1. Field of the Invention
This invention relates to steering wheel sensors that are mounted to rotating shafts that are joined by a torsion bar. In particular, there is an eccentricity compensator that prevents hysteresis between the two rotating shafts when their axes of rotation are co-axial or not co-axial.
2. Description of the Related Art
Various devices and methods of dealing with the joining of two shafts that are rotating in a non-coaxial manner are known. These are called eccentricity compensators. One such device is a universal joint or U-joint. The U-joint is a well known device that typically is used to link between a transmission shaft and an axle shaft to allow the shafts angle to bend.
Unfortunately, the prior art devices as they wear out, create what is called hysteresis. Hysteresis is a backlash or slop between the two rotating shafts. While, some hysteresis may be acceptable in a drive shaft application, in an application such as a steering wheel torque sensor this is unacceptable. A steering wheel torque sensor needs to have very precise position information of the relative rotational positions of the two shafts in order to correctly sense the amount of torque applied to the steering wheel. As the sensor wears, there cannot be excessive rotational movement between the two shafts leading to incorrect torque readings. Referring to FIG. 1, part of a prior art eccentricity compensator 10 is shown. Eccentricity compensator 10 is part of a steering wheel torque sensor that is described in U.S. patent application Ser. No. 09/837,075, filed Apr. 18, 2001 and titled, xe2x80x9cSteering Wheel Torque and Position Sensorxe2x80x9d.
The compensator 10 is shown in an assembled partial end view. The compensator 10 has a shaft 11 that is connected to a carrier or ring 14 by splines (not shown). Ring 14 has four pins 16 that extend upwardly. Another ring 12 is mounted adjacent to ring 14. Ring 12 has four slots 18. Pins 16 are located in slots 18. The rings 12, 14 and pins 16 are formed from injection molded plastic. Over a period of time during use, the eccentricity compensator parts will wear. The result is a gap 19 between pin 16 and a side wall 21. When the gaps 19 form in slots 18, the sensor components can rotate or have hysteresis when there is no actual torque in the steering wheel column. The compensator rotates about an axis or rotation 20. When the compensator is new rings 12 and 14 will rotate together in an original position as indicated by line 22. In compensator 10, there may be initial hysteresis due to a gap 19 due to manufacturing tolerances of the pin and slot. After the compensator 10 has had some wear, the positions of ring 12 and 14 will rotate relative to each other. This is indicated by dashed line 24. The rotational difference or error or hysteresis caused by the wear is indicated as a hysteresis angle 26. Angle 26 is very undesirable as it leads to incorrect torque readings.
The automotive industry has been focusing on electrical assist power steering for vehicles. The electrical assist power steering unit is an electrical motor attached to the steering linkage that operates when assist is required. A large amount of torque on the steering wheel occurs at low speed operation or during parking. The electrical assist power steering is generally not needed during high speed operation such as during highway driving. The major advantages of electrical assist power steering are first, that it only operates during the short time of turning and is inoperative the rest of the time and second that it is simpler to manufacture. In a hydraulic power steering system, the power steering pump is always being turned by the engine and represents an energy drain on the motor all the time even though steering is only performed during a small percentage of the total time a car is operated. An electrical assist power steering system requires sensing of torque applied to the steering wheel. The torque indicates how much force the operator is exerting to move the wheel. The output signal from a torque sensor is fed into a control unit which controls the electrical motor of the assist unit. When the torque sensed is high, the assist applied to the steering linkage will be high. When the torque sensed is low, the assist applied to the steering linkage will be low.
In general, a sensor that measures the relative displacement between two rotating shafts has useful applications in the areas of industrial machinery, aerospace, electrical power generation and transportation.
There is a current unmet need for a device that prevents hysteresis between two non-coaxial rotating shafts. Additionally, there is a current unmet need for a eccentricity compensator to prevent hysteresis in a steering wheel column torque sensor.
It is a feature of the invention to provide a eccentricity compensator that prevents wear induced rotational displacement that is mounted between two rotating shafts that have non-coaxial axes of rotation.
Yet, another feature of the invention is to provide a eccentricity compensator for preventing hysteresis between a first and a second rotating shaft that are joined by a torsion bar. The eccentricity compensator includes a first ring attached to the first shaft and a second ring located adjacent the first ring. A third ring is attached to the second shaft and the third ring is located adjacent the second ring. At least one pin-slot pair is located between any two of the first, second or third rings. A spring is mounted adjacent the pin-slot pair. The spring biases the pin-slot pair such that hysteresis is prevented. The springs and slots are arranged in such a way that wear of the pins in the slots does not result in drift.
The invention resides not in any one of these features per se, but rather in the particular combination of all of them herein disclosed and claimed. Those skilled in the art will appreciate that the conception, upon which this disclosure is based, may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention.