Patent Application
20050023086
The present invention relates generally to braking systems, and in particular, to shape memory alloy (“SMA”)-actuated and to bender-actuated helical brake mechanisms for permitting and inhibiting movement of one or more members relative to each other.
Helical wrap springs are commonly employed in conventional braking mechanisms to control whether two or more moveable members can move relative to each other. In a typical helical spring brake, a helical wrap spring is positioned concentrically about, and in frictional engagement with, the outer surface of a drive member, such as a shaft or drum. The direction of the turns of the helical wrap spring permits rotation of the shaft in one direction relative to the spring (i.e., in a direction that tends to unwrap the helically coiled spring), but those same turns also prevent rotation of the shaft in the opposite direction (i.e., in a direction that tends to wrap the helically coiled spring). By externally applying an expansion or separation force to the ends of the spring, the spring operates frictionlessly, or nearly frictionlessly, with respect to the shaft. As such, the shaft is released to freely rotate in either direction relative to the spring.
Traditionally, actuation mechanisms that expand or separate the spring ends typically rely on a collection of sophisticated linkages (e.g., cables or rods), gears or other relatively complicated mechanical configurations. There are at least as many types of actuation mechanisms for initiating or terminating braking as there are applications for helical spring brakes. Some examples of applications employing helical spring brakes include automobiles, bicycles, elevators, hoists, as well as adjustment mechanisms for window regulators, window shades, car seats, seat head rests, and the like.
But the conventional actuation mechanisms used to activate braking in these applications, although adequate in operation, are typically associated with one or more of the following drawbacks. A first drawback is that conventional actuation mechanisms usually require either numerous quantities of members (e.g., a number of gears) or large-sized members (e.g., elongated activator members, such as rods or cables), or both, to effectuate actuation. As such, conventional actuators for helical wrap spring brakes are generally suboptimal in form factor (i.e., in physical, external dimensions) as well as in simplicity. As an example, consider that window lifters using a helical spring brake are well known in the art to inhibit unintentional motion of the lifting mechanism (and the window). To inhibit the unintentional motion of a payload (i.e., an object being acted upon, such as a window), lifting mechanisms rely on complex configurations using one or more of the following: a set of gears; concentric drums in which an internal helical wrap spring unwinds from a first drum to engage frictionally with the inner surface of a second drum, which is hollow and co-axial with the first; multiple springs for all directions of travel; a relatively heavy motor having enough friction in its off state so that the payload will remain stationary (such heavy motors typically consume more power than is otherwise necessary); and a pin-like latch for locking a part used to lift a payload, the unlocking of which requires an actuator powerful enough to disengage the pin from the locked part as the weight of the payload (e.g., a window) bears down on the pin.
Other drawbacks of current helical spring brake actuators are that the use of linkages and gears tend to either limit the placement of a trigger for the actuator or require sufficient physical dexterity by users to trigger actuation (i.e., by manually separating the spring ends), or both. As such, manual separation of the spring ends can be difficult for elderly or otherwise infirm users. For example, automobile parking brakes or hood latches employing helical spring brakes generally require substantial effort to release the helical wrap spring. Moreover, the triggering means to release parking brakes and hood latches, such as a knob, are inconveniently located underneath the steering wheel and dashboard.
As another example, consider that helical wrap spring actuation mechanisms for seat reclination applications (as well as other applications) typically employ a number of gears or linkages for activating a helical spring brake, which tends to be off-axis to the gears or linkages. This arrangement increases the number of components constituting the actuation mechanism. As with parking brakes and hood latches, a triggering means is used to either recline a seat back or to move the seat forward to enable persons to enter or exit a back seat. An example of such a triggering means is a manual seat latch, which is inconveniently located at the bottom of a seat back, near the floor of a vehicle. So, conventional actuation mechanisms generally limit designers from placing the triggering means in a convenient location. Another drawback is that the mechanical linkages and gears that traditionally constitute these actuators can inadvertently generate audible sounds, as noise, from the interaction of the actuator components.
