Controller output modes, operating equations, cascade control--part 1

Goals

• 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.

Controller output

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. +=+=+=+ 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 section.

Single or stand-alone controller output:

The value of the final output of a single or stand-alone controller is affected by one of the two possible signals:

1. 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.

2. The second one is when the controller is in AUTO mode and the PID actions now start to increment or decrement the MANUAL value in each scan time of the system.

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SP Auto /manual (1) status control ERR OP1, OP1 Auto /manual (2) control status OP1 tracks OP2 if ' both' are in auto (being incremented/ decremented by CV Dual output controller CV = Controller value (PID O/P) PV PID CV S

+=+=+=+ Dual output controller

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Multiple controller outputs

A close inspection shows that this controller can have two or more output blocks, all identical, but totally isolated from each other. If we consider a controller with two (or more) output blocks, which result in final output signals OP1, OP2 to OPN there exists many permutations of possible actions that this type configuration can perform, the most important being listed below: 9.3.1 Multiple controller output configurations

As each output block is independent of all the others that are attached to a controller, their absolute output values can be, and usually are, different from each other. Although the PID controller's action is continually reacting to the SP and PV values on its input, the results of the PID calculations will only affect, (+ /0 /-), a particular output if the mode of that output is set to auto or cascade.

+=+=+=+ the output control strategy of a single or multi-output controller.

Assume initially, the Output of the Controller-1 (OP1) is set to manual mode or initialized and the Controller output (OP1) is cascaded or connected to another controller; and the Controller-2 is in auto mode. The Output of controller-2 (OP2) will be responding to the PID change requirements, but the Controller-1 Output (OP1) will be static at its manual value or initialized value. Only when Controller-1 (OP1) is set to AUTO mode or CASCADE mode, by either the MANUAL or INITIALIZE control signals changing will OP1 (Output of Controller-1) then starts to respond to the PID commands.

The OP1 value will then 'Track' the value of OP2, although they may well have different absolute values. If the PID summing result says 'Increment' by a value of PN in one scan time, then OP1 will increase its value from OP1N to OP1N + PN and OP2 will increase its output value by OP2N to OP2N + PN, i.e. both outputs will change by the same magnitude, but maintain the differential value between them.

Limits of controller outputs:

The controller itself has no knowledge of its final and absolute output value(s) from its output blocks. The result is that these outputs can be driven into saturation at 0% or 100% with the controller still trying to 'drive' them further below zero or above full scale.

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Manual initialize; Open in cascade (SP value) from secondary controller when being initialized; Output(1) Lo limits; Output block 2; Output N; Controller mode(s); Controller manual input; Man OP value register CV S Hi limits P I D Auto

+=+=+=+ 2 Output control, block control and interconnections

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As we may well not know or be aware of this happening, and how and when and with what accuracy the outputs recover back into their operational range, we must be able to 'select' our requirements for the type of output limit calculations we need and why.

Saturation and non-saturation of output limits

There are two principle types of output limit calculations, the first, with output limits, allows saturation of the output based on P and D-control. The second does not allow output saturation to occur under any circumstances.

Saturation of the output

If the output of a controller is allowed to saturate, it allows it in the following manner:

• The controller calculates a VIRTUAL output value independent of any output limit. These may be values far above 100% or far below 0%.

• Only the real output, which is the displayed output value, is limited by pre defined output-limits.

• The real output then awaits the return of the virtual output to within the defined output-limits.

• Then, within the range of the output-limits, the real output follows the virtual output value exactly.

• Controllers driving field output normally use this kind of output-limit handling.

Non-saturation output limit calculation:

Non-saturation of the output is achieved by ensuring that only the real output values are used for the calculation. If a single calculation results in an output value attempting to go beyond the pre-set output-limits, the output value will be set to the value of the output limit it would have violated.

When the controller calculates the output value next time (in the next scan), the real output value (output = output limit) is used. The previous calculation, beyond output limits, has been totally forgotten.

NEXT: part 2
PREV: PID equations: dependent and independent gains

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