GB2064135A

GB2064135A - Interference suppression in measuring apparatus

Info

Publication number: GB2064135A
Application number: GB8037565A
Authority: GB
Inventors: Karl-Heinz Schmall
Original assignee: Schmall K H
Current assignee: Schmall K H
Priority date: 1979-11-30
Filing date: 1980-11-24
Publication date: 1981-06-10
Also published as: DE3042781A1; DE3042781C2; CH656702A5

Abstract

In a compensating arrangement for proximity scanning and control devices, in which a high-frequency measuring field exists between a measuring element 7 and a counter element 1, a compensating field having an inverted phase position and the same effective energy emission is generated by a compensating element 8. The compensating field is here generated so close - compared with the wavelength of the high frequency - to the measuring field that the energy emissions of the compensating field and measuring field approximately compensate each other. <IMAGE>

Description

SPECIFICATION Arrangement for the compensation of interfering emissions of electromagnetic highfrequency oscillations in proximity scanning devices The invention relates to an arrangement for the compensation of the electromagnetic interference field emitted by electrodes and/or feelers (sensors) in proximity scanning and control devices, having at least one high-frequency measurement field between a measuring electrode and a counterelectrode.

In proximity scanning devices, predominantly capacitive or inductive sensors are used which are connected to one or more high-frequency circuits and the electric behaviour of which depends on the relative distance of the surfaces or objects, which are to be scanned, from the sensor.

Thus, for example, the distance between a workpiece and a machining tool is measured by connecting the electric capacitance existing between the two within a circuit arrangement in such a way that a change in the capacitance correspondingly varies the electrical values of the circuit arrangement. The same effect is obtained, for example, by inductive scanning if one side of the measuring length consists of one or more coils, which measuring length has a certain inductivity due to the distance from the workpiece and in which the inductivity is varied correspondingly by a change in distance, this method in general requiring an electrically conductive or ferromagnetic material.

The~chånges in the measuring length frequently are quantitively small changes which, however, must be very precisely detected and evaluated. In most cases, the capacitances and/or inductivities which can be obtained are relatively small, since the mutually opposite surfaces or bodies of the measuring lengths have only relatively small dimensions. For this reason, those circuit arrangements are predominantly used in these devices which work at high frequencies and, due to the desired high signal-to-noise ratios, also at high-frequency voltages which are correspondingly high.

It is known to provide measuring lengths of this type in oscillator circuits, the frequency of which is varied when changes occur in the dimensions to be measured.

In practice, a number of devices of this type are known which, for example for measuring levels in containers with non-conductive materials, for detecting the motion of dial instruments and for measuring the distance between workpieces and tools, utilize capacitive and/or inductive members in the measurement circuit of high-frequency oscillators.

The enumeration is incomplete and is meant only to illustrate a few examples.

In a large number of sensor circuits of this type, which are connected to high-frequency circuits, it is not possible to screen the measuring length itself electrically in such a way that the highfrequency electromagnetic energy emitted by the members of the measuring length remains confined to the immediate surroundings of the measuring length.

This case applies, for example, when a relatively large workpiece, for example a steel plate, represents one side of the measuring length, whilst the other side of the measuring length (electrode) is located, as a metal surface acting capacitively, in the vicinity of the steel plate. In such a case, the measuring length is used, for example, for controlling the position of a tool at a distance from the steel plate in such a way that machining of the steel plate can be carried out in an optimum manner. It is not possible to screen the whole machine, which carries out such machining, in such a way that the high-frequency electromagnetic field existing between the steel plate and electrode remains inactive with respect to the exterior.

If an inductive sensor is used in similar scanning devices, such as, for example, in the scanning of the position of measuring instruments, the electromagnetic field generated by the coils cannot be screened in many cases because the contructional or technological conditions make this impossible.

Therefore, such proximity scanning devices, which cannot be screened, always emit an electromagnetic field, the energy of which depends on the measuring length arrangement and on the power of the high-frequency circuit applied thereto.

It has already been indicated above that, because of the required minimum interference distance as a signal-to-noise ratio, the highfrequency power in the measurement circuit must reach a certain minimum value, ranging in practice from a few milliwatt up to several watt, in order to achieve fault-free functioning in operation.

