Electronic wheel unit for a vehicle wheel, and method for operating an electronic wheel unit of this kind

Abstract

A method for operating an electronic wheel unit disposed on a vehicle wheel of a vehicle includes providing the electronic wheel unit with a detecting device for detecting rotation angle positions of the vehicle wheel that are present at certain detection times, and a radio transmitter device for transmitting a sequence of individual electromagnetic signals which include data representative of the detected rotation angle positions and their associated detection times. The detecting device is further used to detect an amount of a wheel acceleration of the vehicle wheel and to set an interval of time between the detection times of the rotation angle positions to be shorter the greater the amount of wheel acceleration. A corresponding electronic wheel unit and a method and an apparatus for localizing respective installation positions of a plurality of such electronic wheel units on a vehicle are also provided.

Claims

1. A method for operating an electronic wheel unit disposed on a vehicle wheel of a vehicle, the method comprising the following steps: using a detector for detecting rotation angle positions of the vehicle wheel being present at certain detection times; using a radio transmitter for transmitting a sequence of individual electromagnetic signals including data representative of the detected rotation angle positions and their associated detection times; additionally using the detector to detect an amount of a wheel acceleration of the vehicle wheel; and setting an interval of time between the detection times of the rotation angle positions by the detector to be shorter with a greater amount of wheel acceleration.

2. The method according to claim 1, which further comprises taking into account at least one of a longitudinal wheel acceleration or a transverse wheel acceleration for the amount of wheel acceleration.

3. The method according to claim 1, which further comprises periodically detecting the amount of wheel acceleration.

4. The method according to claim 1, which further comprises detecting the amount of wheel acceleration at times being set variably by the detector.

5. The method according to claim 1, which further comprises providing each of the signals with data representing at least three of the detected rotation angle positions and their associated detection times.

6. The method according to claim 1, which further comprises selecting a number of the rotation angle positions represented by the data of a signal, together with associated detection times, as a function of a current rotation angle velocity of a relevant vehicle wheel.

7. An electronic wheel unit for placement on a vehicle wheel of a vehicle, the electronic wheel unit comprising: a detector for detecting rotation angle positions of the vehicle wheel being present at certain detection times; and a radio transmitter for transmitting a sequence of individual electromagnetic signals including data representative of the detected rotation angle positions and their associated detection times; said detector being configured to detect an amount of a wheel acceleration and to set an interval of time between the detection times of the rotation angle positions to be shorter with a greater amount of wheel acceleration.

8. A method for localizing respective installation positions of a plurality of electronic wheel units each being disposed on one of a plurality of vehicle wheels of a vehicle, the method comprising the following steps: using detectors for detecting rotation angle positions of the vehicle wheels being present at certain detection times; using radio transmitters for transmitting a sequence of individual electromagnetic signals including data representative of the detected rotation angle positions and their associated detection times; additionally using the detectors to detect an amount of a wheel acceleration of the vehicle wheels; setting an interval of time between the detection times of the rotation angle positions by the detectors to be shorter with a greater amount of wheel acceleration; providing the vehicle with fixed detectors each being disposed on the vehicle and associated with a respective one of the vehicle wheels for detecting rotation angle positions of a respective relevant vehicle wheel being present at certain detection times; providing the vehicle with a transmitter for transmitting signals including data representative of the rotation angle positions detected by the fixed detectors and their associated detection times; receiving and evaluating the signals transmitted by the electronic wheel units to determine rotation angle positions detected by said detectors of the electronic wheel units and their associated detection times; receiving and evaluating the signals transmitted by the fixed detectors of the vehicle to determine the rotation angle positions detected by the fixed detectors and their associated detection times; comparing the rotation angle positions determined by the detectors of the electronic wheel units and the rotation angle positions determined by the fixed detectors of the vehicle by taking into account their respective detection times, to determine a correlation between the determined rotation angle positions; and analyzing the correlation to make an association between the electronic wheel units and installation positions of the vehicle wheels.

