GB2358065A

GB2358065A - Torque measurement

Info

Publication number: GB2358065A
Application number: GB9929945A
Authority: GB
Inventors: Maurice Allan Yates
Original assignee: ADVANCED ENERGY MONITOR SYST
Current assignee: ADVANCED ENERGY MONITOR SYST
Priority date: 1999-12-18
Filing date: 1999-12-18
Publication date: 2001-07-11
Also published as: GB9929945D0

Abstract

A method is provided for the measurement of the torque transmitted by a rotating shaft. Reference images are obtained of spaced parts of the shaft when it is stationery, and images then obtained when the shaft is rotating. The images are compared to determine the degree of twist in the shaft and the torque transmitted by the shaft is then calculated based on the degree of twist and the shaft parameters.

Description

2358065 TORQUE MEASUREMENT

Field of the Invention

This invention relates to torque measurement and is concerned with the provision of an improved method for the measurement of torque.

The invention has been developed in relation to the measurement of the torque transmitted by means of shafts of substantial size rotating at speeds of up to 1500 r.p.m. in, for example, pumping stations, but is not limited thereto.

Summary of the Invention

According to the present invention there is provided a method for the measurement of the torque transmitted by a rotating element, said method comprising obtaining images of spaced parts of the shaft when stationary, obtaining images of said spaced parts of the shaft when it is rotating, comparing said images to determine the degree of twist in the shaft when it is rotating and calculating the torque in the rotating shaft based on the degree of twist and the shaft parameters.

The images of the spaced parts of the shaft when stationary are preferably obtained simultaneously, as are said images of the spaced parts of the shaft when it is rotating.

A correlation procedure is preferably used to determine the displacement of each of said parts (when the shaft is rotating) from the position which it occupied when the shaft was stationary.

According to a more specific aspect of the present invention there is provided a method for the measurement of the torque transmitted by a rotating element, said method comprising obtaining reference images of first and second parts of the shaft when it is stationary, obtaining images of the first and second parts of the shaft when it is rotating, comparing the images of the first and second parts with the reference images to determine the difference in the displacements of said images from the reference images, determining the degree of twist in the shaft when it is rotating from said difference in the displacements, and calculating the torque in the rotating shaft based on the degree of twist and the shaft parameters.

Brief Description of the Drawings

Figure 1 is a diagrammatic view illustrating the carrying out of a measurement operation, Figure 2 is a diagrammatic illustration of the obtaining of an image, Figure 3 shows the comparison of an image with a reference image,and Figure 4 shows the effects of correlation.

Description of the Preferred Embodiment

As shown in Figure 1, two imaging devices 1 OA and 1 OB are used to obtain images of a shaft 11 of diameter d at two positions spaced apart by a distance L The imaging devices 10A and 10B can be arrays of photodiodes, charge coupled devices or any other suitable form of optical sensor technology. The shaft 11 connects a power source, for example, an electric motor, and a driven element, for example, a pump.

The images are initially obtained by the imaging devices 10A and 1 OB when the shaft 11 is stationary and the initial images are stored in a microprocessor memory for reference purposes.

An imaging device 10A or 10B is shown diagrammatically in Figure 2 and each image 12 which is obtained consists of a series of numbers representing the image brightness or intensity at different physical positions of the imaging device 1 OA or 1 OB.

As the shaft 11 moves, the brightness pattern will appear on different parts of the imaging device and the series of numbers representing the brightness pattern will be substantially the same, but in a different place. The brightness pattern may result from the basic appearance of the shaft 11 but may be implemented by, for example, writing on the surface of the shaft using a felt tip pen.

As the shaft 11 rotates, each imaging device 1 OA or 1 OB will "seen a different part of the shaft 11 as compared to that which it saw initially when the shaft 11 was stationary. A correlation procedure is used to determine the angle through which each portion of the shaft 11 has rotated.

The correlation function is a measure of the degree of similarity of two mathematical functions. As the image is actually a series of discrete numbers, rather than a continuous curve, the correlation in this case is a measure of the degree of similarity of two lists of numbers. If the list of numbers received from an imaging device 10A or 10B when the image is of the original part of the shaft 11 is correlated with the original or reference image, a high value of correlation is obtained.

By comparing the reference image with different parts of the actual image, different values of the correlation function will be obtained and, when the reference image is compared with the part of the image which corresponds to the originally imaged part of the shaft, the correlation function will be a maximum, see Figures 3 and 4. How far down the list the pattern must be shifted until the maximum correlation is obtained is a direct measure of how far the image has moved since the original image was taken and this is a direct measure of how far that portion of the bar has rotated.

A detailed description of the use of correlation in determining displacement is contained in the article by A.J. Pugh, E.A.W maunder and M.R. Belmont entitled "An Optical Displacement Transducer", Measurement, 12 (1993) pages 159 - 170, to which reference should be made.

The measurements obtained with imaging device 10A will provide an indication of how far the particular section of the shaft has rotated since the reference image was obtained. By placing the second imaging device at the known distance 1 along the shaft 11, two values of how far the shaft 11 has rotated from the starting position will be obtained. The difference between the two values is a direct measure of the apparent relative linear displacement of the two parts of the shaft over the distance between the two imaging devices 10A and 10B. From this distance 1, the diameter d of the shaft 11, and the relative linear displacement, it is possible to measure the angle of twist of the shaft 11. From a knowledge of the angle of twist of the shaft 11 and the Young's modulus of the material from which the shaft 11 is formed, it is possible to calculate the torque and hence the power in the shaft.

One of the advantages of the present invention is that no contact with the shaft 11 is required. Another is that no mechanical attachments or modifications to the shaft 11 are required. No special markings on the shaft 11 are required though, in practise, a better signal to noise ratio can be obtained by applying a few random squiggles to the shaft using, for example, a felt tip pen.

No particularly accurate alignment of the two imaging devices 1 OA and 1 OB with the shaft 11 is required. The zero reference is obtained when the shaft 11 is stationary by taking the two initial reference images simultaneously. The measurement images are also taken by simultaneous operation of the two imaging devices 1 OA and 1 OB. In most other measurement systems, either accurate alignment must be achieved with the shaft stationary or a dynamic measurement must be carried out with the shaft rotating under zero load. In the field, both are difficult to achieve.

Claims

Claims:

1. A method for the measurement of the torque transmitted by a rotating element, said method comprising obtaining images of spaced parts of the shaft when stationary, obtaining images of said spaced parts of the shaft when it is rotating, comparing said images to determine the degree of twist in the shaft when it is rotating and calculating the torque in the rotating shaft based on the degree of twist and the shaft parameters.

2. A method as claimed in Claim 1, in which the images of the spaced parts of the shaft when stationary are obtained simultaneously.

3. A method as claimed in Claim 1 or Claim 2, in which the images of the spaced parts of the shaft when it is rotating are obtained simultaneously.

4. A method as claimed in Claim 3, in which a correlation procedure is used to determine the displacement of each of the parts of the rotating shaft from the position which it occupied when the shaft was stationary.

5. A method for the measurement of the torque transmitted by a rotating element, said method comprising obtaining reference images of first and second parts of the shaft when it is stationary, obtaining images of the first and second parts of the shaft when it is rotating, comparing the images of the first and second parts with -7- the reference images to determine the difference in the displacements of said images from the reference images, determining the degree of twist in the shaft when it is rotating from said difference in the displacements, and calculating the torque in the rotating shaft based on the degree of twist and the shaft parameters.

6. A method for the measurement of the torque in a rotating element, said method being carried out substantially as hereinbefore described.

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