The fraction emitted as electromagnetic energy by the measuring length is also smaller or larger, corresponding to this high-frequency power, and the orders of magnitude of this energy are such that they can already be picked up by receiver installations in the vicinity, for example radio or television receivers, which are tuned to the emitted frequencies.

Such frequencies consistently interfere with the desired received signals.

A long time ago, agreements were concluded internationally and worldwide, which are laid down in radio interference regulations within the individual countries and which stipulate certain maximum values for such interfering emissions from proximity scanning devices. Some of these regulations are worded in such a way that it is impossible to ensure that, in certain units, the envisaged purpose of the high-frequency scanning device will be fulfilled, because the high-frequency power required for this is high, the energy fraction emitted from the measuring length exceeds the permitted limiting values and the unit can thus not be approved at all by the supervisory authorities for operation, or it can be approved only under specific conditions.These conditions either differ from country to country or are restricted to certain areas and frequently cause high costs for the necessary official check in each individual case.

A further disadvantage is the restriction to particular frequency ranges, which is provided for in such conditions, and this often entails unsatisfactory functioning of the measuring length.

The present invention avoids these disadvantages, which frequently are economically serious, of the known scanning devices.

According to the invention, this is achieved in particular by an arrangement for generating at least one compensating field having the inverse phase position and the same effective energy emission in order to obtain a resultant interference which tends to approach zero, the compensating field being located at a distance from the measurement field which is small compared with the wavelength of the high-frequency emission.

Thus, the effect of the electromagnetic energy emission by the measuring circuit or scanning circuit, which cannot be prevented because of the laws of physics, on the environment is here compensated by a corresponding energy emission of the same frequency, but of inverse phase, and under otherwise identical or approximately identical conditions for the emission.

The arrangement according to the invention, several examples of which are illustrated in the following text, achieves such an attenuation of the interference fields that it becomes possible, even in the case of relatively strong high-frequency powers in the measuring and scanning circuits, to reach extremely small resultant interference fields which no longer interfere with radio receiver installations, even in the immediate vicinity.

As an example, the design of a capacitive proximity measuring device, which is used for regulating the distance between a tool and a workpiece, will now be described and explained by reference to Figure 1.

Measuring devices of this type are known and conventional in the state of the art.

In Figure 1 , the workpiece, a metal plate, is marked 1. 2 represents the tool, for example a welding torch, which can be moved relatively to the workpiece by a motor 3 via a control device 4.

The control device 4 is connected to a measuring device 5 which has a high-frequency resonant circuit 6 which is interconnected with the electrode 7. The electrode 7 is mechanically joined to the tool 2; consequently, it moves together with the latter. When the tool 2 moves closer to the metal plate 1 or the metal plate 1 moves closer to the tool 2, the distance between the metal plate 1 and the electrode 7 also becomes smaller.

Consequently, the capacitance of the measuring length increases and hence the frequency of the resonant circuit decreases; via the control device 4, this causes the drive 3 to run in a direction which restores the previous distance.

It is evident that the measuring length determined by the distance between the metal plate 1 and the electrode 7 emits energy, because a high-frequency alternating voltage from the resonant circuit is applied to the electrical capacitance of the measuring length.

In this example, this interfering emission is compensated according to the invention by the addition of a second measuring circuit consisting of the electrode 8. The electrode 8 is connected to the opposite end of the resonant circuit and is therefore under a high-frequency alternating voltage of inverse phase. Since the electromagnetic fields emitted by the electrodes 7 and 8 are in mutually inverse phase positions, they are mutually cancelled either completely or very largely. The electrical center of the resonant circuit is connected to earth in order to obtain the inverse phase position at the two ends relative to earth.

In practice, it has therefore proved particularly advantageous when the electrodes 7 and 8 are spatially in close proximity, but do not influence each other.

This example represents a simple variant of the device according to the invention.

In Figure 2, a similar device is described, in which the compensation of the high-frequency field is achieved by a second emitting capacitive field of inverse phase position, which is not connected to the same high-frequency circuit.

In Figure 2, an electrode 10 is located as a measuring electrode next to a workpiece 9. Within the measuring device 11, it is connected to a resonant circuit 12, the output signal of which is fed to an evaluation circuit 1 3 which in turn, at the output 14, generates a control signal which drives the motor 15.

