GB2211603A

GB2211603A - Blade incidence tracking apparatus

Info

Publication number: GB2211603A
Application number: GB8825153A
Authority: GB
Inventors: Maxwell Richard Hadley
Original assignee: Stewart Hughes Ltd
Current assignee: Stewart Hughes Ltd
Priority date: 1987-10-27
Filing date: 1988-10-27
Publication date: 1989-07-05
Also published as: GB8825153D0; GB8725130D0

Abstract

A method and apparatus for tracking a rotating body to determine the angle of the chord of the blade in relation to a reference plane comprises a detector in which at least two photodiodes are arranged to detect the passage of the leading and trailing edges of the blade. The times between events relating to the passage of the edges, including, inter alia, the leading edge entering the field of view of one of the sensors and the trailing edge subsequently leaving said field of view, is measured and the angle of the chord is calculated from the measured times. The height of the edges of the blade and the lead or lag of the blade can also be calculated. The sensors may be arranged to detect the silhouette of the blade against a background illumination such as normal daylight from the sky or light from a light source, e.g. a laser, reflected from reflective areas on the blade. <IMAGE>

Description

BLADE INCIDENCE TRACKER SYSTEM The invention relates to tracker systems for use in for example tracking a rotating blade to determine the angle or incidence of the chord of the blade in relation to a reference blade. The system can be used for tracking the path and incidence angle of the blade of a fan, windmill, propeller or rotor.

Tracker systems have been used extensively to track and monitor the performance of, for example, helicopter blades and have also been suggested for use in diagnostic systems relating to helicopter rotors and the like.

The requirement for such a system is normally that it should have the ability to monitor and measure continuously the blade track and lag during any flight condition and to provide a continuous output representative thereof.

Basic systems have already been proposed in, for example, United States Patent Specification Nos.

2,960,908 and 3,023,317. These specifications disclose a parallax interval sensing device which, as applied to the helicopter field, enables the distances of the individual blades from a sensor to be determined automatically. A passive tracker device is disclosed which utilizes an optical sensing device to determine the transit time of a helicopter blade between two ray bundles with a fixed parallax angle therebetween. Given that the blade velocity is known, its distance from the sensing device is readily determinable. The sensing device has optics designed to define two incoming ray bundles inclined at a defined angle and a photoelectric cell responsive to interruption by a blade of the light paths in the ray bundles to produce a pulsatory signal containing information as to the time taken for the leading (or trailing) edge of the blade to transit between the ray bundles.The further the distance S of the blade from the sensor, the longer will be this transit time, so the measure of transit time provides a measure of blade distance from the sensor.

The arrangement described in these documents requires a high contrast between the blade and the background light in order to provide a clear and discernable signal which changes with time. Where the intensity of the light bundle to be obscured by the blade decreases, then the change of signal is smaller and the accuracy of the measurement and the resolution of the measurement decreases as a result.

Tracker systems can fall into two types. Those that are generally referred to passive which rely on background daylight as the light source for the detectors or sensors which are then obscured by the blade, and those which require an active source where a source of light is deflected by the blade to a sensor as it passes the optical path thereof.

The invention is seen as encompassing both active and passive tracking systems since it is aimed towards deriving information about the behaviour of the blade such as the angle of the chord of the blade, the height of the blade, and the lead or lag of the blade.

The invention lies in the realisation that by measuring to a reasonable degree of accuracy the time interval between events related to the passage of the blade through the fields of view of the detector the required information about the blades behaviour can be determined with a high degree of confidence.

According to one aspect of the invention there is provided an apparatus for tracking a rotating blade to determine the angle of the chord of the blade in relation to a reference plane, the apparatus comprising a detector in which at least two sensing means having respective fields of view are arranged to sense the leading and trailing edges of the blade passing through said fields, timing means for measuring the times between events relating to the passage of the edges through said fields and calculating means for calculating from the measured times the angle of the chord.

According to another aspect of the invention there is provided a method of tracking a rotating blade to determine the angle of the chord of the blade in relation to a reference plane, in which method said, angle of the chord of the blade is calculated from the times between events related to the passage of the edges through the fields of view of at least two sensing means.

According to a further aspect of the invention there is provided an apparatus for tracking a rotating blade, which apparatus comprises detector means for detecting radiation, said means including at least two radiation sensors disposed in a plane and focusing means for focusing said radiation in said plane to define a radiation field associated with each sensor, said detector being disposed in spaced relationship with each blade whereby the radiation field associated with each sensor is traversed sequentially by said blade means for monitoring the signal output of each sensor, and clock means associated with the monitoring means, the arrangement being such that the monitoring means and the clock means record the incidence of the leading edge and trailing edge of the blade sequentially in each of the beams.

