The low temperature interlock seems strange

[hazel]

Dear all,
If the RTD sensor broke in the positive line, what will happen?
I find the signal in the PLC display didn’t change after I disconnect the positive line?
The RTD signal connect to M-system monitor switch for RTD type and output a digital signal to PLC system.
I measure the voltage between (+)(-) from field and get 2 different value. One is 4.8 VDC,The other is 9 VDC.
I'm fraid if the(+)line broke in someday, this interlock will not active.

 

[CMfgE1]

Your information is very confusing. I will assume that your having RTD difficulties.

RTD's (3 or 4 wire) work off the principal of resistance.
The 3rd and/or 4th wires of an RTD are used for compensation of the length of the High and Low signal wires. This makes the OHM value more accurate by not factoring in the lenght of the RTD wire.

Disconnecting any wire from the transmitter should result in a change of resistance and therefore change in temp reading. Of course there might be a fail high, fail low or the display might freeze on the PLC when an error occurs.

The one confusing thing is how the signal becomes digital?

But please post more if this does not answer your question


Good luck!

Quote: "Its not what you know, its who you know"
Everythings a learning experience-Everything

 

[hazel]

This RTD sensor is 3 wire type.
The sensor wire connect to a monitor switch which
output an ON or OFF signal. If the process temperature lower than the set point, then PLC receive an OFF signal.
In normal, the loop is closed. when the moniotr switch detect the signal is lower than set point, the contact open and active the interlock.
My problem is PLC can not detect open loop as I dicsconnect the positive wire.

 

[hazel]

Sorry, make a mistake.
I should say the PLC can not detect an error as the positive wire is disconnected.

 

[buzzp]

Sounds like it is pegged out. In other words, the open circuit resistance (very high) is not affecting the reading. So, it sounds like the wrong RTD values are programmed into the PLC or the conversion from analog to digital is screwed up. Disconnect the RTD and measure the resistance and plot this over a few temperatures to see if it is changing. Measuring voltage will not help you much.

It sounds like you measured the comms line voltage between the controller and PLC.

 

[CMfgE1]

Gotcha, Thanks, I had a hard time visualizing your problem.
And your concerned the loop is not failsafe.

Without knowing the model of the switch, heres my take.

There could be "bucking of power", that is the PLC is providing power in the loop and the switch is also providing power.

It is also possible the switch is not setup/programmed properly.

Check to see if the digital input is sink or sourced wired.
Sink is outside power supplied or source the power comes from the digital input module.

Models and make of the PLC and the switch would help diagnose more. Let me know


Good Luck!!!


Quote: "Its not what you know, its who you know"
Everythings a learning experience-Everything