Industrial controllers , basic theory  

CASCADE CONTROL


 


2.1  GENERAL

A cascade control system is a multiple-loop system where the primary variable is controlled by adjusting the setpoint of a related secondary variable controller. The secondary variable then affects the primary variable through the process.

The primary objective in cascade control is to divide an otherwise difficult to control process into two portions, whereby a secondary control loop is formed around a major disturbances thus leaving only minor disturbances to be controlled by the primary controller.

The use of cascade control is described in many texts on process control applications.

The advantages of cascade control are all somewhat interrelated . They include:

  1. Better control of the primary variable
  2. Primary variable less affected by disturbances
  3. Faster recovery from disturbances
  4. Increase the natural frequency of the system
  5. Reduce the effective magnitude of a time-lag
  6. Improve dynamic performance
  7. Provide limits on the secondary variable

Cascade control is most advantageous on applications where the secondary closed loop can include the major disturbance and second order lag and the major lag is included in only the primary loop. The secondary loop should be established in an area where the major disturbance occurs. It is also important that the secondary variable respond to the disturbance. If the slave loop is controlling flow and the disturbance is in the heat content of the fluid, obviously the flow controller will not correct for this disturbance.

There is only one master controller and usually only one slave controller and only one manipulated variable. See Figure 1 (a), (b), and (c). However, some applications can benefit from the use of more than one slave controller. There will be a separate secondary variable and manipulated variable associated with each slave controller in the system if the slave loops are in parallel as shown in Figure 1 (d). Another configuration, shown in Figure 1(e), is the operating of a cascade system with two slave loops in series.

Figure 1. Cascade control systems .

 

          


 

   

The distinguishing feature of a cascade control system is that the output of the master controller adjusts the setpoint of a slave controller as shown in Figure 2. Figure 3 shows the two slave controllers in parallel and Figure 4 shows the two slave controllers in series.

 

FEEDFORWARD CONTROL

2.1 GENERAL 

Feedforward control is a strategy used to compensate a for disturbances in a system before they affect the controlled variable . A feedforward control system measures a disturbance variable, predicts its effect on the process, and applies corrective action, as shown in Figure 1.

Without the feedforward controller the manipulated variable has a value which is related to the uncontrolled variable to produce the desired value of the controlled variable. In addition-to this there is a disturbance variable which is either a part of the uncontrolled variable or from some other source that enters the system.

The effect of this disturbance on the controlled variable is measured or calculated. The effect the manipulated variable has on the controlled variable must also be measured or calculated. These measurements and/or calculations must include steady state effects as well as dynamic effects (time related) and non-linearities. Feedforward controller FFC (Figure 1) will receive the disturbance variable as an input and must provide an output that results in an equal and opposite effect so as to maintain the controlled variable at a constant value.

The output of feedforward controller FFC includes a bias component required to match the uncontrolled variable as well as a "fudge factor" component that changes in response to the disturbance variable. The "fudge factor" must be the right amount and at the right time.

Given an exact model of the process, the feedforward controller will adjust the manipulated variable so that the controlled variable is unaffected by the disturbance. In fact, the controlled variable has no influence over the control; corrective action is totally in response to the disturbance.

Each disturbance must be treated in the same way if the controlled variable is to be held constant. Feedforward controller FFC may accommodate more than one input.

Feedforward con roller FFC may be a simple relay device having a 4-20-mA input from the disturbance variable transmitter and providing a 4-20mA output for operating a valve in the manipulated variable line. It would be unusual to find that the input and output relationship for feedforward controller FFC had to be one to one and linear to compensate for the disturbance. A gain and bias adjustment is always required to match the manipulated variable to the uncontrolled and disturbance variables. Additionally, it may require lead-lag elements, linearizers, non-linearizers, and a summer.

However, this system has three major drawbacks:

  • The model must be exact (including dynamics and nonlinearities).
  • All instruments in the loop must be perfectly calibrated.
  • Disturbances other than the feedforward variable are not controlled.

