WO2013079530A1

WO2013079530A1 - Dynamic belt monitoring apparatus and method

Info

Publication number: WO2013079530A1
Application number: PCT/EP2012/073836
Authority: WO
Inventors: David Lynn; Jonathan PRIMROSE
Original assignee: Schrader Electronics Limited
Priority date: 2011-11-30
Filing date: 2012-11-28
Publication date: 2013-06-06
Also published as: GB201120623D0; GB2497100A; GB2497100B

Abstract

A belt monitoring apparatus (30) for monitoring a belt in a pulley system comprising at least two pulleys around which the belt is fitted. The belt monitoring apparatus (30) comprises a first sensing device (32) including a transmitter (42) configured to transmit a wireless signal and a receiver (44) configured to receive the wireless signal reflected, in use, by a span of the belt, and to produce a corresponding output signal. The apparatus (30) further includes processing means (36) configured to detect in the output signal a signal component caused by transverse vibration of the span and to determine the frequency of the transverse vibration from the span vibration signal component.

Description

Dynamic Belt Monitoring Apparatus and Method

Field of the Invention The invention relates to belt monitoring in a pulley system, especially in vehicle engines or industrial drive mechanisms. The invention relates particularly to the monitoring of belt tension.

Background to the Invention

It is desirable to monitor characteristics, including tension, of belts for various reasons, including optimizing the efficiency of the system of which the belt is part, determining wear and prolonging the life of the belt. This is particularly true for belts incorporated into the drive system of a motor vehicle or an industrial drive mechanism.

Tension monitoring devices are known but are typically intended for use when the belt is static, or at least when the vehicle is stationary. It would be desirable to provide an improved belt tension monitoring apparatus, especially one suitable for measuring tension when the belt is in use.

Summary of the Invention A first aspect of the invention provides a belt monitoring apparatus for monitoring a belt in a pulley system comprising at least two pulleys around which said belt is fitted, the belt monitoring apparatus comprising a first sensing device including a transmitter configured to transmit a wireless signal and a receiver configured to receive said wireless signal reflected, in use, by a span of said belt, and to produce a corresponding output signal, said apparatus further including processing means configured to detect in said output signal a signal component caused by transverse vibration of said span and to determine the frequency of said transverse vibration from said span vibration signal component. In preferred embodiments, the apparatus further comprises a second sensing device including a transmitter configured to transmit a wireless signal and a receiver configured to receive said wireless signal reflected, in use, by said span of said belt, and to produce a corresponding output signal, said apparatus further including processing means configured to detect in said output signal a belt formation signal component caused by movement of formations, for example teeth, formed on a surface of said belt from which surface said wireless signal is reflected during use, past said transmitter and receiver as said belt revolves during use.

A second aspect of the invention provides a pulley system comprising at least two pulleys around which a belt is fitted, and a belt monitoring apparatus comprising a first sensing device including a transmitter configured to transmit a wireless signal and a receiver configured to receive said wireless signal reflected, in use, by a span of said belt, and to produce a corresponding output signal, said apparatus further including processing means configured to detect in said output signal a signal component caused by transverse vibration of said span and to determine the frequency of said transverse vibration from said span vibration signal component.

A third aspect of the invention provides a method of monitoring a belt in a pulley system comprising at least two pulleys around which said belt is fitted, the method comprising transmitting a wireless signal onto said belt; receiving said wireless signal reflected, in use, by a span of said belt and producing a corresponding output signal; detecting in said output signal a signal component caused by transverse vibration of said span; and determining the frequency of said transverse vibration from said span vibration signal component. A fourth aspect of the invention provides a belt monitoring apparatus for monitoring a belt in a pulley system comprising at least two pulleys around which said belt is fitted, the belt monitoring apparatus comprising a sensing device including a transmitter configured to transmit a wireless signal and a receiver configured to receive said wireless signal reflected, in use, by said span of said belt, and to produce a corresponding output signal, said

apparatus further including processing means configured to detect in said output signal a belt formation signal component caused by movement of formations, for example teeth, formed on a surface of said belt from which surface said wireless signal is reflected during use, past said transmitter and receiver as said belt revolves during use.

A fifth aspect of the invention provides a pulley system comprising the belt monitoring apparatus of the fourth aspect of the invention.

