Testing vehicle tyres

Abstract: Method and apparatus for testing inflated vehicle tyres to determine internal physical characteristics such as tyre pressure comprises ultrasonic waveform transmission and reception apparatus adapted to interpret the attenuation of the ultrasonic waveform produced by the inflated tyre whereby pressure and other internal physical characteristics can be determined.

Description: This invention relates to a method and apparatus for testing vehicle tyres, for example to determine the internal pressure thereof. The method and apparatus is also applicable to the determination of other physical characteristics including tyre thickness, the presence of an inner tube, and the wear characteristic of the tyre after a period of use. An example of a particular application of the invention is in automotive tyre and wheel testing apparatus such as equipment used for determining such characteristics as wheel balance and tyre compliance with official regulations. We have identified a need for an enhancement of the versatility of such apparatus so that there can be carried out conveniently at the same time additional tests such as a determination of tyre pressure, tyre wear and/or related characteristics. Prior proposals in relation to the measurement of tyre pressure have included force detectors and the conventional measurement of the pressure at the inlet valve by means of diaphragm or bourbon type pressure sensors. These latter proposals require an operator to place and fit and remove the sensor in relation to the tyre, and in some cases interpretation of the results are required. All this is a time consuming operation which is nevertheless still susceptible to the production of data errors. From the technical viewpoint, it is desirable that tyre testing is carried out at the recommended working pressure of the tyre in question, whereby the need for rapid and convenient tyre pressure determination is seen to be of considerable importance as one part of a programme of tyre testing. Other prior proposals in relation to tyre pressure sensing include the following. There are disclosed in SU 518 386 and 414 506 apparatus involving drum and roller systems incorporating braking facilities to enable tyre evaluation by sensing distortion and deformation thereof. It is considered that such a mechanical approach to tyre evaluation is unnecessarily costly and complex and not adapted to provide the convenient production of a spectrum of data relating to the tyre under test. There is disclosed in :- US - A - 5 181 423 WO - A - 82 02249 DE - A - 2 936 213 tyre test apparatus comprising oscillator systems in which there are included both rotary and non-rotary sensors, the rotary sensors being mounted on the tyre under test, and coupling means being provided between the sensors, such as an inductive coupling. Tyre evaluation is carried out by means of, for example, strain gauge techniques. It is considered that systems involving the requirement to mount a rotary sensor on the tyre or wheel under test represent an unacceptable level of complication and inconvenience in use. Proposals also exist for the use of techniques for the determination of external tyre dimensions by the use of non-mechanical tyre contact systems, such as an ultrasonic waveform, the waveform being used as a non-contact means permitting the tyre external dimensions to be sensed during rotation by reference to (for example) the linear movement required for movement between positions corresponding to tangential contact of the beam with opposite side walls of the tyre. US - A - 4,089,226 and US-A-4 089 225 both disclose a system for residual tire life prediction by ultrasound. There is disclosed a pulse ultrasonic reflection system in which pulses of ultrasonic energy are transmitted into an area of a tire in a manner to provide pulses of reflected acoustic energy so that each reflected pulse includes reflected ultrasonic energy from the plies of the casing of the tire. The method includes a determination of the difference between the information thus-obtained from a given tire and that obtainable from the same ply or plies of an unused new tire of the same type. US - A-4,469,450 and US - A -4,059,989 make similar disclosures. US - A - 4,472,974 discloses a roller-type ultrasonic tire inspection system in which a roller carrying an ultrasonic transducer engages the tread of a tire to be inspected. US - A - 5,341,687 discloses a three-dimensional pressure sensor utilising ultrasonic energy to stimulate a piezoelectric transducer which senses tire pressure. Prior patents disclosing the use of ultrasonic techniques for tire inspection and requiring the location of one or more ultrasonic transducers within the tire casing include : US - A - 3,882,717 and 4,266,428 and 4,274,289 and 4,275,489 and 4,285,235 and 4,297,876 and 4,327,579 and 4,337,660 and 4,936,138 and 5,095,744. To the best of the Applicants' knowledge no prior disclosure teaches the use of a method for testing inflated vehicle tires in which an externally transmitted ultrasonic waveform is utilised and is interpreted by reference to its decay or attenuation in amplitude with time. An object of the present invention is to provide a method and apparatus offering improvements in relation to the determination of physical characteristics of tyres, such as pressure, without the need for direct mechanical intervention with the tyre during testing as required by prior proposals, or otherwise providing improvements in relation to matters discussed herein. In the embodiment of the invention described below, there are provided advantages in relation to convenience of fitting and use, together with simplification of structure and the provision of more widely ranging information than has been hitherto available from prior systems. According to the invention there is provided a method and apparatus for testing vehicle tyres as defined in the accompanying claims. In an embodiment, there is provided a method and apparatus in which the ultrasonic waveform is transmitted to the tyre to be tested from an external location with respect to the tyre, and the waveform is likewise received at a location external to the tyre. By this means, the advantage is provided that the tyre testing