9. An apparatus for localizing respective installation positions of a plurality of electronic wheel units each being disposed on one of a plurality of vehicle wheels of a vehicle, the apparatus comprising: detectors disposed at the electronic wheel units for detecting rotation angle positions of the vehicle wheels being present at certain detection times; radio transmitters for transmitting a sequence of individual electromagnetic signals including data representative of the detected rotation angle positions and their associated detection times; said detectors being configured to detect an amount of a wheel acceleration and to set an interval of time between the detection times of the rotation angle positions to be shorter with a greater amount of wheel acceleration; fixed detectors each being disposed on the vehicle and associated with a respective one of the vehicle wheels for detecting rotation angle positions of a respective relevant vehicle wheel being present at certain detection times; a transmitter disposed on the vehicle for transmitting signals including data representative of the rotation angle positions detected by said fixed detectors and their associated detection times; and a central control unit and a radio receiving unit for: receiving and evaluating signals transmitted by the electronic wheel units to determine the rotation angle positions detected by said detectors of the electronic wheel units and their associated detection times; receiving and evaluating the signals transmitted by said fixed detectors of the vehicle to determine the rotation angle positions detected by said fixed detectors and their associated detection times; comparing the rotation angle positions determined by said detectors of the electronic wheel units and the rotation angle positions determined by said fixed detectors of the vehicle by taking into account their respective detection times, to determine a correlation between the determined rotation angle positions; and analyzing the correlation to make an association between the electronic wheel units and installation positions of the vehicle wheels.

10. A non-transitory computer-readable medium with instructions stored thereon, that when executed by a processor, perform the steps of the method according to claim 1.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

(1) The invention is described in more detail below on the basis of exemplary embodiments with reference to the enclosed drawings, in which:

(2) FIG. 1 shows a side view of a vehicle wheel equipped with an electronic wheel unit according to one exemplary embodiment,

(3) FIG. 2 shows a block diagram of the electronic wheel unit from FIG. 1,

(4) FIG. 3 is a graph illustrating a detection of rotation angle positions of the vehicle wheel from FIG. 1 and the transmission of a sequence of radio signals over time on the basis thereof,

(5) FIG. 4 is a schematic plan view of a motor vehicle which is equipped with a tire pressure monitoring system and in this regard has vehicle wheels of the type shown in FIGS. 1 and 2 equipped with electronic wheel units, and

(6) FIG. 5 is a flowchart illustrating a localization method performed in the vehicle from FIG. 4.

DESCRIPTION OF THE INVENTION

(7) FIG. 1 shows a vehicle wheel W1 on which an electronic wheel unit 12-1 is arranged, in the illustrated example on an inner side of a tire tread of the vehicle wheel W1 provided with an air-filled tire.

(8) The primary purpose of the electronic wheel unit 12-1 is to measure a tire (inside) pressure p during operation of the relevant vehicle (see e.g. FIG. 4) and from time to time to transmit corresponding measurement results to a vehicle-based radio receiving unit 40, i.e. one arranged in the relevant vehicle, by means of an electromagnetic signal (radio signal) R1.

(9) In the example shown, the radio receiving unit 40 has a communication connection via a digital bus system 30 to a central control unit 20 which evaluates data contained in each radio signal R1, in particular measured values of the tire pressure p in this case, and/or makes them available for further use in other parts of on-board electronics of the relevant vehicle.

(10) The data of the signal R1 further contain an identification code of the wheel unit 12-1, so that each received signal from the control unit 20 can be unambiguously associated with this wheel unit 12-1.

(11) When a plurality of vehicle wheels are equipped with such wheel units, however, there still remains the problem of associating the wheel unit identified on the basis of the data of a signal with the relevant installation position, i.e. the installation position (e.g. in the case of a car: front left, front right, rear left, rear right) of that vehicle wheel on which the wheel unit is actually arranged or installed.

(12) For the purpose of such localization of each wheel unit, such as e.g. the wheel unit 12-1 shown, there is provision in the exemplary embodiment shown for the data of the signal R1 to further each represent a plurality of pieces of rotation angle position information which each indicate rotation angle positions of the vehicle wheel W1 that are detected by means of the wheel unit 12-1 and their associated detection times.

(13) As a departure from this example, the data of the signal R1 could each also represent only a single piece of rotation angle position information (rotation angle position and associated detection time).

(14) The control unit 20 compares the rotation angle position information of the relevant vehicle wheels that is detected in wheel-based fashion with rotation angle position information detected in vehicle-based fashion, so as to determine a correlation between the determined information and finally, by analyzing this correlation, to make an association between the individual wheel units and their installation positions.

(15) FIG. 2 shows the design of the electronic wheel unit 12-1 in more detail.

