Geometrical instruments – Gauge – Angle
Patent
1986-12-01
1990-02-06
Haroian, Harry N.
Geometrical instruments
Gauge
Angle
33512, 33DIG13, A61B 510
Patent
active
048979279
DESCRIPTION:
BRIEF SUMMARY
This invention relates to an electrical sensor for measuring angular displacement. Particularly, but not exclusively, the invention relates to an electro-gonio-meter for use in monitoring joint mobility in patients with ailments such as arthritis which produce abnormal joint movement.
One electro-goniometer for use in medical assessment of joint mobility uses mercury-filled tubes as resistive elements of a Wheatstone bridge circuit. Two or four such tubes are used to provide a bendable beam and a linear relationship between electrical output and angular displacement is given. Problems have been encountered concerning the long-term stability of the electrode/mercury interface and also the well-recognised toxicity of mercury makes its use undesirable.
Another known type of electro-goniometer for orthopaedic diagnosis uses an electrically conductive impregnated plastics strip. Although this arrangement allows a small flexible unit to be made great difficulties with the calibration and electrical stability have been encountered.
An object of this invention is to obviate or mitigate the aforesaid disadvantages.
According to the present invention there is provided an electrical sensor for use in providing a signal indicative of angular displacement between two spaced locations, comprising a flexible carrier of material capable of flexing without permanent plastic deformation when its ends are fixed to said locations and said locations undergo relative rotary movement within a predetermined range; and at least one electrically conductive element having a resistance which is a function of strain affixed to the carrier and extending therealong substantially the whole distance between said locations.
Preferably the carrier is a thin strip of spring steel not exceeding 0.5 mm in thickness. Other possible materials are, for example, non-ferrous metals, acrylic polymers, thermoplastics materials and other plastics materials and fibres.
The strain may be monitored by measuring the electrical resistance of the gauges in a Wheatstone bridge arrangement.
The electrical output may be calibrated to read angular displacement between the ends of the carrier.
The criteria employed in producing the electro-goniometer of this invention were to produce a device which:
(a) required no special alignment with the rotation axis of the limbs,
(b) had low stiffness to facilitate fixation to limbs,
(c) was small enough to be worn beneath a patient's clothing,
(d) has long term electrical stability, and
(e) has infinitely variable electrical output.
The electro-goniometer of the invention uses the `bending beam` approach but replaces the mercury filled tubes of prior devices, suitably with commercially available strain gauges. These gauges have a definite strain limit and have, up till now, been used to measure strain, stress or force where no appreciable angular displacements have occurred. The use of strain gauges where significant deflections would take place is a rather controversial step but the strain magnitude has been reduced sufficiently by decreasing the thickness of the bending beam to very small values.
The resulting device is therefore very flexible and tests have shown the electrical output to be linearly related to angular movement and stability to be excellent. In addition, the method of bending does not influence the output (a Z bend would zero if the top and bottom ends of the carrier were parallel). The size of the unit can be very small and will therefore easily satisfy the criterion of being worn beneath clothing. The device therefore oversomes several of the limitations of previous devices and is convenient for the measuring of joint motion in clinical and research centres.
The mechanics involved in the design of the bending beam originate from the equation for pure bending: ##EQU1## where .sigma.=stress in a particular fibre (usually surface)
For the use of a bending beam as an electro-goniometer it is preferable to have zero bending moment (or resistance to bending movement) therefore the equation: ##EQU2## is used
REFERENCES:
patent: 2885524 (1959-05-01), Eisler
patent: 3305816 (1967-02-01), Doi
patent: 3854112 (1974-12-01), Greenwood
patent: 3991745 (1976-11-01), Yoslow
Haroian Harry N.
University of Strathclyde
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