Articulated mechanism comprising a cable reduction gear for use in a robot arm

A link with a transmission cable (14) between a driving pulley (15) and a driven pulley (22) causing a mobile unit (5) to rotate, has two strands (23, 24) connecting the driven pulley (22) with idle pulleys (17, 18) which are substantially aligned with an axis (Y) of rotation of an part (2) intermediate to the unit (5) and to the base (1) on which the motor (12) is secured. Thus, the motor (12) is positioned on optionally fixed parts and rotation of the support (2) has only very little effect on the tension of the cable (14), so that it may always work properly. The invention is especially applied to articulated robot arms.

FIELD OF THE INVENTION

The subject of this invention is an articulated mechanism notably comprising a cable reducer which may belong to a robot arm.

BACKGROUND OF THE INVENTION

Generally, robots refer to controllably deformable structures for accomplishing certain tasks, as well as to master arms actuated by an operator for transmitting instructions to a slave arm reproducing the movements of the master arm or to a virtual environment via mechanical transmissions or computer interfaces.

Several design constraints weigh heavily on robots and notably on master arms. First of all, the operator of a master arm must experience sensations analogous to those which he would have experienced by directly controlling the slave arm, especially when the latter is working: the stresses received by the slave arm should therefore be reproduced with sufficient accuracy in the master arm. The linkages between the different portions of the robot, essentially the joints, should be provided with little play and fitted out with accurate low inertia transmissions, having little friction but capable of being easily blocked. Finally, it is desirable that the robots be as lightweight as possible which involves not only reducing their own weight but distributing it so that it is only moderately exerted on the control means of the robot used for moving it or, on the contrary, for maintaining it in a stable state by recovering static stress.

Above all, the means used for thereby controlling the joints or the other linkages of the robot, comprise motors which are among the heaviest components of the robot. It is current practice to position them as much as possible on the bases or low parts of the robot in order to reduce or suppress the moments required for lifting or moving them. It is then mandatory to provide a suitable transmission between the motor and the unit of the robot which it drives and which also provides reduction of the angular velocity of the motor. This is easy if the unit is directly connected to the base; but if it is connected to the base via another unit, mobile on the base, and therefore providing another degree of freedom, it is much more difficult to design a suitable transmission because of the much more complex positions which the unit may take up relatively to the base.

The tensioned cables between the shaft of the motor and a fixing portion belonging to the driven unit should be mentioned among the transmissions which may be proposed. Such transmissions are adopted on remote manipulators with master and slave arms, where the cables however suffer from the drawback of being long, which reduces the stiffness of the transmission and of passing through complex trajectories which cause couplings between the movements of different parts of the robot.

The transmission cables positioned between two robot portions which are not directly connected with each other should normally be tensioned on pulleys fixed on intermediate units. Then there occur problems in that the distances between their fixing points on the motor, the intermediate units and the unit which they drive, generally vary, thereby-producing a change in the cable's tension through elasticity, with the drawback that the robot's stiffness is changed. The harm is more marked when the robot is a master arm which is displaced by hand and the motor is a force feedback motor, because the operator can only be uncomfortable by feeling that the mechanical strength opposed by the arm varies with the displacement.

An improved cable reducer drive is provided as an essential component of the invention. In its most general embodiment, it relates to an articulated mechanism which may belong to a robot arm, comprising a base, a support rotating on the base around a first axis and a unit rotating on the support around a second axis, which is not parallel to the first axis, as well as a unit actuator for having it rotate, the actuator comprising: a motor secured to the base; and characterized in that the actuator further comprises a tensioned cable between a shaft of the motor, at least a pair of idle pulleys rotating on the support and a pulley connected to the, unit, with the idle pulleys being substantially tangent to the first axis; the cable forming a pair of strands substantially in extension and collinear with the first axis; the idle pulleys and the pulley of the unit being positioned so that said strands of the pair extend between the pulley of the unit and the idle pulleys, respectively.

