VEHICLE HORN AND CONTROL FUNCTION SWITCH

Abstract: A vehicle horn and control function switch has compensation for wide temperature variations and preloads from surrounding material due to manufacturing processes and material aging. The switch has a force sensor (140) and a temperature sensor (150) mounted in the hub (22) of a steering wheel (10). The force sensor is connected to the input of an operational amplifier that has an offsetting input network and an amplifying feedback network that are controlled from a programmable microcontroller. The output of the operational amplifier and the temperature sensor are connected to the programmable microcontroller (310). When an actuating force signal is applied to the force sensor, the force signal at an input to the programmable microcontroller exceeds a predetermined threshold and the programmable microcontroller activates an output, causing a horn or other control function to actuate.

Description: VEHICLE HORN AND CONTROL FUNCTION SWITCH The present invention relates to a vehicle horn and control function switch apparatus and, more particularly, to a vehicle horn and control function switch apparatus for steering wheel applications with compensation for effects of temperature and preloading. The invention also relates to a method of operation for the compensated switch apparatus. The driver side airbag is located in the center of the vehicle steering wheel, along with the horn switch and other vehicle controls. In order to accommodate this addition, automotive designers turned to very thin electronic switch devices for horn actuation that could easily be positioned over an airbag cover. However, these devices suffered from the effects of temperature changes and static forces from adjacent components resulting from the manufacturing process or material aging characteristics . These very thin electronic switch devices are often fabricated from membrane material, piezoelectric elements, or pressure sensitive resistive ink, and require supporting electronic circuitry in order to function US 5 463 258 and US 5,539,259 disclose identical circuits for sensing a resistive change in a force transducer due to applied forces, for turning on a transistor switch, for energizing a relay, and for activating a horn. Neither patent discloses any means of temperature or preload force compensation. For reliable operation over extreme temperature ranges encountered in vehicle applications, compensation schemes are required for the very thin electronic switch devices. One such compensation scheme is described in US 5 576 684. This scheme makes use of a flexible potentiometer that changes resistance as its shape changes as the user presses against the steering wheel hub cover. Another similar approach is described in US 5 398 962. This apparatus comprises a force sensor fabricated from pressure sensitive resistive material and disposed in a cover assembly mounted on the steering wheel. Yet another approach is disclosed in US 5 489 806, which discloses a pressure or deflection sensitive horn switch with temperature compensation to compensate for changes in the plastic airbag cover stiffness due to changes in temperature. The sensors comprise both a temperature sensing device and a horn switch deflection sensing device operated by applied force mounted on the airbag cover surface and connected to an analog to digital converter controlled by a microprocessor. The microprocessor evaluates and compares the temperature and deflection signals to determine if the deflection of the cover surface is sufficient, when compensating for change in cover stiffness due to temperature change of the cover surface, to actuate a horn. Although these approaches provide for some temperature variation, it is not apparent that the wide sensor conductance variations with temperature of these sensing devices can be accommodated. There is also no disclosure of compensation for variations in preload forces due to manufacturing processes or material aging. Another related problem not addressed is the ability to maintain the maximum dynamic range of the force sensor relative to the fixed operating range of the supporting circuitry for all operating temperatures and preload conditions. For reliable operation over extreme temperature ranges encountered in vehicle applications, there is a need for a horn and control function switch temperature compensation means that enables reliable operation over a temperature range of about -40°C to about +85°C. To account for changes in preload forces resulting from the manufacturing process or vehicle component material aging, a horn and control function switch preload compensation means must enable reliable operation over a range of applied force to the switch sensor of from 1 Kg up to 5 Kg. Since, upon an applied force to the switch sensor of from 1 Kg up to 5 Kg, a typical resistance range of a horn and control function switch sensor is from 39 Kohms to 188 Kohms at -40°C, and from 2 Kohms to 12 Kohms at 85°C, there is a need to match this extremely wide dynamic sensor operating range to the fixed operating range of the control circuitry with suitable dynamic range compensation means. In addition to these requirements, the horn and control function switch and associated circuitry must be easily and inexpensively fabricated, assembled, tested, calibrated, and installed. The present invention is directed to an apparatus and method of operation that satisfies these needs. Such a device is disclosed in the appended main claim and variations are set forth in the subordinate claims. BRIEF DESCRIPTION OF THE DRAWINGS These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: FIG.l shows a vehicle steering wheel with an airbag assembly. FIG. 2 shows a cross sectional view of a steering wheel mounted airbag assembly with force and temperature sensors connected to associated electronics circuitry and horn. FIG. 3 shows the resistance characteristics of a typical force sensitive resistive