METHOD FOR PROCESSING A MEASUREMENT SIGNAL FROM A PRESSURE MEASUREMENT CELL, AND A MEASUREMENT CELL ARRANGEMENT

Abstract

Method for determining a pressure in a pressure cell, wherein the method consists in the fact that a measuring signal (x) is determined, which is at least proportional to a measured pressure in the pressure cell, that an output signal (y) is produced from the measuring signal (x) using a filter unit (10) having a transfer function, in that a noise signal contained in the measuring signal is at least reduced, and preferably eliminated, that a time change of the measuring signal (x) is determined, and that the transfer function is set in a function of the time change of the measuring signal (x). In addition, a measuring cell arrangement is provided.

Claims

1. A method of determining a pressure a pressure cell (2), wherein the method consists in the fact, that a measuring signal (x) is determined that is at least proportional to a measured pressure in the pressure cell (2), that an output signal (y) is produced using a filter unit (10) that has a transfer function from the measuring signal (x), in that a noise signal contained in the measuring signal (x) is at least reduced and preferably eliminated, and is characterized in that a time change of the measuring signal (x) is determined and the transfer function is set in the function of the time change of the measuring signal (x).

2. The method according to claim 1, characterized in that the pressure in the pressure cell (2) is set at least proportional to the output signal (6).

3. The method according to claim 1, characterized in that the transfer function at least in a first order has a low pass characteristic, wherein its time constant is set in the function of the time change (x/T) of the measuring signal (x).

4. The method according to claim 3, characterized in that an average value of the measuring signal (x) is determined, that a difference signal (3) is determined by a difference formation between the measuring signal (x) and the average value of the measuring signal (x), and that the time change (x/T) of the measuring signal (x) is derived at least from the difference signal e.

5. The method according to claim 4, characterized in that the average value of the measuring signal (x) is determined using an exponential average filter, which is defined for a time-discrete measuring signal (x) by
f.sub.n=.sub.1.Math.x.sub.n+(1.sub.1).Math.f.sub.n1 wherein f.sub.n is the time-discrete average of the measuring signal (s), .sub.1 is a variable, x.sub.n is the time-discrete measuring signal (x), and n is a time-dependent index, wherein the variable .sub.1 especially has a value between 1 and 0, especially preferably between 1 and 0.1, and most especially between 0.85 and 0.95.

6. The method according to claim 4, characterized in that the time change (x/T) of the measuring signal (x) is determined by formation of an average of the difference signal (e).

7. The method according to claim 4, characterized in that the time change (x/T) of the measuring signal (x) is determined using an exponential average filter, which is defined for a time-discrete difference signal (e) by
(x/T).sub.n=.sub.2e.sub.n+(1.sub.2).Math.(x/T).sub.n1 where (x/T) is the time-discrete time change of the measuring signal (x), .sub.2 is a variable, e is the time-discrete difference signal and n is a time-discrete index, wherein the variable .sub.2 especially has a value between 1 and 0, especially preferably between 0.5 and 0.01, and most especially preferably between 0.05 and 0.15.

8. The method according to claim 1 , characterized in that the transfer function is defined by the formula
y.sub.n=.Math.x.sub.n+(1).Math.y.sub.n1 wherein y is the time-discrete output signal, x is the time-discrete measuring signal, is a variable whose value depends on the time change of the measuring signal (x), and n is a time-dependent index.

9. The method according to claim 8, characterized in that the measuring signal (x) is processed in a fast path (20) for producing an output pulse (FC), wherein the output pulse (FC) of the fast path (20) is active at least as long as measuring signal change measured during at most three sampling intervals is greater than the noise measured in the same time period in the measuring signal (x) or in the measuring signal change.