By having the cable form a pair of strands substantially in extension and collinear with the first axis, it is guaranteed that the extension and the tension of the cable are only very slightly changed when the intermediate support rotates around the first axis. Advanced designs of the mechanism further enable this undesirable extension to be reduced.

According to certain advantageous features of the invention, the shaft of the motor is perpendicular to the axes of rotation of two of the idle pulleys between which and the shaft of the motor, the cable forms two rectilinear strands; and a linear transmission exists between the shaft of motor and the cable.

An articulated mechanism which may belong to a robot arm, comprising a base, a support rotating on the base around a first axis, is provided in a more complex embodiment of the invention, characterized in that it comprises two pulleys for controlling an arm through a linkage mechanism and two actuators of the control pulleys, the actuators each comprising: a motor secured on the base; a tensioned cable between a shaft of the motor, a pair of idle pulleys rotating on the support and one of the control pulleys; the idle pulleys being substantially tangent to the first axis; the idle pulleys and the control pulleys being positioned so that the cables form pairs of strands substantially in extension and collinear with the first axis which each extend between one of the control pulleys and one of the idle pulleys; advantageously, the link mechanism comprises a unit which is stiffly connected to one of the control pulleys and an articulated connecting rod at the other of the control pulleys, the arm being articulated at the connecting rod and at the unit; the control pulleys being parallel.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

Three preferred embodiments which will be described successively and which are always more advanced, comprise certain common components which are disclosed inFIG. 1, including a base1which may be fixed or not, a support2mounted on the base1by rotating around a pivot3orientated along a first axis Y, and a unit5mounted on an arm6of support2through a pivot7rotating around a second axis Z. Here, axes Y and Z remain perpendicular in spite of possible movements of support2and unit5; in other embodiments, they are simply not parallel. A first motor8drives support2into rotation via an elementary transmission consisting of a pinion9on the output shaft10of the motor8and an engaging toothed crown11cut on the perimeter of the support2.

A second motor12drives the unit5; like the first motor8, it is secured on the base1and therefore can only drive unit5through a more complicated transmission; whereas the previous transmission might comprise a belt, a cable, etc., instead of a gear, it is recommended that above all, the composite transmission13between the second motor12and the unit5should comprise a tensioned cable14between a driving pulley15positioned on the output shaft16of the motor12, a pair of idle pulleys17and18mounted on an arm19of base1through respective pivots20and21, and a driven pulley22depending on unit5and stiffly fixed to it. Cable14is wound around the driven pulley22and covers a sufficient portion of a turn so as to obtain the desired clearance and around the driving pulley15covering several turns so that sufficient friction retains the cable14from sliding. Pivots20and21of the idle pulleys17and18are perpendicular to the first axis Y and extend at a little distance from it so that the tensioned cable14between the driven pulley22and each of the idle pulleys17and18forms a pair of strands23and24substantially in extension and coinciding with the first axis Y.

Rotation of support2around the first axis Y only very slightly changes the position of the strands23and24and therefore has hardly any influence on the tension of cable14, which is the sought-after result. It will be noted that cable14will generally be wound with several turns around the driving pulley15so that it may drive the cable with sufficient friction, and that the corresponding strands of cable14respectively leading to the idle pulleys17and18, and noted as25and26, will therefore not be coplanar. Advantageously, pulleys17and18will be placed in planes respectively formed by strands23and25or14and24in the middle of the motor's travel.

However, the axes of pivots20and21are then not parallel, which may be disadvantageous.

The embodiment ofFIG. 2differs from the previous one by the position of the second motor as well as by the shape of the cable which does not provide the drawback as indicated above; the components corresponding to those of the first embodiment but for which the description should be amended or completed will bear a reference number increased by100.

Now the cable114is tensioned between two further idle pulleys30and31positioned between the shaft116of motor112and the previous idle pulleys17and18, respectively. It forms a rectangle between the driving pulley115and the driven pulley22.