The output of the evaluation circuit 1 3 is also connected to an amplifier 1 6 which leads to the compensating electrode 17. The output voltage of the amplifier is variable via a preselection device 1 8. It is thus possible to adjust the high-frequency voltage on the output 14.

The amplifier 16 inverts the phase by 180 degrees. In this example, the electrode 17 connected to the amplifier output is not located directly opposite the workpiece, but it is spatially in the immediate vicinity of the electrode 10, without having an action on this field. The machine earth to which the workpiece 9 is also connected is opposite the electrode 17.

Thus, only the electrode 10 with a defined interference emission field is present in the measuring length. The voltage on the compensating electrode 17 is adjusted by means of the preselection device 1 8 in such a way that the powers of the two fields of the electrodes 10 and 1 7 are measurably canceled. The rigid 1 80 degree phase position between the two electrodes is obtained by connecting the resonant circuit 12 to the amplifier 1 6 so that the phase position thereof always effects a phase displaced by precisely 180 degrees in the amplifier output.

According to the invention, the preselection device 18 can be connected to a field strength measuring instrument which is not shown and which measures the resultant interference field and feeds back to the preselection device in such a way that the latter is readjusted until the resultant interference field in the particular case is zero.

As a further example of the arrangement according to the invention, Figure 3 shows an inductive proximity measuring length, the interference field of which is compensated.

Figure 3 shows a metallic object 19 which, depending on the distance, exerts an effect, which either increases or decreases the inductivity, on the sensor which is constructed as a ring coil 20.

In this example, the ring coil 20 is connected in series to a ring coil 21. The two coils are connected to an oscillator 22 in which their resultant inductivity determines the frequency of the oscillator circuit.

The ring coil 20 and ring coil 21 are arranged in such a way that their windings run in opposite directions. In the example of Figure 3, the ring coil 21 is located above the ring coil 20 so that the ring coil 20 mainly functions as the sensor, since it is located closer to the metallic object 19.

The electromagnetic interference field emitted by the ring coil 20 is compensated, according to the invention, in this arrangement by the field of the ring coil 21 of exactly opposite phase.

According to the invention, it is also possible, analogously to the example of Figure 1, to arrange the two rings coils 20 and 21 next to each other and as equivalent sensors, the current flowing through the ring coil 21 again having the opposite phase.

An embodiment of the invention is likewise possible, for example, in which the ring coil 21 is not fed in series with the ring coil 20, but completely separately from the latter.

Corresponding to Figure 2, the ring coil 21 is then connected to the output of an amplifier, preferably an amplifier having an adjustable degree of amplification, and thus always receives a high-frequency alternating current of a phase inverted by exactly 180 degrees in order to generate the compensating field which, according to the invention, reduces the resultant highfrequency interference field to very small values.

This arrangement is analogous to the capacitive design and thus corresponds to Figure 3, so that a detailed explanation is unnecessary.

All the arrangements according to the invention, the above-mentioned examples only representing a few possible applications thereof, are based on the fact that the compensation of the high-frequency interference fields is effected by inverted-phase, capacitive and/or inductive excitation circuits which are arranged to be immediately adjacent in such a way that their mutual distance is very small compared with the wavelength of the high-frequency energy emission. The 180 degree phase condition can be virtually fully met in this way.

In the case of capacitive feelers, an embodiment of the invention with a concentric arrangement of the capacitive electrodes, for example a small circular metal plate P as one electrode, and the other electrode, which is fed in inverted phase, for example as a surrounding ring R, has proved particularly suitable. If the thickness of ring and small plate are here selected to be very small, compared with the diameter, the uncompensated field generated between the two electrodes is so weak that it does not cause interference.

Figure 4 shows such an arrangement.

Obviously, the electrodes can, however, also be arranged next to each other and can then be designed, for example, as small rectangular plates.

In the case of inductive electrodes, in particular ring coils, the constructional form according to Figure 5 has proved particularly suitable. In this case, a body 32 of insulating material is wound with a coil 33 which is very close to a metal surface 35 which is to be proximity-scanned. The coil 34 is wound above this on the same body 32, but at a distance from the coil 33 and the metal surface 35, so that its interaction with the coil remains small.

The mutual distance is, however, very small compared with the wavelength of the highfrequency energy used. For example, the distance between the coils 33 and 34 is only 10 mm at a wavelength of 30 m.

