GB2582282A

GB2582282A - Torque sensor calibration

Info

Publication number: GB2582282A
Application number: GB1903163.2A
Authority: GB
Inventors: Habens Malcolm
Original assignee: Datum Electronics Ltd
Current assignee: Datum Electronics Ltd
Priority date: 2019-03-08
Filing date: 2019-03-08
Publication date: 2020-09-23
Also published as: WO2020183140A1; GB201903163D0

Abstract

An apparatus and corresponding method for determining the shear modulus of a drive shaft 1, such as those used in a ship, by applying a compressive force P, which may be provided by a compression jack, perpendicular to the longitudinal axis of the shaft, and using an arrangement of shear strain gauges 2,3 attached to the shaft. The arrangement may comprise four gauges, two measuring shear due to compression and two measuring shear due to tension. The arrangement may also comprise two distinct entities which may have a compression (fig 2, C1, C4) and torsion (fig 2, T1, T3) sensor each. The apparatus may also comprise a support structure R to receive the shaft. The entities may be attached 180o apart. The compression load may be applied at multiple angularly spaced locations, which may be equally spaced. The shear modulus determined by this apparatus and method may be used to determine shaft power and torque when calibrating drive shafts.

Description

TORQUE SENSOR CALIBRATION

Technical Field

The present invention relates inter alia to the measurement of shear modulus of drive shafts, in particular, although not exclusively, to measurement of shaft power and torque.

Background

Accurate measurement of shaft power and torque in large drive systems has become of greater importance in verifying power levels and in fuel saving programs. In the context of a ship or waterborne vessel, it is important to ascertain the power which is transferred from the engine to the propeller.

The world's largest torque test machine is currently 1.2MNm. The largest currently planned is 5MNm. However, these machines have limited physical capacity so cannot he used to calibrate the largest torque sensors.

In addition to the limitations when calibrating large torque transducers, the known test machines cannot he used to calibrate large drive shafts, typically found on ships. Such large drive shaft on ships typically have a diameter of 400mm and a length of over 5 meters. Essentially any shaft that would not fit or could be economically transported to a test machine presents a problem in terms of how to perform a calibration process thereon.

Installing a complete flanged torque sensor on a ship is possible, however it is very costly in both manufacture and installation. Dependent on their physical size these sensors cannot he calibrated using the available test machines. For the currently available test machines, they can only cater for 1.2mtr diameter shafts and in relation to shaft weight the largest machine currently available can only cater for 2000Kgs.

Measuring shaft power on large drive shafts is normally completed using shaft power meters.

Shaft power meters measure the twist (or torsional shear) in the shaft when under load using a range of techniques including strain gauges, static displacement and phase displacement. In all cases these shaft power meters are dependent on a knowledge of the shaft shear modulus.

The shear modulus of a drive shaft can be measured by testing a (steel) sample. However, the accuracy of this is often questionable as the sample needs to have come from the same steel batch and manufacturing process to match the average of the outer layers of the shaft (dependent on diameter).

Also, in known systems, the current errors are present: Gauge Factor -this can range from +1-1.0% to +1-0.2% Shear Modulus -may have a degree of uncertainty of up to 4% -Note ITTC and ISO 15016 v 2015 Instrument errors -systems should offer 0.1% however with a combination of installation and instrument errors we would expect variation of over 0.5% We have devised an improved apparatus and method for determining actual the shear modulus in-situ of a shaft. This can then be used in relation to measuring the torque of a drive shaft.

Summary

According to a first aspect of the invention there is provided apparatus for determining the shear modulus of a drive shaft, comprising a compression apparatus which is arranged to apply a shear compression force substantially perpendicular to a longitudinal axis of the drive shaft, the apparatus further comprising a shear strain gauge arrangement configured to he attached to the shaft.

The applied compression force, or shear load, may be viewed as transverse to the longitudinal axis of the drive shaft.

The strain gauge arrangement may comprise a strain gauge bridge.

The invention may be viewed as the calibration of strain gauges for use in measuring torque and 20 power.

The strain gauge arrangement may comprise four strain gauge sensors or elements, two of which are intended in use to measure compression and two of which are intended in use to measure tension.

