Cascade Control

This folder contains the source files for the demonstration project on cascade control. Cascade control is used when measurement of process variable can be supported by the measurement of additional process variable with lower dead-time. For industrial application, the integral term wind up of PIDU_P and bumpless switching between MAN/AUT modes of PIDU_S must be handled.

The whole cascade control structure is stored in the CASC subsystem. The models of processes are in the cascade_process task.

When CNB_CAS = 1 the primary (outer) PIDU_P controller generates setpoint for the secondary (inner) controller PIDU_S. Both controllers work in automatic mode and the cascade structure is ON. When switching to manual (MAN) mode both blocks PARR_hv and PARR_sp are switched to local tracking mode, hence the output of secondary controller PIDU_S will not change. In manual mode, the hand value can be changed as a local parameter of PARR_hv block. Moreover, the setpoint for primary controller is reset to pv by PARR_sp block. The internal state of PIDU_S is tracked to its mv. The internal state of PIDU_P is tracked to additional (secondary) process variable. This allows the bumpless switching of both controllers back to automatic mode. When switching CAS = OFF, the outer loop is bumpless opened and the inner control loop can be tuned. In such case the setpoint is changed in PARR_sp_S block. The state of primary controller PIDU_P is tracked to the secondary controller setpoint. This allows the bumpless closing of outer control loop.

Timing of the project

The algorithm runs each 100 milliseconds (0.1 s). See the EXEC function block,
tick x ntick0 = 0.05 x 2 = 0.1

Prerequisites

• REXYGEN Runtime Core must be installed and running on the target device.

Running the example

• The *exec.mdl file is the project main file.
• Open it with REXYGEN Studio, compile and download it to the target device.
• Switch to Watch mode and observe the algorithm.
• Open trend diagnostic window for the block TRND_CASC.
• In REXYGEN Studio, select the CASC subsystem and enter it (Ctrl+U).
• Turn on the CNB_MAN block and observe the bumpless switching between MAN/AUT modes.
• In MAN mode, change the hand value in PARR_hv block. The internal state of PIDU_P will be tracked to additional (secondary) process variable.
• Switch both controllers back to automatic mode (CNB_MAN = off).
• Switch the CNB_CAS off. The outer loop is bumplessly opened now and the inner control loop can be tuned. In such case the setpoint is changed in PARR_sp_S block.
• The state of primary controller PIDU_P is tracked to the secondary controller setpoint and the outer control loop can be bumplessly closed again by CNB_CAS = on.

Documentation

• Press F1 for help on the selected function block in the REXYGEN Studio.
• PIDU function block documentation
• PARR function block documentation
• Function blocks of REXYGEN
• REXYGEN Studio User Guide
• Complete documentation of REXYGEN

Additional information

• Visit the REXYGEN webpage for more information about the example projects and developing advanced automation and control solutions using REXYGEN.