(16) The wheel unit 12-1 comprises a pressure sensor 14 for measuring the current tire pressure p and an acceleration sensor 15 for measuring a current radial acceleration a.

(17) A rotation of the vehicle wheel W1, symbolized by an arrow in FIG. 1, results in the acceleration sensor 15 measuring the radial acceleration a present at a site 13-1.

(18) Since the radial acceleration a is made up of a component caused by the gravitation and a component caused by the rotation of the vehicle wheel W1, an appropriate evaluation of the sensor signal (e.g. by means of extraction of the gravitation component) can determine the current rotation angle position.

(19) Moreover, a tire contact surface formed in the lower circumferential region of the wheel W1 results in corresponding signal characteristics in the sensor signal delivered by the acceleration sensor 15 whenever the wheel unit 12-1 or the site 13-1 passes through the region of this tire contact surface.

(20) These signal characteristics, which arise periodically e.g. when traveling at constant speed, can therefore also be taken as a basis for determining when the site 13-1 passes through the tire contact surface and, by virtue of a further evaluation of the sensor signal, the current rotation angle position of the vehicle wheel W1 in a simple manner at any time.

(21) As a departure from the use of an acceleration sensor, it would e.g. also be possible for what is known as a shock sensor or a deformation sensor or another suitable sensor to be used in order to realize detection of a rotation angle position at a certain detection time by means of evaluation of the sensor signal of said sensor.

(22) In the wheel unit 12-1, the measured values representative of the tire pressure p and the acceleration a, as can be seen in FIG. 2, are communicated to a program-controlled evaluation device, in this case a microcontroller 16, which takes them as a basis for producing the data and communicates said data to a radio transmission unit 19 by means of which the signal R1 containing these data is transmitted. As already mentioned, the data in this case include a plurality of pieces of rotation angle position information.

(23) A trait of the electronic wheel unit 12-1 is that a variable time difference between successive detection times of the rotation angle positions is provided. Specifically, in the example shown, this involves the electronic wheel unit 12-1 being used to detect an amount of longitudinal wheel acceleration of the vehicle wheel W1 and an interval of time between the detection times of the rotation angle positions tends to be set to be shorter the greater the amount of longitudinal wheel acceleration.

(24) Alternatively or additionally, the electronic wheel unit 12-1 could also be used to detect an amount of transverse wheel acceleration of the vehicle wheel W1 and the interval of time between the detection times of the rotation angle positions could tend to be set to be shorter the greater the amount of longitudinal wheel acceleration.

(25) Further, there could additionally also be provision for the interval of time between the detection times to be dependent on the rotation angle velocity of the vehicle wheel W1.

(26) The detection of the longitudinal wheel acceleration and the rotation angle velocity of the vehicle wheel W1 can be accomplished by means of the microcontroller 16 by virtue of appropriate evaluation of the sensor signal delivered by the acceleration sensor 15.

(27) If the aim is (also) for transverse wheel acceleration to be detected, it is advisable either to suitably design the acceleration sensor 15 contained in the electronic wheel unit 12-1 for measuring transverse acceleration (also) or to integrate a separate acceleration sensor (not shown) for measuring transverse acceleration into the electronic wheel unit 12-1.

(28) In respect of setting the detection times, the microcontroller 16 in the example shown is connected to a clock 17 contained in the wheel unit 12-1 for the purpose of timekeeping. Alternatively, such a clock 17 could e.g. also be implemented as a partial functionality of the microcontroller 16 by the latter itself.

(29) In the example shown, one or more detected rotation angle positions are initially accumulated by buffer-storing the corresponding rotation angle position information in a memory unit 18, connected to the microcontroller 16, that e.g. can also store a program code for sequence control for the microcontroller 16.

(30) At predetermined transmission times (e.g. periodically), the microcontroller 16 initiates the transmission of a signal R1 containing the corresponding data. In each case, the data contain the rotation angle position information acquired and buffer-stored since the last transmission.

(31) The individual detection times (based on the operation of the wheel-based clock 17) can be encoded within the data of the relevant signal R1 e.g. by appropriate time information (timestamps), so that this time information can be decoded again at the receiver and used in the localization method.

