I have seen the circuitry of a couple of programmable motion controllers. They were built around around microcontrollers like STM. So do I guess, the PLCs are also built around some microcontrollers. PLEASE correct me if I am wrong.

My concern is that how do the input ports of such motion controllers and PLCs are secured against EMI. What kind of protective circuit is used to avoid false triggering? Does the STM microcontrollers or others used in PLCs have some built-in protection? How can we secure AVR microcontrollers against EMI?

 

Generally PLC input ports are Opto-isolated and these are generally considered low impedance devices, so inherently non susceptible to EMI .

 

Generally PLC input ports are Opto-isolated and these are generally considered low impedance devices, so inherently non susceptible to EMI .

View attachment 294669

I assume that both grounds are separate otherwise no isolation.

Is this opto-isolation the only protective network or there are some other too?

I have been using such opto-isolation with Arduino but does not provide 100% protection. The input ports of Motion Controllers and PLCs are more secure than (opto-isolated) input ports of Arduino, when operating in the same environment.

 

...
I have been using such opto-isolation with Arduino but [it] does not provide 100% protection. The input ports of Motion Controllers and PLCs are more secure than (opto-isolated) input ports of Arduino, when operating in the same environment.

What is your evidence for this claim?

 

What is your evidence for this claim?

Well, I had one Chinese set-up - A power press equipped with a servo-based strip feeding unit. A 3-pin NPN NO inductive proximity sensor is installed on the press such that when the mold lifts an inch high after the stroke, the sensor would issue pulse to a programmable controller which turns the servo to move the strip forward to a user-defined length for next stroke.

I replicated the entire set-up except the controller for which I used Arduino. Between the input pin and the proximity sensor I used PC817 optocoupler as below:


However, I was experiencing false triggering. I enclosed the sensor cable in (24V supply) grounded flexible metal conduit. The conduit was grounded at the PCB side. Despite all these, I could not avoid false triggering.

A few weeks ago, the Chinese machine's servo failed. I had to replace the servo set but unfortunately, I could not program the controller to control the new servo so I replaced with my Arduino based controller. But again, the false triggering issue appeared on the Chinese machine too.

SUMMARY:
Chinese machine with a programmable Controller + 3-wire sensor => No false triggering.
Chinese machine with Arduino based Controller (with opto isolation) + 3-wire sensor => False triggering present. (The operating environment and conditions are the same).
This hints that opto isolation is not securing the input of Arduino.