WO2013040634A1

WO2013040634A1 - Capacitive sensors and associated methods

Info

Publication number: WO2013040634A1
Application number: PCT/AU2012/001122
Authority: WO
Inventors: Phillip HARMAN; Gordon PHELPS
Original assignee: Romteck Pty Ltd
Priority date: 2011-09-21
Filing date: 2012-09-19
Publication date: 2013-03-28

Abstract

In one preferred form there is provided a capacitive sensor (42). The capacitive sensor (42) includes a body element (44) of a relatively thin configuration having a first face (46) and a second face (48). A sensor electrode (50) is arranged on the first face (46). A guard electrode (52) is arranged on the first face 46 and the second face (48).

Description

CAPAC TIVE SE N S ORS AND AS S O C IATE D ME THOD S FIE LD OF THE INVE NTI ON

In preferred forms the present invention relates to capacitive sensors and associated methods. BAC KGROUND T O T HE INVE NTI ON

Capacitive sensors are often used in the measurement of distance. Capacitive sensors are able to measure other characteristics such as thickness and density.

Capacitive sensors are used in numerous applications including semiconductor manufacture, disk drive sensing, machine tool metrology and quality control. In precise positioning applications capacitive sensors are often users to measure down to the micrometer or nanometer level.

Capacitive measurement systems readily available in the market are considered to almost universally utilize a metallic capacitive sensor or probe. The range of capacitive sensors manufactured by Micro-Epsilon Messtechnik GmbH and Co of Konigbacher, Germany provides a number of examples. Figure 1 illustrates a table showing a number of sensors, the table being taken from the Micro-Epsilon Messtechnik GmbH and Co website (www.micro-epsilon.com).

Whilst a background to the invention has been provided, it is to be recognised that any discussion in the present specification is intended to explain the context of the invention. It is not to be taken as an admission that the material formed part of the prior art base or relevant general knowledge in any particular country or region.

S UMMARY O F T HE INVE NTI ON

According to a first aspect of preferred embodiments herein described there is provided a capacitive sensor comprising: a body element having a first face and a second face; a sensor electrode arranged on the first face; a guard electrode arranged on the first face; and a guard electrode arranged on the second face. Preferably the guard electrode on the first face is connected to the guard electrode on the second face by an electrical connection extending through the body element from the first face to the second face.

Preferably the body element includes electrical connection elements for connecting the guard electrode on the first face to the guard electrode on the second face.

Preferably the sensor electrode is provided as a layer on the first face of the body element; the guard electrode on the first face is provided as a layer on the first face; and the guard electrode on the second face is provided as a layer on the second face.

Preferably the body element comprises circuit board material. Preferably the body element is less than 3 mm in thickness.

Preferably the body element is less than 2mm in thickness.

Preferably the body element is less than 1mm in thickness.

Preferably the body element is of a relatively thin configuration.

Preferably the sensor electrode and the guard electrodes extend 1mm or less in thickness away from the periphery of the body element.

According to a second aspect of preferred embodiments herein described there is provided a method of providing a capacitive sensor comprising: providing a body element having a first face and a second face; forming a sensor electrode on the first face; forming a guard electrode on the first face; and forming a guard electrode on the second face. Preferably the method includes providing electrical connections in the body element for connecting the guard electrode on the first face of the body with the guard electrode on the second face of the body.

Preferably the method includes providing the body element as circuit board material; providing the sensor electrode as a layer on a first face of the body element; providing the guard electrode on first face of the body element as a layer; and providing the guard electrode on second face of the body element as a layer.

Preferably the body element is of a relatively thin configuration. According to a third aspect of preferred embodiments herein described there is provided a capacitive sensor including: a sensing trace on the front of a circuit board, and a guard trace on the front of the circuit board; and a correction trace on the rear of the circuit board.

Preferably the size of the correction trace covers the area of the sensing trace and guard trace on the front of the circuit board.

Preferably the capacitive sensor includes vias for connecting the guard trace on the front of the circuit board with correcting trace on the rear of the circuit board.