A sixth aspect of the invention provides a method of monitoring a belt in a pulley system comprising at least two pulleys around which said belt isfitted, the method comprising transmitting a wireless signal onto said belt; receiving said wireless signal reflected, in use, by a span of said belt and producing a corresponding output signal; detecting in said output signal a belt formation signal component caused by movement of formations, for example teeth, formed on a surface of said belt from which surface said wireless signal is reflected during use, past said transmitter and receiver as said belt revolves during use. Preferred features are recited in the dependent claims.

Advantageously, preferred embodiments of the invention allow the determination of belt tension while the belt is moving in application, e.g. while a vehicle is being driven or a machine is in use. In particular, preferred embodiments of the invention are configured to monitor the frequency of vibration of a span of the rotating belt. From that measurement other drive properties, such as span tension and drive loading or torque can be calculated.

In preferred embodiments, dynamic span vibration frequency is measured, i.e. the vibration frequency of a span of the belt between pulleys during use, from which the span tension may be calculated.

Optionally, embodiments of the invention may be configured to calculate drive torque from one or more belt tension measurements. Typically this is achieved by providing a respective belt tension measuring apparatus for each span of a belt or belt train and calculating drive torque from the respective tension measurements. Further preferred features of the invention will become apparent to those ordinarily skilled in the art upon review of the following description of a preferred embodiment and with reference to the accompanying drawings.

Brief Description of the Drawings

An embodiment of the invention is now described by way of example and with reference to the accompanying drawings in which:

Figure 1 is a schematic representation of a pulley system;

Figure 2 is a schematic representation of an optical sensing device suitable for use with preferred embodiments of the invention;

Figure 3 is a block diagram of a signal processing apparatus suitable for use with preferred embodiments of the invention; and

Figure 4 is a block diagram of a belt monitoring apparatus embodying one aspect of the invention. Detailed Description of the Drawings

Referring now to Figure 1 of the drawings there is shown, generally indicated as 10, a belt and pulley system comprising first and second pulleys 12, 14 and a belt 16 coupling the pulleys 12, 14 so that rotation of one can be imparted to the other. In typical embodiments, the belt and pulley system 10 is part of a drive system which further includes drive means, e.g. an engine or motor (not shown), coupled to one of the pulleys 12, 14 (which may be referred to as the drive pulley). As the drive means rotates the driven pulley, the belt imparts rotational movement to the other pulley 14, 12 (which may be referred to as the idle pulley). In typical embodiments, the pulley system 10 is part of a vehicle's engine or the drive system of an industrial machine.

In the illustrated embodiment, the belt 16 is provided with formations in the form of teeth 18 (Figure 2) which engage with corresponding formations, e.g. teeth (not shown), provided around the circumference of the pulleys 12, 14 to allow force and therefore movement to be imparted between the belt and pulleys. In the illustrated embodiment, adjacent teeth 18 are separated by a respective gap 19. In alternative embodiments, the belt may take any suitable other form, e.g. a flat belt, a V-belt or a groove belt.

The portion(s) of a belt 16 that extend between adjacent pulleys 12, 14 may be referred to as spans. In the illustrated embodiment, belt 16 has two spans 20, 22. It will be understood that the invention may be used with pulley systems having a plurality of pulleys (i.e. is not limited to the illustrated 2 pulley system) coupled to a belt, in which case there may be more than two spans.

In use, as the belt 16 revolves around the pulleys 12, 14 the spans 20, 22 vibrate, or oscillate, in a transverse direction, i.e. a direction that is perpendicular to the direction in which the span 20, 22 extends between the pulleys 12, 14. This is illustrated in Figure 1 by respective representations 22', 22" of span 22 at opposite extremes of its vibration travel. Figure 1 also shows an enlarged view of part of span 22 in which the direction of vibration is shown by arrow A. The span vibration frequency (SVF) is designated as F₀.

