GB2151775A - Apparatus for measuring torque of a shaft - Google Patents

Abstract

Torque of a shaft 12 is measured by providing a radiation source 16 and a radiation sensor 17 and interposing two radially slotted discs 10B, 11B each respectively mounted at one end 10A, 11A on the shaft 12 but rotatable at the disc ends on bearings 14 on the shaft. When the shaft rotates torque causes relative angular movement between the normally aligned radial slots and a reduction in radiation reaching the sensor 17; and electronic circuit responsive to the output of the sensor provides an indication of the torque. The apparatus may be calibrated statically and can be used for detecting small angular displacement with high shaft vibration. <IMAGE>

Description

SPECIFICATION Improvements in or relating to measurement of torque This invention relates to the measurement of torque.

One form of measurement of torque of a rotating shaft is a non-contact method using photo-electric or magnetic transducers. Two uniformly slotted wheels mounted on the shaft but axially separated respectively pass through two spaced operational zones to generate two series of pulses. In a photo-electric arrangement each zone is between a light source and a light sensor. Torque in the shaft causes the shaft to twist thus creating a proportional change in the delay between two pulses generated by the two wheels. The magnitude of the delay is dependent upon the vibration both radial and angular (circumferential) of the respective wheels. The power delivered to the shaft can be monitored by using this delay and the known rate of rotation of the shaft.

This method is not satisfactory for small angular displacements with high shaft vibration.

According to the invention apparatus for measuring torque of a shaft comprises a radiation source, a radiation sensor, first and second means adapted for mounting on the shaft at axially spaced locations between the source and sensor, relative angular movement between the first and second means changing the radiation received by the sensor from the source, and means for deriving from said change a measure of the torque on the shaft.

The first and second means may be slotted discs.

The slots may be rectangular.

The slots may have radial sides.

The deriving means may comprise electronic means having an output signal proportional to the change in torque.

The invention may be performed in various ways and one specific embodiment with possible modifications will now be described by way of example with reference to the accompanying drawings, in which Figure l is a longitudinal section-through one arrangement, Figure 2 is an axial view of a disc; Figure 3 is a curve of voltage output against torque; Figure 4 is a schematic-showing of voltage pulse and light source.

Referring to the drawings Figure 1 shows a schematic diagram of a torque measuring device or torque meter. Two aluminium cylinders 10, 11 are clamped each at one end 10A, 11A onto a shaft 12 whose torque is to be measured. Each cylinder is in two halves held together at one end by the clamps (eg bolted together) which hold the cylinder on the shaft, the clamps are shown schematicaliy at 9. The cylinders are fixed to the shaft only at ends 10A, 11A. At the other end of each cylinder are respectively fixed annular aluminium discs 1 OB or 11 B axial with the respective shaft, and also split into halves for assembly purposes, with for example a separation of less than 0.1cm between the discs. Clamps 9a hold the cylinder halves and disc halves together at the disc ends but leave the cylinders and discs free of the shaft at this end.Each disc has a narrow radial slot or aperture 1 3A, 1 3B machined into it such that by suitably orientating the discs these slots may be brought into alignment. Also situated at the disc end of each cylinder is a PTFE annular bearing 14 which makes a light, almost friction-free contact with the shaft 12. The purpose of these bearings is to prevent any radial movementofthe cylinders and hence the discs due for example to shaft vibrations.

The cylinders and discs rotate with the shaft and pass through an operational field 15 (defined by the broken lines in Figure 1) created by a uniform rectangular light source 16 and a photo-detector 17. When the overlapping slots 13A, 13B pass through the field 15 illumination of the photo-detector 17 will take place for a period equal to the transition time of the slots through the field. The output from the detector will be in the form of a voltage pulse whose amplitude will be proportional to the overlapping slot width. This will be a maximum if the slots are completely aligned and fall to zero as they become misaligned. A suitable source is a uniform rectangular tungsten filament source.

If during rotation torque is generated in the shaft 12 then a small degree of angular displacement between the discs 10B, 11 B will result creating a proportional increase in slot misalignment and hence a corresponding reduction in pulse amplitude at the output of photo-detector 17. By calibrating the pulse amplitude against torque statically as described later, the torque may be monitored under rotating conditions.

It can be shown that for a hollow shaft under torsional strain Mt, the angular deflection

where G = shear modules d1 = outside diameter d2 = inside diameter thus

If 8+ is the total angular twist over a length e then: 34) 80 = + and

Consider now the overlapping slots let a = the length of the slots and t = the width where a > > t If the slots are rectangular but t/a is small and the slots are displaced some distance from the axis of the discs we may consider the sides to be radial since the included angle # will be very small, see Figure 2. The area A of each slot is thus given by: A=4)#a[2r+a] 2 Where r is the inner radius of the slot.This represents the effective aperture through which light may be transmitted from the source to the detector when the slots are aligned.

