Monitoring a sensor

[johnnyboy1852000]

Hi,

In our plant we are having issues with operators unplugging sensors so there machine don't stop. The purpose of the sensor is to stop the machine if tooling comes lose so we don't break tooling. The problem I am running into is how to monitor to make sure they have the sensor plugged in. The input doesn't come on until the machines moves forward and when it does its only for a split second. what they want me to do is if the sensor is unplugged to not even be able to turn the machine on. Anybody have any ideas on how to accomplish this?

Thanks,

John Schweitzer

 

[Ken Roach]

Welcome to the PLCTalk community !

In general, the simplest way to make a sensor "removal proof" is by using a normally-closed type sensor.

If you have a PLC controller monitoring the sensor, it should be straightforward to condition the logic to look for a FALSE state only for a short period of time after the machine starts moving, and to prohibit the machine from starting if the sensor is continuously in a FALSE state.

The other solution is to promptly fire any operator who is on shift when a sensor is found unplugged. It's not fair or elegant, but it works.

 

[KuulKuum]

The other solution is to promptly fire any operator who is on shift when a sensor is found unplugged. It's not fair or elegant, but it works.

+1 vote for this solution

 

[Racer5039]

If you can't get to the PLC/ control program, you could change the sensor to have both N.O. and N.C. Use the N.O. ( or N.C.) just like it is used now, set up the other side of the sensor to run a relay, relay contact into the e-stop or run control. No sensor plugged in, no run signal.

 

[Steve Bailey]

GE's Genius I/O has embedded diagnostic capabilities. For inputs it detects the leakage current across solid state sensors and reports an open wire fault if there is no current. I would not recommend it for new installations as it will be phased out soon.
Genius I/O has been around since the mid-1980s. Even though it was always significantly more expensive per point than standard I/O, I am somewhat surprised that the other major players in the PLC marketplace never offered similar functionality and also that GE never deployed it on any of their other I/O offerings.

 

[dmargineau]

GE's Genius I/O has embedded diagnostic capabilities. For inputs it detects the leakage current across solid state sensors and reports an open wire fault if there is no current. I would not recommend it for new installations as it will be phased out soon.
Genius I/O has been around since the mid-1980s. Even though it was always significantly more expensive per point than standard I/O, I am somewhat surprised that the other major players in the PLC marketplace never offered similar functionality and also that GE never deployed it on any of their other I/O offerings.

Allen-Bradley ControlLogix 1756-IA8D (8 Point AC Input with Diagnostic) and 1756-IB16D (16 Point DC Input with Diagnostic) have Open Wire detection provisions.