Systems and methods according to preferred embodiments of the present invention herein described preferably provide:

(i) Capacitive sensors advantageously manufactured using conventional printed circuit board fabrication techniques;

(ii) Capacitive sensors that provide advantageous flexibility in terms of sensor shape and accurate surface mapping capabilities;

(iii) Capacitive sensors having relatively advantageous precision in terms of the form of sensor provided;

(iv) Capacitive sensors that are associated with advantageous manufacturing processes and costs; and

(v) Capacitive sensors that are readily manufactured in a slim-line configuration.

It is to be recognised that other aspects, preferred forms and advantages of the present invention will be apparent from the present specification including the detailed description, drawings and claims.

The present invention is to be construed beneficially to the applicant. B RIE F D E S C RIPTIO N OF D RAWINGS

In order to facilitate a better understanding of the present invention, several preferred embodiments will now be described with reference to the accompanying drawings, in which Figure 1 provides a tabular view showing a number of existing capacitive sensors;

Figure 2 provides a cross-sectional view of a capacitive probe described in US4311959;

Figure 3 provides a perspective view of a conventional capacitive probe; Figure 4 provides a schematic view showing the conceptual effect of guarding;

Figure 5 provides a circuit diagram showing the operation of a conventional probe;

Figure 6 shows an alternate configuration to that shown in Figure 5;

Figure 7 provides a perspective view illustrating the construction of a conventional capacitive probe;

Figure 8 provides a side view of a capacitive sensor according to a first preferred embodiment of the present invention;

Figure 9 provides a cross-sectional view of the capacitive sensor shown in Figure

8;

Figure 10 to 12 provide several perspective views illustrating the arrangement of the capacitive sensor show in Figure 8;

Figures 13 and 15 illustrate the structure and operation of a non-preferred variation;

Figures 14 and 16 illustrate the structure and operation of a preferred capacitive sensor according to a further preferred embodiment of the present invention;

Figures 17 and 18 provides provide several views of capacitive sensors according to a further preferred embodiments of the present invention; and

Figure 19 provides a general view of a method according to a further preferred embodiment of the present invention. DE TAILE D DE S C RIPTIO N O F T HE E MB O DIME NT S

It is to be appreciated that each of the embodiments is specifically described and that the present invention is not to be construed as being limited to any specific feature or element of any one of the embodiments. Neither is the present invention to be construed as being limited to any feature of a number of the embodiments or variations described in relation to the embodiments.

The preferred embodiments of the present invention detailed below describe particular capacitive sensors and methods of manufacture using advantageous printed circuit board (PCB) fabrication techniques.

The embodiments are considered to demonstrate unique and flexible application features in comparison to conventional capacitive sensors. It is considered that the embodiments provide advantageous flexibility in terms of sensor shape and accurate product surface mapping capabilities with substantially reduced manufacturing and replacement cost.

Figure 1 is described above. Figure 2 shows a capacitive probe described in US431 1959 to E. Riessland et al filed 14 June 1979. As shown in Figure 2 the probe has a number of conventional features.

Firstly there is provided a hollow cylindrical metallic shell that is grounded. The shell houses a number of electrodes and includes dielectric separating layers. The probe is of a conventional cylindrical shape and is formed from metallic material.

The inventors of the present invention consider that there are a number of problems associated with the use of electrodes and dielectrics as conventionally employed by capacitive sensors. Figure 3 shows the general form of a conventional capacitive probe 10. Again the probe 10 provides a metallic shell 12. A central electrode 14 is used to provide an electrical field to target 16. A guard electrode 1.8 is used to influence the electric field between central electrode _.1_.4 and the target 16. This provides improved linearity with respect to capacitance and distance measurements inferred by the probe _.10. Referring to Figure 4 the guard electrode 1.8 can, on one level, be considered to collimate the electric field from the electrode 14 to a sensing area 20. The electric field between the central electrode 1_.4 and the target 16 is accordingly collimated by the influence of the guard electrode 1_.8 such that electric field between the electrode 14 is generally limited from spreading outwardly and influencing adjacent areas of the target 16. One circuit that could be used in the conventional probe 10 is shown in Figure 5. The interaction between the electrode 14 and the target 16 provides a capacitance 22. A constant current source 24 is connected to an input 26 of a high input impedance unity gain amplifier 28 and a capacitive probe 30. The constant current source 24 provides an alternating frequency current supply between 10kHz and 500kHz. The current supply could be sinusoidal, triangular or square or otherwise in nature.