Further inventive features are set forth with particularly in the appended claims.

The invention is particularly useful in measuring the incidence of a blade, that is to say the angle of the chord extending between the leading and trailing edges of the blade, during rotation.

In order that the invention may be well understood, exemplary embodiments and methods of the invention will be described in greater detail hereinafter, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic view of a typical detector in accordance with the present invention; Figure 2 is a diagram showing the track sensor optical geometry relative to the angle of incidence of a blade; Figure 3 is an exploded view of a detector for use in accordance with the present invention; Figure 4 is a block diagram of the track sensor circuit; Figure 5 is a track sensor light response curve as detected by the circuit of Figure 4; Figure 6 is a diagram showing the geometry of the blade passing through the sensor field of view; Figure 7 is a diagram showing the error in torsional tracking due to blade curvature;; Figures 8 and 9 show the pitch at root versus pitch at 75% span of yellow and green blades respectively; Figure 10 is a variation of incidence along the span of a yellow blade with an angle of incidence of 100 at route; and Figure 11 illustrates the variation of incidence along the blade span for a green blade.

Referring now to the drawings, Figure 1 shows in schematic form, a sensor indicated generally at 10 comprising a camera lens 11, which may for example be a 50mm lens having a maximum aperture of F1.8. The lens is mounted at the upper end of a housing 12 having a back plate 13, the upper surface 14 of which constitute the focal plane of lens 11. The upper surface 14 of back plate 13 carries a pair of photodiodes 21 and 22 in spaced relation.

The sensor is passive in that it requires ordinary daylight to provide a light stimulus through the lends to the photodiodes. Thus the operation of this sensor is dependent upon the ambient conditions of illumination. Preferably the photodiodes have the ability to recognise both the case of a high resolution image with good contrast between the blade and the sky brightness and the opposite case which can occur, for example, when a low sun under a generally overcast sky significantly reduces or even reverses the contrast between the blade and its background.

The detector 10 is adapted to be rigidly mounted on the body of a helicopter so that the optical plane of lens 12 is approximately 1 to 1.5 metres from the track of helicopter blade indicated generally at 30.

From Figure 1 it can be seen that the detector 10 when mounted on the body of the helicopter is arranged so that rotor blade 30 will pass sequentially through the fields of view 1,2 of each of the two photodiodes 21 and 22. As the blade passes through a field of view light from the sky is occluded from entering the photodiodes resulting in a drop in the output from the photodiode.

One aspect of the invention lies in the realisation that if the time duration of light occluded from each of the photodiodes 21, 22 can be determined and the length of chord of the rotor blade is known, then the blade velocity can be calculated from this information. Furthermore, by measuring the period of time between the blade obscuring the fields of view of the two photodiodes that is to say in general terms the period of time between the blade leaving the first field and entering the second field, the height of the leading and trailing edges of the blade can be calculated. This is because the angle between the fields of view are known and once the velocity of the blade has been calculated the distance between the photodiodes and the edges of the blade becomes a simple trigonometry problem.Moreover, by providing an event marker related to the relation of the shaft associated with the blade, such as may be provided by a tachometer, the lead or lag of the blade, that is to say the distance between the position of the blade as detected by the detector in relation to the expected position as can be derived from the tacho event marker, can also be calculated.

The photodiodes form part of a circuit, as shown in Figure 4, which process the signals generated by the photodiodes in response to events relating to the passage of the blade through the two fields of view 1, 2. Each photodiode has associated with it a currentvoltage conversion stage 40 based on an FET operational amplifier which converts the photo current from the diode into a voltage level. The transresistance of this stage is varied by an automatic gain control (AGC) stage 41. The control element of the AGC stage 41 is a P channel MOS FET the gate voltage of which is variable in order to maintain the output from the converter amplifier 40 at an optimum level. A second FET operational amplifier 42 amplifies the voltage output from the conversion stage 40 and limits the signal bandwidth. The signal bandwidth is limited to about 16 kHz in the specific example illustrated.The output from the amplifier 42 provides an input to both a level detector 43 and to the automatic gain control stage 42.

The operation of the AGC stages 41 is designed to compensate for changes in the level of sky brightness, thereby to maintain an output from the amplifier 42 which remains substantially the same over a wide range of sky brightnesses. Accordingly, the input to the level detector 43 will be a signal which varies between a maximum when the appropriate field of view is not obscured by the blade and a minimum when the said field is fully obscured by the blade.

