FLOW RATE CONTROL DEVICE

Abstract

An operator is able to easily perform down-tuning while still achieving superior responsiveness and, in some cases, obtain an equivalent responsiveness as that obtained from the flow rate control device currently being used. In a flow rate control device that performs feedback control of a fluid control valve such that a measured flow rate closely approximates a target flow rate, there are provided a response lag input section that inputs a response lag set value, which is a value showing a response lag that an operator wishes to set, and a response lag generating section that generates response lags used in the feedback control in accordance with the response lag set values.

Claims

1. A flow rate control device that performs feedback control of a fluid control valve such that a measured flow rate closely approximates a target flow rate, comprising: a response lag input section that inputs a response lag set value, which is a value showing a response lag that an operator wishes to set; and a response lag generating section that generates response lags used in the feedback control in accordance with the response lag set values.

2. The flow rate control device according to claim 1, wherein the response lag set value is a value that shows a first order lag, a second order lag, or a lag of a greater order than this from which dead time has been excluded.

3. The flow rate control device according to claim 2, wherein the value showing the first order lag, the second order lag, or the lag of a greater order than this is a time constant.

4. The flow rate control device according to claim 1, wherein there is further provided a response lag measurement section that measures a response lag by performing feedback control, and the response lag generating section calculates, based on measured response lags measured by the response lag measurement section and on a feedback control coefficient set at that time, a new feedback control coefficient that forms the set response lag shown in the response lag set values, and replaces the current feedback control coefficient with the new feedback control coefficient.

5. The flow rate control device according to claim 1, wherein the feedback control includes at least proportional control, and is set such that a response lag excluding dead time is changed to a first order lag by using IMC.

6. A storage medium on which is stored a program for a flow rate control device that performs feedback control of a fluid control valve such that a measured flow rate closely approximates a target flow rate, wherein the program enables the flow rate control device to perform the functions of a response lag input section that inputs a response lag set value, which is a value showing a response lag that an operator wishes to set, and of a response lag generating section that generates response lags used in the feedback control in accordance with the response lag set values.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] FIG. 1 is a typical overall view showing a flow rate control device according to an embodiment of the present invention.

[0022] FIG. 2 is a function block diagram of a control mechanism according to the same embodiment.

[0023] FIG. 3 is a control block wiring diagram of a valve control section according to the same embodiment.

[0024] FIG. 4 is a data exemplary diagram showing data (structure) stored in a response lag storage section according to the same embodiment.

[0025] FIG. 5 is a control block wiring diagram according to another embodiment of the present invention.

BEST EMBODIMENTS FOR IMPLEMENTING THE INVENTION

[0026] A flow rate control device according to an embodiment of the present invention will now be described with reference made to FIG. 1 through FIG. 4.

[0027] A flow rate control device 100 is provided on each supply flow path (not shown in the drawings) along which flows a fluid such as, for example, various types of material gas used in semiconductor manufacturing, and is used to control the flow rate of each material gas at a predetermined flow rate. Note that examples of a fluid include liquids, slurries, and the like as well as gases.

[0028] A more detailed description will now be given.

[0029] As is shown in FIG. 1 and FIG. 2, this flow rate control device 100 is provided with a roughly rectangular parallelepiped-shaped body 1 inside which is formed a flow path 11, a thermal flow rate sensor 2 and a valve 3 that are mounted on the body 1, and a control mechanism 4 that controls the valve 3 based on outputs from the flow rate sensor 2.

[0030] An intake aperture 12 and a discharge aperture 13 that are used respectively for introducing and discharging fluids are both provided in a bottom surface of the body 1, and the flow path 11 is formed so as to connect the intake aperture 12 to the discharge aperture 13. The valve 3 is provided at the most downstream side of the flow path 11, and the flow rate sensor 2 is provided upstream from the valve 3.

[0031] The flow rate sensor 2 is a thermal based of flow rate sensor that is formed by a flow splitting element 21 in the form of a fluid resistor provided inside the flow path 11, a narrow tube 22 that is provided so as to bypass the flow splitting element 21 at the front and rear thereof, a detecting mechanism 23 that is formed by a pair of coils 23a (see FIG. 2) that are provided in the narrow tube 22 and that detects values related to flow rates, and a flow rate calculating section 41 that is created by utilizing a calculation function of the control mechanism 4 and that calculates flow rates based on outputs from the detecting mechanism 23.