(32) As a departure from the example shown, in which the data in each case explicitly indicate detection times and associated rotation angle positions, it would also be possible for the data transmitted using the signal R1 to explicitly indicate the detection times only, whereas the associated rotation angle positions are implicitly represented by the data by virtue of said data e.g. being fixed. By way of example, there could be provision for all detections to always take place at one and the same fixed rotation angle position.

(33) In the example shown, the signal R1, which contains data relating to rotation angle position information acquired previously (since the last signal transmission), is transmitted again after every 10 s.

(34) FIG. 3 illustrates this operation of the wheel unit 12-1 by way of example.

(35) FIG. 3 is a timing diagram, the upper part of which shows rotation angle positions .sub.i, detected at discrete times, that are denoted by dots and have been detected at associated detection times t.sub.i. The index i in this case denotes a serial number for the corresponding detection (measurement). FIG. 3 shows the measurements for i=1, 2, 3, . . . 7, 8, 9.

(36) In this case, each rotation angle position .sub.i can assume values in a range from 0 to 360 (corresponding to one full revolution of the vehicle wheel W1). The value of 0, equivalent to a value of 360 on the basis of the periodicity, can be associated, according to any convention, with a specific rotational position of the wheel W1, for example a position at which the site 14-1 (see FIG. 1) of the electronic wheel unit 12-1 is at the very top (or alternatively e.g. at the very bottom).

(37) As can be seen from FIG. 3, the measurements of the rotation angle position are performed at variable intervals of time (t.sub.2t.sub.1, t.sub.3t.sub.2, t.sub.4t.sub.3, . . . ) over time t. In the example shown, the interval between the times t.sub.1, t.sub.2, t.sub.3, . . . is always in the range from 0.2 s to 10 s and, as mentioned, tends to be set to be shorter the greater the amount of aforementioned (current) wheel acceleration, in this case e.g. longitudinal wheel acceleration.

(38) In the example shown, this could involve e.g. a value of the longitudinal wheel acceleration below a first threshold value resulting in the interval of time (t.sub.i+1t.sub.i) between successive detections being set at 10 s and a value of the longitudinal wheel acceleration above a second threshold value resulting in said interval of time being set at 0.2 s, there possibly being provision for e.g. a linear dependence (decrease in the interval of time as longitudinal wheel acceleration increases) for the intermediate range (first threshold value longitudinal wheel acceleration second threshold value) in the simplest case. However, the latter dependence could also be provided for differently and/or be modified by further dependences (e.g. on the transverse wheel acceleration and/or the wheel rotation angle velocity).

(39) The variation of the intervals of time t.sub.i+1t.sub.i that is apparent by way of example from FIG. 3 could therefore be based e.g. on the scenario that in the period from approximately t.sub.1 to t.sub.5 the driver increasingly accelerates (or brakes) the vehicle and in the period from approximately t.sub.6 to t.sub.9 he reduces this acceleration (or braking) again, so that relatively quickly successive detections of the rotation angle positions (.sub.4 to .sub.6) are effected in the period from approximately t.sub.4 to t.sub.6.

(40) As is further apparent from FIG. 3, the rotation angle positions .sub.1 to .sub.4 collected up to a certain transmission time are picked up, together with their associated detection times t.sub.1 to t.sub.4, from the electronic wheel unit 12-1 in data, which are transmitted to the radio receiving unit 40 using a radio signal R1.sub.1-4.

(41) The rotation angle positions .sub.5 to .sub.9 which are then collected in the same way are picked up together with their associated detection times t.sub.5 to t.sub.9 by the electronic wheel unit 12-1 in data, which are transmitted to the radio receiving unit 40 using a subsequent radio signal R1.sub.5-9.

(42) A start time ts of each signal R1, as shown in FIG. 3 a start time ts.sub.1-4 of the signal R1.sub.1-4 and a start time ts.sub.5-9 of the signal R1.sub.5-9, is correlated in a predetermined manner with the data included by the relevant signal R1 in the example shown. This correlation consists in a time difference t between the last of the detection times contained in the data in each case (in this instance e.g. the times t.sub.4 and t.sub.9) and the start time ts of the relevant signal (in this instance ts.sub.1-4 or ts.sub.5-9) being fixed. This time difference t is e.g. approximately 0.4 s in the example shown.

(43) In addition, there is provision in the example shown for the start times ts of successive signals R1, in this case that is to say e.g. the start times ts.sub.1-4 and ts.sub.5-9, to be periodically successive with a predetermined time difference ts. In the example shown, this time difference ts=15 s.

