Material-handling Apparatus

Abstract

An articulated boom having an upper arm element and a lower arm element is secured to a support member at one end thereof to pivot about a first horizontal axis and has an end effector assembly at the other end thereof. The upper arm element and the lower arm elements are pivotally connected about a second horizontal axis. An intermediate member and a lower arm support member are provided for moving the lower arm element about the second horizontal axis. The intermediate member is pivotal about the first horizontal axis and the lower arm support member is pivotally connected at its ends to the lower arm elements and the intermediate member. An actuator pivotally connected between the support member and the intermediate member causes the upper arm element to swing about the second horizontal axis. Another actuator pivotally connected between the upper arm element and the support member pivotally swings the upper arm element about the first horizontal axis.

Description

The present invention relates in general to material handling apparatus or manipulators of the kind in which the movements of a control or master member produces corresponding movements amplified in respect to the force exerted thereby of a controlled or slave member.

An object of the present invention is to provide a manipulator which is relatively simple in construction yet which has high performance and load-carrying capabilities.

Another object of the present invention is to provide a manipulator of the character described in which the load-handling capabilities of the manipulator are augmented without requiring increased power-delivering capabilities of the power components of the manipulator.

Another object of the present invention is to provide a manipulator which has a more efficient working volume for the end effector thereof for given size arm elements than prior art manipulators.

A further object of the present invention is to provide an organization of the elements in a manipulator in which the inertia factor in the movements thereof is reduced thereby providing not only better response in the manipulator but also facilitating stabilization of the manipulator.

In carrying out the present invention in accordance with one illustrative embodiment thereof, there is provided a control or slave member having an upper arm element and a lower arm element. One end of the upper arm element is pivotally connected to a slave support member and pivotal about a first horizontal axis thereof. The other end of the upper arm element is pivotally connected to one end of the lower arm element and pivotal about a second horizontal axis. The longitudinal axes of the slave arm elements lie in a vertical plane which is perpendicular to the aforementioned first and second horizontal axes. Means are provided for moving the lower arm element about the second horizontal axis and includes an intermediate member pivotal about the first horizontal axis and a lower arm support member having one end pivotally connected to one end of the lower arm element and pivotal about a third horizontal axis longitudinally spaced from the second horizontal axis and having the other end pivotally connected to the intermediate member and pivotal about a fourth horizontal axis. An actuator is provided having one element thereof pivotally connected to the support member and the other element thereof pivotally connected to the intermediate member. Accordingly, energization of the actuator causes the intermediate member to pivot about the first horizontal axis and to produce a corresponding angular displacement of the lower arm element about the second horizontal axis. A second actuator is provided having one element thereof pivotally connected to the support member and the other element thereof pivotally connected to the upper arm element to move the upper arm element about the first horizontal axis.

The novel features which are believed to be characteristic of the present invention are set forth in the appended claims. The invention itself, however, together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawings wherein:

FIG. 1 is a perspective view of a manipulator embodying the present invention.

FIG. 2 is a side view of the apparatus of FIG. 1;

FIG. 3 is a side view of a portion of the master member assembly of manipulator of FIG. 1 particularly showing the organization of the arm elements and the manner in which the orientation of the arm elements thereof is sensed;

FIG. 4 is a block diagram of the electrohydraulic servosystem for controlling the motion of the lower slave arm element in response to movements of the lower master arm element.

FIG. 5 is a block diagram, similar to the block diagram of FIG. 4, of the electrohydraulic servosystem for controlling the movement of the upper slave arm element in response to movements of the upper master arm element.

FIG. 6 is a line diagram of the elements of the manipulator of FIG. 1 showing in simplified form the organization of the elements thereof.

FIG. 7 is a line diagram of a manipulator similar to the manipulator of FIG. 6 in which the organization of the elements is modified.

