Optical sensor

Details Optical sensor Optical sensor

2001-07-27

2003-09-23

Nguyen, Tu T. (Department: 2877)

Optics: measuring and testing

Position or displacement

Reexamination Certificate

active

06624900

ABSTRACT:

TECHNICAL FIELD
This invention relates to optical sensors for determining the position of one or more moveable surfaces where the surfaces comprise patterned regions of high and low reflectivity to electromagnetic radiation (EMR). Such moveable surfaces are found in displacement sensors, angle sensors and torque sensors.
BACKGROUND
Typically, such sensors consist of at least one EMR source illuminating a patterned region on at least one moveable surface, at least one lens or other focussing means to focus the EMR reflected from the surface(s), and at least one EMR sensitive array to receive the focussed image. The pattern consists of regions of high and low reflectivity to the EMR emitted from the source(s), and is typically either marked on, attached to, or otherwise applied to the respective moveable surface. The pattern may have a constant period, but may also encrypt absolute position information via a formatted barcode. Such sensors also require means of processing the image to decrypt meaningful information relating to the position of the moveable surface(s), and also means of outputting this information. In co-pending International Patent Application No. PCT/AU98/00645 these functions are achieved by the array architecture forming part of an Application Specific Integrated Circuit (ASIC). In the present specification, the combination of all these components, including the respective moveable surface(s), will be termed a “sensing system”.
In the case of linear displacement sensing systems, the pattern is applied to the surface of a translating plate. An example of a linear displacement sensing system using this principle is disclosed in International Patent Publication WO97/03338. In this patent the moveable surface has two patterns, each of high and low reflectivity. A coarse pattern is used for gross position measurement and a fine pattern for accurate position measurement.
In the case of angular displacement sensing systems, the pattern is usually applied to a cylindrical surface, with the illumination and reflection of EMR occurring in a substantially radial direction, or on a disk-like surface, with the illumination and reflection of EMR occurring in a substantially axial direction. Many other axisymmetric shapes can also be used. An example of an angular displacement sensing system of the type described is disclosed in German Patent Application DE19705312. In this patent the arrangement of EMR illumination, focussing lens, array and processing architecture is clearly shown.
A torque sensing system can also be constructed by utilising multiple arrays (or, for example, a single two dimensional array) and having patterns which change position circumferentially relative to each other as a function of applied torque. An example of such a device is disclosed in co-pending International Patent Application No. PCT/AU98/00645. This patent shows a number of possible arrangements of utilising the basic principle described above to measure torque.
In all of these embodiments, the sensing system is supported within an enclosure. This enclosure serves to eliminate contamination of the moveable patterned surface(s) by foreign material or extraneous EMR. The electrical components which make up the sensing system, are usually mounted within this enclosure on a Printed Circuit Board (PCB), while the other fixed components such as the focussing lens (and ancillaries) are usually mounted separately within the enclosure. The fixed components of the sensing system require accurate alignment to each other and, in turn, these require to be correctly spatially positioned with respect to the moveable surface(s). All previous designs of such devices have required disassembly of the enclosure to repair or replace all or part of the sensing system which, if not carried out by skilled personnel, may result in corruption of this alignment and spatial positioning of the fixed and moveable sensing system components. This, in turn, will dramatically degrade the optical and electronic performance of the sensing system.
The essence of the present invention resides in the provision of a removable sensor module containing all the fixed components of the sensing system. These fixed components are therefore accurately mutually aligned and integrated within the housing of the sensor module. The sensor module can be then installed into the enclosure, the latter surrounding the moveable surfaces of the sensing system, via an aperture in the enclosure designed to provide accurate “datums” for mounting the sensing module. In this way the fixed components of the sensing system (in the sensor module) are aligned with, and accurately spatially mounted with respect to, the moveable surface(s) within the enclosure. Moreover, replacement of the fixed components of the sensing system can now be readily achieved by untrained personnel via replacement of the sensor module as a single component, while maintaining accurate alignment and spatial positioning of the fixed and moveable components of the sensing system.
SUMMARY OF INVENTION
The present invention consists of a sensor for determining the position of a moveable surface having patterned regions of high and low reflectivity to EMR, the sensor comprising an ASIC, at least one lens, and at least one EMR source, the ASIC comprising at least one array of EMR sensitive detectors and processing means, the EMR source facilitating illumination of the surface and the at least one lens facilitating the focussing of reflected EMR from the surface and generating an image on the at least one array of EMR sensitive detectors corresponding to the pattern on the surface, characterised in that the ASIC, the at least one lens, and the at least one EMR source are all enclosed in a single housing providing accurate optical alignment of these elements and integrated as a single replaceable module, and the processing means of the ASIC facilitates processing of the image to determine the position of the pattern on the surface.
It is preferred that the housing of the sensor also comprises an electrical connector and the processing means also facilitates the outputting of a digital or analog electrical representation of the position to the electrical connector.
It is preferred that the electrical connector comprises a multi-pin plug.
It is preferred that the EMR emitted by the at least one EMR source passes through a light guide.
It is preferred that the multi-pin plug also provides electrical power to the sensor.
It is preferred that the at least one lens forms part of a lens system, the lens system comprising at least two lenses separated by an iris.
It is preferred that the at least one lens is any one of a refractive, reflective or diffractive optical component.
It is preferred that the at least one EMR source comprises a Light Emitting Diode (LED).
It is preferred that the ASIC is mounted on a PCB and the PCB is mounted in the housing.
In some embodiments of the present invention all the opto-electronic components which make up the sensor module, including the ASIC and LEDs, are mounted on a single PCB using Surface Mount Devices (SMDs). The LEDs may have focussing lenses integrated into their bodies or, alternatively, one or more light guides may be used to convey EMR from the LEDs to the moveable surface(s) in order to minimise optical losses. This light guide may consist of a moulded transparent plastic tubular or solid section or, alternatively, fibre optic technology may be employed.
Opto-electronic position sensing systems, of the type described in reference to the present invention, rely on a discrete (ie. non-continuous) image sampling process. It is therefore preferred that the LEDs are intermittently pulsed according to a predetermined duty cycle with a high “on” current for a very short time period. The “on” current can in fact equate to many times the steady state current capability of the LEDs. This allows much higher instantaneous optical power emissions to be achieved without damaging the LEDs. It also “strobes” the pattern on the moveable surface(s) and hence reduce

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