Need two-input analog comparator with display

drforsythe

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I am trying to help out a company who has no technical support and they are far away from my locale. We are trying to sense when a roll of paper is getting low. I plan to use either an ultrasonic or laser distance sensor with 4-20mA output. I do this all the time with a PLC and HMI. The operator can either change the setpoint via recipe selection or manual entry. Either technology works fine in the PLC applications I have done. I would like to do the same thing, only this time without the PLC and HMI.

I would like to find a device (like a panel meter) that can take a potentiometer signal into one channel and the distance sensor into a second channel. The potentiometer input would be displayed and act as the setpoint. The distance sensor would also be displayed and act as the process value. When the process value is greater than the setpoint (or lesser, depending on configuration), I would like an output to turn on.

I looked around a bit on the web and checked out Red Lion, but have not found something yet. I have a feeling the low technology that I am requiring is going to be the roadblock. If anyone has a suggestion, I would appreciate it. Thank you in advance.
 
What you describe is a remote setpoint on-off controller. Most stand-alone controllers use a user-entered SP, so you need one with a 2nd analog input that functions as a remote setpoint.

Honeywell UDC 2500 has a 4-20mA input (input #2) for remote setpoint (potentiometer).
The other signal, distance sensor, is the primary process variable on Input #1.

They obviously have to be scaled so the numbers make sense when compared.

An alarm is configured for deviation mode (which tracks the setpoint as a reference)

When PV < SP, then alarm = ON (some hystesis/deadband needed to turn it off.)
 
Precision Digital's PD6060-6R2 1/8 DIN panel meter has two inputs with a subtract (math) function.



I see that Channel C is the math channel, so with Channel C to an alarm relay, you can configure the level at which the alarm trips.



Presumably the change from a positive difference to a negative difference is what you're looking for.
 

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