BALL SCREW ASSEMBLY WITH INTEGRATED FORCE MEASUREMENT

Abstract

BALL SCREW ASSEMBLY WITH INTEGRATED FORCE MEASUREMENT The invention relates to a ball screw assembly having a ball screw nut comprising an external surface, an internal surface defining a central bore, and a ball race defined by the internal surface, a ball screw shaft comprising an external surface defining a ball race, the ball screw shaft being disposed in the central bore such that the two ball races form a ball bearing track, a first stop disposed at a first end of the ball screw nut inside the ball bearing track, a plurality of main balls forming a ball train disposed in the ball bearing track, a main spring assembly disposed in the ball bearing track between a first end of the ball train and the first stop,and an elongation detection element affixed to a portion of a flat surface defined by the external surface of the ball screw nut.

Description

Title of the invention: TITLE OF THE INVENTION BALL SCREW ASSEMBLY WITH INTEGRATED FORCE MEASUREMENT PRIORITY CLAIM

[0001] The present application claims priority from U.S. Provisional Patent Application No. 63 / 188,546 filed on May 14, 2021. FIELD OF THE INVENTION

[0002] The present invention relates generally to mechanical linear actuators. More particularly, the present invention relates to designs and methods for assembling ball screw assemblies and sensing linear forces therein. BACKGROUND OF THE INVENTION

[0003] Conventional electromechanical braking systems 90, as shown in [Fig. 6], require actuators that can provide linear force, as well as brake application force feedback to associated vehicle control systems. An exemplary linear actuator 91 is a ball screw assembly having a ball train interposed between a ball track formed in an outer surface of a ball screw shaft and a ball track formed in an inner surface of a ball screw nut. As shown, the current state of the art can accomplish linear force sensing by positioning a dedicated force measuring cell 93 between the actuator 91 and a housing 92. However, the force measuring cell 93 adds to the overall length of the assembly, thereby occupying valuable system space, and adds to the complexity of the system.Furthermore, dimensional alignment of the force measuring cell 93 to its mating components is critical for accurate and repeatable force sensing as well as the requirement of maintaining minimum preload.

[0004] The present invention recognizes and addresses considerations of prior art constructions and methods. Summary of the invention

[0005] One embodiment of the present disclosure relates to a ball screw assembly having a ball screw nut having a first end, a second end, an outer surface, an inner surface defining a central bore, and a ball race defined by the inner surface, a ball screw shaft having an outer surface defining a ball race, the ball screw shaft being disposed in the central bore of the ball screw nut such that the ball race of the ball screw nut and the ball race of the ball screw shaft form a ball race, a first stop disposed at the first end of the ball screw nut within the ball raceway, a plurality of main balls forming a ball train, the ball train being disposed in the ball raceway, and an elongation detecting element affixed to a flat surface portion defined by the outer surface of the ball screw nut.

[0006] Another embodiment of the present disclosure relates to a brake assembly having a caliper having an arm portion, a piston disposed within the caliper, the piston being axially movable along a longitudinal central axis of the piston, a first brake pad affixed to the arm portion of the caliper, a second brake pad affixed to an end surface of the piston, a brake disc disposed between the first brake pad and the second brake pad, and a ball screw assembly having a ball screw nut having a first end, a second end, an inner surface defining a central bore, and a ball race defined by the inner surface, a ball screw shaft having an outer surface defining a ball race,the ball screw shaft being disposed in the center bore of the ball screw nut such that the ball race of the ball screw nut and the ball race of the ball screw shaft form a ball raceway, a first stopper disposed at the first end of the ball screw nut within the ball raceway, a plurality of main balls forming a ball train, the ball train being disposed in the ball raceway, a main spring assembly disposed in the ball raceway between a first end of the ball train and the first stopper, and an elongation detecting element affixed to a flat surface portion defined by the outer surface of the ball screw nut.

[0007] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate one or more embodiments of the invention and, together with the description, serve to explain the principles of the invention. Brief Description of the Drawings

[0008] A complete and enabling disclosure of the present invention, including the best mode thereof, addressed to those skilled in the art, is set forth in the specification, which refers to the accompanying drawings, in which;

[0009] [Fig.l] is a schematic view of a brake assembly comprising an embodiment of a ball screw assembly in accordance with an embodiment of the present disclosure;

[0010] [Fig.2] is a partial cross-sectional view of the brake assembly shown in [Fig.l];

[0011] [Fig.3] is a side plan view of the ball screw assembly of the brake assembly shown in Figures 1 and 2;

[0012] [Fig.4] is a side plan view of a ball screw assembly in accordance with an alternative embodiment of the present invention;

[0013] [Fig.5] is a side plan view of a ball screw assembly in accordance with an alternative embodiment of the present invention; and

[0014] [Fig.6] is a schematic view of a brake assembly comprising a ball screw assembly and a prior art force measuring cell configuration.