The strain gauge arrangement may comprise two integral, physically distinct, entities, each comprising two strain gauge sensors which are incorporated into each entity. Preferably for each entity comprises a strain gauge for measuring compression and a strain gauge for measuring tension.

Strain Gauges are preferably applied at 180 degrees to monitor the shear stresses due to torsion the shear calibration loads.

The gauges installed at each location are preferably a minimum of a half shear bridge at each location. For many practical installations full bridges of shear strain gauges complete with their own signal conditioning are installed at each location.

When loaded in torque the strain gauge bridge (half or full bridge) outputs are preferably of the same sign relative to rotation and when loaded by a shear load perpendicular to the axis of the shaft, the shear the bridge outputs are preferably of opposite signs.

The shaft may therefore he calibrated by applying a perpendicular shear load as the shear strains seen at the two location 180 degrees apart have a direct relationship to the shear strain created by applying torsion to the same shaft. The relationship is described in equation xx The apparatus may comprise a support structure which is arranged to receive and support a drive shaft for the purpose of applying a shear load. The support structure may be termed a rig.

The compression apparatus may comprise a jack. The jack may he a compression jack. The jack may comprise a mechanical component or assembly arranged to apply a compression force. The compression apparatus may comprise a screw or hydraulic assembly arranged to generate a compression force.

The apparatus may comprise a load cell to measure the load applied by the compression machine.

The apparatus may be arranged to be used or installed to a chive shaft in situ, i.e. in its operation location. Alternatively or in addition, the apparatus may be configured to be used with a chive shaft which is remote from, or not in its, operation position. For example, this could he the case for a drive shaft before its installation and commission for operational use, or for a drive shaft which has been removed from its operational position.

In one embodiment of the invention there is provided the use of a loading jig on a shaft in situ that allows a perpendicular shear load to be applied at a known point. The jig may comprise a hydraulic or mechanical jack to apply the load and a load cell to measure the load applied.

A first shear strain gauge assembly and a second shear strain gauge assembly may be fixedly secured to an outer surface of the drive shaft at substantially one hundred and eighty degrees apart. The strain gauge assemblies may each be arranged to measure strain at portions of the chive shaft which intersect planes which are substantially parallel.

According to a second aspect of the invention there is provided a method for determining the shear modulus of a drive shaft, the method comprising applying a shear compression force to the drive shaft, which compression force is substantially perpendicular to a longitudinal axis of the drive shaft, and the method comprises using a shear strain gauge arrangement attached to the drive shaft to determine shear strain when the compression force is applied.

The shear strain gauge arrangement may comprise a first set of two strain gauges (or gauge elements) and a second set of two strain gauges. For each set of gauges one strain gauge may he arranged to measure compression and the other gauge is arranged to measure tension. Each set of gauges may be an integral entity or assembly. with both gauges incorporated thereinto.

The method may comprise applying a compression load to the drive shaft at multiple angularly spaced locations. Where there are three or more locations, the positions are substantially equally angularly spaced. The method may comprise determining one or more averaged strain values measured in response to the applied compressive loads at the different locations. In this regard the method may comprise applying the compressive load at each location, recording a strain measurement from the strain gauges, and then performing an averaging operation to obtain one or more averaged values or measures of shear strain.

According to a third aspect of the invention there is provided a method of measuring torque or torsional shear experienced by a drive shaft using the strain gauges of the second aspect of the invention. In other words, using one-and-the-same strain gauges as used to determine the shear modulus of the shaft.

The invention may he viewed as a method of calibrating torque measurement using a compression machine to apply shear loads.

1. Shear strains developed by a perpendicular shear force applied to a drive shaft measured by a strain gauge bridge are directly related to the shear strains developed by applying a torsional load to the same drive shaft.

2. The same strain gauge element that measures the static perpendicular shear can be used to measure torque.

3. The use of the same strain gauge elements within the calibration eliminates the uncertainty of the strain-gauge gauge factor.

4. Using the perpendicular shear force to apply shear stress to the gauges provided accurate is a valid method of calibration Other aspects of invention, or modified/enhanced versions of above aspects of the invention, may include one or more features described in the description and/or as shown in the drawings, either individually or in combination.

