Control arithmetic apparatus and method

Details Control arithmetic apparatus and method Control arithmetic apparatus and method

2000-10-18

2004-06-22

Patel, Ramesh (Department: 2121)

Data processing: generic control systems or specific application

Generic control system, apparatus or process

Optimization or adaptive control

C700S028000, C700S037000, C700S046000, C700S052000, C700S074000, C700S089000, C708S505000, C708S523000, C708S530000, C714S006130, C714S746000, C714S751000

Reexamination Certificate

active

06754542

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a control arithmetic apparatus for performing, for example, PID (Proportional, integrational and Differential) control or IMC (Internal Model Control) to calculate a manipulated variable on the basis of the error (deviation) between a set point and a controlled variable or the like and, more particularly, to a control arithmetic apparatus having the function of suppressing an overshoot accompanying the application of an disturbance and a control arithmetic method.
Conventionally, an apparatus using PID control is generally used as a versatile control arithmetic apparatus that can be used for an indefinite controlled system. As shown in
FIG. 5
, a conventional PID control arithmetic apparatus
10
is comprised of a host computer
11
for outputting an error Er by subtracting a controlled variable PV from an input set point SP and a control arithmetic section
12
for calculating a manipulated variable MV from the error Er output from the subtracting section
11
and outputting it to a controlled system
13
.
Letting C
PID
be the transfer function of the control arithmetic section
12
and P be the transfer function of the controlled system
13
, the manipulated variable MV is obtained from the transfer function C
PID
of the control arithmetic section
12
according to equation (1):
MV=C
PID
(
SP−PV
)  (1)
The transfer function C
PID
can be expressed as
C
PID
=Kg
{1+(1
/Tis
)}(1
+Tds
)/(1
+&eegr;Tds
)  (2)
where Kg is a proportional gain, Ti is the integration time, Td is the differentiation time, and &eegr; is the constant (e.g., &eegr;=0.2).
In this case, in controlling temperature and pressure, the transfer function P of the controlled system
13
can be approximated by equation (3):
P =Kp
exp(−
Lps
)/(1
+Tps
)  (3)
where Kp is the gain, Tp is the time constant, and Lp is the dead time. The gain Kp provides static characteristics for the controlled system
13
; the time constant Tp, time delay characteristics (dynamic characteristics); and the dead time Lp, dead time characteristics (dynamic characteristics).
According to the known adjustment formula in a control theory or the known IMC theory proposed by M. Morari, in order to satisfy both stability and quick response of control, PID parameters (proportional gain Kg, integration time Ti, and differentiation time Td) are preferably given as follows:
Kg =&agr;Tp
/(
KpLp
)  (4)
Ti=&bgr;Tp  (5)
Td=&ggr;Lp  (6)
where &agr;, &bgr;, and &ggr; are constants (e.g., &agr;=0.6, &bgr;=1, and &ggr;=0.5). If PID parameters are set for the control arithmetic section
12
, excellent control characteristics can be normally obtained.
According to the above conventional PID control arithmetic apparatus, in a steady state of temperature control, when a disturbance occurs, e.g., a temporary drop in temperature occurs, the manipulated variable MV is updated to restore the temperature. At the time of occurrence of a disturbance like a temporary drop in temperature, the temperature is automatically restored without updating the manipulated variable MV after a lapse of a relatively long period of time. In such a case, updating the controlled variable PV by using the PID control unit apparatus amounts to excessive manipulated variable correction. Consequently, as a phenomenon reflecting in the controlled variable PV, an excessive control response represented by an overshoot occurs, as shown in FIG.
6
A.
As an example of a temporary drop in temperature as a disturbance, a phenomenon occurs, in which the temperature in a reaction furnace temporarily drops when a boat on which a plurality of semiconductor wafers are mounted is inserted into the reaction furnace in a batch type CVD (Chemical Vapor Deposition) furnace used for the manufacture of semiconductors. In the batch type CVD furnace shown in
FIG. 7
, reference numeral
61
denotes a boat,
62
, a reaction furnace,
63
-
1
to
63
-
5
, heaters for heating zones
1
to
5
in the reaction furnace
62
;
64
-
1
to
64
-
5
, sensors for measuring the temperatures (controlled variables PV) in the zones
1
to
5
, and
65
-
1
to
65
-
5
, PID control arithmetic apparatuses. The PID control arithmetic apparatuses
65
-
1
to
65
-
5
calculate the manipulated variables MV and output them to the heaters
63
-
1
to
63
-
5
to set the temperatures in the zones
1
to
5
to the temperature designated by the set point SP.
For temperature control in a semiconductor manufacturing process of forming a thin film by using a chemical reaction, an overshoot is a serious undesired phenomenon. Under the circumstances, a control arithmetic apparatus is disclosed in Japanese Patent Laid-Open No. 4-039701 (reference
1
). This apparatus has the function of suppressing an overshoot by temporarily correcting the set point SP supplied to a PID control arithmetic section to a value near the controlled variable PV when a situation in which the controlled variable PV approaches the set point SP is detected, as shown in FIG.
8
A.
The control arithmetic apparatus disclosed in reference
1
corrects the set point SP upon detection of a situation in which the controlled variable PV approaches the set point SP. For this purpose, the overshoot suppressing function is activated in the second half of last period in a series of controlled variable changes in the time interval between the instant at which the controlled variable PV deviates from the set point SP upon application of a disturbance and the instant at which the controlled variable PV is settled at a value near the set point. For this reason, at the time of occurrence of a disturbance for which the controlled variable PV changes at a high speed, like a temporary drop in temperature at the time of loading of a boat into the batch type CVD furnace, the overshoot suppressing function does not work for an excessive manipulated variable correction immediately after the application of a disturbance, which should be suppressed.
Consequently, as shown in
FIGS. 8A and 9A
, only a control result that does not differ much from that obtained by a general control arithmetic apparatus having no overshoot suppressing function can be obtained. That is, even if the overshoot suppressing function is activated upon detection of a situation in which the controlled variable PV approaches the set point SP, it is substantially too late to perform effective control. As described above, in the conventional control arithmetic apparatus, if a disturbance for which the controlled variable PV changes at high speed is applied, an overshoot cannot be suppressed.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a control arithmetic apparatus and method which can suppress an overshoot even if a disturbance for which the controlled variable changes at high speed is applied.
In order to achieve the above object, according to the present invention, there is provided a control arithmetic apparatus comprising first calculation means for calculating an error of a controlled variable on the basis of a controlled variable and set point for a controlled system, detection means for detecting, in control cycles, on the basis of the error output from the first calculation means whether a disturbance is applied, second calculation means for calculating an error correction amount on the basis of a magnitude of the error output from the first calculation means when application of a disturbance is detected, convergence operation means for performing convergence operation such that the error correction amount output from the error correction amount calculation means gradually converges to 0, and control arithmetic means for calculating a manipulated variable on the basis of the error output from the first calculation means and an error correction amount after the convergence operation which is output from the convergence operation means.


REFERENCES:

Affiliated with

Also associated with

Rating

Control arithmetic apparatus and method does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Control arithmetic apparatus and method, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Control arithmetic apparatus and method will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3353934