[0015] The repeated use of reference characters in this specification and the drawings is intended to correspond to identical or analogous features or elements of the invention according to the disclosure.

[0016] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] Reference will now be made in detail to the presently preferred embodiments of the invention, one or more examples of which are illustrated in the accompanying drawings. Each example is provided for the purpose of explaining, not limiting, the invention. Indeed, it will be apparent to those skilled in the art that modifications and variations may be made to the present invention without departing from the scope and spirit thereof. For example, features illustrated or described in one embodiment may be used in another embodiment to result in yet another embodiment. Thus, it is intended that the present invention covers such modifications and variations falling within the scope of the appended claims and their equivalents.

[0018] As used herein, terms referring to a direction or position relative to the orientation of the ball screw assembly in a brake system, such as, but not limited to, "vertical," "horizontal," "top," "bottom," "above," or "below," refer to directions and positions relative to the orientation of the ball screw assembly in its expected normal operation, as shown in Figures 1-3. Thus, for example, the terms "vertical" and "top" refer to the vertical orientation and relative top position in the perspective of Figures 1-3 and should be understood in that context, even with respect to a ball screw assembly that may be arranged in a different orientation.

[0019] .

[0020] Referring now to the drawings, [Fig.l] is a schematic sectional view of a brake assembly 1 having a ball screw assembly 100 in accordance with a first embodiment of the present disclosure. As shown, the brake assembly 1 selectively applies a friction braking force to a rotating disc 2 unified with a wheel of an automobile or the like. The brake assembly 1 has a caliper 3, a first backing plate 4 and a second backing plate 5, and a first pad 6 and a second pad 7. The caliper 3 is supported so as to movable by a stub axle (not shown), and the first backing plate 4 and the second backing plate 5 are arranged on the caliper 3 so as to sandwich the brake disc 2 therebetween. The first pad 6 and the second pad 7 are fixed to the first backing plate 4 and the second backing plate 5, respectively, and can press respective side surfaces of the disc 2.

[0021] The caliper 3 comprises a first body 8, a second body 9 and a cover 10. The first body 8 and the second body 9 are fixed together. The cover 10 is fixed to the second body 9. The first body 8 comprises a body part 11 and an arm part 12. One end of the second body 9 is fixed to the body part 11. The arm part 12 is orthogonally coupled to the body part 11. The second support plate 5 is fixed to the arm part 12. The second body 9 comprises a brake cylinder 13 and an extension plate 14. The cylinder 13 is fixed to the body part 11 of the first body 8. The extension plate 14 extends from the cylinder 13.

[0022] The cylinder 13 has a first end 41 and a second end 42 which are opposite to each other in an axial direction. The cylinder 13 has a cylindrical portion 15 which is open at the first end 41 and an end surface plate 16 coupled to the second end 42 of the cylindrical portion 15. A piston 17 which is movable in the axial direction ST is housed in the cylinder 13. One end 73 of the piston 17 projects toward the disc 2 through an opening portion at one end of the cylinder 13 (which corresponds to the first end 41 of the cylindrical portion 15) and is fixed to the first support plate 4.

[0023] A sealing member 18 is interposed between a cylindrical outer surface 67 of the piston 17 and an inner surface of the cylinder 13 (which corresponds to an inner surface 69 of the cylindrical portion 15) to seal the space between the outer surface 67 and the inner surface 69. The sealing member 18 may be an O-ring housed in a housing groove formed in the inner surface 69 of the cylinder 13. The outer surface 67 of the piston 17 and the inner surface 69 of the cylinder 13 are coupled together via a key 19 formed in key grooves formed in the outer surface 67 and the inner surface 69. A key coupling using the key 19 allows the movement of the piston 17 in the axial direction ST to be guided and also allows the rotation of the piston 17 relative to the cylinder 13 to be regulated.

