Electricity: electrical systems and devices – Electrostatic capacitors – Variable
Patent
1989-11-29
1991-04-09
Griffin, Donald A.
Electricity: electrical systems and devices
Electrostatic capacitors
Variable
73718, 307116, G01L 912, H01G 700
Patent
active
050069527
DESCRIPTION:
BRIEF SUMMARY
This invention relates to capacitive transducers, for example for use in displacement-responsive devices.
Capacitive displacement transducers are known for use in displacement-responsive devices such as measurement probes and joysticks, where a stylus or lever is movable in the directions of two or more orthogonal axes. Normally there would be one or more separate capacitive transducers for each axis of movement. Each such transducer comprises at least a pair of capacitor plates which are relatively movable. In some known devices, e.g. as shown in EP No. 0239337 A, there are three or more capacitor plates in each transducer, so that the transducer comprises a differential pair of capacitors. Such an arrangement has improved performance.
In conventional displacement-responsive devices with capacitive transducers, the transducer associated with each axis has a separate signal processing circuit which determines the capacitance of the transducer as the same varies with displacement. Such a signal processing circuit is known from British Patent No. GB 1 366 284, in which phase and antiphase AC input signals are applied to a differential capacitive transducer, and subsequently demodulated synchronously by a phase sensitive rectifier, and then integrated. The resulting DC signal is used to control the differential AC inputs to the transducer so as to produce a null output from the transducer, and the size of the DC control signal required to do this provides an output proportional to displacement. British Patent No. GB 2 036 982 B describes similar circuits, and additionally suggests that differential transducers in three channels may be multiplexed together in a time-sharing arrangement, so that the demodulation and control circuitry may be shared between the three channels. However, such a time-sharing arrangement does not provide a continuous output for each channel.
A first aspect of the present invention provides a circuit for providing outputs from at least two capacitive transducers, the circuit comprising: signal thereto; transducers; and said common means and demodulating the received signal synchronously with the frequency of the respective input signal to that transducer; sub-harmonics of each other.
As noted above, a known capacitive displacement transducer may comprise three or more plates, arranged as a differential pair of capacitors. Thus, a two axis device normally requires six capacitor plates, and because of the requirements for relative movement of the plates on two separate axes, it follows that each of the six plates forms a separate sub-assembly of the device. In the case of a three axis device, the situation is even worse because nine capacitor plates are required, each forming an individual sub-assembly. Construction of the device is therefore complicated and relatively expensive.
A second aspect of the present invention provides a device which is responsive to displacement in at least two axes, the device comprising a capacitive displacement transducer for each said axis, each transducer including at least two opposed capacitor electrodes, one electrode of each transducer being common with an electrode of the or each other transducer.
Preferably the capacitor electrodes of the transducers which are not common are provided on a common printed circuit board. Preferably there are two such printed circuit boards, on opposing sides of the common electrode, giving each transducer a differential arrangement in which the common central electrode is relatively movable between the two outer electrodes on the printed circuit boards.
Examples which incorporate both aspects of the invention will now be described, with reference to the accompanying drawings, wherein:
FIG. 1 is a vertical section through a joystick incorporating capacitance transducers,
FIGS. 2 and 3 are sections on the lines II--II and III--III respectively in FIG. 1,
FIG. 4 is a diagrammatic isometric view of the arrangement of electrodes in FIGS. 1 to 3,
FIG. 4A is a view corresponding to FIG. 4, but showing an alternative electrod
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patent: 4629957 (1986-12-01), Walters et al.
patent: 4719538 (1988-01-01), Cox
patent: 4806783 (1989-02-01), Anderson
IBM Technical Disclosure Bulletin vol. 25 No. 3B: Aug. 1982.
Griffin Donald A.
Renishaw plc
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