WO2002084222A1

WO2002084222A1 - Capacitive angular position detector

Info

Publication number: WO2002084222A1
Application number: PCT/US2002/012701
Authority: WO
Inventors: David C. Brown
Original assignee: Gsi Lumonics Corporation
Priority date: 2001-04-11
Filing date: 2002-04-10
Publication date: 2002-10-24

Abstract

A capacitive angular position detector (30) comprising, in stacked planar order, a stationary first detector plate (32), a first lobed dielectric rotor (34), a double-sided drive plate (36), a second lobed dielectric rotor (34), and a second detector plate (32). The first detector plate, first lobed dielectric rotor, and a first side of the double-sided drive plate form a first detector section (D1). The second detector plate, second lobed dielectric rotor, and a second side of the double-sided drive plate form a second detector section (D2). The detector sections are electrically connected so as to multiply the sensitivity of the output signal of the capacitive angular position sensor.

Description

CAPACITIVE ANGULAR POSITION DETECTOR

This application claims the benefit of U.S. Provisional Patent Application No. 60/283,035, filed on April 11, 2001.

FIELD OF INVENTION

The present invention is in the field of rotary position detectors for devices of limited angular rotation, and more particularly, to capacitive angular position detectors with stacked lobe rotors, detector plates, and drive plates.

BACKGROUND OF THE INVENTION

It is common for limited rotation motors, such as those used in galvanometric optical scanning devices, to be configured with capacitive angular position detectors for measuring the precise angular position of the rotor of the motor with respect to the stator of the motor. Capacitive angular position detectors of the planar type utilize a stationary excitation or drive plate, a stationary detector plate, and an intermediately positioned moving plate or armature, such as a lobed rotor. The capacitive angular position detector has an angular sensitivity which is proportional to the dielectric constant of the materials, the frequency of the driving current, and the area and spacing of the detector plates. Since high sensitivity is critical to increasing the signal-to-noise ratio of the detector, and thus its effective resolution, capacitive position detectors are commonly configured with small spacings, large plates, and are driven at high frequencies .

In addition to the difficulties and costs associated with spacing moving parts close together, there are sources of error associated with these close spacings which are in addition to the errors associated with the temperature coefficients of the materials themselves. In the limit, the spacing of the plates is bounded on the lower side by the electrical breakdown potential of the dielectric, usually air.

In addition, the change in sensitivity produced by the thermal expansion of the parts is roughly proportional to the percentage change of the gap dimension between the parts. Any position measuring distortions produced by differential thermal expansion of the parts is also manifested in proportion to the percentage of the gap that these changes represent . Several techniques have been used to increase the sensitivity of capacitive angular position detectors. For example, it has become commonplace to make the moving plate or armature multi-lobed, and the stationary detector plate multi- sectored, in an attempt to increase the effective area of the detector plates. This process trades angular range for sensitivity, with each doubling of the number of lobes halving the effective angular range. It has also become commonplace to drive the capacitor (s) of the detector at higher and higher frequencies, now in the several tens of megahertz range. Unfortunately, high frequency drive currents tend to radiate "noise" to susceptible nearby circuitry in proportion to frequency, and so cost, space, and effort must be devoted to shielding this noise.

The clearance between the moving and the stationary plate (s) has now become one to a few thousandths of an inch, requiring great care in tolerance control to prevent rubbing, and at the same time, introducing the possibility of thermally-induced degradation of performance. Many capacitive angular position detectors are now enclosed in a thermally- controlled housing to alleviate these effects.

SUMMARY OF THE INVENTION

The present invention provides a capacitive angular position detector comprising, in stacked planar order, a stationary first detector plate, a first lobed dielectric rotor, a double-sided drive plate, a second lobed dielectric rotor, and a second detector plate, the first detector plate, first lobed dielectric rotor, and a first side of the double- sided drive plate forming a first detector section, the second detector plate, second lobed dielectric rotor, and a second side of the double-sided drive plate forming a second detector section. There can be N such detector sections, formed by stacking further components in the same fashion.

In addition, the present invention provides a capacitive angular position detector comprising, in stacked planar order, a first drive plate, a first lobed dielectric rotor, a double- sided detector plate, a second lobed dielectric rotor, and a second drive plate, the first drive plate, first lobed dielectric rotor, and a first side of the double-sided detector plate forming a first detector section, the second drive plate, second lobed dielectric rotor, and a second side of the double-sided detector plate forming a second detector section. Again, there can be N such detector sections, formed by stacking further components in the same fashion.

The present invention further provides a method of forming a capacitive angular position detector comprising: providing, in stacked planar order, a first detector plate, a first lobed dielectric rotor, a double-sided drive plate, a second lobed dielectric rotor, and a second detector plate ; forming a first detector section using the first detector plate, the first lobed dielectric rotor, and a first side of the double-sided drive plate; and forming a second detector section using the second detector plate, the second lobed dielectric rotor, and a second side of the double-sided drive plate. The present invention additionally provides a method of forming a capacitive angular position detector comprising: providing, in stacked planar order, a first drive plate, a first lobed dielectric rotor, a double-sided detector plate, a second lobed dielectric rotor, and a second drive plate; forming a first detector section using the first drive plate, the first lobed dielectric rotor, and a first side of the double-sided detector plate; and forming a second detector section using the second drive plate, the second lobed dielectric rotor, and a second side of the double-sided detector plate.