[0024] Hydraulic pressure that biases the piston 17 toward the disc 2 can be supplied to the cylinder 13 via a hydraulic path not shown in the drawings. In this case, the cylinder 13 and the piston 17 form a hydraulic actuator. The caliper 3 has the function of pressing the two pads 6 and 7 against the disc 2 to generate a braking force. The caliper 3 comprises an electric motor 20, a speed reduction apparatus 21 and the ball screw apparatus 100. The speed reduction apparatus 21 reduces the rotational speed of the electric motor 20. The ball screw apparatus balls 100 converts a rotary motion transmitted from the electric motor 20 via the speed reduction apparatus 21 into a linear motion of the piston 17 in the axial direction ST.

[0025] The electric motor 20 comprises a motor housing 23 and an output shaft 24. The motor housing 23 is fixed to the extension plate 14 of the second body 9. The speed reduction apparatus 21 comprises a drive gear 25, an idler gear 26 and a driven gear T1. The drive gear 25 is integral with one end of the output shaft 24 of the electric motor 20 so as to be rotatable together with the output shaft 24. The idler gear 26 meshes with the drive gear 25. The driven gear 27 meshes with the idler gear 26. The idler gear 26 is pivotally carried by the second body 9 so as to be rotatable. The cover 10 is fixed to the second body 9 so as to cover the speed reduction apparatus 21.

[0026] Referring further to [Fig. 2], the ball screw assembly 100 includes a ball screw shaft 110 and a ball screw nut 130. The ball screw shaft 110 is an input member. The ball screw nut 130 is a rotatable output member threaded onto the ball screw shaft 110 via a plurality of main balls 140. The ball screw shaft 110 is inserted through the ball screw nut 130. The ball screw shaft 110 is carried by the second body 9 ([Fig. 1]) of the brake assembly such that the ball screw shaft 110 is rotatable relative to the second body 9 but axially restricted. The ball screw nut 130 is carried inside the second body 9 by the piston 17 so as to be movable in the axial direction and to be non-rotatable relative to the second body 9.

[0027] As shown, the ball screw shaft 110 is carried by a roller bearing 32 retained adjacent to a support hole 31 formed in the end surface plate 16 of the cylinder 13 so that the ball screw shaft 110 is rotatable and stationary in the axial direction (ST axial direction). The driven pinion 27 is coupled to one end 112 of the ball screw shaft 110 so as to rotate together with the ball screw shaft 110. As shown, the ball screw nut 130 has a cylindrical outer surface 132 and a cylindrical inner surface 134. A helical-shaped ball race 136 is defined by the inner surface 134. Similarly, the ball screw shaft 110 has a cylindrical outer surface 114 in which a helical-shaped ball race 116 is formed.The plurality of main balls 140 form a ball train which is disposed in a ball race 150, as shown in [Fig. 2], defined between the ball track 116 and the ball track 136.

[0028] The outer surface 132 of the ball screw nut 130 also includes a rotation regulating portion (not shown). The rotation regulating portion is engaged with a rotation regulating portion (not shown) of an inner surface 77 of the piston 17. The outer surface 132 of the ball screw nut 130 is fitted into a bore defined by the cylindrical inner surface portion of the piston 17. The engagement between the rotation regulating portions prevents rotation of the piston 17 and the ball screw nut 130 relative to each other. In alternative embodiments, the outer surface of the ball screw nut 130 may have flat surfaces 134 ([Fig. 3]) that engage correspondingly shaped surfaces in the bore of the corresponding piston (not shown) to prevent rotation of the ball screw nut 330 relative to the piston.

[0029] Still referring to Figures 1 to 3, the ball screw nut 130 has a first end 137 and a second end 139, the first end 137 of the ball screw nut 130 being disposed axially inward in the piston 17 compared to the second end 139. A retaining ring (annular member) 40 fitted into an annular groove formed in the inner surface 77 of the piston 17 is engaged with an end surface of the second end 139 of the ball screw nut 130. Thus, the piston 17 and the ball screw nut 130 are coupled together so as to move together in the axial direction ST (ball nut axial direction X).When a rotation of the output shaft 24 of the electric motor 20 is transmitted to the ball screw shaft 110 via the speed reduction apparatus 21 to rotate the ball screw shaft 110, the ball screw nut 130 moves in the ball nut axial direction X (axial direction ST). At this time, the piston 17 is guided by the key 19 and moves together with the ball screw nut 130 in the axial direction ST.