10. The method according to claim 9, characterized in that the measuring signal (x) further is processed in a slow path (30) for producing a switching signal (SC), wherein the switching signal (SC) of the slow path (30) is active at least as long as change in the measuring signal (s) measured for longer than 2*TS is greater than the noise in the measuring signal (x) measured in the same time period or in the measuring signal change, wherein TS is a predetermined minimal pulse width of the output pulse (FC) and that the variable obtains a value depending on an OR operation between the output pulse (FC) and the switching signal (SC).

11. The method according characterized in that the variable at least after a predetermined transition time after a switching process assumes either the value .sub.1 or the value .sub.2, wherein the value for .sub.1 lies especially in the range of 0.01 to 0.9 and wherein the value for .sub.2 lies especially in the range of 0.0001 to 0.01.

12. The method according to claim 8, characterized in that switching from a value .sub.1 to a value .sub.2 takes place over a timespan F.sub.in and/or that switching from a value .sub.2 to a value .sub.1 takes place over a timespan F.sub.out.

13. A measuring signal arrangement with a pressure cell (2) and a membrane pressure measuring cell (2) that is functionally connected to the pressure cell (2) and produces a pressure-dependent measuring signal (x), which is applied to a filter unit (10) having a transfer function for producing an output signal (y), characterized in that a time change of the measuring signal (x) may be determined and that the transfer function may be set in the function of the time change of the measuring signal (x).

14. The measuring cell arrangement according to claim 12, characterized in that the output signal (y) may be used for setting the pressure in the pressure cell (2).

15. The measuring cell arrangement according to claim 13, characterized in that the transfer function has at least in the first order a low pass characteristic, wherein its time constant may be set in the function of the time change (x/T) of the measuring signal (x).

16. The measuring cell arrangement according to claim 15, characterized in that an average of the measuring signal (x) may be determined, that a difference signal (e) may be determined by means of a difference formation between the measuring signal (x) and the average of the measuring signal (x), and that the time change (x/T) of the measuring signal (x) may at least be derived from the difference signal (e).

17. The measuring cell arrangement according to claim 16, characterized in that the average of the measuring signal (x) may be determined using an exponential average filter, which is defined for a time-discrete measuring signal (x) by
f.sub.n=.sub.1.Math.x.sub.n+(1.sub.1).Math.f.sub.n1 wherein f.sub.n is the time-discrete average of the measuring signal (x), .sub.1 is a variable, x.sub.n is the time-discrete measuring signal (x), and n is a time-dependent index, wherein the variable .sub.1 especially has a value between 1 and 0, especially preferably between 1 and 0.1, and most especially preferably between 0.85 and 0.95.

18. The measuring cell arrangement according to claim 17, characterized in that the time change (x/T) of the measuring signal (x) may be determined by formation of an average of the difference signal (e).

19. The measuring signal arrangement according to claim 16, characterized in that the time change (x/T) of the measuring signal (x) may be determined using an exponential average filter, which is defined for a time-discrete difference signal (e) by
(x/T).sub.n=.sub.2e.sub.n+(1.sub.2).Math.(x/T).sub.n1 wherein (x/T) is the time-discrete time change of the measuring signal (x), .sub.2 is a variable, e.sub.n is the time-discrete difference signal (e) and n is a time-dependent index, wherein the variable .sub.2 especially has a value between 1 and 0, especially preferably between 0.5 and 0.01, and most especially preferably between 0.05 and 0.15.

20. The measuring cell arrangement according to claim 14, characterized in that the transfer function is defined by the formula y.sub.n=.Math.x.sub.n+(1).Math.y.sub.n1 wherein 6 is the time-discrete output signal, x.sub.n is the time-discrete measuring signal (x), is a variable whose value depends on the time change of the measuring signal (x), and n is a time-dependent index.

21. The measuring cell arrangement according to claim 20, characterized in that that the measuring signal (x) is applied to a fast path (20) to produce an output pulse (FC), wherein output pulse (FC) of the fast path (20) is active for at least as long as the measuring signal change measured during at most 3 sampling intervals is greater than the noise measured in the measuring signal (x) or in the measuring signal change in the same time period.

