Abstract:
The robot arm is composed of two parallel branches ( 2, 3 ) formed from a reduced number of segments ( 5, 6, 7 ) and connected to each other by a wrist ( 5 ) through a wrist holder ( 8 ), the directions of which are kept approximately constant by transmissions ( 12  to  16 ). The working space of this control arm is large and it is free of any kinetic singularities except at the end positions, and good decoupling of translation and rotation movements is achieved.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
   The present application claims priority from French Patent Application No. 00 06364, filed May 18, 2000. 
   FIELD OF THE INVENTION 
   The present invention relates to robotic arms. More particularly, the present invention relates to a control arm comprising two branches in parallel. 
   BACKGROUND OF THE INVENTION 
   The function of the control arms is to transfer movements applied to them by an operator into control instructions for an instrument or a system, usually a remote robot called a slave arm or a computer simulation. When the control arm is provided with a sufficient number of degrees of freedom, the operator can control it in translation and in rotation in space. 
   There is a wide variety of forms of anus used in robotics. In the most traditional form, they are composed of a chain of segments connected to each other by articulations or sometimes by other types of joints; this arrangement is called “in series”. But there are disadvantages with this type of arm whenever there are a large number of segments. 
   Thus, there is play at joint mechanisms which accumulate and eventually produce a significant imprecision on the position of the free end of the arm. Furthermore, the motors that normally need to be added to the arm to control the states of the joints in order to modify their configuration or to hold them in a fixed position regardless of the external forces applied to them, and which are frequently the heaviest part of the arms, cause excessive bending moments that can make it necessary to reinforce the structure of the segments and therefore further increase the weight of the arm, making it inconvenient to manipulate. It was proposed to displace the motors onto the fixed base on which the arm is mounted, but this solution would require installing transmissions between the motors and the joints controlled by them, which is not always possible and in any case contributes to making the arm complex. 
   This is why in the more recent history of robotics, designers have been interested in constructions using several (two or more) arms in parallel, in which the distal ends are connected together. For a similar number of degrees of freedom, the branches of arms built in this way are less complex than in robots in series, which significantly attenuates the disadvantages of the imprecision in the position of the arm and the weight of the branches. However, there are specific limitations with this type of robot. They may thus be difficult to control to a given state, due to the fact that their kinetic complexity is greater; their working range is usually smaller than the working range of arms in series, since it is limited by the working range of different branches in parallel and by collisions between segments of the different branches; finally, a fault correlated to the previous fault is that the number of singularities, which are configurations that should be avoided since the robot may be subjected to uncontrolled movements at these points, is generally greater. 
   Singularities correspond to local disappearances of degrees of freedom or uncontrolled movements. Singularities corresponding to uncontrolled movements are specific to parallel robots, but all singularities restrict the usage range of the arm. This disadvantage is more pronounced with master arms, which are not designed to apply repetitive movements or movements known in advance and that are controlled by hand without thinking about singularities, and which therefore may occur unexpectedly. This is why they must be limited, while transferring them to the ends of the working range. It is concluded from these various remarks that arms in parallel are attractive as master arms due to the convenience in manipulating them, but in reality their specific defects make most of them inappropriate for this application. The invention deals with a particular arrangement of arms in parallel, for which the main advantages are a significant reduction in singularities and considerable ease of control, which means that the movement required to reach a required state can be imposed without difficulty. 
   BRIEF SUMMARY OF THE INVENTION 
   These objectives are achieved with a control arm comprising two branches extending between a common base and a common wrist, characterized in that it comprises wrist holders arranged between the wrist and the branches, means of keeping constant angles between the wrist holders and fixed planes, such that the wrist holders make it possible to separate the elements responsible for the translation control from the elements responsible for controlling degrees of freedom of rotation, within each of the two branches. This separation in itself contributes to reducing singularities. The document in prior art closest to this concept is possibly an article by Iwata entitled “Pen-based haptic virtual environment” (IEEE-ICRA 1993, p. 287 to 292) that describes a parallel robot with two branches connected at a wrist but without a wrist holder according to the invention. This arm comprises singularities. 
   Branches advantageously have segments articulated to each other, to the corresponding wrist holders and to a portion of the base through three parallel axes; this portion of the base is then advantageously installed free to rotate on a fixed portion of the base around an axis orthogonal to the previous parallel axes, which confers three degrees of freedom in translation of the branches at the wrist holder; the wrist is then composed of articulated portions, so that there are two or three degrees of freedom in rotation. The means of keeping the angles mentioned above between the wrist holders and the fixed planes constant may comprise transmissions extending on the branches, and more precisely pulleys located on the successive articulation axes of branch segments and a series of tensioned belts between the pulleys of the successive articulation axes; the pulleys are usually free to rotate, except for the last two pulleys which are fixed to the wrist holder and to an immobile structure fixed with respect to the fixed base or to the first body of each of the branches, respectively. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated into and constitute a part of this specification, illustrate one or more embodiments of the present invention and, together with the detailed description, serve to explain the principles and implementations of the invention. 
     In the drawings: 
       FIG. 1  is a diagram schematically illustrating an arm assembly in accordance with one embodiment of the present invention. 
