WO2003100283A1

WO2003100283A1 - An electromechanical actuator

Info

Publication number: WO2003100283A1
Application number: PCT/IB2002/001906
Authority: WO
Inventors: Richard Hannis; Richard Corbett
Original assignee: SKF Industrie SpA
Current assignee: SKF Industrie SpA
Priority date: 2002-05-27
Filing date: 2002-05-27
Publication date: 2003-12-04
Anticipated expiration: 2004-11-27
Also published as: AU2002307842A1

Abstract

The electromechanical actuator, particularly for a drum parking brake of a motor vehicle, comprises an electric motor with a housing (11, 13) fixable to the vehicle. A first shaft (12) is driven for rotation by the electric motor about a first axis (a1). A second shaft (17) is attached to a cable (21) in turn attached to a brake member to be actuated. A gear reduction mechanism, interposed between the first and the second shafts (12, 17), includes a stationary internal gear (14) defining a fixed central axis, and a mobile pinion (15) connected eccentrically to and driven by the first shaft (12) and engaging the internal gear (14) so as to perform a motion compounded of rotation about the axis (a2) of the pinion (15, 15´) and revolution about the central axis of the internal gear (14). Recesses (16) and pins (18) are interposed between the second shaft (17) and the pinion (15), for rotating the second shaft at the same rotational speed of the pinion.

Description

An electromechanical actuator

The present invention refers to an electromechanical actuator. More particularly, although not exclusively, the invention refers to an electromechanical actuator well suited for mounting on motor vehicles for actuating, for example, drum parking brakes .

In the motorcar industry today there is an ever increasing demand for remote control of devices on board of the vehicle by means of electromechanical actuators electrically connected to control buttons located near the driver ' s seat .

It is an object of the present invention to provide an electromechanical actuator of compact dimensions, adapted to fit easily in a variety of different positions on the vehicle, according to space requirements.

Another object of the invention is to provide an electromechanical actuator having a small electric motor yet capable of developing high actuating forces via a high ratio gear reduction mechanism.

A further object of the invention is to provide a cost- effective yet reliable actuator made of few parts.

According to the present invention, these objects are achieved by means of an electromechanical actuator having the features defined in independent claim 1. Preferred embodiments are defined in the dependent claims .

The invention will now be described, purely by way of non- limiting example, with reference to the appended drawings, in which: figure 1 is partially sectioned perspective view of an electromechanical actuator in accordance with the present invention; figure 2 is a partial transverse cross-section taken through the lines II-II in figure 1, showing the gear reduction mechanism of the actuator; figure 3 is an axial sectional view schematically showing the working principle of the gear reduction drive of the actuator of figures 1 and 2 ; and figure 4 is a transverse cross-section of a variant of the actuator of figures 1 to 3.

In the text below, the structure and operation of the actuator will be described only to such an extent as is necessary for the understanding of the invention.

Referring initially to figure 1, an electromechanical actuator in accordance with the present invention is designated overall 10. Actuator 10 includes a small electric motor (not shown) mounted within a housing 11. A printed circuit board and/or other electronic means for controlling the operation of the electric motor may be accommodated within the housing 11. The electric motor drives a shaft 12, herein termed "input" shaft, for rotation about the fixed axis ai (figure 3) . Mounted to the housing 11 is a substantially cylindrical cap 13 on the inner periphery of which there is provided an internal gear 14, secured to or integral with the cap 13 or the housing 11.

A pinion 15 is eccentrically mounted to the input shaft 12. Particularly, as shown schematically in figure 3, the pinion 15 is mounted in a non-rigid manner to an eccentric end portion 12a of the input shaft 12. A bearing (not shown) may be mounted between the eccentric portion 12a and the pinion 15.

The number of pinion teeth Zi is chosen to be lower than the number of internal gear teeth z₂, such that rotation of the input shaft 12 about the fixed axis causes the pinion 15 to travel around the inside of the internal gear 14 (whose axis is fixed) , following a motion compounded of a revolution about the fixed, central axis of internal gear 14 and a rotation about the pinion axis a .

A plurality of cylindrical recesses 16 are formed angularly equally spaced around the axis a₂ of the pinion on the pinion side facing away from the input shaft 12. As better shown in figure 3, an output shaft 17 carries a corresponding number of pins 18, each of which is fitted with radial play in one of the recesses 16 of pinion 15. The output shaft 17 is axi- ally aligned with the input shaft 12 and mounted coaxially in the cap or housing 13 by means of a bearing 19. A pulley element 20 is mounted to or formed integrally with the outer end of the output shaft 17 so as to be fast for rotation therewith and project from an end opening formed in the end of the cap 13 (to the right in figure 1) .

An end of a flexible transmission means 21, such as a belt, wire or cable, is anchored to the pulley element 20, such that rotation of the output shaft 17 pulls the cable 21 and winds it around the pulley element 20 or releases it and unwinds it, in accordance with the direction of rotation imparted to the output shaft . The other end of the flexible cable 21 is anchored to one or more actuated members (not shown) , for example of a drum parking brake. The constructional and functional features of parking brakes are known and will not therefore be described herein.

