PID controlling meter

A PID controlling meter is provided with unit for displaying an amount of operation representing the sum total of the bias constant and the operation result of three terms, i.e., a proportional term, an integrating term and differentiating term each of which is a function of deviation. The meter also has a constant setting unit for setting displayed amount of operation as a control parameter; unit for displaying set values of the constant setting a unit; a unit for reading the set values of the constant setting unit when a power source is closed; and a unit for determining bias constant when the power source is closed on the basis of the constant set values read by the reading unit.

BACKGROUND OF THE INVENTION 
This invention relates to a PID controlling meter, and more particularly, 
to a technique for controlling overshoot generated when a power source is 
closed. 
As is well known, PID controlling meters have advantages in that less 
steady state deviation occurs and in that correction of disturbance can be 
quickly accomplished, and therefore, PID controlling meters are 
extensively used for various process controls such as temperature control. 
In PID controlling meters of this kind, over-shoot often occurs in control 
operations performed when a power source is closed. This results from the 
fact that when the power source is cut off, the integrated values so far 
presented are lost and when the power source is again closed, integrating 
operation restarts from an initial integrating value (normally, zero) and 
as a consequence, the number of required integration operations increases. 
To minimize such over-shoot and to shift the condition to the previous 
stabilized control condition quickly, the integrated value from outside 
need merely be corrected, but to perform a correcting operation upon every 
closure of the power source is very cumbersome and operability is poor. 
In view of the foregoing, in prior art PID controlling meters, an antireset 
wind up function is utilized wherein an amount of upper limit operation 
and an amount of lower limit operation in the range capable of being 
integrated are determined from the amount of operation under the 
stabilized control condition, and the integrating operation is effected 
only when the amount of operation is within said range when the power 
source is reclosed, wherby the over-shoot resulting from over-integration 
is minimized to eliminate the necessity of a correcting operation by the 
user. 
However, PID controlling meters which use anti-reset wind-up functions have 
the following problems. 
First, since the integrating operation is not performed when the amount of 
operation is other than the above described set range, said set range has 
to be extended to prevent an increase in steady-state deviation due to the 
influence of disturbances. This is not favorable in terms of the initial 
object which is to minimize overshoot. In addition, in order to determine 
the above described set range, a high degree of expertise is required 
using data for obtaining the stabilized control condition in respect of 
objects to be controlled and also data concerning the condition 
disturbance and bringing appropriate judgment on the basis of said data. 
Thus, it is not possible for the user to simply make a decision. Finally, 
two kinds of settings are necessary, thus making the structure of the 
controlling meter quite complex. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a PID controlling meter 
which can minimize over-shoot without complicating the structure of the 
controlling meter and with a simple setting operation. 
In accordance with the present invention, values displayed on the 
controlling meter are merely set to a single setting unit, and therefore, 
the setting operation is simple and the structure of the controlling meter 
is not complex. 
Moreover, bias constant is to be determined on the basis of the amount of 
operation under the stabilized control condition obtained in the early 
stage of operation, and therefore, the over-shoot generated when the power 
source is closed in the operations performed after the second operation 
and thereafter, can be reduced. 
Other and further objects of this invention will become apparent upon an 
understanding of the illustrative embodiments described below or will be 
indicated in the appended claims, and various advantages not referred to 
herein will occur to one skilled in the art upon employment of the 
invention in practice.

DETAILED DESCRIPTION OF THE INVENTION 
As is well known, the control operation of a PID controlling meter is given 
by 
##EQU1## 
where M refers to the amount of operation. In Equation (1), Kp represents 
the proportional gain, Ki the integrating gain, Kd the differentiating 
gain, e the deviation, and M0 the bias constant. 
Under the stabilized control condition, a temperature detector has a value 
approximately equal to a target value, and in this case, it can be assumed 
that e.apprxeq.0 and (de)/(dt).apprxeq.0 i.e., that these values are 
approximately equal to 0. Therefore, Equation (1) is given by 
##EQU2## 
where Ma represents the integrated value leading to the stabilized control 
condition. 
