Proportional-plus-reset controller

The main limitation of the pilot-operated regulator is stability. When the gain in the pilot amplifier is raised too much, the loop can become unstable and oscillate or hunt. The two-path pilot regulator (see b) is also available. This regulator combines the effects of self-operated and the pilot-operated styles and mathematically produces the equivalent of proportional plus reset control of the process pressure. 

Figure 22 Response of Proportional Plus Reset Control. 33...

Proportional plus reset control is a combination of the proportional and integral control modes. 

EO 1.4 DESCRIBE the characteristics of the following types of automatic control systems d. Proportional plus reset control system... 

Let s once more refer to our heat exchanger example (see Figure 23). This time we will apply a proportional plus reset controller to the process system. 

The proportional action of the proportional plus reset controller, if acting alone, would respond to the disturbance and reposition the control valve to a position that would return the hot water out to a new control point, as illustrated by the response curves. You ll note that a residual error would still exist. 

Proportional plus reset controllers act to eliminate the offset error found in proportional control by continuing to change the output after the proportional action is completed and by returning the controlled variable to the setpoint. 

The proportional plus reset control mode is summarized below. 

Proportional plus reset control eliminates any offset error that would occur with proportional control only. 

Reset windup is an inherent disadvantage of proportional plus reset controllers that are subject to large error signals. 

For processes that can operate with continuous cycling, the relatively inexpensive two position controller is adequate. For processes that cannot tolerate continuous cycling, a proportional controller is often employed. For processes that can tolerate neither continuous cycling nor offset error, a proportional plus reset controller can be used. For processes that need improved stability and can tolerate an offset error, a proportional plus rate controller is employed. 

Most commonly it is known as proportional-plus-reset controller. Its actuating signal is related to the error by the equation... 

FZG 1.12. A plot of gain vs. to for the proportional-plus-reset controller shows the contributions of the components. 

The gain curve for the proportional-plus-reset controller (Fig. 1.12) can be roughly approximated by the asymptotes ... 

A plot of Gi vs. To In Fig. 1.18 shows a curve which is complem enta to that of a proportional-plus-reset controller. 

The same process is to be controlled with a proportional-plus-reset controller, adjusted for a reset phase lag of 60°, Calculate the settings required for °/o-amplitude damping, and check your answer against Table l. l. 

A certain process consists of a 1-min dead time and a 30-min lag. Estimate the period and settings for J-i-amplitude damping under proportional-plus-derivative control. Repeat for a proportional-plus-reset controller, assuming 45 phase lag in the controller. 

A volume booster installed at the inlet to the valve motor of Example 3.2 reduces its time constant to 0.5 sec. Predict the period of oscillation that will result from the change, allowing 45 phase lag in the proportional-plus-reset controller. Calculate the proportional band and reset time for i. -amplitude damping. 

The choice of integrated error as a performance index has a very prae-tical aspect, in that it can be readily calculated from controller settings. In a proportional-plus-reset controller. 

FIG 4.13. A proportional-plus-reset controller is the complement of a single-capacity process. 

A proportional-plus-reset controller applied to the same process, and adjusted to produce 22.5 phase lag, can serve as a reference for compaii son. The values of reset time and proportional band required for )- 4-ainplitude damping were calculated for selected ratios of trated error per unit load chan was then found as the PR product, to compare with that obtainable through complementary feedback. This information is plotted in Fig. 4.16, with coordinates... 

Again, half this value leaves the loop undamped. Critical damping was obtained with a conventional proportional-plus-reset controller whose settings were related to both dead time and sample interval. But with the sampling controller, reset alone is required, and dead time can have any value less than At — At, without affecting the closed loop. 

Loops containing two integrations are capable of a limit cycle, however. An example would be a non-self-regulating process such as liquid level, with a proportional-plus-reset controller. The gain product of the two integrating elements will vary as the square of the period, more than can be offset by the gain of hysteresis. Under these conditions, loop gain varies inversely with amplitude. 

Consider a loop consisting of a dead-time plus integrating process of time constant t, hysteresis, and a proportional-plus-reset controller. Let the reset time be set for 30 phase lag and the proportional band for ( -amplitude damping at A/H of 2. Table 5.2 summarizes the effect of hysteresis. 

The forces affecting the motion of a valve stem are principally friction and pressure drop. It has been pointed out that friction is the cause of hysteresis. And hysteresis can cause limit cycling in the presence of two integrating elements, such as a liquid-level loop with proportional-plus-reset control. But a positioner is stable in the presence of hysteresis and will succeed in eliminating from the primary loop that, source of phase shift. 

The obvious solution is to add a second integral mode. But double integral by itself is unstable, in that it produces 180 phase lag at all periods. But if the first integral, i.e., the volume error, is acted upon by a proportional-plus-reset controller, the system can be stable. Such an arrangement is functionally described in Fig. 6.13. 

A control system has been developed which incorporates valve sequencing for wide range along with compensation for the nonlinear curve. It features a small equal-percentage valve driven by a proportional pH controller. The output of the pH controller also operates a large linear valve through a proportional-plus-reset controller with a dead zone. The system is shown in Fig. 10.15. 

Vector diagram, for first-order lag, 22 for proportional-plus-reset control, 16 for three-mode controller, 99 Velocity limit, 65 Volatility, relative, 291 Volume booster, 67... 

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