CAPACITIVE DISTANCE SENSOR

Abstract

A capacitive distance sensor for determining distances from an object, wherein the distance sensor has at least one sensor electrode and at least one compensation electrode. Deviations from an ideal parallel orientation of distance sensor and object and/or errors caused by capacitive edge effects can be compensated by means of the compensation electrode.

Claims

1-15. (canceled)

16. A capacitive distance sensor for measuring a distance to an object, the capacitive distance sensor comprising: at least one first planar sensor electrode, wherein the first planar sensor electrode is designed, together with an opposite area of the object, to form a capacitance dependent on the distance, such that the distance is determined on the basis of a capacitance measurement; and at least one separate tilt compensation electrode assigned to the first sensor electrode, wherein the at least one planar sensor electrode and the at least one separate tilt compensation electrode are disposed in a plane, and therefore measurement errors which are caused by deviations from an ideal parallelism of the at least one planar sensor electrode and opposite object area are compensated on the basis of the capacitance of the at least one tilt compensation electrode.

17. The capacitive distance sensor according to claim 16, wherein the at least one first tilt compensation electrode is also designed for the compensation of capacitive edge effects, or the edge effect compensation electrode is also designed for the compensation of deviations from an ideal parallelism.

18. The capacitive distance sensor according to claim 16, wherein the capacitive distance sensor also comprises at least one second planar sensor electrode which in comparison to the first planar sensor electrode has the same capacitance and is electrically responsive identically but oppositely, such that the distance is determined with the aid of the at least one first planar sensor electrode and the at least one second planar sensor electrode on the basis of a differential capacitance measurement.

19. The capacitive distance sensor according to claim 18, wherein the distance sensor has at least two separate tile compensation electrodes and further comprises four integrators, such that in each case the first and second planar sensor electrode and two compensation electrodes are read simultaneously.

20. The capacitive distance sensor according to claim 18, wherein the first and second planar sensor electrodes and compensation distance electrodes together form a circular arrangement which is symmetrical in the plane, wherein the first and second planar sensor electrodes form circle segments and the compensation electrodes are arranged externally, concentrically around the first and second planar sensor electrodes.

21. The capacitive distance sensor according to claim 18, wherein the first and second planar sensor electrodes and the compensation electrodes have contact holes for electrical contacting and the number of contact holes in the compensation electrodes is equal to the number of contact holes in the first and second planar sensor electrodes, wherein the first and second planar sensor electrodes have the same number of contact holes.

22. The capacitive distance sensor according to claim 16, wherein the capacitive distance sensor further comprises electrically shielded lines for electrically contacting the fat least one first planar sensor electrode or at least one separate tilt compensation electrode, wherein the electrically shielded lines can be switched to the same electrical potential as the at least one first planar sensor electrode or at least one separate tile compensation electrode or capacitive distance sensor has at least one passive electrical shielding electrode at a fixedly defined electrical potential.

23. The capacitive distance sensor according to claim 16, wherein the capacitive distance sensor is designed for continuous real-time measurement of the distance from the object or has a linear response behavior by means of the compensability.

24. A precision bearing with two parts movable relative to one another, wherein the precision bearing has at least one capacitive distance sensor according to claim 16 for measuring at least one distance between the two movable parts.

25. The precision bearing according to claim 24, wherein the precision bearing has at least two capacitive distance sensors arranged at a distance from one another, such that a deviation from an ideal parallelism of the two movable parts relative to one another can be compensated.

26. The precision bearing according to claim 24, wherein the precision bearing is designed as a translation bearing, which has at least two capacitive distance sensors arranged at a distance from one another in an axis (y) perpendicular to the linear axis (x), such that rotations about the linear axis (x) can be compensated.

27. The precision bearing according to claim 24, wherein the precision bearing is intended for use in a high-precision meter.

28. A method for continuously determining a distance between two machine parts, which are intended for parallel displacement relative to one another, wherein a capacitive distance sensor according to claim 16 is used to continuously determine the distance between two machine parts.

