Actuator with variable mechanical gain, and associated method

Abstract

An actuator for providing a thrust force over a determined travel, comprising a nut, a screw, a sleeve configured to surround the screw in an axial direction (X-X) of the screw, a plurality of rollers, the nut being configured to cooperate with the screw, the nut being secured to the rollers, which are free to rotate, the rollers being configured to each move in particular in the one of at least one helical guide or the sleeve, the screw being configured in order, when it is turned, to operate the actuator, to rotate the nut when it bears via the rollers on a profile of the helical guide and to thus advance in the axial direction (X-X) of the screw at greater or lesser speed along a slope of the profile of the helical guide. The actuator further comprises a motor configured to turn the screw. Successive values of the slope all along the profile or profiles of respectively the one or more helical guides ae adapted to ensure that the slope systematically compensates for at least one peak of the desired thrust force in order to carry out clipping of the maximum values of the thrust force such that the motor, the screw and the nut are dimensioned to a motor torque value corresponding to the value of the thrust force after clipping.

Claims

1. An actuator for producing a thrust force over a particular travel, comprising: a nut, a screw, a bush configured to surround the screw in an axial direction (X-X) of the screw, a plurality of rollers, the nut configured to cooperate with the screw, the nut fastened to the rollers, free to rotate, and each roller configured to move in one of at least one helical guide of the bush, the screw configured so that when the screw is turned the screw causes the actuator to function to drive the nut in rotation when the nut bears via the rollers on a profile of the helical guide and therefore to advance in the axial direction (X-X) of the screw more or less quickly depending on the slope of the profile of the helical guide, the actuator further comprising a motor configured to turn the screw, wherein the profile of the at least one helical guide follows a hyperbolic law.

2. The actuator as claimed in claim 1 in which the slope all along the profile of the helical guide is at each point inversely proportional to a force to be supplied at each of the respective points.

3. The actuator as claimed in claim 1 in which the actuator includes the helical guide or a plurality thereof and one or more rectilinear guides, each helical guide having its own variable slope profile, the number and the distribution of the one or more rectilinear guides and the number and the distribution of the one or more helical guides of the actuator and the profile of each individual helical guide being determined as a function of the position and the value of various loads along the travel of the actuator.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be better understood from the following detailed description of embodiments of the invention given with reference to the figures, in which

(2) FIG. 1 depicts a prior art actuator device with variable mechanical gain;

(3) FIG. 2 contains a graph showing a force F that varies linearly from 0 to 10,000 N over a travel of 2 mm in accordance with one example of the invention;

(4) FIG. 3 contains a graph depicting the slope of the guide in an actuator device with mechanical gain as a function of the travel in accordance with one example of the invention;

(5) FIG. 4 contains a graph depicting the guide profile obtained by applying the calculation of the slope from FIG. 3 to a rotation of 2.5 radians;

(6) FIG. 5 contains a graph depicting a constant motor torque as a function of an angle; and

(7) FIG. 6 contains a flowchart to depict a method of optimization of at least one slope of the profile of a helical guide in an actuator.

(8) The same references may be used to designate the same elements or similar elements that appear in more than one figure.

DETAILED DESCRIPTION

(9) The kinematics defined in the actuator device with variable mechanical gain described in the publication EP1370388B1 are optimized by variation of the mechanical gain of the kinematic, which corresponds to adding an additional adjustment parameter compared to the prior art device. FIG. 1 repeats basic elements known from the aforementioned patent EP1370388B1.

(10) A motor M actuates directly a recirculating ball screw 10 in a first member 30. The recirculating ball screw 10 must have a very good inverse yield. A nut 12 collaborates with the recirculating ball screw 10, that nut 12 being fastened to a first piece 14 that transmits the thrust to a sub-assembly 36 that is in fact a second member that delivers an output thrust. Thrust ball bearings and a guide 24 guide a second part 42 of the second member that produces an output thrust force over a particular travel (the particular travel is not depicted in FIG. 1). The nut 12 also carries a plurality of rollers 28 that are fastened to it but free to rotate. Each roller 28 rolls in a guide that may have a rectilinear part 34L and that has a helical part 34H the pitch of which is reversed relative to that of the recirculating ball screw 10. It is the helical part 34H of the guides that generates almost all of the force for which the device was designed, the rectilinear part 34L essentially producing the closer movement.

(11) The pitch of the screw 10 is large so that its inverse yield is good and an axial force applied to the nut 12 is sufficient to drive rotation of the nut 12 on the screw 10. In the following analysis this makes it possible to consider that the axial force generated by the screw 10 is negligible compared to the axial force produced by the rollers 28 when those rollers 28 are in the helical guides 34H. When the rollers 28 are in the helical guides 34H the rotation of the nut 12 relative to the screw 10 enables movement of the nut 12 to cause it to follow the profile of the guide 34H.

(12) The present invention implies an analysis of the slope all along the profile of the helical part 34H of the guides in order to adapt the variations of the slope all along those guides to the force to be produced and thus to optimize the kinematic and the drive system to render the kinematic drive assembly of the actuator device with variable mechanical gain as small as possible, as light as possible, as performing as possible and of the lowest possible cost.

(13) The objective of adapting the successive values of the slope all along the profile of the guides (34H) one at a time is for the guides (34H) systematically to compensate the peaks of the desired thrust force in order to produce true clipping of the maximum values of the motor torque so that, in contrast to the prior art device, it is not these peak values that impose the definition thereof. This maximum motor torque value will therefore be systematically lower than that established with a normal kinematic corresponding to the peak values of the cycle.

