VALVE ASSEMBLY AND METHOD FOR REGULATING THE PRESSURE OF A FLUID

Abstract

A valve assembly includes a proportional valve having an opening cross section that can be continuously varied by an actuator; a sensor for sensing the valve output pressure; a digital regulating device; and a switching valve disposed parallel to the proportional valve. The opening cross section of the switching valve is smaller than the maximum opening cross section of the proportional valve. The regulating device is programmed (i) to automatically calculate, at runtime, using the currently given valve output pressure and the current position of the actuator, the maximum working pressure achievable at the maximum opening of the proportional valve with the valve, (ii) and to additionally open the switching valve when the computed maximum achievable working pressure falls below a predefinable target working pressure by a definable deviation value.

Claims

1. A valve assembly for controlling a pressure of a fluid, comprising: an actuator; a proportional valve operably connected to the actuator, the proportional valve having an opening cross section that can be continuously varied by the actuator; a sensor operably connected to the proportional valve and configured to sense a valve output pressure of the proportional valve; a digital controlling device; and a switching valve arranged parallel to the proportional valve and having an opening cross section, wherein the opening cross section of the switching valve is smaller than a maximum opening cross section of the proportional valve, wherein, during a running time, the digital controlling device is configured to automatically calculate a working pressure achievable as a maximum with the proportional valve when the proportional valve is configured at the maximum opening cross section based on a currently given valve output pressure from the sensor and a current position of the actuator, and wherein the digital controlling device is further configured to open the switching valve when the calculated maximum achievable working pressure falls below a predefinable target working pressure by a determinable deviation value.

2. The valve assembly as claimed in claim 1, wherein the calculation of the maximum achievable working pressure is further based on a pneumatic resistance of a further routing of a line from a valve output to an end of the line on a working side of the valve assembly.

3. The valve assembly as claimed in claim 2, wherein the calculation of the maximum achievable working pressure is further based on an outflow characteristic of the end of the line on the working side of the valve assembly.

4. The valve assembly as claimed in claim 1, wherein the calculation of the maximum achievable working pressure is further based on a drop in supply pressure occurring when there is an increase in the opening cross section of the proportional valve.

5. The valve assembly as claimed in claim 1, wherein, to stabilize a control behavior, a determination of an opening time of the switching valve is based on a switching hysteresis.

6. A method for controlling a pressure of a fluid by a valve assembly, the method comprising: controlling an opening cross section of a proportional valve of the valve assembly using an actuator; sensing, using a sensor, a valve output pressure of the proportional valve, wherein a switching valve of the valve assembly is arranged parallel to the proportional valve, wherein an opening cross section of the switching valve is smaller than a maximum opening cross section of the proportional valve; automatically calculating at a running time, using a digital controlling device, a working pressure achievable as a maximum with the proportional valve when the proportional valve is at a maximum opening cross section based on a currently given valve output pressure and a current position of the actuator; comparing the calculated working pressure with a predefinable target working pressure; and generating a manipulated variable for an additional opening of the switching valve when the calculated maximum achievable working pressure falls below a predefinable target working pressure by a determinable deviation value.

7. The method as claimed in claim 6, wherein the calculation of the maximum achievable working pressure is further based on a pneumatic resistance of a further routing of a line from a valve output to an end of the line.

8. The method as claimed in claim 7, wherein the calculation of the maximum achievable working pressure is further based on an outflow characteristic of the end of the line.

9. The method as claimed in claim 6, wherein the calculation of the maximum achievable working pressure is further based on a drop in supply pressure occurring when there is an increase in the opening cross section of the proportional valve.

10. The method as claimed in claim 6, further comprising: stabilizing a control behavior of the valve assembly by determining an opening time of the switching valve based on a switching hysteresis.

Description

[0019] Further advantages of the invention are presented in more detail below together with the description of a preferred exemplary embodiment of the invention on the basis of the figures, in which

[0020] FIG. 1 shows an equivalent circuit diagram of a pneumatic valve assembly;

[0021] FIG. 2 shows an electrical equivalent circuit diagram of the valve assembly as shown in FIG. 1;

[0022] FIG. 3 shows a diagram to illustrate the composition of the air volume delivered by the valve assembly.

