Data processing: generic control systems or specific application – Specific application – apparatus or process – Product assembly or manufacturing
Reexamination Certificate
2001-07-16
2004-09-28
Picard, Leo (Department: 2125)
Data processing: generic control systems or specific application
Specific application, apparatus or process
Product assembly or manufacturing
C700S129000, C162S262000
Reexamination Certificate
active
06799082
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
Profile control in the cross-machine direction of a paper machine involves a long dead time and a large time constant. As a result, this type of control takes a long time to stabilize for such events as a setpoint change, thus decreasing production efficiency. Another problem has been that a manipulated variable may overshoot after paper threading due to sheet break or after grade change. As a result, the manipulated variable takes a long time to settle, thereby also decreasing production efficiency.
The present invention relates to a process control method based on a finite settling-time response method. More particularly, the invention relates to a process control method and a process control apparatus preferred in controlling paper thickness profiles in the cross-machine direction of a paper machine.
2. Description of the Prior Art
FIG. 1
is a diagrammatic view showing the configuration of a system for controlling the paper thickness (CLP) profiles of a paper machine. In the figure, produced paper
51
is smoothed in its entirety and tuned in its thickness profile by a calender
53
, and wound onto a reel
55
. At this point, the thickness profile in the cross-machine direction of the produced paper
51
is measured by a CLP sensor
54
placed immediately before the reel
55
. The CLP sensor
54
repeats a scan (round travel) across the paper
51
in the cross-machine direction at a speed of approximately 30 seconds per one-way travel, thus measuring the thickness profile of the paper
51
.
A CLP measurement signal, which is the output of the CLP sensor
54
, is input to a measurement computation unit
56
, where the CLP measurement signal is converted to a digital signal at a maximum speed of approximately 60 Hz. Thus, a maximum of 1200 items of measured CLP data is obtained for each one-way travel. Such a collection of CLP measurement signals as acquired for each one-way travel is referred to as a CLP profile. The CLP profile is regarded as representing the cross-section of the paper
51
in the cross-machine direction, and used for the purpose of paper quality control. The CLP profile is considered especially important in the case of printing paper. For example, the quality of newspaper is stipulated such that a variable R, which is the deviation between the maximum and minimum variances in paper thickness, be kept at or below 1 &mgr;m for an average paper thickness of 80 &mgr;m.
The CLP profile is input to a control computation unit
57
, where a controlled/manipulated variable for controlling a hot-air heater
52
is calculated. This controlled/manipulated variable is input through an interface panel
58
to the hot-air heater
52
. The hot-air heater
52
blows air as hot as 40 to 400° C. onto a calender
53
in order to change the diameter thereof by means of thermal expansion, thereby tuning the CLP profile. The heater is segmented into multiple zones at a spacing interval of 75 to 100 mm and the temperature setpoint of hot air is changed on a zone-by-zone basis, so that the diameter of the calender
53
in the cross-machine direction is varied and thereby the CLP profile is properly adjusted.
Now, zone-specific profiles used in CLP profile control will be explained with reference to FIG.
2
. In the figure, a numeral
61
indicates paper, the width thereof being divided into N zones. A numeral
62
indicates the measurement points of paper thickness, which amount to P
t
points. Since the number of measurement points is greater than the number of zones under normal conditions, a plurality of measurement points are allocated to each zone.
FIG. 2
shows that five measurement points are allocated to each zone. Although dependent on the scale of a paper machine, N has a value from 50 to 100 in normal applications. Likewise, P
t
has a value of 1200 at the largest, though this value depends on the length of frame used.
A CLP profile PRF(i) is the value obtained by subtracting the average of all the values of a CLP process variable RPV(i) from the value of the CLP process variable RPV(i) of each measurement point. Thus, the CLP profile PRF(j) is expressed as equation 1 below.
PRF
(
i
)=
RPV
(
i
)−
PV
AVE
(
i=
1, . . . ,
Pt
) (1)
where
PV
AVE
=
1
Pt
∑
j
-
1
Pt
RPV
(
i
)
A measurement point of a CLP profile positioned at the midpoint of an ith zone is referred to as the position-specific measurement point of that zone, and is represented as PC(i). The number of measurement points included in one zone is determined from the mechanical width of the hot-air heater and the spacing between the measurement points of paper thickness, and is represented as AP. By way of simplifying later discussion, AP is defined as an odd number. If any calculated value of AP happens to be even, 1 is added to the value to make it odd.
The zone-specific CLP profile of a zone i is defined as ZP(i), and expressed as equation 2 below.
ZP
(
i
)
=
1
AP
∑
j
=
-
BP
BP
PRF
(
PC
(
i
)
+
j
)
(
2
)
where
i=1, . . . , N
PC(i)=Position-specific measurement point of ith zone
BP
=
AP
-
1
2
CLP profile control refers to the type of control wherein a hot-air heater is controlled so that the CLP profile ZP(i) of each zone is flattened as much as possible.
Next, CLP profile control will be explained with reference to FIG.
3
. The input to a controller
71
is a paper thickness deviation variable E(s), where a manipulated variable W(s) is calculated and output. The manipulated variable W(s) is input through a hold unit
72
to a process
73
. The output of the process
73
is paper thickness C(s), which is fed back to the controller
71
. G(s), H(s) and P(s) denote the transfer functions of the controller
71
, hold unit
72
and process
73
, respectively.
The transfer function P(s) of the process
73
can be approximated using a combination of a dead time and a first-order delay. Process measurement carried out in a certain paper machine using a step response method results in the dead time=5 min, time constant=8 min, and process gain=0.1 &mgr;m/%.
In order to carry out such control as described above, a sampling PI control method has been used conventionally. More specifically, it has been the common practice of such control to calculate a change in the controlled/manipulated variable &Dgr;W
n
(i) using equation 3 below.
Δ
W
n
(
i
)
=
100
PB
(
Δ
E
n
(
i
)
+
TC
TI
×
E
n
(
i
)
)
(
3
)
where
&Dgr;W
n
(i)=Change in the manipulated variable of ith zone (%)
&Dgr;E
n
(i)=E
n
(i)−E
n−1
(i)
E
n
(i)=Zone-specific CLP profile of ith zone
E
n−1
(i)=Zone-specific CLP profile of ith zone measured one control period earlier
TC=Control period, i.e., time required for a one-way travel of scan (sec)
PB=Proportional band (%)
TI=Integral time (sec)
FIG. 4
shows the result of simulating the response of the process to a steplike disturbance, using the Chien-Hrones-Reswick method which is a typical method for determining a PI gain from a step response. The horizontal axis represents time in units of a control period (number of scans). Note that in this simulation, we defined the dead time L as 300 sec, time constant T as 480 sec, proportional band as 167×L/T, and integral time TI as T. This combination of settings minimizes the response to 20% overshoot. In
FIG. 4
, a numeral
81
indicates a control deviation variable and a numeral
82
denotes a change in the manipulated variable. As is evident from the figure, the process requires as many as 25 scans (750 sec) for 80% response and as many as 45 scans (1350 sec) for 90% response. This means that the process has extremely poor response.
SUMMARY OF THE INVENTION
The present invention is intended to provide a highly responsive method of control based on a finite settling-time response method. More specifically, the invention provides a method for
Kojima Moonray
Kosowski Alexander
Picard Leo
Yokogawa Electric Corporation
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