[0032] More specifically, the respective coils 23a are both electrical heating wires, and a temperature control circuit (not shown in the drawings) is connected thereto so as to maintain the respective coils 23a at a predetermined temperature. The values of the voltages that are applied to each coil 23a by this temperature control circuit are output from the detecting mechanism 23 to the flow rate calculating section 41, and the flow rate calculating section 41 calculates the flow rates based on the respective voltage values.

[0033] Note that this flow rate sensor is not limited to being a thermal based of flow rate sensor, and various types of flow rate sensor such as differential pressure flow rate sensors, ultrasonic wave flow rate sensors, and Coriolis flow rate sensors and the like may also be used.

[0034] The control mechanism 4 is an electronic circuit that is provided with a CPU, memory, an A/D-D/A converter, input/output means, and drivers and the like. As a result of the CPU and peripheral devices thereof operating in cooperation with each other based on a program for the flow rate control device that is stored in the memory, the control mechanism 4 demonstrates functions of a calculating circuit of the valve control section 42 and the like as well as of the flow rate calculating section 41, which are component elements of the flow rate sensor 2.

[0035] As is described above, the flow rate calculating section 41 calculates flow rates of fluids flowing through the flow path 11 based on outputs from the detecting mechanism 23, and outputs the resulting values (hereinafter, these are referred to as ‘measured flow rate values’).

[0036] As is shown in FIG. 2, the valve control section 42 receives target flow rates input by an operator or supplied via communication from an external device or the like, and measured flow rates output from the flow rate calculating section 41, and performs PID feedback control on the aperture angle of the valve 3 such that deviations between the target flow rates and the measured flow rates are minimalized.

[0037] More specifically, the valve control section 42 has a structure that, as is shown by the control block wiring diagram in FIG. 3, in addition to a PID controller 421, also includes an IMC (Internal Model Controller) 422.

[0038] In FIG. 3, G.sub.p(s) is a transfer function of the actual processing (i.e., of a system including the valve 3 and the flow rate sensor 2), and varies depending on the type of apparatus.

[0039] G.sub.c(s) is a transfer function of the PID controller 421 and is expressed by the following Formula (1).

G.sub.c(s)=K.sub.p(1+1/T.sub.i.Math.s+T.sub.d.Math.s)  (1)

[0040] Here, K.sub.p is the proportional gain, T.sub.i is the integrated time, and s is the Laplace transform.

[0041] G.sub.p′(s) is a transfer function of a processing model 423.

[0042] The IMC 422 is a system expressed by a transfer function G.sub.IMC(s) obtained from G.sub.p(s), G.sub.c(s), and G.sub.p′(s).

[0043] In this embodiment, by appropriately setting the processing model 423 so as to determine G.sub.IMC(s), a transfer function serving as a feedback system which includes the valve control section 42, the valve 3, and the flow rate sensor 2 is constructed so as to provide a first order lag if dead time is disregarded.

[0044] Namely, if a transfer function of the valve 3 is taken as 1/(f.sub.1.Math.s+1), and a transfer function of the flow rate sensor 2 is taken as 1/(e.sub.1.Math.s+1), then the transfer function Gp(s) is expressed as in Formula (2).

G.sub.p(s)=(1/(f.sub.1.Math.s+1))(1/(e.sub.1.Math.s+1))  (2)

[0045] In addition, G.sub.IMC(s) is set in the flowing manner.

G.sub.IMC(s)=(x.sub.1.Math.s+1)(x.sub.2.Math.s+1)/(T.Math.s)  (3)

[0046] Here, x.sub.1 and x.sub.2 are set such that they form the same constants as f.sub.1 and e.sub.1 respectively.

[0047] As a result, the transfer function G.sub.1(s) obtained by connecting in series the IMC 422 and the valve 3 and flow rate sensor 2 is expressed as in the flowing Formula (4).

G.sub.1(s)=G.sub.IMC(s).Math.G.sub.p(s)=1/(T.Math.s)  (4)

[0048] In addition, as is described above, the transfer function G(s) of the overall system including the feedback system provides a first order lag as is shown below in Formula (5) if the dead time is disregarded.

G(s)=1/(1+T.Math.s)  (5)

[0049] This can be rewritten in the manner shown by the following Formula (6).