In view of the foregoing, what is needed is an improved actuation mechanism for operating helical spring brakes to overcome the drawbacks of conventional actuators.
In one embodiment of the present invention, a shape memory alloy (“SMA”)-actuated helical spring brake comprises a rotatable member and a helical wrap spring arranged concentrically about the rotatable member. The spring has a first spring end and a second spring end and it includes a number of turns configured to frictionally engage the rotatable member by means of an inwardly-directed radial bias. The turns permit rotation of the rotatable member in a first direction and inhibit rotation in a second direction. The SMA-actuate helical spring brake also includes an anchor point coupled to the second spring end, and an SMA actuator having an output drive member coupled to the first spring end. The SMA actuator is configured to, for example, deflect the first spring end to permit the rotatable member to rotate. The rotatable member is typically a shaft.
In a related embodiment, the SMA-actuated helical spring brake can include a motor rigidly coupled to the rotatable member for providing a motive torque to rotate the rotatable member. The SMA-actuated helical spring brake can also include a payload rigidly coupled to the rotatable member. In some cases, the payload exerts a bias torque sufficient to rotate the rotatable member in the second direction when the helical wrap spring permits the rotatable member to rotate. As such, this SMA-actuated helical spring brake is suitable to operate on a payload that is a window. In another embodiment, the SMA-actuated helical spring brake further comprises a biasing device configured to induce rotation of the rotatable member in the second direction when the helical wrap spring permits the rotatable member to rotate. Consequently, an SMA-actuated helical spring brake with a biasing device is suitable to operate on a pin-latch mechanism, a door opening mechanism or a seat back safety lock release mechanism. In some embodiments of the present invention, a bender actuator is substituted for an SMA actuator to govern actuation of a helical spring brake.
The invention is more fully appreciated in connection with the following detailed description taken in conjunction with the accompanying drawings, in which:
Like reference numerals refer to corresponding parts throughout the several views of the drawings.
Embodiments of the present invention relate to SMA-actuated and bender-actuated helical spring brakes for imparting motion to a payload. According to some embodiments of the invention, an SMA-actuated helical spring brake can be configured to permit or inhibit a rotatable member to move a payload, such as window, a seat back, etc. Advantageously, an SMA-actuated helical spring brake according to some embodiments of the present invention can be formed with a relatively compact form factor so as to preserve space that otherwise would be consumed by sophisticated linkages, gears or other relatively complicated mechanical configurations. As such, embodiments of the present invention facilitate the miniaturization of helical spring brake actuator mechanisms. Without the need for physical efforts to release a helical spring brake in accordance to some embodiments, SMA-actuated helical spring brakes facilitate actuation for elderly and infirm users. Also, an SMA-actuated helical spring brake according to some embodiments of the present invention can be triggered remotely without requiring mechanical linkages from the site of remote activation to the brake, thus allowing convenient placement of a trigger for activating SMA-actuated helical spring brakes. Moreover, an SMA-actuated helical spring brake according to some embodiments of the present invention can include a deflection detector or a rotation detector to reduce the amount of time that an SMA actuator remains activated, thus increasing the life expectancy of the SMA actuator.
A trigger 101 is configured to trigger activation of SMA actuator 102, which in turn releases helical spring brake 100. Trigger 101, such as a push-button, can be located at a location convenient to a user without regard to linkages, gears, or some other mechanical member extending from trigger 101 to SMA-actuated helical brake, as typically required by conventional helical wrap spring actuation mechanisms. Trigger 101 generates an electrical signal that causes SMA actuator 102 to activate.
