Electronic steering wheel technique for passing information with a programmable current source and detector

A steering wheel control system (10) that employs a method for passing push-button commands from a steering wheel to a vehicle system, wherein the method is not dependant upon the contact resistance of a sliding contact (32) located in the steering wheel column. The control system (10) includes a constant current source circuit (20) that generates a unique constant current corresponding to each command. The sliding contact (32) passes the constant current between the steering wheel and the vehicle system, and a calibrated resistance circuit (40) converts the unique constant current into a discrete voltage. The discrete voltage is also converted into a digital signal for use by a microcontroller (60) to activate the vehicle systems. The use of constant currents therefore avoids operational problems associated with decreases in the overall system noise margin due to large voltage drops across the sliding contact (32). Furthermore, the use of off-the-shelf constant current generation and conversion components allows for an overall reduction in system cost and increase in system reliability.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to vehicle steering wheels. More 
particularly, the present invention relates to an improved technique for 
passing electronic information between a steering wheel electronics 
assembly and a vehicle system that employs a programmable current source. 
2. Discussion of the Related Art 
In the automotive industry, steering wheels commonly provide drivers with a 
number of automotive controls such as cruise control, braking, flashing 
front lights, and flashing rear lights that are operated by electronic 
push buttons located on the steering wheel. These push buttons generate 
electronic information to be used by the vehicle when performing the 
various automotive functions. Existing techniques pass the electronic 
information from a steering wheel electronics assembly to an electronic 
control box, located under the dashboard or elsewhere in the vehicle. The 
steering wheel electronics assembly is generally located in the center of 
the steering wheel and can have the various electronic push buttons and 
lights mounted on it to operate the vehicle systems. The rotary motion of 
the steering wheel in relation to the steering wheel column typically 
requires the placement of one or more sliding contacts in the column in 
order to pass the electronic information from the steering wheel to the 
electronic control box. Typically, the steering wheel electronics assembly 
uses a time bit sequence of information and signal magnitude to convey to 
the electronic control box which particular button was pushed. In turn, 
the electronic control box decodes the electronic information and 
activates the appropriate automotive circuit within the vehicle. This 
circuit technique is fairly complex and difficult to maintain particularly 
due to the bit sequencing algorithms and circuitry required. 
Another significant problem with this circuit technique for transmitting 
signals from steering wheel to a vehicle system concerns the contact 
resistance of the sliding contacts. The sliding elements of the sliding 
contacts are typically spring loaded and very prone to oxidation and other 
causes of surface degradation. If the electrical resistance between the 
sliding elements of a contact is too large, there will be an increased 
voltage drop across the sliding elements which decreases the overall 
system noise margin. The result is that the electronic control box cannot 
interpret the signal magnitude, and the system will either fail to operate 
or operate intermittently. This failure to operate properly is a source of 
great frustration to the driver, reduces system reliability, and causes 
numerous and costly repairs. Thus, there is a need to combat the problem 
created by high contact resistance of sliding contacts without increasing 
the cost of overall system operation. 
SUMMARY OF THE INVENTION 
The steering wheel control system of the present invention passes 
electronic information between a steering wheel push-button and a vehicle 
system by generating a constant current corresponding to the electronic 
information, passing the constant current from the steering wheel through 
a sliding contact and converting the constant current into a discrete 
voltage that is used by a microcontroller for identification of the 
electronic information. 
The present invention therefore identifies the electronic information by a 
current level rather than a voltage level, which minimizes the effect of 
large voltage drops across the sliding contact. A particular current 
corresponds to each steering wheel push-button rather than a sequence of 
signal magnitudes. An electronic control box can then simply look for the 
presence of particular current levels rather than the typical time bit 
sequence of information at a minimum required magnitude. The present 
invention also uses a much simpler circuit than in existing technologies 
because of the off-the-shelf nature of constant current generation and 
current conversion components. This simplicity makes the present invention 
less expensive and easier to maintain. Furthermore, the present invention 
allows for the continued use of current sliding contacts. 
Further objects, features and advantages of the invention will become 
apparent from a consideration of the following description and the 
appended claims when taken in connection with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The following discussion directed to an electronic steering wheel technique 
for passing information with a programmable current source and detector is 
merely exemplary in nature, and is in no way intended to limit the 
invention or its applications or uses. 
Turning now to FIGS. 1 and 2, a steering wheel control system, indicated 
generally at 10, according to the invention, includes a constant current 
source circuit 20, a signal transmission assembly 30, and a calibrated 
resistance circuit 40 for passing electronic information from a steering 
wheel push-button (not shown) to a vehicle system (not shown). The 
steering wheel push-buttons are used to operate various vehicle systems, 
such as the vehicle cruise control system, emergency flashers, etc. The 
constant current source circuit 20 includes a plurality of contact 
switches 21 and programming resistors R1 for each separate push-button on 
the steering wheel, where each programming resistor R1 has a different 
value. The particular value of the resistor R1 associated with each 
contact switch 21 allows the constant current source 20 to generate a 
different constant current depending on which steering wheel push-button 
is pressed. In other words, each switch on the vehicle's steering wheel is 
assigned a particular current level, so that when a steering wheel push 
button is activated, the associated contact switch 21 is closed, and the 
value of the resistor R1 associated with that contact switch 21 determines 
the current level. 
