WO2002026546A1

WO2002026546A1 - Electric power steering rack and pinion with central motor and primary reducer

Info

Publication number: WO2002026546A1
Application number: PCT/US2000/026562
Authority: WO
Inventors: Zdravko Menjak; Ratko Menjak; James M. Card
Original assignee: Delphi Technologies, Inc.
Priority date: 2000-09-27
Filing date: 2000-09-27
Publication date: 2002-04-04

Abstract

An electric power steering assist mechanism (10) comprising an electric motor (16) removably disposed on a front steering wheel assembly (12) of a vehicle, the electric motor (16) is energized through an Electronic Control Unit (20) receiving signals from a plurality of torque detecting sensors (40) on a steering wheel shaft (32, 34), the electric motor (16) engaging a primary gear reducer which increases the output torque of the electric motor (16), the primary reducer engaging a ball-and-nut mechanism (22) that integrate a rack shaft (18) through which the electric motor's rotation is transformed into a linear axial motion of the rack shaft thereby ultimately assisting in turning the wheels.

Description

ELECTRIC POWER STEERING RACK AND PINION WITH CENTRAL MOTOR AND PRIMARY REDUCER

TECHNICAL FIELD

The present invention relates to an electric power steering assist mechanism. More particularly, the invention relates to a motor driven power steering arrangement coupled with a reducer arrangement to facilitate employment of a smaller motor.

BACKGROUND OF THE INVENTION

There are many prior art electric power steering assist mechanisms that assist a driver in turning a steering wheel of a vehicle. Generally, a number of torque sensors determine the torque on a steering shaft and send a signal to an Electronic Control Unit (ECU). The ECU in turn sends a corresponding signal to an electric power source, which sends a current signal to an electric motor. The electric motor is then energized and its rotational power is transferred to a rack shaft of the steering system through a ball-and-nut mechanism, thereby assisting the driver in steering the vehicle.

An average size vehicle requires an axial force of approximately 7000N to 9500N exerted on its steering rack shaft to be able to effectively steer its wheels. For an electric motor to generate a force of this magnitude while directly engaging the steering rack shaft, the electric motor has to produce an output torque in a range between 6 to 8 Nm. Such an electric motor is quite large in size, i.e., has a large diameter, and requires a high operating voltage delivered by special electrical wiring. The size of the electric motor and additional special electrical components cause installation problems, particularly in smaller vehicles where space is at a premium.

Moreover, an electric power steering mechanism with an electric motor directly engaging the rack shaft through a ball and nut mechanism generates a large inertia on the steering wheel assembly causing a negative steering feel for the driver while operating the vehicle.

Some other prior art assemblies utilize space more efficiently by integrating an electric motor inside a front wheel assembly. However, this requires specific and tailored electric motor part designs for every different car model and, therefore, an increase in production cost. It is noted that certain difficulties also arise in vehicle serviceability due to the need for disassembling the whole mechanism in the event of a motor or other component failure. Therefore, the use of a removably installed small electric motor coupled with a primary gear reducer for an electric power steering assist mechanism that operates with the conventional electrical components and electric power supply in a small vehicle would utilize space more efficiently for installation of the electric power steering assist mechanism and eliminate the above mentioned shortcomings of the prior art.

SUMMARY OF THE INVENTION

The present invention discloses an electric power steering assist mechanism assembled on a front steering system of a vehicle. The mechanism comprises a removable electric motor coaxially installed along a casing. The electric motor is adapted to engage a primary gear reducer reducing output speed, and increasing the torque output. Particular speed reduction and torque multiplication is accomplished by selecting a gear ratio for the primary gear reducer. The primary gear reducer preferably comprises a planetary gear assembly, double belt assembly, or double gear assembly. The primary gear reducer in turn is connected to a ball-and-nut mechanism, which transforms the rotary movement of the electric motor into a linear axial movement of a rack shaft that is movably disposed inside the casing. The ball- and-nut mechanism is also positioned inside the casing and comprises a ball nut rotating around a ball screw and moving the ball screw in an axial direction. The ball screw is integrated with the rack shaft at one end and connected to a tie rod at the opposite end.

A steering wheel is attached to an upper shaft portion of a steering shaft, which is connected to a lower shaft portion by a torsion bar. The steering torque is detected by a plurality of sensors disposed on the torsion bar sending a signal to an Electronic Control Unit (ECU). The ECU then signals an electric power source to energize the electric motor, which in turn actuates the primary gear reducer engaged with the rotor shaft of the electric motor. The primary gear reducer increases the torque output of the electric motor and mechanically connects the motor to a ball-and-nut mechanism, rotation of wliich urges the rack shaft in a direction commensurate with input of the operator, ultimately assisting in the turning of the wheels of the vehicle.

