Electric power assist steering systems (EPAS) have been around since the 1960's. Hydraulic power assist steering has traditionally dominated the market. Hydraulic systems have high parasitic energy loss when the hydraulic pump is pumping, but power assist is not required. Early attempts to eliminate this parasitic loss involved fitting an electric motor to the pump and only driving the pump when necessary.
Electric hydraulic assisted power steering systems use an electric motor to drive a hydraulic pump to feed a hydraulic power steering system. These systems are an intermediate step by the industry and their use will likely fade with the increased use of SPAS. EPAS systems allow realization of reduced noise, reduced energy use, active safety features, and adjustability to meet driving conditions. However, the use of these systems has remained limited until recent C.A.F.E. requirements became more difficult to meet. This is driving automotive manufactures to turn to EPAS systems more and more in an effort to improve vehicle fuel economy. SPAS systems eliminate the parasitic losses typically found in hydraulic assist power steering systems. System manufacturers such as Nexteer make claims of 6% fuel economy improvements.
For example, one difficulty that slowed implementation of EPAS systems was meeting the power requirement with a 12 volt electric motor. Recently systems have been developed that successfully solve this problem. Further, all SPAS systems require a control module to sense driver input and control the electric motor to provide the desired assist. The control module measures driver input torque and uses this to determine the amount of assist required. Assist can be tuned to meet the drivers need depending on driving conditions. The system can even have a tunable “feel” available to the driver.
Even though the main driver for automotive SPAS is fuel economy improvement, SPAS has additional benefits. The system can make steering assist available even when the vehicle's engine is not running. It also enables the use of the automatic parallel parking systems available today.
There are two main types of EPAS systems; column assist and rack assist. Rack assist SPAS systems have an electric motor that is connected to the steering rack. The electric motor assists the rack movement usually through driving a lead screw mechanism. Column assist EPAS systems have an electric motor connected to the steering column. The electric motor assists the movement of the column shaft usually through a worm gear type arrangement. One advantage of these types of systems is the electric motor can be placed in the passenger compartment freeing up valuable space under the hood. This also keeps any sensitive electrical components out of the harsh under hood environment.
Worm drive column assist systems are usually used in small cars where the assist power requirements are lower than what would be needed in a large heavy vehicle. These systems are limited by the speed of the steering wheel and the ratio of the worm drive. The steering wheel at its fastest speed rotates relatively slowly at approximately 60 rpm. With a 60 rpm speed of the steering wheel and a worm drive ratio of 15:1, the max speed of the electric motor would only be 900 rpm. Worm drives are limited to ratios under 20:1 because ratios higher than that cannot be back-driven.
The steering system must be able to be operated with no power. This requires the worm drive be able to operate with the gear driving the worm (back-driven). Having a low motor speed and limited ratio worm drive causes the need for high torque motor. Even with a high torque motor, these types of systems have not been made successful on heavy vehicles. Small vehicles are light and require less steering effort thus enabling the use of these systems. Worm drive column assist EPAS systems are the lowest cost systems and thus also lend themselves to smaller less expensive vehicles.
Typical steering systems with worm drive assists are limited in their efficiency. EPAS systems must be designed to operate when there is no power available. Due to the nature of worm drive's tendency to lock up during back driving when ratios exceed approximately 20:1, worm drive EPAS systems efficiency is not greater than approximately 85% and nearer to 65% during back-driving conditions.
Today there are no column assist EPAS systems commonly available that use anything other than a worm drive to facilitate the assist. These column systems are unable to provide enough assist for large heavy vehicles.
Representative of the art is U.S. Pat. No. 7,887,446 which discloses a helically-toothed-belt transmission device. A backlash “D” is selectively enlarged in a helically-toothed-belt transmission device that transmits drive force by meshing between a helically toothed belt and a helically toothed pulley, that is, a tooth helix angle “θ” is set in a range of −0.2≦1−Wxθ/Pt≦0.75, with “Pt” being a tooth pitch, “θ” a tooth helix angle, and W the width of the belt. The backlash “D” between the helically toothed belt and the helically toothed pulley is set to be 1.6%-3% of the tooth pitch “Pt”.
What is needed is a vehicle steering system transmission comprising an input shaft and an output shaft coupled by a first pair of sprockets having a ratio, a second pair of sprockets having a ratio and the third pair of sprockets having a ratio. The present invention meets this need.