device over a normal range of applied force at three operating temperatures . FIG.4 shows the conductance characteristics of a typical force sensitive resistive device over a normal range of applied force at three operating temperatures . FIG. 5 shows the conductance characteristics of a typical force sensitive resistive device at a temperature of 25°C over a normal range of applied force at four different levels of preload forces. FIG. 6 shows a block diagram of the amplifying and offsetting electronics circuitry for a temperature and preload compensated vehicle horn switch. FIG. 7 shows a programmable microcontroller with associated electronic circuitry for a temperature and preload compensated vehicle horn switch. FIG. 8 shows a block diagram of the amplifying and offsetting electronics circuitry for a temperature and preload compensated vehicle horn and control function switch. FIG. 9 shows a flow chart that depicts the sequence of operational steps performed by the programmable microcontroller. DETAILED DESCRIPTION OF THE INVENTION FIG. 1 is a view of a vehicle steering wheel assembly 10 having an outer perimeter 20 and a hub 22. Within the center of the hub 22 is an airbag assembly 100 molded into the hub 22 with molding material 25. Lines 2-2 are cutting plane lines identifying the location of the sectional drawing shown in FIG. 2. FIG. 2 shows a sectional view of a steering wheel mounted airbag assembly 100 with force sensor 140 and temperature sensor 150. The airbag assembly 100 has an airbag inflator 110 and an airbag 120 contained within a housing 130. Mounted to the housing 130 is a force sensor 140 and a temperature sensor 150 that are electrically connected to associated electronics circuitry 300 by force signal connections 212 and temperature signal connections 210. The airbag assembly 100, force sensor 140, and temperature sensor 150 are enclosed in molding material 25 and mounted onto a steering column 30. The electronics circuitry is electrically connected to a horn 500 by a horn actuation signal 214. Although not shown, there may be more than a single force sensor in the configuration shown in FIG. 2. These other sensors are connected to electronics circuitry in a similar fashion to that shown in FIG. 2, but are used to actuate other control functions such as lighting control functions, entertainment audio control functions, speed control functions, and temperature control functions. FIG. 3 shows the resistance characteristics of a typical force sensitive resistive sensor over a normal range of applied force at three operating temperatures. The upper curve 610 depicts the variation of resistance with applied force from about 1.0 Kg to about 5.0 Kg at a temperature of -40°C. The middle curve 620 depicts the variation of resistance with applied force from about 1.0 Kg to about 5.0 Kg at a temperature of 25°C. The lower curve 630 depicts the variation of resistance with applied force from about 1.0 Kg to about 5.0 Kg at a temperature of 85°C. The wide variations in resistance due to temperature at any given force are apparent from this depiction. FIG. 4 shows the conductance characteristics of a typical force sensitive resistive sensor over a normal range of applied force at three operating temperatures. These curves depict the same information as depicted in FIG. 3, but plotted as conductance which is the reciprocal of resistance. These curves depict more clearly the effective change in the sensitivity or gain with temperature with no preload forces. The upper curve 650 depicts the variation of conductance with applied force from about 1.0 Kg to about 5.0 Kg at a temperature of 85°C. The middle curve 660 depicts the variation of conductance with applied force from about 1.0 Kg to about 5.0 Kg at a temperature of 25°C. The lower curve 670 depicts the variation of conductance with applied force from about 1.0 Kg to about 5.0 Kg at a temperature of -40°C. By connecting an input of a variable gain amplifier to this force sensitive resistive sensor and selecting an appropriate gain of the amplifier corresponding to each temperature shown, the curves could be made to coincide with one another, thus compensating for the variation in conductance (resistance) with temperature. Note that with no preload force, these three curves would have zero conductance at zero applied force, and therefore, passing through the origin. FIG. 5 shows the conductance characteristics of a typical force sensitive resistive sensor device over a normal range of applied force at four different levels of preload forces at a temperature of 25°C. The upper curve 662 depicts the variation of conductance with applied force from about 1.0 Kg to about 5.0 Kg at 25°C with a preload force of 5.0 Kg. The upper-middle curve 664 depicts the variation of conductance with applied force from about 1.0 Kg to about 5.0 Kg at 25°C with a preload force of 3.0 Kg. The lower-middle curve 666 depicts the variation of conductance with applied force from about 1.0 Kg to about 5.0 Kg at 25°C with a preload force of 1.0 Kg. The lower curve 660 depicts the variation of conductance with applied force from about 1.0 Kg to about 5.0 Kg at 25°C with a preload force of 0.0 Kg. This latter curve 660 is the same as the middle curve 660 shown in FIG. 4. The quiescent state of the force sensor is where the applied force of these curves is zero and intersects with the vertical axis. Thus, the quiescent force signal of the upper curve 662 is 5.0 Kg, the quiescent force signal of the upper middle curve 664 is 3.0 Kg, the quiescent force signal of the lower middle curve 666 is 1.0 Kg, and the quiescent force signal of the lower curve 660 is 0.0 Kg. By connecting an input of an amplifier having an adjustable offset to this force sensitive resistive sensor and selecting an appropriate offset of the amplifier corresponding to each preload force shown, the curves could be made to coincide with one another, thus compensating for the variation in conductance (resistance) with