procedure requires no intervention in a mechanical sense with the tyre at all and can all be carried out externally thereof. The ultrasonic transmitter and receptor apparatus may be located in contact with the tyre or at a distance of up to approximately 4 inches (10 centimetres) from the tyre. In the case of non-contact between the transmitter/sensor apparatus and the tyre, the tyre can of course rotate during the test. In the described embodiment, the interpretation of the received waveform is carried out by reference to its decay or attenuation in amplitude with respect to time. The decay of the waveform between the inner walls of the tyre is expressed by the following function relating the pressure (P) within the tyre to an attenuation constant ( α ) and the tyre inner width (r) between the tyre walls, between which the pulse travels :- P cs (e -4αr ) OR P = K (e -4αr ) Also in the embodiment, the interpretation step of the method is carried out with reference to the decay of the intensity or amplitude of the ultrasonic waveform peaks, as they are affected by the tyre structure. Thus, for example, this approach enables the condition and spacing of the tyre braid or plies to be determined. The embodiments of the invention described below utilise an ultrasonic transducer as an aid for vehicle tyre pressure determination, and likewise to detect the presence of an inner tube or foreign material within the tyre, or indeed the nature of the metallic lattice or ply structure within the polymeric wall material of the tyre, together with the detection of internal flaws within the tyre wall. The embodiments also provide a general statement with respect to the condition of the material of the tyres. The term "ultrasonic" as used in this specification refers to a waveform having a frequency above the usual upper human hearing limit of approximately 21,000 Hertz. The embodiments of the invention described below are applicable to the measurement of tyre pressure in vehicle diagnosis techniques, and are particularly applicable to vehicle wheel balance apparatus and test lanes. Indeed, the embodiments provide advantages whenever there is limited access to the tyre inflation valve and can also be used to determine tyre thickness, the presence of an inner tube and the distance between tyre walls. In the embodiments, the apparatus is incorporated into wheel balancing apparatus so as to provide an automated tyre pressure reading facility which removes the requirement for operator intervention for this purpose. Moreover, the reliability of the pressure measurement is increased in this way since the measurement can be taken at closely spaced time intervals and the result averaged and displayed. Moreover, the apparatus is able to provide, in addition to a determination of tyre pressure, data or a visual display enabling identification of tyre defects, even where these have no visible external physical manifestation. In an envisaged alternative embodiment, the waveform transmitted for the purpose of tyre testing, or indeed for use in relation to the testing of other articles, is at a frequency differing from that of ultrasonic energy. Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which :- Fig 1 shows a section through a vehicle tyre and an associated wheel rim, the figure showing an ultrasonic transducer positioned in relation to the tyre for testing purposes; Fig 2 shows the waveform obtained from the apparatus of Fig 1 for the purpose of determining tyre pressure; Fig 3 shows a corresponding trace from the transducer and utilised to determine the condition and spacing of the internal tyre braid within the tyre; Fig 4 shows a corresponding waveform obtained from the apparatus and illustrating a defect such as a rip detected in the internal tyre wall; Fig 5 shows a block-type circuit diagram for ultrasonic waveform transmitter/receiver apparatus and associated waveform interpretation apparatus; and Fig 6 shows a flow chart for the software algorithms utilised in the waveform interpretation process. As shown in Fig 1, apparatus 10 for testing an inflated vehicle tyre 12 having an air space 13, to determine an internal physical characteristic thereof, such as the internal pressure, is provided in the form of an ultrasonic transducer 14 to produce waveform pulses 15. Tyre 12 is mounted on the usual wheel rim 16 having edge regions 18 and 20 to receive the complementary tyre bead 22, 24. Tyre 12 comprises tyre walls 26, 28 and a central body portion 30 having a road-contacting tread (not shown). The ultrasonic transducer 14 will now be described further. Transducer 14 comprises combined ultrasonic transmitter and receiver apparatus provided with an electric supply 32 and adapted to generate and transmit ultrasonic waveforms at a chosen frequency or frequencies. The transducer likewise comprises ultrasonic receptor apparatus positioned alongside or integrated with the transmitter apparatus and screened therefrom and adapted to receive and detect an attenuated waveform after its transmission and interaction with the inflated tyre. As shown in Fig 1, transducer 14 is located externally of the tyre. In this embodiment the transducer is shown in contact with tyre wall 28. In an alternative embodiment (not shown) the transducer is spaced a short distance from the tyre wall, whereby the tyre can be rotated during testing. This latter approach enables an averaged determination to be made of the air pressure within the tyre. A series of readings is obtained and averaged (to eliminate or reduce measurement errors) and the pressure determined in accordance with the equation quoted above. There is shown in Fig 5 the circuit diagram for transducer 14 and its associated waveform interpretation apparatus. As indicated in Fig 5 transducer 14 is directly connected to associated radio frequency oscillator and amplifier functions, 15 and 17, which are associated with the transmission and reception functions of transducer 14. Amplifier 17 cooperates with associated filtering and adding and rectifying and analogue-to-digital conversion functions 19, 21, 23 and 25 respectively, followed by processing and display functions 27 and 29. An output from oscillator 15 is associated with addition function 23. With respect to interpretation of