Not Sensing versus No Sensor

slackjaw

Member
S
Join Date
Feb 2007
Location
Massachusetts
Posts
15
Hi all,

I am hoping some one among you may have thought up a clever solution for this: I am working on a line that uses a photoelectic sensor to detect the presence of tablets. The sensor itself works just fine - when a tablet passes between the emitter and the receiver, the output of the sensor is set (i.e. TRUE of high). Otherwise, when no tablets are present, the output of the sensor is reset (i.e. FALSE or low)

However, the line is periodically dismantled and then reassembled for maintenance activities. Recently, the line was reassembled, but the photoelectric sensor was not plugged in (yes, I am serious).

Because the sensor is only set when it senses a tablet, the automation system cannot tell the difference between:

(1) the sensor is operating normally, but there is no tablet present
(2) the sensor is not powered

Is there some standard way for automation logic to verify the presence of a digital input in this scenario? For instance, if the sensor were normally set, and only reset when a tablet was present, then I could add logic to verify that the sensor was at least present and operating. Sadly, there is no option to change the sensor behavior.

I could source a new sensor to suit my needs, but there would be a tremendous amount of paperwork involved...
 
So, you could have your sensor operate normally closed. Then add logic that detects the NO condition, and if it persists too long, raise a warning that either the product is Stuck in front of it or the sensor is disconnected/broken.
 
Hi there,
If you are aware in your program of tablets being placed on the conveyor somewhere upstream of your tablet sensor, then you have your basis to reasonably expect tablets to be present, correct?
So if your PLC knows tablets are on conveyor, start a timer. Set the preset to the reasonable amount of time for a tablet to travel to sensor, plus a buffer. Reset the timer when the sensor detects a tablet. If timer times out, then your sensor is not working.
That's one down and dirty way.
 
If it was me I would look at something else that would provide a running bit, like a conveyor.

If you don't see a sensor high whilst the conveyor is running within say 20 seconds of start up - sound a no sensor alarm.
 
However, If you are talking tablets and a qualified system(pharma), its going to be paperwork either way!
So in THAT case might as well go w/ a sensor that offers dual outputs, 1 NO 1 NC, and use the combination of the 2 outputs to do proper detection.
 
Dravik said:
So, you could have your sensor operate normally closed. Then add logic that detects the NO condition, and if it persists too long, raise a warning that either the product is Stuck in front of it or the sensor is disconnected/broken.
Click to expand...

Precisely.

Except that I am not going to be permitted to use a different sensor, and the existing sensor is not configurable, so I am essentially stuck with what I have.
 
dcooper33 said:
Hi there,
If you are aware in your program of tablets being placed on the conveyor somewhere upstream of your tablet sensor, then you have your basis to reasonably expect tablets to be present, correct?
So if your PLC knows tablets are on conveyor, start a timer. Set the preset to the reasonable amount of time for a tablet to travel to sensor, plus a buffer. Reset the timer when the sensor detects a tablet. If timer times out, then your sensor is not working.
That's one down and dirty way.
Click to expand...

Thanks for the suggestion. We could do that, and in fact, we already do a variation of this. But this scenario wreaks havoc with our product tracking. What we need is a way to verify during startup of the line that the sensor is in place and powered. Currently we have added a human verification step to our initialization process, but that process is already cumbersome.
 
Dravik said:
However, If you are talking tablets and a qualified system(pharma), its going to be paperwork either way!
So in THAT case might as well go w/ a sensor that offers dual outputs, 1 NO 1 NC, and use the combination of the 2 outputs to do proper detection.
Click to expand...

Yes, it is a qualified pharma system.

You are correct, paperwork either way. But FAR LESS paperwork to add an informational warning than to modify a component in our existing product tracking model.
 
slackjaw said:
Yes, it is a qualified pharma system.

You are correct, paperwork either way. But FAR LESS paperwork to add an informational warning than to modify a component in our existing product tracking model.
Click to expand...

Hmm, maybe put the sensor power in series w/ a relay and feed the relay output back to an input on your control system. Use that to at least verify there is power...
 
slackjaw said:
Hi all,

I am hoping some one among you may have thought up a clever solution for this: I am working on a line that uses a photoelectic sensor to detect the presence of tablets. The sensor itself works just fine - when a tablet passes between the emitter and the receiver, the output of the sensor is set (i.e. TRUE of high). Otherwise, when no tablets are present, the output of the sensor is reset (i.e. FALSE or low)

However, the line is periodically dismantled and then reassembled for maintenance activities. Recently, the line was reassembled, but the photoelectric sensor was not plugged in (yes, I am serious).

Because the sensor is only set when it senses a tablet, the automation system cannot tell the difference between:

(1) the sensor is operating normally, but there is no tablet present
(2) the sensor is not powered

Is there some standard way for automation logic to verify the presence of a digital input in this scenario? For instance, if the sensor were normally set, and only reset when a tablet was present, then I could add logic to verify that the sensor was at least present and operating. Sadly, there is no option to change the sensor behavior.

I could source a new sensor to suit my needs, but there would be a tremendous amount of paperwork involved...
Click to expand...

Since you are using Emitter/Receiver type photoeyes just set the Receiver as Light Operated (vs. currently used Dark Operated).

In Light operation mode the Receiver will send a High signal (ON) only when it is illuminated by its Emitter; set this condition (photoeye ON) as a requirement for the conveyor operation thus ensuring the tablet detection functionality at run commencement.

Then, within your application programming, replace each current occurrence of the XIC (Examine If Closed or Normally Open Contact) addressed to the corresponding photoeye system Input with a XIO (Examine If Open or Normally Closed Contact) and reciprocally, any current XIO instruction with an XIC.

The photoeye will detect a tablet when the Emitter signal is Low (OFF).
 
Dravik said:
Hmm, maybe put the sensor power in series w/ a relay and feed the relay output back to an input on your control system. Use that to at least verify there is power...
Click to expand...

I was contemplating something along these lines, but I wasn't sure if there might be a more elegant answer. I figured, these "Dark On" sensors are in wide use - surely I am not the first person to have this problem...
 
dmargineau said:
Since you are using Emitter/Receiver type photoeyes just set the Receiver as Light Operated (vs. currently used Dark Operated).

In Light operation mode the Receiver will send a High signal (ON) only when it is illuminated by its Emitter; set this condition (photoeye ON) as a requirement for the conveyor operation thus ensuring the tablet detection functionality at run commencement.

Then, within your application programming, replace each current occurrence of the XIC (Examine If Closed or Normally Open Contact) addressed to the corresponding photoeye system Input with a XIO (Examine If Open or Normally Closed Contact) and reciprocally, any current XIO instruction with an XIC.

The photoeye will detect a tablet when the Emitter signal is Low (OFF).
Click to expand...

Thanks for the detailed suggestion. The sensor is "Dark On" only. The amount of paperwork required to replace the sensor will likely prevent me from doing so.
 
BTW: Thank you all very much for your prompt replies. I have been typing fast to keep up, and forgot to thank each of you along the way.

I truly appreciate your time and your suggestions.
 

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