preload force. The conclusion from these considerations of FIGS. 4 and 5 is that both temperature and preload variations on the conductance of a force sensitive resistive sensor can be compensated for by connecting the force sensor to an input of an amplifier having adjustable gain and offset, and appropriately adjusting the amplifying (gain) and offsetting of the amplifier. FIG. 6 shows a block diagram of the amplifying and offsetting electronics circuitry for a temperature and preload compensated vehicle horn switch. A force sensor 140 provides a force signal 212 to the input of an amplifier 350 having adjustable amplifying and offsetting capability. The amplifier 350 provides a compensated force signal 328 to a first input 312 of a programmable microcontroller 310. A temperature sensor 150 provides a temperature signal 210 to a second input 313 of the programmable microcontroller 310. The programmable microcontroller 310 provides an amplifying control signal 340 to the amplifier 350 from a first output 315, and an offsetting control signal 330 to the amplifier 350 from a second output 316. The programmable microcontroller 310 also provides a horn activating signal 322 to a horn actuator 320 from a third output 317. The horn actuator 320 provides a horn actuating signal 214 to a horn 500. The operation of the programmable microcontroller 310 may be described by six steps. The programmable microcontroller 310 first reads the compensated force signal 328 at the first input 312 of the programmable microcontroller 310 to determine if the compensated force signal 328 exceeds a predetermined threshold value, indicating an actuating compensated force signal. Second, if the compensated force signal 328 exceeds the threshold value in the first step indicating an actuating compensated force signal, the programmable microcontroller 310 activates the third output 317 to generate a horn activating signal 322 and returns to the first step. Third, if the compensated force signal 328 is less than the threshold value in the first step indicating a quiescent compensated force signal, the programmable microcontroller 310 deactivates the third output 317 to silence the horn 500. Fourth, the programmable microcontroller 310 adjusts the offset of the amplifier 350 by energizing the offset control signal 330 from the second output 316 of the programmable microcontroller 310, based on the quiescent compensated force signal 328 from the first step. Fifth, the programmable microcontroller 310 reads the temperature signal 210 at the second input 313 of the programmable microcontroller 310 and adjusts the amplifying of the amplifier 350 by energizing the amplifying control signal 340 from the first output 315 of the programmable microcontroller 310 based on the temperature signal 210. The control signals from the programmable microcontroller 310 are selected by being connected to ground potential by the programmable microcontroller 310. The sixth step is to return to the first step. FIG. 7 shows a preferred embodiment of the device is shown in accordance with the present inventive concepts. Employing a programmable microcontroller with associated electronic circuitry for a temperature and preload compensated vehicle horn switch. A force sensor 140 provides a force signal 212 to the negative input 354 of a biased inverting operational amplifier 352. The opposite side of the force sensor is connected to a ground potential . The negative input 354 of the biased inverting operational amplifier 352 is also connected to an offsetting input resistor network that determines the offsetting of the biased inverting operational amplifier 352, comprising resistor Rl 360, resistor R2 362, resistor R3 364, and resistor R4 366. The offsetting input resistor network is controlled by a second output 316 of the programmable microcontroller 310, the second output 316 being connected to a first offset control line 332 connected to resistor R4 366, a second offset control line 334 connected to resistor R3 364, a third offset control line 336 connected to resistor R2 362, and a fourth offset control line 338 connected to resistor Rl 360. The opposite side of the resistors are connected together and connected to the negative input 354 of the biased inverting operational amplifier 352. A positive input 356 of the biased inverting operational amplifier 352 is connected to a bias voltage. The biased inverting operational amplifier 352 provides a compensated force signal 328 to a first input 312 of the programmable microcontroller 310. A temperature sensor 150 provides a temperature signal 210 to a second input 313 of the programmable microcontroller 310 and to one side of a voltage divider resistor R9 380, the opposite side of resistor R9 380 being connected to ground potential. The temperature sensor 150 and the resistor R9 380 form a voltage divider configuration that is commonly used with temperature sensors. The opposite side of the temperature sensor 150 is connected to a reference voltage. An amplifying determining feedback resistor network comprising resistor R5 370, R6 372, R7 374, and R8 376 is connected between the negative input 354 and the output of the biased inverting operational amplifier 352. It should be understood that while the preferred embodiment described above utilizes individual resistors and control lines for the gain and offset networks, these functions could also be provided using programmable digital potentiometers such as Xicor's model X9312. The amplifying determining feedback resistor network is controlled from a first output 315 of the programmable microcontroller 310, comprising a first amplifying control signal 342 connected to resistor R5 370 and a second amplifying control signal 344 connected to resistor R6 372. The programmable microcontroller 310 also provides a horn activating signal 322 to a horn actuator 320 from a third output 317. The horn actuator 320 provides a horn actuating signal 214 to a horn 500. The operation