       FIG. 2  is a diagram schematically illustrating a wrist of the arm assembly in accordance with one embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   With respect to  FIGS. 1 and 2 , the arm is installed on a fixed base  1  and comprises two arms  2  and  3  arranged in an arbitrary manner between this base  1  and a wrist  4  that connects them together. Advantageously, these two branches may be superposed. Advantageously, they may also be similar. In the reference configuration shown, in which the wrist  4  is perpendicular to them, the branches  2  and  3  are in parallel planes. Each of the branches comprises an arm segment  5 , a forearm segment  6 , and also an attachment body  7  forming a moving base connected to the fixed base  1  and a wrist holder  8  connected to the wrist  4 . The attachment body  7  rotates about the base  1  about an X 1  axis. The arm segment  5  is articulated to the attachment body  7  and to the forearm segment  6  by the X 2  and X 3  articulation axes respectively. Advantageously, these axes will be parallel to each other and perpendicular to the X 1  axis. The distal end of the forearm segment  6  is articulated to the wrist holder  8  by an articulation axis X 4  which will advantageously be parallel to the X 2  and X 3  axes. The wrist holders  8  comprise ends  9  rotating about an axis X 5  coincident with a general direction of elongation of the wrist holder  8 , end pieces  10  are attached to the ends  9  with the capability of rotating about themselves about X 6  axis perpendicular to the X 5  axis, and a handle  11  connects the end pieces  10  to each other, keeping them aligned with each other, while being able to rotate about an X 7  axis coaxial to them. Advantageously, this axis is orthogonal to the previous pairs of X 5  and X 6  axes in the reference configuration. The handle  11  is free to pivot about the X 7  axis without changing the distance between the X 5  axes or the wrist holders  8 , unlike the handle in the previous article which included a screw connection that could be the cause of singularities. 
   An essential element of the invention is that the wrist holders  8  and their X 5  axes form a constant angle with fixed planes, in this case plans perpendicular to the X 1  axis; if this angle is a right angle, the wrist holders  8  remain at a fixed orientation in space. This is achieved by means of a transmission comprising a holding pulley  12  coaxial with the X 2  axis and fixed to the attachment body  7 , a return pulley  13  coaxial with the X 3  axis and free to rotate freely about segments  5  and  6 , a holding pulley  14  coaxial with the X 4  axis and fixed to the wrist holder  8 , and two belts  15  and  16  tightened between pulleys  12  and  13 , and between pulleys  13  and  14  respectively, thus forming a chain, for each of the branches  2  and  3 . Regardless of the movement applied to segments  5  and  6 , the action of the belts  15  and  16  holds the X 5  axis in a direction identical to the plane formed by the segments, since the pulley  12  remains fixed. 
   Movements of the handle  11  are produced by movements of segments  5  and  6  and attachment bodies  7  about the X 1 , X 2  and X 3  axes. Translation movements are obtained by moving the wrist holders  8  of the two branches in the same direction, and rotation movements (except for rotation about the axis of the handle) are achieved by moving them in opposite directions. The result is that five degrees of freedom can be controlled by acting on the position of the two wrist holders  8  with motors on or close to the base. The branches  2  and  3  remain well separated from each other and do not come into collision except for very large inclinations of the handle. Furthermore, movement singularities can only occur at extreme tipping positions of the wrist  4 . 
   We will now describe the means of actuating the arm. Motors are used to feedback forces felt at the slave arm or generated by a computer simulation, to the operator. These motors  17  are arranged on the fixed base  1  and help tilt the attachment bodies about the X 1  axis by a gear, a belt or other transmission, motors  18  are placed on the X 2  axes and help in tipping the segments of the arm  5  with respect to the attachment bodies  7 , other motors  19  are also placed on the X 2  axes but in reality are used to adjust the angles made between the X 3  axis and the arm segment  5  and forearm segment  6 , using a pulley  20  connected to their output spindle, a pulley  21  connected to the forearm segment  6  and a tensioned belt  22  between these pulleys. The pulleys  20  and  21  and the belt  22  may be replaced by any other appropriate device such as a connecting rod, which will lead to a parallelogram type of assembly well known to those skilled in the art. 
   There is no need to place a motor for the X 4  axes which are controlled otherwise, for the X 5  or X 6  axes since rotations about these axes are derived only from movements at the ends of the forearm segments  6 ; but a motor  25  designed for use by the handle  11  may be added in order to control a pivoting degree of freedom about the X 7  axis. 
   Advantageously, the motor  25  may be fixed to a gripping duct  23  on the handle  11  installed on end pieces  10  free to rotate about a bearing  24 , while the output shaft from the motor  25  is connected to the opposite end piece  10 . It may also be mounted on a moving base  7 , but this requires a device for the transmission forces from the motor  25  to the handle. 
   Sensors such as angular position encoders are associated with the different motors to measure their movements and to indicate the state of the arm and imposed controls, but once again these techniques are known and will not be mentioned in this text. If a degree of freedom is superfluous, it is advantageous to manage without the pivot control of the handle  11  which is the most difficult to produce precisely and comfortably. In any case, the other motors are fixed on the fixed base  1  or the attachment body  7  supported directly by the base, so that their weight is not supported by segments  5  and  6 , which may be fairly lightweight as a result. 
   Motors  18  and  19  controlling the arms  5  and the forearms  6  that are located along the X 2  axis can be offset from the X 2  axis if a reduction gear has to be added. They may also be installed on the fixed base  1  provided that a suitable transmission device is inserted as far as the X 2  axis, but this device is not described in this document. The terminal device on the arm is not necessarily a handle, it may be a pen, a ball, pliers, etc., depending on the envisaged application, for example games, simulation devices, remote manipulation, remote operation or remote displacement for various industries.