The probe 30 is connected to the junction of the output of the constant current source 24 and the input 26 of the high input impedance amplifier 28 via a length of coaxial cable 32. If the outer sheath of the coaxial cable were to be connected directly to a ground potential, as shown in Figure 6, the inherent capacitance of the cable would electrically shunt the capacitance of the capacitive sensor and would, for small surface area capacitance probes, severely limit the maximum probe measurement range (the distance to be measured from the sensor grounded object) over which the capacitance probe will operate satisfactorily.

Notably the capacitive distance measurement system when connected in the configuration shown in Figure 6 will function normally in the sense of the output voltage from the amplifier varying as the distance of the capacitive sensor from the grounded object is altered. Nonetheless, the output voltage from the amplifier will become non-linear with respect to distance as the capacitance between the sensor and ground approaches the capacitance of the connecting coaxial cable. The capacitance of the connecting coaxial cable can be substantially nullified by connecting the output of the high input impedance amplifier to the outer sheath of the cable as shown in Figure 5. In capacitive probes this is know as 'guarding' with the output of the unity gain amplifier being known as the 'guard signal'.

With a constant current 'i' at a fixed frequency 'f generated by the constant current source 24 the output voltage 'V from the amplifier can be shown to be

V = i .d /(2. π. f. Sr.So. A) Where

1) Sr is the relative permittivity of the medium the sensor is operating in (for air this is unity); 2) So is the permittivity of free space (8.85 x 10^" );

3) A is the area of the capacitive sensor that faces the grounded object; and

4) d is the distance between the surface of the sensor and the object.

Since π, f, Sr, So and A are constant the output voltage V is proportional to the separation distance d; written as V ~ d

As shown V is directly and linearly proportional to d.

In a well engineered Capacitive Displacement Measurement system, using an operating frequency of 25kHz and a capacitive sensor of 10mm diameter, a distance accuracy of +/- lum over a range of 0 to 1mm is readily obtainable.

The construction of a conventional capacitive sensor as described above is illustrated in Figure 7. The construction basically includes an extension of the coax cable and consists of an inner metallic cylinder 34, connected to the inner of the coax cable 36. An outer metallic cylinder 38 surrounds the inner cylinder 34 and is connected to the guard signal via the outer of the coax cable.

The inner cylinder 34 and outer cylinder 38 are separated by an insulating dielectric medium 40 that prevents them shorting together. Such an insulating medium is typically less than 0.1mm thick and fashioned from a stable insulating material such as nylon, PTFE etc. In operation, the capacitive sensor 30 is mounted perpendicular to the grounded surface of the object 16 to which the distance is to be measured. Both the signal from the constant current source 24 and that from the guard signal cause an electric field to be established between the cylindrical capacitive sensor 14 and the grounded object 16.

In the embodiment, since the signal applied to the inner and outer cylinders 34, 38 are identical no electric field will exist between them. In particular no electric field will exist between the two cylinders at the junction of the surface of the disk and the medium (e.g. typically air). By surrounding the inner cylinder 34 with an outer cylinder 38 connected to a guard signal this will tend to ensure that an electric field from the inner cylinder 34 to the grounded object 16 remains perpendicular to the surface of the inner cylinder 34. Maintaining the inner electric field so that it is perpendicular to the surface of the inner cylinder 34 greatly enhances the linear relationship between the distance to the grounded object 16 and the output voltage.