SMA actuator 102 can be of any configuration that uses one or more SMA elements as the primary force generation means. An SMA “element” refers to an SMA material of elongate form, capable of contraction and elongation along the longitudinal axis. The element may have a circular cross-section, as is the case for an SMA wire, or any of a variety of cross-sections such as elliptical, square, rectangular, or the like. Shape memory alloy (“SMA”) refers to metals, which exhibit two very unique properties, pseudo-elasticity, and the shape memory effect. Pseudo-elasticity refers to the almost rubber-like flexibility of SMAs. The shape memory effect refers to the unique ability of shape memory alloys to be severely deformed and then returned to their original shape simply by heating them. By way of example and not limitation, shape memory alloys include NiTi (Nickel-Titanium), CuZnAl, and CuAlNi among others. Although other means are known in the art, heating is commonly accomplished by passing an electric current through the wire. For purposes of concreteness, the description of the present invention invokes mainly electric or ohmic heating of the SMA wire.
To effectuate actuation of SMA actuator 102, its SMA elements (e.g., SMA wires) are heated by passing a current through the elements, causing an output drive member 103 of SMA actuator 102 to retract. Output drive member 103 is typically a plate, rod, or some other member configurable to drive a load upon which SMA actuator 102 operates. A perspective view of output driver member 103 is shown in
Motor 104 is a drive mechanism for rotating shaft 106. The drive mechanism can operate by converting electrical energy, mechanical energy, thermal energy, or some other type of energy into mechanical energy for imparting a rotational force upon shaft 106. In some embodiments, motor 104 operates to turn shaft 106 in one direction of rotation while payload 110 tends to cause shaft rotation in the other direction. In some cases, motor 104 can be a motive force provided by a human being (i.e., a person causes rotation of shaft 106). Payload 110 is the object that helical spring brake 100 operates upon via shaft 106 to either permit that object to move or to inhibit its motion. Examples of such objects include windows, window regulators, window shades, car seats, seat head-rests, latches, vehicle power door locks, power glove-box locks, gas-tank flaps, trunk lids, or any other type of object where it is desirable to control the motion of that object. In some cases, payload 110 provides its own bias force that biases rotation of shaft 106 in direction D1. For example, if payload 110 is a window, gravity acting on the mass of the window is the bias force for that window.
Helical wrap spring 108 is configured to maintain frictional engagement with shaft 106 when spring end 108a is not being acted upon by SMA actuator 102. Specifically, when SMA actuator 102 is in its extended state (i.e., inactive or unpowered), helical wrap spring 108 remains firmly wrapped in a direction around shaft 106 such that if the bias of payload 110 causes rotation of shaft 106, the frictional engagement between shaft 106 and helical wrap spring 108 strengthened. As such, payload 110 cannot rotate in direction “D1.” But when SMA actuator 102 is in its contracted state (e.g., when it is triggered or activated by a user), output drive member 103 of SMA actuator 102 acts upon spring end 108a to deflect it by an amount that tends to unwrap spring 108 from shaft 106. This deflection permits helical wrap spring 108 to operate in frictionless engagement with shaft 106 so that shaft 106 is free to rotate in either direction “D1” or “D2.”
In accordance with various embodiments of the present invention, helical spring brake 100 provides a braking mechanism regardless of whether motor 104 is found in any of three states. In a first state, motor 104 is unpowered and does not engage or rotate shaft 106. In a second state, motor 104 is configured to rotate shaft 106 against the bias of payload 110. And in a third state, motor 104 is configured to rotate shaft 106 in the same direction as the bias of payload 110.
In the first state, consider that motor 104 and SMA actuator 102 are both inactive. In this state, the bias of payload 110 can cause shaft 106 to begin rotating if helical wrap spring 108 is not sufficiently engaged with shaft 106. Because of the orientation of the wrapped turns of helical wrap spring 108, friction between shaft 106 and helical wrap spring 108 acts to wind the latter more tightly around the former until rotational movement of shaft 106 ceases completely. Helical wrap spring 108 can be configured so that its winding about shaft 106 is almost imperceptible and its response time is almost instantaneous when braking has been initiated.