[0023] FIG. 1 shows a simplified pneumatic equivalent circuit diagram of the valve assembly 1. The valve assembly 1 comprises the proportional valve 2, the switching valve 3 and the microcontroller 4. The essential functional components of the valve assembly 1 are formed in a common housing, symbolized by a dashed frame. The valve assembly 1 is connected via the working line 5 to a volume V (not shown in any more detail in FIG. 1), which is an open volume, such as for example the processing head of a laser cutting device. For this purpose, the working line 5 opens into the volume V at its line end, the line end of the working line 5 in the exemplary embodiment according to FIG. 1 being designed as an open diaphragm 6, which describes a sudden cross-sectional constriction at the end of the line. The proportional valve 2 is designed as a 3/3-way proportional valve. In its rest position in the de-energized state, the proportional valve 2 assumes its closed central position (NC), in which the working line 5 is shut off. To act on the working line 5, the proportional valve 2 can be switched by the proportional solenoid 7a against the spring loading of the mechanical return spring 8b into a first switching position, in which it connects the working line 5 to the compressed air supply 9 with a variable valve opening cross section. To vent the working line 5, the proportional valve 2 can be switched by the proportional solenoid 7b against the spring loading of the mechanical return spring 8a into a second switching position, in which it connects the working line 5 to the vent 10 with a variable valve opening cross section. The proportional solenoids 7a and 7b are electrically activated via the microcontroller 4 integrated in the housing of the valve assembly 1, which serves as a digital controlling device of the valve assembly 1. The microcontroller 4 is formed as a “single-board computer (SBC)” in which all the electronic components necessary for operation (CPU, memory, input and output interfaces, A/D converter, DMA controller, etc.) are combined on a single PCB. The microcontroller 4 receives from the pressure sensor 11 a continuous electrical signal, which represents the respectively current valve output pressure at the proportional valve 2. The microcontroller 4 is also formed with the data communication interface 12, with which it can be connected to a higher-level programmable logic controller (PLC, not shown in any greater detail in FIG. 1). The microcontroller 4 receives from the programmable logic controller at the running time a dynamically predefinable target working pressure p.sub.CMD. The microcontroller 4 is programmed to automatically calculate the working pressure achievable as a maximum in each case when the proportional valve has its maximum opening width at the running time on the basis of the currently given valve output pressure and the current position of the actuator of the proportional valve 2 known to it from the current activation state, and to compare it with the respectively predefined target pressure. Furthermore, the microcontroller 4 is programmed additionally to open the switching valve 3 when the calculated maximum achievable working pressure falls below the currently predefined target working pressure p.sub.CMD by a determinable deviation value in that it activates the switching magnet 13. The switching valve 3 is arranged in the bypass line 14 running between the compressed air supply 9 and the working line 5 and blocks it in its rest position in the de-energized state. In its switching position, the switching valve 3 opens an additional opening cross section between the compressed air supply 9 and the working line 5. The cross-sectional constriction 15 in the bypass line 14 serves to symbolize the characteristic that the switchable line cross section of the bypass line is smaller than the maximum opening width of the proportional valve 2. This can be achieved in practice by the nominal width of the switching valve 3, the cross section of the bypass line 14 or a diaphragm or throttle. The condition that the switchable line cross section of the bypass line 14 is smaller than the maximum opening width of the proportional valve 2 must be satisfied in order that the controlled combined air volume can be increased seamlessly.

[0024] FIG. 2 shows an electrical equivalent circuit diagram of the valve assembly 1 as shown in FIG. 1 to illustrate the parameters to be taken into account when designing a suitable technical calculation model. The pneumatic application system is acted upon on the input side by the compressed air supply 9 with the supply pressure p.sub.1.sub.0. Until it enters the valve assembly 1, the air stream is subjected to the pneumatic resistance R2, which is characterized by the cross section and the nature of the corresponding line routing. The compressed air supply to the valve assembly 1 takes place on the supply side with the pressure p.sub.1. Starting from the valve assembly 1, compressed air is introduced into the working side of the pneumatic application system as a mass flow {dot over (m)}.sub.in with the pressure p.sub.2 controlled by the valve assembly 1. Up to the end of the line, the air stream is subjected to the further pneumatic resistance R1, which is characterized by the cross section and the nature of the corresponding further line routing. As a mass flow {dot over (m)}.sub.out, the air stream finally passes through the end of the line designed as a diaphragm 6, wherein the outflow characteristic of the diaphragm 6 can be characterized by the conductance G and leads to the application of pressure to the volume V, which is identified in the electrical equivalent circuit diagram of FIG. 2 as capacitor C.

[0025] On the basis of the electrical equivalent circuit diagram in FIG. 2, a suitable technical calculation model is shown below: the air stream introduced through the valve assembly 1 into the working line 5 can generally be described as follows as mass flow {dot over (m)}.sub.in:

{dot over (m)}.sub.in=Ψ(p.sub.1.sub.A, p.sub.2.sub.A).Math.p.sub.1.Math.(f.sub.1( . . . ).Math.x(t)+f.sub.2( . . . ).Math.A.sub.V.Math.u(t)),

with [0026] Ψ(p.sub.1.sub.A, p.sub.2.sub.A)—outflow function, [0027] f.sub.1( . . . )—general functional relationship of proportional valve 1, [0028] f.sub.2( . . . )—general functional relationship of switching valve 3, [0029] x(t)—variable opening cross section of proportional valve, [0030] u(t)—switching state of switching valve, [0031] A.sub.V—opening cross section of switching valve, [0032] R—general gas constant, [0033] ν—temperature.