Referring not to FIG. 1, there is shown a manipulator having an articulated boom or slave member 10 including an upper arm element 11 and a lower arm element 12. A control or master member 15 including an upper arm element 16 and a lower arm element 17 is also provided. One end of the upper arm element 11 of the slave member is pivotally connected to a slave support member 20 about a first horizontal axis 21 to form a shoulder joint. The slave support member 20 is supported in the bearing member (not shown) which is connected to a base or mount member 22. The support member 20 is pivotal on mount member 22 about a vertical axis 13 thereof by means (not shown) which may, for example, be such means as disclosed in copending patent application Ser. No. 33,442, filed Apr. 30, 1970 and assigned to the assignee of the present invention. The other end of the upper arm element 11 is pivotally connected to one end portion of lower arm element 12 to form an elbow joint and is pivotal about a second horizontal axis 23. The other end portion of the lower arm element has connected thereto an end support plate 24 pivoted about a horizontal axis 30, end effector positioning apparatus 25 secured to the plate 24, and end effector 26 secured to the positioning apparatus 25. The end effector positioning apparatus 25 comprises three rotary elements 27, 28 and 29 mechanically connected in series so as to permit movement of the end effector 26 about any one of three mutually perpendicular axes. The end effector 26 shown is one in which suction cups 31-34 are provided for interfacing and secured thereto objects to be moved by the manipulator. One end of the upper arm element 16 of the master control member is pivotally connected to a master support member 18 and is pivotal about a horizontal axis. The master support member 18 in turn is pivotally mounted on the slave support member and is displaced horizontally therefrom so as to be pivotal with respect thereto about a second vertical 19 axis. The other end of the upper arm element 16 is pivotally connected to one end of the lower arm element 17 and pivotal about another horizontal axis. Handle 36 is provided on the other end of the lower arm element 17 and includes buttons which are responsive to operator finger action to effect through appropriate electrical and hydraulic control elements (not shown) the operation of the end effector positioning elements 25 and the end effector 26. Also connected to the slave support member and displaced horizontally from the slave support member 20 so that the control member 15 is positioned between the operator of the manipulator and the slave member is a platform 37 or operator support member shown in the form of a seat for the operator and a footrest so that the operator is oriented in the direction in which the end of the slave member is oriented and moves in azimuth so as to maintain such orientation.

The front face 35 of the end support plate 24 is automatically maintained perpendicular with respect to a horizontal plane means of a pantograph assembly including a triangular plate 40 pivotal about the second horizontal axis 23 and a lower pantograph member 41 and an upper pantograph member 42. The lower pantograph member 41 is pivotally mounted at one end to the support plate 24 and at the other end to the triangular plate 40 so that bar member 41 is maintained parallel to the lower slave arm element 12. Similarly, the upper pantograph member 42 has one end pivotally mounted to the triangular plate 40 and the other forked end thereof mounted to the slave support member 20 so that the lower pantograph member 42 is maintained parallel to the upper slave arm element 11. The pivotal axis of the upper slave arm element 11 and the upper pantograph member 42 intersect a vertical plane through the longitudinal axes thereof which form the corners of a parallelogram. Similarly, the pivotal axes of the lower arm element 12 and the lower pantograph member 41 intersect a vertical plane through the longitudinal axes thereof which form the corners of a parallelogram. Such pantograph structure is described and claimed in a copending patent application Ser. No. 33402, filed Apr. 30, 1970 and assigned to the assignee of the present invention.

The upper slave arm element 11 is pivoted about the first horizontal or shoulder axis 21 by means of a hydraulic actuator 45 having a cylinder 46 pivotally mounted to the slave support member 20 and having a piston 47 assembly pivotally mounted to the lower arm member 11 as more clearly shown in FIG. 2. Means are provided for moving the lower slave arm element 12 about the second horizontal or elbow axis 23 and includes an intermediate member 48 and a lower arm support member 49. The intermediate member 48 is pivotally connected to the slave support member 20 so as to be pivotal about the first horizontal axis 21. The lower arm support member 49 has one end pivotally connected to the aforementioned one end portion of the lower arm element 12 and pivotal about a third horizontal axis 50 longitudinally spaced from the second horizontal axis 23. The other end of the lower arm support member 49 is pivotally connected to the intermediate member 48 and pivotal about a fourth horizontal axis 51. The third horizontal axis 50 lies between the second horizontal axis 23 and the other end of the lower slave arm element 12. The first, second, third and fourth horizontal axes 21, 23, 50 and 51 are parallel and intersect a vertical plane through the longitudinal axes of the lower slave arm element 12 and the upper arm element 11 at points forming the corners of a parallelogram.