22. The measuring cell arrangement according to claim 21, characterized in that the measuring signal (x) is further applied to a long path (30) for producing a switching signal (SC) wherein the switching signal (SC) of the slow path (30) is active at least as long as the change in the measuring signal (x) measured for longer than 2*TS is greater than the noise measured in the same time period in the measuring signal (x) or in the measuring signal change, wherein TS is a predetermined minimal pulse width of the output pulse (FC) and that the variable obtains a value depending on an OR operation between the output pulse (FC) and the switching signal (SC).

23. The measuring cell arrangement according to claim 20, characterized in that the variable at least after a predetermined transition time following a switching process assumes either the value .sub.1 or the value .sub.2, wherein the value for .sub.1 especially lies in the region of 0.01 to 0.9 and wherein the value for .sub.2 especially in the region of 0.0001 to 0.01.

24. The measuring cell arrangement according to claim 20, characterized in that between the filter unit (10) and the decision unit (12) a transition unit (51) is provided, in which switching from a value .sub.1 to a value .sub.2 over a timespan F.sub.in takes place and/or switching from a value .sub.2 to a value .sub.1 over a timespan F.sub.out.

Description

[0051] Below embodiments of the present invention are explained in detail by drawings. Wherein:

[0052] FIG. 1 shows a measuring cell arrangement with a membrane pressure cell, connected to a process chamber, with which a measuring signal is determined, which after processing according to the invention is supplied to a valve,

[0053] FIG. 2 shows a block diagram of the signal processing unite according to FIG. 1 with a calculation unit for processing of the measuring signal,

[0054] FIG. 3 shows a block diagram of a first embodiment of the calculation unit according to FIG. 2,

[0055] FIG. 4 shows a block diagram of a second embodiment of the calculation unit according to FIG. 2,

[0056] FIG. 5 shows a signal flow diagram that depicts the method steps occurring in a decision unit,

[0057] FIG. 6 shows a block diagram of a further embodiment for the calculation and decision unit according to the invention with a fast and a slow signal path,

[0058] FIG. 7 shows a block diagram for the fast signal path according to the invention according to FIG. 6,

[0059] FIG. 8 shows a block diagram for the slow signal path according to the invention according to FIG. 6, and

[0060] FIG. 9 proceeding from FIG. 2, a further embodiment with a transition unit between the calculation unit and filter unit.

[0061] FIG. 1 shows in a highly simplified block diagram a measuring cell arrangement with a process chamber 1, a membrane pressure cell 2, a vacuum pump 3, a signal processing unit 4, a control unit 5, a valve actuator 6, and a valve 7. The membrane pressure cell 2 is used for determining the pressure in the process chamber 1, in which a pressure predetermined in accordance with the specification of a vacuum process is set. Vacuum processes comprise the most diverse methods, such as for example coating procedures, etching procedures, heat treatment of workpieces, etc. Vacuum processes are also often carried out with supporting gases, which are needed both actively as a reactive gas or as an inert case in the process. The gases are supplied for this purpose to the process chamber 1 via the valve 7 controlled by the valve actuator 6, with which the gas supply and the pressure in the process chamber 1 may be controlled. By means of the membrane pressure cell 2, a measuring signal x is produced which is processed in the signal processing unit 4 and the control unit 5 to a control signal s for the valve actuator 6. For process control, it is necessary that the membrane pressure measuring cell 2 on the one hand be as precise as possible, but on the other hand also makes quick measurements in order to be able to react as quickly and precisely as possible to pressure changes in the process chamber 1.

[0062] It is also conceivablein a simplified embodiment of the present inventionthat the output signal y of the signal processing unit 4 is not used for controlling the pressure in a process chamber. In this case this is not a closed system but an open system. Here a pressure is measured in a pressure cell of any desired typesimilarly to in the process chamber according to FIG. 1with a pressure cell 2. The measuring signal x measured with the pressure cell 2 is likewise processed in a signal processing unit 4 in order to obtain an output signal y that is stable and noise-free but reacts quickly to changes.

