Torque transducer

Details Torque transducer Torque transducer

2000-05-03

2002-09-17

Fuller, Benjamin R. (Department: 2855)

Measuring and testing

Dynamometers

Responsive to torque

Reexamination Certificate

active

06450044

ABSTRACT:

TECHNICAL FIELD
This invention relates to torque transducers 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 an intermediate shaft and Hookes joint arrangement 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. The input shaft is at least partially surrounded by the fixed housing of the steering gear. It is a requirement of the electric power steering servo system to accurately measure the continuously varying torque in this rotating shaft. Conventionally torque applied to the shaft causes it to is angularly deflect, such deflection causing one part of the shaft to angularly displace with respect to another part, and this displacement is sensed to provide a measurement of this torque.
The sensing means needs to allow for rotation of the shaft within the housing, usually employing non-contact or mechanical signal transmission means. Non-contact means include optical aperture based devices and magnetic devices such as magnetostrictive or variable reluctance couplings. Mechanical means include slidably connected potentiometers and other indicating devices.
To improve the accuracy of such sensing means a torsionally compliant coupling in the form of a torsion bar is used to connect the two input members at either end of the shaft When torque is applied between the two input members the torsion bar deflects causing an increased angular displacement, which allows the use of less sensitive, or less accurate sensing means.
The torsion bar may be in the form of a separate element as in the case of a conventional rotary hydraulic power steering valve. Alternatively, in the case of some proposed electric power steering systems, the torsion bar may in fact be integral with the shaft member and be a relatively torsionally compliant (ie. less torsionally stiff) portion of the shaft member which couples substantially rigid torque input members at each end of the shaft member. The shaft member in these latter systems can be readily machined as a single steel component, and the only requirement is that the angular deflection of the relatively torsionally compliant coupling portion, connecting the two substantially rigid torque input member portions, has sufficiently low torsional stiffness that the sensing system is able to accurately measure its angular deflection.
Generally, the use of a torsion bar requires the use of a failsafe mechanism, being a torque limiting device to prevent failure of the torsion bar when unavoidable torque overload conditions occur.
Such torque limiting devices are well known in the art of vehicle steering, and will therefore not be described in this specification.
The prior art, which is most closely related to that of the present invention, is described in U.S. Pat. No 5,369,583 and International Patent Application PCT/GB95/02017 which show sensors employing optical disc apertures for measuring torque.
The essence of the present invention resides in the provision of grating elements comprising surfaces composed of alternating regions of high and low reflectivity connected by a torsionally compliant coupling. These surfaces are illuminated by a source of electromagnetic radiation (EMR), typically UV, visible or IR light, which generates patterns on one or more arrays of detectors sensitive to the EMR. Arrays include CCD devices, VLSI vision chips, one and 2 dimensional photodetector arrays and lateral effect photodiodes (commonly referred to as PSD's or position sensitive devices). The disposition of the patterns is a function of torque applied to the shaft, and the output of the one or more arrays can be processed to produce a measure of the torque applied to the shaft. It is distinguished from other reflective torque transducers by use of an reflective imaging approach which does not rely on Moire fringes, speckle patterns or other diffraction gratings. As it uses photo detector arrays, EMR reflected from the gratings provide an instantaneous image which allows a much faster and more complete means of interpreting the information than is possible with individual photo-detectors. In the latter case it is necessary to count successive changes of EMR intensity incident on the photo-detector, which is slower and more prone to error.
Another reflective torque transducer that uses arrays is described in U.S. Pat. No. 5,490,430. This relies on a change in diffraction angle of two or more diffraction gratings that are torsionally strained by the application of torque. This device is prone to error due to misalignment and bending load and requires a collimated and monochromatic source of EMR. The regions of high and low reflectivity 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 at any instant in time regardless of the angular position of the shaft, as the limited array dimensions may not allow the complete circumference or radial face to be viewed by the arrays. The advantages of such a construction over that disclosed in U.S. Pat. No. 5,369,583 and International Application Number PCT/GB95/02017 may arise as one or more of the following:
Firstly, the use of reflective grating elements allows simpler and more compact construction by the use of a cylindrical grating element arrangement, which is not readily achievable using disc apertures as shown in the prior art without requiring a significantly increased diameter. It also allows the EMR source(s) and array(s) to be packaged in the same assembly with further savings in space and cost. Secondly, it allows for easy assembly and disassembly of the transducer, as the grating elements can be removed from one end of the transducer in an axial direction without disturbing the EMR source(s) or array(s).
Thirdly, another advantage with the use of reflective grating elements is that the EMR is reflected from the surface, and is not affected by edge scattering as is the case with apertures with a non-zero thickness. Such scattering limits the maximum resolution of the device. Fourthly, the use of reflective grating elements allows the use of well known and accurate photographic or metallising techniques, for example metal on glass. The use of these techniques with apertures may result in loss of resolution or other problems from internal reflection, diffraction or degradation over time as the EMR has to travel through the glass between the metallised regions.
Finally, the use of reflective grating elements allow the use of intermeshed castellations which can provide a lost motion connection limiting the maximum angular deflection of the torsion bar, thereby eliminating the need for a separate torque limiting device and reducing the cost and complexity of the transducer.
DISCLOSURE OF INVENTION
The present invention consists in a torque transducer comprising a rotating shaft at least partially surrounded by a fixed housing, the axis of rotation of the shaft fixed with respect to the housing, 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 torque input member, the first grating element comprising a first surface and the second grating element comprising a second surface, the transducer also comprising one or more electromagnetic radiation (EMR) sources and one or more arrays of EMR sensitive detectors, characterised in that each source irradiates one or both of the surfaces and each array rece

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