A CABLE ACTUATOR WITH IMPROVED FORCE SENSITIVITY

A cable actuator (100) comprising:          a frame (10);     a screw (2) rotatably mounted on the frame (10) and extending along a first axis (Ox);     a nut (4) cooperating with the screw (2);     means (30) for estimating the angular movement of the nut (4) about the first axis (Lx) relative to the frame (10); and     a distance sensor (30) secured to the frame (10) and having a winder (33) for winding a thread (32), one end (32.1) of the thread (32) connected to the nut (4) at a connection point (4.1);       wherein the distance sensor (30) is arranged in such a manner that the thread (32) changes curvature at a first point (34) situated in a first plane (P1) orthogonal to the first axis (Ox).

FIELD OF THE INVENTION

The invention relates to a cable actuator having a screw-and-nut assembly in which the nut is movable in translation and is coupled by a cable to an element that is to be moved and that is provided with a force sensor. The invention relates more particularly to cable actuators in which the cable performs an anti-rotation function for preventing the nut from turning relative to the screw.

BACKGROUND OF THE INVENTION

Cable actuators are known that comprise a screw mounted on a frame and a nut cooperating with the screw. The nut is associated with anti-rotation means, such that rotation of the screw relative to the nut causes the nut to be moved axially. One or more cables associated with the nut are connected to an outlet of the actuator, which may be rotary (when the cables are connected to pulleys), or linear (when the cables are connected directly to the load that is to be manipulated).

Force sensors for such actuators are generally mounted directly on the outlet of the actuator, and they are found to be bulky, expensive, and/or not very accurate. Also, since such force sensors are coupled directly to the segments of the articulated arm, they need to withstand the impacts and the vibration that come from the segments and the loads they support. In order to avoid being too fragile, they must therefore be over-sized, thereby increasing their volume and decreasing their sensitivity. Thus, although cable actuators present characteristics that are advantageous, in particular in terms of compactness, it is difficult and expensive to operate them with force control, which restricts their distribution.

OBJECT OF THE INVENTION

An object of the invention is to improve the accuracy and the manufacturing and/or maintenance costs of a cable actuator.

SUMMARY OF THE INVENTION

To this end, there is provided a cable actuator comprising:a frame;a screw rotatably mounted on the frame and extending along a first axis;a nut cooperating with the screw;a first cable coupled to the nut and functionally connected to an outlet of the actuator;a second cable coupled to the nut and functionally connected to the outlet of the actuator; anda motor arranged to drive the screw in rotation;

the first cable being arranged to exert forces that oppose the nut being driven in rotation by the screw so as to constitute anti-rotation means such that turning of the screw under drive from the motor causes the nut to move along the screw between a first extreme position and a second extreme position that define a stroke for the nut; and

the cable actuator also comprising:means for estimating the angular movement of the nut about the first axis relative to the frame; andmeans for estimating the force being applied to the outlet of the cable actuator as a function of the angular movement of the nut about the first axis;

wherein the means for estimating the angular movement of the nut comprise a distance sensor secured to the frame and using a thread to sense distance, one end of the thread being connected to the nut at a connection point. According to the invention, the distance sensor is arranged in such a manner that the thread changes curvature at a first point situated in a first plane orthogonal to the first axis, the plane being situated at a first distance from the first extreme point that lies in the range 30% to 70% of the stroke.

An actuator is thus obtained that is provided with a sensor that is simple and that measures the angular position of the nut accurately. The location of the first point improves the sensitivity of the sensor by reducing the maximum stroke of the sensor. The ratio of the working movement of the sensor divided by its total movement is representative of the signal-to-noise ratio of the sensor, and by means of the invention, this is improved.

Advantageously, the first distance lies in the range 40% to 60% of the stroke, and is preferably 50%.

Advantageously, the first point is situated at a nonzero second distance from a straight line that connects the first axis to the connection point.

Advantageously, the change of curvature of the thread is obtained by a drum of the winder.

It is possible to adapt the location of the sensor as a function of other design requirements when the change in the curvature of the thread is obtained by means of a thread deflector, which could possibly be a pulley.