So, the Ma (Ma=M-M0) is utilized as the bias constant when the power source 
is closed, and the integrating gain is decreased until the value draws 
near the target value for the PID control condition. In this way, control 
is carried out so that the magnitude of over-shoot being previously 
generated may be minimized, and the control operation can be quickly 
shifted to the previous stabilized control condition. This can also be 
applied to the case where the target value is changed to obtain a new 
stabilized control condition. 
The PID controlling meter of the present invention has been achieved in 
view of the above mentioned fact and comprises means for displaying said 
amount of operation m, a constant setting unit for setting the amount of 
operation M as a control parameter, means for displaying the set value Mc 
of said constant setting unit, means for reading the set value Mc of the 
constant setting unit when the power source is closed, and means for 
determining said bias constant Ma at the time of closing the power source 
on the basis of said constant setting value Mc read by said reading means. 
A specific embodiment is described below. 
FIG. 1 is a schematic block diagram showing a PID controlling meter to 
which the present invention is applied. In FIG. 1, the present apparatus 
basically comprises a detector 2 for detecting a temperature of an 
electric furnace 1 as an object to be controlled, an amplifier 3, an A/D 
converter 4, a PID controlling section 5, and an output operating section 
6. The A/D converter 4 is provided with setting units 7 and 8 for setting 
a PID control parameter and said bias constant Ma in the form of an analog 
amount. The PID control section 5 is provided with a digital setting unit 
9 for setting a target temperature, a numeral display 10 and a display 
switching section 11. 
The numeral display 10 displays the detected temperature value of the 
electric furnace 1, the set value of the setting units 7, 8, 9, and said 
amount of operation M obtained as the result of operation of the PID 
control section 5. Switching of said display is carried out by the 
switching operating section 11. 
The PID control section 5 is composed of a microcomputer and comprises a 
central processing unit (CPU) for executing said PID operation and the 
aforesaid various means in accordance with the present invention, a ROM 
for storing system programs, and a RAM which functions as a working 
memory. 
FIG. 2 shows the structure of said RAM, which is shown attaching importance 
to parts of the present invention. In FIG. 2, reference character A1 
denotes a temperature input value storage area, A2 a target temperature 
value storage area, A3, A4 and A5 storage areas for storing parameters of 
PID control, respectively, A6 a storage area for storing values of bias 
constant (Mc), A7 a storage area for storing values of an amount of 
operation M, A8 a reading counter (CR) area, A9 a display counter (CD) 
area, A10 a sampling counter (CS) area, A11 a storage area for storing a 
flag (F) for discriminating closure of power source, A12 a storage area 
for storing values of operation constant (Mi), and A13 a storage area for 
storing values of an amount of stabilized operation (Mi0). 
FIG. 3 is a flow chart showing the control operation carried out by the PID 
control section 5 attaching importance to part of the present invention. A 
description will be made hereinafter in accordance with this flow chart. 
When the power source is closed, the known initialized processing is 
executed in Step 300. At this time, the flag F for discriminating the 
closure of the power source is set. 
Thereafter, routines (Steps 301, 302) for retrieving counted values of 
reading counter CR are executed. Reading counter CR is provided to divide 
sampling time by a value stored in sampling counter CS to designate the 
routines of control operation carried out by the PID control section 5, 
said operation being repeatedly carried out every sampling time. 
When CR=0 is detected in Step 301, operation proceeds to Step 303 where the 
temperature input value is stored in the storage area A1 of RAM, and the 
reading counter CR is incremented in step 304. 
Subsequent Steps 305, 306 involve routines for retrieving the counted 
values of the display counter CD. The counted value of the display counter 
CD is incremented and set by a push-button switch provided on the display 
switching operating section 11. Thereby, display routines of display of 
temperature input value (Step 308), display of an amount of operation 
(Step 309) and display of bias constant set value MC (Step 310) are 
executed for the numeral display 10. 
When CR=1 is detected in Step 302, the target temperature value reading 
routine (Step 312) is executed, with the control operation then proceeding 
to Step 313. 
