Reset mode, controllers

The pressure controller (controller block) amplifies the transmitter signal and sends a modified signal to the final element. Depending on the system requirements, the controller block may include additional correction factors, integral and derivative (reset and rate). This is called a three-mode controller. 

The combination of the two control modes is called the proportional plus reset (PI) control mode. It combines the immediate output characteristics of a proportional control mode with the zero residual offset characteristics of the integral mode. 

The integral (I) control mode (sometimes called reset mode, because after a load change it returns the controlled variable to set point and eliminates the offset) generates an output (m) according to the equation ... 

With the exception of derivative action any of these control modes may be used alone in certain applications. Integral and derivative actions are most usually combined with proportional control to give proportional plus integral control (proportional control with automatic reset) proportional plus derivative control or three-mode control, which is proportional plus integral plus derivative. 

Proportional plus Rate Response. To decrease the recovery time, derivative or rale response may be added to a proportional controller. The control valve is moved at a rate proportional to the rate of change of the controlled variable with time, i.e., derivative. Thus, the valve has its greatest movement when the controlled variable is changing fastest and this type of anticipation and response reduces the recovery time of the system (Fig. 9-2Id). The addition of the reset mode, giving three-term control, will eliminate the offset shown in this example. 

Describe the various controller modes, rate modes, reset modes, and proportional bands and how each complements the other. 

The reset (or integral) mode is designed to reduce the difference between the setpoint and the process variable by adjusting the controller output continuously until the offset is eliminated. The reset mode responds proportionally to the size of the error, the length of time that it lasts, and the integral gain setting. 

Measure the period of oscillation t and set derivative and reset time both to t /2t. (The optimum value of D and R in Table 4.2 is 0.64Td, which is 2Td/x or To/2it.) For a two-mode controller, set R at t<,/2.4. When adjusting a three-mode pneumatic controller with antiwindup, always keep R>2D to retain proportional stability. 

Although there is no need to use a complementary controller on simple processes, it is nevertheless interesting to speculate on its configuration. If the process is a first-order lag, its complementary controller turns out to be proportional-plus-reset. In fact, pneumatic two-mode controllers are made this way, as shown in Fig. 4.13. 

In general, better performance will be obtained on more difficult applications by using delayed reset,4 as shown in Fig. 4.17. This is obviously a compromise between two-mode control and complementary feedback. 

The degree of improvement will vary with the difficulty of the process. On processes that, are fairly easy to control, improvement over two-mode control may be marginal. In fact, derivative would normally be of more value. But where dead time is dominant, or where derivative cannot be used because of noise level, delayed reset may be of considerable worth. 

Two-mode control combines the speed of response of proportional action with the elimination of offset brought about by automatic reset. The proportional mode is just as valuable in a sampled dead-time loop as it was in one without sampling. In fact, proportional action enables any loop whose dead time is less than the sampling interval to he critically damped. Figure 4.22 shows how this is done. 

Find the optimum combination of proportional and reset for a dead-time process from the information given in Table 1.1. Why is it different from the situation described for the two-mode controller in Table 4.2 ... 

Given a process controlled with a two-mode controller whose proportional band is 200 percent and whose reset time is 10 min, estimate the maximum error developed by a step load change of 5 percent. What would the error be if the same load change were made gradually over an interval of 30 min What would the error be if the load change entered as a sine wave of 5 percent amplitude and 2-hr period ... 

If limit cycling and offset are both unacceptable, a two-mode controller can be added to the loop. This controller would actuate the three-state device which in turn drives the valve, as shown in Fig. 5.12. Reset action will keep driving the output of the controller out of the dead zone until the error is reduced to zero. Only then will the loop reach a steady state. Proportional action is necessary for stability, for without it, the double integration of reset and motor would cause an undamped cycle. The availability of a proportional band adjustment eliminates the need for an adjustable dead zone, since the two effects are similar. 

Recall the specifications which were set, forth at the beginning of the section on dual-mode control. 3 laximum speed has been provided by the on-off controller. The programmed switching critically damps the loop as the set point is approached. Offset is eliminated by reset iu the... 