If the slots are caused to become misaligned due to torsional loading of the shaft on which they are mounted then a reduction ## in the included angle # will result creating a proportional reduction 8A where #A=a/2[2r+a]## rand a being a constant.

Since the photo-detector 17 will generate an output voltage V proportional to the incident light flux then 3V 8A ~ ## V A + where #V/V, 8A/A and ##/# are the corresponding fractional changes in V,A and # giving 3V ##=##/V# If t is the average slot width then

radians so that 3V 2t 8+=vv (2r + a) and

Since all the quantities except aVN on the right hand side of this equation are constant for a given shaft then Mt=k8V V where

constant.

Thus from a knowledge of k the torque may be calculated from a measure of the fractional change in detector output voltage pulse height.

In practice V is also constant making Mt proportional to the absolute change in pulse height 8V.

Alternatively the dynamic torque (ie that generated in a rotating shaft) may be deduced from a static calibration. In this case the stationary shaft is subjected to a series of increasing torsional loads generated for example, by two weighted horizontal radial arms firmly clamped on to the shaft on either side of the torquemeter and generating equal but opposite moments about the axis of the shaft. The orientation of the shaft is chosen such that the aperture 13A lies approximately midway through the light field 15. By rotating the shaft back and forth through a small angle the maximum detector exposure, and hence voltage output, for a given aperture area may be obtained, this corresponding to the maximum pulse height generated during dynamic conditions.

Figure 3 shows a typical calibration curve for the torquemeter derived by the above procedure indicating the expected linearity between torque and voltage output.

It is possible to monitor the photodetector output pulses (via a control unit) on an oscilloscope.

A peak detector type circuit could be used to generate an analogue output and allow readings to be made on a panel mounted digital voltmeter.

Figure 4 shows the circuit of a control unit to which the source and detector are connected. A photodiode with integral amplifier 40 has a temperature compensated reference voltage VR, obtained using temperature compensated zener diodes 41, which can be adjusted by continously variable resistor 42. The voltage output Vo of the diode 40 is kl-VR where k is the responsivity over the whole spectrum of illumination and lis the visible light illumination in watts. Aswitched gain control 44 is provided for V, in which R2R3R4=Rf, in association with a fairly low drift operational amplifier 45 whose output voltage 46 is

The supply to tungsten filament lamp 16 can be varied by resistor Rc to vary the illumination level. A continuously variable gain control could be used.The photo diode 40 has good temperature stability, adequate time response, satisfactory spectral matching with selected light sources, and good responsivity.

In the case of the torque meter the pulse period ET (defined for example as the periods corresponding to half the pulse height) is not controlled by the aperture width defined by the overlapping slots but the size of the light source, the geometrical response of the detector which in fact follows a normal type distribution and the overall geometry of the source, aperture and detector arrangement.The distance travelled by the aperture through the source/detector field between the half pulse height levels is in fact of the same order as the source width W (see Figure 5) giving a period of 8T where:-

where T = period of rotation of the disc r = inner radius of the slots a = length of the slots For a typical rotational speed of 6000 rpm T would equal 1 Oms and for a value of W = 18mm (the value for the tungsten filament source) 8t=550us which is slow enough for the photodiode employed to experience negligible time constant errors. In operation the voltage pulse height from the photodetector corresponding to zero torsional loading would first be measured and then off-set to zero by suitable adjustment of the reference voltage VR as indicated above. All subsequent pulse height levels corresponding to finite torque in the shaft would then be measured with respect to this zero reference. This would allow relatively high amplifier gains to be employed within the saturation limits of the amplifier.

The apparatus may for example be used with a propeller shaft of a hollow thick-walled cylinder, which is rotated to produce hoop stresses in the cylinder similar to those in a pressure vessel, to assess the power dissipated by the rotating cylinder.

The torque can be monitored in the presence of high levels of vibration and with a low response time equal to the period of rotation of the shaft. Intermittent loads created for example by jet impingement to give thermal transients can also be measured if their period is greater than this response time.

Claims (6)

1. Apparatus for measuring torque of a shaft comprising a radiation source, a radiation sensor, axially spaced first and second means between the source and sensor and mounted on the shaft at axially spaced locations, relative angular movement between the first and second means changing the radiation received by the sensor from the source, and means for deriving from said change a measure of the torque on the shaft.

2. Apparatus as claimed in Claim 1, in which the first and second means comprise slotted discs.

3. Apparatus as claimed in Claim 2, in which the slots in the discs are rectangular.

4. Apparatus as claimed in Claim 2, in which the slots have radial sides.

5. Apparatus as claimed in any preceding claim, in which the deriving means comprises electronic means having an output signal proportional to the change in torque.

6. Apparatus for measuring torque of a shaft substantially as hereinbefore described with reference to and as shown in Figures 1 and 2, or Figure 4, of the accompanying drawings.