In the second state, consider that SMA actuator 102 is inactive (i.e., in an extended and slack state) and motor 104 is active or powered to rotate shaft 106. Motor 104 drives shaft 106 to slightly unwind helical wrap spring 108 in direction D2, thereby significantly reducing the friction between shaft 106 and helical wrap spring 108. Thereafter, the load presented to motor 104 is primarily that of payload 110. So to rotate shaft 106 in direction D2, the torque output of motor 104 should exceed the torque load of payload 110. Once the torque generated by motor 104 surpasses that of the torque load of payload 110 by an amount at least equal to the residual frictional torque of helical wrap spring 108 about shaft 106, shaft 106 rotates in direction D2. For example, consider that payload 110 is a window. Once motor 104 generates enough torque to overcome the weight of the window (as a bias) and the slight residual friction of helical wrap spring 108, the window will move from a rolled-down position to a rolled-up position.
In the third state, consider that SMA actuator 102 is active and in a state of contraction, whereas motor 104 is inactive (i.e., unpowered). In this state, SMA actuator 102 moves spring end 108a in an unwinding direction. This movement causes the frictional force exerted by helical wrap spring 108 on shaft 106 to be significantly reduced. If the bias of payload 110 is sufficient to overcome other frictional torques of the system (e.g., that of an unpowered motor 104), then shaft 106 rotates in direction D1. Continuing with the previous example, the weight of the window as payload 110 causes the window to move from a rolled-up position to a rolled-down position. In some embodiments, motor 104 can be activated in this state to urge movement of payload 110 if the bias of payload 110 is insufficient to overcome the frictional forces in the system or if greater speed is desired.
Deflection detector 202 is communicatively coupled to SMA actuator 102, and is configured to detect when spring end 108a deflects by the distance, d, from a first position, “P1,” to a second position, “P2.” Position P1 indicates the position of spring end 108a at which helical wrap spring 108 frictionally engages shaft 106 to inhibit rotation, whereas position P2 indicates the position of spring end 108a at which shaft 106 freely rotates. Deflection detector 202 can include a wiper 205 configured to maintain contact with output drive member 203 as it retracts and extends. Wiper 205 sends a signal indicating contact with either point 202a or point 202b. Point 202a is coincident with the beginning of travel for output drive member 203 as well as position P1, and point 202b is coincident with the end of travel for output drive member 203 as well as position P2. Given this, deflection detector 202 permits SMA actuator 102 to operate between a beginning and an end of travel respectively coincident with position P1 and position P2.
In operation, spring end 108a typically starts at position P1. Deflection detector 202 will indicate this position to SMA actuator 102. When a user triggers SMA actuator 102, output drive member 203 moves spring end 108a to position P2 as SMA actuator 102 contracts. A suitable trigger can be a push button conveniently accessible to a user, or the trigger can be any other mechanism for closing an electrical circuit so that current will pass through the SMA elements of SMA actuator 102. When deflection detector 202 detects point 202b (i.e., end of travel has been reached, spring end 108a is at position P2), then SMA actuator 202 powers down its SMA wires so as not to overheat them. A suitable circuit for practicing one embodiment of deflection detector 202 in accordance with the present invention is disclosed in U.S. patent application Ser. No. 10/080,640, titled “SMA Actuator with Improved Temperature Control” and filed on Feb. 21, 2002, the disclosure of which is incorporated herein by reference in its entirety. In some embodiments, SMA-actuator 102 powers its SMA elements for a duration of time necessary to deflect spring end 108a from P1 to P2, after which deflection detector 202 powers down those SMA elements. So, even if an obstruction hinders deflection of spring end 108a, the SMA wires of SMA actuator 102 will not overheat.
