Description

The invention relates to a method for generating a horn actuation signal using a load cell being placed inside a steering wheel and an apparatus for carrying out this method according to claim 7.

Technical background

In almost every steering wheel of a passenger car, a bus or a truck, a horn switch is located. For this purpose a movable part of the steering wheel is held on a non-movable part of the steering wheel in such a way that it can be pressed down against the force of a spring. This non-movable part is often the so-called steering wheel body and the movable part is often a part of an airbag module, for example the housing of the module (so called floating module concept) or the cover of the airbag module (so called floating cover concept). Often more than one horn switch is present, but for the sake of simplicity reference is made to only one such horn switch in the following.

In the simplest case, the horn switch is comprised of a contact at the stationery part of the steering wheel and the mating contact on the movable part of the steering wheel. As long as no force is applied to the movable part, said contacts are spaced part from another by means of the spring. If the movable part is pressed down against the force of said spring the two contacts come in contact with one another and close a circuit which leads to an actuation of the horn. One drawback of this design is that the contacts can be subjected to mechanical or electrical wearout. The generic EP 2 326 534 B1 suggests to use a load cell in form of a strain gauge in a horn switch of a steering wheel. Load cells are resistors that change their electrical ohmic resistance in response to mechanical stress applied to them. A strain gauge is an example of a load cell. In the steering wheel de- scribed in EP 2 326 534 B1 , a strain gauge is placed between the stationery part of the steering wheel and a force transmitting part of the steering wheel - which is a movable part - in such a way that the strain gauge is stressed when the movable part is pressed down against the stationery part. The change of the electrical resistance of the strain gauge can be measured and the result of the measurement can be used for generating the horn actuation signal. In order to generate the horn actuation signal in response to the change of the electrical resistance of the strain gauge one needs an electrical circuit, for example in form of a control and an evaluation unit. In the generic EP 2 326 534 B1 , the electrical resistance of the strain gauge is measured by using a Wheatstone bridge with the strain gauge being part of said wheat Wheatstone bridge. At the time the vehicle is started (and the horn is not pressed down) this control and evaluation unit measures the resistance of the strain gauge in its (not stressed) basic state and computes a zero value from this voltage. The resistance of the strain gauge is permanently determined by measuring an actual voltage and this actual voltage is compared to this zero value. The measured changes are interpreted and a horn actuation signal is derived from this. Starting from this prior art it is the task of this invention to provide an enhanced method for generating a horn actuation signal using a load cell. Especially, it is a task to provide a more reliable horn actuation signal and to make it possible to use cost effective standard electronic elements. This task is solved by a method as defined in claim 1 and by an apparatus as defined in claim 7. It has been found out that - at least as long cost effective standard electric and electronic components, especially load cells, are used - a single calibration at the time at which the vehicle is started, is often not sufficient in order to generate the horn signal in a reliable way. So, according to the invention it is suggested to permanently calibrate the system by means of a closed circuit. This is achieved by the following measures: An adjustable voltage generated by an adjustable voltage generation means is provided such that the actual voltage depends additionally to the ohmic resistance of the load cell on that adjustable voltage. The result of the measurement of the actual voltage is permanently compared to a defined value, wherein "permanently" in most cases mean with a certain frequency. In response to the difference between the result of the measurement of the actual voltage and the defined value, the adjustable voltage is permanently adjusted, such that a closed loop for the actual voltage is generated and a permanent calibration takes place, at least as long as no horn actuation signal is generated.

By means of that permanent calibration, changes of the environmental parameters, especially of thermal drift, can be compensated. The method works with price effective standard electronic parts.

It is preferred to run an initialisation before the "regular" mode is started. During this initialisation a start value for the actual voltage is searched and the generation of the horn actuation signal is preferably deactivated.

Since the actual voltage can vary relatively strong when the force transmission part acts on the load cell, it is preferred that the permanent adjustment of the actual voltage is interrupted when a horn actuation is detected.

Further preferred embodiments of the invention are defined in the further subclaims and follow from the preferred embodiment described below in view of the drawings. The invention is now explained in detail by means of a preferred embodiment in view of the drawings.

The figures show the following:

Figure 1 shows a schematic view of a part of a steering wheel comprising a horn switch and a schematic representation of a circuit for the horn switch.

Figure 2 what is shown in Figure 1 when a movable part of the steering wheel is pressed down against a stationery part, alternative embodiment in a representation according to Fig

Figure 4 what is shown in Figure 3 in a state according to Figure 2,

Figure 5 a still schematic but more detailed representation of the electronics used for generating a horn actuation signal based on the ohmic resistance of a load cell being part of the horn switch.

