Bypassing a sensor

[Skiroy]

I was recently asked a question that I wasnt sure was the proper way to answer it. I was asked if there was a input sensor malfunctioning and I had to go into the PLC program to bypass the input what would I do?

I said I would just make a parralell branch around the sensor. I have never done this so I dont know if the program would download without a input on the parralel branch or not.

How would you guys do it?

 

[GlennA]

Before bypassing the sensor, monitor it; is the sensor changing state when you move a target in front of and away from it ?

 

[brendan.buchan]

I was recently asked a question that I wasnt sure was the proper way to answer it. I was asked if there was a input sensor malfunctioning and I had to go into the PLC program to bypass the input what would I do?

I said I would just make a parralell branch around the sensor. I have never done this so I dont know if the program would download without a input on the parralel branch or not.

How would you guys do it?

There are multiple ways you could do this depending on the type of PLC and how it is programmed.

How many times is the sensor used in the program? If it is used a bunch of times, simply forcing it ON could be easiest option - as for best practice, that depends on who you ask.

Most PLCs won't let you put in an "empty" parallel branch.

Some PLCs have internal system bits that are "Always False" or "Always True". You can use these bits to bypass and disable things.

Generally in every program I do, I generate my own "Always False" bit by having an unconditional rung that unlatches or resets a bit. I'll then call it something like "sys_BB" and have my name and number in the description.

I'll then use this bit to mark things and bypass/disable things in my program.

If I ever forget to remove something and someone else stumbles across them in a program, they know who put it there.

 

[RheinhardtP]

I was recently asked a question that I wasnt sure was the proper way to answer it. I was asked if there was a input sensor malfunctioning and I had to go into the PLC program to bypass the input what would I do?

I said I would just make a parralell branch around the sensor. I have never done this so I dont know if the program would download without a input on the parralel branch or not.

How would you guys do it?

I would fix the sensor.. Seriously.. bypassing is not a road you want to go down. It will only set a bad precedent, before you know it half the sensors will be bypassed in the plant.

But if it is critical and NOT a safety related sensor in which case i will refuse to do it, i would use a memory bit in the PLC.

Be sure to add the date time and who approved you to bypass the sensor, when your process fails due to this sensor not working you dont want to be held accountable for this.

 

[jvdcande]

I personally NEVER force inputs. On a written and signed order I only bypass sensors at the connection. That way, when a new sensor arrives and I (or one of my colleagues) have to connect it, we are sure not to forget to remove the bypassing wires.

For a safety sensor I always demand a court order to bypass, arguing that I don't want to be held responsible for accidents. So far nobody did deliver, so I didn't have to bypass any safety sensors so far .

Kind regards,

 

[Andybr]

I personally NEVER force inputs. On a written and signed order I only bypass sensors at the connection. That way, when a new sensor arrives and I (or one of my colleagues) have to connect it, we are sure not to forget to remove the bypassing wires.

For a safety sensor I always demand a court order to bypass, arguing that I don't want to be held responsible for accidents. So far nobody did deliver, so I didn't have to bypass any safety sensors so far .

Kind regards,

I generally try to follow this line but in addition I use a long piece of vivid purple cable to link out the sensor so that it cannot possibly be taken for any sort of permanent fix. It should be obvious what it is as soon as the panel door is opened.

 

[shooter]

I would use another input and make it XOR with the sensor.
i do this a lot to simulate.

 

[daba]

Another approach would be to put permanent code in the PLC so that sensors can be put into maintenance or simulation mode. The plc code can then use this mode to determine when the input should be on or off, and set that status automatically. This mode should only be used until the sensor is changed.

I would also flag up that a device is by-passed on the operator interface, HMI, SCADA, or even a flashing lamp on the control panel.