Preferably, the distance sensor comprises a winder for winding thread on a drum, and the change of curvature of the thread is obtained by the drum of the winder. The thread may occupy a plurality of turns around a drum of the winder.

Alternatively, the distance sensor using a thread comprises a linear movement sensor.

Other characteristics and advantages of the invention appear on reading the following description of a particular and nonlimiting embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

With reference toFIG.1, the actuator of the invention, given overall reference100, comprises a frame10, specifically a portion of a right cylinder11having a base12with a bearing13at its center receiving a screw2to rotate about a horizontal first axis Ox. The screw2is a ball screw of pitch p2that is driven in rotation by an electric motor3having a first rotary encoder3.1. A nut4co-operates with the screw2and has a first eyelet5projecting radially from the nut4. A first cable6extends parallel to the first axis Ox and includes a first segment6.1that is held at its first end6.2in the first eyelet5by a first crimp7.1. The second end6.3of the first segment6.1of the first cable6is crimped to a first pulley14that is secured to a first shaft16rotatably mounted on the frame10to rotate about an axis perpendicular to the first axis Ox. The first cable6extends substantially parallel to the first axis Ox on both sides of a plane P that is orthogonal to the first axis Ox and that contains the first eyelet5, and the first cable6has a second segment6.4that is held at its first end6.5in the first eyelet5by the first crimp7.1. The second end6.6of the second segment6.4of the first cable6is crimped to a second pulley15that is secured to a second shaft17rotatably mounted on the frame10to rotate about an axis perpendicular to the first axis Ox.

The nut4has a second eyelet8projecting radially from the nut4so as to be diametrically opposite from the first eyelet5. A second cable9extends parallel to the first axis Ox and includes a first segment9.1of the second cable9that is held at its first end9.2in the second eyelet8by a second crimp7.2. The second end9.3of the first segment9.1of the second cable9is crimped to a third pulley18that is secured to the first shaft16that is rotatably mounted on the frame10to rotate about an axis perpendicular to the first axis Ox.

The second cable9likewise extends substantially parallel to the first axis Ox on both sides of the plane P that is orthogonal to the first axis Ox and that also contains the second eyelet8, and the second cable9has a second segment9.4that is held at its first end9.5in the second eyelet8by the second crimp7.2. The second end9.6of the second segment9.4of the second cable9is crimped to a fourth pulley19that is secured to the second shaft17that is rotatably mounted on the frame10to rotate about an axis perpendicular to the first axis Ox.

Each of the first and second cables6and9is pre-loaded to a pre-loading tension t6,9that is equal to half the total pre-loading t0, e.g. by acting on the distance between the first and second shafts16and17.

The actuator100also has a fifth pulley20and a sixth pulley21that are secured respectively to rotate with the first shaft16and with the second shaft17. A third cable22extends between the fifth and sixth pulleys20and21and has a first end22.1crimped to the fifth pulley20and a second end22.2crimped to the sixth pulley21.

A support22.3is crimped on the third cable22in order to constitute an outlet22.4of the actuator100that is for connecting to a load101that is to be moved.

The motor3and its encoder3.1are connected to a control unit90comprising a unit91for estimating the position of the nut4, a comparator92, calculation means93, a memory94, and a display95. A control handle96is also connected to the control unit90.

Since the first and second cables6and9are under tension, they exert forces that oppose the screw2turning the nut4while the motor3is rotating in order to move the nut4in either direction relative to the screw2. Under such circumstances, in addition to performing their function of transmitting movement forces from the nut4to the load101, the cables also perform an anti-rotation function such that the screw2rotating under drive from the motor3causes the nut4to move relative to the screw2between a first extreme position E1and a second extreme position E2for the nut4, as shown by chain-dotted lines inFIG.1. The first and second positions E1and E2are spaced apart by a stroke C. The cable actuator100of the invention enables the load101to be moved in two opposite directions.

In a first embodiment of the invention, as shown inFIG.2, a distance sensor30having a winder31for winding a thread or filament32is secured to the frame10. The thread or filament32could be a wire. The thread32has a first end32.1of the thread32connected to the nut4at a connection point4.1. The thread32is engaged on a drum33of the winder31at a first point34(a point of tangency) and it occupies a plurality of turns around the drum33. The drum33has a diameter D33. A spiral spring35exerts a return force on the drum33and keeps the thread32permanently under tension. A rotary encoder40measures rotation of the drum33. The rotary encoder40of the distance sensor30is connected to a processor unit41, itself connected to the control unit90. The curvature of the thread32changes at the first point34while the thread is being wound on the drum33.