The level detector 43 provides an output signal which depends upon whether the pulse signal is above or below a mid-way threshold point between a bright level and a dark level. In other words, when the photodiode is receiving maximum light a high signal will be generated and when the photodiode is receiving minimum light, i.e it is obscured by a blade, a low signal will be generated by the level detector 43.

The outputs from the two level detectors 43 are input to a logic circuit 44 which logically OR's the signals together to provide a squarewave signal such as is shown in Figure 5. The events relating to the changes in state of the waveform in Figure 5 correspond to the leading edge L entering the fields Lel, Le2 and the trailing edge T leaving the fields Tel, Te2. The logic is further arranged to generate a pulse of 270 microseconds duration (not shown) each time a change in state in the shown signal occurs.

Thus a pulse is generated each time the leading edge of the blade enters one of the two fields of view and each time the trailing edge leaves one of the fields of view.

The fields of view are spaced apart so that the trailing edge of the blade will leave the first field of view at time Tel before the leading edge enters the second field of view at time Le2. This gives rise to three discrete time intervals T1, T2, T3 which can be measured, the time intervals so obtained subsequently being used in the calculation of the required information about the rotor blade as will be explained hereinafter.

The power supply unit 50 develops all the regulated and smoothed voltages required by the circuit directly from the 28V nominal supply onboard the aircraft in which the tracker is to be used.

Input filtering and reverse voltage protection are also incorporated into the power supply.

Having obtained the time intervals T1, T2, T3 it is possible to calculate therefrom, in combination with other known data such as the angular spacing of the two fields of view, etc, as has already been mentioned, the average velocity of the blade as it passes over the two sensors the height of the leading and trailing edges of the blade and hence to calculate the angle of the chord of the blade, and the lead or lag of the blade.

The following is a brief description of the algorithms employed in calculating this information.

Considering the track sensor optical geometry set out in Figure 2 it is possible to consider the sensor device as two sensitive photodiodes whose purpose it is to generate logic signals each time it receives a given light level. The refractory properties of the lens cause the sensor field to be a divergent cone radiating from the lens. Thus it is convenient for the purpose of explanation to regard the fields of view of the two photodiodes as two diverging pencil beams of light whose angular spearation is determined by the optical configuration of the detector as a whole. The effective angular separation of these two pencil beams is given by the expression: 2n = 2Tan-l d(u-f v.f where all terms are as illustrated in Figure 2.

In terms of the sensor illustrated in Figures 2 and 6 of the accompanying drawings, the sensor configuration has the following typical values, namely: d = 5mm, f = 50mm, v = 10mm. Inserting these values into the equation and assuming a typical value for u gives 2n Z 110.

At time Le1 (as shown in Figure 5) the leading edge of the blade impinges the first beam or field of view and the first photodiode is occluded for a duration Te1 - Le1 as shown in Figure 5, until the trailing edge of the blade leaves the first beam or field at time Tel. This process is repeated by the second beam or field of view Le2, Te2. It can be shown that the average velocity v is related to these events by the equation: v = 2c T1 + T3 where c is the blade chord. Thus the average velocity of the blade v as it passes between the fields of view of the two sensors can be readily derived.

Referring to Figure 6, the blade is at an angle a to a predefined plane of reference, normally a horizontal plane with respect to the detector. Since the two pencil beams are divergent it follows that the distance 211 travelled by the leading edge is greater than the distance 212 travelled by the trailing edge in crossing the two beams.

It can be shown that the respective heights h1, h2 of the leading and trailing edges are related to the measured times T1, T2, T3 by the equations: h1 = (T + T2C cot n (T1 + T3) and h2 = (To + TsC cot n (T1 + T3) Thus the heights hl, h2 of the leading and trailing edges can be readily calculated.

Assuming that the angle of the balde CL is small, which indeed it is since the typical value of 110 already mentioned is for the purpose of this explanation sufficiently small, then it follows from simple trigonometry that CL = T1 - T3 cot n T1 + T3 Thus the angle of the chord of the blade is related only to the two time intervals T1 and T3 and to the angle between the two divergent fields of view.

It follows that the angle of the chord of the blade can be readily calculated.

An inherent limitation to the accuracy of the obtained measurements is that of the accuracy to which the time intervals can be measured. Typically a given time interval can be determined to within one microsecond and this places a limit on the accuracy with which the blade incidence or angle CL may be determined.

