Torque limiting clutch for orthotic and prosthetic devices

A torque limiting assembly for use with an actuator having a torque transfer output, the assembly comprising a brake pad, a band connected at a first extremity to a first extremity of the brake pad and a cam linking a second extremity of the band to a second extremity of the brake pad. The brake pad and the band are configured to wrap around the a torque transfer output and the positioning of the cam in a first position frictionally engages the brake pad and the band with the a torque transfer output while the positioning of the cam in a second position disengages the brake pad and the band with the a torque transfer output.

TECHNICAL FIELD

The present disclosure relates to a torque limiting clutch for orthotic and prosthetic devices.

BACKGROUND

Rotating systems can have enough rotating energy (i.e. inertia) to cause significant machine damage during a jam-up or system crash. This inertia varies based on the RPM and rotating mass for each application. A high mass low speed application could do more damage than a low mass high speed one during a crash. In order to prevent damage, a torque limiter, which is basically a mechanical fuse, is used to shut down the machine and allow the rotating energy to dissipate without causing excessive damage.

Different types of torque limiters are currently being used in the industry, namely disconnect, shear pin, synchronous magnetic, ball detent, pawl and spring, etc.

The only type of torque limiters that allow for the dissipation of rotation energy through friction is the disconnect types (i.e. torque limiting clutches) who “wait” for the torque to diminish to a predetermined level before reengaging.

The main limitations with the existing disconnect type torque limiters are that they are cumbersome (vs. torque capacity) and typically cannot be engaged or disengaged manually. Furthermore, they do not actively dissipate rotational energy.

Accordingly, there is a need for a torque limiting clutch that is compact, can be engaged or disengaged manually and dissipates rotational energy.

SUMMARY

The present disclosure provides a torque limiting assembly for use with an actuator having a torque transfer output, the assembly comprising:

a brake pad;

a band connected at a first extremity to a first extremity of the brake pad; and

a cam linking a second extremity of the band to a second extremity of the brake pad;

wherein the brake pad and the band are configured to wrap around the torque transfer output and whereby the positioning of the cam in a first position frictionally engages the brake pad and the band with the torque transfer output, and positioning of the cam in a second position disengages the brake pad and the band with the torque transfer output.

There is also provided a torque limiting assembly as described above, further comprising a tension adjustment mechanism connecting the first extremity of the band to the first extremity of the brake pad, wherein the tension adjustment mechanism is configured so as to augment or diminish the frictional engagement of the brake pad and the band with the torque transfer output when the cam is in the first position.

There is further provided a torque limiting assembly as described above, further comprising a liner positioned on a surface of the band and/or the brake pad that is in frictional engagement with the torque transfer output when the cam is in the first position, in one or more embodiments the liner is made of a material having a high friction coefficient and/or wear resistant and/or compressible.

There is still further provided a torque limiting assembly as described above, wherein the cam is composed of two eccentric baring surfaces off center with regard to one another.

There is also provided actuated orthotic device, comprising:a proximal and a distal brace structures for attachment to a limb of a user;an actuator operatively connecting the proximal and the distal brace structures, the actuator including a torque transfer output connected to one of the proximal and the distal brace structures to impart movement to the limb of the user; anda torque limiting assembly as described above, positioned around the torque transfer output, the torque limiting assembly limiting the maximum force exerted on the user.

DETAILED DESCRIPTION

Generally stated, the non-limitative illustrative embodiment of the present disclosure provides a torque limiting clutch consisting in a band brake mechanism for an actuator that starts slipping when the torque is higher than a preset torque. The torque limiting clutch serves to:protect the actuator (transmission, motor and other mechanical components) from external high impact shocks;protect the actuator from inertial loads when the actuator is hitting a travel extension limit (hard stop) at high speed. In this case extreme loads can occur if the kinetic energy is not dissipated somewhere; andprotect the environment from high forces that could be exerted by the actuator.

Another feature of the torque limiting clutch is to allow manual disengagement of the actuator in case of malfunction of the actuator or power failure.

Referring toFIGS. 1 and 2, the torque limiting clutch10consists of a band12that is wrapped partly around an actuator torque transfer output, which in the illustrative embodiment is a rotating drum14, and a brake pad24. It is to be understood that the torque transfer output could be another rotating mechanism such as, for example, a rotating shaft. Tension can be applied to the band12using, for example, a lever16attached to a cam18linking the band12to a first extremity of the brake pad24. It is to be understood that mechanisms other than a lever16may be used to operate the cam18. The tension can be adjusted using a tension adjustment mechanism here in the form of a screw20operatively connecting the band12to a second extremity of the brake pad24, wherein rotation of the crew20in a first direction pulls the band12into a cavity25within the brake pad24, therefore augmenting the frictional engagement of the brake pad24and the band12with the rotary drum14rotation the opposed direction allows the band12to pull out of the cavity25, therefore diminishing the frictional engagement of the brake pad24and the band12with the rotary drum14. A liner22, for example made of a high friction material, serves as a friction surface. The liner22is advantageously wear resistant and somewhat soft or compressible in order to “encapsulate” any dirt or particles that may be trapped between the band12, the brake pad24and the rotating drum14. The band12, the brake pad24and the rotating drum14are advantageously made from the same material, for example Alu 7075-T6, to ensure that the braking force does not change significantly when the temperature fluctuates due to, for example, thermal expansion. The rotating drum14may be, for example, hard anodized (i.e. type II, 50 μm) for wear resistance.