As the output of the proportional controller drives the small valve to either of its limits, the dead zone of the two-mode controller is exceeded. Then the large valve is moved at a rate determined by the departure of the control signal from the dead zone and by the values of proportional and reset. When the control signal reenters the dead zone, the large valve is held in its last position. The large valve is of linear character istic, because the process gain does not vary with flow, as some gains do. 

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... 

The emergency brake control task implements the safety-critical (SIL-4) functionality of the system. It receives the information about the position and speed estimated by the odometry system, the Standby and Supervision activation signals from the mode control imit, and the reset corrunand from the DMI. When the operational mode control unit sends the Standby activation signal the emergency brake is activated. When the system is set to Supervision mode, the estimated distance and speed are compared to a pre-defined braking-curve that sets a maximum speed for each point in the track. If the maximum speed is exceeded the emergency brake is activated. The brake is only deactivated if a reset command is received from the DMI when the train is stopped. 

Most combustion equipment is not controlled by means of a feedback from flue gas analysis but is preset at the time of commissioning and preferably checked and reset at intervals as part of a planned maintenance schedule. It is difficult to set the burner for optimum efficiency at all firing rates and some compromise is necessary, depending on the control valves used and the control mode (e.g. on/off, fully modulating, etc.). 

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

Because of reset windup, this control mode is not well-suited for processes that are frequently shut down and started up. 

The proportional plus reset control mode is summarized below. 

However, there are some processes that cannot tolerate offset error, yet need good stability. The logical solution is to use a control mode that combines the advantages of proportional, reset, and rate action. This chapter describes the mode identified as proportional plus reset plus rate, commonly called Proportional-Integral-Derivative (PID). 

The term T is the integral or reset time setting of the controller. If the bias (b) is zero, this mode acts as a pure integrator, the output of which reaches the value of the step input during the integral time. The integral mode eliminates the offset of plain proportional control because it continuously looks at... 

In selective and cascade control loops, external feedback is the most-often-applied solution. Here, instead of looking at its own output, which can be blocked, the integral mode of the controller looks at an external feedback signal (such as the opening of the valve), which cannot be blocked. In surge control or reactor heat-up applications, the chosen solution usually is to use the slave measurement as the external reset signal to prevent saturation. 

Part (c) in Figure 2.85 illustrates a triple cascade loop, where a temperature controller is the slave of an analyzer controller while the reflux flow is cascaded to temperature. Because temperature is an indicator of composition at constant pressure, the analyzer controller serves only to correct for variations in feed composition. Cascade loops will work only if the slave is faster than the master, which adjusts its set point. Another important consideration in all cascade systems (not shown in Figure 2.85) is that an external reset is needed to prevent the integral mode in the master from saturating, when that output is blocked from reaching and modulating the set point of the slave (when the slave is switched to local set point). 

The first block DIV is a frequency divider. This block has 2 modes of operation, the normal mode and the test mode. In the test mode, the UART chip runs 16 times faster than in the normal mode. Also, the transmission data rate of the UART chip is 16 times faster than the receiving rate. Each block is initialized by setting the reset line low by applying a 0 to port MR. The TX block accepts 8-bit parallel data from the microprocessor interface (MP) block and transmits it serially to the RS-232 port through port DOUT. Conversely, the RX block receives serial data input, and sends it in 8-bit parallel format to the MP block. Again, the transmitter runs at 16 times the speed of the receiver. The microprocessor interface (MP) block asynchronously controls the parallel data flow between the RX / TX blocks and the microprocessor data bus. 

The Integral mode is sometimes referred to as reset because it continues to take action over time until the error between measurement and setpoint is eliminated. The parameter to specify this action is Integral time, which can be thought of as the length of time for the controller to repeat the initial proportional response if the error remained constant. Note that as this parameter is made smaller, the reset increases as the control action is repeated in a shorter period of time. Some controllers use an alternate parameter, Reset, that is the reciprocal of Integral time and is referred to as repeats/unit time. This latter approach is perhaps more intuitive in that as the Reset parameter is increased, there is more reset action being applied. 

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