A small electric motor coupled with a primary gear reducer can be configured to produce an amount of torque on the rack shaft similar to a large electric motor if the larger motor directly engaged the rack shaft. One advantage of this mechanism is its capability to operate with the existing electrical components in a vehicle without the need for special wiring and power source generally associated with large electric motors. Since space is at a premium in small car designs, the use of a small electric motor coupled with a primary gear reducer is preferable over the use of a large electric motor. Also, due to employment of a small electric motor coupled with a primary gear reducer, the large inertia is eliminated and negative steering feel is reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described, by way of example, with reference to the accompanying drawings in which:

Figure 1 is a cross sectional view of the electric power steering assist mechanism assembled on a front steering wheel system with a planetary gear assembly as a primary reducer;

Figure 2 is a cross sectional view of the electric power steering assist mechanism assembled on a front steering wheel system with a double belt assembly as a primary reducer; and Figure 3 is a cross sectional view of the electric power steering assist mechanism assembled on a front steering wheel system with a double gear assembly as a primary reducer.

DETAILED DESCRIPTION OF THE INVENTION

Referring to Figure 1, an electric power steering assist mechanism 10 according to an embodiment of the present invention is illustrated. Mechanism 10 is assembled on a front steering wheel assembly 12 of a vehicle. The electric power steering assist mechanism 10 generally comprises a casing 14, an electric motor 16, a rack shaft 18, an Electronic Control Unit (ECU) 20, and a screw mechanism, preferably a ball-and-nut mechanism 22, for converting rotational movement into linear movement.

An upper shaft ASM for an electric power steering assist mechanism 10 generally comprises a housing 30, which encloses an axially rotatable steering shaft having an upper shaft portion 32 connected to a lower shaft portion 34 via a torsion bar 36. A steering wheel (not shown) is through steering shaft and intermediate shaft attached at one end (the other end being attached to torsion bar 36) to a top end of the upper shaft portion 32 inside the vehicle. The lower shaft portion 34 is coaxially coupled with a pinion gear 38 at one end thereof (the other end being attached to the same torsion bar 36) wherein the pinion gear 38 engages the rack shaft 18. A plurality of torque detecting sensors 40 is disposed on the torsion bar 36 between the upper shaft portion 32 and the lower' shaft portion 34 within the housing 30. The torque sensors 40 measure the torque exerted on the steering shaft by determining the relative motion between the upper shaft portion 32 and the lower shaft portion 34. The torque sensors 40 are wired to the ECU 20 and send a signal corresponding to the measured torque value. The ECU 20 in turn sends a command signal to an electric power generator to energize the electric motor 16.

The casing 14 on the front steering wheel assembly 12 extends laterally enclosing the rack shaft 18 therewithin. The casing 14 is attached to the upper shaft housing 30 at one end (right in Figure 1) permitting the pinion gear 38 to extend therethrough to engage the rack shaft 18. The pinion gear 38 is in constant meshing engagement with the rack shaft 18 functioning as a conventional rack and pinion system. The rack shaft 18 is coupled with the tie rod 24 at one end (right in Figure 1), and extends along the casing 14 to engage the ball-and-nut mechanism 22 at the other end. The rack shaft 18 is moveable in an axial direction within the casing 14 where its movement corresponds to the turning of the road wheels.

The electric motor 16 is preferably removably housed along the casing 14 in a coaxially symmetrical manner. The symmetrically aligned electric motor 16 along the casing 14 saves space needed for its installation in smaller vehicles. The electric motor 16 comprises a hollow rotor shaft 46 with a toothed outer surface, which is extended along and housed around the rack shaft 18. Disposed at the two opposite ends of the electric motor 16 is a pair of elastic supports 26 securing the casing 14 and the electric motor 16 to the vehicle, preferably to the vehicle's chassis.

The embodiment illustrated in Figure 1 employs a planetary gear assembly 28, which functions as a primary gear reducer between the electric motor 16 and the ball-and-nut mechanism 22. The planetary gear assembly 28 comprises a plurality of planetary gears 29, preferably two, each having a toothed outer perimeter. The planetary gears 29 effectively reduce the angular speed of the hollow rotor shaft 46 and increase output torque thereof, thereby allowing for the use of a small electric motor in the electric power steering assist mechanism 10. The degree of the increase in the output torque is determined by the gear ratio employed between gears of casing 14and the hollow rotor shaft 46. When the electric motor 16 is energized, it causes the hollow rotor shaft

46 to rotate. When the hollow rotor shaft 46 rotates, the toothed outer surface of the shaft 46 actuate the toothed outer perimeter of the planetary gears 29 causing the planetary gears 29 to rotate and therefore reduce the angular speed of the shaft 46 and increase the output torque thereof. The planetary gears 29 are securely disposed within the casing 14, and in meshing engagement with the toothed portion of the inner diameter of the casing 14 wherein the planetary gears 29 rotate perimetrically about the cross-section of the hollow rotor shaft 46 while engaging the corresponding inner diameter teeth of the casing 14. Each planetary gear 29 is preferably disposed and freely rotatable on a corresponding spindle 31. Each spindle 31 is attached to a ball nut 22a component of the ball-and-nut mechanism 22 through a connecting shaft 33. As the planetary gears 29 rotate around the hollow rotor shaft 46 by engaging the toothed outer diameter of the shaft 46, each planetary gear 29 freely rotates about its corresponding spindle 31 and the connecting shaft 33 rotates coaxially with hollow rotor shaft 46, and ultimately causing ball nut 22a to rotate about a ball screw 22b through the connecting shafts 33. The hollow motor shaft 46 is not connected to the ball nut 22a as shown by line 35 in Figure 1.