The hydraulic actuator 55 having a cylinder element 56 pivotally mounted to the slave support 20 and having a piston element 57 pivotally mounted to the intermediate member 48 provides a means for pivotally moving the intermediate member about the first horizontal axis 21 to cause the lower arm element to execute a corresponding angular displacement about the second horizontal axis 23 as can be clearly seen by reference to FIG. 2. FIG. 2 is a side view of the manipulator of FIG. 1 in which the end effector positioning apparatus 25 and the end effector 26 have been eliminated and in which the organization of the mounting of the linear actuators 45 and 55 for effecting movement of the slave upper arm element 11 and slave lower arm element 12 are clearly shown.

Also shown in FIG. 2 are a pair of potentiometers 60 and 61 for providing electrical signals corresponding to the angular orientation of the lower slave arm element 12 and the upper slave arm element 11, respectively. The potentiometer 60 includes a housing to which the fixed resistive element 62 thereof is mounted and a shaft 63 to which a movable tap 64 is connected (See FIG. 4). The shaft 63 is pivotally linked by means of links 65 and 66 to the intermediate member 48. The pivotal axis of the shaft 63 of the potentiometer 60, the pivotal axes of links 65 and 66 and the horizontal axis 21 intersect a vertical plane through the longitudinal axis of the upper arm element 11 at points which form a parallelogram. Accordingly, a predetermined angular displacement of the intermediate member 48 about the first horizontal axis 21 corresponding to a predetermined angular displacement of the lower arm element 12 about the second horizontal axis 23 produces a predetermined angular displacement of the shaft 63 of the potentiometer about its axis. The potentiometer 61 also includes a housing to which the resistive element 67 thereof is mounted and a movable tap 68 connected to a shaft 69. The shaft 69 is pivotally mounted through pivotal links 70 and 71 to the slave upper arm element 11. The first horizontal axis 21, the axis of the shaft 69 of the potentiometer 61, and the pivotal axes of the links 70 and 71 intersect a vertical plane extending through the longitudinal axes of the upper slave arm element 11 at points which form the corners of a parallelogram. Accordingly, a predetermined angular displacement of the upper slave arm element 11 about the first horizontal axis 21 produces a corresponding angular displacement of the shaft 69 of the potentiometer 61.

Referring now in particular to FIG. 3, which is a side view of the master control arm assembly of FIG. 1 showing the organization of the elements thereof. The upper arm element 16 of the master control arm member 15 is pivotally connected to a master support member 18 and is pivotal about a fifth horizontal axis 75. The master support member 18 in turn is pivotally mounted to the slave support member by means (not shown) and may be such means as disclosed in the aforementioned patent application Ser. No. 33442. The pivotal mounting point of member 18 is displaced horizontally from the slave support member 20 so as to be pivotal with respect thereto about the second vertical axis 19. The other end of the upper arm element 16 is pivotally connected to an intermediate point on the lower arm element 17 and pivotal about a sixth horizontal axis 76. A second intermediate member 77 is provided pivotally connected to the master support member 18 and pivotal about the fifth horizontal axis 75. A link member 78 is provided having one end pivotally connected to one end of the master lower arm element 17 adjacent the sixth pivotal axis 76 thereof and pivotal about a seventh horizontal axis 79. The other end of the link member 78 is pivotally connected to the second intermediate member 77 and pivotal about an eighth horizontal axis 80. The fifth, sixth, seventh, eighth horizontal axes are parallel and intersect in a vertical plane through the longitudinal axes of the elements 16 and 17 at points which form the corners of a parallelogram. Accordingly, as the control member 15 is moved, the upper arm element 16 and the lower arm element 17 are pivoted about the fifth and sixth horizontal axes 75 and 76, respectively. The movement of the upper arm element 16 is sensed by means of a potentiometer 85 and movement of the lower arm element 17 is sensed by the potentiometer 85. The potentiometer 85 includes a housing in which the resistive element 87 thereof is mounted and a slidable tap 88 connected to a shaft 89. The housing of the potentiometer 85 is mounted to the master support assembly 18 and the shaft 89 is pivotally connected by means of links 90 and 91 to the second intermediate member 77 at a point displaced from the fifth horizontal axis 95. The fifth horizontal axis 75, of the shaft 89 of the potentiometer, and the axes associated with the links 90 and 91 intersect a vertical plane through the longitudinal axis of the lower arm element in points which form the corners of the parallelogram. Accordingly, a given displacement of the lower arm element 17 about the sixth horizontal axis 76 produces a corresponding rotation of shaft 89 of the potentiometer about its axis. The potentiometer 86 also includes a housing connected to the master support member 18 and includes mounted therein a resistive element 92 and a slidable tap 93. The tap 93 is connected to a shaft 94 which is pivotally linked through links 95 and 96 to the upper master arm element 16. The fifth horizontal axis 75, the axis of rotation of the shaft 94 and the pivotal axes of the links 94 and 95 intersect a vertical plane through the upper master element 16 at points which form the corners of a parallelogram. Accordingly, a predetermined angular displacement of the upper master element 16 produces a corresponding angular displacement of the shaft 94 of the potentiometer 86.