[0063] The invention now relatesagain with consideration of the embodiments according to FIG. 1two processing of the measuring signal x in the context of conditions present in a vacuum process and above all is intended for optimal signal processing of the measuring signal x, as it can occur as the pressure signal in such vacuum processes. Here the signal processing in the signal processing units 4 can basically be in analog or digital form, wherein in the following we shall not go into the special preparations that are carried out when signal processing is in analog or digital form as such preparations (analog/digital conversion, filtering to avoid aliasing, selection of sampling frequency etc.) are sufficiently known to the person skilled in the art.

[0064] The output signal y of the signal processing unit 4 is processed further in the control unit 5 for example with a so-called P-, PI-, PID- or state controller. The controller implemented in the control unit 5 is responsible in particular for optimal tracking of the control signal s for the valve actuator 6 or for the valve 7.

[0065] Basically, the statements regarding processes in the signal processing unit 4 and their block diagram images both for the embodiments in a closed system and for the embodiments in an open system are valid.

[0066] FIG. 2 shows schematically and in simplified form a block diagram for illustration of the processing steps that are carried out in the signal processing unit 4 (FIG. 1) according to the present invention. For implementing the processing steps of the algorithm according to the invention for example a signal processor is used, which is accordingly programmed. Naturally the signal processor can also perform other tasks if the processor capacity is sufficient. In particular it is conceivable that the controller of the control unit 5 is implemented in the same signal processor.

[0067] As is plain from FIG. 2, the measuring signal x of a filter unit 10 is supplied that produces the output signal y. The filter unit 10 with the measuring signal x and the output signal y form the actual signal path of the signal processing unit 4 (FIG. 1). The remaining components that are still to be explained such as the calculation unit 11 and the decision unit 12 are provided for establishing the characteristic of the filter unit 10.

[0068] The filter unit 10 has a filter characteristic that is defined in a time-discrete system for example according to the following equation:

y.sub.n=.Math.x.sub.n+(1).Math.y.sub.n1

[0069] Here y is the time-discrete output signal, x is the time-discrete measuring signal, n is a time-dependent index, and is a variable whose value decisively determines the time constant of the filter unit 10. The object of the present invention is optimal setting of the value for the variable in such a way that a noise signal in measuring signal x is suppressed to the extent possible or even eliminated, but at the same time a changing pressure in the process chamber is identified so as to be able to react to it with the appropriate speed.

[0070] The mentioned equation with the variable has as the filter characteristic for suppression of the noise signal portion a low pass characteristic, wherein the time constant can be defined for a first-order filter as follows:

[00003]$=.Math..Math.T.Math.\frac{1-}{}$

[0071] The choice of values for the variable is decisive for the present invention. When there is a stable pressure value, if the measuring signal x receives only a noise signal, the value of is to be chosen as small as possible (.sub.min), for example 0.01. In this way, the noise signal presence in the measuring signal x is maximally suppressed and the filtered output signal y is preferably suited for use in the downstream controller of the control unit 5 (FIG. 1), for a stable output signal leads to lower activity of the valve actuator 6 or of the valve 7 and thus to a reduced load on these components were by their probability of failure with respect to known systems is considerably reduced.

[0072] On the other hand, a change in the measuring signal x based on an actual pressure change in the process chamber must be detected without delay, which makes a different value for the variable necessary, namely for example of value for between 0.3 and 1.0 (.sub.max).

[0073] The adjustment of the value for the variable is implemented according to the invention depending on the time change of the measuring signal x, which is explained below in detail.