The constant current circuit 20 further includes a constant current 
mechanism 23 that generates the constant current based on which contact 
switch 21 is pressed, and a precision band-gap reference diode 27 that 
provides a reference voltage for the current mechanism 23. The constant 
current mechanism 23 includes a biasing resistor R2 and a bipolar junction 
transistor (BJT) 24. Alternately, the current mechanism 23 includes a 
field-effect transistor (FET) 26 and a programming operational amplifier 
25, as shown in FIG. 2. If the constant current mechanism 23 includes the 
BJT 24, then the biasing resistor R2 properly biases the BJT 24. If the 
constant current mechanism 23 includes the FET 26 and the programming 
operational amplifier 25, then the programming operational amplifier 25 
biases the FET 26 while the biasing resistor R2 biases the programming 
operational amplifier 25. It should be noted that use of the programmable 
operational amplifier 25 at a low power eliminates the base-emitter 
voltage drift in the BJT 24, and allows programmable currents to be much 
higher than required for circuit operation. 
The signal transmission assembly 30 includes a first sliding contact 32 
that transfers the current signal from the constant current source circuit 
20 to the calibrated resistant circuit 40. Additionally, the signal 
transmission assembly 30 includes a second sliding contact 34 that 
connects the source circuit 20 to an electrical ground. The source circuit 
20 would be located in the vehicle steering column, and the sliding 
contacts 32 and 34 would allow the electrical signal to be transferred 
from the vehicle steering column to a non-rotating electrical connection. 
The calibrated resistance circuit 40 includes a plurality of differential 
resistors R3 through R7 and a differential operational amplifier 41. The 
resistor R7 receives the current signal through the sliding contact 32, 
and provides a voltage drop depending on that current level. The resistors 
R3-R6 act as a voltage divider for the operational amplifier 41 that 
receives the voltage signal from the resistor R7. Therefore, the output of 
the amplifier 41 is an analog voltage output representative of the voltage 
drop across the resistor R7 for each particular current level. The circuit 
40 also includes an analog-to-digital (A/D) converter 50 and a 
microcontroller 60. The A/D converter 50 receives the analog signal from 
the operational amplifier 41 and converts it to a digital signal, and the 
microcontroller 60 receives the digital signal and transmits it to the 
corresponding vehicle system through a relay (not shown) to operate that 
vehicle system depending on the digital value of the particular current 
level. The microcontroller 60 has an auto calibration output which is 
connected to a calibration switch 42 to calibrate the voltage divider. 
In operation, the steering wheel control system 10 has an ambient current 
on the order of a few milliamps in its non-activated state before the 
driver activates a push button on the steering wheel. When a push button 
is activated, the corresponding contact switch 21 closes and the control 
system 10 passes electronic information from the steering wheel to the 
selected vehicle system. The constant current source 20 generates a 
constant current corresponding to the electronic information based on the 
value of the resistor R1 associated with the particular switch 21. The 
programming resistor R1 thus generates a programmable current 
corresponding to the electronic information when the contact switch 21 
closes. The constant current mechanism 23 then generates the constant 
current in response to the programmable current and the precision band-gap 
reference 27 provides a reference voltage to the constant current 
mechanism 23. Therefore, depending on which contact switch 21 is closed, 
the constant current mechanism 23 generates a different constant current. 
The sliding contact 32 then passes the constant current from the steering 
wheel to the calibrated resistance circuit 40 that converts the constant 
current into a discrete voltage. 
The voltage divider provided by the differential resistors R3 through R6 
creates a unique differential voltage based on the constant current 
flowing through resistor R7. Specifically, the resistor R7 drops a certain 
amount of voltage across it, while resistors R3 through R6 operate as a 
voltage divider for the differential operational amplifier 41. The 
differential operational amplifier 41 then creates a discrete voltage 
based on the differential voltage. The A/D converter 50 converts the 
discrete voltage into a digital signal, and the microcontroller 60 
processes the digital signal. The microcontroller 60 also has the auto 
calibration output which calibrates the calibrated resistance circuit 40 
in response to resistance changes by triggering the calibration switch 42 
located in the calibrated resistance circuit 40. 
It should be appreciated that due to the use of a constant current to pass 
the electronic information, the control system 10 can operate effectively 
at contact resistances as high as approximately 50 ohms whereas existing 
methods encounter transmission problems above approximately 1 to 10 ohms. 
It is to be understood that the invention is not limited to the exact 
construction illustrated and described above, but that various changes and 
modifications may be made without departing from the spirit and scope of 
the invention as defined in the following claims.