Thus, feedforward by itself is insufficient control. However, combined with conventional feedback, it can be a powerful control tool. If a load change in a process occurs so frequently that the controller cannot keep up, or if disturbance is so large that the controlled variable cannot be held within tolerable limits, and if the disturbance variable itself cannot be controlled, consider adding feedforward control to the system.

The system of Figure 2 can be viewed as separate feedforward and feedback control independently adjusting the valve.

* The feedback controller does the same job and has the same responses and settings as if it were acting alone. It just doesn't have as much work to do.

 

* The feedforward control cancels the effect of the measured disturbance. Since feedback acts as the system's watchdog, the process model need not be exact. In fact, simple gain and lead-lag elements will usually suffice.

The effect of load changes other than the measured disturbance will be corrected by the feedback system.

Feedforward and cascade control systems are often confused because of their similarities: two measured variables, one manipulated variable, one independent set-point . But cascade systems control both measured. variables, with the master determining the set-point of the slave. In contrast, feedforward and feedback corrections independently adjust the control valve, and there is no control applied to the feedforward variable.

Since feedforward control when used is almost always used with feedback control the Taylor 170OR Controller has a feedforward input port which allows the feedforward signal to be combined with the controller output. Figure 3 shows this portion of the controller block diagram.

If the feedforward control requires only gain, bias or linearization (characterization) it can all be-accomplished in the 1700R Controller used for feedback control. The feedforward controller function FFC shown in Figure 2 , is all inside the feedback controller FBC, which makes for a simple installation.. The disturbance signal is connected to an analog input which becomes the source for the linearizer which is configured for the required gain, bias and/or characterization. This linearizer channel is the feedforward variable source.

When the feedforward calculation is more complicated, also requiring lead-lag and additional disturbance inputs, the whole calculation can be accomplished in a  Taylor 1700N Math Unit which may be connected. to the feedforward input of the 170OR Controller.

There are two choices on how the feedforward compensation signal combines with the feedback controller output: Add and Multiply.

 

 

Add means the feedforward signal is added to the controller algorithm output on a one for one basis. See Figure 4. If it is necessary to subtract, the feedforward input action should be reversed. The feedforward input port will not accept a signal with reverse polarity.

Multiply means that the final output follows the equation:

where FF% is the feedforward input in percent and FB% is the output of the controller algorithm in percent. The multiplying factor changes over a range of 0 to 2, with 1 at midrange. It does not actually go to 0 since it has low cutoff at 0.01 See Figure 5.

This equation can be interpreted as the feedforward input being able to control the gain between the controller algorithm output and the final output over a range of 0 to 2 or as the controller algorithm output being able to control the gain between the feedforward input and the final output over a range of 0 to 2.

 

The choice between Add or Multiply depends on what is required in the. control process This is determined by making an analysis of how the manipulated variable must be adjusted in response to the disturbance variable and what effect this adjustment has on the feedback control loop gain

In general, if a change in the disturbance variable has no effect on the loop gain, the feedforward calculation should be Add. If a change in the disturbance variable does effect the loop gain the feedforward calculation should be Multiply. However, the adaptive gain capability with the 1700R controller provides another means for loop gain correction.

When the feedforward input signal joins the feedback controller output at the location shown in Figure 3, output limiting, output tracking capability and bumpless transfer from any allowable mode to feedforward or feedforward-feedback applies to the combined output.

To summarize, feedback control is the first choice and most commonly used type of control, but when the controller must operate with low values of gain and reset for optimum settings, disturbances can cause large upsets and the system may take a long time to recover.

Use feedforward control to accommodate the major disturbance; but it will be successful only if the major disturbance can be sensed and a correction made quickly in the manipulated variable.

Feedforward control is most successful in applications where the feedback control loop is slow responding and the feedforward path is fast responding.


Many industries utilize the cascade control system, like manufacturing industry and auto industry. If you want to find out more, all it takes is a little online industry research to see if industries like the pharmaceutical industry use the same types of controls and mechanisms as manufacturing plants.


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