In preferred modes of use where belt tension is monitored while the belt 16 is running, the transverse vibration is caused by mechanical interaction between the belt 16 and the pulleys 12, 14 as the belt 16 runs around the pulleys 12, 14, e.g. the engagement and disengagement of the belt teeth with the pulleys 12, 14 in the present example. Alternatively, the transverse vibration may be caused by application of an external force on the span, e.g. by plucking or striking by any suitable instrument such as a finger, hammer, actuator, air jet or the like. Figure 4 illustrates a belt monitoring apparatus 30 embodying one aspect of the invention. The belt monitoring apparatus 30 is configured to monitor belt vibrations, i.e. the transverse vibration in the or each span 20, 22, and so may be said to comprise a belt vibration monitoring apparatus. In particular, the belt monitoring apparatus 30 is configured to measure the frequency of the belt vibrations. In this example, the belt vibration monitoring apparatus comprises a vibration sensing device 32 and signal processing apparatus, which preferably comprises signal processing circuitry 34 and/or a suitably programmed programmable processor 36, typically a digital signal processor. The

programmable processor 36 may take any suitable form, e.g. a suitably programmed microcomputer or microcontroller.

In preferred embodiments, the apparatus 30 is configured to determine the tension in the, or each, span 20, 22 from the or each respective measured vibration frequency. Conveniently, this may be achieved by suitable

programming of the processor 36. As such, the apparatus 30 may be said to comprise a belt tension monitoring apparatus.

Optionally, the apparatus 30 may be configured to determine drive torque in the pulley system using the respective belt span tensions. Conveniently, this may be achieved by suitable programming of the processor 36. As such, the apparatus 30 may be said to comprise a drive torque monitoring apparatus, which is described in more detail hereinafter. More generally, depending on the application, the system may be configured to produce one or more different output values, some of which are shown in Figure 3 by way of example.

In preferred embodiments, the apparatus 30 includes a second sensing device 38, which may optionally be used to determine belt span vibration as is described in more detail hereinafter.

Referring now to Figure 2, there is shown a sensor unit 40 which includes sensing device 32 and, preferably also, sensing device 38. In preferred embodiments, the sensors 32, 38 are located adjacent one another and face in substantially the same direction. The unit 40 may be mounted on any convenient component of the pulley system 10, or of the system (e.g. engine or drive system) of which the pulley system 10 is part. In the case where the belt 16 has teeth, or other formations to be detected by the sensor 38, the unit 40 is positioned so that at least sensor 38 faces the surface of the belt 16 that includes the formations. In preferred embodiments, a respective sensor unit 40 is provided for each span 20, 22 of the belt 16. Alternatively, a sensor unit may be provided only for the (or each) tight side of a belt, i.e. a span held under tension during use.

Conveniently, the unit 40 may include all of the components of apparatus 30. Alternatively, some of the components of apparatus 30 may be located remotely from the unit 40, and communicate with the components in the unit 40 by a wired or wireless communications link.

The vibration sensing device 32 is intended for use in the detection of the vibration frequency of the span 20, 22 with which it is associated. As such it may be referred to as a span frequency detector. The vibration sensing device 32 comprises a wireless transmitter 42 and corresponding receiver 44. The transmitter 42 and receiver 44 are preferably of a type that transmit and receive, respectively, optical signals, especially infrared (I ) signals. Hence, the preferred sensing device 32 may be said to comprise an optical sensor.

Alternatively, the transmitter 42 and receiver 44 may be of a type that transit and receive, respectively, other types of electromagnetic signals or other wireless signals, for example acoustic signals, lasers, UV light or visible light. In some applications, the use of IR is advantageous since it is not susceptible to interference from heat, visible light or noise pollution.

The sensor unit 40, and in particular the transmitter 42 and receiver 44, is located and positioned such that in use wireless signals emanating from the transmitter 42 are reflected by the respective span 20, 22 and are received by the receiver 44. The sensing device 32 is configured to transmit wide-angle, or divergent, radiation such that the wireless signals irradiate a relatively wide area on the surface of the belt 16 (as opposed to a relatively narrow area that would be irradiated by, for example, a focussed beam). To this end the transmitter 42 may be configured to transmit wide-angle, or divergent, radiation and/or a lens device or other means (not present in the illustrated embodiment) may be provided to produce a divergent beam from the device 32. In any event, it is preferred that the transmitted wireless signals are non-focussed. It is

advantageous to the subsequent frequency measurement that the radiation is allowed to flood the belt, as the signal is considered 'low definition', i.e. the angular resolution of the radiation from the sensor 32 is sufficiently large (wide angle) that the span vibration frequency is present and detectable in signals produced by the sensor 32. A focused beam would filter the span vibration, making it difficult or impossible to detect any signal component representing the span vibration frequency.