of the programmable microcontroller 310 may be described by six steps. The programmable microcontroller 310 first reads the compensated force signal 328 at the first input 312 of the programmable microcontroller 310 to determine if the compensated force signal 328 exceeds a predetermined threshold value, indicating an actuating compensated force signal. Second, if the compensated force signal 328 exceeds the threshold value in the first step indicating and actuating compensated force signal, the programmable microcontroller 310 activates the third output 317 and generates a horn activating signal 322 and returns to the first step. Third, if the compensated force signal 328 is less than the threshold value in the first step indicating a quiescent compensated force signal, the programmable microcontroller 310 deactivates the third output 317 to silence the horn 500. Fourth, the programmable microcontroller 310 adjusts the offset of the biased inverting operational amplifier 352 by selectively energizing the control lines of the second output 316 of the programmable microcontroller 310, comprising the first offset control signal 332, the second offset control signal 334, the third offset control signal 336, and the fourth offset control signal 338. The selection of energizing control lines of the second output 316 of the programmable microcontroller 310 is based on the quiescent compensated force signal 328 from the first step. Fifth, the programmable microcontroller 310 reads the temperature signal 210 at the second input 313 of the programmable microcontroller 310 and adjusts the amplifying of the biased inverting operational amplifier 352 by selectively energizing the first amplifying control signal 342 and the second amplifying control signal 344 from the first output 315 of the programmable microcontroller 310 based on the temperature signal 210. The control signals from the programmable microcontroller 310 are selected by being connected to ground potential by the programmable microcontroller 310. The sixth step is to return to the first step. FIG. 8 is a block diagram of an alternative the amplifying and offsetting electronics circuitry for a temperature and preload compensated vehicle horn and control function switch. Although FIG. 8 depicts only one control function force sensor and one control function actuator, it is apparent to one skilled in the relevant art that there could be a plurality of control function force sensors and a corresponding plurality control function actuators as well. A horn force sensor 140 provides a force signal 212 to the input of an amplifier 350 having adjustable amplifying and offsetting capability. The amplifier 350 provides a compensated force signal 328 to a first input 312 of a programmable microcontroller 310. A control function force sensor 160 provides a force signal 213 to the input of an amplifier 390 having adjustable amplifying and offsetting capability. The amplifier 390 provides a compensated force signal 329 to a third input 311 of the programmable microcontroller 310. A temperature sensor 150 provides a temperature signal 210 to a second input 313 of the programmable microcontroller 310. The programmable microcontroller 310 provides an amplifying control signal 340 to the amplifier 350 from a first output 315, and an offsetting control signal 330 to the amplifier 350 from a second output 316. The programmable microcontroller 310 also provides an amplifying control signal 341 to the amplifier 390 from a sixth output 319, and an offsetting control signal 331 to the amplifier 390 from a fifth output 314. The programmable microcontroller 310 also provides a horn activating signal 322 to a horn actuator 320 from a third output 317, and a control function activating signal 326 to a control function actuator 324 from a fourth output 318. The horn actuator 320 provides a horn actuating signal 214 to a horn 500 and the control function actuator 324 provides a control function actuating signal 216 to a control function 510. FIG. 9 shows a flow chart that depicts the sequence of operational steps performed by the programmable microcontroller. The first step performed by the programmable microcontroller is to read a compensated force signal . The compensated force signal is then compared to a predetermined threshold value. If the compensated force signal is equal to or greater than the predetermined threshold value indicative of an actuating compensated force signal, the horn is actuated and control is returned to the first step. If the compensated force signal is less than the predetermined threshold value indicative of a quiescent compensated force signal, the horn is silenced. Next, the programmable microcontroller selects output control signals to adjust an input resistor network of a biased inverting operational amplifier to provide offset compensation based on the quiescent compensated force signal. Next, the programmable microcontroller reads a temperature signal. The programmable then selects output control signals to adjust a feedback resistor network of a biased inverting operational amplifier to provide amplifying compensation based on the temperature signal. Control is then returned to the first step. Advantages of the present invention include reliable operation over extreme temperature ranges and a compensation scheme that accounts for changes in preload forces resulting from the manufacturing process or vehicle component material aging. This temperature and preload compensation scheme also reduces the resolution requirements of the electronic circuitry resulting in reduced costs. By providing dynamic compensation, the compensation scheme also enables optimization of the dynamic range of the force signal at the input to the programmable microcontroller. This arrangement also provides for ease of initial setup in the manufacturing operation. The result is an apparatus that is easily and inexpensively fabricated, assembled, tested, calibrated, and installed.