Brief description of the Drawings

Various embodiments of the invention will now be described, by way of example only, with reference to the following drawings in which: Figure 1 is a perspective view of a drive shaft to which shear strain gauges are attached, Figure 2 is an enlarged view of a portion of the drive shaft of Figure 1, showing the strain sensors attached thereto, Figure 3 is a side elevation view of the drive shaft which schematically shows a compression load applied, Figure 4 is a circuit diagram showing an electrical circuit which includes the strain gauges which are attached to the drive shaft to measure shear strain, Figure 5 is a representation of the shear to which would he measured by the strain gauges, Figure 6 is a perspective view of a schematic representation of the drive shaft being subjected to an applied torque, Figure 7 is a representation of torsional shear experienced by the drive shaft in response to an applied torque, and Figure 8 is a circuit diagram showing an electrical circuit which includes the strain gauges which are attached to the drive shaft to measure torsional shear

Detailed Description

There is now described a method and apparatus for determining the shear modulus of a drive shaft using strain gauge sensors attached to the drive shaft, typically in relation to a drive shaft of a waterborne vessel. Having determined the shear modulus in this way, an accurate determination of power and torque transmitted through the shaft can he achieved using the same strain gauges so as to allow an accurate assessment of the operational performance characteristics of the shaft.

With reference initially to Figures 1 and 3, there is shown a drive shaft 1 of a ship. As will be described in more detail below, the shear modulus of the shaft is determined by the application of a shear force which is applied in a direction which is perpendicular to the longitudinal (rotational) axis A-A of the shaft.

The bobbin or coupling (not illustrated) provided at one or both distal ends of the drive shaft that carries the torque can include either loading adaptors so that it can have shear loads applied or he provided with flats, or flattened portions, so that shear loads can be applied to the shaft with the shaft suitably restrained and/or supported. The flats may comprise preformed flattened surfaces which enable the drive shaft to be maintained in position or stabilised when the compressive load is applied.

The drive shaft 1 has one or two sets of shear gauges attached to the shaft. One full set of shear strain gauges comprising two half bridges on the same section at 180 degrees provides a good level of accuracy and confidence in the measurement. Additional strain gauge installations further improve the statistical confidence in the final measurement. When testing an element or a shaft in a compression test machine three sets of strain gauges at 120 degrees can he used to mimic key elements of DIN 51309. In the Figures, two sets of strain gauges are shown, referenced as 2 and 3. Each set is an entity or integral assembly in which two strain gauge elements are incorporated, adjacent and side-by-side, one element for measuring the shear strains due to tension and the other for measuring the shear strain due to compression. Each set or pair of strain gauge elements is fixedly attached to an outer surface of the drive shaft 1. Specifically, the strain gauge entity 2 comprises a compression strain gauge Cl and a tension strain gauge T2, and the strain gauge entity 3 comprises a compression strain gauge C2 and a tension strain gauge Tl.

The four strain gauge elements are connected together by way of a bridge circuit as shown in Figure 4.

The gauge assemblies 2 and 3 are located accurately at substantially180 degrees opposed or substantially diametrical, about the outer surface of the drive shaft 1. The use of two sets of gauges located on parallel planes each side of the applied load reduces the uncertainty of the overall measurements. The use of two complete sets of strain gauges and then averaging the results removes any errors due to the longitudinal position that the compressive load is applied, with a perfect/idealised jig the compressive load will he applied in the centre of the load support frame.

Further reductions in uncertainty can be made by adding additional pairs of strain gauges to the shaft aligned at 120-degree intervals around the shaft. This allows for the load to be rotated in steps of 120 degrees. The average of the three shear measurements is used to calculate the shear modulus/strain gauge ratio.

The individual strain elements are wired so that they measure shear due to the compressive forces. Having individual strain values allows the system gTeater detail when documenting and evaluating the test data.

The drive shaft I equipped with the shear strain gauges affixed thereto is now loaded onto a compression machine and reference is now made in particular to Figure 3. The compression machine is configured to apply a shear load P to the centre section of the drive shaft using a compression rig, which provides support shown at R. The compression machine comprises a mechanical or hydraulic device which is arranged to apply the compression load P. With the load P applied to the drive shaft 1, the shear strain experienced by the drive shaft can be measured by taking readings output by the strain gauge circuitry at S-and S+.