Figure 1 shows a very schematic representation of a part of a steering wheel comprising a horn switch, as well as an also very schematic representation of the electronics for this horn switch. This steering wheel comprises a stationery part 10 which can be part of a steering wheel body, and a force transmission part 14 in form of a movable part 14' being held on the stationery part 10 of the steering wheel and being connected to the stationery part of the steering wheel 10 by means of a spring 18. The movable part 14' can especially be a part of an airbag module, for example a housing of an airbag module or a cover of an air- bag module. A horn switch is provided such that a horn actuation signal can be generated when the movable part 14' of the steering wheel is pressed down. This horn switch comprises a load cell 20 in form of a strain gauge 20' fixed to the stationery part of the steering wheel 10 and a post 16 extending from the movable part 14' such that when the movable part 14' is pressed down this post acts on the strain gauge 20' and the strain gauge is stretched and thus its ohmic resistance is increased (Figure 2). Of course other mechanical constructions can be chosen, for example the strain gauge 20' can be fixed to the movable part 14', or the strain gauge can extend between the stationary part 10 and the movable part 14'. The only essential thing is that the strain gauge 20 is stressed (in the example of Figures 1 and 2 deformed), when the movable part 14' is pressed down such that it changes its ohmic resistance. So the mechanical layout shown in Figures 1 and 2 is only one example.

Figures 3 and 4 show an example in which the force transmitting part 14 does not need to be movable and which does not need a spring. Here, the load cell 20 is permanently in direct contact of both: the stationary part 10 and the force transmissitting part 14. The stationary part 10 and the force transmitting part 14 can be directly connected to each other at least in sections, as is shown in Figure 3, so that no clearance between the stationary part 10 and the force transmitting part 14 is needed. When a pushing force is applied to the force transmitting part 14 in the area of the load cell (Figure 4), this force is transmitted by the force transmitting part 14 to the load cell 20 such that the load cell 20 is stressed an its ohmic resistance changes. During the load transmission, the load transmitting part 14 can be slightly elastically deformed.

The load cell 20 is electrically connected to an electronic circuit which can be looked at as being comprised of a constant voltage source 30 and a control and evaluation unit which has an output 48a for the horn actuation signal. In praxis most of the control and evaluation unit as well as the constant voltage source 30 can be a part of a micro controller 60 as is shown in Figure 5. But basically it would also be possible to design this control and evaluation unit of discrete elements, but still it should be emphasised that the use of a micro controller is preferred. Although at least partially integrated in in a micro controller, the functional elements are referred to as "units" in Figure 5 and in its description for the sake of linguistic clearness. But since this "units" do not need to be physical units, the more general term "means" is used in the claims. With reference to Figure 5 a preferred embodiment of the electronics (meaning basically the control and evaluation circuit) is described. As has already been mentioned most of the components can be integrated in a micro controller 60, but in order to explain the electronic circuit, discrete units are shown although most of them can be part of the aforementioned micro controller 60.

First, the constant voltage source 30 is connected to the load cell 20 via a resistor 32, which can for example have a resistance of 10kQ. The load cell 20 is connected to the resistor 32 and to ground and can for example have an ohmic resistance of for example 480Ω in its basic state. The voltage between the load cell 20 and the first resistor 32 is supplied to a differentiating circuit 40. This voltage is referred to as raw voltage Vr since it only depends on the constant voltage, the resistance of the resistor 32 and the resistance of the load cell 20. The other input of the differentiating circuit 40 is connected to an adjustable voltage generation unit 34 whose output voltage is denoted as adjustable voltage V _a . This adjustable voltage generation unit 34 is basically a digital to analogue (A/D) converter and in the embodiment shown it is comprised of a pulse width modulation (PWM) output 36 of the microcontroller 60 and of a filter 38 which "flattens" the PWM signal. This kind of generating an adjustable voltage is known in the art.

The differentiating circuit 40 subtracts either the raw voltage Vr from the adjustable voltage Vad or vice versa, such that the output voltage of this differentiating circuit 40 is the difference between these two voltages. This voltage is referred to as actual voltage V _a . In the embodiment described, the raw voltage V _r is subtracted from the adjustable voltage Vad. This actual voltage V _a is amplified by an amplification unit which is in this embodiment made up of two operational amplifiers 42 and 44. The output voltage of this amplification unit - referred to as am- plified actual voltage Vamp -, which is a direct function of the actual voltage V _a , is fed into an analogue to a digital (A D) converter 46. So, the amplification unit and the A/D converter form a measuring unit (or means) for the measurement of the actual voltage Va. The digital output signal of this A/D converter represents the result of the measurement of the actual voltage. The digital output of the A/D converter 46 is connected to a horn actuation detection unit 48 and to a voltage correction unit 50. The horn actuation unit 48 has an output 48a for out- putting the horn actuation signal and the voltage correction unit 50 has an out- put which is connected to the adjustable voltage generation unit 34. Further, the horn actuation detection unit 48 and the first voltage correction unit 50 can be bi-directionally connected to one another.