A capacitive sensor 42 according to a first preferred embodiment of the present invention is shown in Figure 8. The sensor 42 includes a body element 44 of a relatively thin configuration having a first face 46 and a second face 48. In the sensor 42 a sensor electrode _5_0 is arranged in a central location on the first face 46. A guard electrode 52 is advantageously arranged on both the first face 46 and the second face 48. As would be apparent the guard electrode 52 could be considered as constituting an electrode on each face.

The guard electrode 52 on the second face 48 substantially spans both the sensor electrode 50 and the guard electrode 52 on the first face 46.

The body element 44 is provided as a sheet of substantially non-conductive PCB material. The first face 46 and the second face 48 are provided on opposite planar sides of body element 44. More particularly, in the embodiment, a stable PCB substrate is used to provide the body element 44 (e.g. such as FR4). The substrate is of a thickness such that the PCB does not substantially flex during use which would result in inaccurate and variable distance readings.

As shown the sensor electrode 50 is provided as a layer 54 on the first face 46 of the body element 44. The guard electrode 52 is provided as a layer 56 on the first face 46 as well as a layer 58 on the second face 48. The provision of the guard electrode 52 on both the first face 46 and the second face 48 is considered to provide a capacitive sensor 42 that has a number of advantageous characteristics.

The sensor electrode 50 is advantageously provided on the PCB using standard PCB design techniques such as etching. Referring to Figure 10 the PCB sensor 50 consists of a circular copper inner trace 60 surrounded by an outer ring 62 in the form of the guard electrode 52. The remainder of the surface 64 is bare PCB material e.g. fibreglass.

Looking in plan view this presents a similar geometry to that of the conventional dual cylinder capacitive sensor shown in Figure 7. In fact, the advantageous approach adopted by the present embodiment makes it practical to manufacture a PCB sensor with exactly the same planer dimensions as a regular metallic cylinder based sensor.

Referring to Figure 8 a small gap 6_.6 is left between the inner circular trace 60 and outer ring 62 which corresponds to the insulation between the inner and outer cylinders of the conventional cylindrical metallic sensor. Of course a suitable dielectric material could be located in this gap.

Figure 9 shows a cross section of the PCB capacitive sensor 42. As shown the capacitive sensor 42 includes the sensor electrode 50 and the guard ring 52. The body element 44 includes a number of elements 68 allowing the guard electrode 52 to be electrically connected through the body element 44 from the first face 46 to the second face 48. In this embodiment the passages each provide an electrical connection using a plated through hole or 'via connection' (vertical interconnect access) on the rear of the PCB that electrically connects to the copper traces on the front of the PCB.

An additional element 70 is used to connect the sensing trace 60 to the rear of the PCB to enable a coax cable to be connected. As shown in Figures 9, 10 and 11 the guard electrode 52 on the second face 48 does not cover the connection 70. A gap 72 is provided. As would be apparent the guard electrode 52 on the first face 46, in embodiments, need not be directly connected to the guard electrode 52 on the second face 48,

Referring to Figure 12 it is shown that the body element is initially provided with the connection elements 68 and 70 formed therein.

The inner circular PCB trace 60 is connected to the output of a constant current source and the outer ring 62 to the guard signal in the same manner as a conventional metallic cylinder based sensor (See Figure 5). The connections to the PCB capacitive sensor may be via guarded coax cable such that the sensor may be located some distance from the remainder of the circuitry. With the body element 44 comprising printed circuit board material and etching constituting the sensor and guard forming process this allows for a relatively inexpensive construction. This is considered to be quite unlike conventional systems which generally utilise a capacitive sensor that is metallic in nature. The present embodiment has a number of comparative advantages.

In comparison the present approach is considered to provide a relatively inexpensive and flexible construction method. Cost is advantageously reduced in applications which often require the use of multiple sensors to cover a given surface and the subsequent averaging of sensor outputs. In comparison to conventional capacitive sensors which are expensive and frequently damaged in use, the embodiment provides a sensor that is relatively inexpensive to manufacture and can be produced in a range of topological profiles.

