Measuring and testing – Dynamometers – Responsive to torque
Reexamination Certificate
1999-03-12
2002-09-17
Fuller, Benjamin R. (Department: 2855)
Measuring and testing
Dynamometers
Responsive to torque
C073S862000, C073S862080, C073S862334
Reexamination Certificate
active
06450045
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a torque sensing calibration unit for calibrating or confirming the accuracy of calibration of torque sensing equipment such as a calibrated torque wrench or a torque sensor.
All measuring equipment if certified to operate to a particular degree of accuracy must be true to that standard of accuracy both immediately after manufacture and initial sale and after prolonged use. It is therefore necessary for all measuring or sensing equipment which carries such a certificate of accuracy to be regularly checked to confirm that the accuracy is maintained. Alternatively the equipment must be regularly recalibrated to the stated degree of accuracy. In the case of torque sensing equipment, such as torque indicating torque wrenches, frequent recalibrations are desirable. A typical calibration standard for a torque wrench would be a long arm beam designed to resist applied torque by means of a reaction torque generated by a proof mass suspended in a scale pan a significant distance, probably more than 2 meters, from a fulcrum of the beam. Torque is applied to the beam about an axis that coincides with the pivotal axis of the beam, and beam movement is prevented by the careful addition of proof masses to the scale pan at one end of the long arm beam until a balancing reaction torque is obtained. The entire calibration must be carried out in a temperature-controlled room, in conditions of controlled humidity, the room having a stable floor that is substantially free of vibration. The recalibration process is slow, as it involves the addition of proof masses individually to the scale pan. The recalibration can only be carried out at a certified testing site where the massive calibration equipment is permanently installed and where the force of gravity, g, is accurately known. The reaction torque &tgr; applied by the long arm beam can however be derived directly as the product
&tgr;=
d×M×g
where
d is the distance from the fulcrum of the beam to the scale pan,
M is the total proof mass added to the scale pan to prevent angular movement of the beam in response to an applied torque, and
g is the acceleration due to gravity at the test site.
That measured reaction torque can be compared with the corresponding value as indicated by the torque sensor on the torque wrench, as a direct check on the accuracy of calibration of the sensor.
New proposals for desirable standards of calibration equipment and calibration procedures have suggested that any single calibration or recalibration of a torque measuring instrument should check the accuracy of the instrument's calibration in a series of measurements spanning a torque range of from 10% of the maximum rated torque of the instrument being calibrated to about 100%, in 10% increments. Furthermore, it has been suggested that each individual measurement should be repeated a number of times, with an average or a statistically weighted average being used to indicate the reaction torque being applied by the calibration equipment. It is impractical to use existing long arm beam calibration equipment in such repetitive calibration procedures, and there has emerged a need for an automatic machine capable of making a series of torque measurements as a predetermined automatically run cycle, each measurement being made to a certified degree of accuracy. Furthermore that accuracy needs to be at least as good as that of existing long arm beam calibration equipment. Preferably such automatic equipment should be able to permit automatic repeat measurements so that each torque measurement is taken a number of times to check and confirm its accuracy, and should be capable of automatically scanning across a defined range of torque loads, in a series of defined increments. Other desiderata attaching to the design of any such calibration equipment are that it should preferably be portable, should not require a knowledge of the acceleration due to gravity, g, at the test site, and that it should be capable of recognizing and allowing for the existence of side loads.
The invention is based on the realization that a secondary measuring instrument may be acceptable as a definitive reference for torque calibration if the accuracy of that secondary instrument is significantly greater than that of the primary torque reference measurement calibration apparatus.
BRIEF SUMMARY OF THE INVENTION
The invention provides a torque sensing calibration unit comprising a hollow torsion bar to which torque is to be applied, associated with at least two pairs of position sensors, each pair being arranged to measure movement of the bar on diametrically opposite sides of a central axis through the bar, and the pairs of sensors being arranged to measure movement in mutually perpendicular directions.
Preferably the hollow torsion bar has a disc-shaped terminal end and a first one of the two or more pairs of sensors is arranged to detect rotary movement about the central axis of the torsion bar. That can be achieved by using the first pair of position sensors to monitor the positions of a pair of flat coplanar faces of side lugs of the disc-shaped terminal end, the plane of the flat faces of the side lugs being coincident with the central axis of the torsion bar. The lugs are preferably equidistant from the central axis, so that pure rotations of the hollow torsion bar would move the lugs in equal and opposite directions. A side load applied to the torsion bar in a direction perpendicular to the plane of the flat faces would however cause deflection of the hollow torsion bar so as to move the lugs in the same direction, so that an analysis of the lug movements relative to one another gives accurate and independent identifications of both rotary movement and linear transverse movement of the torsion bar in the direction of measurement due to side loads.
A second pair of the position sensors is preferably arranged to detect rotary movement about an axis transverse to the central axis of the torsion bar, by monitoring the positions of a pair of target zones on the face of the disc-shaped terminal end of the torsion bar or on the side lugs, on diametrically opposite sides of the central axis. Analysis of the sensor outputs of the second pair gives an accurate identification of linear movement of the torsion bar due to side loads in a direction perpendicular to the side loads producing the transverse movement measured by the first pair of position sensors.
The third pair of the position sensors is arranged similarly to the second pair, but in a mutually perpendicular plane.
As an alternative to the second and third pairs of position sensors described above, a second pair of the position sensors may be arranged to monitor the positions of a pair of flat coplanar faces of a second pair of side lugs of the disc-shaped terminal end, the plane of the flat faces of the second pair of side lugs being coincident with the central axis of the torsion bar and perpendicular to that of the flat faces of the first pair of side lugs. In such an arrangement the analysis of the movement of the second pair of lugs relative to one another gives an accurate measure of rotary torsion induced movement and of linear side load induced movement of the torsion bar, independently of one another. Together, the first and second pairs of sensors therefore measure, independently, movement due to torque and movement due to side loads in two mutually perpendicular directions. A third pair of sensors could if appropriate monitor movement due to side loads along the third axis.
For ease of processing the data from the two or more pairs of sensors, all preferably monitor movement of areas of the terminal end of the torsion bar at equal distances from the central axis. Preferably the second and third pairs are arranged in a horizontal plane and in a vertical plane respectively through the central axis.
Because the analysis of data from the position sensors includes data identifying torque (rotational) loads and data identifying side loads in each of two or three three
Crane David Ogilvie
Quinton Hedley Lloyd
Allen Andre
Crane Electronics Ltd.
Fuller Benjamin R.
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