Patent Application
20060265112
The present invention relates to a method and system for monitoring vehicle speed and more particularly to a method and system for calibrating a vehicle speed.
Vehicle speed, as displayed on or by a vehicle speedometer, is based on wheel speed as sensed by wheel speed sensors at each wheel of the vehicle. A control module determines vehicle speed based on the number of revolutions of a wheel, as indicated by the wheel speed sensors, calculated over a predetermined period of time. The accuracy of the vehicle speed is dependent upon the accuracy of the size of the tire associated with the wheel speed sensor. Thus, since the tire size associated with the wheels is known by the manufacturer of the vehicle at the time of assembly of the vehicle, the speed of the vehicle is pre-calibrated based on the known tire size.
If a customer replaces the tires with non-standard tires, such as aggressive off-road tires or snow tires, the speedometer display can be inaccurate if the control module is not updated with the new tire size. It is possible to have the vehicle speed recalibrated upon installation of new tires, as suggested in U.S. Pat. No. 6,313,742. However, such a system requires an affirmative action taken in order to recalibrate the vehicle speed determination including actuation of the calibration system as well as the entering of specific tire data.
Thus, it is an object of the present invention to provide a method and system for automatically calibrating a vehicle speed, as may be displayed on a vehicle speedometer, to accommodate any tire size without requiring a user to initiate the calibration.
The objects, features, and advantages of the invention will be apparent from the written description that follows. The invention provides a method and system for automatically calibrating the speed of a vehicle. Wheel speed sensors coupled to each of the wheels of the vehicle provide a first vehicle speed based on an average tire size associated with the wheels. A GPS sensor provides a second vehicle speed. A control module automatically calibrates the speed of the vehicle based on a comparison of the first and second vehicle speeds, which can then be displayed on a vehicle speedometer. Automatic calibration of the vehicle speed is initiated upon ignition start of the vehicle after predetermined events have occurred.
The tire size can be updated to insure that the vehicle speed is accurate. An estimated tire size error is determined based on the first and second vehicle speeds. The estimated tire size error is compared to a predetermined error threshold. If the tire size error exceeds the error threshold, then the tire size value is corrected for future use in determining the speed of the vehicle.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.
The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:
The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.
Referring now to the drawings, a preferred embodiment of the invention and its manner of use will be described.
Further coupled to the vehicle 10 is a GPS sensor 20 for use by the control module 16 in determining a second vehicle speed. GPS sensor 20 provides position information to the control module 16. Control module 16 then determines vehicle speed by dividing the change of position by a predetermined time period.
The control module 16 can then compare the two vehicle speed values to determine a tire size error. If the tire size error exceeds a predetermined threshold error, then the tire size data stored in the control module 16 is updated for future use in determining vehicle speed via the wheel speed sensors 14.
Turning now to
Next, at step 114, the wheel speed data is read from wheel speed sensors 14. The vehicle speed, SWSS, is then determined at 116 by dividing the average wheel speed by the average tire size. At 118, if a display 18 is available, the vehicle speed is displayed on display 18.
The method proceeds to step 120 where it is determined if the vehicle is traveling in a straight direction. This condition must be met before the method continues in order for the calibration of the vehicle speed to be reliable. Determination of whether or not the vehicle is traveling in a straight direction can be done by determining a distance traveled by each of the wheels, i.e., the number of revolutions over a predetermined period of time, and determining if any of the distances traveled differs from any of the other values since they would be equal if traveling in a straight direction. Alternatively, if a vehicle has a steering position sensor (not shown), then an input from that sensor could be used to determine straight line direction. If a determination is made that the vehicle is traveling in a straight direction, the method proceeds to step 122 and the vehicle speed, SGPS, is determined using the GPS sensor 20 as described above. If the vehicle is not traveling in a straight direction, the method returns to step 114.
As another measure to insure reliability of the vehicle speed calibration, at step 124, the method proceeds to determine if the vehicle is traveling reasonably, e.g., no parking lot speeds. Thus, the vehicle speed as determined by the GPS sensor 20 is compared to a predetermined speed threshold, such as 20 mph in the preferred embodiment. If the vehicle speed threshold is not exceeded, the calibration returns to step 114.
If the vehicle speed threshold is exceeded, the method proceeds to step 126 so that a tire size error, if any, can be determined. This would be the case if new tires were installed that were not equivalent in size to tires originally installed by the vehicle manufacturer and pre-calibrated into the control module 16. Tire size error, ETS, is determined according to the following, ETS=(Average Wheel Speed/SWSS)−(Average Wheel Speed/SGPS).
The tire size error is compared to a predetermined threshold at step 128. In the preferred embodiment, the threshold is 4 revolutions/mile. If the tire size error does not exceed the threshold, the method returns to step 114. If, on the other hand, the tire size error does exceed the threshold, then a tire size correction term, TCOR, is determined at step 130 by multiplying the tire size error by a proportional control, e.g., 5%. It is preferable to adjust the new tire size over several passes of the algorithm rather than immediately in one step, therefore, only 5% of the error is corrected during each pass of the method. Thus, any single GPS reading that may be in error is filtered out.
The new tire size value is then determined at 132 by subtracting the tire size correction term to the original tire size. The method then returns to step 114 and repeats the process if the ignition remains on as indicated at step 134. If the ignition is turned off, the new tire size value is stored in memory and the method ends until the ignition is turned back on. Thus, the vehicle speed is constantly recalibrated to take into consideration tire sizes that may not be standard to the vehicle.
The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.