Referring now in particular to FIG. 4, there is shown a servosystem in schematic form responsive to the electrical signals developed at the electrical output of the lower slave arm potentiometer 60 of FIG. 2 and the lower master arm potentiometer 85 of FIG. 3 for energizing the lower slave arm actuator 55 to bring the lower slave arm element 12 into correspondence with the orientation of the lower master arm element 17. One such servosystem is described and claimed in a copending application Ser. No. 76,562, filed Sept. 29, 1970, and assigned to the assignee of the present invention. Each of the elements of FIG. 4 identical to the elements of FIGS. 2 and 3 are denoted by the same numeral. The servosystem includes the master potentiometer 85, slave potentiometer 60, the master amplifier 100, the slave amplifier 101, a summing network 102, a current amplifier 103, fluid flow control valve 104 and the slave linear fluid actuator 55. The potentiometer 60 includes a resistive element 62 having positive and negative terminal and center taps and includes a slidable tap 64 which is coupled to the shaft of the potentiometer and moves in accordance therewith to provide a voltage output, the polarity and magnitude of which with respect to the center tap varies in accordance with the magnitude and direction of displacement of the shaft 63 in respect to a predetermined reference direction. Similarly, the master potentiometer 85 includes a resistive element 87 having a positive terminal, a negative terminal and a center tap. It also includes a variable tap 88 which is mechanically coupled to the shaft 89 of the potentiometer. Accordingly, as the shaft of the potentiometer is angularly displaced from a predetermined reference direction an output is developed between the center tap and the movable tap, the magnitude and direction of which corresponds to the magnitude and direction of displacement of the shaft from the predetermined reference orientation. The slave linear actuator 55 includes a cylinder member 56 and a piston assembly 57. The cylinder 56 is pivotally connected to the slave support member 20 and the piston assembly 57 is pivotally connected to the intermediate member 48. A port 105 is provided at one end and another port 106 is provided at the other end of the cylinder 55 for directing fluid in one direction or the other in the cylinder to cause the piston assembly 57 to move therein in one direction or the other. The electrohydraulic fluid flow control valve 104 includes a current input circuit 107 and a pair of fluid output ports 108 and 109, a pressure source port S and a fluid return port R. The pressure source port S is adapted to be connected to a suitable pressure source and the fluid return port R is adapted to be connected to a fluid return for the source. The valve 104 provides a flow or pressure output at the output ports thereof the magnitude and direction of which in respect to the output ports is a function of the magnitude and direction of the current applied to the input circuit of the control valve. Each of the output ports 108 and 109 is connected to a respective port of the actuator 55. The output of the master potentiometer 85 is applied to the input of the master amplifier 100 and similarly the output of the slave potentiometer 60 is applied to the input of a slave amplifier 101. The outputs from the amplifiers 100 and 101 are applied to the summing network 102 which develops a difference output, the magnitude and polarity of the difference depending upon the difference of the signals obtained from the potentiometers 60 and 85 and corresponding to the magnitude and direction of displacement of the slave element 12 with respect to the master element 17. The output from the summing network is applied to current amplifier 103, the output of which in turn is applied to the input circuit 107 of the control valve 104 to develop at the output ports of the control valve a flow or pressure output, the magnitude and direction of which is dependent upon the magnitude and direction of the difference in the signals obtained from the potentiometers 60 and 85. The pressure output from the control valve is phased or applied to the actuator 55 in a direction to cause the actuator to move the slave element 12 in a direction to reduce the displacement of the slave element 12 with respect to the master element 17 and hence into correspondence therewith.