[0034] The pressure increase {dot over (p)}.sub.2 for a constant working volume V (=capacitor C in the equivalent circuit diagram of FIG. 2) can be described by:

[00001]${\stackrel{.}{p}}_2=\frac{R_{\vartheta }}{V}.Math.\left({\stackrel{.}{m}}_{\mathrm{in}}-{\stackrel{.}{m}}_{\mathrm{out}}\right)=\frac{R_{\vartheta }}{C}.Math.{\stackrel{.}{m}}_{\mathrm{in}}-\frac{R_{\vartheta }}{C}.Math.\frac{G}{R1.Math.G+1}.Math.p_2=c_1.Math.{\stackrel{.}{m}}_{\mathrm{in}}-c_2.Math.p_2,$

with the variables c.sub.1 and c.sub.2 having to be determined approximately as constants for the respective pneumatic application system.

[0035] In order that a predefinable target working pressure p.sub.CMD can be achieved with the valve assembly 1, the condition {dot over (m)}.sub.in>{dot over (m)}.sub.out must be satisfied. It follows from this that the switching valve 3 must always be opened when the following applies:

[00002]${\stackrel{.}{m}}_{{\mathrm{out}.Math.}_{p=p_{\mathrm{CMD}}}}=\frac{G}{R1.Math.G+1}.Math.p_{\mathrm{CMD}}=c_2.Math.p_{\mathrm{CMD}}>{\stackrel{.}{m}}_{{\mathrm{in}}_{\max }}=\Psi \left(p_{1_A},p_{2_A}\right).Math.p_1.Math.x_{\max }=c_3\left(p_{1_A},p_{2_A}\right).Math.p_{\max },$

where the variable c.sub.3 is to be determined approximately as a constant for the respective pneumatic application system.

[0036] In order that the controlled combined controlled air volume of the proportional valve 2 and the switching valve 3 can be increased seamlessly, it is also necessary to satisfy the condition that the switchable line cross section of the bypass line 14 is smaller than the maximum opening width of the proportional valve 2:

f.sub.1( . . . ).Math.x.sub.max>f.sub.2( . . . ).Math.A.sub.V.

[0037] With the increase in the flow through the proportional valve 2, the supply pressure p.sub.1.sub.0 drops, which can be described in a simplified form as follows:

p.sub.1 ({dot over (m)}(x))=p.sub.1.sub.0−a.sub.p1{dot over (m)}(x).fwdarw.p.sub.x(x)≈p.sub.1.sub.0−a.sub.px.Math.x.

[0038] When the switching valve 3 is switched off with u=0, the equation for the mass flow {dot over (m)}.sub.in introduced into the working line 5 changes as follows:

[00003]${\stackrel{.}{m}}_{\mathrm{in}}=\frac{\Psi \left(p_{1_A},p_{2_A}\right).Math.p_{10}.Math.f_1(.Math.).Math.x\left(t\right)}{1+\Psi \left(p_{1_A},p_{2_A}\right).Math.a_{p1}.Math.f_1(.Math.).Math.x\left(t\right)}\approx \Psi .Math.f_1.Math.p_1.Math.x\left(t\right)-\Psi .Math.f_1.Math.a_{\mathrm{px}}.Math.x^2\left(t\right).$

[0039] Based on this, a suitable calculation algorithm for the working pressure achievable as a maximum with the proportional valve 2 when it has the maximum opening width can be derived as follows:

[0040] From the equations shown for the pressure increase {dot over (p)}.sub.2 and the mass flow {dot over (m)}.sub.in introduced into the working line 5 when the switching valve 3 is switched off, taking into account the supply pressure drop, the calculable pressure increase {dot over (p)}.sub.2 results in a simplified form in:

{dot over (p)}.sub.2≈c.sub.1 (c.sub.3.sub.1.Math.x(t)−c.sub.3.sub.2.Math.x.sup.2(t)−c.sub.2.Math.p.sub.2(t)).

so that the maximum achievable working pressure p.sub.2.sub.max can be determined as follows:

[00004]$p_{2_{\max }}\approx \frac{c_{3_1}}{c_2}.Math.x_{\max }-\frac{c_{3_2}}{c_2}.Math.x_{\max }^2.$

[0041] The above equation can be discretized as follows

[00005]$p_k=\frac{c_{3_1}}{c_2}.Math.x_k-\frac{c_{3_2}}{c_2}.Math.x_k^2-\frac{1}{c_1.Math.c_2}{\stackrel{.}{p}}_{2_k}.$

[0042] The results of the above calculation are continuously accumulated in a buffer memory during the running time. On the basis of the buffer data collection, to estimate the maximum achievable working pressure at the running time the parameters

[00006]$a_0=\frac{c_{3_1}}{c_2}\mathrm{and}{a}_1=\frac{c_{3_2}}{c_2}$

required for the actuation of the switching valve 3 are continuously solved as an optimization task

[00007]$J=\frac{1}{2}\underset{a_0,a_1}{\min }\left\{{\underset{\_}{e}}^T\underset{\_}{e}\right\}$$\mathrm{with}$$\underset{\_}{e}=\underset{\_}{p}-a_0.Math.\underset{\_}{x}+a_1.Math.\underset{\_}{x^2}.$

Both the data accumulation and the optimization calculation are performed at the running time as background processes.