As can be seen inFIG.2, when the nut4is at half-stroke, the point34is situated in a plane P1orthogonal to the first axis Ox and the plane P1is situated at a first distance d1from the first extreme point E1. In this configuration, the distance d1is equal to 50% of the stroke C. Thus, in this example, the point34is situated in a midplane P1of the stroke C of the nut4. The point34is also situated at a nonzero second distance d2from a straight line D1that connects the connection point4.1to the axis Ox.

In operation, a user acts on the handle96to cause the load101to move. The unit90then causes the motor3to rotate. Under drive from the motor3, turning of the screw2causes the nut4to turn identically as a result of contact friction between the screw2and the nut4. This turning tensions the first and second cables6and9, which then exert forces opposing the nut4being driven in rotation by the screw2. In addition to performing their function of transmitting movement forces to the load101, the first and second cables6and9then also perform an anti-rotation function such that the screw2rotating under drive from the motor3causes the nut4to move relative to the screw2.

When the load101reaches the position desired by the user, the user ceases to act on the control96. During a first step, the unit91estimates a theoretical position for the nut4of the screw2based on the numberNof revolutions performed by the motor as measured by the encoded3.1. The unit91thus establishes a theoretical linear position for the nut4of the screw2along the first axis Ox, and also a theoretical angular position for the nut4about the first axis Ox. The theoretical linear position of the nut4along the screw2corresponds to the position along the first axis Ox that the nut4would occupy on the screw2afterNrevolutions while unloaded, i.e. for a load101of zero mass. The theoretical angular position of the nut4around the axis Ox corresponds to the position around the axis Ox that the nut4would occupy on the screw2after N revolutions while unloaded, i.e. for a load101of zero mass. This theoretical angular position can vary as a function of the theoretical linear position of the nut4along the screw2. For convenience of description, it is assumed that the angular and linear positions are measured in a rectangular reference frame (Ox, Oy, Oz) associated with the nut4.

The actual position of the nut4on the screw2is estimated by the number of revolutions of the drum33as measured by the rotary encoder40. The processor unit41measures the rotation α of the rotary encoder40and transmits it to the control unit90. The comparator92compares the actual angular position of the nut4about the axis Ox with the theoretical angular position of the nut about the axis Ox and, by subtraction, the comparator92obtains a value δang4for the deviation of the angular position of the nut4.

The calculation means93then estimate the force being applied to the support22.3by the load101as a function of the value δang4of the deviation of the angular position of the nut4.

This estimation may be performed in particular by solving the following equation 27 for nut equilibrium:

in which:C corresponds to the torque applied to the first pulley14;R corresponds to the radius of the first pulley14;E corresponds to the distance between the points of tangency of the first cable6on the first pulley14and on the second pulley15;α corresponds to the angle of rotation of the nut4relative to the frame10;ρ corresponds to the anchor radius where the first cable6is anchored relative to the axis of the nut4(or of the screw);d corresponds to the distance from the center of the nut4to the point of tangency of the first cable6on the first pulley14;pcorresponds to the “reduced pitch” of the screw-and-nut system2,4i.e. to p2/2π;

k1corresponds to the stiffness of the shorter of the strands of the first cable6from among the strand between the nut4and the first pulley14and the strand between the nut4and the second pulley15;k2corresponds to the stiffness of the longer of the strands of the second cable9from among the strand between the nut4and the first pulley14and the strand between the nut4and the second pulley15; andt0corresponds to the total pre-loading tension shared over the first and second cables6and9.

The approximations leading to this equation or enabling it to be sold (e.g. limited developments) may depend on the linear position of the nut4of the screw2.

A cable actuator100is thus obtained in which the sensor30serves to estimate the tensions in the first and second cables6and9, thus making it possible to deduce the force being exerted on the outlet22.4of the actuator100. The position of the point34of the drum33varies as a function of the position of the nut4of the screw2. Thus, the position of the plane P1, and thus the distance d1, varies during movement of the nut4of the screw2. To a first approximation, it is possible to estimate that the distance d1varies over an amplitude range that is substantially equal to half the diameter D33.