In some embodiments, an end-of-travel switch may be conveniently located to detect the movement of the actual payload. Specifically, an end-of-travel having a similar structure to deflection detector can operate to detect a position indicating the end of movement for a payload. In the case of a window lifter mechanism, for instance, such a switch might detect the arrival of the window to its fully lowered position. Also, a power circuit (not shown) can be configured using conventional design techniques to disable its power so that any further triggering (e.g., push-button depression) at trigger 101 by a user will not cause any additional power to flow to the SMA actuator, thereby preserving the longevity of the SMA elements of SMA actuator 102, among other things.
After SMA actuator 102 is triggered to release helical wrap spring 108, output drive member 203 begins retracting to deflect spring end 108a from position P1 to position P2. In one embodiment, SMA actuator 102 deflects spring end 108a relatively quickly so shaft 106 begins rotating expeditiously. Rotation detector 254 detects rotation of shaft 106 and permits SMA actuator 102 to continue operation, without regard to the rate of rotation, until shaft 106 stops rotating. Once rotation detector 254 detects no rotation, it will disable current from being applied through the SMA elements. As shaft 106 rotates in the other direction, rotation detector 254 is configured to determine whether SMA actuator 102 was powered before rotation begins. If it was not powered before shaft 106 begins rotation, then rotation detector 254 refrains from powering the SMA elements of SMA actuator 102. This means the shaft is rotating in a direction opposite than before. In another embodiment, rotation detector 254 is configured to detect a specific rate of rotation (or a range of rates of rotation) and modulates the passage of current into SMA actuator 102 to maintain shaft 106 rotating at a desired rate of rotation, until shaft 106 ceases rotation. A variety of other sensors and signals may also be configured to control the flow of power to the SMA actuator. For instance, a window lift end-of-travel switch may work in conjunction with rotation detector 254. In this case, if the rotation stops before the end-of-travel switch is triggered, an unsafe condition may be inferred (such as due to a trapped human limb), and power to the motor may be interrupted to curtail the unsafe situation. A similar condition may be concluded based of reading the signal from a load cell mechanically placed in series between the motor and the window.
Window lifting mechanism 400 operates as follows. When SMA actuator 402 is inactive with its output drive member 403 extended and slack, helical wrap spring 408 is an frictional engagement with shaft 406. Helical wrap spring 408 is coupled at spring end 408a to output drive member 403 and is coupled at spring end 408b to anchor point 414. With the direction of the wrapped turns of helical wrap spring 408 shown in
Vehicle seat back release mechanism 800 includes an SMA-actuated helical spring brake composed of SMA actuator 802, helical wrap spring 808 and anchor member 828. Helical wrap spring 808 frictionally engaged with shaft 806 when SMA actuator 802 is inactive, and has a first spring end 808a coupled to an output drive member of SMA actuator 802 and a second spring end 808b anchored at anchor member 828. SMA actuator 802 and anchor member 828 are rigidly coupled to seat back 804. Seat back 804 can be viewed as a motor that provides a motive torque to vehicle seat back release mechanism 800. A human hand typically applies the motive torque applied to seat back 804.
Shaft 806 is rigidly coupled to coupling 820 and energy storage device 830. In this example, energy storage device 830 is a torsion spring for storing potential energy. Torsion spring 830 is thus configured to provide a bias torque to shaft 806 when helical wrap spring 808 is released. The bias torque from torsion spring 830 is designed to operate upon coupling 820, which is the previously-described payload. Coupling 820 engages a lock release 822 that is configured to release safety lock 824. Safety lock 824 prevents inadvertent movement of seat back 804, especially during moments of extreme deceleration.
Vehicle seat back release mechanism 800 operates as follows. First, consider that seat back 804 is in an upright position and safety lock 824 is locked to prevent inadvertent forward or rearward movement of seat back 804. Also, torsion spring 830 has already been charged upon previously rotating seat back 804 from a forward position to the upright position in direction “charge” 832. Typically, the bias torque of torsion spring 830 is generated by human hand. This bias force causes shaft 806 to rotate against the turns of helical wrap spring 808, thereby causing helical wrap spring 808 to be in its extra wrapped state. As such, torsion spring cannot discharge while shaft 806 is in frictionally engaged.