Operation of the actuator is as follows.

When the electric motor is activated, driving the input shaft for rotation about fixed axis a_x, the pinion 15 wobbles within the stationary internal gear 14 performing the aforementioned compound motion. The speed of rotation of the pinion 15 is reduced with respect to the rotational speed of the input shaft 12 according to a reduction ratio

^zτ ^{~ z}\ wherein: i = reduction ratio

Zi = number of pinion teeth (inner gear) z₂ = number of internal gear teeth (gear ring) .

The output shaft 17 is driven at the same reduced rotational speed of the pinion 15. The pins 18 and the corresponding recesses 16 in which they are loosely accommodated are so dimensioned as to ensure smooth rotation of the output shaft 17. The number, location and dimension of the pins 18, which have to sustain the full input torque, are determined by the maximum contact force. This depends on the input torque and the distance of the main axis to the contact point of each pin and its recess. The maximum force is reached once by each turn of the impulse drive. So the load has a pulsating character. However, the pins are mostly loaded only from one direction, which is caused by the higher brake torque compared to the back drive torque .

As shown in figure 1, mounted on the housing 11 is a locking mechanism 22 comprised of a spring-loaded locking pin 23 and a solenoid (not shown in detail) . The locking pin is constantly biased in a radial direction towards the output shaft 17 by means of a spring so as to engage shaft 17 and lock it mechanically in a given angular position, which, in this example, corresponds to a braking condition. When the solenoid is activated, it withdraws pin 23 against the biasing action of the spring and releases the output shaft 17. As will be apparent, the actuator of the present invention enables the generation of gear ratios up to 150:1 with an efficiency of as much as 90 percent or more.

The compact design of the actuator allows it to be fitted in a variety of positions, in view of surrounding components and according to any vehicle design requirement.

Although the invention is described herein in the context of its application to vehicles and drum parking brakes, it will be understood that reference to these possible fields of use should not be construed as in any way limiting the scope of the patent .

While a specific embodiment of the invention has been disclosed, it is to be understood that such disclosure has been made merely for the purpose of illustration and that the invention is not to be limited in any manner thereby. Various modifications will be apparent to those skilled in the art. For example, as shown in figure 4, the gear reduction mechanism may be arranged with an inner gear 14' fixed to the housing and a wobbling outer gear 15' eccentrically mounted on the input shaft (not shown) .

Claims

1. An electromechanical actuator comprising: an electric motor with a housing (11, 13) ; a first shaft (12) driven for rotation by the electric motor about a first axis (a_x) ; a second shaft (17) attached to a flexible transmission means (21) in turn attached to at least one member to be actuated; a gear reduction mechanism, interposed between the first and second shafts (12, 17), the mechanism including: a stationary gear member (14, 14') defining a fixed central axis, and a mobile gear member (15, 15') connected eccentrically to and driven for rotation by the first shaft (12) and engaging the stationary gear member (14, 14') so as to perform a motion compounded of rotation about the axis (a₂) of the mobile gear member (15, 15') and revolution about the central axis of the stationary gear member (14, 14'); non-rigid connecting means (16, 18), interposed between the second shaft (17) and the mobile gear member (15, 15'), for rotating the second shaft at the same rotational speed of the mobile gear member.

2. The actuator of claim 1, wherein the stationary gear member is an internal gear (14) fast with the housing (11, 13) and having a given number of teeth (z₂) , and wherein the mobile gear member (15) is a pinion travelling around the inside of the internal gear and having a number of teeth (zi) which is lower than that of the internal gear.

3. The actuator of claim 1, wherein the non-rigid connecting means comprise a plurality of recesses (16) formed in the mobile gear member (15, 15') and a corresponding plurality of engaging members (18) accommodated with radial play within the recesses (16) and fast for rotation with the second shaft (17) .

4. The actuator of claim 1, further including a pulley portion (20) fast for rotation with the second shaft (17) for winding the flexible transmission means (21) .

5. The actuator of claim 1, further including a locking mechanism (22) with a spring-loaded locking pin (23) and a solenoid, wherein the locking mechanism is capable of assuming: a locking condition in which the locking pin (23) is spring-biased so as to engage the second shaft (17) locking it in a given angular position; and a released condition in which the solenoid is activated to disengage the locking pin (23) , against said spring- biasing action, from the second shaft.

6. The actuator of claim 1, wherein the mobile gear member (15, 15') is mounted in a non-rigid manner to an eccentric portion (12a) of the first shaft (12) .

7. The actuator of any of the preceding claims, characterised in that it is mounted on a motor vehicle and operatively connected to at least one member to be actuated on board of the vehicle.

8. The actuator of claim 7, characterised in that it is operatively connected to a brake member to be actuated.

9. The actuator of claim 8, characterised in that it is operatively connected to a member of a drum parking brake,