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Browse Prior Art Database

PIEZOELECTRIC MAGNETIC ROTARY SENSOR

IP.com Disclosure Number: IPCOM000026796D
Original Publication Date: 1993-Oct-31
Included in the Prior Art Database: 2004-Apr-06
Document File: 4 page(s) / 166K

Publishing Venue

Xerox Disclosure Journal

Abstract

High resolution digital radial encoders (either incremental or absolute) are often not cost effective or small enough when used in conjunction with modern cost effective hybrid circuitry. In many applications, sensor costs are far greater than the supporting control system. The piezoelectric rotary sensor depicted in Figures 1,2A and 2B, could offer unique solutions to cost effective, manufacturable, miniature incremental encoding.

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Page 1 of 4

XEROX DISCLOSURE JOURNAL

PIEZOELECTRIC MAGNETIC Proposed Classification ROTARY SENSOR U.S. C1.116/284 Thomas H. Peterson Int. C1. Gold 13/00

FIG. 2A

- 16,18

20

FIG. I FIG. 2B

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Page 2 of 4

PIEZOELECTRIC MAGNETIC ROTARY SENSOR(Cont'd)

High resolution digital radial encoders (either incremental or absolute) are often not cost effective or small enough when used in conjunction with modern cost effective hybrid circuitry. In many applications, sensor costs are far greater than the supporting control system. The piezoelectric rotary sensor depicted in Figures 1,2A and 2B, could offer unique solutions to cost effective, manufacturable, miniature incremental encoding.

When certain crystals are placed between two plate electrodes and a force is applied to the plates, the stress and corresponding deformation of the crystal produces a potential difference between the surfaces of the crystal. This phenomenon is called the piezoelectric effect.

This disclosed sensor incorporates a disk that magnetically excites a pair of piezoelectric elements, 16 and 18, in such a way that incremental position and rotational direction information is given. Two additional elements, 22 and 24, give hemispherical location. Since the device has a resolution of 0.5", the circumference of the disk is approximately 150mm. A 0.2m.m by 0.2- cube face for the crystal would be necessary for this resolution. Since

E =gtP, with E = OutputVoltage g = Voltage
t = Thickness
P = Pressure = Force/Area

the necessary force required on a 0.2mm square x-cut quartz crystal 2mm in length would be:

EA (20x10 -3u) (0.0002m )'

F=-= = 7.273~ 10 -6 N gt (- 0.055~dN) (0.002m)

This force would produce a 20mv signal which could be used to open the source and drain on a small mosfet device and produce a digital signal.

The device shown in the figures has at least two magnetically excited piezoelectric sensors, 16 and 18, that are held within .5mm of a magnetic cog 20 of .3mm thickness. It has been shown through empirical experimentation that the force generated with a small common magnet mounted on a similar disk was several orders of magnitude greater than the required force. Even with magnetic field decay at elevated temperatures, the force is more than sufficient.

When the teeth of cog 20 align with the piezoelectric crystal, as shown in Figure 2A, a small sputtered foil on the end of the crystal is attracted towards the magnetized cog and an axial strain results....