[0074] FIG. 3 shows in a schematic illustration a block diagram of a first embodiment for defining the time change of the measuring signal x. Essentially the measuring signal x is smoothed out in a smoothing unit 13, for example using an average filter. It has been shown that here in particular a so-called exponential average filter is especially suited. The output signal f of this filter is sent to a difference unit 14, which determines the difference from the unchanged measuring signal x, which is below also defined below as a difference signal e.

[0075] The difference signal e is a measure for the time change of the measuring signal x and is used in this embodiment according to the invention for setting the value for the variable in the filter unit 10 (FIG. 2), wherein at most scaling is still required.

[0076] The smoothing unit 13 implemented by means of an exponential average filter is defined by the recursive formula

f.sub.n=.sub.1.Math.x.sub.n+(1.sub.1).Math.f.sub.n1

[0077] wherein f is the time-discrete output signal, .sub.1 is a variable, x is the time-discrete measuring signal and n is a time-dependent index, wherein the variable .sub.1 with the exponential average filter for producing the difference signal f preferably has a value between 1 and 0, especially preferably between 1 and 0.1, and most especially preferably between 0.85 and 0.95.

[0078] The calculation unit 11 according to FIG. 2 in the embodiment according to FIG. 3 thus produces a difference signal e as follows:

e.sub.n=f.sub.nx.sub.n

[0079] FIG. 4 shows, again in schematic representation, a block diagram of a further embodiment of the calculation unit 11 (FIG. 2). This is about product of the time change of the measuring signal x, which may also be terms x/T, in two stages, wherein the first stage is identical to the individual stage according to FIG. 3 According to FIG. 4, a further processing of the difference signal e with an average filter 14 is carried out, which average filter 15 can again for example be implemented as an exponential average filter. Consequently, the same definitions apply; see above formulations, as they were as early as the first stage. Individually the value for the variable , which was accordingly designated 2, has a different value: the variable .sub.2 preferably acquires with the exponential average filter for determining the time change of the measuring signal x a value between 1 and 0, especially preferably between 0.5 and 0.1, most preferably a value between 0.5 and 0.15.

[0080] The time change x/T of the measuring signal x can thus be defined as follows from the time-discrete difference signal e:

(x/T).sub.n=.sub.2e.sub.n+(1.sub.2).Math.(x/T).sub.n1

[0081] wherein (x/T) is the time-discrete time change of the measuring signal x, e.sub.n is the time-discrete difference signal e, and n is time-dependent index.

[0082] Also in this embodiment of the present invention, the time change x/T or (x/T).sub.n hereby obtained of the measuring signal x at most has to be scaled, as was already explained in connection with the embodiment according to FIG. 3.

[0083] FIG. 5 shows a further embodiment of the present invention, wherein this is a special embodiment of the decision unit 12 (FIG. 2). According to the embodiment already explained with reference to FIGS. 3 and 4, the decision unit 12 contains at most a scaling of the time change x/T of the measuring signal x obtained by the calculation unit 11. In continuation of this embodiment of the invention, it is recommended that the range of values for the values a be limited to the lower and upper end. Accordingly, in accordance with the further embodiments of the present invention shown in FIG. 5, it is provided that an .sub.min and an .sub.max be provided, wherein these take effect in accordance with the flow diagram shown in FIG. 5:

[0084] The time change x/T of the measuring signal x is scaled with a factor k (as already explained in connection with the embodiments shown in FIGS. 3 and 4). The scaled time change x/T is designated as . There now follows a series of decisions that have the objective of limiting the establishment of within a range of values between a minimal value .sub.min and a maximal value .sub.max. Between the extreme values .sub.min and .sub.max, the value of a is accordingly set to the result of the calculation unit 11 (FIG. 2), whether the latter was implemented according to FIG. 3 or according to FIG. 4.

[0085] FIG. 6 shows a further embodiment of the present invention, wherein here in schematic representation a block diagram is shown for the calculation unit 11 and the decision unit 12 shown in FIG. 2, which in FIG. 2 are enclosed in a broken line, and below may also be designated as the slope determination unit 50.

