Method of Detecting a Deflated Tire on a Vehicle

This invention relates to a method for detecting a deflated tire on a vehicle and provides a system suitable for cars and trucks and the like.

European Patent Publication No. 291217 describes a system primarily aimed at vehicles having electronic anti-lock braking systems although it may be used as a stand alone system.

Two versions are disclosed one of which uses a multi-pulse signal generating device means at each wheel. The other is a single-pulse per wheel revolution generator. Further details of these systems are disclosed in the publications for co-pending European Patent Applications No. 91300939.9 and 91300938.7 both dated 5th February 1991.

The multi-pulse signal based unit of the above described systems have been found to have a minimum speed above which the system operates satisfactorily of about 10 km/h when a wheel speed generator of 96 teeth is used. In the case of a 48 tooth generator the minimum speed is of the order of 18 km/h. Thus multi-pulse systems are restricted from operating satisfactorily at low speeds. By contrast a single pulse per revolution system, which is also described in the earlier applications and is based on time per revolution, has been found to operate well at low speeds but at high speeds, where the time periods to be measured are very short, it is necessary to use comparatively expensive computer systems and thus the system is not financially efficient.

An object of the present invention is to provide a deflation warning system for a tire of the above type, that overcomes the above discussed speed induced limitations by functioning over a wide effective speed range at minimum cost.

One preferred embodiment of the present invention, includes a method for detecting a partially deflated tire on a vehicle having four tires each mounted on a wheel each wheel having a signal generator for producing a series of digital signals or pulses spaced apart by equal increments of angular rotation of the respective tire and wheel assembly. The method first includes calculating vehicle speed from one of series of signals the selecting an appropriate interval for deflation detection, measuring for each of the four wheels a speed of rotation signal for said interval, storing the speed signal found, repeating the measuring for at least three consecutive equal chosen intervals, averaging the speeds to produce an average speed signal for each wheel, processing the four signals in a processing unit which subtracts the sum of the signals from one pair of diagonally opposite wheels from the sum of the signals from the other pair of diagonally opposite wheels, sensing when the magnitude of the result is between 0.05% and 0.6% of the mean of the two sums and when if sensed within the range operating a warning device to indicate a tire is partially or completely deflated.

Further aspects of the present invention will become apparent from the following description of an embodiment of the invention by way of example and in conjunction with the attached diagrammatical drawings in which:

FIG. 1 is a schematic illustration of the four wheel positions on a vehicle.

FIG. 2 is a block diagram of the calculation elements.

FIG. 3 is a flow chart illustrating operation of the apparatus and method

FIG. 1 shows a vehicle having four wheels, the front wheels being No's 1 and 2 and the rear wheels No's 3 and 4. Each wheel is provided with a multi-tooth pulse generator of the same type as used on electronic ABS (Automatic Braking System) equipped vehicles. Suitable generators produce 96 pulses for each revolution of the wheel.

The four output chains of pulses 1', 2', 3', 4' are fed into a car mounted computer illustrated in FIG. 2. Firstly the output signal 1' from the front left hand wheel is fed into a speed calculating unit 5 where the speed of the vehicle is calculated. The resultant speed is then fed to a sense control unit.

The sense control unit 6 determines from a premapped set of characteristics, a suitable appropriate interval for measuring the speed of the vehicle wheels and carrying out the desired deflation sensing check. In the preferred arrangement this interval is a number of pulses. For example 128 pulses is a low speed of typically 4-15 km/h, 768 pulses at 20 to 95 km/h, and 2048 pulses is 50 to 260 km/h. This gives time periods of 0.5 to 2.5 secs for sensing.

Thus having chosen the number of pulses for which measurements will be carried out the pulse counter unit 7 operates taking output signals from each wheel and processing them in timing units. Four separate timing units are provided in the pulse counting unit 7 (one of each of the speed input signals 1',2',3',4'). These log the time from each wheel. As will be appreciated the time for each wheel is directly proportional to the speed of the respective wheel, and these signals are then used to determine whether or not there is a relative deflation on one or more of the tires of the vehicle. An important feature of the present invention is that for different vehicle speeds as measured by the speed calculating unit 5 operating on the speed signal 1' from the front left hand wheel No. 1 , the sense control unit 6 determines a different number of pulses within which the time will be measured for the four wheels. This allows the system to accurately sense a small change in radius caused by a small deflation of the tire (for example 0.6 bar) by choosing the number of pulses for calculating the wheel speeds.