Referring now particularly to FIG. 5, there is shown a servosystem in schematic form responsive to the electrical signals developed at the electrical output of the upper slave arm potentiometer of FIG. 2 and the upper master arm potentiometer of FIG. 3 for energizing the upper slave arm actuator 45 to bring the upper slave arm 11 into correspondence with the orientation of the upper master arm element 16. The servosystem of FIG. 5 is similar to the servosystem of FIG. 4. Each of the elements of FIG. 4 identical to the elements of FIGS. 2 and 3 are denoted by the same numeral. The servosystem includes the master potentiometer 86, slave potentiometer 61, the master amplifier 110, the slave amplifier 111, a summing network 112, a current amplifier 113, a fluid flow control valve 114 and the slave linear fluid actuator 45. The potentiometer 61 includes a resistive element 67 having positive and a negative terminal and a center tap and includes a slidable tap which is coupled to the shaft of a potentiometer and moves in accordance therewith to provide a voltage output, the polarity and magnitude of which with respect to the center tap varies in accordance with the magnitude and direction of displacement of the shaft 69 in respect to a predetermined reference direction. Similarly, the master potentiometer 86 includes a resistive element 92 having a positive terminal, a negative terminal and a center tap. It also includes a variable tap 93 which is mechanically coupled to the shaft 94 of the potentiometer 86. Accordingly, as the shaft of the potentiometer 86 is angularly displaced from a predetermined reference direction an output is developed between the center tap and the movable tap, the magnitude and direction of which corresponds to the magnitude and direction of displacement of the shaft from the predetermined reference direction. The slave linear actuator 45 includes a cylinder member 46 and a piston assembly 47. The cylinder 46 is pivotally connected to the slave support member 20 and the piston assembly 47 is pivotally connected to the upper slave arm element 11. A port 115 is provided at one end and another port 116 is provided at the other end of the cylinder 45 for directing fluid in one direction or the other in the cylinder to cause the piston assembly 87 to move therein in one direction or the other. The electrohydraulic fluid flow control valve 114 includes a current input circuit 117 and a pair of fluid output ports 118 and 119, a pressure source port S and a fluid return port R. The pressure source port S is adapted to be connected to a suitable pressure source and the fluid return port R is adapted to be connected to a fluid return for the source. The valve 114 provides a flow or pressure output at the output ports thereof the magnitude and direction of which in respect to the output ports is a function of the magnitude and direction of the current applied to the input circuit of the control valve. Each of the output ports 118 and 119 is connected to a respective port of the actuator 45. The output of the master potentiometer 86 is applied to the input of the master amplifier 110 and similarly the output of the slave potentiometer 61 is applied to the input of a slave amplifier 111. The outputs from the amplifiers 110 and 111 are applied to a summing network 112 which develops a difference output, the magnitude and polarity of the difference depending upon the magnitude and polarity of the difference of the signals obtained from the potentiometers 61 and 86 and corresponding to the magnitude and direction of displacement of the slave element 11 with respect to the master element 16. The output from the summing network 112 is applied to a current amplifier 113, the output of which in turn is applied to the input circuit 117 of the control valve 114 to develop at the output of the control valve a flow or pressure output the magnitude and direction of which is dependent upon the magnitude and direction of the difference in the signals obtained from the potentiometers 61 and 86. The pressure output from the control valve is phased or applied to the linear actuator 45 in a direction to cause the actuator to move the slave element 11 in a direction to reduce the displacement of the slave element with respect to the master element 16 and hence into correspondence therewith.

Reference is now made to FIG. 6 which shows a simplified, line diagram of the assembly of the elements associated with the slave member of the manipulator of FIGS. 1 and 2. In this figure each of the elements identical to the elements of FIGS. 1 and 2 is indicated with the same reference numeral. In the organizational arrangement of FIG. 6, movement of the lower slave arm element 12 is effected by the actuator 55 operating with reference to ground rather than with reference to or against the upper slave arm element 11 as in prior art manipulators. Such an arrangement has several advantages, one of which is that it reduces the load which must be supported and moved by the upper slave arm actuator 55. With the arrangement shown in FIG. 6 any load appearing at the end of the lower slave arm 12 is moved about the second horizontal axis 23 simply by actuation of the actuator 55 and the load is shared by both the upper arm element 11 and the lower support arm member 49. The actuator 45 simply functions to hold the upper slave arm 11 in position. The work of moving the load about the second horizontal axis is performed by the actuator 55. In view of the fact that smaller torques are applied by the actuator 45 to move a given load at the end of the lower slave arm element, stabilization of the servo apparatus of the manipulator is easier to accomplish. In addition, the response of system is improved as the inertia of the upper arm slave servosystem is smaller than it would be in such prior art arrangements. An additional advantage of the combination set forth in FIG. 6 is that the end of the lower slave arm element 12 can be made to encompass an essentially rectangular area thereby optimizing the working volume of the manipulator system of which it is a part. In the position shown, generally horizontal movements of the end of the lower slave arm element are effected by actuation of essentially actuator 45. Similarly, generally vertical movements of the end of the lower arm element 12 is effected by energization essentially of the actuator 55.

