自动化专业英语 王树青Unit 1.5Controller Tuning

Unit 1.5 Controller Tuning

1.What are the commonly encountered process control system in a chemical plant ?

2.What must be adjusted after control system in is installed?

3. What method is often used for tuning of a controller?

4. Could you please give the typical approach and steps?

After a control system is installed the controller settings must usually be adjusted until the control system performance is considered to be satisfactory. This activity is referred to as controller tuning or field tuning of the controller. Because controller tuning is usually done by trial and error , it can be quite tedious and time-consuming. Consequently, it is desirable to have good preliminary estimates of satisfactory controller settings. A good first guess may be available from experience with similar control loops. Alternatively, if a process model or frequency response data are available, some special design methods can be employed to calculate controller settings. However, field tuning may still be required to fine rune the controller, especially if the available process information is incomplete or not very accurate General guidelines for selection of controller type (P ,PI.etc. ) and choice of settings are available for commonly encountered process variables: flow rate, liquid level. gas pressure, temperature, and composition. The guidelines discussed below are useful for situations where a process model is not available. However, they should be used with caution because exceptions do occur Similar guidelines are available for selecting the initial controller settings for the startup of a new plant.

Flow Control

Flow and liquid pressure control loops are characterized by fast responses (on the order

of seconds), with essentially no time delays The pfo.ess dynamics are due to compressibility (in a gas stream) or inertial effects (in a liquid). The sensor and signal transmission line may introduce significant dynamic lags if pneumatic instruments are used. Disturbances in flow-control systems tend to be frequent but generally not of large magnitude Most of the disturbances are high-frequency noise (periodic or random) due to stream turbulence, valve changes, and pump vibration. PI flow controllers are generally used with intermediate values of the controller gain Kc. The presence of recurring high-frequency noise rules out the use of derivative action.

Liquid Level

A typical non-self regulating liquid-level process has been discussed, because of its integrating nature. a datively high-gain controller can be used with little concern about instability of the control system. In fact, an increase in controller gain often brings an increase in system stability. while low controller gain can increase the degree of oscillation Integral control action is normally used bur is not necessary if small offsets in the liquid level (±5%) can be tolerated Derivative action is not normally employed in level control, since the level measurements often contain noise due to the ,plashing and turbulence of the liquid entering the tank .

In many level control problems, the liquid storage tank is used as a surge tank to damp out fluctuations in its inlet streams If the exit flow rare from the tank is wed as the manipulated variable , then conservative controller settings should be applied to avoid large ,rapid fluctuation. in the exit flow rate. This strategy is referred to as averaging control if level control also involves heat transfer, such as m a vaporizer or evaporator, the process model and controller design be.ome much more complicated In such situations special control methods can be advantageous.