Figures 13 and 14 respectively illustrate a first capacitive sensor 74 and a second capacitive sensor 76. The second capacitive sensor 76 accords with a further preferred embodiment of the present invention. The capacitive sensor 74 is considered to perform relatively poorly in comparison to the second capacitive sensor 76. As shown in Figures 15 and 16 the provision of the guard sensor 78 on the reverse second face 80 provides advantageous linearity to the electric field provided by the sensor electrode 82 (of the second capacitive sensor 76) in comparison to the sensor electrode _,84_...(of the first capacitive sensor 74).

The first capacitive sensor 74 exhibits a high static capacitance even when placed a considerable distance from a grounded target. The reason for this poor performance is due to the presence of an electric field on the rear surface of the PCB trace as shown in Figure 15. This is advantageously prevented by the guard electrode 78 in the second capacitive sensor 76.

In the first capacitive sensor 74 the electric field extends through the PCB material and finds its way to the grounded target. In the second capacitive sensor 76 the guard electrode 78 extends through the PCB and provides an advantageous blocking function as shown in Figure 16. Computer modelling of the electric fields associated with objects constructed as described is possible using conventional techniques. An example of such modelling software is 'Electric Fields' provided by Field Precision LLC of the USA (www.fieldp.com).

As discussed the embodiment of the second capacitive sensor 76 includes an additional PCB trace, on the rear of the PCB material that is connected to the guard signal. The size of the additional trace covers the area of the traces on the other side of the PCB. The additional trace can be considered to provide a 'correction trace' that advantageously provides advantageous performance.

It is to be appreciated that embodiments are considered to provide greater flexibility in terms of sensor shape and accurate product surface mapping capabilities with substantially reduced manufacturing and replacement cost.

Returning to Figure 8, the body element 44 is of an advantageously thin configuration being less that 2mm in thickness. Including the additional thickness provided by the sensor electrode 50 and guard electrode 52, which extend away from the periphery of the body element 44, the thickness of sensor 76 is still less than 4mm in thickness.

Whilst the body element 44 is formed of relatively inflexible material, it is to be appreciated that in other embodiments flexible materials may be used. PCB capacitive sensors comprising sensor, guard and correction traces in preferred embodiments may be constructed using such materials. Furthermore both regular and irregular shaped sensors may be constructed using flexible PCB processes.

Figure 17 illustrates a cross sectional view of the construction of a flexible PCB capacitive sensor 86 according to a further preferred embodiment of the present invention. The sensor 86_is advantageously able to measure the distance from an irregular shaped target 88. In Figure 17 there is provided a grounded metal target 88, the flexible capacitive sensor 86 and backing material 90, which may be conductive or an insulator and coax lead 92. If the backing material 90 is conductive then this may be connected to the guard signal.

Embodiments also allow for the shape of preferred PCB capacitive sensors to be irregular. An example of an irregularly shaped sensor 94 is shown in Figure 18. The irregular sensor 94 consists of a sensing trace 96, a guard trace 98, a guard correction trace 100 (on the rear of the PCB), an insulated area 102 between the sensing trace 96 and guard trace 98 and vias 104,

The sensor, guard and correction traces do not need to be contiguous. A non-contiguous PCB capacitive sensor can be constructed from a number of individual sensor, guard and correction traces that are subsequently interconnected.

The PCB capacitive sensors according to embodiments can be constructed from individual PCB sensors, with may each be regular, irregular or flexible in nature and either individual traces interconnected or the output from their associated electronics mathematically processed in a required manner. This is in distinction to conventionally constructed capacitive sensors based on regular shaped structures typically using round section metallic material.

Figure 19 illustrates a method 106 according to another preferred embodiment of the present invention. In the method 106 a body element 108 is provided as circuit board material. A sensor electrode .1.1.0 is provided as a layer on a first face .1.12 of the body element 108 with a gap 1 14. A guard electrode 1 16 is provided as a layer on first face 1 12 of the body element 108 and as a layer on a second face 1 18 of the body element 108. A number of electrical connections 120 are formed in the body element 108 to connect the layers of the guard electrode 1 16, The guard electrode 1 16 on the first face 1 12 surrounds the electrode 1_.1_.0. The guard electrode 1_.1_.6 on the second face _.1_.1_.8 follows the periphery of the guard electrode 1_.1_.6 on the first face 1_.12, A connection 120 connecting the electrode .1.1.0 on the second face 1 1.8 is isolated from the guard electrode on the second face 1.18,

Various methods according to preferred embodiments provide an advantageous methodology for the fabrication of capacitive probe sensors (capacitive probes) for use with a capacitive displacement measuring systems, which uses Printed Circuit Board fabrication techniques.