29. A capacitive distance sensor for measuring a distance to an object, wherein the capacitive distance sensor comprises: at least one first planar sensor electrode, wherein the at least one first planar sensor electrode is designed, together with an opposite area of the object, to form a capacitance dependent on the distance, such that the distance is determined on the basis of a capacitance measurement; and at least one separate edge effect compensation electrode assigned to the at least one first planar sensor electrode, wherein the first planar sensor electrode and the separate edge effect compensation electrode lie in a plane and have the same circumference with different area, such that measurement errors which are caused by capacitive edge effects of the first planar sensor electrode is compensated on the basis of the capacitance of the separate edge effect compensation electrode.

30. A computer program product which is stored on a machine-readable carrier, or computer data signal, with program code, which is suitable for carrying out the method according to claim 28.

Description

[0049] FIG. 1a shows, in a schematically simplified manner and in a side view, a capacitive distance sensor 100 of the prior art, wherein, for simpler illustration, only the most essential components are shown. The sensor 100 is used to measure the distance d from an object 50 or to measure the gap d between the object 50 and a second object (not shown), to which the distance sensor 100 is attached. Both objects or the sensor 100 and the object 50 are displaceable relative to one another, parallel to the plane 52. For example, the two objects are the two parts of a precision bearing, for example the two sliding faces of an air bearing, which are displaceable in translation, for example linearly, along the direction 52 and have the air gap d from one another. Alternatively (also by way of illustration), the bearing is not a translatory bearing, but a rotary bearing, such that the plane 52 is curved and the sensor 100 can perform, for example, a 360 rotary movement along the plane. Precision bearings are installed for example in precision-measuring machines, such as coordinate measuring devices, total stations, laser trackers or laser scanners, and also in robot arms or CNC precision manufacturing machines.

[0050] In the ideal case, the objects are always moved relative to one another in parallel, i.e. the distance d is ideally constant (in the linear case there is ideally only one degree of freedom for the relative movement). In reality, however, deviations herefrom occur, i.e. the size d of the gap deviates over the movement range (displacement path) as a result of the manufacturing process or also as a result of ambient influences, such as changes in temperature or ageing-related phenomena. These deviations, for example in the case of a bearing of a precision-measuring machine, falsify the measurement results, or, in the case of a manufacturing facility, the accuracy of the workpiece manufactured by said facility. In order to therefore measure and/or monitor the distance d (for example in the form of a bearing monitoring) and thus be able to take into consideration and/or compensate the distance d or changes or deviations thereof from the target or ideal distance, the capacitive distance sensor 100 is now used.

[0051] The capacitive distance sensor 100 has a planar sensor electrode 5 with the length or diameter 2R. This forms, together with the (surface) area of an object 50 distanced therefrom, a capacitor or capacitance C. The magnitude of the capacitance C, as is known, is dependent on the distance d. The distance d can thus be determined by measuring the capacitance C.

[0052] With a prior art sensor 100, errors are nevertheless possible, however, since an erroneous consideration or determination of the distance d occurs if the orientation of the sensor electrode 5 relative to the object face 51 is not parallel thereto, but instead, as shown in FIG. 1a, is tilted and the two areas enclose an angle with one another.

[0053] Such an undesirable tilt is created for example again by imprecise manufacture or also by ambient influences/ageing.

[0054] The actual capacitance C is then not easily calculated by

[00001]$C\frac{1}{d},$

but instead in accordance with:

[00002]$\begin{array}{cc}\begin{array}{c}C{}_{-R}^R.Math.\frac{1}{d+.Math.x}.Math.\mathrm{dx}=.Math.\frac{2}{}.Math.\mathrm{arc}.Math..Math.\tanh .Math.\frac{.Math.R}{d}\\ .Math.\frac{2.Math.R}{d}+\frac{2}{3}.Math.{\left(\frac{r}{d}\right)}^2.Math.{}^2+O\left({}^3\right)\end{array}& \left(1\right)\end{array}$

[0055] FIG. 1b shows, in a schematically simplified manner, a first embodiment of a capacitive distance sensor 4 according to the invention, with which any tilt or deviation from the ideal position of the distance sensor 4 or the sensor electrode 5 can be compensated or taken into consideration. The distance sensor 4 according to the invention for this purpose, in addition to the sensor electrode 2, has a tilt compensation electrode 1, which is separate from the sensor electrode 2 and which in the example is divided into two parts 1a, 1b in such a way that these parts are assigned to an end of the sensor electrode 2 each and are separated therefrom. Both electrodes 1, 2 lie in a plane 53. The compensation electrodes 1a and 1b form a capacitance C1 or C2 respectively with the object surface 51. Deviating from the illustration, embodiments in which the plane 53 is slightly curved or the sensor and compensation electrodes 1, 2, 3 are slightly curved, such that for example the three electrodes 1, 2, 3 form the arc of a circle, are also possible depending on the technical field of application. Such embodiments are advantageous for example for determining distance in the case of rotary relative movements, for example for determining distance within a rotary bearing (see FIG. 10).