Reference is now made to FIG. 7 which is a line diagram of an organization of the elements of the slave member 10 of the manipulator similar to the organization set forth in FIG. 6 with parts of FIG. 7 corresponding to parts of FIG. 6 indicated by the same numeral. The organization of FIG. 7 is identical with the organization of FIG. 6 except in that the location of the upper slave arm element 11 and lower slave arm support member 49 have been interchanged so that where in FIG. 6 the lower arm support member functioned as a member in compression in supporting loads out on the end of the lower slave arm element in FIG. 7 the lower arm slave support functions as a member in tension in supporting loads appearing at the end of the lower slave arm element.

While the invention has been described in specific embodiments it will be appreciated that many modifications may be made by those skilled in the art and we intend by the appended claims to cover all such modifications as fall within the true spirit and scope of the invention.

Claims

What we claim as new and desire to secure by Letters Patent of the United States is:

1. In combination,

a slave support member,

a slave member including an upper arm element and a lower arm element, one end portion of said upper arm element pivotally connected to said support member and pivotal about a first horizontal axis, one end portion of said lower arm element pivotally connected to the other end portion of said upper arm element and pivotal about a second horizontal axis, the longitudinal axes of said arm elements lying in a first vertical plane perpendicular to said first and second horizontal axes,

means for moving said lower arm element about said second horizontal axis including

an intermediate member pivotally connected to said slave support member and pivotal about said first horizontal axis,

a lower arm support member having one end pivotally connected to said one end portion of said lower arm element and pivotal about a third horizontal axis longitudinally spaced from said second horizontal axis and having the other end pivotally connected to said intermediate member and pivotal about a fourth horizontal axis,

said first, second, third, and fourth horizontal axes being parallel and the intersection thereof with said vertical plane forming the corners of a parallelogram,

an actuator having one element thereof pivotally connected to said support member and the other element thereof pivotally connected to said intermediate member, whereby energization of said actuator causes said intermediate member to pivot about said first horizontal axis and produce a corresponding angular displacement of said lower arm element about said second horizontal axis.

2. The combination of claim 1 in which is included

another actuator having one element thereof pivotally connected to said support member and the other element thereof to said upper arm element,

whereby energization of said other actuator causes said upper arm element to pivot about said first horizontal axis.

3. The combination of claim 1 in which said second horizontal axis intersects said lower arm member between the intersection of said third horizontal axis of said lower arm member and said other end portion of said lower arm member.

4. The combination of claim 1 in which said third horizontal axis intersects said lower arm member between the intersection of said second horizontal axis of said lower arm member and said other end portion of said lower arm member.

5. The combination of claim 1 including

a master support member connected to said slave support member,

a master member including an upper arm element and a lower arm element, one end portion of said upper master arm element pivotally connected to said master support member and pivotal about a sixth axis, one end portion of said lower master arm element pivotally connected to the other end portion of said upper master arm element and pivotal about a fifth horizontal axis, the longitudinal axes of said arm elements lying in a second vertical plane perpendicular to said fifth and sixth horizontal axes,

a second intermediate member pivotally connected to said master support member and pivotal about said fifth horizontal axis,

a link member having one end pivotally connected to said one end portion of said lower master arm element and pivotal about a seventh horizontal axis longitudinally spaced from said sixth horizontal axis and having the other end pivotally connected to said second intermediate member and pivotal about an eighth horizontal axis,

said fifth, sixth, seventh, and eighth horizontal axes being parallel and forming the corners of a second parallelogram,

a first servo loop means responsive to the displacement of said upper master arm element about said fifth horizontal axis to maintain alignment of the longitudinal axis of said upper slave arm element with the longitudinal axis of said upper master arm element,

a second servo loop means responsive to the displacement of said lower master arm element about said sixth horizontal axis to maintain alignment of the longitudinal axis of said lower slave arm element with the longitudinal axis of said lower master arm element.