The second sensing device 38 is intended for use in monitoring the teeth 18 in order to determining one or more characteristics of the belt 16, for example, speed, tooth position (from which missing teeth can be identified) and/or tooth profile (from which wear can be determined). As such it may be referred to as a tooth detector. The tooth sensing device 38 comprises a wireless transmitter 46 and corresponding receiver 48. The transmitter 46 and receiver 48 are preferably of a type that transmit and receive, respectively, optical signals, especially infrared (I ) signals. Hence, the preferred sensing device 38 may be said to comprise an optical sensor. Alternatively, the transmitter 46 and receiver 48 may be of a type that transit and receive, respectively, other types of electromagnetic signals or other wireless signals, for example acoustic signals, lasers, UV light or visible light. Advantageously, the radiation transmitted from the transmitter 46 is focussed onto the surface of the belt 16. This may be achieved by one or more lens 50 or other focussing device where necessary (if for example a laser or other inherently focussed (i.e. narrow beam as opposed to wide-angle) radiation source is used, additional focussing may not be required). Focusing the radiation provides the sensor 38 with a relatively small angular resolution, which increases the definition of the tooth profile and facilitates analysis of the signals received by the receiver 48. It is preferred that the signals received by the receiver 48 are focussed onto the receiver 48, e.g. by one or more lens 51. In preferred embodiments, information on tooth position, speed and profile can be extracted from the signals received by the sensor 38.

In preferred embodiments, the sensors 32, 38 are located in respective compartments in the unit 40, separated by a partition 52 for reducing radiated cross-talk between the sensors 32, 38. To this end the partition 52 is

preferably formed from a material that is impermeable or relatively

impermeable to the relevant wireless radiation (I in this example). Each sensor 32, 38 generates an electrical output signal corresponding to the reflected signal that it receives from the belt 16. The electrical output signal produced by the sensor 38 comprises a relatively high definition signal because the sensor 38 directs radiation with relatively small angular resolution, i.e.

relatively focussed, onto the belt 16 and so allows relatively small belt movements, e.g. the passing of teeth, to be readily detectable from the reflected signal while larger movements, such as oscillations of the span are not readily detectable. As a result, the output signal produced by the sensor 38 comprises a signal component (which may be referred to as the Tooth Meshing Frequency (TMF) component) corresponding to the detected passing by of teeth 18 with respect to the sensor 38 as the belt 16 moves. Hence, the signal produced by the sensor 38 contains relatively high definition data representing the teeth 18 passing the sensor 38.

The electrical output signal produced by the vibration sensor 32, comprises a relatively low definition signal because the sensor 32 directs radiation with relatively large angular resolution, i.e. relatively un-focussed, onto the belt 16 and so allows relatively large belt movements, such as oscillations of the span, to be readily detectable from the reflected signal while smaller movements, e.g. the passing of teeth, are less detectable. Hence, the output signal produced by the vibration sensor 32 comprises a signal component (which may be referred to as the Tooth Meshing Frequency (TMF) component) corresponding to the detected passing by of teeth 18 with respect to the sensor 32 as the belt 16 moves, and a signal component (which may be referred to as the span vibration frequency (SVF) component) corresponding to the detected transverse vibration of the belt 16. Hence, the signal produced by the sensor 32 contains relatively low definition data representing the span vibration frequency together with relatively low definition data representing the teeth 18 passing the sensor 32. The TMF signal component generally has a higher frequency than the SVF component and can be extracted by any convenient filtering means, for example mathematically, by digital signal processing or by analogue electronic circuitry.