It is well known in the art that the ball-and-nut mechanism 22 transforms the rotary motion of the electric motor 16 into a linear axial movement of the rack shaft 18. The ball nut 22a has a plurality of inner spiral channels and ball bearings 22d, which are kept in meshing engagement with the ball screw 22b having a ball groove on its outer perimeter. A ball nut and ball screw is known to one of ordinary skill in the art. The ball nut 22a is supported by a plurality of angular contact bearings 22c disposed between the outer perimeter of the ball nut 22a and the inner surface of the casing 14 permitting the ball nut 22a to securely rotate about the ball screw 22b thereby moving the ball screw 22b in a lateral direction. The ball screw 22b is preferably just an extended portion of the rack shaft 18 which is engageable with the ball nut 22a. Stated somewhat differently, although the ball screw 22b may be integrated with the rack shaft 18 at one end and connected to a tie rod at the other end, it is preferable that the ball screw 22b be a portion of the rack shaft 18 which is engagable with the ball nut 22a. Therefore, the lateral movement ball nut 22b actually corresponds to turning a road wheel at one end and moving the rack shaft 18 at the other end, and ultimately assisting in turning the road wheels. Some factors, such as precision in the operation of the mechanism 10 or manufacturing costs dictate the utilization of different primary gear reducers. Referring to Figure 2, illustrated is the electric power assist mechanism 10 according to another embodiment of the present invention using a double belt assembly 42 as a primary gear reducer. Those parts shown in Figure 2, which are identical to those shown in Figure 1 are denoted by identical reference numerals, and will not be described in detail below. The double belt assembly 42 has a rotatable shaft 42a having two opposite ends. A pulley 42b is fixedly disposed at one end of the shaft 42a and another pulley 42d is fixedly disposed the other end of the shaft 42a. The rotation of the shaft 42a corresponds to the rotation of the pulleys at each end. Pulley 42b is rotatably connected to the hollow rotor shaft 46 of the electric motor 16 via a belt 42c. Pulley 42d is rotatably connected to the ball nut 22a via a belt 42e. A proper gear ratio, i.e. the relative size of between pulley 42b and pulley 42d determines the output torque required to rotate the ball nut 22a as the rotation of the hollow rotor shaft 46 is transformed to the rotation of the ball nut 22a.

Referring to Figure 3, an electric power steering assist mechanism 10 according to another embodiment of the present invention is illustrated using a double gear assembly 44 as a primary gear reducer. Those parts shown in Figure 3, which are identical to those shown in Figure 1 are denoted by identical reference numerals, and will not be described in detail below. The double gear assembly 44 generally has the same set up as the double belt assembly 42 but instead employs a plurality of gears 44a and 44c rather than pulleys and belts. The double gear assembly 44 comprises a rotatable shaft 44b having two opposite ends. Disposed at one of the shaft 44b is the gear 44a directly in meshing engagement with the hollow rotor shaft 46, and disposed the other end of the shaft 44b is the gear 44c in direct meshing engagement with the ball nut 22a. A proper gear ratio between gear 44a and gear 44c in the double gear assembly 44 determines the output torque needed to rotate the ball nut 22a, and ultimately the rack shaft 18, as the rotation of the hollow rotor shaft 46 of the electric motor 16 is transformed to the rotation of the ball nut 22a.

While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustration only, and such illustrations and embodiments as have been disclosed herein are not to be construed as limiting to the claims.

Claims

1. An electric power steering assist mechanism having an electric motor and a rack shaft movably disposed within a casing on a vehicle wheel assembly comprising: a primary gear reducer engaged with said electric motor; a screw mechanism engaged with said primary gear reducer, said screw mechanism translating rotary movement of said electric motor to linear movement of said rack shaft.

2. The electric power steering assist mechanism recited in claim 1 wherein said screw mechanism is a ball screw and a ball nut.

3. The electric power steering assist mechanism recited in claim 1 wherein said primary gear reducer comprises a planetary gear assembly having a plurality of planetary gears driven by said motor disposed within said casing and rotatable perimetrically within said casing.

4. The electric power steering assist mechanism recited in claim 1 wherein said primary gear reducer is a double gear assembly.

5. The electric power steering assist mechanism recited in claim 1 wherein said primary gear reducer is a double belt assembly.

6. The electric power steering assist mechanism recited in claim 1 further comprising a plurality of elastic supports disposed on said casing for attaching said casing to a vehicle frame structure.

7. The electric power steering assist mechanism recited in claim 2 wherein said ball nut and said ball screw are integrated with said rack shaft at one end and attached to a tie rod at an opposite end.

8. The electric power steering assist mechanism recited in claim 1 wherein said electric motor is removably coaxially installed along said casing.