Claims: CLAIMS 1. A vehicle horn and control function switch, comprising: (a) a means (140) for measuring a force applied to a surface of a steering wheel and generating a force signal; (b) a means (150) for measuring a temperature of the surface of the steering wheel and generating a temperature signal; (c) a means (300) for receiving the force signal and amplifying and offsetting the force signal; (d) a means (300) for receiving the temperature signal and compensating the amplifying and offsetting of the force signal depending upon the temperature signal and a quiescent force signal; and (e) a means (310) for determining when the compensated force signal exceeds a threshold value and actuating a horn (500) . 2. The vehicle horn and control function switch described in claim 1 wherein the means (140) for measuring a force applied to a surface of a steering wheel is a pressure sensor. 3. The vehicle horn and control function switch described in claim 1 wherein the means (140) for measuring a force applied to a surface of a steering wheel is a force sensing resistive device. 4. The vehicle horn and control function switch described in claim 1 wherein the means (150) for measuring a temperature of the surface of the steering wheel is a temperature sensor. 5. The vehicle horn and control function switch described in claim 1 wherein the means (150) for measuring a temperature of the surface of the steering wheel is a thermistor. 6. The vehicle horn and control function switch described in claim 1, wherein the means for amplifying and offsetting the force signal is a biased inverting operational amplifier with adjustable offset and gain. 7. The vehicle horn and control function switch described in claim 1, wherein the means (310) for receiving the temperature signal and compensating the amplifying and offsetting of the force signal is a programmable microcontroller. 8. The vehicle horn and control function switch described in claim 1, wherein the means for receiving the temperature signal and compensating the amplifying of the force signal is by a programmable microcontroller that selects resistors in a feedback network of a biased inverting operational amplifier based on the received temperature signal, the compensated force signal at the biased inverting operational amplifier output being applied to an input of the programmable microcontroller. 9. The vehicle horn and control function switch described in claim 8, wherein the resistors in a feedback network are a programmable digital potentiometer in a feedback network. 10. The vehicle horn and control function switch described in claim 1 wherein the means for receiving the temperature signal and compensating the offsetting of the force signal is a programmable microcontroller (310) that selects resistors in an input network of a biased inverting operational amplifier based on the received quiescent force signal, the compensated force signal at the biased inverting operational amplifier output being applied to an input of the programmable microcontroller. 11. The vehicle horn and control function switch described in claim 10, wherein the resistors in an input network are a programmable digital potentiometer in an input network. 12. The vehicle horn and control function switch described in claim 1, wherein the means for determining when the compensated force signal exceeds a threshold and actuating a horn (500) is a programmable microcontroller. 13. The vehicle horn and control function switch described in claim 1, wherein the means for determining when the compensated force signal exceeds a threshold value and actuating a horn further comprises a means for determining when the compensated force signal exceeds a threshold value and actuating a control function selected from the group consisting of a lighting control function, an audio entertainment control function, a speed control function, and a temperature control function.

Inventors: MONROE TEX K, PODOLOFF ROBERT M, MONROE, TEX, K., PODOLOFF, ROBERT, M.
Applicants: BREED AUTOMOTIVE TECH, BREED AUTOMOTIVE TECHNOLOGY, INC.
Application year: 1999
Main classification: B60R21
IPC codes: B60Q5, B60R21