The torque limiting clutch10can be disengaged, for example by lifting in direction “L” the lever16(seeFIG. 3), positioning the cam (18) in a position that allows for completely free motion of the rotating drum14. The torque limiting clutch10can be disengaged in case of, for example, malfunction or power failure.

The cam18provides the tension in the band12when the lever16is engaged. The cam18is designed with two eccentric bearing surfaces18aand18b(best seen inFIGS. 2 and 3) that are off center with regard to one another. After assembly, the lever16is advantageously micro-welded onto the cam18thus making them unitary. The tension adjustment mechanism, i.e. screw20, is used to adjust the band12tension in order to get the desired braking torque.

Referring now toFIGS. 4 and 5, there is shown an illustrative example of the torque limiting clutch10integrated with an actuated orthotic device30having proximal32and distal34brace structures for attachment to the limb of a user. The proximal32and distal34brace structures are operatively linked together via an actuator36designed to impart movement to the limb of the user. The torque limiting clutch10is located inside the housing of the actuator36and ensures that the actuator36, as well as the other components of the orthotic device30(i.e. brace structures32and34, torque sensor38, etc.) are not damaged by external shocks, such as when the user lands after a jump, or when the orthotic device30reaches its extension limit. Furthermore, the torque limiting clutch10limits the maximum force exerted on the user, thus reducing the risk of injury in the event of shocks.

Referring toFIG. 5, the actuator36comprises a housing assembly40a,40bfor holding the various components of the actuator36, namely the torque sensor beam38a, configured to be connected to the brace structure32, the torque sensor magnet38b, the motor stator42and rotor44, the transmission wave generator46, the transmission circular spline48, the rotating drum14with transmission flex spline50, the center shaft52with conical bearings54, and the angle sensor56. In the present illustrative embodiment, the torque limiting clutch10further comprises an angle sensor magnet26mounted on a shaft28supported by the brake pad24. The brake pad24being configured to be connected to the distal brace structure34, the angle sensor magnet26and the angle sensor56cooperate together in order to provide a measure of the angle between the proximal32and distal34brace structures.

Braking Force and Stress

Referring toFIG. 6, there is shown a conceptual representation for the computation of the braking force of the torque limiting clutch10and the stress in its various components for the above described illustrative example.

Equation 1 illustrates the relation between band12tensions, coefficient of friction of the liner22and wrapping angle:
T1/T2=eμθtEquation 1
where:

μ is the friction coefficient of the liner22;

θtis the wrapped angle of the band12.

In the illustrative embodiment ofFIGS. 4 and 5, the desired braking torque capability is established at 75 Nm, although the desired braking force is of 50 Nm. It is to be noted that the brake pad24to which is attached the band12also provides a braking force. The total force applied on the brake pad24being roughly T1+T2, we find the total braking torque to be:
Torque=(T2−T1)×D/2+μ(T2+T1)×D/2  Equation 2

Using Equations 1 and 2 with θ1220°, μ=0.1, D=0.065 m (where D is the diameter of the rotating drum14) and Torque=75 Nm, we find the following tensions in the band:
T1=4987N
T2=3571N

In the illustrative example, the cam18is designed with two eccentric bearing surfaces18aand18b(best seen inFIGS. 2 and 3) that are off center by 0.5 mm. The stress level in the band12area is of about 200 MPa, while the stress in the cam18area is up to about 315 MPa. Considering that the band12is not submitted to a large amount of cycles, this stress level is considered acceptable. Assuming the maximum stress to be 350 MPa and the minimum stress to be 0 MPa, it can be estimated that the band12can resist at least 50 K engaging/disengaging cycles.

The average pressure on the liner22is found to be of about 16 MPa (with the band12width being of 8 mm). This pressure level is acceptable for the use of, for example, Acetal plastic which has a limit of 70 MPa.

The tension stress in the adjustment screw20is computed at 580 MPa. The stress level in the adjustment screw20being important, a screw with a grade of at least 8.8 is recommended.

The above calculations were calculated using a finite element analysis.

In the illustrative example, the linear displacement of the extremity of the band12being of 0.18 mm and the deformation of the rotating drum14providing a dimensional change of the same order of magnitude, the cam18must provide sufficient travel to compensate for the extension of the band12and compression of the rotating drum14in order to engage and disengage efficiently.

It is to be understood that the various materials, values and measurements of the components of the torque limiting clutch10have been described in accordance with an illustrative embodiment of the present disclosure and that these may vary depending, for example, on the specific application.

It is further to be understood that the torque limiting clutch10may be adapted for applications other than orthotic and prosthetic devices without departing from the scope of the present disclosure.

Although the present disclosure has been described with a certain degree of particularity and by way of an illustrative embodiments and examples thereof, it is to be understood that the present disclosure is not limited to the features of the embodiments described and illustrated herein, but includes all variations and modifications within the scope and spirit of the disclosure as hereinafter claimed.