(44) After the signals R1 are received by the radio receiving unit 40 and the data contained therein are supplied to the central control unit 20, which has its own vehicle-based clock, an evaluation of the data involves the detection times t.sub.i determined directly from the received data (and referenced to the wheel-based clock in the wheel unit 12-1) being converted into detection times t.sub.1* which are referenced to the operation of the vehicle-based clock.

(45) Such clock synchronization between the wheel-based clock 17 and the vehicle-based clock can be realized on the vehicle e.g. by virtue of the time difference ts between successive start times (in this instance e.g. between the times ts.sub.1-4 and ts.sub.5-9) being measured by means of the vehicle-based clock and compared with the setting ts=15 s, with a (typically occurring small) disparity between the nominal value of 15 s and the time difference actually measured at the vehicle then being able to be taken as a basis for establishing the operating speed difference between the wheel-based clock 17 in the wheel unit 12-1 and the vehicle-based clock.

(46) Assuming that the vehicle-based measurement of the time difference ts=15 s results in a period of ts*=14.8 s referenced to the vehicle-based clock, this means that the wheel-based clock 17 is running too fast by a factor of ts/ts*=15 s/14.8 s=1.014.

(47) Based on the conversion factor thus calculated, in this instance 1.014, it is then possible for e.g. the real time t.sub.4* (referenced to the vehicle-based clock) to be calculated as the time ts.sub.1-4* measured at the vehicle less 1.0140.4 s.

(48) Finally, it is then possible for e.g. the remaining detection times t.sub.1 to t.sub.3 to be calculated e.g. on the basis of the corresponding time differences t.sub.4t.sub.1, t.sub.4t.sub.2 and t.sub.4t.sub.3 likewise corrected by the factor 1.014. The time t.sub.1* referenced to the vehicle-based clock is thus obtained e.g. as: t.sub.1*=t.sub.4*1.014 (t.sub.4t.sub.1).

(49) Such clock synchronization can be carried out continuously or from time to time (update of the conversion factor).

(50) The clock synchronization explained above using the example of the wheel unit 12-1 of the vehicle wheel W1 can be performed in a corresponding manner for the wheel units installed on one or more further vehicle wheels of the same vehicle.

(51) Based on the detection times thus referenced to the common vehicle-based clock for all vehicle wheels, it is then possible for a localization method to be performed to associate each electronic wheel unit with the correct installation position in each case.

(52) Since the intervals of time between the detection times t.sub.1, t.sub.2, t.sub.3 . . . are provided for variably in each case, depending on a current amount of wheel acceleration (and possibly e.g. additionally depending on a current rotation angle velocity of the relevant vehicle wheel), the electronic wheel unit can advantageously be used to obtain and transmit particularly meaningful rotation angle position information.

(53) The arrangement of a plurality of electronic wheel units on a corresponding plurality of vehicle wheels and a localization method carried out in this case are explained below with reference to FIGS. 4 and 5.

(54) FIG. 4 shows a motor vehicle 1, in this instance e.g. a car, in which four vehicle wheels W1 to W4 are arranged at installation positions front left, front right, rear left and rear right.

(55) In this example, it will be assumed that the vehicle wheel W1 together with the electronic wheel unit 12-1 is the vehicle wheel already described with reference to FIGS. 1 to 3 and that the further vehicle wheels W2 to W4 together with their electronic wheel units 12-2 to 12-4 are of respectively corresponding design, so that a detailed description of the wheels W1 to W4 together with their wheel units 12-1 to 12-4 will be dispensed with at this juncture.

(56) The radio signals transmitted by the wheel units 12-1 to 12-4 (e.g. all periodically) are denoted by R1 to R4 in FIG. 4.

(57) The radio receiving unit 40 transmits the data received by means of these signals R1-R4 to the central control unit 20 via the digital bus system 30 (e.g. LIN bus system or the like), which central control unit is equipped with a computer unit 22 and a digital memory unit 24 for the purpose of evaluating the data (concerning tire pressures and rotation angle positions) of the vehicle wheels W1-W4. In this case, the memory unit 24 contains in particular a program code for sequence control of the evaluation in the control unit 20.

