This invention relates to a method of calibrating vehicle speed signals.

It has become increasingly common for modern vehicles to be equipped with electronic speed sensors that generate an electromagnetic signal which varies in response to the speed of a corresponding wheel. These wheel speed signals are used, for example, in adaptive braking and/or adaptive traction systems to effect electronic control of the vehicle's brakes to prevent skidding and to transfer power from one wheel to the other if one wheel loses traction on the driving surface. Wheel speed signals may also be used to drive an electronic speedometer.

In general, wheel speed sensors consist of an electromagnetic pickup mounted adjacent a toothed tone wheel which is mounted for rotation with the corresponding vehicle wheel. The change in magnetic reluctance due to the constantly changing magnetic path depending upon whether a tooth or a space is moving across the electro- magnetic pickup induces a pulsed output from the electro¬ magnetic pickup. This output signal is transmitted through a calibration circuit which generates a square wave output signal from the input signal generated by the electromagnetic pickup. The frequency of this square wave signal is a function of instantaneous wheel speed. The automotive industry has adopted a standard for electronic speedometers and other devices in which such devices are designed to receive a nominal number of pulse counts per unit of distance traveled by the vehicle. The electronic speedometer is designed to indicate speed as a function of the frequency of this signal. However, the number of pulses generated by any typical speed sensing unit per unit of distance traveled can vary substantially, depending upon the size of the tire, diameter and number of teeth in the tone wheel, and tire inflation.

Accordingly, a calibration factor must be used to convert the actual pulse counts per unit of distance traveled by

the wheel into a signal that the electronic speedometer is designed to display that accurately reflects vehicle speed.

Determination of this calibration factor has proven to be in practice quite difficult. Of course, the correction factor can be calculated from known information, including the number of teeth on the tone wheel, rolling radius of the tire, etc., but this calculation is only approximate at best, and cannot take into account varying factors, such as tire inflation. Furthermore, a change in tire size will affect the calibration factor, and since changes in tire size are not recognized by the vehicle operator as affecting the speedometer calibration, it is doubtful that recalibration would be effected. Another way of establishing a calibration factor is to count the pulses as the vehicle is driven, calculate the calibration factor, and then set dip switches in the electronic hardware to set the new calibration factor. Obviously, this method also has disadvantages, and is not likely to be performed, on a regular basis. The present invention permits establishment of the calibration factor to be effected quickly and easily by the vehicle operator, does not require complicated calculations, and does not require special instruments or the setting of switches inside of the electronic hardware. The calibration procedure is made sufficiently simple that the vehicle operator can recalibrate the speedometer on a regular basis, thus insuring an accurate calibration if tire sizes are changed, or tire inflation is changed.

These and other advantages of the invention will become apparent from the following description, with reference to the accompanying drawings, in which;

Figure 1 is a diagrammatic illustration of the vehicle wheel speed sensor, the conditioning circuit, the microcontroller, and associated switches used for perfor- ming the method according to the present invention; and Figure 2 is a diagrammatic illustration of the sequence comprising the method pursuant to the present invention.

Referring now to the drawing, a wheel speed sensor generally indicated by the numeral 10 includes a toothed tone wheel 12, which is mounted for rotation proportional to the wheel rotation of a vehicle, and a conventional electromagnetic pickup 14. The magnetic pickup 14 responds to the change in reluctance of the magnetic path due to whether a tooth or a space between two adjacent teeth of the tone wheel 12 is brought into registry with the end of the magnetic pickup 14 to generate an output signal which varies in response to the speed of rotation of the tone wheel 12. This signal is fed through a conditioning circuit 16, which generates a square wave output indicated at 18, in which the pulse width will be a function of the speed of rotation of the tone wheel 12. The signal 18 is fed to a high speed input terminal 20 of a microprocessor 22. Microprocessor 22 generates a signal on output port 24 which drives an electronic display 26 in the operator's compartment of the vehicle to display vehicle speed. The microprocessor 22 creates the signal actuating the display 26 in response to the pulsed output signal 18 and a calibration factor as will be hereinafter described.

