Creating and Using Lookup Tables for Resistive AI Devices in Sigma Controllers

[Product_Support]

Issue

Sigma controllers require accurate interpretation of resistive analogue input (AI) signals. When using non-standard sensors such as 10k Type 2 or Type 3 thermistors, incorrect resistance values can trigger hardware alarms or result in inaccurate readings. A custom lookup table must be created to ensure proper operation.

Product Line

Satchwell BAS & Sigma

Environment

Satchwell BAS & Sigma UNC controllers

Cause

Sigma controllers interpret resistance values as a percentage of a 2V signal at the A/D converter. The default lookup tables are designed for specific sensor types (e.g., D?T range). When integrating other resistive sensors like 10k Type 2 or Type 3 thermistors, the controller may not recognize the input correctly, leading to hardware alarms or misread values. A custom lookup table is required to translate resistance values into valid input percentages.

Resolution

Step-by-Step Process to Create a Lookup Table

1. Gather Sensor Data
   Obtain resistance values at various temperatures from the sensor’s datasheet

2. Use the Sigma Calculation Formula

   R1*250 / R1+14700

   Where:

   • R₁ = Sensor resistance
   • 14700 = Resistor value on the AI card

   The circuit shown below is a typical example of an AI channel for a resistance input.

   

   To ensure the circuit resistance does not exceed 10kΩ, it is recommended to place a 10kΩ shunt resistor in parallel with the sensor.

3. Create the Lookup Table Object

   • Enter the calculated input values into a new lookup table object in Sigma.
   • Ensure the table covers the full operating temperature range of the sensor.

4. Assign the Table to the AI Object

   • Link the custom lookup table to the relevant analogue input object in Sigma.

5. Download and Apply

   • Use Sigma System Setup and Diagnostics to build and download the new lookup table to the controller.

6. Verify Operation

   • Check that the AI object displays correct temperature values.
   • Use programmable objects if additional formatting or scaling is needed.

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Examples

• Type 2 Resistances.pdf
• Type 3 Resistances.pdf

• Lookup Table Details

  • Column C: Raw sensor resistance
  • Column D: Controller input resistance with 10kΩ shunt resistor
  • Columns E & F: Example lookup table values (percentage of 2V signal)