[0030] Accurate measurements of the linear forces that are applied in a braking system are often required to provide feedback for the proper operation of the braking system. As such, ball screw assemblies in accordance with embodiments of the present invention preferably include a multi-function actuator ball screw nut 130 or a ball screw nut / piston combination. The ball screw nut 130 is preferably designed for multiple functions such as its original function, acting as a mating component with the ball screw shaft 110, and further, providing a force measuring cell 200 for measuring forces applied within the system. The ball screw nut 130 can function as a force measuring cell 200 because all materials undergo physical deformation (elongation) under stress. As evidenced by [Fig.3], the ball screw nut 130 is aligned with the applied linear force of the ball screw assembly (arrows 210 and 211), which makes the ball screw nut 130 a desirable location for measuring applied forces within the braking system.

[0031] Referring specifically to Figures 2 and 3, the force measuring cell 200 preferably includes a strain sensing portion 221 formed by a groove 223 and one or more bores 225 formed therein, each bore being configured to receive a strain sensing element 220 therein. As shown, the annular groove 223 formed around the ball screw nut 130 between the first end 137 of the nut and the leading end of the ball train allows for greater measurement sensitivity due to the thinned wall portion of the ball screw nut 130. In addition, the plurality of bores 225 formed in the bottom wall of the groove 223 also allow for greater deformation of the leading end 137 of the ball screw nut, thereby improving sensitivity. Note that the ball train 140 is omitted in [Fig.3].As shown, one or more strain sensing elements 220, such as a strain gauge disposed within a cup-shaped receptacle, are further received in corresponding ones of the bores 225. Each strain sensing element 220 is connected by wires 240 that pass through an axial groove 241 to an external control system (not shown) for processing. Further, a protective strip (not shown) of material may be disposed within the annular groove to protect the strain sensing elements 220 from debris, impact, etc.

[0032] The ball screw assembly 100 with integrated force measurement can be encompassed in a shorter linear space via existing assemblies ([Fig. 6]) by eliminating traditional externally positioned force measurement cells 13, creating a compact actuation system. Because the strain measurement is now part of the ball screw nut 130, the disclosed ball screw nut 130 that acts as a force measurement cell does not require additional system alignment and dimensional stacking challenges as found in prior art configurations ([Fig. 6]). The ball screw nut 130 force measurement cell 200 has a high tolerance to debris because the sensing element(s) 220 are embedded within the ball screw nut 130, with only the connecting wires 240 exposed to the system environment.The ball screw nut 130 may contain any type of sensing elements, such as elongation, piezoelectric, capacitive, or inductive.

[0033] Referring now to [Fig. 4], a force measuring cell 203 in accordance with an alternative embodiment of the invention preferably includes a strain sensing portion 321 formed by a groove 223 and one or more bores 125 formed therein, adjacent bores 325 being separated by flat portions 327 which are configured to receive a strain sensing element 320 thereon. As shown, the annular groove 323 formed around the ball screw nut 330 between the first end 337 of the nut and the front end of the ball train 140 ([Fig. 2]) allows for greater measurement sensitivity due to the thinned wall portion of the ball screw nut 330. In addition, the plurality of bores 325 formed in the bottom wall of the groove 323 also allow for greater deformation of the front end 337 of the ball screw nut, thereby improving sensitivity. Note that the ball train 140 is omitted in [Fig. 4]. As shown, one or more strain sensing elements 320, such as strain gauges, are received on the corresponding flat portions 327 of the groove 323. Each strain sensing element 320 is connected by wires 340 that pass through an axial groove 341 to an external control system (not shown) for processing.In yet another embodiment, signal amplification elements (not shown) may be attached to the flat portions 327 as well to reduce the need for external signal processing. Further, a protective strip (not shown) of material may be disposed within the annular groove to protect the strain sensing elements 320 from debris, impact, etc.

[0034] Referring specifically to [Fig. 5], a force measuring cell 400 preferably includes a strain sensing portion 421 formed by a groove 423 that is configured to receive one or more strain gauges 320, as shown in [Fig. 4], on one or more strain sensing elements 220, as shown in [Fig. 3], therein. When using strain sensing elements 220, one or more bores (not shown) are formed in the groove 423, each bore configured to receive a strain sensing element 220 thereon. As shown, the annular groove 423 formed around the ball screw nut 430 between the first end 437 of the nut and the front end of the ball train 140 ([Fig.2]) allows for greater measurement sensitivity due to the thinned wall portion of the ball screw nut 430.Additionally, when present, the plurality of bores (225 in [Fig. 3]) formed in the bottom wall of the groove 423 also allow for greater deformation of the leading end 337 of the ball screw nut, thereby improving sensitivity. Note that the ball train 140 ([Fig. 2]) is omitted in [Fig. 5]. As shown, the groove 423 is oblique to the planar leading end 437 of the ball screw nut 430 at an angle (α) equal to that of the helical ball raceway 450. As such, a constant distance exists between each strain sensing element in the groove 423 and the ball train 140 ([Fig. 2]) at which the force represented by the arrow 210 is transmitted to the ball train at the arrows 211, as shown in [Fig. 2]. In this way, a very precise measurement of stress is accomplished.

