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CM3 Thermocouple Module Concept

Consecutively Changing​


An analog value is a consecutively changing value, such as voltage, current, temperature, velocity, pressure, and flow rate.

For example, a temperature value is consecutively changing with time.
However, as the changing temperature value is directly input into the PLC, its equivalent analog amount of DC voltage (0V ~ +10V) or DC current (4mA ~ 20mA) is input into the PLC via a transducer.

Non-Consecutively Changing​

A constant is a non-consecutively changing value, such as 0, 1, 2, or 3. These values are called digital values.

The digital value 0 represents an OFF signal.
The digital value 1 represents an ON signal.

binary-coded decimal (BCD) and binary values are also considered digital values.

The CPU module is unable to directly input analog values to calculate their corresponding digital values. Therefore, the system converts analog values into digital values and then inputs the digital value into the CPU.

It is necessary to convert the digital value of the CPU into the analog value to output analog values to the external side.

Compensating Cable​

  • There is a cable to compensate deviation (temperature variations) due to distance in the thermocouple input module.
    • This shows almost the same thermo-electrical characteristics in temperature:
      • -20°C ~ 90°C (-4°F ~ 194°F) for G type.
      • 0 °C ~ 150°C (32°F ~ 302°F) for H type.

Thermocouple​

  • Thermocouples are temperature sensors consisting of two different metals attached.
  • Thermocouples use the thermoelectric effect which makes thermoelectricity flow by generating an electromotive force due to a difference of temperature between both contact points.
  • The strength of the electromotive force is determined by the types of attached metals and temperature for both contact points.
    • It is not affected by shape, dimension, and temperature variation of metals.

Temperature Conversion Characteristic​

  • The temperature conversion to thermocouple input has non-linear characteristics.
  • The CM3-SP04ETO converts the thermocouple input with non-linear characteristics into analog-to-digital (AD) and outputs the temperature conversion that is linearly treated.

Disconnection Detection Function​

  • When disconnection occurs between a thermocouple or parts of the compensating cable and module, the internal disconnection detection circuit detects the short circuit by identifying larger voltage than measurement range.

Reference Junction Compensation (RJC)​

  • The sensors for various types of thermocouples output voltage corresponding to difference in temperature between both ends, actual temperature is measured in a way that a temperature sensor is added to the current temperature in the PLC.
  • Currently, the CM3-SP04ETO module performs reference junction compensation (RJC) by using a thermistor.