[0086] All of the following specific numerical examples (in particular for the time constant ) procedure from a typical sampling time T (which may also be termed the cycle time) of 1 ms. Naturally the sampling time T of 1 ms only serves as an example. The sampling time T is basically selected within the scope of the available calculating performance and the necessary reaction time of the entire system.

[0087] As is plain in FIG. 65, the slope determination unit 50 initially comprises two functional blocksnamely the fast block 20 and the slow block 30. These two functional blocks 20 and 30, which are yet to be explained, are also designated the fast path 20 and the slow path 30.

[0088] In terms of quality, the following principles can be formulated with regard to parameters .sub.1 and .sub.2 of filter unit 10 (FIG. 2):

[0089] Measuring signals x, which do not change in the time function (i.e., if no pressure changes are present), can be filtered intensely so as to maintain a maximal noise suppression. It has been shown that for the parameter .sub.2, values between 0.0001 (10 s) and 0.01 (100 ms) are suited. A preferred value for the parameter .sub.2 here is 0.001 (1 s).

[0090] Measuring signals x that change in a function of time (i.e., if pressure changes are present) need to be less intensely filtered. In this case, the parameter .sub.1 defines the damping factor. This is therefore typically chosen to be larger than the parameter .sub.2. It has been shown that for the parameter .sub.1, values between 0.1 (100 ms) and 0.9 (0.1 ms) are suitable. A preferred value for the parameter .sub.1 here is 0.1 (9 ms).

[0091] As already mentioned, the slope determination unit 50 initially consists of the two functional blocks fast 20 and slow (30, wherein the functional block fast produces an output signal FC for fast changes and the functional block slow 30 an output signal SC for slow changes, from which a control signal SW is obtained by an OR operation as follows:

SW=FC OR SC

[0092] The result of an active control signal SWas follows from the flow diagram of FIG. 6 after the OR gate is that the value .sub.1 is used in the filter unit 10 (FIG. 2). On the other hand, .sub.2 is used in the filter unit 10 when the control signal SW is inactive.

[0093] The functional block fast 20 detects and reacts within a sampling interval T (wherein the sampling interval T again is 1 ms, for example) to fast changes of the measuring signal x, but is relatively insensitive to slow or constant measuring signal changed. The slow or constant measuring signal changes are detected by the functional block slow 30.

[0094] The boundary between slow and fast measuring signals x is indicated by the functional block fast 20:

[0095] If the frequency of the measuring signal x is smaller than

[00004]$\frac{1}{2.Math.\mathrm{TS}},$

from the standpoint of the functional block fast 20 this is a slow measuring signal x; otherwise it is a fast measuring signal x. The meaning of these statements and the resultant reaction will be examined in detail in connection with the explanations of the functional blocks 20 and 30.

[0096] FIG. 7 shows the functional block fast 20 in accordance with FIG. 6. The measuring signal x is sent to the calculation unit 11, which is shown in FIG. 4 and is explained in detail. Accordingly, those statements are also valid for this embodiment of the invention.

[0097] The output signal x/T of the calculation unit 11 is sent to a value unit 21, in which the value of x/T is determined and sent to an addition unit 25. In a further value unit 22, the value of the difference signal e likewise determined in the calculation unit 11 is obtained. The value signal |e| is then again smoothed in an average filter 23 with the parameter .sub.3 in accordance with the following formula:

h.sub.n=.sub.3.Math.|e.sub.n|+(1.sub.3).Math.h.sub.n1

[0098] wherein the output signal h.sub.n after scaling with the factor CF in a multiplication unit 24 is sent to the additional unit 25, where the difference between the value signal

[00005]$.Math.\frac{.Math..Math.x}{.Math..Math.T}.Math.$

and CF.Math.h is determined. The result is sent to a threshold value detector 26, which produces a trigger when a predetermined threshold value is exceeded, which is sent to a monoflop 27. The monoflop 27, which is formed for example as a retriggerable monoflop, after receipt of a trigger at the input, produces an output pulse CF, whose length may be set over the pulse width TS. In this regard, retriggerable means that a trigger arriving during the time process restarts the internal time of the monoflop each time, and the active switching state is accordingly extended in time.