The present invention provides a capacitive angular position detector having a signal sensitivity improvement of N, and a signal to noise improvement of the square root of N, wherein N is equal to the number of detector sections. In addition, the capacitive angular position detector of the present invention provides thermal drift error cancellation. Further, the capacitive angular position detector of the present has a large active angular range, compared with its sensitivity and signal to noise ratio.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the present invention will best be understood from a detailed description of the invention and embodiments thereof selected for the purpose of illustration and shown in the accompanying drawings in which:

Fig. 1 is an exploded view of the principal components of a related art capacitive angular position detector. Fig. 2 is an exploded view of the principal components of a capacitive angular position detector having two detector sections in accordance with the present invention.

FIG. 3 is an exploded view of the principal components of a capacitive angular position detector having four detector sections in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The features of the present invention are illustrated in detail in the accompanying drawings, wherein like reference numerals refer to like elements throughout the drawings.

Although the drawings are intended to illustrate the present invention, the drawings are not necessarily drawn to scale.

An exploded view of a related art capacitive angular position detector 10 is illustrated in FIG. 1. Such a capacitive angular position detector 10 is commonly used, for example, in a galvanometer scanner to determine the angular position of a motor shaft 12 on which a light-directing component 14, usually a mirror, is attached. Surrounding the motor shaft 12, and mounted to the stator of the motor 16 in some convenient manner, is a drive plate 18, with the electrically conductive surface 20 of the drive plate 18 facing away from the stator of the motor 16. A four-lobed rotor 22 is mounted to the motor shaft 12 in such a manner as to establish the necessary clearance between the drive plate 18 and the dielectric material of the lobes 24 of the rotor 22. An eight quadrant detector plate 26 is mounted to the stator of the motor 16 behind or below the rotor 22, and suitably insulated from the drive plate 20, again with the required clearance, so that the dielectric rotor 22 is

"sandwiched" in between the drive plate 18 and the detector plate 26. The required drive and signal processing electronics are mounted in a conventional manner either to the top of the detector plate 26 or remotely as desired. An exploded view of a capacitive angular position detector 30 in accordance with the present invention is illustrated in FIG. 2. Using the same galvanometer application example as described in reference to FIG. 1, a four quadrant detector plate 32 is mounted to, and faces away from, the stator of the motor 16. The detector plate 32 is followed by a two-lobed dielectric rotor 34, a double-sided drive plate 36, having an electrically conductive surface on both sides, and mounted to the stator of the motor 16, a second two-lobed dielectric rotor 34, and finally a second four-quadrant detector plate 32 that faces the double-sided drive plate 36. Each dielectric rotor 34 with its respective detector plate 32 and drive plate 36 can be considered a detector section D. In FIG. 2, two detector sections Dl and D2 are shown. Of course, the sections D can be configured with dielectric rotors 34 having any number of lobes and detector plates 32 having any number of quadrants. In addition, more than two detector sections D (e.g., Dl, D2 , ..., DN) may be used. The effect of stacking two detector sections Dl, D2 , or a multiplicity of N detector sections Dl, D2 , ..., DN, to form the capacitive angular position detector 30 of the present invention is to multiply the sensitivity of the capacitive angular position detector 30 by N. Unlike related art capacitive angular position detectors, such as the detector 10 shown in FIG. 1, the increase in sensitivity is provided without having to resort to small clearances, high frequencies, high voltages, high dielectric constant material, or reducing the angular range of the detector. The capacitive angular position detector 30 of the present invention, however, can also be configured with small clearances, high voltages, high frequencies, high dielectric constant materials, etc., and will benefit from these measures to exactly the same degree as do the capacitive angular position detectors of the related art .

There are several features of the capacitive angular position detector 30 of the present invention that are unique to its design, and which are not shared by the capacitive angular position detectors of the related art, such as the detector 10 shown in FIG. 1. For example, by using two detector plates 32 positioned oppositely to each other, a temperature drift canceling effect is provided. In particular, when differential expansion between the length of the motor shaft 12 and the stator of the motor 16 takes place, one dielectric rotor 34 is brought closer to a first side of the double-sided drive plate 36, while the other dielectric rotor 34 is moved further away from the opposing side of the double-sided drive plate 36 by substantially the same amount.

This has the effect of canceling the change in sensitivity of the capacitive angular position detector 30 resulting from the differential thermal expansion.

Another feature provided by the capacitive angular position detector 30 of the present invention is the averaging of minor defects in the edges and surfaces of the detector plates and the dielectrics. To the degree that these defects are randomly located, only one detector plate will be affected at a time, so as a percentage of the output signal of the capacitive angular position detector 30, the effect of the defect is reduced by the square root of the number of detector sections N. As a result, in addition to the improvement in its output signal, the signal to noise ratio of the capacitive angular position detector 30 is improved by the square root of N.