The data from the shear strain gauges provides a (direct) correlation between the force applied and the shear strain, as provided by the relationship: Shear Strain/Shear Stress = Shear Modulus (where Shear Stress = Force/Area) The modulus of the central (coupling) portion of the drive shaft 1 (which is intermediate of the bobbins/couplings or which transmits the torque) can thereby he determined.

For yet further accuracy, the drive shaft can he rotated by 120 degrees, and the compression load applied at each angular position. The average of the three readings provided by the sensors are taken for the calibration.

In an alternative implementation of the above method, in a further embodiment, three pairs of strain gauge installations applied at 120 degree intervals around the drive shaft and the load can also be applied at 120 degree intervals. This provides a more complete map of the shear strains to which the drive shaft is subjected. Measurements taken from the gauges not parallel to the applied load are reduced and proportional to the signal seen from the gauges parallel to the load.

Having accurately determined the shear modulus of the drive shaft, a torque is then applied to the shaft, and the extent of twist or torsional shear which the shaft experiences can be measured using the strain gauges 2 and 3 which are attached to the shaft 1. Reference is made to Figures 6 and 7. Using the output signals from S-and 5+ of the bridge circuit connected gauges 2 and 3, the following relationship can be used: mathematical relationships used in processing said outputs from the strain gauges, both in case when a compressive load is applied and when a torque is applied to the drive shaft, are shown in Appendix A. It will appreciated that substantially the same principles apply to calibration in relation to hollow shafts It will be appreciated that one-and-the-same set of strain gauges is used for both measuring compression shear and for measuring torsional shear. This is achieved by changing the connections of the strain bridge to be 'reconfigured' to measure direct shear or torsional shear. This can be achieved by either adding a switch to the circuit or physically/manually re-wiring. Figure 8 shows the connections of the bridge of strain gauges as connected for measuring torsional shear.

It will further be appreciated that the above embodiments may be implemented by affixing the strain sensors and applying the compression load with the drive shaft not in situ, e.g. before the drive shaft is installed operationally. Or, this could include removing a drive shaft from its operational location and applying the compression load using a compression machine to determine shear modulus before the drive shaft is re-installed to its operational location. As yet a further alternative, the strain gauges could be attached to a drive shaft at the time of manufacture, and the shear modulus of the drive shaft determined prior to its installation.

Alternatively, the embodiments above a testing tool could be used with the drive shaft in situ that allows compression load to he applied. Such a tool may comprise a screw or hydraulic jack. A ship shaft using either a screw or hydraulic jack. Such a tool is of particular advantage in measuring the shear modulus of the drive shaft of a ship, for which it may not he feasible to remove the drive shaft (e.g. causing excessive operational downtime) to enable calibration of torque sensors.

Some of the advantages of the use of a compression machine to apply shear force perpendicular to the axis of the drive shaft include: 1. The shear force can be applied to a higher level of accuracy and confidence than most torsional loads. The accuracy of the system has a very high level of confidence based on the accuracy of the compression machine in applying a known compression load.

2. The shear strain developed when a shear force is applied is directly proportional to the shear strain developed by applying a torsional load.

In the above described embodiments, there is also the advantage in pure accuracy of the shear strain being measured using one-and-the-same gauge elements as arc used to measure the torque, since the gauges are fixedly attached to the drive shaft. This advantageously eliminates any error relating to the gauge factor.

Further advantageously, calibrating in accordance with the above described embodiments reduces overall errors to: Load Applied -can be measured to 0.1% (0.01% on some test machines) Instrumentation -better than 0.1 % Overall, errors due to Modulus, gauge factor and installation are removed or significantly reduced in the calibration according to the above embodiments, and levels of confidence in power calculations (where shaft power {W} = shaft torque {Nim} * RPM/30 * 7r) using the same arc substantially 0.25%.