In the illustrated embodiment, the filtering of the output of the vibration sensor 32 is performed by an analogue electronic circuit designed to filter and typically also amplify the signal received from the sensor 32. Figure 3 illustrates a suitable circuit in block form, which is suitable for use as the signal processing circuit 34 of Figure 4. The circuit 34 comprises a differentiator 62 for detecting variations in the input signal and which may conveniently comprise a capacitor (not shown) for this purpose. An integrator is provided for filtering out any DC or common mode offsets. In the illustrated example, integrator circuitry 64 with two time constants T = X or T = Χ^Λ2 is arranged to provide differential signals across a difference amplifier 66, the amplifier 66 amplifying the difference in the differential signals. Integrator 68 may be provided for further filtering. The circuit 34 exhibits a low frequency response configured to attenuate the relatively high frequency TMF component and pass the relatively low frequency SVF

component.

Once the SVF signal component has been isolated, its frequency can be determined. In preferred embodiments, this is performed by the processor 36 which is programmed to perform suitable digital signal processing and/or mathematical analysis of the digitised SVF signal component. In the illustrated embodiment, the circuitry 34 produces the SVF signal component as an analogue signal, which may be digitized for use by the processor 36 by any suitable means, for example by signal sampling. In alternative embodiments, the filtering performed by circuitry 34 is alternatively performed digitally, e.g. by processor 36, in which case the isolated SVF component is already in digital form (the output from the sensor 32 having been digitized by any convenient means). In any event, the processor 36 is programmed to perform spectral analysis, e.g. Fourier analysis, on the SVF signal component to determine the SVF value. The processor 36 may additionally be programmed to perform signal pre-processing prior to spectral analysis to condition the SVF signal as required, e.g. to remove DC offsets or common mode signal.

For example, in the illustrated embodiment, 512 samples of the output signal from the circuitry 34 may be taken at 1024Hz and stored by the processor 36. A soft window may be applied to the stored data samples prior to high-pass filtering, using an MR representation of a Butterworth 4 pole filter with a pass band starting at 0.5Hz. This removes any remaining common mode, or DC, signal. Once conditioned, the data samples are processed using Fourier Analysis to extract the frequency components.

Once determined the span vibration frequency can be used to determine other characteristics of the belt 16, for example belt tension. In particular, the span vibration frequency is directly proportional to the tension in the span 20, 22. Span tension may be calculated as:

T = 4ml²?

Where T is span tension, f is span vibration frequency, / is the length of the relevant span 20, 22, and m is mass per unit length of the belt 16.

Hence, the processor 36 may be programmed to calculate span tension from the span vibration frequency.

In some applications, including vehicle engines, the nature of the application dictates that span tension will change as the load changes on the pulley system, which will cause the SVF to change. In such cases it is preferred to provide a tracking device to lock to the SVF, tracking it and eliminating instances of mistaking the TMF for the SVF. Any suitable conventional tracking device may be used, for example arranged to define a frequency window to monitor frequencies close to the SVF, to detect changes in the frequency of the SVF signal component and to track the new value of the SVF. The tracking device may conveniently be implemented by the processor 36, e.g. in software.

In cases where the TMF signal component and the SVF signal component of the output from sensor 32 are sufficiently close in frequency that they are difficult to separate by the filtering methods described above, the TMF signal component can be mathematically extracted from the sensor output to leave the SVF component. This can be achieved using the output of the tooth sensor 38 which, because its detection is focussed on the teeth 18, comprises a high fidelity TMF signal (not combined with an SVF signal component). Hence, the processor 36 is able to use the high definition TMF signal from the tooth sensor 38 to separate the SVF signal component from the output of the vibration sensor 32. For example, the TMF signal from the sensor 38 may be converted into a suitable digital form by any convenient means (e.g. sampling and Fourier analysis). The processor 36 may be programmed to mathematically remove the TMF component from the digitized data representing the output of the vibration sensor 32 using the data representing the TMF component that has been obtained from the output of the tooth sensor 38.

Drive torque can be inferred if the static and dynamic tension of each span of a drive system are known. The static tension is the installation tension, or hub load, of the drive belt 16. On a simple drive system 10, consisting of a drive pulley 14 and an idle pulley 12, the belt 16 is divided into two spans 20, 22 - the tight side and the slack side. They are known as such because, once rotating, the belt span that is spooling on to the drive pulley is in tension while the span spooling off the driver pulley is out of tension.

When the drive pulley 14 is stationary, the static tension is equally divided across the tight and slack side spans 20, 22. Under load, the tight side tension is increased by the tension applied by the driven load and the slack side span decreases below the static tension by the tension applied by the driven load. Therefore, if all three elements are known - static tension, tight side tension and slack side tension - the drive loading can be calculated. For such applications, a respective sensor unit 40 is provided for each span.

