Connection between thermocouple and PLC is an important thing to consider. The cost & efficiency both factors are needed to take in to consideration. Also the environment (temperature) of the sensor location, distance to the PLC is a factor.
Thermocouple connection in a system comprises with connection channel cable and cold end temperature generator usually pt-100 temperature sensor. As pt-100 temperature sensor has this option to directly making interface with PLC plus it is cheap, it is more popular to use thus.
Extension is normal copper cable:Thermocouple connection-
- Cold end compensation have to do in connection box for example by using Pt-100 sensor
Extension is thermocouple metal:Thermocouple connection-
- Extension cable is compensation cable (same material)
- Cold end is generated into PLC-terminals
- Cold end compensation is done inside PLC by internal temperature measurement (Pt-100)
- Relatively expensive set-up
- Compensation cable is difficult to get in some countries
- Not so flexible to handle and do connection than Copper
Amplifier used in Extension channel:Thermocouple connection-
- Amplifier takes care about “cold end”- compensation
- We can use analog input card for receiving mA-signal
- Useful for marine installations where limited space
- Testing can be done by shorting thermocouple input, so amplifier should send internal temperature signal forward to PLC.
- More costs, because amplifier is needed in every measuring channel
CAN bus used as extension channel:Thermocouple connection-
- Data transfer with digital CAN-bus, better EMC behavior
- Cold end is generated into WECS/UNIC terminals
- Cold end compensation is done inside WECS/UNIS by internal temperature measurement (Pt-100)
Why CAN bus as extension channel with thermocouple is better:
This is a better efficient and cheap connection.Only one Pt-100 temperature sensor used to generate cold end temperature.Any number of incoming thermocouple can be used for this one.Thus a common single junction box with single pt-100 and multiple incoming thermocouple.
Plus the the CAN bus or controller area network bus is fast ,efficient comparing to its pricing.