The processing of the four speeds signals come from the pulse counter unit 7 is basically the same as that in the prior applications in that the processing unit 8 calculates factors for the lateral and longitudinal acceleration of the vehicle by comparing the angular velocity signals for the wheels on each side of the vehicle, and then compares the signals from the front and rear pair of wheels with the forward speed calculated from the mean of the angular velocities of all four wheels. The lateral and longitudinal acceleration factors, which may be accelerations or other acceleration dependent factors, are compared with a predetermined value for the vehicle concerned. If an excessive longitudinal acceleration factor is found then the deflation warning calculated is inhibited. If a lateral acceleration greater than .apprxeq.0.06 g is sensed then once again sensing of a deflation warning is inhibited. These calculation avoid false deflation warning signals due to weight transfer and tracking effects caused by a high vehicle lateral and longitudinal accelerations, because such signals cause similar changes in rolling radius to a deflation.

The system then calculates an error signal dT by comparing the times (which are proportional to the angular velocities) for the wheels according to the formula: ##EQU1## where F14=F1+F4 F23=F2+F3 and F1, F2, F3, F4 are the times for the number of wheel pulses chosen. By monitoring the dT signal, the processing unit, prepares to sense deflation if the signal is greater than 0.05% and less than 0.6%. If such a signal is found then the existence of a puncture is known and the next step is to find out which tire is punctured.

The monitoring unit 8 then calculates the difference between each wheel speed in turn and the average speed of the four wheels, and if the difference between any one wheel and the average is more than 0.1%, a second signal is generated.

If both dT and the different signals exist in the ranges mentioned, then a deflation is sensed and the wheel concerned determined. An output signal is then given to the alarm 9 which is conveniently a light on the dashboard of the vehicle to show the deflation and four other lights to show which wheel has the deflation.

As a safety guard against spurious signals the warning signal is preferable delayed until 3 or more consecutive warning results have been obtained in the processing unit 8.

To fully allow for a tire and vehicle characteristics it is necessary to trigger manually or otherwise a calibration initiating signal. This is done after changing one or more tires of the vehicle, when the vehicle is new, and, may in some circumstances, be necessary after reflation although this is not usual.

FIG. 3 represents a flow chart illustrating the steps involved in detecting a deflated tire.

In step 10, pulse 1' is sent to the speed calculation unit 5 where the speed of the vehicle calculated is determined.

In step 11, the sense control unit 6 establishes a value related to the vehicle speed calculated by the speed calculating unit 5 from the chain of pulses from one wheel (in this particular embodiment signal 1' from wheel 1). The sense control unit 6 determines these values as compared to a set of pre-mapped speed related values for which the appropriate measuring intervals for carrying out the deflation sensing check have been set. In one preferred embodiment, these appropriate measuring intervals are the number of pulses to be counted, e.g., for low speed counts 128 pulses, for any intermediate speed counts 768 and for high speed counts 2,048. Thus, the speed control unit determines the appropriate measuring interval based upon the speed of the vehicle. This measuring interval is then sent from the sense control unit 6 to the pulse counter unit 7. In step 12, the pulse counter unit 7 counts the chain of pulses 1', 2', 3' and 4' from each wheel and logs the time taken to receive the appropriate number of signals set by the sense control unit 6 from each wheel. As identified in step 24, the pulse counter unit has four separate timing units, one for each wheel. While these timing units would usually use the same clock frequency for timing, particularly in the case of a single purpose built computer, it is not necessary and different clock frequencies may be used for timing the pulses for different wheels.

As illustrated collectively in steps 13, 14 and 15, the rotation speed of each wheel in the first, second and third intervals are separately stored for determining the average rotational speed of each wheel in step 16.

Once the average rotational speed is determined, in step 17, an error signal DT is calculated. DT represents the formula discussed above.

In step 18, it is determined whether the error signal DT is within the range of between 0.05% to 0.6%.

If DT is not within the range then control is passed back to step 12. If DT is within the prescribed range, control then proceeds to step 9 for a comparison of each wheel with the average wheel speed.

In step 20, it is determined whether one wheel is different from the average by greater than 0.1%. If not, then control passes again to step 12. If the difference exceeds 0.1%, an output signal is sent in step 21.

Step 22 is determined whether three consecutive output signals have been issued. This assists in guarding against false deflation signals or signals which occur falsely as a result of normal use of the tire. If the system detects three consecutive output signals, then the deflation warning alarm is signaled.

As an alternative to the time measuring system which operates as described above the system may select from the calculated vehicle speed a suitable time interval and then for each of the four wheels count the number of pulses and process these and signals in the same way.