Accordingly, in preferred embodiments, the processor 36 is programmed to calculate drive torque. The apparatus 30 may also include a heat detector 60 especially a non-contact thermal radiation detector, or pyrometer, for example a passive I pyrometer. The heat detector 60 is mounted in any suitable location close to but not touching the belt 16, e.g. on the sensing unit 40. The output of the heat detector 60 may be used by the processor 36 to provide a measurement of the belt surface temperature.

Hence, in Figure 3, the apparatus 30 is shown as providing multiple output measurements, including TMF (determined from the output of the tooth sensor 38), SVF (determined from the output of the vibration sensor 32 and, if required the tooth sensor 38) and belt surface temperature (determined from the output of the heat detector 60).

The tooth detector 38 may be used to monitor other characteristics of the belt 16, for example tooth count and tooth wear. In particular the output from the detector 38 can be used to detect average tooth wear while the belt 16 is moving during use. As the belt wears during its normal application, the tooth profile reduces, therefore reducing the amount of load each individual tooth can carry before failure. This process is expedited if the drive preload is insufficient or if the dynamic loading is in excess of the design parameters of the belt drive.

The tooth detector 38 produces an output signal whose characteristics vary depending on whether a tooth 18 or gap 19 is detected. Typically, a peak h the output signal corresponds to the detection of a tooth 18 while a trough in the output signal corresponds to the detection of a gap between adjacent teeth 18. Hence, as the belt passes by the sensor 39, the tooth detector 38 generates an output signal comprising a series of alternating peaks and troughs, e.g. a generally sinusoidal signal. The processor 36 may be programmed to detect one or more characteristics of the output signal and to infer one or more characteristics of the belt/teeth from the detected characteristics. In preferred embodiments, the processor 36 is programmed to compute an average tooth width. This may be calculated as an average of a plurality of individual tooth widths measured from the sensor output.

In order to extract tooth data from the sensor output signal, a selected fiducial point is detected in respect of each peak of the output signal. Any conveniently detectable point of the signal may be chosen as the fiducial point, e.g. a turning point, a maxima, the crest of the peak (which is the preferred choice in this embodiment), or a minima. Analysis of the sensor output is typically performed in respect of data gathered by the processor 36 from the sensor output over a period of time during which multiple peaks will be detected by the detector 38. In respect of the analysis period, the processor 36 obtains digitized data representing the output signal during that period. The digitized representation of the output signal may be obtained by any convenient means, e.g. samping at a suitable sampling rate. The fiducial points allow the processor 36 to associate the digitized data with respective peaks of the output signal (and therefore with respective passing teeth 18). This allows the digitized data to be aligned, or otherwise organised, peak-by-peak to enable an averaging function to be performed.

The processor 36 adds the respective digitized data for each peak together. Because the peak data is aligned, respective data values for corresponding points in the respective peaks are added together to created a respective aggregate value for each point of an aggregate peak. The aggregate point values are then divided by the number of detected peaks to provide a respective average point value for each point of an averaged peak.

Conveniently, to process the digitized peak data a (mathematical) window is defined, the width of which determines the number of digital data values used to define each peak. The window is preferably centred around the crest of the respective peaks. Conveniently, the window for each peak can be defined with respect to the fiducial point of each peak. For example, in the preferred embodiment, the fiducial point is chosen to be the respective crest of each peak, and the window is defined as encompassing a respective number (preferably the same number) of data values on either side of the fiducial point.

Accordingly, in the preferred embodiment, the processor 36 applies a window function to the digitized senor output data, based around the respective fiducial point, to obtain a respective set of data points for each peak. Like points in each data set are added together to create an aggregate data set. The data values in the aggregate data set are then each divided by the number of detected peaks to create an average data set.

The averaged peak data provides an averaged representation of the teeth 18 detected during the analysis period, with the level of non-coherent noise significantly reduced. Characteristics of the averaged data may then be determined to infer characteristics of the teeth 18. For example, the width of the peak defined by the averaged data may be calculated (the width may for example be defined by the window used in the above calculation or by a window of any other selected size, conveniently based around the crest of the peak). The averaged data may be used to obtain an indication of average peak width, which may be used as an indication of tooth wear.