(14) The kinematic drive assembly defined in this way will therefore be lighter, smaller and therefore less costly than the prior art system, and the motor will include fewer magnets (rare earthssamarium cobalt). Moreover, the lower inertial level of the kinematic drive assembly will confer on that assembly a much higher level of performance, in particular in respect of bandwidth.

(15) If the motor produces a constant torque, the latter will have the mean value corresponding to the transfer during the cycle of the energy necessary to produce the expected function and therefore correspond to the minimum possible value of said motor torque.

(16) As a general rule, if the motor produces a constant torque, there should be localized variations of the slope of the guides inversely proportional to those of the force to be delivered.

(17) Thus if the force to be delivered is proportional to the travel, the slope of the guides 34H will follow a hyperbolic law in order for the product of the force by the slope to remain constant.

(18) Thus the invention aims to optimize the profile of the guides 34H, and this variation of the successive values of the slope all along the profile of these guides, and therefore of the mechanical gain of the kinematic system, corresponds to a real additional adjustment parameter compared to the prior art device. It is this specific feature that makes it possible to provide the expected function with a motor and kinematic lighter, less costly and better performing than what can be achieved using the prior art system.

(19) The same actuator may include no, one or a plurality of rectilinear sectors or rectilinear guides 34L and one or more variable slope sectors or helical guides 34H, each variable slope sector 34H being able to have its own individual profile. The number and the distribution of said rectilinear sectors 34L and variable slope sectors 34H of said actuator and the individual profile of each of the variable slope sectors 34H are determined as a function of the position and the value of the various loads along the travel of the load to be driven.

(20) FIG. 2, provided by way of example, shows a force F that various linearly from 0 to 10,000 N over a travel of 2 mm.

(21) In this example the force is given by the mathematical formula:
F=KY where F is the force in Newtons; K is the coefficient of the function in Newtons per meterin the example 5 10.sup.6 Newtons per meter; Y is the travel in metersin the example 0.002 m; and x is the sign signifying multiplied by. In accordance with the invention the slope of the guide at a given point is inversely proportional to the force at that point and the mathematical formula is
Py=C/Fy where Py is the slope of the guide at the position Y in meters per radian; C is the motor torque in Newton meters; Fy is the force at the position Y in Newtons; and / is the sign signifying divided by.

(22) The energy W is the integral of the function 5 10.sup.6*Y, Y varying from 0 to 0.002 m, i.e. 5 10.sup.6*(0.002).sup.2/2=10 Joules.

(23) To obtain an energy value of 10 Joules over a rotation of 2.5 radians at constant torque that constant torque must be 10 Joules/2.5 radians=4 Newton meters.

(24) With this 4 Newton meters value of the torque the slope of the guide may be calculated at any point. For example at the following points: at the position Y=1 mm (mid-travel) the force is 5000 N and the slope of the guide is therefore equal to 4/5000=0.0008 m/radian; at the position Y=2 mm (end of travel) the force is 10,000 N and the slope of the guide is therefore equal to 4/10,000=0.0004 m/radian.

(25) FIG. 3 represents the slope of the guide as a function of the travel.

(26) The start of the travel when the slope of the guide is greater than 4 mm/radian is not represented. In effect, for very low values of the force F the slope of the curve theoretically becomes very high. But in practise and in the context of the prior art device the guide profile becomes rectilinear and parallel to the screw and when the guides are rectilinear it is the screw that produces the movement of the load. With a screw slope of 4 mm/radian the force supported and supplied by the screw is 4 Nm/0.004=1000 N, i.e. 1/10.sup.th of the maximum force.

(27) FIG. 4 represents the guide profile obtained by applying the slope calculation from FIG. 3 to a rotation of 2.5 radians. The point 0,0 is offset from the axes for greater clarity.

(28) The guide profile represented allows a constant torque of 4 Nm and, by causing the rollers to turn in the guide by 2.5 radians, a force to be applied increasing from 0 to 10,000 N over a travel of 2 mm.

(29) FIG. 5 represents the motor torque that is constant.

(30) Other examples could be given, like a force varying sinusoidally. This is the case of an aircraft flap or a vertical control surface that turns about an axis perpendicular to the flow of air; the axis of a flap is horizontal and the axis of a vertical control surface is vertical. The master torque of a flap or vertical control surface (the surface exposed to the flow of air) varies sinusoidally as the angular position of the flap or vertical control surface increases. The force exerted by the airflow is proportional to the master torque and therefore varies sinusoidally with the angle. To cause the flap or vertical control surface to turn a linear actuator acts at a point situated at a certain distance from the rotation axis. The force that the actuator must exert is sinusoidal and it will be very beneficial to apply the present invention. In all cases, the slope of the guide at each point is calculated by dividing the motor torque by the force at this point; the motor may be sized according to the mean value.

(31) FIG. 6 contains a flowchart to illustrate the method of optimizing at least one slope of the profile of a helical guide in an actuator as described hereinabove. The method comprises the following steps: definition 600 of a function F(y) of a force F to be supplied by the actuator as a function of the travel y travelled by the actuator determination 601 of the slope Py of the helical guide at each position y as being inversely proportional to the force F(y) at that position y for a constant torque C of the motor (M) in accordance with the formula Py=C/F(y) determination 602 of a predetermined energy value E.sub. in Joules to be supplied over one rotation of the nut 12 by radians as being linked to the constant torque C in Newton meters in accordance with formula C=E.sub./.