[0056] The first and second compensation electrode parts 1a, 1b or the tilt compensation electrode 1 formed therefrom are shown here in such a way that the following is true:

[00003]$\begin{array}{cc}C-C_1-C_2\frac{2.Math.R}{d}+O\left({}^3\right)& \left(2\right)\end{array}$

[0057] With the distance sensor 4 according to the invention, the tilt of the sensor electrode 5 or of the plane 53 relative to the object area 51 is thus compensated, that is to say counterbalanced, and the tilt angle is determined and is taken into consideration when determining the sought distance d.

[0058] FIG. 2 shows a schematically simplified depiction in cross-section of a second embodiment of a capacitive distance sensor 4 according to the invention for measuring the distance d from an object 50. Besides the above-mentioned falsification of the measurement result on account of deviations from an ideal parallel orientation of the sensor 4 or the sensor electrode 5 relative to the object area 51, deviations from the ideal dependency

[00004]$C\frac{1}{d}$

also occur due to capacitive edge effects.

[0059] Since the extent of the sensor electrode 5 (along the direction 53) is limited, the entire capacitance C thereof, besides being composed of the ideal capacitance Cp of the inner region of the electrode 5, is also composed of electrical fields and/or capacitances Cf of the edge of the sensor electrode 5, such that (in accordance with the shown two-dimensional simplification) the following is given:

C=C.sub.p+2C.sub.f (3)

[0060] For compensation of these edge effects, the distance sensor 4 according to the invention, besides the sensor electrode 5, now has an edge effect compensation electrode 3, which lies in the same plane 52 as the sensor electrode 5 and is distanced therefrom or separated therefrom. The compensation electrode 3 is selected here such that it has an effective area of the inner region different from the sensor electrode 5, but the same circumference. In other words, the length of the edge of both electrodes 5 and 3 is the same, however their areas are different. The capacitance Cp1 of the inner region of the edge effect compensation electrode 3 thus differs from the inner capacitance Cp of the sensor electrode 5, the edge capacitances Cf, however, are the same. The capacitive value of the electrode edges can thus be compensated by forming a difference between the overall capacitances of the sensor electrode 5 and compensation electrode 3 or can be calculated as follows:

C=C.sub.p+2C.sub.f(C.sub.p1+2C.sub.f)=C.sub.pC.sub.p1 (4)

[0061] A total capacitance C is thus obtained which is independent of edge effects or edge capacitances. A capacitive sensor 4 of which the ideal or linear behavior is not disturbed by such edge effects is thus advantageously provided.

[0062] FIG. 3 schematically shows the linear behavior of a capacitive distance sensor according to the invention resulting from the above-described compensation of tilt errors or deviations from the ideal parallel orientation and compensation of capacitive edge effects. As a result of the above-proposed measures of the assignment of at least one appropriately designed compensation electrode, the sought distance d is dependent linearly on the measured capacitance c this (or inverse thereof), moreover across the entire measurement range.

[0063] This in particular, there is thus no need for lookup tables for calibration or fault fixing, which disadvantageously are relatively computing intensive and elaborate in respect of calibration. In order to ultimately determine the distance value d it is merely necessary to determine two parameters k and c) in a calibration process in accordance with the following equation (5), which for example is achieved by measuring the sensor capacitances cm1 and cm2 at just two known distances d1 and d2. From the relationship

[00005]$\begin{array}{cc}{d}_1=\frac{k}{c_{m.Math..Math.1}-c_0},d_2=\frac{k}{c_{m.Math..Math.2}-c_0}& \left(5\right)\end{array}$

the parameter k and c0 are given as follows:

[00006]$\begin{array}{cc}k=\frac{d_1.Math.d_2}{d_1-d_2}.Math.\left(c_{m.Math..Math.2}-c_{m.Math..Math.1}\right)& \left(6\right)\\ c_0=\frac{1}{d_1-d_2}.Math.\left(c_{m.Math..Math.2}.Math.d_2-c_{m.Math..Math.1}.Math.d_1\right)& \left(7\right)\end{array}$

[0064] Thus, the present invention advantageously makes it possible to provide a simple compensation model for which merely the two above-mentioned premises have to be determined, which can be achieved in a very simple way.