Step 313 involves a routine for judging whether or not the setting of the 
target temperature value has been changed. If the set value has not been 
changed, operation proceeds to Step 315 while jumping Step 314 to read 
parameters set in the setting units 7 and 8, with the control operation 
then proceeding to Step 304. 
When CR=2 is detected in Step 302, operation proceeds to Step 316. Step 316 
involves the routine for judging whether or not the flag F for 
discriminating power source closure is set, said flag F being set when the 
power source is re-closed (Step 300). As a result, routines of Steps 317, 
318 and 319 are executed unless the flag F is set, with control operation 
returning to the initial Step 300. 
The above described routines are repeatedly executed, in the PID operation 
processing routine of Step 317 in said process, the amount of operation M 
shown in Equation (1) is successively calculated and controlled so that 
the temperature of the electric furnace may be stabilized at the target 
temperature. After the stabilized control condition has been passed for a 
given time (for example, continued for a duration of one minute under the 
condition that deviation is within 0.3.degree. C.), a value representing 
the amount of stabilized operation Mi0 is transferred to the storage area 
A13 of the RAM. At this time, the bias constant M0 shown in Equation (1) 
is preset to 50% 
In this way, if the stabilized control condition has been obtained, the 
push-button switch of the display switching operating section 11 is 
operated to obtain CD=1. Then the amount of stabilized operation Mi0 is 
displayed on the numeral display 10 by means of the routine of Step 309. 
The operator operates the push-button switch to have CD=2 in order to set 
the aforesaid value to the bias constant settin unit 8. As a consequence, 
the set value Mc is displayed on the numeral display 10 in Step 310, and 
therefore the operator can confirm and correct the set value. This set 
value Mc is stored in RAM in the routine of Step 315. This bias constant 
setting unit 8 comprises a volume or the like, which display unit of 
graduations is made the same as that of the numeral display 10 so as to 
minimize an extent wherein correction is made in the operation of 
confirmation. 
Next the case will be described in which the determined work is finished, 
the power source of the PID controlling meter is opened and the power 
source for subsequent work is re-closed. In this case, the flag F for 
determining the closure of power source is set in the initialized 
processing routine 300, and therefore, this can be detected in Step 316 in 
the initial sampling time, with flag F being set in Step 320 control 
operation then proceeds to Step 321. In Step 321, operation is performed 
to obtain Mc-50. The result of this operation is stored (as operation 
constant Mi0) in the RAM storage area A12 in Step 322. 
Now, F is reset (F=0) in Step 316 within the same initial sampling time, 
and therefore, temperature control routines after Step 317 are executed. 
In the PID operation processing routine in Step 317, an operation for 
obtaining a solution to Equation (1) is carried out, and the bias constant 
term is determined to be M0+Mi=50+(Mc-50)=Mc. Accordingly, the integrated 
value under the stabilized control condition can be determined and thus, 
temperature control at the time of reclosure of the power source is 
accomplished so as to decrease the over-shoot previously generated. 
As described above, in the PID controlling meter of the present invention, 
the over-shoot at the time of closing the power source in the operations 
after the second operation can be decreased in a simple procedure wherein 
the stabilized control condition in the early stage of operation is 
obtained, and the amount of operation at that time is set to the bias 
constant setting unit. 
Even in the case where a target value is changed to obtain a new stabilized 
control condition, over-shoot occurs. This is treated in the following 
manner. When a change of a setting of a target value is detected in Step 
313, the amount of stabilized operation Mi0 prior to said change is 
transferred to the storage area A12 in Step 314, with operation then 
proceeding to Step 315. Accordingly, in the PID operation processing 
routine of Step 317, the operation constant, Mi, is utilized as the bias 
constant in a manner similar to that of the former case, in order to 
decrease the over-shoot at the time of change in setting. 
While in this embodiment, the bias constant setting unit comprises an 
analog amount setting unit such as a volume, it should be understood of 
course that said unit in not limited thereto in the present invention, but 
a digital amount setting unit, such as a digital switch, can also be used.