TABLE OF CONTENTS As a result of studying this chapter, and after having completed the relevant exercises, the student should be able to:
Demonstrate a clear understanding of controllers with multiple and independent outputs
Clearly distinguish between saturation and non-saturation output limits
Describe the concept and strategy of cascade control
Select, and apply correctly, the controller options of initialization, PV-tracking and type of control equation
Describe the concept of cascade control with multiple secondaries
Demonstrate how to tune all controllers within a cascade control system.
In order to enable controllers to be cascaded together certain system design requirements have to be made available. The most important one centers on the output section of a controller, in particular the primary one. Figure 9.1 shows a typical output section, or block, of a PID controller, illustrating the control signals and actions they perform upon the final output value. The functions of each of these will be discussed in this chapter.
The value of the final output of a single or stand-alone controller is affected by one of the two possible signals:
The first one is derived from the MANUAL mode, where a set or static value can manually be placed in the output, this value being considered a live zero The controller itself has no knowledge of what this value is, and it can be anywhere for 0 to 100% of the output range.
The second one is when the controller is in AUTO...
