|Title of Invention||
AN ELECTROMECHANICAL LINEAR ACTUATOR FOR A PARKING BRAKE
|Abstract||The actuator comprises a screw mechanism (13) having a threaded nut (16) rotatable by means of an electric motor (11) about a longitudinal axis (x) and a screw (17) translatable along said axis between an axially extended position and an axially retracted position. The screw (17) is connectable to a wire (W) of a Bowden cable (C) having an outer sheath (S). The actuator has a casing (20) with a first, fixed transversal surface (18). A cup (50) is mounted axially slidable on the casing (20) and has a second transversal surface (51) adapted for resting against an end of the sheath (S). A spring (53) is compressed between the first, fixed transversal surface (18) and the sliding cup (50). The cup (50) is capable of reaching an axially extended position, corresponding to a released brake condition, and an axially retracted position towards the first fixed transversal surface (18), against the bias of the spring (53), corresponding to a brake actuated condition.|
|Full Text||An electromechanical linear actuator for a parking brake
The present invention refers to an electromechanical linear actuator for a parking brake of a motor vehicle.
There are known braking devices associated with electrome-chanical linear actuators comprised of an electric motor and a drive assembly which converts rotary motion imparted to the rotor of the electric motor into a linear movement of an out-put member which applies a braking force to the parking brakes of the vehicle through a Bowden cable. See, for exam-ple, WO-A-98/56633 and WO-A-2005/061293. The actuators are associated with sensors that supply signals indicative of the amount of braking force being applied.
It is a general object of the invention to provide an actua-tor of simple construction, which allows to obtain accurate and reliable signals about the application of the requested braking force. A particular object of the invention is to at-tain such signals also in those cases where the actuator is associated with an extremely simplified electronic control system, also in the event of a failure or absence of a sensor of the aforementioned type.
These and other objects and advantages, which will be better understood herein after, are achieved according to the inven-tion by an electromechanical actuator having the features de-fined in the appended claims.
The constructional and functional features of a preferred but not limiting embodiment of an actuator according to the in-vention will now be described, reference being made to the accompanying drawings, in which:
figure 1 is a partially sectioned perspective viiw of an actuator according to the invention;
figure 2 is an enlarged, partially sectioned perspective view of part of the actuator of figure 1;
figure 3 is a further enlarged, partially sectioned per-spective view of part shown in figure 2;
figure 4 is an axial cross-sectional view of the part of the actuator shown in figure 2; and
figure 5 is a partially sectioned perspective view of some of the components shown in figure 2.
With reference initially to figure 1, numeral 10 indicates overall an electromechanical actuator according to the inven-tion, intended to actuate the parking brakes (not shown) of a motor vehicle. The actuator 10 includes an electric motor 11 which, through a gear reduction assembly 12 and an associated screw mechanism 13, imparts linear movements to a Bowden ca-ble C connected to the parking brakes and comprising a metal wire W and a sheath S.
The electric motor 11 is preferably a permanent magnet direct current motor, which can selectively rotate in either direc-tion depending on a control signal provided by an electronic control module (not shown). An output shaft 15 of the motor drives for rotation, through the reduction assembly 12, a nut 16 of the screw mechanism 13. This screw mechanism can be a friction screw, or a ballscrew, or a roller screw mechanism.
The screw mechanism converts rotary movement of the nut 16 into linear translating movement of a screw 17 coupled to an end of the metal wire W of Bowden cable C. In figure 4, the screw 17 is shown in its axially retracted position that causes tensioning of the wire W and actuates the parking brakes. In figures 1-3, the screw 17 is shown in its axially extended position that causes untensioning of the wire W and releases the brakes.
The actuator 10 has a casing, indicated overall at 20, com-prising some parts of plastic material and some parts of me-tallic material, as explained in the following. The electric motor 11 and the gear reduction assembly 12 are accommodated within a cylindrical casing body 21 of plastic material with a cover 22. The nut 16 is rotatably supported by means of a bearing 30 within a tubular body 23 of plastic material fixed to or formed integral with the cover 22.
As an alternative to the screw mechanism here described, the invention can be equally implemented with a telescopic screw mechanism of the type disclosed in WO-A-2005/005212, wherein the translating output member is coupled to a tubular screw performing a screwing movement about and along a longitudinal axis.
The tubular body 23 extends axially for a length suitable to contain the screw 17 in its fully extended position (figure 1). In its axially retracted position (figure 4) the screw is contained in a chamber 24 of the casing, alongside the elec-tric motor 11. A metal reinforcing guide tube 25 is coaxially fixed within the plastic tubular body 23, and extends from the free end of the tubular body 23 to the bearing 30, in or-der to transfer to this the axial loads during operation of the actuator, as discussed hereinafter. In the example shown in the drawings, the guide tube 25 is fixed to a conical mem-ber 26 of sheet metal resting against the stationary outer race of the bearing 30. As shown in figure 5, the guide tube 25 forms an axially extended rectilinear channel 27. A pin 19 transversally projecting from the axially outer end of the screw 17 slides along the channel 27 to prevent the screw form rotating about its axis of translation.