[0065] FIG. 4 shows, in a schematically simplified manner, and advanced development of a capacitive distance sensor 4 according to the invention in cross-section. Besides the compensation electrodes according to the invention as described above (omitted in the drawing for the sake of clarity), this distance sensor also has a second sensor electrode 6. This second sensor electrode is preferably arranged in the same plane 53 is the first sensor electrode and also corresponds otherwise to the first sensor electrode 5. In any case, the second sensor electrode 6 is arranged and embodied in such a way that the two capacitances are the same, i.e. the capacitance C1 of the first sensor electrode 5 is the same as the capacitance C2 of the second sensor electrode 6: C5=C6.

[0066] By means of the two sensor electrodes 5 and 6, a differential capacitance measurement or measurement evaluation is made possible, wherein the charge q5 of the first sensor electrode 5 is equal and opposite to the charge q6 of the second sensor electrode 6: q5=q6. The charge of the object 50 moving or sliding relative to the electrodes 5, 6, induced in the counter electrode or counter area 51, is on the whole equal to zero: q5+q6=0. Due to the charge neutrality of the counter area, the measurement is on the one hand independent of the impedance conditions of the counter area. In addition, the mutual influencing of a plurality of sensors measuring over the same counter area is reduced or eliminated.

[0067] FIG. 5 schematically shows, in plan view, a preferred embodiment of a capacitive distance sensor 4 according to the invention. The distance sensor 4 has a first planar sensor electrode 5 and a second planar sensor electrode 6 for differential capacitance measurement as described above. The first and second sensor electrodes 5, 6 are arranged here in the form of three geometric electrodes or three electrode parts electrically contacted with one another and arranged in alternation and in a circular manner in the form of equal circle segments. With a distance sensor 4 of this kind, a measurement range in the micrometer range for example is covered, e.g. up to at most 100 micrometers or at best 1000 micrometers, wherein a measurement accuracy of less than one micrometer is achieved.

[0068] Compensation electrodes 2a, 2b are arranged in an annular manner around the sensor electrodes 5, 6. The ring 8 of compensation electrodes one, 3 is concentric with the circular arrangement of the sensor electrodes 5, 6.

[0069] The compensation electrodes 2a, 2b are used here both for tilt compensation, as described in relation to FIGS. 1a and 1b, and for compensation of capacitive edge effects, as described in relation to FIG. 2. The compensation electrodes 2a, 2b thus perform a dual function. They are selected such that on the one hand the relationship between sensor electrodes 5, 6 and compensation electrodes 2a, 2b is satisfied both in the sense of the above equation (2) and in the sense of the above equation (4), wherein the equations (2), (4) are adapted accordingly from the above-presented simple two-dimensional case for the three-dimensional case of the present example.

[0070] in the example, the compensation electrodes for this purpose are divided into twelve (geometric) electrodes 2a, 2a, 2b, 2b, wherein every two parts 2a, 2a and 2b, 2b are assigned to a sensor electrode part 5 or 6. The division into two compensation electrode parts 2a, 2a and 2b, 2b (instead of a single electrode) is implemented in order to provide the circumference necessary for the compensation function of edge effects: the circumference of two parts 2a, 2a or 2b, 2b in each case corresponds together to the circumference of one sensor electrode part 5 or 6.

[0071] On the whole, each first or second sensor electrode 5 or 6 is thus assigned, three times, two electrical compensation electrodes 2a and 2a or 2b, 2b, such that ultimately two compensation electrodes 2a and 2b are provided which are each assigned to one sensor electrode 5 or 6 and both compensate for tilt effects as well as edge effects.

[0072] Due to the provided capacitances of the compensation electrode ring 8, both a possible deviation from the ideal parallel orientation of the sensor 4 relative to the measurement area or object surface, and also the influence of the edges of the first and second sensor electrodes 5, 6 is compensated, for example by means of subtraction of the inner sensor electrode capacitance present in each case.