Qout=1/(1+T/s).Math.Qset=1/(1+1/Kp.Math.s).Math.Qset  (6)

[0050] Here, Qset is the target flow rate value and Qout is the measured flow rate value. 1/Kp is an inverse function of the time constant T.

[0051] Moreover, in this embodiment, as is shown in FIG. 2, the control mechanism 4 is further provided with the functions of a response lag input section 43, a response lag measurement section 44, a response lag storage section 45, and a response lag generating section 46.

[0052] The response lag input section 43 receives response lag setting values, which are values showing response lags of the flow rate control device 100, via an input means such as a keyboard or the like, or via communication. The response lag setting values are designated by an operator and, here, are values relating to first order lag elements of the response lag in the measured flow rate relative to the target flow rate. More specifically, the response lag setting values are time constants (i.e., a time from a flow rate start until 63% of a target flow rate is achieved). In addition to this, the response lag setting values may also be approximately a four-fold time constant (i.e., a time from a flow rate start until 98% of a target flow rate is achieved), or may also be a proportion (i.e., %) of the current response lag. Hereinafter, the response lag setting values may also be referred to as a set time constant.

[0053] The response lag measurement section 44 measures the response lag (here, this is a time constant of the first order lag and, hereinafter, is referred to as a measurement time constant) resulting from the control being performed at the current point in time. To achieve this, the response lag measurement section 44, for example, supplies a step signal as the target flow rate and, by measuring the indicial response at that time, calculates the measurement time constant. In addition to this, the measurement response may also be calculated using an impulse response or a ramp response or the like. The measurements of the response lag by the response lag measurement section 44 are made both before the actual flow rate control is started and during idle periods.

[0054] The response lag storage section 45 is set in a predetermined area of the memory, and is where the measurement time constants measured by the response lag measurement section 44 are stored. At this time, the response lag storage section 45 stores conditions at the time of measurement together with the respective measurement time constants as a pair in table form such as is shown, for example, in FIG. 4. These conditions are ambient conditions such as the type of fluid, the temperature, and the pressure and the like.

[0055] The response lag generating section 46 adjusts control parameters in the valve control section 42 such that the time constants resulting from this control are the same as the set time constants received by the response lag input section 43.

[0056] More specifically, as is described above, the responsiveness from the valve control section 42 is set as a first order lag by the IMC 422. Accordingly, a relationship between the time constant T and the proportional gain Kp generated by the PID controller 421 is as follows.

Kp=1/T  (7)

[0057] Therefore, the response lag generating section 46 refers to the response lag storage section 45, and calculates the ratio from the measurement time constant in the same type of measurement conditions and the proportional gain at that time. The response lag generating section 46 then calculates the proportional gain to form the set time constant from this ratio, and replaces the proportional gain currently being used with this new proportional gain.

[0058] The formula used to calculate the theoretical new proportional gain is shown in Formula (8), and the response lag generating section 46 performs an equivalent calculation to this in order to set the new proportional gain.

Kp.sub.1=Kp.sub.0.Math.T.sub.1/T.sub.0  (8)

[0059] Here, Kp.sub.1 is the new proportional gain, Kp.sub.0 is the current proportional gain, T.sub.1 is the set time constant, and T.sub.0 is the measurement proportional gain.

[0060] Therefore, according to this type of structure, all an operator needs to do is to input a desired set time constant, and the response lag of this flow rate control device 100 is auto-tuned to this set time constant.

[0061] Accordingly, according to the flow rate control device 100 of the present embodiment, simply by setting the same responsiveness as the responsiveness of the flow rate control device currently being used, the current flow rate can be easily replaced without having to perform any other measures such as, for example, readjusting the target flow rate.

[0062] Moreover, conventionally, in the field of flow rate control devices in which fast responsiveness has hitherto been sought, this effect, namely, the fact that constant responsiveness down-tuning by an operator is now possible for the first time can be said to be a remarkable breakthrough.

[0063] On the other hand, if faster responsiveness is sought, then it is sufficient if the value of the input set time constant is reduced to its performance limit, and this enables the capabilities inherent in this flow rate control device to be fully utilized.

[0064] Moreover, because the time constants that are used as response lag set values are values that enable a user to intuitively ascertain the responsiveness with ease, they also have the advantage of being easy to use.

