TG - TC - Difference Between At-1 (40% AT) and At-2 (100% AT) Auto-Tuning

Purpose

This guide explains the difference between At-1 (40% AT) and At-2 (100% AT) auto-tuning types available in Omron digital temperature controllers. Understanding these modes helps users select the most appropriate tuning method for their application.

Supported Devices

This functionality applies to E5@C Series Digital Temperature Controllers (e.g., E5CC, E5EC, E5AC) and other Omron models that support selectable 40% or 100% autotuning.

Note: Some models (such as E5CS, E5CSV, and E5CB) support only basic ON/OFF auto-tuning and do not allow selection between At-1 and At-2.

Requirements

Before performing auto-tuning:

• The controller is correctly installed and wired.
• The controller must be powered ON and in RUN mode.
• Control must be set to PID mode (not ON/OFF).
• The process (e.g., heater and controlled load) must be powered and stable.
• Access to the controller’s AT Execute/Cancel parameter via the keypad or communication interface.
• Be aware that during AT execution, only a limited set of parameters (RUN/STOP, AT Execute/Cancel, Program Start, and Communications Writing) can be changed.
• Basic understanding of how PID control affects system behavior.

Procedure

Step 1: Understanding At-1 and At-2 Auto-Tuning

When you execute auto-tuning (AT), the controller automatically determines the optimal PID constants (P, I, D) for stable control. This is done using the limit cycle method, in which the manipulated variable (MV) is deliberately varied to measure the process response.

The key difference between At-1 and At-2 lies in how much the MV is varied during tuning and the resulting trade-off between stability and tuning speed.

At-1 (40% AT) – Gentle Tuning

The controller varies the manipulated variable (MV) by about 40% of its full range. This smaller amplitude results in smaller temperature fluctuations during tuning.

Because the changes are gentler, tuning generally takes longer. The duration may also depend on the initial process deviation (difference between the set point and actual value).

Best suited when minimizing disturbances is important, such as when controlling processes with sensitive materials, where large swings in temperature could be harmful or inefficient.

At-2 (100% AT) – Fast Tuning

The controller varies the manipulated variable (MV) by 100% of its full range. This produces larger temperature fluctuations during tuning.

Because the variation is larger and the process responds more quickly, tuning usually completes in less time compared to At-1. Unlike At-1, the amplitude cannot be adjusted; it is always 100%.

Recommended when shorter tuning time is a priority, and process fluctuations during tuning are acceptable, for example, in robust systems or during initial equipment setup.

Key Differences: 

At-1 (40% AT): Longer tuning time, smaller fluctuations, safer for sensitive processes.

At-2 (100% AT): Shorter tuning time, larger fluctuations, suitable for stable systems where disturbances during tuning are acceptable.

Step 2: Select the Desired Auto-Tuning Mode

Enter the Adjustment Level on the controller.

Locate the parameter “AT Execute/Cancel” (At).

Choose one of the following:

At-1 (40% AT) – Executes auto-tuning with a 40% manipulated variable (MV) amplitude.

At-2 (100% AT) – Executes auto-tuning with a 100% MV amplitude.

The TUNE indicator on the controller lights up during AT execution.

Allow the process to complete. When finished, the setting will return to Off automatically, and PID constants (P, I, D) will be updated.

Note: The actual time required for tuning depends on system conditions

Date/Revision History

First review 9/15/2025 V1.0

Author: 

Aimee Rivera, Technical Support Engineer

References

E5CC - User's Manual