Tuning, controller

How do we choose the values of the controller parameters Kc, ii and td They must be chosen to ensure that the response of the controlled variable remains stable and returns to its steady-state value (disturbance rejection), or moves to a new desired value (set point tracking), quickly. However, the action of the controller tends to introduce oscillations. 

How quickly the controller responds and with how little oscillation, depends on the application. The following are some of the available tuning methods, of which we will consider the last two in detail (see textbooks1-11 for numerous other methods)  

The settings obtained by this method are good initial estimates but are not optimal and some retuning may be necessary. Note that this method 

2 Process Reaction Curve Method (Cohen-Coon Tuning). For some processes, it may be difficult or hazardous to operate with continuous cycling, even for short periods. The process reaction curve method obtains settings based on the open loop response and thereby avoids the potential problem of closed loop instability. The procedure is as follows  

The purpose of controller tuning is to choose the correct controller constants to obtain the desired performance characteristics. This usually means that the control variables should be restored in an optimal way to acceptable values, following either a change in the set point or the appearance of an input disturbance. Simulation examples TEMPCONT and CONTUN, provide exercises for controller tuning using the methods explained below. 

There are a variety of feedback controller tuning methods. Probably 80 percent of all loops are tuned experimentally by an instrument mechanic, and 75 percent of the time the mechanic can guess approximately what the settings will be by drawing on experience with similar loops. We will discuss a few of the time-domain methods below. In subsequent chapters we will present other techniques for tinding controller constants in the Laplace and frequency domains. 

A word needs to be said about controller type and controller tuning. Controller algorithm selection and tuning are important to the success of any control system. Two features should be recognized about the Eastman process. First, it is an integrating process with little selfregulation in terms of pressure, liquid levels, and chemical components. Second, there are no tight specifications on any variables. 

Tuning was performed by increasing the controller gain and testing the dynamic response to a step change in setpoint until the loop became too oscillatory. Reactor temperature was tuned first, followed by pressure, separator temperature, stripper temperature, component A composition, and component B composition. No claim is made that these are the best settings, but they give adequate control and required little time to tune. 

A plantwide control design procedure was used to develop a simple but effective regulatory control system for the Eastman process with an on-demand product control objective. With this strategy, control of production rate is essentially instantaneous. Drastic upsets and disturbances are handled by simple proportional-only overrides. 

Controller parameters are tuned to provide both performance and stability and there are many different rules for the tuning parameters of a control loop [6, 7, 22]. After deciding on the controller structure, one decides on the desired dosed-loop response criteria. Then one must distinguish between processes where there is at least an approximate modd with known parameters and the case when the process model is unknown. 

Process model is known An approximate model of the process may be obtained by the step response test noted earlier (see Section 12.4.2.4) (or from first prindples). When an approximate modd of the process is known we may obtain the tuning parameters directly. We use here the example of the first-order plus dead time process, since its dynamics are so representative of the polymer equipment dynamics. Here we have chosen for the tuning criteria to minimize the integral of the time-weighted absolute error (ITAE) [Eq. (96)] [7]. 

Ogunnaike, W. H. Ray, Process Dynamics, Modeling, and Contrd, Copyright 1994 Oxford University Press. This material is used by permission of Oxford University Press. 

Responses for step-change in set point with ITAE first-order plus dead time 

The minimum ITAE tuning rules based on a FOPDT process are given in Table 12.1 [22]. Note that the tunings in Table 12.1 should only be applied in the range 

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The dynamics of the secondary control loop should be approximately two to four times as fast as the dynamics of the primary control loop in order to achieve stable control. The secondary controller is actually part of the primary controller s process system. Hence, changes in the secondary controller tuning constants change the process system of the primary controller. Therefore, cascade control loops should always be tuned by first tuning the secondary controller and then the primary controller. If the secondary controller tuning is changed for any reason, the primary controller may need to be retuned also. 

The performance of a controller depends as much on its tuning as its design. Tuning must be apphed by the end user to fit the controller to the controlled process. Tnere are many different approaches to controller tuning based on the particular performance criteria selected. 

Basic process control system (BPCS) loops are needed to control operating parameters like reactor temperature and pressure. This involves monitoring and manipulation of process variables. The batch process, however, is discontinuous. This adds a new dimension to batch control because of frequent start-ups and shutdowns. During these transient states, control-tuning parameters such as controller gain may have to be adjusted for optimum dynamic response. 

Fig. 4.35 Closed-loop step response of temperature control system using PID controller tuned using Zeigler-Nichols process reaction method.

Adequate PC and its associated instrumentation are essential for product quality control. The goal in some cases is precise adherence to a single control point. In other cases, maintaining the temperature within a comparatively small range is all that is necessary. For effortless controller tuning and the lowest initial cost, the processor should select the simplest controller (of temperature, time, pressure, melt-flow, rate, etc.) that will produce the desired results. 

Anodic shipping voltammetry (ASV) is the most widely used form of stripping analysis, hi this case, the metals are preconcenhated by elechodeposition into a small-volume mercury electrode (a tiiin mercury film or a hanging mercury drop). The preconcenhation is done by catiiodic deposition at a controlled tune and potential. The deposition potential is usually 0.3-0.5 V more negative than E° for the least easily reduced metal ion to be determined. The metal ions reach die mercury electrode by diffusion and convection, where diey are reduced and concentrated as amalgams ... 

One should note the use of engineering units in the controller tuning parameters. The units become important when one must compare different controllers tuning parameters using different units for the PID calculation. 

The cost functional is the indicator of how well the control loop is functioning. The lAE criterion used essentially says "measure the cumulative difference between the actual value and the desired set point" this cumulative score is a measure of control system performance. With this code in the model, the commands to do the controller tuning are ... 

Figure 5.164. Tank temperature versus time for two values of Kc (1.5 and 2.0), with XI = 10000. The changes at T=10 and T=20 are programmed step changes in the inlet water flow rate. Oscillations and offset are caused by sub-optimal controller tuning.

Trial and error method controller tuning 101 Tube 623... 

With higher order models, we can construct approximate reduced-order models based on the identification of dominant poles. This approach is used later in empirical controller tuning relations. 

At this point, one may be sufficiently confused with respect to all the different controller tuning methods. Use Table 6.3 as a guide to review and compare different techniques this chapter and also Chapters 7 and 8. 

This process also lasts several minutes, which makes it globally a very penalizing step and relatively complex to implement from an engine control/tuning point of view. 

How could one control/tune electron transport through single molecules ... 

Continuous Binary Distillation Column 496 Controller Tuning Problem 427 Three-Stage Reactor Cascade with Countercurrent Cooling 287... 

Then in Chaps. 7 and 8 we will look at closedloop systems. Instrumentation hardware, controller types and performance, controller tuning, and various types of control systems structures will be discussed. 

In this chapter we will study control equipment, controller performance, controller tuning, and general control-systems design concepts. Some of the questions that wc will explore are how do we decide what kind of control valve to use what type of sensor can be used and what are some of the pitfalls that you should be aware of that can give faulty signals what type of controller should we select for a given application and how do we tune the controller. 

Therefore, the pairing of Xj, with V and Xg with R gives a closedloop system that is integrally unstable for any controller tuning. 

This selection of control structure is independent of variable pairing and controller tuning. The MRI is a measure of the inherent ability of the process (with the specified choice of manipulated variables) to handle disturbances, changes in operating conditions, etc. 

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