The modes of operation of the described circuit are as follows:

There are two operation modes, namely the non-permanent initialisation mode which can for example be performed every time when the vehicle is started, and a permanent operation mode which is performed during driving of the vehicle, after the initialisation mode is finished. First, the initialisation mode is described:

The aim of this initialisation mode is to adjust the adjustable voltage Vad such that the actual voltage V _a (or respectively the amplified first voltage Vamp) is adjusted such that the result of the measurement of the actual voltage (meaning the digital output) is inside an interval around a defined value. For the further explanation is it assumed that the differentiating circuit 40 subtracts the raw voltage V _r from the adjustable voltage Vad such that the actual voltage V _a is positive as long as the value of the adjustable voltage Vad increases the value of the raw voltage Vr. When the initialisation mode is started, the adjustable voltage generation unit 34 generates a starting voltage of the adjustable voltage Vad. This starting voltage is - in the embodiment described - selected such that it is surely higher than the raw voltage Vr, so that a positive actual voltage Va is generated. This actual voltage V _a is amplified via the amplification unit and the amplified actual voltage Vamp is fed into the A/D converter 46. The digital signal from the A D converter - which corresponds to the result of the measurement of the actual voltage V _a - is fed into the voltage correction unit 50 and compared to a defined value, which corresponds to a set voltage. In the following, "comparing the actual voltage to the set voltage" and "comparing the result of the measurement of the actual voltage to a defined value" is used interchangeably, since the meaning is the same. If it is found that the actual voltage is higher than the set voltage, the first voltage correction unit 50 controls the adjustable voltage generation unit 34 in such a way that the adjustable voltage Vad is reduced. In the embodiment shown this means that the pulse widths outputted from the output of the microcontroller are reduced. So, a new actual voltage V _a is generated and again compared with the set voltage. If it is found that the actual voltage is lower than the set voltage, the voltage correction unit 50 controls the adjustable voltage generation unit 34 such that the adjustable voltage Vad is increased (meaning that the pulse widths of the PWM output are increased by one or several steps), if it is found that the actual voltage is higher than the set voltage, the first voltage correction unit 50 controls the adjustable voltage generation unit 34 such that the adjustable voltage Vad is decreased (meaning that the pulse widths of the PWM output are decreased by one or several steps), and so forth. This adjustment of the adjustable voltage Vad is executed with a defined frequency of for example several kHz. This frequency must be distinguished from the frequency of the PWM signal which is usually by more than a magnitude higher. This initialisation mode is run until the result of the measurement of the actual voltage is stably inside an interval around the defined value (or in other words: until the actual voltage V _a has stably reached a value which is inside an interval around the desired set voltage). During the initialisation mode it is preferred that the adjustment performed by the voltage correction unit 50 starts with a large interval and then cuts the preceding intervals in half every round. This leads to a quick finding of the searched actual voltage. During this initialisation mode the horn actuation detection unit 48 is disabled.

After the initialisation mode is finished, the system switches to the permanent operation mode. During this mode, the system continues to adjust the actual voltage V _a to the desired set voltage as described above via the closed loop as described above. During the permanent operation mode it can be preferred that the adjustment performed by the voltage correction unit 50 takes place in con- stant intervals of for example one PWM step. In an alternative, the size of the intervals can vary in dependence of the measured difference between the actual voltage V _a and the set voltage. So, changes of the basic resistance of the load cell 20 occurring because of, for example, changes in temperature, are compensated. When the hom actuation detection unit 48 detects a change of the actual voltage V _a that indicates the actuation of the horn by the driver, it sends out a horn actuation signal via its outlet port 48a. Additionally, it stops the closed loop adjustment, so that the adjustable voltage Vad is kept constant such that the system does not try to adjust the actual voltage Va to the set voltage, as long as the load cell 20 is stressed due to a pressing down of the force transmitting part of the steering wheel. As soon as the horn actuation detection unit 48 finds that the horn actuation has ended, the closed loop adjustment is again started. One typical event that can be judged as a horn actuation by the horn actuation detection unit 48 can be that the actual voltage changes rapidly and that this change has a duration of at least a predefined period of time, for example half a second.

It is possible that the sampling rate (this is the frequency in which the adjustable voltage Vad is adjusted) is higher during the initialisation than during the perma- nent operation.

The system can be adjusted in sensitivity (by a threshold parameter definition) at the steering wheel integration test. Because the actual voltage is initially and permanently adjusted, the system is insensitive against changes in ambient temperature and series deviations of the components. List of reference numbers 10 stationary part of steering wheel

12 fixations for strain gauge

14 force transmitting part

14' movable part of steering wheel

16 post

18 spring

20 load cell

20' strain gauge

30 constant voltage source

32 first resistor

34 adjustable voltage generation unit

36 PMM output of microcontroller

38 filter

40 differentiating circuit

42 first operational amplifier

44 second operational amplifier

46 A/D converter

48 horn actuation detection unit

48a output of the horn actuation detection unit

50 voltage correction unit

60 micro controller

Vr raw voltage

Vad adjustable voltage

V _a actual voltage

Vamp amplified actual voltage