Assuming that this accuracy results in an angular resolution of SCL, it can be shown that SCL " 216T1 where ST is the smallest CL T1 + T3 measurement time increment.

This error will be small when the time intervals T1 and T3 are large, namely when the speed of rotation of the blade is small and the blade chord is large.

For example for the case where the blade tip speed v is 220 m/s, the viewing angle is 750 the chord of the blade is o.5m in length, and ST = 10-6 secs, solving the equations gives SCL " 0.0060.

A number of errors may result from the blade leading curvature edge. The curvature of the blade leading edge means that it impacts the two beams at different points on its surface thus incurring a bias error in the estimate of the blade incidence o. This error is readily evaluated from Figure 7 indicating an exaggerated chord section impacting the beams prematurely at the points X and Y and not the desired common point P at the chord tip.

The blade always impacts the beams before the chord tip P. Since the blade incidence algorithm is proportional to the difference between the velocity times T1 and T3, for a blade constant velocity the error in T1 - T3 equals BCv where v is the average velocity.

Since PN = Rsin# then BC = 2Rsin8Tann The error in T1 - T3, ST = 2RsinTann (T1 + T3) 2c The error in 8, 68 = cotn ST T1 + T3 68 = cotn 2Rsin#Tann(T1 + T3) (T1 + T3)2c = R/c sine Thus, in a typical example, for a typical blade of thickness/chord ratio of 10% 2R = 0.1, which gives c 68 = sine Z 8 20 20 which is significant for large 8.

The true blade incidence 8 is related to the estimated blade incidence 8 through the expression A # = # + ## For small angles: # = # + R8 c A so that 8 = 8 1 - Ric For a 10% thickness/chord ratio, Ric = 0.05 so that: A 8 " 8(1 + R/c + neglecting higher order terms # # 1.05 # It is clear from the abvoe that this bias error is readily incorporated into the final incidence algorithm.

This technique for the remote measurement of blade incidence was validated experimentally by measurements of the blade incidence at 75% span compared against that obtained from pitch potentiometer measurements at the blade root.

The results obtained from two blades are shown in Figures 8 and 9, each showing similar trends. above four degrees, the measurement points lie on a straight line whose gradient is less than unity as might be expected due to washout. Below four degrees, however, a departure from linearity is observed indicating the presence of an additional applied torque at this point along the span and collective pitch. As the incidence at the root is decreased, the data points return to the straight line as might be expected from a linearity twisted blade.

Maintaining a constant 100 collective pitch at the blade root, the torsional sensor was scanned slowly along that interval of the blade span where the light levels were of sufficient contrast to produce consistent data. The results obtained from two quite separate blades are shown in Figures 10 and 11 indicating the variation of blade incidence as a function of the blade span from 0.5R to 1R. Again the curves are non-monotonic suggesting the presence of a spurious additional applied torque at these points along the blade span.

In an experiment by the inventors the described detector set up on the fuselage of a helicopter to measure substantially instantaneous rotor blade behaviour. The obtained measurements were found to be capable of providing data concerning blade bending motions as set out in European Patent Application No.

86303447.6 and attitude measurement.

In another embodiment of the invention (not shown) the described passive detector is replaced by an active detector and effective material is provided at areas of the blade along a common chord. From the above equations it should be apparent that the areas if reflective material should be placed at or near to the leading and trailing edges of the blade.

An active detector as has already ben explained, comprises, in addition to the abovedescribed photodiodes and lens arrangement, etc, a light source which directs light towards the reflective areas on the blade. some of the light is reflected by the reflective areas and is seen by the photodiodes as an increase in received light intensity. Thus, it follows that the response of. the photodiodes will be the reverse of that described in relation to Figures 4 and 5. that is to say, the presence of the leading edge or the trailing edge of the blade in the field of view will result in an increase in output from the appropriate photodiode and associated amplifiers.

However, the logic circuits and subsequent analysis circuits connected to the output thereof can be readily modified to take account of this by those possessed of the relevant skills and no further explanation herein is regarded as being necessary.

The light source used in the active detector may be any one of a wide range of different devices currently available such as for example an infrared diode or laser diode or a laser.

The use of an active detector in which the light source produces a collimated light beam, such as would be the case for a laser, offers the advantage that by making the reflective areas small in relation to the length of chord of the blade the events of the leading edge and trailing edge entering and leaving a field of view can be made wholly discrete from each other. It follows that the time intervals T1, T2, T3 can be readily calculated from these events and that the chord angle, the leading and trailing edge heights, the blade velocity and the blade lead or lag can also be readily calculated.

