The present invention relates to a method of calibrating an acceleration sensor and more particularly to providing a multiple point calibration system.
Acceleration sensors, also called accelerometers, are typically used in motor vehicle applications including, for example, anti-lock braking systems. Signals from the acceleration sensor indicating the instantaneous acceleration of the vehicle are used in the control logic for controlling anti-lock braking.
An acceleration sensor senses linear acceleration parallel to its sensing plane. Internally, some known acceleration sensors contain a silicone proof mass that is suspended between two stationary capacitor plates by cantilever beam springs. When the sensor body is accelerated, the movement is transmitted to the mass via the springs. The springs bend to balance the inertial force of the mass. Due to this bending the relative position of the mass between the two plates is changed. This change causes a change in the capacitance between the two plates which is measured. The internal sensor electronics calculate the acceleration by solving the engineering formulas for force equals a spring constant times displacement and force equals mass times acceleration. Capacitance is a function of displacement and can be measured and converted to a voltage that is readable by an analog-digital converter. The sensor further contains certain parameters for converting capacitance to voltage. The parameters include acceleration-to-voltage ratios and the zero position output value. Thus, capacitance can be converted to voltage which can then be converted to acceleration.
In vehicle brake systems, it is important to have an accurate indication of instantaneous acceleration to provide accurate and consistent braking control actions. In the past, it has been known to calibrate an acceleration sensor with the sensor at a zero degree angle and using only a single point of reference. Thus, correction is only provided for the zero position output voltage at a zero degree angle. Known acceleration sensors for vehicles do not account for a sensor being at an inclined angle, including accounting for a pitch angle or a yaw angle.
The present invention is directed to a method of calibrating an acceleration sensor for a vehicle, the acceleration sensor being connected to an electronic control unit, comprising the steps of positioning the acceleration sensor at a first angle A1 and communicating the first angle A1 to the electronic control unit. Measuring a first output voltage V1 of the acceleration sensor at the first angle A1. Positioning the acceleration sensor at a second angle A2 and communicating the second angle A2 to the electronic control unit. Measuring a second output voltage V2 of the acceleration sensor at the second angle A2 and calculating a sensitivity factor as a ratio equal to a difference between the first and second output voltages V2xe2x88x92V1 divided by a difference between the first and second angles A2xe2x88x92A1. Another step includes calculating a zero position output voltage which is equal to the first output voltage minus a product of the sensitivity factor times a sine of the first angle.
A further method according to the present invention includes installing the acceleration sensor onto a vehicle and then further calibrating the sensor comprising the steps of: placing the vehicle at a predetermined third angle A3 and communicating the third angle A3 to the electronic control unit. Measuring a third output voltage of the acceleration sensor at the third angle A3. Calculating a second sensitivity factor as a ratio equal to a difference between the third output voltage and one of the first and second output voltages V3xe2x88x92V2 (or V1) divided by a difference between the third angle and one of the first and second angles A3xe2x88x92A2 (or A1). Also, calculating a second zero position output voltage which is equal to the third output voltage minus a product of the sensitivity factor times a sine of the third angle.
Any or all of the first, second and third angles can be pitch angles and/or yaw angles.
According to another embodiment of the present invention, there is a method of determining a characteristic line for an acceleration sensor comprising: positioning the acceleration sensor at a first angle A1 and measuring a first output voltage V1 of the acceleration sensor at the first angle A1. Positioning the acceleration sensor at a second angle A2 and measuring a second output voltage V2 of the acceleration sensor at the second angle A2. Calculating a sensitivity factor, m, as a ratio according to the equation:
m=(V2xe2x88x92V1)/(A2xe2x88x92A1);
calculating a zero position output voltage V0 according to the equation:
V0=V1xe2x88x92(mxc3x97sin(A1));
determining a characteristic line for the acceleration sensor using the equation:
Vi=V0+(mxc3x97Xg)
wherein variable Vi is instantaneous output voltage, V0 is the zero position output voltage, m is the sensitivity factor and Xg is an amount of instantaneous acceleration force.
Optionally, but preferably, the method of calibrating an acceleration sensor for a vehicle includes an acceleration sensor having a silicon proof mass resiliently suspended between a pair of capacitor plates and connected to an electronic control unit for accurate sensing of vehicle acceleration. In addition to the multiple steps of communicating, measuring and calculating various parameters, the present method contemplates selectively storing any or all parameter values.
Thus, the present invention provides a method for more accurately calibrating an acceleration sensor by utilizing two or more reference points that correspond to different angular positions of the acceleration sensor. In addition, a further calibration is contemplated after installation of the sensor on a vehicle. Accordingly, the present invention provides a method for calibrating an acceleration sensor that improves the accuracy of the sensor output and the accuracy and efficiency of the accompanying vehicle systems, such as braking control, which rely on acceleration input values.