the waveform received by the combined transmitter and receptor apparatus during use, this function is performed by a data processing circuit forming part of processor 27 and provided with software for determination of tyre pressure and/or other selected characteristics in accordance with the equation/expression quoted above or related functions in which the received waveform is interpreted by reference to the decay or attenuation of the amplitude of the waveform with time. Calibration of the apparatus is carried out at the pre-production stage. The flow chart of the software algorithm for these interpretation functions is provided in Fig 6. As shown in Fig 6, the flow chart of the software algorithms comprises the following steps :- acquire data set; find peaks; is it largest peak? YES/NO; if yes, record time and magnitude; if no, is it associated with inner tyre wall? if yes, ignore; if no, is it associated with second tyre wall? if yes, record time and magnitude; if no, return to find peaks step; calculate difference in time and magnitude; apply calculations to formula; apply formula result to look up table to give pressure. In the above steps, the application of the formula is in accordance with matters discussed below. As shown in Fig 2, the output of transducer 14 may be displayed on an oscilloscope visual display unit if desired, although display is not a requirement for pressure determination. Trace 34 comprises peaks 36 representing wave reflection at the tyre walls 26, 28. As shown in Fig 2, the waveform intensity attenuation "A" between successive peaks 36 is proportional to an exponential function of an attenuation constant ( α ) for the tyre in question and the tyre inner width (r) :- A cs e -4αr OR A = K e -4αr Therefore, since the tyre pressure is likewise proportional to the same function, the attenuation provides a measure of tyre pressure, which is susceptible to calibration. Fig 3 shows a typical trace from the transducer 14, as displayed upon an oscilloscope vdu during use to determine the condition and spacing of the tyre braid or reinforcement material within the polymeric body of the tyre. Trace 38 shows multiple peaks 40 corresponding to the braid structure of the tyre. Fig 3 shows the spacing 42 between successive peaks 40 which may be employed as an element in the interpretation of the trace. On the basis of a knowledge of the original braided structure of the tyre, interpretation of the trace enables evaluation of the condition of the tyre. The interpretation apparatus may be pre-programmed with data relating to the original tyre structure for comparison purposes. The trace 44 in Fig 4 shows at 46 the region of a tyre defect, for example a rip in the tyre wall which produces an increase in intensity of the trace due to the reduced distance between the tyre/air interface and the transducer 14. Tyre blisters produce likewise readily recognisable traces including a return echo for each blister. Amongst other modifications which could be made in the above embodiments while remaining within the scope of the invention as it is defined in the appended claims are the use of ultrasonic transmitter and receiver apparatus at opposite sides of the tyre.

Claims: A method of testing inflated vehicle tyres (12) to determine an internal physical characteristic thereof, the method comprising :- a) transmitting a waveform; b) subjecting the tyre to be tested to said waveform; c) receiving the waveform from said tyre and interpreting the received waveform; d) said step of transmitting said waveform comprising transmitting an ultrasonic waveform from an external location with respect to said tyre; and e) said step of receiving said waveform comprising receiving said ultrasonic waveform at a location external to said tyre; characterised by f) said step of interpreting the received waveform being carried out to provide a quantitative measure of said internal physical characteristic by reference to the decay or attenuation of the amplitude of said waveform with respect to time. A method according to claim 1 characterised by said step of interpreting the received waveform comprising interpreting the decay or attenuation of the intensity or amplitude of the peaks of said waveform as affected by the structure of the tyre (12) under test. A method according to claim 2 characterised by the step of displaying said received waveform. A method according to any one of claims 1 to 3 characterised by the step of calculating the air pressure in said tyre (12) on the basis of said attenuation of said amplitude of said received waveform. Apparatus (10) for testing inflated vehicle tyres (12) to determine an internal physical characteristic thereof, the apparatus comprising :- a) waveform transmission apparatus (14) adapted to subject a tyre to be tested to a transmitted waveform; b) waveform reception apparatus (14) adapted to receive a waveform from a tyre under test; and c) waveform interpretation apparatus (17 to 27) adapted to permit the interpretation of the received waveform; d) said waveform transmission apparatus being adapted to transmit an ultrasonic waveform from a location external to the tyre under test; e) said waveform reception apparatus being adapted to receive the ultrasonic waveform from said tyre at a location external thereto; characterised by f) said waveform interpretation apparatus being adapted to interpret said received waveform to provide a quantitive measure of said internal physical characteristic with reference to the decay or attenuation of the amplitude of said waveform with respect to time. Apparatus (10) according to claim 5 characterised by said waveform interpretation apparatus comprising visual display means (29) whereby said waveform can be interpreted by visual inspection. Apparatus (10) according to claim 5 or claim 6 characterised by said waveform interpretation apparatus (17 to 27) comprising signal processing means (27) adapted to calculate tyre pressure on the basis of said attenuation of said amplitude of the received waveform.

Inventors: JONES BARBARA LYNN, WYLIE THOMAS FRANCIS, JONES, BARBARA LYNN, WYLIE, THOMAS FRANCIS
Applicants: SNAP ON EQUIPMENT LTD, SNAP-ON EQUIPMENT LIMITED
Application year: 1996
Main classification: G01M17
IPC codes: G01M17, G01L17