Overshoot

At finite positive and negative charge densities on the electrode, the counterion density profiles often exhibit significantly higher maxima, i.e. there is an overshoot, and the derived potential actually shows oscillations itself close to the electrode surface at concentrations above about 1 M. 

In the following sections examples of the application of this procedure to the analysis of specific phenomena such as wall slip and stress overshoot which affect polymeric flow processes are illustrated. 

Prediction of stress overshoot in the contracting sections of a symmetric flow domain... 

Figure 5.13 The predieted pattern of the stress overshoot in example 5.3.1...

Modem process facHities are computer controHed. Temperature and AT are programmed upward during the start of a growth cycle, pressure and temperature are monitored and controHed, and pressure and temperature overshoot alarms and overrides are provided. Such systems also store data from previous mns for correlations with properties or for identical repHcation of past conditions. 

Foxboro developed a self-tuning PID controller that is based on a so-called expert system approach for adjustment of the controller parameters. The on-line tuning of K, Xi, and Xo is based on the closed-loop transient response to a step change in set point. By evaluating the salient characteristics of the response (e.g., the decay ratio, overshoot, and closed-loop period), the controller parameters can be updated without actually finding a new process model. The details of the algorithm, however, are proprietary... 

The speed control operates the governing valve to maintain steam flow commensurate with load demand while holding speed essentially constant. For sudden load changes there will be a short-time overshoot, and a special case is the instantaneous loss of load, load dump at full load. The usual specification states that the overshoot on load dump must not exceed 9 to 10 percent of rated speed. The settled speed rise will of course be equal to the regulation, 4 or 6 percent for a NEMA Class C or B governor and less than I percent for Class D. 

The scheme is termed a closed transient switching. A comparison of the two methods in terms of voltage transients and current overshoots is given in Table 4.1. In an AIT starter The same logic can be applied as discussed above. The star point of the AIT is opened and connected through the main contactor C3 to provide a near replica to a Y A switching. Figure 4.7 illustrates the revised scheme. 

Current overshoot during changeover from Y to A, point, a, on Y current curve (Figure 4.. ))... 

The current curve becomes abed (Figure 4.5) and current overshoots from on to ob momentarily, which may exceed 14-20/ ... 

The current curve becomes ah h h c d (Figure 4.5). There is no current overshoot beyond the normal current in A... 

The upper limit of the starling torque chosen at 180% appears to be too low and can be raised to, say, 210%. This will also help in reducing the number of steps. Number of steps at 4-6 are preferable to economize on the cost of switchgears, and yet provide a reasonably smooth (free from overshoots) starling torque. 

The higher Ihe value of C. the lower will be the voltage overshoots in the rectified voltage and the inverter circuit would be fed by an almost cotistant voltage soitrce. The capacitor iti the circuit also provides an indirect protection from the voltage surges. 

Instead of a charging capacitor C. a large size series inductor L is introduced in the d.c, link (Figure 6.29).. Since V = L di/d/. the larger the value of L, the lower will be the current overshoots (d/7d/) and a near-constant d.c. link current source is obtained for the inverter unit. 

The object may just reach its original position. By the time it does so, it loses all its restoring force due to damping and does not overshoot. Sueh systems do not oscillate. For critically damped systems... 

In the event of an expander trip, the regenerator pressure decreases by 46 mbar, and then increases to overshoot the steady-state value by 16 mbar. This constitutes a substantial improvement over the previously described process, but still did not meet the specifications of the end-user. 

The turbine temperature, flow, and speed increases in a very short time of about three to five minutes to the full rated parameters. There is usually a short period of time where the temperature may overshoot. If supplementary firing or steam injection for power augmentation is part of the plant system, these should be turned on only after the gas turbine has reached full flow. The injection of steam for power augmentation, if done before full load, could cause the gas turbine compressor to surge. 

This is a mode of control that anticipates when a process variable will reach its desired control point by sensing its rate of change. This allows a control change to take place before the process variable overshoots the desired control point. You might say that derivative control gives you a little kick ahead. 

P.B. = proportional band setting overshoot = amount - -l- of setpoint, °F span = transmitter span, °F... 

Another unorthodox application is to form a U-F foam on airport runways to act as an arrester bed to stop aircraft that overshoot during emergency landings or abortive take-offs. 

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