[0065] In addition, in order to obtain the same functions as the present flow rate control device 100, it is also possible to calculate and fix the respective control coefficients (K.sub.p, T.sub.i, T.sub.d) of the PID control such that they are the same as the input set time constants (i.e., the desired response lag). However, the respective control coefficients can be freely supplied independently from the control targets, and, currently, optimum values for each cannot be calculated using an algebraic method but must be determined by trial and error. Therefore, there are times when an excessive amount of time is needed in order to calculate the response lag settings.

[0066] In contrast to this, in the present embodiment, because the IMC 422 is provided in addition to the PID controller 421, and a structure is provided beforehand in which the responsiveness obtained from valve control provides a first order lag, as is described above, of the three control coefficients, only the proportional gain Kp needs to be adjusted in order to set the time constant. Accordingly, without placing any heavy load on the calculation circuits, the response lag can easily be set automatically such that is the same as the set time constant.

[0067] Note that the present invention is not limited to the above-described embodiment.

[0068] For example, in the above-described embodiment, the response lag generating section 46 refers to the response lag storage section 45, and retrieves the measurement time constant in the same type of ambient conditions from the past as well as the proportional gain from that past time, and then, based on these, determines the proportional gain, however, it is also possible to determine the measurement time constant by matching the ambient conditions in advance to the ambient conditions during the actual flow rate control, and then calculating the proportional gain based on these such that the set time constant is established. If this type of structure is employed, then there is no need to store past ambient conditions in the response lag storage section.

[0069] It is also possible to set the proportional gain by performing feedback control to gradually change the proportional gain such that there are minimal deviations between the measurement time constant and the set time constant.

[0070] During the actual flow rate control, if the temperature of the fluid or the pressure of the fluid before and behind the valve varies, then in order to maintain responsiveness, it is also possible to correct the proportional gain K.sub.p1 in the response lag generating section using either one or more of these. For example, the following Formula (9) shows an example of a correction calculation based on variations in the pressure p.

K.sub.p1′=(X.sub.np.sup.n+X.sub.n-1p.sup.n-1+ . . . +X.sub.1p)K.sub.p1+b  (9)

[0071] Here, X.sub.1˜X.sub.n are coefficients determined in advance via experiment and the like, and K.sub.p1 is the proportional gain after the correction calculation.

[0072] Moreover, as is shown in FIG. 5, it is also possible for the response speed to be set by adding a filter 47, which is a calculation circuit that implements first order lag processing and the like on target flow rate values, and then setting the time constants. In this case, the target flow rate values are corrected by the filter 47.

[0073] Furthermore, because the responsiveness of the flow rate control device, in particular, the responsiveness of the valve 3 is changed by the pressure of the fluid (i.e., the pressure on the upstream side and/or the pressure on the downstream side of the valve), it is more preferable for control to be performed such that these pressure changes are canceled out and the response lag is maintained (i.e., is compensated) at a set value. Specifically, a structure may be employed in which, for example, the time constant of the filter 47 shown in FIG. 5 changes in accordance with the measured fluid pressure (here, the pressure on the upstream side) so that changes in the responsiveness caused by pressure are compensated.

[0074] It is also possible for either limits to not be set or for limits to be set for response lag values that are able to be input using the response lag input section (for example, set time constants).

[0075] In the case of the former, for example, an input in which the value of the response lag is 0 is also received, however, in this case, the flow rate control device operates in a state of maximum responsiveness, namely, in a state in which the response lag is at a minimum. In other words, if a value that exceeds the maximum possible responsiveness of the flow rate control device is input as the response lag, then this value is disregarded, and the flow rate control device operates at its maximum possible responsiveness.

[0076] In the case of the latter, even if a response lag such as, for example, 0 that exceeds the maximum possible responsiveness of the flow rate control device is input, then instead of accepting this, either a new input may be asked for, or else the minimum response lag value that is possible in accordance with the specifications may be displayed, and then the flow rate control device may be operated using this response lag.

[0077] Furthermore, it should be understood that the present invention is not limited to the above-described embodiment, and that various modifications and the like may be made thereto insofar as they do not depart from the spirit or scope of the present invention.

DESCRIPTION OF THE REFERENCE NUMERALS

[0078] 100 . . . Flow rate control device

[0079] 3 . . . Fluid control valve

[0080] 43 . . . Response lag input section

[0081] 44 . . . Response lag measurement section

[0082] 45 . . . Response lag storage section

[0083] 46 . . . Response lag generating section