Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
2000-04-07
2002-12-31
Lee, John R. (Department: 2881)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S492100, C250S231160
Reexamination Certificate
active
06501068
ABSTRACT:
This invention relates to a method for manufacture of optical torque transducers. Such torque transducers are used for measuring the magnitude of torque in shafts, in particular rotating shafts such as found in electric power steering systems in vehicle applications.
BACKGROUND
Electric power steering systems conventionally incorporate an input shaft element, connected via a Hooke's joint arrangement and intermediate shaft to the steering wheel. The input shaft therefore needs to rotate through an angle typically one to two revolutions either side of the on-centre steering position. It is a requirement of the electric power steering system to accurately measure the continuously varying torque in this rotating shaft. Conventionally torque applied to the shaft causes it to angularly deflect, such deflection causing one part of the shaft to angularly displace with respect to another part, and this displacement may be sensed to provide a measurement of this torque.
The sensing means needs to allow for rotation of the shaft, preferably employing non-contact signal transmission means for reliability or simplicity. Said sensing means includes aperture based and reflective optical devices and magnetic devices such as magnetostrictive or variable reluctance couplings.
To improve the accuracy of such sensing means the torque transducer may incorporate a shaft assembly which comprises two grating elements attached to or integral with two torque input members joined by a torsionally compliant coupling. When torque is applied between the two torque input members the torsionally compliant coupling deflects causing an increased relative angular displacement of the two grating elements that allows the use of less sensitive sensing means.
This method relates to the manufacture of torque transducers that use reflective or transmissive optical sensing means, which comprise grating patterns consisting of alternating regions of high and low reflectivity or transmissibility (respectively) for a given incident electromagnetic radiation (EMR). High and low reflectivity includes variations in direct, specular reflection and variations in diffuse reflection. The grating patterns are illuminated by a source of EMR, typically UV, visible or IR light, which are interrogated by one or more arrays of detectors sensitive to the EMR. Arrays include Charge Couple Devices (CCD), Very Large Scale Integration (VLSI) vision chips and one or two-dimensional photodetector arrays. The output of the one or more arrays interrogating the patterns can be processed to produce a measure of the torque applied to the shaft.
The regions of high and low reflectivity or transmissibility can be arranged axially or radially about the axis of rotation of the shaft, and are of such a nature that allows a continuous output of the arrays regardless of the angular position of the shaft, as the array dimensions may not allow the complete circumference or radial face to be viewed by the arrays at any instant in time.
The prior art which is most closely related to that of the present invention is described in U.S. Pat. No. 4,406,939 (Golker) entitled “Method for the Manufacture of a Code Disk for Optical Incremental Shaft Encoders and Absolute Shaft Encoders” which shows the use of laser techniques for manufacturing of shaft encoder patterns.
The essence of the present method resides in the application of radiation, and particularly optical laser radiation, patterning techniques simultaneously to two mutually adjacent grating elements of a shaft assembly. The shaft assembly is assembled prior to irradiation, and comprises the two grating elements attached to or integral with two torque input members that are connected by a torsionally compliant coupling. The simultaneous irradiation of the two grating elements of the shaft assembly provides very accurate control of the relative disposition of the two grating patterns thus generated for the zero torque alignment condition. Further, since the grating patterns are mutually adjacent, a single radiation source can be used for both patterns with further improvement of pattern disposition. This contrasts to the use of optical patterning methods as disclosed in the aforementioned prior art which, if employed, would require the independent “prepatterning” of two grating elements (referred to as “code disks”) prior to assembly of the shaft, with subsequent introduction of inaccuracies in the relative disposition of the two grating patterns during the subsequent assembly operation.
Radiation in the present specification includes optical electromagnetic radiation with wavelengths in vacuum between 40 nm and 1 mm, as defined by The International Society for Optical Engineering.
The advantages of a method according to the present invention to construct a “reflective grating” torque transducer (as disclosed in co-pending International Patent Application No. PCT/AU98/00645) or a “transmissive grating” torque transducer (as disclosed in co-pending Australian Provisional Patent Application No. PP0984 filed Dec. 17, 1997 entitled “Transmission Path Torque Transducer”) accrue from the generation of the patterns after assembly of the shaft. This means that the relative dispositions of the two patterns are determined with great accuracy, which is not disturbed by subsequent assembly operations as would be the case if the patterns were generated prior to assembly of the shaft.
Firstly, calibration of the finished transducer is not required thus saving cost and time in manufacture.
Secondly, the shafts can be interchanged without recalibration or accurate assembly methods, and the arrays are likewise interchangeable without recalibration or accurate assembly methods.
Thirdly, the transducer can be disassembled and reassembled if required for maintenance or repair purposes without the need for special tools or accurate assembly methods.
Fourthly, the “locked in” nature of the relative pattern dispositions means that adjustment features such as screws and stops are not required, which therefore eliminates the possibility that the transducer can be inadvertently miscalibrated by the user or untrained repair personnel.
Fifthly, the use of irradiation allows patterns to be generated on the grating elements with great accuracy and speed.
Finally, the use of irradiation allows complex and angularly non-repetitive patterns, such as barcodes, to be flexibly generated, for example by manipulation of a laser. This allows the torque transducer shaft to be of simpler construction as “aliasing” problems can be eliminated as the individual marks (or groups of regions of high and low reflectivity or transmissibility) have a unique, coded identity. Aliasing is defined in this application as the situation where sufficient angular deflection occurs between the two grating elements that the disposition of the respective patterns is identical to their disposition for a lesser magnitude of angular deflection, thereby yielding indeterminancy and potentially a false measurement of torque in the shaft. Barcodes provide an absolute indication of which mark is being viewed by the array hence eliminate indeterminacy caused by aliasing. Marks in the form of barcoded patterns also allow easy determination of absolute shaft angular position, providing additional functionality of the transducer, or elimination of the need for external angle encoders.
SUMMARY OF INVENTION
The present invention consists in a method of manufacturing grating patterns on the surfaces of grating elements of a torque transducer, the torque transducer comprising a shaft with a longitudinal axis, the shaft comprising first and second substantially rigid torque input members which are connected by a torsionally compliant coupling, the coupling thereby enabling angular deflection of the first torque input member relative to the second torque input member as a function of the magnitude of the torque in the shaft, a first grating element attached to or integral with the first torque input member and a second grating element attached to or integral with the second to
Arent Fox Kintner Plotkin & Kahn PLC
Bishop Innovation Limited
Lee John R.
Quash Anthony
LandOfFree
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