It will be apprecaited from the foregoing, therefore, that in addition to the tracking of the height of the blade and allowing for the coning of the blade it is also possible to measure the angle of incidence of the blade as it passes through the fields of the sensor. This allows fine adjustment of individual angles of incidence of the individual blades within the helicopter rotor, thus providing improved tracking and control facilities for the system.

Embodiments of the invention having been described, it will be apparent to those possessed of the appropriate skills that modifications are possible without departing from the ambit of the invention as claimed.

Claims

CLAIMS:

1. An apparatus for tracking a rotating blade to determine the angle of the chord of the blade in relation to a reference plane, the apparatus comprising a detector in which at least two sensing means having respective fields of view are arranged to sense the leading and trailing edges of the blade passing through said fields, timing means for measuring the times between events relating to the passage of the edges through said fields and calculating means for calculating from the measured times the angle of the chord.

2. An apparatus according to claim 1 in which said sensing means each comprise photodiodes.

3. An apparatus according to claim 1 or 2 wherein the detector further comprises focusing means positioned between said sensing means and the blade for focusing light towards said sensing means and thereby defining the respective fields of view.

4. An apparatus according to claim 1 or 2 or 3, in which the timing means is arranged to time the interval between the leading edge of the blade entering at least one field of view and the trailing edge subsequently leaving said field of view.

5. An apparatus according to claim 4, in which said timing means is further arranged to time the interval between the trailing edge ieaving one of said fields of view and the leading edge entering the other of said fields of view.

6. An apparatus according to any of the preceding claims, in which the calculating means is arranged to calculate also the height of the leading and trailing edges of the blade in relation to the reference plane.

7. An apparatus according to any of the preceding claims in which the sensing means lie in the reference plane.

8. An apparatus according to any of the preceding claims further comprising tachometer means for producing an event marker in relation to the rotation of a shaft associated with said blade and wherein the calculating means is arranged to calculate from the measured times and a related event marker the lead or lag of the blade.

9. An apparatus according to claim 2 or any of claims 3 to 8 as dependent thereon, in which each of the sensing means further comprise a threshold sensor for sensing when the output from the photodiodes exceeds a threshold level and means for adjusting said threshold level according to ambient light conditions.

10. An apparatus according to any of the preceding claims further comprising a light source associated with the sensing means and arranged to illuminate reflective areas located on the blade such that light from said light source will be reflected into the said fields of view thereby to facilitate sensing of the leading and trailing edges.

11. An apparatus according to claim 10 in which the reflective areas are located at or near to the edges of the blade.

12. An apparatus according to claim 10 or 11 in which the light source comprises an infra red diode.

13. An apparatus according to claim 10 or 11 in which the light source comprises a laser.

14. A method of tracking a rotating blade to determine the angle of the chord of the blade in relation to a reference plane, in which method said angle of the chord of the blade is calculated from the times between events related to the passage of the edges through the fields of view of at least two sensing means.

15. A method according to claim 14, in which said events include the leading edge of the blade entering at least one field of view and the trailing edge of the blade subsequently leaving said field of view.

16. A method according to claim 14 or 15, in which said events include the trailing edge of the blade leaving one field of view and the leading edge of the blade subsequently entering the other field of view.

17. A method according to claim 14, 15 or 16 in which the height of the leading and trailing edges of the blade are also calculated from said times between events related to said passage of the edges.

18. A method according to any of claims 14 to 17 in which the lead or lag of the blade is calculated from said times between events and events related to the rotation of a shaft associated with said blade.

19. A method according to any of claims 14 to 18 in which light from a light source illuminates reflective areas on the blade to facilitate detection of said events.

20. A method according to claim 19 wherein the reflective areas are located at or near to the edges of the blade.

21. An apparatus for tracking a rotating blade, which apparatus comprises detector means for detecting radiation, said means including at least two radiation sensors disposed in a plane and focusing means for focusing said radiation in said plane to define a radiation field associated with each sensor, said detector being disposed in spaced relationship with each blade whereby the radiation field associated with each sensor is traversed sequentially by said blade means for monitoring the signal output of each sensor, and clock means associated with the monitoring means, the arrangement being such that the monitoring means and the clock means record the incidence of the leading edge and trailing edge of the blade sequentially in each of the beams.

22. An apparatus or method substantially as described herein with reference to the accompanying drawings.