The implementation of Printed circuit techniques within the design of the capacitive probe fabrication allows for the creation of many different shapes of capacitive probes and capacitive probes with distinct topological features. Systems and methods according to preferred embodiments of the present invention herein described preferably provide:

It is to be recognised that various alterations and equivalent forms may be provided without departing from the spirit and scope of the present invention. This includes modifications within the scope of the appended claims along with all modifications, alternative constructions and equivalents.

There is no intention to limit the present invention to the specific embodiments shown in the drawings. The present invention is to be construed beneficially to the applicant and the invention given its full scope.

In the present specification, the presence of particular features does not preclude the existence of further features. The words 'comprising', 'including' and 'having' are to be construed in an inclusive rather than an exclusive sense.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS :

1. A capacitive sensor comprising: a body element having a first face and a second face; a sensor electrode arranged on the first face; a guard electrode arranged on the first face; and a guard electrode arranged on the second face.

2. A capacitive sensor as claimed in claim 1 wherein the guard electrode on the first face is connected to the guard electrode on the second face by an electrical connection extending through the body element from the first face to the second face.

3. A capacitive sensor as claimed in claim 1 or 2 wherein the body element includes electrical connection elements for connecting the guard electrode on the first face to the guard electrode on the second face.

4. A capacitive sensor as claimed in claim 1, 2 or 3 wherein the sensor electrode is provided as a layer on the first face of the body element; the guard electrode on the first face is provided as a layer on the first face; and the guard electrode on the second face is provided as a layer on the second face.

5. A capacitive sensor as claimed in any one of claims 1 to 4 wherein the body element comprises circuit board material.

6. A capacitive sensor as claimed in any one of claims 1 to 5 wherein the body element is less than 3 mm in thickness.

7. A capacitive sensor as claimed in any one of claims 1 to 6 wherein the body element is less than 2mm in thickness.

8. A capacitive sensor as claimed in any one of claims 1 to 7 wherein the body element is less than 1mm in thickness.

9. A capacitive sensor as claimed in any one of claims 1 to 8 wherein the body element is of a relatively thin configuration.

10. A capacitive sensor as claimed in any one of claims 1 to 9 wherein the sensor electrode and the guard electrodes extend 1mm or less in thickness away from the periphery of the body element.

11. A method of providing a capacitive sensor comprising: providing a body element having a first face and a second face; forming a sensor electrode on the first face; forming a guard electrode on the first face; and forming a guard electrode on the second face.

12. A method as claimed in claim 11 including providing electrical connections in the body element for connecting the guard electrode on the first face of the body with the guard electrode on the second face of the body.

13. A method as claimed in claim 11 or 12 including providing the body element as circuit board material; providing the sensor electrode as a layer on a first face of the body element; providing the guard electrode on first face of the body element as a layer; and providing the guard electrode on second face of the body element as a layer.

14. A method as claimed in claim 11, 12 or 13 wherein the body element is of a relatively thin configuration.

15. A capacitive sensor including: a sensing trace on the front of a circuit board, and a guard trace on the front of the circuit board; and a correction trace on the rear of the circuit board.

16. A capacitive sensor as claimed in claim 15 wherein the size of the correction trace covers the area of the sensing trace and guard trace on the front of the circuit board.

17. A capacitive sensor as claimed in claim 15 or 16 including vias for connecting the guard trace on the front of the circuit board with correcting trace on the rear of the circuit board.

18. A method substantially as herein described with reference to the accompanying drawings

19. A system substantially as herein described with reference to the accompanying drawings.