[0099] As already explained above, the signal x/T constitutes a measure for the change in the measuring signal x. By filtering of the amount of the difference signal e with the average filter 23 and subsequent scaling with the factor CF, the signal CF.Math.h is obtained. This is now a measure for the basic noise of the measurement of the measuring signal change x/T. By comparing the signals CF.Math.h and the amount of x/T one thus obtains the binary control signal trigger, which is used to control the monoflop 27.

[0100] It has been shown that the damping factors .sub.1 and .sub.2 and .sub.3 in particular should have the following values:

[0101] For .sub.1 in a range of 0.1 to 0.001 (9 ms to 1 s), especially 0.01 (100 ms) as a typical value; for .sub.2 in the range of 01 to 0.001 (9 ms1 s), especially 0.01 (100 ms) as a typical value, and for .sub.3 in the range of 0.01 to 0.0001 (100 ms to 10 ms), especially 0001 (1 s) as a typical value.

[0102] Proceeding from the pressure monitoring and pressure setting system shown in FIG. 1, the following can be established: each fast pressure change (pressure jump) in the process chamber 1 produces a trigger pulse. The width of this pulse depends on the selected damping factors .sub.1 and .sub.2. Especially small pressure changes in real vacuum systems have time constants in the range >10 ms. In order to ensure that there is no switching within a pressure change (flank) between the fast and slow filter, a retriggerable monoflop 27 is used to ensure that the output pulse FC has at least the pulse width TS. The preferred value range for the pulse width TS lies for example between 50 ms (especially with small, fast vacuum systems) and 5 s (especially for a large, slow vacuum systems). A typical value for the pulse width TS is 500 ms. The scaling factor CF for example has a value of 0.15.

[0103] For measuring signals x which have frequencies smaller than

[00006]$\frac{1}{2.Math.\mathrm{TS}},$

the functional block fast 20 does not respond or responds with lower reliability than desirable. This is in particular because the pulse width TS of the monoflop 27 (FIG. 7) is too short so as to be able to cover a full signal period. Measuring signals x with a frequency of smaller than

[00007]$\frac{1}{2.Math.\mathrm{TS}}$

are therefore in the embodiment shown in FIG. 6 processed slowly by the functional block. It is expressly pointed out, however, that already with the functional block fast 20 alonei.e., without the functional block slow 30very good results can be obtained.

[0104] One embodiment for the functional block slow 30 (see FIG. 6) is shown in FIG. 8. Here the functional block slow 30 is set up for measuring signals x with a frequency of smaller than

[00008]$\frac{1}{2.Math.\mathrm{TS}},$

wherein the frequencies typically are small than 1 Hz, assuming a pulse width TS of 500 ms for example. The functional block slow 30 calculates a switching signal SC as an outpatient as follows:

[00009]$\mathrm{SC}=\{\begin{array}{c}\mathrm{active}.Math..Math.\mathrm{if}.Math..Math.\mathrm{SSN}<\mathrm{SS}\\ \mathrm{inactive}.Math..Math.\mathrm{if}.Math..Math.\mathrm{SSN}\mathrm{SS}\end{array}$

[0105] wherein SS is a measure for the change of the measuring signal x over a longer time period, which for example is longer than 2.Math.TS (i.e., double the pulse width TS) and thus typically amounts to seconds, and wherein SSN is a measure for the noise of the measuring signal x. Both SSN and SS are determined using the average filters of the already described type. The transfer function of the average filter was explained in connection with the description of the filter unit 10 of FIG. 2.

