Electrical computers: arithmetic processing and calculating – Electrical digital calculating computer – Particular function performed
Reexamination Certificate
2000-06-07
2001-07-17
Malzahn, David H. (Department: 2121)
Electrical computers: arithmetic processing and calculating
Electrical digital calculating computer
Particular function performed
Reexamination Certificate
active
06263355
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a control method of a chemical process such as that used in a chemical reactor. More specifically, the present invention relates to a method for controlling a chemical reactor using a feedforward subroutine for calculating parametric balances responsive to multivariable inputs which takes advantage of system knowledge and a rapid noise filtering subroutine. The present invention is particularly applicable to real time automatic control systems and apparatus and more specifically to a class of controllers utilizing dynamic system prediction techniques employing on-line parametric balances and non-linear modeling. A filter and corresponding filtering method are also disclosed.
2. Brief Discussion of Related Art
In the control art, traditional or classic feedback controllers dominate control practice. Traditional feedback controllers include linear controllers, such as the proportional (P) controller, the proportional-integral (PI) controller, or the proportional-integral-derivative (PID) controller, all of which are discussed below, and non-linear controllers, such as the fuzzy logic (FL) controller. A high level partially block, partially schematic diagram of a hypothetical chemical reactor utilizing P-type feedback control is shown in
FIG. 1
, which illustrates a process whereby the liquid level in a conical tank
10
is being maintained by controlling the feed rate Vf of the influent liquid. More specifically, a level controller
12
provides signals indicative of the level in tank
10
to a flow controller FC
14
, which senses the liquid feed Vf and provides positioning signals to feed control valve
16
to control the liquid feed Vf to the tank
10
. It will be noted that, in the system depicted in
FIG. 1
, the liquid out of the tank is not controlled by the flow controller
14
.
It will be appreciated that when the level in the conical tank is above its set-point SP, the controller
14
will reduce the fresh feed to the tank, i.e., decrease Vf, and when it's too low, the controller
14
will increase the flow, i.e., increase Vf. The magnitude of this adjustment is determined by the tuning parameters used, the most important of which is the gain, i.e., the proportional term (“P”) in PID. In this case, the gain would be specified is units of (liters/hr)/(% level). By specifying the gain, the operator specifies how much the liquid feed Vf will be adjusted for a change in the percentage (%) deviation of the level in tank
10
from the predetermined set-point SP.
It should also be noted that the other two terms denoted by the term PID are the integral term and the derivative term. The integral term, as its name implies, keeps track of how long the level has been away from the predetermined set-point SP. As the area between the set-point curve and the present valve curve increases, the integral term (I) begins commanding larger changes to the liquid feed Vf. In contrast, the derivative term (D) specifies the adjustment amount for the liquid feed Vf when the level of tank
10
is accelerating or decelerating, e.g., the change in Vf would be a first value when the level of tank
10
is increasing at an increasing rate and a second value different from the first value when the level of tank
10
is increasing at a decreasing rate.
PID is a conventional control strategy that has been around since the 1930's; PID control is still predominant in the chemical manufacturing industry. It will be appreciated that PID control has several advantages, the greatest of which is that it does not require any special knowledge or models of the system; PID control merely requires that the operator have a deft hand at tuning the system. This strength is also its biggest weakness. More specifically, PID control doesn't take advantage of what the operator does know about the system. Thus, for the hypothetical control system illustrated in
FIG. 1
, the system does not take into account the fact that the tank
10
is conical. As shown in
FIG. 1
, the conical tank
10
is draining under gravity with the level controller
14
adjusting the flow rate of fresh feed, Vf. Because the tank
10
is conical, the change in the hold-up required to change the level is much greater when the level is high. Intuitively, this presents a problem in that a much larger adjustment to the flow Vf will be required to rectify a level deviation of 1% when the tank
10
is nearly full than when it is nearly empty. For this reason, any single set of tuning parameters for the FC controller
14
will not work for all values of the level set-point SP. Thus, it is generally not be possible to use a single set of tuning parameters for all levels of the tank
10
. Therefore, the larger “transitions” from one level to another are principally done manually by the operator since one set of parameters will not work for both the low and high levels of tank
10
.
The situation illustrated in
FIG. 1
is further complicated if the control system has other control loops operating with respect to the tank
10
, i.e., a temperature loop as shown in FIG.
2
. In
FIG. 2
, the temperature
10
is preferably being controlled by adjusting the temperature Tj of the jacket fluid Vj. More specifically, a temperature sensor
18
provides a temperature signal to a temperature controller
20
controlling a heater
22
, which heater heats the fluid Vj provided to the jacket
10
′ of tank
10
. To control the tank temperature to 70° C., the temperature controller
20
changes the set-point on the jacket fluid temperature, Tj. It should be noted that controller
14
and
20
, although physically isolated from one another, are nonetheless operatively coupled to one another. To help demonstrate the concept of “controller coupling” on this system, assume that the fresh liquid is being fed to tank
10
at 20° C. and assume that the tank fluid density is a function of temperature. Therefore, any change in the feed Vf will affect the temperature of the fluid in tank
10
, which will affect the density of the fluid in tank
10
, which will subsequently affect the level in tank
10
, and so on. As shown in
FIGS. 3 and 4
, instability can result, as discussed in greater detail below. The relational diagram of
FIG. 4
further illustrates controller coupling due to the strong coupling of the level and temperature controllers that occurs through the density of the liquid in the tank
10
.
It will be appreciated that the amount of heat transferred to the tank
10
will depend on the temperature difference (T
jacket
−T
tank
) and the surface area of contact, i.e., the heat exchange surface area. Moreover, the temperature rise in the tank
10
depends on the mass of liquid in the tank and the heat capacity of the liquid. It will be noted that both the surface area for heat exchange and the liquid mass are strong, non-linear functions of tank level; it follows, just by inspection, that any disturbance in or change to the level in tank
10
will upset the temperature of the liquid in tank
10
.
Furthermore, assuming that the liquid density is a function of temperature, any change in the temperature of tank
10
will affect the level of liquid in tank
10
, which will, in turn, affect the fresh liquid feed Vf, which will further upset the temperature in tank
10
. In conventional PID control, this is what is known as controller coupling leading to either sustained oscillations in the system or outright instability of the system. This is shown in
FIGS. 3 and 4
for a change of level set-point SP. From these latter Figures, it will be appreciated that while the tuning of controller
14
was acceptable for higher levels of tank
10
, the much faster response of the system at lower levels of tank
10
caused severe system instability. The solution to controller coupling for systems with PID control such as illustrated in
FIG. 2
is to “detune” one of the controllers
14
,
20
, i.e., to reduce the ability of controller
20
, for example, to contr
Harrell Douglas G.
Williams Dennis C.
Malzahn David H.
Montell North America Inc.
Powell, Jr. Raymond H. J.
Westerlund Robert A.
Westerlund & Powell P.C.
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