To enter or exit rear seat, a passenger triggers or activates SMA actuator 802 to cause helical wrap spring 808 to unwrap slightly so as to release its grip on shaft 806. When helical wrap spring 808 releases shaft 806, the bias torque from torsion spring 830 causes coupling 820 to disengage lock release 822, thereby causing safety lock 824 to unlock. Since unlocking of safety lock 824 permits relative rotation between seat back 804 and shaft 806, the bias torque from torsion spring 830 does not itself affect the position of seat back 804.
When a user tilts seat back 804 in a forward direction for entry or exit, anchor member 828, which is firmly attached to seat back 804, follows along in unison. The unwrapping action expected primarily at spring end 808b, however slight, permits a relative rotation of both seat back 804 and helical wrap spring 808 about shaft 806, which remains stationary. Although SMA actuator 802 can be typically inactive with its output drive member relaxed or extended during the forward movement of seat back 804, SMA actuator 802 can optionally be activated to reduce friction with shaft 806.
Next, when a user again brings seat back 804 to an upright position, the wrapping action of the coil of helical wrap spring 808 at end 808b causes it to frictionally engage shaft 806. SMA actuator is unpowered during the return of seat back 804 to its upright position. So, as seat back 804 continues moving to its upright position, shaft 806 transfers torque to recharge torsion spring 830. Then, coupling 820 returns to its initial state of engaging lock release 822. Once seat back 804 is returned it its upright position, safety lock 824 moves in direction “reset” 826 to again lock seat back 804 against inadvertent seat back movement.
Bender actuators are well known and are constructed from any of several technologies, such as piezoelectric, bi-metal (thermally bendable), as well as SMA actuators. Bender actuators typically consist of two elongated members joined mechanically either at both ends, or entirely along their common length. Generally, the two members differ in their responsiveness to external stimuli. For instance, in a piezoelectric bender actuator, one member can be a piece of active ceramic whereas the other is a passive element. The passive element can be any of the following: a passive ceramic, a passive metal, or an active ceramic, but with opposite sign of responsiveness. When an appropriate stimulus is applied to a bender actuator as actuator 902, bender actuator either bends or tilts (i.e., does not necessarily contract or expand linearly). For example, if actuator 902 is a bender actuator, then it has one end fixed to support 910 via insulator 912b. The other end of actuator 902 is coupled to first spring end 908a via insulator 912a. As such, the bender actuator is able to perform mechanical work, even though it can generate forces along a curved path that is generally perpendicular to the long direction of actuator 902. Conceptually, first spring end 908a of actuator 902 can be viewed as being part of actuator 902, when configured as a bender actuator.
Regardless, the bending action inherent in bender actuators works by deflecting the movable end (i.e., first spring end 908a) in a direction that unwraps the coil of helical wrap spring 908. This action advantageously includes both an angular tilt component and a small tangential translation component of first spring end 908a. Both of these components work in the direction of unwrapping helical wrap spring 908. The balance between the tilt and translation components depends on the initial distance, “id,” from a connection point associated with insulator 912b to a connection point associated with insulator 912a. The availability of both of these components means that distance “id” need not be set with excessively tight tolerance. This, in turn, makes the manufacturability of products more forgiving of assembly inaccuracies.
The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that specific details are not required in order to practice the invention. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed; obviously, many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, they thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. Any feature of any specific embodiment of the present invention can be employed in any embodiment described herein. It is intended that the following claims and their equivalents define the scope of the invention.
This application claims benefit under 35 U.S.C. 119(e) of United States Provisional Patent Application Number 60/484,021 filed Jun. 30, 2003 entitled “SMA-Actuated Helical Spring Brake,” the disclosure of which is incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 60484021 | Jun 2003 | US |