[0106] SS is obtained with the further average filters 35 and 38 (FIG. 8) analogously to the average filters 13 of FIGS. 4 and 15 of FIG. 7. The only difference lies in the size of the damping factors .sub.5 and .sub.6, which are no optimized for smaller frequency ranges.

[0107] It has been shown that the damping factors .sub.5 and .sub.6 should especially have the following values:

[0108] For .sub.5 in the range of 0.01 to 0.0001 (100 ms to 10 s), especially 0.001 (1 s) as a typical value; and for .sub.6 in the range of 0.1 to 0.0001 (100 ms to 10 s), especially 0.001 (1 s) as a typical value.

[0109] The signal calculated in the just described manner is substantially a measure for the sum of the change of the measuring signal x and the noise of the measuring signal x. With a high pass filter 31 and a further average filter 33, the independent signal SSN is now calculated from (slow) changes of the measuring signal x. This is thus a measure for the noise of the measuring signal x, and by comparison with the signal SS, one obtains the desired switching signal SC according to the conditions indicated above.

[0110] It has been shown that the damping factors .sub.4 for example should lie in a range of 0.005 to 0.0005 (200 ms to 20 s), in particular should be equal to 0.0005 (2 s).

[0111] The output signal of the average filter 33 is connected for scaling to a multiplier unit 34, whose second input is acted on with a scaling factor CS for producing the output signal SSN. It has been shown that the scaling factor CS has a value of 50, for example.

[0112] The task of the high pass filter 34 is separating noise and slow changes in the measuring signal x. Assuming that the noise of the measuring signal is normally distributed over the assessable frequency range of 0-1 kHz (with a typical sampling interval T of 1 ms), a high pass filter 34 in accordance with the following configuration has proven to be suitable:

[0113] Filter type: high pass filter

[0114] design method: elliptical

[0115] sampling frequency: 1 kHz

[0116] cutoff frequency in pass band: 400 Hz

[0117] oscillations in pass band range: 3 dB

[0118] cutoff frequency in stop range: 250 Hz

[0119] damping in stop range: 73 dB

[0120] Under these conditions, a fourth-order high pass filter is provided, which can be implemented problem-free, can be designed and implemented with acceptable expense.

[0121] FIG. 9 shows a further embodiment of the present invention, in which the filter unit 10 according to the invention switches optimally quickly and accordingly abruptly between the two damping factors .sub.1 and .sub.2.

[0122] It is conceivable that this abrupt switching is not tolerated by all of the subsequent controllers in the control unit 5 (FIG. 1). Therefore, the switching between .sub.1 and .sub.2 according to the now further embodiment of the invention can be less abrupt by activation of a fade-in/fade-out option. Toward this endas is plain from FIG. 9a transition unit 51 is provided between the determining unit 50 and the filter unit 10. The transition unit 51 is assigned two additional parameters F.sub.in and F.sub.out.

[0123] In the following, the function of the transition unit 51 is explained: the two additional parameters F.sub.in and F.sub.out define two timespans, which are used in switching of the damping factor of .sub.1 to .sub.2 or vice versa, wherein depending on the transition direction either the timespan F.sub.in or the timespan F.sub.out is definitive: If the switch has to be made from .sub.2 to .sub.1 (thus a pressure change occurs), the timespan F.sub.in is used during which a soft transition from .sub.2 to .sub.1 is carried out. In the reverse direction thus when stable pressure conditions again dominate after a pressure change the switch must be from .sub.1 to .sub.2. This happens according to this embodiment likewise no longer abruptly, but within the timespan defined by F.sub.out. Again a softer transition from .sub.1 to .sub.2 occurs.

[0124] It has been shown that for the two timespans F.sub.in or F.sub.out, for example, the following values are suitable:

[0125] For the timespan F.sub.in in the range of 0 to 100 ms, especially 10 ms as a typical value; and for the timespan F.sub.out in the range of 0 to 10 s, especially 1 s is a typical value.