Double-sided detector plates as well as doubled sided drive plates may be incorporated into the stacked design scheme of the present invention to yield N detector sections D. For example, the capacitive angular position detector may include an even number N of opposing detector sections D, configured in N/2 pairs, each pair of detector sections D having a common, double-sided drive plate, and each pair of detector sections D being divided by and sharing a common, double-sided detector plate. Other variations and configurations will be readily apparent to one skilled in the art .

An example of the use of a double-sided detector plate 38 is shown in FIG. 3. In particular, the capacitive angular position detector 30 illustrated in FIG. 3 includes a four quadrant detector plate 32 that faces away from the stator of the motor 16, followed by a two-lobed dielectric rotor 34, a double-sided drive plate 36, a second two-lobed dielectric rotor 34, a double-sided detector plate 38, a third two-lobed rotor 34, a second double-sided drive plate 36, a fourth two- lobed rotor 34, and a detector plate 32 that faces the second double-sided drive plate 36. Each of the detector plates 32, 38, and drive plates 36 are mounted to the stator of the motor 16 in any suitable manner. This configuration of the capacitive angular position detector 30 comprises four stacked detector sections Dl, D2 , D3 , and D4. The double-sided detector plate 38 is common to detector section pairs D1-D2 and D3-D4.

The foregoing description of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and many modifications and variations are possible in light of the above teaching. Such modifications and variations that may be apparent to a person skilled in the art are intended to be included within the scope of this invention.

Claims

1. A capacitive angular position detector comprising, in stacked planar order, a first detector plate, a first lobed dielectric rotor, a double-sided drive plate, a second lobed dielectric rotor, and a second detector plate, the first detector plate, the first lobed dielectric rotor, and a first side of the double-sided drive plate forming a first detector section, the second detector plate, the second lobed dielectric rotor, and a second side of the double- sided drive plate forming a second detector section.

2. A capacitive angular position detector according to claim 1, comprising N detector sections, and having a signal sensitivity improvement of N, and a signal to noise improvement of the square root of N.

3. A capacitive angular position detector according to claim 1, wherein the detector sections are configured in opposing pairs to provide thermal drift cancellation.

4. A capacitive angular position detector according to claim 1, wherein the detector has a large angular range compared with its sensitivity and signal to noise ratio.

5. A capacitive angular position detector comprising, in stacked planar order, a first drive plate, a first lobed dielectric rotor, a double-sided detector plate, a second lobed dielectric rotor, and a second drive plate, the first drive plate, the first lobed dielectric rotor, and a first side of the double-sided detector plate forming a first detector section, the second drive plate, the second lobed dielectric rotor, and a second side of the double-sided detector plate forming a second detector section.

6. A capacitive angular position detector according to claim 5, comprising N detector sections, and having a signal sensitivity improvement of N, and a signal to noise improvement of the square root of N.

7. A capacitive angular position detector according to claim 5, wherein the detector sections are configured in opposing pairs to provide thermal drift cancellation.

8. A capacitive angular position detector according to claim 5, wherein the detector has a large angular range compared with its sensitivity and signal to noise ratio.

9. A method of forming a capacitive angular position detector comprising : providing, in stacked planar order, a first detector plate, a first lobed dielectric rotor, a double-sided drive plate, a second lobed dielectric rotor, and a second detector plate; forming a first detector section using the first detector plate, the first lobed dielectric rotor, and a first side of the double-sided drive plate; and forming a second detector section using the second detector plate, the second lobed dielectric rotor, and a second side of the double-sided drive plate.

10. The method of claim 9, further comprising: increasing a signal sensitivity of the capacitive angular position detector by N, and increasing the signal to noise ratio of the capacitive angular position sensor by the square root of N, by forming N of the detector sections.

11. The method of claim 9, wherein the detector sections are configured in opposing pairs to provide thermal drift cancellation.

12. The method of claim 9, wherein the capacitive angular position detector has a large angular range compared with its sensitivity and signal to noise ratio.

13. A method of forming a capacitive angular position detector comprising: providing, in stacked planar order, a first drive plate, a first lobed dielectric rotor, a double-sided detector plate, a second lobed dielectric rotor, and a second drive plate; forming a first detector section using the first drive plate, the first lobed dielectric rotor, and a first side of the double-sided detector plate; and forming a second detector section using the second drive plate, the second lobed dielectric rotor, and a second side of the double-sided detector plate.

14. The method of claim 13, further comprising: increasing a signal sensitivity of the capacitive angular position detector by N, and increasing a signal to noise ratio of the capacitive angular position sensor by the square root of N, by forming N of the detector sections.

15. The method of claim 13, wherein the detector sections are configured in opposing pairs to provide thermal drift cancellation.

16. The method of claim 13, wherein the capacitive angular position detector has a large angular range compared with its sensitivity and signal to noise ratio.