This layout is meaningful if the motor112is orientated so that its shaft116is perpendicular to the second axis Z, when the latter is placed in the configuration ofFIG. 2, as well as to the first axis Y and so that its pulley115is substantially tangent to the plane passing through the additional idle pulleys30and31: strands32and33of the cable114leading from the additional idle pulleys30and31to the driven pulley115are then substantially coplanar, as well as the remainder of cable114, by the way, which is tensioned while remaining straight in the grooves of the idle pulleys17,18,31and32.

Referring back, please, toFIG. 1, it will be seen that rotation of the driving shaft16brings about an unwinding of the turns on one side of the pulley115and a winding of additional turns on the other side, i.e. a displacement of the stacking of the turns in the X direction or along the driving shaft16. This is likely to vary the tension of the cable114and thereby prevent us from completely achieving the goal which we had set. This residual extension of the cable114still exists in the embodiment ofFIG. 2, but as it is executed without substantially moving the cable114from the plane in which it extends, better stability of the mechanism's operation is achieved all the same.

This residual extension of the cable is completely obviated by adopting the layout ofFIG. 3, which differs from the previous one in that the cable (here214) is not wound on a driving pulley but is fixed to a linear transmission such as a rack40associated with an additional cable41wound around a pulley215of the driving shaft216. In such a system, the rotation of the driving shaft216displaces the rack40linearly (vertically) and has the cable214slide on itself without not in the least changing its shape; the deformations consecutive to the displacement of the stacking of the turns are withstood by the additional cable41. In this embodiment, even less than in the previous ones, the tension of the cable214does not depend on the movements of support2or unit5.

It should be noted that other idle pulleys may be added if the driving motor12,112or212should be placed further away from the unit in another position. Also, the additional idle pulleys may be present in an uneven number. In the same way, idle pulleys may be positioned on the body2in order to place the Z axis of rotation of the body5in another position or orientation.

Up to now, the invention has been described for a master arm with force feed back motors. It may also be applied to robots with displacements controlled by these same motors.

A robot arm articulation layout including the previous mechanism may be described by means ofFIG. 4.

In fact, this is a development of the embodiment ofFIG. 1, where in addition to unit5, to the second motor12, etc., an analogous mechanism is found which comprises a third motor60, an output shaft61with a driving pulley62, a pair of idle pulleys66and67and a driven pulley65, and parallel to the previous driven pulley22by rotating around the second axis Z. A cable63is tensioned between the driving pulley62and the driven pulley65by having it pass through the idle pulleys66and67, which are adjacent to the idle pulleys17and18of the other mechanism, respectively; the strands70and71of the cable63connecting the idle pulleys66and67to pulley65themselves also substantially coincide with the second axis Y. Cables14and63are located in planes forming an acute angle between them, in order to slightly move the motors12and60away from each other while keeping each of them mounted on the base1.

A connecting rod68may be articulated with the driven pulley65. This parallelogram type mechanism may also be achieved through cables; for this, one or more cable strands are secured on pulleys attached to bodies65and69and the axes of which respectively pass through the (Z) axis and through the linkage axis of bodies5and69, thereby achieving transmission of the movement of the control pulley65to the body69. It is seen that the displacement of the arms69in a vertical plane may be controlled by the joint movements of motors12and60, which cause both driven pulleys22and65to rotate independently and that the position of the pivoting plane of arm69may be changed with motor8. Both transmission systems controlled by motors12and60both benefit almost entirely from the advantages of the invention because of the proximity of the strands of cables14and63which are adjacent to the driven pulleys22and65and of the Y axis. It will be noted that the end of the arm69may take up all the positions in space, whereas the three motors which control this position are all secured to base1and form a compact assembly. The thereby formed robot will not be very bulky, the arm69being exposed and the motors being grouped together at a fixed (or at least mobile) base portion of the robot; and the unit5, the connecting rod68and the arm69will not have to withstand the stresses generated by the motor weight and may be built so as to be more lightweight. For instance, to obtain a robot with six degrees of freedom, a handle may be added to the robot inFIG. 4.