PID Loop - Metasys - LIT-12012005 - Field Device - N2 Controller - 13.0

Modernization Guide for N2 Controllers - Metasys

Product name
N2 Controller
Document type
User Guide
Document number
Revision date

When replacing a DCM, the PID Loop calculations can be duplicated easily with a PID Control in CCT. The DCM101 contains 16 PID Loops while the DCM140 holds 20. Each PID Loop object can accept up to six AI, and issue commands to a maximum of eight AOs, sent to Analog Output Digital (AOD) objects or other PID Loops.

Output Processing: In addition to providing closed loop control, the DCM's PID Loop allows for the execution of multiple commands on a priority basis by the stages in the following figure. Additional logic may be required in CCT to duplicate the DCM PIDL Output Processing functionality. When there are not enough switches or selectors in a single PIDL object, you can add another PIDL object to allow for more, which is a special configuration known as the Pass-through PIDL.

Figure 1. Output Processing

Auxiliary Signal Switch: The Auxiliary Signal Switch provides the option of overriding the PID calculation with an alternate signal. The Auxiliary Signal Switch outputs one of two inputs (PID Calc Output and the Aux Sw Input) based on the value of the Aux Signal Enable attribute.

The Output Filter is a simple exponential filter that has the effect of altering the rate of change of the output. This effect filters out short duration variations in the output of the PID algorithm.

Selector provides the option of establishing a minimum or maximum control signal by overriding the PID calculation or the Aux Switch Input Value. When enabled, the selector function outputs either the higher value or the lower value of two inputs.

The Reliability Switch can override the Selector Output when this signal is unreliable. The Reliable attribute is set to No if an error occurs in the PID calculation, or if a reference object providing inputs to any of the PIDL ports becomes unreliable.