As used herein, terms and expressions indicating positions and orientations, such as "axial", "radial" and "transver-sal", should be construed as referring to the axis of trans-lation X of the screw 17. Likewise, such expressions as "axi-ally extended" and "axially retracted" are to be interpreted with respect to the fixed casing 20 of the actuator.
The screw 17 is removably connected to the wire W by means of a locking device 40 (figure 3). Fixed onto the axially outer end of the screw is a sleeve 41 with one or more radial through bores 42 in which balls 43 are held engaging a groove G formed in the end head H of wire W. The balls are held in the locking position shown in figure 3 by a ring 44 fitted around the sleeve 41 and retained in a locking position around the balls by a spring 45 interposed between a shoulder 4 6 of the sleeve and the ring 44 itself. In order to connect or disconnect the wire VJ to or from the screw 17, the ring 44 has to be urged manually, compressing the spring 4 5 until an inner enlarged part of the ring is brought at the level of the bores 42, whereby the balls 43 move radially outwardly, disengaging the groove G and therefore releasing the head H from the screw 17.
An end of the sheath S of the cable C (figures 2 and 4) rests against a radial or transversal shoulder surface 51 of a cup-like member 50 having an opening 52 for the passage of the wire W. The cup-member 50 is mounted axially slidably with respect to the casing of the actuator, advantageously on the outside of the casing at an easily accessible position. As shown in figure 3, the cup member 50 is mounted slidably co- axially on a second cup member 54 having a stepped profile. Cup member 54, which is preferably metallic, is fixed to the axially outer end of the guide tube 25. The first cup member 50 can slide over the free end of the tubular body 23. A com-pression spring 53 is interposed between the sliding cup 50 and a transversal opposing surface 18 fixed with respect to casing 20. In the example shown in the drawings, the opposing surface 18 is formed by the second, fixed cup 54.
The sliding cup 50 carries a pin 55 projecting in a radially inward direction and sliding in a longitudinal slot 55 of the inner fixed cup 54. The pin 55 acts as a stopping member that holds the sliding cup 50 on the casing when the spring 53 and the sheath S are not axially compressed.
When the screw 17 is in an axially extended position (figures 1-3) , which corresponds to the release of the parking brake, the wire W is untensioned and the spring 53 urges the sliding cup 50 away from the actuator casing. When the screw 17 is in an axially retracted position (figure 4), that corresponds to actuation of the brake, the wire W is tensioned; the sheath S is axially compressed and urges the sliding cup 50 towards the actuator casing, compressing the spring 53 until the rear transversal surface 57 of the sliding cup 50 abuts against the front transversal surface 58 of the fixed cup 54. It will be noted that the position of the sliding cup 50 with respect to the actuator casing is indicative of the amount of the braking force applied through the Bowden cable.
The sliding cup 50 can carry a permanent magnet 60 generating a magnetic field that can be detected by a fixed linear sen-sor 61 (figure 4), for example a Hall effect sensor, mounted on the tubular body 23 of the casing and electrically con-nected to the control module of the electric motor. By de-tecting changes in the intensity of the magnetic field owing to the sliding movements of cup 50 upon tensioning and unten- sioning the cable, the sensor 61 generates an electric signal that allows to accurately detect the amount of force applied to the brakes by the actuator.
As will be appreciated, this result is attained by means of an actuator of simple design made up of a low number of com-ponents, few of which are mobile elements.
Further, it will be appreciated that the abutment between the cups 50 and 54 allows to detect, in an extremely simple man-ner, that the required actuating force for the brakes has been applied. As a matter of fact, when the wire W is ten- sioned and the spring 53 is progressively compressed, the current absorbed by the electric motor increases almost pro¬portionally to the degree of compression of the spring. Once the sliding cup 50 has abutted the fixed cup 54, the spring 53 is no further compressed, and the current absorbed by the electric motor 11 increases abruptly. This increase, indicat¬ing that the braking force has been applied, can be detected by the electronic control module also in the event of a fail¬ure of the sensor 61, or if this sensor is absent, by simply monitoring the current absorbed by the electric motor. Know¬ing the modulus of spring 53, by means of an algorithm, the value of the braking force applied to the brakes is easily calculated. On the one hand, therefore, the actuator of the present invention is suitable for use with a non sophisti¬cated (and therefore inexpensive) electronic control system; on the other hand, the aforesaid abutment provides the elec¬tronic control system with a redundant datum, informing the control system that the braking force has been applied also in the event of a failure of the sensor. It will also be noted that the magnet 60 and the sensor 61, if provided, are located at easily accessible positions for carrying out main¬tenance or replacement in the event of a failure. Also the spring 53 is easily interchangeable, whereby a same actuator can be promptly adapted to different applications, according to the braking force required by motor vehicle manufacturers.