[0043] On the basis of the calculation bases presented above, the switching time for opening the switching valve 3 is defined by

p.sub.CMD>p.sub.2.sub.max=f(p.sub.1, x.sub.max).

[0044] On the basis of this, the following switching rules can be formulated in practice to further improve the switching behavior:

[0045] To make the switching behavior dynamic, first the general parameter p.sub.offset is introduced, with the effect that the switching behavior is brought forward in time by an additional, predefinable deviation value>0, i.e. the switching valve is already opened before the maximum achievable working pressure falls below the predefinable target working pressure:

[00008]$u=\{\begin{array}{c}1,p_{\mathrm{CMD}}-p_{\mathrm{offset}}\ge p_{2_{\max }}\\ 0,\mathrm{otherwise}\end{array}$

[0046] Bouncing of the valve assembly 1 caused by frequent switching on and off can be avoided by introducing a switching hysteresis with the additional parameters p.sub.thr.sub.upper and p.sub.thr.sub.lower as a result of the following switching rules that stabilize the switching behavior:

[00009]$\{\begin{array}{c}1,u_{k-1}=0\wedge p_{\mathrm{CMD}}-p_{\mathrm{offset}}+p_{{\mathrm{thr}}_{\mathrm{upper}}}\ge p_{2_{\max }}\wedge p_{\mathrm{CMD}}>p_2\\ 1,u_{k-1}=1\wedge p_{\mathrm{CMD}}-p_{\mathrm{offset}}+p_{{\mathrm{thr}}_{\mathrm{lower}}}\ge p_{2_{\max }}\\ 0,u_{k-1}=0\wedge p_{\mathrm{CMD}}-p_{\mathrm{offset}}+p_{{\mathrm{thr}}_{\mathrm{upper}}}<p_{2_{\max }}\\ 0,u_{k-1}=1\wedge p_{\mathrm{CMD}}-p_{\mathrm{offset}}+p_{{\mathrm{thr}}_{\mathrm{lower}}}\ge p_{2_{\max }}\vee p_{\mathrm{CMD}}<p_{{\mathrm{CMD}}_{\min }}\\ 0,{\forall }_{k-1}\le 0\end{array}$

[0047] The parameter p.sub.CMD.sub.min is an additional minimum setpoint, below which the switching valve is switched off and the setpoint pressure p.sub.CMD is only set via the proportional valve 2.

[0048] FIG. 3 shows a diagram of the air stream controlled by the valve assembly 1 in dependence on the predefinable target working pressure p.sub.CMD, the x axis representing the increasing default value p.sub.CMD and the y axis the air volume {dot over (V)} delivered by the valve assembly into the working side of the pneumatic application system. The straight line A shows the total air volume introduced from the valve assembly into the working side of the pneumatic application system with increasing default value p.sub.CMD. With increasing default value p.sub.CMD, the total air volume A is made up of the partial air volume B delivered by the proportional valve 2 and the partial air volume C additionally delivered by the switching valve 3 as from the opening of the switching valve 3 at the switching time T. Up until the switching time T, the total air volume A delivered by the valve assembly is identical to the partial air volume B delivered by the proportional valve 2, which is why, starting from the origin of the coordinates up to the switching time T, graph B initially runs identically to straight line A, i.e. is congruent with it. With the opening of the switching valve 3, the supply pressure p.sub.1.sub.0 drops, which is why, from this time, the partial air volume B delivered by the proportional valve 2 also initially drops. Since, from this time, the partial air volume C delivered by the switching valve 3 at the same time suddenly increases, the combined total air volume A delivered overall by the valve assembly 1 continues to increase linearly.

LIST OF REFERENCE SIGNS

[0049] 1 Valve assembly [0050] 2 Proportional valve [0051] 3 Switching valve [0052] 4 Microcontroller [0053] 5 Working line [0054] 6 Diaphragm [0055] 7a, 7b Proportional solenoid [0056] 8a, 8b Return spring [0057] 9 Compressed air supply [0058] 10 Venting [0059] 11 Pressure sensor [0060] 12 Data communication interface [0061] 13 Switching magnet [0062] 14 Bypass line [0063] 15 Cross-sectional constriction