Appendix A In the following shear / torque cross-calibration method analysis the following is assumed: 1. Isotropic Shaft Material 2. Ideal Shaft Geometry 3. Linear Elastic Loading Regime 4. Same Strain Gauge Setup between measurements 5. Strain Gauges have identical gauge factors In both cases of loading the chive shaft, the shaft has radius (r), area (A), Shear Modulus (G), and the strain gauges have gauge factor (gf) (which is supplied by the manufacturer of the strain gauge). For the case of a round beam with a shear load (L). the maximum shear stress (rs,max) is given to the left, below. The ease of a round beam with an applied torque (T). the maximum shear stress (rt,n tax) is given to the right, below.

Shear Measurement 4L Ts,max = 3 A Torque Measurement 2T Tt,max = A r The measured strain (r) relates, in both cases, to the maximum shear stress by: T = 2 G E 2L ES,771CLX 3 G A Et,max = G A r The measurement signal (Vsig) relates to the measured strain through the gauge factor: Vsig = gf E Vsigs = 2 gf L gf T 3 G A VsiatG Ar A calibration factor (C * s) is then defined, based on the shear measurement: 2 gf Cf 3 G A = L 3Cf, Vsig't = 2 r T The calibration factor from the shear measurement can be applied as part of the calibration factor for the torque measurement. The crosscalibration method begins with determining the calibration factor when the shear loading is applied, and then using the calibration factor (Cf,$) to the torque loading case. These two calculations are, including the relative uncertainties: V stq s CV stq SL) cf S L V st,q L s fir * 2 r Vsig,t stg,t f,$) T = + +SV -+SC - 3 Cf., r Vsig,t Crs D V.sta t (SD SVsiq 61) " +-+2-+ 3 Cf., D Vsig L Where D is the shaft diameter and it is also assumed that the uncertainty in both of the strain gauge signal measurements are identical.

Claims

CLAIMSI. Apparatus for determining the shear modulus of a drive shaft, comprising a compression apparatus which is arranged to apply a shear compression force substantially perpendicular to a longitudinal axis of the drive shaft, the apparatus further comprising a shear strain gauge arrangement configured to he attached to the shaft.
2. Apparatus as claimed in claim 1 in which the strain gauge arrangement comprises a strain gauge bridge.
3. Apparatus as claimed in claim 1 or claim 2 in which the strain gauge arrangement comprises four strain gauge sensors or elements, two of which are intended in use to measure the shear due to compression and two of which are intended in use to measure the shear due to tension.
4. Apparatus as claimed in any preceding claim in which the strain gauge arrangement comprises two integTal, physically distinct, entities, each comprising two strain gauge sensors which arc incorporated into each entity.
5. Apparatus as claimed in claim 4 in which each entity comprises a strain gauge for measuring shear due to compression and a strain gauge for measuring shear due to tension.
6. Apparatus as claimed in any preceding claim which comprises a support structure which is arranged to receive and support a drive shaft for the purpose of applying a shear load.
7. Apparatus as claimed in any preceding claim in which the compression apparatus may comprises a compression jack.
8. Apparatus as claimed in any preceding claim which comprises a first shear strain gauge assembly and a second shear strain gauge assembly which arc fixedly secured to an outer surface of the drive shaft at substantially one hundred and eighty degrees apart.
9. A method for determining the shear modulus of a drive shaft, the method comprising applying a shear compression force to the drive shaft, which compression force is substantially perpendicular to a longitudinal axis of the drive shaft, and the method comprises using a shear strain gauge arrangement attached to the drive shaft to determine shear strain when the compression force is applied.
10. A method as claimed in claim 9 in which the shear strain gauge arrangement comprises a first set of two strain gauges (or gauge elements) and a second set of two strain gauges.
11. A method as claimed in claim 10 in which for each set of gauges one strain gauge is arranged to measure compression and the other gauge is arranged to measure tension.
12. A method as claimed in any of claims 9 to 11 which comprises applying a compression load to the drive shaft at multiple angularly spaced locations. 1I
13. A method as claimed in claim 12 in which where there are three or more locations, the locations are substantially equally angularly spaced.
14. A method as claimed in claim 12 or claim 13 which comprises determining one or more averaged strain values measured in response to the applied compressive loads at the different locations.
15. A method of measuring torque or torsional shear experienced by a drive shaft using the strain gauges referred to in claims 9 to 14.