The casing 20, owing to the metal parts 25, 26 and 54, pro-vides a loop-like path through metal components capable of withstanding high temperatures that may possibly occur within the vehicle in the event of a failure or accident. Indeed, the axial forces pass from the wire W to the screw 17, the nut 16, through the bearing 30, the cone 26, the reinforcing tube 25, the dual diameter cup 54, the spring 53, the sliding cup 50, to return to the sheath S.
The invention is not intended to be limited to the embodiment described and illustrated herein, which should be considered as an example of the actuator; rather, the invention may be modified with regard to the shape and arrangement of parts and to constructional and functional details, as will be ap-parent to those skilled in the art. For example, in an alter-native embodiment, the opposing surface for the sliding cup 50 may consist of a shoulder 28 formed by the tubular body 23. Likewise, the fixed cup 54 may be formed as a single piece with the metal tube 25. In a still different embodi-ment, the axial stop of the sliding cup 50 can be performed by the same spring 53 in its completely compressed condition, axially interposed between the cup 50 and the fixed surface.
1. An electromechanical linear actuator for a parking brake, comprising:
an electric motor (11);
a screw mechanism (13) having a first threaded element
(16) rotatable by means of the electric motor about a longi-tudinal axis (x) and a second threaded element (17) translat-able along said axis between an axially extended position and an axially retracted position, the second threaded element
(17) being connectable to a wire (W) of a Bowden cable (C) having an outer sheath (S);
a casing (20) with a first, fixed transversal surface
characterized in that the actuator further comprises:
a member (50) mounted axially slidable on the casing (20) and having a second transversal surface (51) adapted for resting against an end of the sheath (S); and
elastic means (53) acting between the first, fixed transversal surface (18) and the sliding member (50);
wherein the sliding member (50) is capable of reaching an axially extended position, corresponding to a released brake condition, and an axially retracted position towards the first fixed transversal surface (18), against the bias of the elastic means (53), corresponding to a brake actuated condition.
2. An actuator according to claim 1, characterized in that the sliding member (50) is slidably mounted on an end of an axially elongated portion (23, 25) of the casing (20).
3. An actuator according to claim 2, characterized in that the axially elongated portion (23, 25) of the casing com¬prises a metal tube (25) .
4. An actuator according to claim 3, characterized in that the metal tube (25) has opposite ends cooperating in an axial thrust relationship respectively with the first transversal surface (18) and a bearing (30) mounted in the casing (20) for rotatably supporting the first threaded element (16) of the screw mechanism (13) .
5. An actuator according to claim 3, characterized in that the axially elongated portion of the housing comprises an outer body of plastic material (23) located coaxially outside the metal tube (25).
6. An actuator according to claim 3, characterized in that the metal tube (25) forms an axially extended rectilinear guide (27) for axially slidingly engaging a member (19) transversely projecting from the second threaded element (17) .
7. An actuator according to claim 1, characterized in that the sliding element (50) carries a permanent magnet (60) operatively coupled to a sensor or magnetic-electric trans¬ducer (51) fixed to the housing and adapted to provide an electric signal indicative of the axial position of the slid¬ing member (50) with respect to the casing (20).
8. An actuator according to claim 1, characterized in that the casing has a further fixed transversal surface (58, 28) providing an abutment surface for stopping axial movement of the sliding member (50) in its fully retracted axial posi¬tion.
9. An actuator according to claims 3 and 8, characterized'^- in that the first fixed transversal surface (18)- and the fur-ther transversal abutment surface (58) are formed by a metal member (54) secured to or integral with the metal tube (25).
10. An actuator according to claim 1, characterized in that the sliding member (50) has substantially the shape of a cup with an opening (52) for allowing the passage of the wire (W) of the Bowden cable (C) .
11. An actuator according to claim 1, characterized in that the sliding member (50) is located on the outside of the cas-ing (20) .
12. An actuator according to any one of the preceding claims, characterized in that the first threaded element (16) is an axially stationary nut rotatably mounted in the casing (20) by means of a bearing (30), and in that the second threaded element (17) is a screw axially translatable with respect to the casing (20) ,
13. An actuator according to claim 1, characterized in that the screw mechanism (13) is a friction screw, or a ballscrew, or a roller screw mechanism.
|Indian Patent Application Number||4916/CHENP/2008|
|PG Journal Number||09/2014|
|Date of Filing||16-Sep-2008|
|Name of Patentee||AKTIEBOLAGET SKF|
|Applicant Address||S-415 50, GOTEBORG,|
|PCT International Classification Number||B60T13/74|
|PCT International Application Number||PCT/EP07/51418|
|PCT International Filing date||2007-02-14|