The processor 36 may use the digitized output signal data to determine other characteristics of the belt 16 or teeth 18. For example, the number of teeth passing the sensor 38 during a specified time period may be counted by counting the number of peaks in the output signal during that period.

Performing peak detection on the digitized data may be performed by any convenient mathematical or digital signal processing method. The number of teeth passing in the specified time period may then be used to determine the speed of the belt. Another option is to detect missing teeth by detecting missing peaks in the output signal. This can be performed in any convenient manner. For example, the processor 36 may be provided with a value representing an expected delay between peaks and to determine that a tooth is missing if said delay is exceeded, or exceeded by more than a threshold amount.

Alternatively, the processor 36 may be programmed to calculate an average time between peaks and to determine that a tooth is missing if said average time is exceeded, or exceeded by more than a threshold amount.

The above description of preferred embodiments of the invention assumes that teeth 18 are provided in the belt 16. It will be understood however that the invention may be applied in respect of belts having other detectable formations, i.e. formations that cause a corresponding detectable feature (e.g. peak or trough) in the output of the sensor(s) while the belt is moving. The invention is not limited to the embodiments described herein which may be modified or varied without departing from the scope of the invention.

Claims

CLAIMS:

1 . A belt monitoring apparatus for monitoring a belt in a pulley system comprising at least two pulleys around which said belt is fitted, the belt monitoring apparatus comprising a first sensing device including a transmitter configured to transmit a wireless signal and a receiver configured to receive said wireless signal reflected, in use, by a span of said belt, and to produce a corresponding output signal, said apparatus further including processing means configured to detect in said output signal a signal component caused by transverse vibration of said span and to determine the frequency of said transverse vibration from said span vibration signal component.

2. A belt monitoring apparatus a claimed in claim 1 , wherein said sensing device is arranged to cause said wireless signal to diverge in a direction away from said transmitter.

3. A belt monitoring apparatus as claimed in claim 1 or 2, wherein said sensing device is arranged to cause said wireless signal to be substantially un- focussed on said span.

4. A belt monitoring apparatus as claimed in any preceding claim, wherein said wireless signal comprises an optical signal, preferably an infra-red (I ) signal.

5. A belt monitoring apparatus as claimed in any preceding claim, further including a low pass filtering means for detecting said span vibration signal component.

6. A belt monitoring apparatus as claimed in claim 5, wherein said low pass filtering means is arranged to remove a component of said output signal caused by movement of formations, for example teeth, formed on a surface of said belt from which surface said wireless signal is reflected during use, past said transmitter and receiver as said belt revolves during use.

7. A belt monitoring apparatus as claimed in any preceding claim, wherein said processing means is arranged to perform spectral analysis of said span vibration signal component to determine the frequency of said transverse vibration.

8. A belt monitoring apparatus as claimed in any preceding claim, wherein said processing means is arranged to determine the tension in said span from the frequency of said span vibration signal component.

9. A belt monitoring apparatus as claimed in claim 8, wherein said processing means is arranged to calculate said tension as being proportional to the square of said frequency.

10. A belt monitoring apparatus as claimed in any preceding claim, further comprising a second sensing device including a transmitter configured to transmit a wireless signal and a receiver configured to receive said wireless signal reflected, in use, by said span of said belt, and to produce a

corresponding output signal, said apparatus further including processing means configured to detect in said output signal a belt formation signal component caused by movement of formations, for example teeth, formed on a surface of said belt from which surface said wireless signal is reflected during use, past said transmitter and receiver as said belt revolves during use.

1 1 . A belt monitoring apparatus as claimed in claim 10, wherein said second sensing device is arranged to cause said wireless signal to be focussed on said span.

12. A belt monitoring apparatus as claimed in claim 10 or 1 1 , wherein said wireless signal comprises an optical signal, preferably an infra-red (I ) signal.

13. A belt monitoring apparatus as claimed in any one of claims 10 to 12, wherein said processing means is configured to determine said span vibration signal component by removing said belt formation signal component from the output of said first sensing device.