This ensures that the coil 34 excited in inverted phase generates a field which cancels the interference effect of the field of the coil 33.

The directions of the currents flowing in the two coils are marked by arrows. The solid vertical lines represent the field of the coil 33, and the broken lines represent the field of the coil 34. The two fields cancel each other's action relative to the exterior. The coil 33, however, acts as a feeler electrode towards the plate 35.

Figure 6 shows an illustrative embodiment of the arrangement, according to the invention, of a proximity measuring electrode and a compensating electrode on a flame-cutting machine.

The sensor electrode 36 and the compensating electrode 37 are connected in inverted phase to a resonant circuit 38 which is accommodated in the immediate vicinity of the electrodes, for example on the torch which is marked 2 in Figure 1. The resonant circuit is interactively connected to the oscillator 39. The cable connection leads to the evaluation circuit 40. A particular advantage in this illustrative embodiment is that only one cable connection is required, which can be provided as a coaxial cable.

The invention is, of course, not restricted to the illustrative embodiments shown. Thus, for example, not only resonant oscillator circuits with frequency-determining circuits in the measuring length, but also passive sensor circuits, which act as mistuning voltage dividers or capacitive/inductive voltage dividers, can be fitted out in accordance with the invention.

Claims

1. Arrangement for the compensation of the electromagnetic interference field emitted by electrodes and/or feelers (sensors) in proximity scanning and control devices, having at least one high-frequency measurement field between a measuring electrode and a counter-electrode, characterized by an arrangement for generating at least one compensating field having the inverse phase position and the same effective energy emission in order to obtain a resultant interference field which tends to approach zero, the compensating field being located at a distance from the measurement field which is small compared with the wavelength of the highfrequency emission.

2. Arrangement according to Claim 1, characterized in that, to generate the compensating field, a further electrode is located in the immediate vicnity of the sensor electrode, and the further electrode is excited with alternating energy of the same frequency, inverse phase and such a power that the two electromagnetic interference fields between the electrodes and the common reference potential cancel each other, at least for the major part, relative to the exterior.

3. Arrangement according to Claim 2, characterized in that a second emission electrode with inverse phase excitation is arranged in such a way that it is located concentrically to the first sensor electrode.

4. Arrangement according to Claim 2, characterized in that both the electrodes are constructed as inductive ring electrodes in coil form and are located concentrically to one another and/or on the same common central axis.

5. Arrangement according to Claim 3, characterized in that the second, compensating electrode is located to the side of the sensor electrode and is used in the same way as an additional sensor electrode.

6. Arrangement according to Claim 1, characterized in that the second electrode with inverse phase excitation is connected to the same resonant circuit but to the opposite pole thereof, and that the electrical center of the resonant circuit is at the reference potential, preferably earth.

7. Arrangement according to Claim 6, characterized in that the position of the electrical center is adjustable to make it possible to set full compensation.

8. Arrangement according to Claim 2, characterized in that the second, compensating electrode is arranged in a position which is constant relative to the common reference potential, in such a way that it does not act as a second sensor electrode.

9. Arrangement according to Claim 2, characterized in that the compensating electrode is connected via an amplifier, of which the output energy has the inverse phase position relative to the sensor electrode and the amplification factor is adjustable.

10. Arrangement according to Claim 9, characterized in that the amplification and/or phase position of the amplifier are varied via an externally located field strength-measuring instrument with a control instrument in series in such a way that, in operation, a compensating energy which reduces the resultant interference field to the minimum possible value is always delivered by the amplifier output leading to the compensating electrode.

11. Arrangement according to one of the preceding claims, characterized in that the two electrodes are arranged in devices for regulating the distance between tools and workpieces, preferably on flame-cutting machines and welding machines.

12. Arrangement according to Claim 11, characterized in that two electrodes in closely adjacent arrangement are each provided with one feed line from the energy source.

13. Arrangement according to Claim 3 and 11, characterized in that the electrodes are connected to a common resonant circuit which is located in the immediate vicinity of the two electrodes and is connected directly or via an oscillator circuit to a feed line leading to the evaluation circuit.

14. Arrangement for the compensation of the electromagnetic interference field emitted by electrodes and/or feelers (sensors) in proximity scanning and control devices substantially as herein described with reference to and as illustrated by Figures 1, 2, 3,4, 5 or 6 of the accompanying drawings.