[0035] Although one or more preferred embodiments of the invention are described above, it will be appreciated by those skilled in the art that various modifications and variations may be made to the present invention without departing from the scope and spirit thereof. For example, the disclosed concept is not restricted to use in only a ball screw assembly system but, instead, may be applied to a standardized thread (ACME, et al.) or roller screw system. Furthermore, the present concept is also not limited to braking applications but, instead, may be used anywhere a screw and nut type system requires force feedback. It is intended that the present invention covers such modifications and variations falling within the scope and spirit of the appended claims and their equivalents.

Claims

Claims

1. A ball screw assembly (100) comprising: a ball screw nut (130) having a first end, a second end, an outer surface (132), an inner surface (134) defining a central bore, and a ball race (136) defined by the inner surface; a ball screw shaft (110) having an outer surface (114) defining a ball race (116), the ball screw shaft being disposed in the central bore of the ball screw nut such that the ball race of the ball screw nut and the ball race of the ball screw shaft form a ball race; a first stopper disposed at the first end of the ball screw nut within the ball race; a plurality of main balls (140) forming a ball train, the ball train being disposed in the ball race;and an elongation detecting element (220) affixed to the ball screw nut; the ball screw nut further comprising a groove (223) formed in an outer wall of the ball screw nut and disposed between a first end of the ball screw nut and the ball train, and the elongation detecting element being disposed inside the groove.;

2. The ball screw assembly of claim 1, wherein the groove has a bottom wall and the elongation detecting element is affixed to the bottom wall of the groove.

3. The ball screw assembly of claim 2, wherein the groove is disposed in a plane that is perpendicular to a longitudinal central axis of the ball screw nut.

4. The ball screw assembly of claim 2, wherein the groove forms an angle with a longitudinal center axis of the ball screw nut that is equal to an angle formed by the ball race and the longitudinal center axis.

5. A ball screw assembly according to claim 4, wherein the ball raceway is helical in shape.

6. The ball screw assembly of claim 1, further comprising a bore (225) extending inwardly from a bottom wall of the groove.

7. The ball screw assembly of claim 6, wherein the elongation sensing element is disposed within the bore.

8. A brake assembly (1) comprising: a caliper (3) having an arm portion; a piston (17) disposed within the caliper, the piston being axially movable along a longitudinal central axis of the piston; a first brake pad (6) affixed to the arm portion of the caliper; a second brake pad (7) affixed to an end surface of the piston; a brake disc (2) disposed between the first brake pad and the second brake pad; and a ball screw assembly (100) comprising: a ball screw nut (130) having a first end, a second end, an outer surface, an inner surface defining a central bore, and a ball race defined by the inner surface;a ball screw shaft (110) having an outer surface defining a ball race, the ball screw shaft being disposed in the center bore such that the ball race of the ball screw nut and the ball race of the ball screw shaft form a ball raceway; a first stopper disposed at the first end of the ball screw nut within the ball raceway; a plurality of main balls (140) forming a ball train, the ball train being disposed in the ball raceway; and an elongation detecting element (220) affixed to a flat surface portion defined by the outer surface of the ball screw nut;the ball screw nut further comprising a groove (223) formed in an outer wall of the ball screw nut and disposed between a first end of the ball screw nut and the ball train, and the elongation detecting element being disposed inside the groove.;

9.

10.

11.

12.

13.

14. A brake assembly according to claim 8, wherein the groove has a bottom wall and the elongation sensing element is affixed to the bottom wall of the groove. A brake assembly according to claim 9, wherein the groove is disposed in a plane which is perpendicular to a longitudinal central axis of the ball screw nut. A brake assembly according to claim 9, wherein the groove forms an angle with a longitudinal central axis of the ball screw nut which is equal to an angle formed by the ball race and the longitudinal central axis. A brake assembly according to claim 11, wherein the ball race is helical in shape. The brake assembly of claim 8, further comprising a bore (225) extending inwardly from a bottom wall of the groove. A brake assembly according to claim 13, wherein the elongation sensing element is disposed within the bore.

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