[0073] In the example the electrodes 2a, 2b have 3, 5 or 6 contact zones 7, for example contact bores/contact holes, for their electrical contracting. In order to ensure, in a simple manner, the above-mentioned proportionalities of the sensor electrodes 5, 6 to the compensation electrodes 2a, 2b in respect of the compensation or fault elimination, the sensor electrodes 5, 6 have two such contacts. The same contacting conditions for a particular region or a particular electrical polarity are thus provided, and the contact areas 7 do not have to be taken into consideration separately in the evaluation of the measured capacitances.

[0074] FIG. 6 schematically shows an example of an evaluation circuit of a capacitive distance sensor 4 according to the invention as described beforehand in conjunction with FIG. 5. By means of wires 11, the first and the second sensor electrode 5, 6 as well as the two compensation electrodes 2a, 2b of the compensation electrode ring 8 are contacted, such that four of these independent electrodes are charged simultaneously by means of the voltage sources 10 and then the corresponding charge or capacitance of these four electrodes 5, 6, 2a, 2b can be determined for example by means of a charge amplifier.

[0075] The electrodes or capacitors in question are charged by means of the voltage sources 10 with appropriately attached electronic switches, such that each electrode 5, 6, 2a, 2b is charged with the voltage +VR or VR. The capacitance measurement is taken via four resettable analogue integrators 9, which convert the capacitive charges into voltage values. These voltages are then digitalized by an ADC converter.

[0076] FIG. 7 schematically shows a development of the capacitive distance sensor 4 according to the invention in accordance with FIG. 6 above. In addition to the aforementioned features, the distance sensor 4 has shields 11s and 13, which shield the electrical connections between sensor electrodes 5, 6, 2a, 2b and evaluation electronics 9 ADC against external electromagnetic influences and thus reduce noise of the measurement signals.

[0077] On the one hand, electrically shielded lines 11s are used. In order to avoid parasitic capacitances, these are switched by means of additional switches or voltage sources 12 to the same potential +VR or VR as the sensor electrodes 5, 6, 2a, 2b, such that an active shielding is provided.

[0078] On the other hand, the distance sensor 4 optionally has an additional passive shielding 13, which lies at a fixed or defined electrical potential 14. Influences disturbing the capacitance measurement are thus reduced even further. In the example, the passive shield 13 is configured such that all electrical connections are shielded thereby at once. Alternatively, the distance sensor 4 can have a plurality of such passive shieldings 13, such that for example each line 11s has its own shielding 13.

[0079] FIG. 8 schematically shows an example of a switching sequence of a capacitive distance sensor 4 according to the invention with active shielding according to the previous FIG. 7, wherein, for the sake of simpler illustration, only the circuit the compensation electrode ring 8 has been shown. With regard to the reading process, the (inner) first and second sensor electrodes 5, 6 and the outer ring 8 formed from compensation electrodes 2a, 2b can be treated as two capacitive sensors independent of one another.

[0080] FIG. 8 shows, at the top, the electrical circuit with lines 11s, potential sources 10 and 12, and integrators 9, which deliver the output signals VA and VB, wherein the necessary switches are denoted by A1-A7 and B1-B7. A switching sequence of the switches A1-A7 and B1-B7 and the resulting output voltages VA and VB are shown qualitatively therebeneath in part. Inter alia, it can be seen that the electrodes are excited by opposite polarity, such that a differential measurement process is made possible. In addition, the switches are managed in such a way that charge affects which are generated by deviations from theoretical or ideal switching processes can be compensated in a post-processing (correlated double sampling).

[0081] FIG. 9 shows, in an exploded oblique view, a preferred embodiment of a capacitive distance sensor 4 according to the invention. The cylindrical distance sensor 4 has at the top the actual effective circular area with the first and second sensor electrodes 5, 6 and the ring 8 of compensation electrodes (see FIG. 5). For shielding, there are a total of three (layers of) active shielding electrodes 15 and two (layers) with passive shielding 13. These are arranged successively in alternation, wherein a conductor layer 11 for contacting the sensor and compensation electrodes 5, 6, 8 is arranged directly between two layers of active shielding 15. Due to the arrangement of the uppermost layer with the sensor and compensation electrodes 5, 6, 8 and the conductor layer 11 adjacent only to layers of active shielding 15, temperature effects over the extent of the carrier (printed circuit board) are advantageously minimized. The present structure additionally enables a compact design, with the greatest possible shielding of interfering electrical fields.