14. A belt monitoring apparatus as claimed in claim 13, wherein said processing means is configured to create a data representing said output signal from said first sensing device and data representing said belt formation signal component and to remove said belt formation signal component from the output of said first sensing device by mathematical processing or digital signal processing of said respective data.

15. A belt monitoring apparatus as claimed in any one of claims 10 to 14, wherein said processing means is configured to obtain from said belt formation signal component a plurality of data sets each representing a respective detected belt formation, to average said data sets to create an average data set, and to analyse said average data set to determine wear of said formations.

16. A belt monitoring apparatus as claimed in claim 15, wherein each of said data sets comprises data representing a peak of said belt formation signal component, said processing means being arranged to determine wear of said formations by determining a width of the peak defined by said average data set.

17. A belt monitoring apparatus as claimed in claim 15 or 16, wherein said processing means is arranged to obtain each of said plurality of data sets by applying a window function to data representing said belt formation signal component.

18. A belt monitoring apparatus as claimed in claim 17, wherein said processing means is arranged to apply said window function to data representing respective peaks in said belt formation signal component.

19. A belt monitoring apparatus as claimed in any one of claims 10 to 18, wherein said processing means is configured to detect components of said belt formation signal component corresponding to respective detected belt formations and to count successive detected belt formations.

20. A belt monitoring apparatus as claimed in claim 19, wherein said processing means is arranged to determine a speed of said belt by

determining the rate of detection of successive belt formations.

21 . A belt monitoring apparatus as claimed in any one of claims 10 to 20, wherein said processing means is configured to detect components of said belt formation signal component corresponding to respective detected belt formations, and to determine that a belt formation is missing from said belt by detecting a corresponding missing component of said belt formation signal component.

22. A belt monitoring apparatus as claimed in any one of claims 10 to 21 , wherein said first and second sensing devices are mutually spaced apart in the direction of travel of said belt, preferably side-by-side, each facing the same surface of said belt.

23. A belt monitoring apparatus as claimed in any one of claims 10 to 22, wherein said first and second sensing devices are located in respective compartments separated by a partition.

24. A belt monitoring apparatus as claimed in any preceding claim, wherein said processing means is configured to determine the frequency of said transverse vibration from the frequency of said span vibration signal component.

25. A pulley system comprising at least two pulleys around which a belt is fitted, and a belt monitoring apparatus comprising a first sensing device including a transmitter configured to transmit a wireless signal and a receiver configured to receive said wireless signal reflected, in use, by a span of said belt, and to produce a corresponding output signal, said apparatus further including processing means configured to detect in said output signal a signal component caused by transverse vibration of said span and to determine the frequency of said transverse vibration from said span vibration signal component.

26. A pulley system as claimed in claim 25, wherein said belt monitoring apparatus is fixed with respect to the pulley system and positioned such that said first sensing device faces a surface of said belt span.

27. A pulley system as claimed in claim 25 or 26, wherein said belt includes formations on at least one surface, said belt monitoring apparatus being positioned such that said first sensing device faces said formations.

28. A pulley system as claimed in any one of claims 25 to 27, wherein a respective belt monitoring apparatus is provided for a tight span and a slack span of said belt, said processing means being configured to determine the respective tension in said tight and slack spans from the respective frequency of the respective span vibration signal component, and to determine the drive torque of said pulley system using the respective tension values.

29. A method of monitoring a belt in a pulley system comprising at least two pulleys around which said belt is fitted, the method comprising transmitting a wireless signal onto said belt; receiving said wireless signal reflected, in use, by a span of said belt and producing a corresponding output signal; detecting in said output signal a signal component caused by transverse vibration of said span; and determining the frequency of said transverse vibration from said span vibration signal component.

30. A belt monitoring apparatus for monitoring a belt in a pulley system comprising at least two pulleys around which said belt is fitted, the belt monitoring apparatus comprising a sensing device including a transmitter configured to transmit a wireless signal and a receiver configured to receive said wireless signal reflected, in use, by said span of said belt, and to produce a corresponding output signal, said apparatus further including processing means configured to detect in said output signal a belt formation signal component caused by movement of formations, for example teeth, formed on a surface of said belt from which surface said wireless signal is reflected during use, past said transmitter and receiver as said belt revolves during