We want to interface a pulse output from an optical shaft encoder to PLC. The shaft generates 500 pulses per revolution and the speed of shaft is 1 revolution per minute. We want to measure the RPM in PLC. We are using Ab's SLC. Should we use DI module or a high speed counter for the same? Please advice. Thanks & Regards Aniruddha Joshi [email protected] www.transtechprojects.com

 

You can follow as follows: At 500 pulses/rev , and at a speed of 1 rpm , it means you will have a pulse rate of 500/60 per second i.e equivalent to a pulse period of 120ms. Considering a 50-50% duty cycle, the PLC should be able to handle reading of input within 60ms. While calculating the speed of PLC , you should take into account the on-time,off time delays for the type of DI module which you plan to use,and worst case scan time of the PLC program( which is dependent on the CPU type and the size of the program itself). Looking at the 60ms restriction, i feel you may not need a hi-speed counter module. Also, some micro-plcs do have built-in high speed counters or some DI inputs have the ability to be used as high speed inputs.please check these specs from the catalogues of the manufacturer. hope this would help your cause.

 

At 1 RPM and 500 PPR, you have 8.333 pulses per second, or a period of 120 mSec per pulse. Assuming a square wave pulse shape, you have 60 mSec ON, and 60 mSec OFF. That should be enough time to be detected using a discrete input module unless your scan time is unusually long. One revolution per minute is pretty slow, so if you meant to say that the shaft speed is 1 revolution per second, then the period is 2 mSec per pulse and you'll need a high speed counter. The PLC interrogates the DI module once per scan. To detect a signal, it must be ON (and OFF) for a period of time at least equal to the PLC's scan time, but that leaves you no margin of safety. For added reliability, you should design for the signal to be in the same state for at least twice as long as the scan time. The assumption about a square wave pulse shape is also critical. The same factors apply to the OFF state as to the ON state of the pulse.

 

as described there will be approx. 9 pulses/sec. if the scan time of the PLC is < 60 msec then you may use a DI else use a HSC module. ron

 

We want to interface a pulse output from an optical shaft encoder to PLC. The shaft generates 500 pulses per revolution and the speed of shaft is 1 revolution per minute. We want to measure the RPM in PLC. We are using Ab's SLC. Should we use DI module or a high speed counter for the same? Please advice. The DI card would work OK up to about 50 HZ, as long as the encoder can put out 24VDC. Check the input response of the card. Use a STI routine for a consistent time base to read the encoder. You could read the free running clock to count time between pulses. If the encoder is TTL (5VDC) level, I think that AB makes a TTL card for the SLC. Bill Sturm

 

I designed and had manufactured a board to take the quadrature signal of the shaft encoder and output 12V directional pulse signals. If you turn it CW, only one output will produce a pulse signal corresponding to full quadrature cycles. If you turn it CCW, then the other output does the same. The circuit also has the ability to pulse for every state change of the quadrature cycle with respect to direction as well. We use this circuit to decode shaft encoder base float level sensors for water well measurement. The PLC we use is a Control Microsystems Micro16. If you have any questions you can contact me at [email protected]