[0082] FIG. 10 schematically shows in cross-section and in plan view an example for the application of a distance sensor 4 according to the invention in a precision bearing 20. The bearing 20 is a rotary bearing. The sensor 4 is used to monitor the distance d between the inner ring 50 and outer ring 21 of the bearing 20. In accordance with the invention the sensor 4, besides the main electrode 5, which together with the opposite inner ring 50 forms a capacitance, also has a compensation electrode 2 divided into two parts, which together with the inner ring 50 forms a further capacitance. The electrode 2 is separated from the sensor electrode 5 and is assigned thereto in such a way that the two electrodes 2, or the three electrodes form a curved plane 53. The compensation electrode 2 is formed in accordance with the above-explained principles in such a way that both deviations of the sensor 4 from an ideal position and edge effects of the sensor electrode 5 can be compensated on the basis of the compensation capacitance of the compensation electrode 2, such that the distance d can be monitored in a highly precise manner.

[0083] FIG. 11 schematically shows a side view of a precision bearing system 16 of a linear axis x, which is equipped with capacitive distance sensors according to the invention 4z, 4y1, 4y2. The precision bearing 16 in the example is an air bearing, which for example is used in the linear drive of a portal coordinate measuring machine or CNC machine.

[0084] What is shown is a rectangular beam structure, which is used as a guide 17 in the direction x of the desired linear displacement movement. The object that is displaceable relative to the object 17 is the carrier 18 provided for movement in the x direction. This carrier is supported at a distance from the guide 17 by compressed air openings 19 generating an air cushion, such that practically friction-free movement as possible.

[0085] The spacing or distance of the two objects 17 and 18 in the z direction (dz) and y direction (dy1 dy2) provided in a particular x position, in the case of application in a CMM, influences the 3D coordinates to be determined of the workpiece to be measured, which is situated on the measurement table; the distances life example in the order of a few micrometers to 100 micrometers or at most 1000 micrometers. In order to compensate for deviations from the target distances or ideal distances or in order to determine the particular distance dz and dy1, dy2, for example with nanometer precision and include this in the calculation of 3D coordinates, the bearing 16 in the example has three capacitive distance sensors according to the invention 4z, 4y1, 4y2, which are attached to or in the carrier 18 and progressively or continuously measure the distance dz and dy1, dy2 from the guide 17 perpendicular to the direction of movement x, or monitor it in real-time or in situ, i.e. also during the movement of the two objects 17 and 18 relative to one another. For example, the distances dz and dy1, dy2 can thus also be determined with each 3D coordinate acquisition or can be recorded with a certain measurement frequency during the entire measurement procedure or the entire production process of the workpiece.

[0086] The measurement or manufacture for example of a coordinate measuring machine or manufacturing machine into which the bearing system 16 is installed can thus advantageously be performed precisely even in the event of adverse ambient influences, such as fluctuations in temperature or vibrations or as a result of imprecise manufacture of the bearing 16, and the demands on manufacturing tolerances of the bearing 16 can be reduced; this is also possible particularly since, on account of the tilt compensation according to the invention (see FIG. 1b) of the capacitive distance sensor 4z, 4y1, 4y2, even the installation thereof on or in the carrier 18 can be provided with a relatively large tolerance.

[0087] The measured distances dy1, dy2 and dz are thus used for precise position determination of the carrier 18 and monitoring of the positioning accuracy thereof. Any deviations in the y direction and z direction can thus be determined and compensated.

[0088] In addition, by means of the two capacitive distance sensors 4y1, 4y2 mounted along the z direction, further deviations of the linear bearing 16 are additionally determined and compensated. By means of the two sensors 4y1, 4y2 along the z axis, rotation such as rolling errors about the z axis are monitored and compensated. As a further option (not shown), the bearing has further additional distance sensors, mounted on the carrier 18 for example in a manner distanced in the x direction, which are used for compensation of further errors (pitching, yawing) and to further increase the precision.

[0089] It goes without saying that these shown figures schematically show merely possible exemplary embodiments. The various approaches can also be combined with one another in accordance with the invention and combined with corresponding devices and methods of the prior art.