Reset time

Detuning a controller (e.g., using a smaller controller gain or a larger reset time) tends to reduce control loop interactions by sacrificing the performance for the detuned loops. This approach may be acceptable if some of the controlled variables are faster or less important than others. 

The reset time, which is nser-adjnstable, can range from 0.05 seconds to 80 minutes or more, depending on controller design. The reset time constant, when converted to frequency 1/2(Tr) Hz (where Tr is the reset time in seconds), determines the frequency where the reset and proportional response characteristics of the controller merge (see Fig. 8-64Z ). Tuning the reset adjustment on the controller moves the reset frequency to the left or right along the frequency axis and thereby affec ts the reset ac tion of the controller. 

Q Residual of phase-equilibrium X Feedback-reset time s t... 

Eodt (13-174) where V and are initial values, Kc and T are respectively feed-back-controller gain and feedback-reset time for integr action, and E is the error or deviation from the set point as given by ... 

Controllers can be adjusted by changing the values of gain Kp, reset time Xi and derivative time Td- The controller can be set by trial and error by experimenting, either on the real system or by simulation. Each time a disturbance is made the response is noted. The following procedure may be used to test the control with small set point or load changes ... 

Roughly, the reset time is the time that it takes the controller to repeat the proportional action. This is easy to see if we take the error to be a constant in the integral. 

PI controllers are most common. They eliminate offsets and have acceptable speeds of response in most industrial settings. We usually pick a low to intermediate gain (wide proportional band, PB 150) to reduce the effect of noisy signals (from flow turbulence also why we do not use D control). We also use a low reset time ( 0.1 min/repeat i.e. relatively large I action) to get fast set-point tracking. 

K, = fe back controller gain (dimensionless) r, = feedback controller integral time constant or reset time (minutes)... 

A- FLOW LOOPS. PI controllers are used in most flow loops. A wide proportional band setting (PB — ISO) or low gain is used to reduce the effect of the noisy flow signal due to flow turbulence. A low value of integral or reset time (t, - 0.1 minute per repeat) is used to get fast, snappy setpoint tracking. 

The dynamics of the process are usually very fast. The sensor sees the change in flow almost immediately. The control valve dynamics are the slowest element in the loop. So a small reset time can be used. 

Tray 4 temperature on the Lehigh distillation column i controlled by a pneumatic Pf controller with a 2-mipute reset time and a 50 percent proportional band. Temperature controller output (COr) adjusts the Ktpoint of a steam flow controller (reset time 0.1 min and proportional band 100 percent). Column base level is controlled by a pneumatic proportional-only controller setting bottoms product withdrawal rate. 

JO. A process has an openloop transfer function that is a first-order lag with a time constant and a steadystate gain K, . If a PI feedback controller is used with a reset time r, sketch root locus plots for the cases where ... 

Thus we have found that the appropriate structure for the controller is PI, and we have solved analytically for the gain and reset time in terns of the parameters of the process model and the desired closedloop response. 

J9. A process has a transfer function = 1/Wr,s + 1)]- It is controlled by a PI controller with reset time t,. Sketch the G B plot of phase angle versus frequency, the Nyquist plot of and a toot locus plot for the case where (< )... 

If a PI controller is used, find the smallest value of the ratio of the reset time T/ to, the process time constant for which a maximum closedloop log modulus of + 2 dedbds is attainable. 

One of the major questions in multivariable control is how to tune controllers in a diagonal multiloop SISO system. If PI controllers are used, there are 2N tuning parameters to be selected. The gains and reset times must be specified so that the overall system is stable and gives acceptable load responses. Once a consistent and rational tuning procedure is available, the pairing problem can be attacked. 

Then all feedback controller reset times T/,- are calculated by multiplying the Ziegler-Nichols reset times t Ni W same factor F. 

There are three loci (when t, r,). They begin at z = 0, z = -I-1 and z = b. One path ends at the zero of. The root loci for several values of r, are shown in Fig. 19.6. Decreasing the reset time t, decreases stability and ultimate gain K . 

Temperature, target Decoupler transfer function Base temperature Exhaust temperature Reset time... 

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... 

Control objectives related to the operation of the process units and the process itself (production rate, product quality, unit-level, and total inventory) should be addressed in the fast time scale. For instance, when a multi-loop linear control strategy is considered, the reset time for the controllers should be of the order of magnitude of the time constants of the individual process units. 

Finally, we analyzed the control implications of the presence of impurities in a process, concluding that the control of impurity levels must be addressed over an extended time horizon using the flow rate of the purge stream as a manipulated input. To close the impurity-levels loop, one should resort either to an appropriately tuned linear controller (e.g., a PI controller with long reset time) or to a (nonlinear) model-based controller that uses (an inverse of) the reduced-order model of the slow dynamics - as developed in this chapter - to compute the necessary control action. 

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