(58) The vehicle 1 has provision for vehicle-based rotation speed sensors 10-1 to 10-4, each of which is associated with one of the vehicle wheels W1-W4 as shown, that can be used to perform time-resolved detection of the rotation angle positions of the individual vehicle wheels W1 to W4.

(59) Based on the speed sensors 10-1 to 10-4, corresponding data (i.e. rotation angle positions together with their associated detection times) D1-D4 are transmitted to the central control unit 20 via the digital bus system 30.

(60) The localization method carried out in the control unit 20 comprises the following steps: receiving and evaluating the signals R1-R4 transmitted by means of the electronic wheel units 12-1 to 12-4 in order to determine the rotation angle positions .sub.i detected by means of the respective wheel-based detecting means 15, 16, 17, 18 of the electronic wheel units 12-1 to 12-4 and their associated detection times t.sub.i, receiving and evaluating the signals D1-D4 transmitted by means of the vehicle-based speed sensors 10-1 to 10-4 of the vehicle 1 in order to determine the rotation angle positions detected by means of the speed sensors 10-1 to 10-4 and their associated detection times, comparing the rotation angle positions determined by means of the electronic wheel units 12-1 to 12-4 and the rotation angle positions determined by means of the speed sensors 10-1 to 10-4 of the vehicle 1 by taking account of their respective detection times, in order to determine a correlation between the determined rotation angle positions, and analyzing the correlation in order to make an association between the electronic wheel units 12-1 to 12-4 and the installation positions (in this case: front left, front right, rear left and rear right) of the vehicle wheels W1-W4.

(61) FIG. 5 shows essential steps of this localization method again in a flowchart.

(62) In a step S1, the receiving and evaluating of the signals D1-D4 transmitted by means of the vehicle-based speed sensors 10-1 to 10-4 is effected.

(63) In a step S2, the receiving and evaluating of the signals R1-R4 transmitted by means of the wheel-based electronic wheel units 12-1 to 12-4 is effected, this step S2 consisting of two substeps S2-1 and S2-2.

(64) In substep S2-1, the rotation angle positions contained in the received data are extracted, together with their associated detection times.

(65) In substep S2-2, vehicle-based measuring of at least one time and/or of a time difference in consideration of the received signal and converting of these detection times into detection times referenced to a vehicle-based clock are effected.

(66) Finally, in a step S3 composed of substeps S3-1, S3-2 and S3-3, the actual algorithm for localizing the installation positions of the wheel units 12-1 to 12-4 is performed.

(67) In substep S3-1, for each of the wheel units or for each of their identification codes, the rotation angle positions determined by means of the wheel units and the rotation angle positions determined by means of the vehicle-based fixed detecting means are subjected to a comparison taking into consideration their respective detection times. In this case, it is possible for e.g. a plurality of distribution patterns corresponding to the number of vehicle wheels considered to be determined which show e.g. for each identification code how greatly rotation angle position information determined for the relevant identification code in wheel-based fashion differs from each of the pieces of rotation angle position information determined in vehicle-based fashion.

(68) In substep S3-2, a result of the comparison is taken as a basis for determining, e.g. separately for each identification code, a correlation between the rotation angle position information determined on the one hand in wheel-based fashion and on the other hand in vehicle-based fashion. For this purpose, it is possible for e.g. predetermined correlation parameters to be calculated, e.g. parameters which are characteristic of the probability of a particular wheel unit or its identification code being associated with a particular installation position.

(69) Finally, in substep S3-3, the correlation(s) is/are analyzed to make an association between the wheel units and the installation positions. In this case, it is possible for the association e.g. to be based on the statistically most likely installation position in each case being associated for each wheel unit or its identification code and/or vice versa the statistically most likely wheel unit being associated for each installation position.

LIST OF REFERENCE SIGNS

(70) 1 motor vehicle W1 to W4 vehicle wheels 10-1 to 10-4 speed sensors D1 to D4 signals of the speed sensors 12-1 to 12-4 electronic wheel units 13-1 site 14 pressure sensor 15 acceleration sensor 16 evaluation device (microcontroller) 17 wheel-based clock 18 memory unit 19 radio transmission unit R1 to R4 signals of the electronic wheel units 20 central control unit 22 computer unit 24 memory unit 30 digital bus system 40 radio receiving unit t time rotation angle t.sub.1, t.sub.2, t.sub.3, . . . detection times t time difference ts start time te end time ts time difference