Set-Point Response

Fig. 2. Unit step set point response for IPDT system (proposed continuous line, Luyben [10] dotted line).

FIG. 8-23 Both load regulation and set-point response require high gains for the feedback controller... 

FIG. 8-29 Tuning proportional and integral settings to optimize set-point response degrades load response using a separate set-point gain adjustment allows both responses to be optimized. 

The tuning settings based on the process reaction curves obtained by the open-loop tuning method, in addition to the Ziegler-Nichols method (Table 2.38), can also be selected by other methods. Figure 2.39 compares the load responses and Figure 2.40 compares the set point responses of these methods. 

Set-point responses of the different turning techniques (example). 

The Redwood Mass Flow Controllers (MFCs) were able to control mass flow, but with some limitations. In particular, great care had to be exercised when calibrating the MFCs since the calibration curve sometimes exhibited an inflection point in the flow rate versus set point response. A few MFCs did have linear calibration curves, but many exhibited this nonlinearity to varying extents. This was probably due to the flow measurement and control techniques of these MFCs, which operated by controlling the pressure drop across an orifice. Discussions with Redwood staff indicated that adjusting the internal flow control and measmement parameters for these controllers required a considerable amoimt of work. 

Set-point control (see Servo problem) Set-point response with feedback controller, 260, 267, 271, 273, 276... 

Figure 1.19 illustrates the set-point response of a single-eapaeity proeess to zero proportional band. As soon as the set point is ehanged, the valve will open wide, dehvering maximum inflow. The level will rise as rapidly as possible, which is a function of both k and the present value of level. If no control were provided, the measurement would follow the projected path. But when the new set point is reached, the inflow will be reduced instantaneously to a value equal to the outflow. This assumes that all elements in the loop, excepting the tank, are capable of instantaneous response. If this is not so, the process is not single-capacity. 

Let us study the effect of zero proportional band on this process. The set-point response is given in Fig. 1.21. When the measurement is below the set point, the fill valve will be wide open, delivering flow F. If the load (outflow) is 50 percent of F, the rate of rise of level will be... 

In the foregoing analysis of sampled systems, discussion was centered around set-point response, with the set point being introduced an instant prior to sampling. In this way, the eontroller saw the error as soon as it occurred. 

Further simplification can be made if proportional action is applied only to the measurement, although set-point response suffers somewhat ... 

This algorithm is a poor choice where set -point response is important, as in a cascade system (see Chap, 6). 

This is one control function whose exact settings can be determined numerically for a process with three dynamic elements, But however difficult the process, settings for the switching parameters can be found which will provide absolute optimum set-point response using this system. 

The dual-mode system gives the best set-point response attainable. 

As with other nonhnear controllers, set-point response exceeds what is obtainable with Hnear modes. This is because set-point changes are... 

FIG 5.21. A three-mode nonlinear controller esdiibits better set-point response bat poorer load response than its linear counterpart. 

Equation (8.8) shows that the set-point response is delayed by r, and that a load change will induce a transient of duration Tj — Tm and magnitude r dq/q. Both responses appear in Fig. 8.7. 

FIG 8.16. If set-point response is important, the set point should go directly into the forward loop. 

It has thus been shown that the dual algorithm can handle both set-point and load disturbances satisfactorily without having to adjiist the parameters. It is markedly better than a discrete PID algorithm tuned for set-point response. 

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