By industry standard, the display.26 is designed to respond to a nominal pulse count of 30,000 pulses per mile (18,641 pulses per kilometer). However, the number of pulses actually generated per mile by the wheel speed sensor 10 will be a function of the number of teeth on the tone wheel, tire size, tire inflation, etc. Accordingly, the microprocessor 22, in addition to measuring the signal frequency, also uses a calibration factor stored internally in 34 in order to calculate the signal transmitted to the display 26. Accordingly, the present invention relates to a method for determining this calibration factor. As discussed hereinabove, prior art method were not satis- factory for a number of reasons. Accordingly, the microprocessor 22 includes terminals connected respectively to the vehicle ignition switch 28, a calibration switch 30, and a momentary contact pushbutton switch 32. When

recalibration of the calibration constant is effected, the momentary contact switch 32 is momentarily closed after which the calibration switch 30 is closed and the vehicle is driven through a measured distance. Referring now to Figure 2, the manner in which the calculation of a new calibration factor is effected will be described in detail. When the vehicle operator effects a calibration, he depresses the momentary contact switch 32, as indicated at 34 in Figure 2. At the beginning of the aforementioned measured distance, the calibration switch 30 is turned on as indicated at 36. The microprocessor 22 then begins counting pulses, as indicated at 38. The microprocessor 22 continues to count pulses, until the calibration switch 30 is turned off, as indicated at 40 in Figure 2, which occurs at the end of the measured distance. At that time, counting is stopped, as indicated at 42, and the number of pulses is stored in memory as indicated at 44. The new calibration constant is calculated, as indicated at 46, as equal to the nominal number of pulse counts P (30,000 pulses per mile), divided by the number of pulses stored in memory at 44.

Accordingly, the microprocessor 22 will now retain in memory both the old calibration constant (hereinafter referred to as Fo) and the new calibration constant (hereinafter referred to as F„n) .

Microprocessor 22 then calculates a reference speed RS, using the old calibration constant and a predetermined speed as indicated at 48. The predetermined speed is a speed stored in memory, for example, 55 miles per hour (88 kilometers per hour) to effect the calculations indicated at 48 and 50 in Figure 2. As indicated at 50, the same predetermined speed which was used to calculate the reference speed RS, as indicated at block 48, is also used to calculate reference speed RS, in block 50. However, RS ₂ is calculated using the new calibration constant F . As indicated at 52, the error is calculated as being equal to the difference between R_^ and RS-. This error which may be a

positive or negative, is then tested as indicated at 54, to determine if it is within predetermined narrow limits. If the error is not within the limits, the calculation is aborted as indicated at 56. If the error is within the limits, the error, since it normally would be quite small, is multiplied by a predetermined constant C at step 58 to form Error _2< If the error calculated at step 52 is, for example, a plus 1 mile per hour, and the predetermined constant C is 5, Error ₂ at step 58 will be equal to a plus 5 miles per hour, so that a plus 1 mile per hour deviation in the reference speed between calculations using the old and new calibration constant will appear as a plus 5 miles per hour error in the calculation in block 60. As indicated at 60, a display error speed DSP is calculated by adding Error- to a predetermined zero error speed. Zero error speed used at block 60 may be the same as or different than the reference speed used to calculate RS, and RS ₂ - For example, if the predetermined zero error speed to be used is 45 miles per hour (72 kilometers per hour), and Error ₂ is a plus 5, the display error speed DSP will be 50 miles per hour (80 kilometers per hour). As indicated at 62, the vehicle operator is required to close the calibration switch 30 while vehicle speed is zero in order to display on display 26 the display error speed DSP, which may be greater than or less than the zero error speed (chosen arbitrarily here as 45 miles per hour). Since the vehicle operator knows the predetermined zero error speed (chosen here as 45 miles per hour), the magnitude of difference between the display error speed DSP and the zero error speed allows the vehicle operator to determine the acceptability of the calculation of the new calibration constant.

If the vehicle operator decides that the new calibration is not acceptable in view of the display error speed DSP displayed as indicated at 64, being significantly different than the known zero error speed, indicating a procedure error or major change in the vehicle, the operator may abort the calibration as

indicated at 66 by turning the ignition switch 28 off, as indicated at 70. If the DSP is acceptable to the vehicle operator, the vehicle operator momentarily closes the momentary switch 32 as indicated at 68. When this occurs, the new calibration constant F.n replaces the old calibration constant F _Q , as indicated at 72. Accordingly, future speedometer displays will be calculated using the new calibration constant.