encoders & omron CJ1M PLC

[hps]

Hi, I need to read inpulses from a 50 pulse per cycle encoder which operates the clock of a shift register (for a Conveyor belt). Everything works OK at lower speeds but I loose the shift register clock when I crank the conveyor speed up. I reckon its a 5mS pulse time at the required speed. I'm using one pulse lead from the encoder to input 0.00 of a standard input module. Any help wwould be appreciated.

 

[Terry Woods]

If the input is ON everytime the PLC looks, then there is no OFF-to-ON transition. The shift register needs to see an OFF-to-ON transition in order to shift.

How fast is the encoder turning?

What kind of resolution do you really need?

If you geared down the encoder speed you would increase the chances of catching the transitions... but would you still have the resolution you need?

 

[hps]

At the conveyor running at the required speed, I expect a 5mS encoder pulse which equals just under 2mm on the belt.The application requires a vision inspection system to look at product on the belt so resolution is important.

 

[Terry Woods]

If I assume that your pulse is 5mS ON and 5mS OFF then you need a scan-time that is less than 10mS... what is your scan time?

BTW, while on-site, you have to hit the refresh button once in a while to see if there are any responses.

 

[BobB]

CX-Programmer has a differential monitor. The count is cached in the PLC. Right click on the bit and you can select differential monitor. You also have the option of selecting diff up or diff down. It is an accurate count of what the PLC program sees, not the actual input turning on and off.

Next issue, what is the scan time? Which CJ1M CPU are you using? How much program?

The higher end CJ1M CPUs are very very fast. I have one with a scan time considerably lower than the 5 ms. You will need to have the input on for approx twice the scan length to never miss a pulse.

You can also configure a card to have varying time filters on the inputs in the settings window. I believe the default filter is 8 ms. Set it low and try that.

Perhaps an interrupt card may be needed to allow you to perform the task. Then when your input turns on, an interrupt routine can be enabled to pulse your shift register.

 

[hps]

Thanks for your ideas. I'll be working on the project Monday.
Thanks
Clint

 

[john paley]

I have run in to a similar app problem, using AB though. I ended up installing the fastest input module available (IQ, I think) input module and using a 2 Ms timed interrupt. This interrupt contained the counting part of the process. I suppose your's would contain that and the vision trigger. If you use this method, make sure to immediade update the input first in the interrupt file, and immediate update outputs last.

I'm not familiar with the Omron, but if it doesn't offer these features, your only alternitive is to sacrifice resolution to get all the counts.

Check your scan time as well as the input specs for time.

 

[Jay Anthony]

Both a scheduled interrupt and immediate update I/O are available on the CJ1M PLC. Your concept is probably his only hope.