

[I am turning into a dog with a bone on this one ...]


Another reason for buying a -VHSC card is that someone else should have figured out all of the pitfalls.

Analogous to the noise/wobble in the Freq.-to-Volt. circuit described earlier will be all sorts of issues if you try to count (de-rated) pulses off that pulser disk with a prox and try to calculate speed (i.e. pulse frequency) in PLC code; PLCs are not precision instruments. Unless the code can timestamp edges with interrupts, the pulses are controlled by the IO task. Even with a scheduled task, the code only knows the edge count since the last task to +/-1 edge, so it will have to make a tradeoff between measurement rate and accuracy. I don't know what the scan times are but with 5ms as a guess that is 100edges/s max and at least 1% = 35rpm+ noise at a 1Hz measurement rate. And time is another issue: of course there is the GSV of CST CurrentValue with μs resolution, but the code can only sample that in the task, not at the edge.


And on it goes. Unless the customer will be either satisfied with 1% noise or a longer measurement sample time, or is impressed by the use of duct tape over money, the cost of a -VHSC card will be far cheaper than the time it takes to do almost anything else.
 
The built in high speed counters are just not fast enough to catch
at 3600 rpm and just 1 pulse per rev you need a update time of less then 20 ms to even have a chance to catch all the pulses that scan and update the input table and and scan the ladder code and update counters. most plc's only update the inputs at 100 ms intervals to reliable read the inputs for what you need update the table 2 timer for each transition that would mean the update time would be 400 ms at that rate you will miss about half the input pulses and that would be if the input pulse remains on that long
you would need a pulse for 50% of the rotation
if just not reliably doable a high speed counter that can count the edge's of the input
good luck trying it i have seen may try and many fail
 
Originally posted by GaryS:

The built in high speed counters are just not fast enough to catch...

From the Rockwell 1769 technical specs publication:

Capture.JPG

The 1769-L2X plc has a variant with a built-in high speed counter, similar to what parky mentioned with Mitsubishi. The pulse edges are detected and counted in hardware.

The plc program doesn't need to be able to process every pulse. the hardware just can't miss any pulses.

Keith
 
Parky is absolutely right, and there are some A-B controllers with build-in hardware interrupts and hardware high speed counters of various sorts.

But not the 1769-L18ER CompactLogix. Its onboard I/O are ordinary DC inputs and output modules permanently mounted to the POINTBus. And the 1734-IB8 is an ordinary DC input module without high-speed circuits.

The 1734-IJ (5V) and -IK (24V) counter modules are a decent way to get a velocity input, setting them for "Rate" mode. Max single-channel rate count is 1 MHz.
 
I was not referring to AB specifically but many PLC's do have them and they do not use the high speed counters they have hardware ones built into the hardware where the accumulators etc. are accessible in the PLC Memory.
In general it tends to be the smaller types that have this built in.
 
I was not referring to AB specifically but many PLC's do have them and they do not use the high speed counters they have hardware ones built into the hardware where the accumulators etc. are accessible in the PLC Memory.
In general it tends to be the smaller types that have this built in.


Yes, e.g. MicroLogix 1100 has a High-Speed Counter, cf. here, but it would still need ladder code to do the rate calculation; the free-running clock at 10kHz would be reasonably accurate, but the sampling accuracy might be limited by the scan rate.


Update:


Ah! But t can generate an interrupt, and the interrupt routine can do a decent calculation; Ken Roach wrote a program, HSC_frequency.RSS/.PDF on MrPLC ca. October, 2009.
 
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