In the description that follows of second, third, and fourth embodiments of the invention, elements that are identical or analogous to those described above are given the same reference numerals.

In a second embodiment as shown inFIG.3, the actuator100includes a thread deflector implemented in the form of a return pulley36. The thread32extends from the connection point4.1to the pulley36that it engages at the point of tangency34of the thread32of the pulley36. The thread32then changes curvature at the point34. On leaving the pulley36, the thread32extends to the drum33, and in this example It does so along a direction that is substantially parallel to the axis Ox.

As can be seen inFIGS.4and5, the distance d1between the plane P1and the extreme position E1varies with the position of the nut4along the screw2, while remaining in the range 40% (FIG.4) to 60% (FIG.5) of the stroke C.

Depending on the dimensions of the various components of the actuator100, the thread32may have a first portion32.2extending between the points32.1and34, and a second portion32.3extending between the point31and the pulley36, which presents a deflection relative to a second plane P2for winding the thread32on the pulley36. Under such circumstances, and by way of example, in a third embodiment the pulley36is mounted in a clevis that is mounted to pivot about a pivot axis that is orthogonal to the axis of rotation of the pulley36so that the plane P2always contains the portions32.2and32.3of the thread32. In this third embodiment, the pivot axis needs to be parallel with and close to the portion32.3, and ideally is concentric therewith. For example, if the pivot axis is mounted on ball bearings, even a very low level of tension in the thread32suffices to keep the pulley36automatically in the plane of the two portions32.2and32.3.

In a fourth embodiment, the distance sensor30using the thread32may comprise a linear sensor with return that is connected to a second end of the thread32.

Naturally, the invention is not limited to the embodiments described, but covers any variant coming within the ambit of the invention as defined by the claims.

In particular:although above, the frame is cylindrical in shape, the invention applies equally well to frames of other shapes, e.g. such as a plate, a square tube, or any shape;although above, the axis of rotation of the screw extends horizontally, the invention applies equally to other orientations for the axis of rotation of the screw, e.g. such as a vertical orientation, an orientation of 45°, or any orientation;although above, the cable actuator has two cables connected to the nut, the invention applies equally to an actuator in which the nut is connected to a single cable or to more than two cables;although above, the outlet of the cable actuator is connected to a cable in order to deliver movement in translation, the invention applies equally to an outlet that is constrained to rotate with one of the shafts of the actuator in order to deliver a movement in rotation;although above, the user acts on a handle in order to control the actuator, the invention applies equally to other control means, e.g. such as a switch, or even by voice;although above, estimation of the force exerted by the actuator is described for when the actuator is stabilized in a given position, the invention applies equally to measuring the force dynamically while the actuator is moving;although above, the actuator includes a ball screw, the invention applies equally to other types of screw, such as for example a screw with threads only, or a roller screw;although above, the first cable is coupled to the nut by crimping to an eyelet secured to the nut, the invention applies equally to other means for coupling a cable to the nut at a first connection point connecting the first cable to the nut, such as for example a ring welded on the nut, crimping in a hole made in the nut, round turns in a drill hole, fastening to an intermediate support;although above, the cables extend parallel to the first axis, the invention applies equally to other configurations of the cables in which a cable can adopt any orientation relative to the first axis;although above, the first and third pulleys are secured to the same shaft, the invention applies equally to pulleys mounted on independent shafts;although above, all of the cables of the actuator are pre-loaded, the invention also applies to a single pre-loading cable, to no pre-loading cable, or to only a fraction of the cables being pre-loaded;although above, the actuator includes a pulley for effecting the thread, the invention applies equally to the thread being defected in other ways, e.g. by a rotatably-mounted shaft, a metal or synthetic eyelet, a stationary shaft made of low-friction material of bronze or polytetrafluoroethylene (PTFE) type; andalthough above, the screw is mounted in a bearing, the invention applies equally to other means for rotatably mounting the screw on the frame, e.g. such as a bronze bushing, or a bearing having needles, balls, or conical rollers.