GB2454721A

GB2454721A - Controlling the translation and orientation of a moveable platform with six-degrees-of--freedom

Info

Publication number: GB2454721A
Application number: GB0722552A
Authority: GB
Inventors: Dafydd Roberts
Original assignee: Individual
Current assignee: Individual
Priority date: 2007-11-19
Filing date: 2007-11-19
Publication date: 2009-05-20
Also published as: GB0722552D0

Abstract

A device for controlling the translation and orientation of a moveable platform 20 with six-degrees-of-freedom comprises three or more jointed limbs each connected between a fixed base 0 and the moveable platform 20 where each limb is constrained using revolute joints and at least six of the revolute joints 4, 5, 6, 10, 11 and 12 being active. The distal end of each limb is connected to the platform 20 by joints 16, 17, 18 with three degrees of freedom. The axis of rotation of each shoulder actuator 4, 5, 6 is perpendicular to the axis of rotation of a corresponding revolute joint at the proximal end of each upper arm member 7, 8, 9 and the axes or rotation of the revolute joints at the proximal and distal ends of each upper arm member are parallel. Each of the revolute joints may include an angular position sensor and the axes of rotation of the revolute joints at the proximal end of each upper arm member 7, 8, may be coplanar with the origin axis of the fixed base frame.

Description

Device for controlling and monitoring the translation and orientation of an element in space.

TEChNICAL FIELD

This invention relates in general to a device for controlling and monitoring the translation and Orientation ofa moveable platform, and in particular a device which moves a platform with six degree-of-freedom (DOE) covering a large workspace, with a high load to weight ratio and high stilThess, for the accurate positioning of scientific instruments and the like.

BACKGROUND ART

The commonest devices used to provide Six degree-of-freedom (DOF) are Stewart platforms.

The Stewart platform has been studied extensively for use as a flight simulator and as a parallel manipulator (Stewart, D. 1965, "A Platform with Six Degrees of Freedom," Proc. liistitute of Mechanical Engr., London, England, Vol. 180, pp. 371-386).

Other variations of the Stewart platform have also been proposed. Most of these six-DOE parallel manipulators consist of' six limbs connecting a moving platform to a fixed base by spherical joints.

These six-limbed manipulators suffer the following disadvantages: 1. Their direct kinematics are very difficult to solve.

2. Their workspace is relatively small.

3. Spherical joints are difficult to manufacture with high precision.

4. Prismatic actuators are bulky aiid have limited travel The only six-limbed, six-DOE parallel manipulators for which closed-form direct kinematic solutions have been reported in the literature are special forms of the Stewart platform where limbs have concentric spherical joints at the connection to the moving platform or concentric spherical joints at both the fixed base and the nioving platform. The manufacture of concentric spherical joints present design problems.

As to the general Stewart platfbrm, researchers have to resort to numerical techniques for closed-form direct kinematic solutions.

A six-degree-of-freedom (6-DOF) parallel kinematics machine (PKM) described by G. L. Yang, I. M. Chen, W. II. Chen and W. Lin, in their paper "Kinematic Design of a Six-DOE Parallel-Kinematics Machine with Decoupled-Motion Architecture", IEEE Transaction on Robotics' and Automation, Vol. 20(5), pp. 876-884, (2004). has simple kinematics, large cylindrical reachable workspace, and high stiffliess in the vertical direction and presents a promising platform structure, hut is reliant on the use of a bulky and expensive p1smatic actuator to provide vertical motion for each limb.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide an accurate manipulator with a large workspace, a high load to weight ratio and high stiffness, for the positioning of scientific instruments and the like.

Another object of the invention is to provide a six degree of freedom platform with closed-form kinematics solutions.

A further object of the invention is to provide a large workspace for the positioncr A still further object of the invention is to utilize mostly revolute joints to increase the workspace and to reduce the manulacturing cost ofilie positioner.

A still fijrther object of the invention is to utilize angular position sensors at a plurality of the revolute joints to enable the rapid computation of the exact position and orientation of the moving platform.

These objects are achieved by providing a novel form ot Stewart Platform comprising a number (in a preferred embodiment three) of jointed limbs between a fixed base and a moving platform. The extension, elevation (tilt) and lateral rotatioii (swivel) of each limb is constrained using revolule joints. PreIrably, each limb is connected to the moving platform by suitable coupling means, for example a universal-type coupling or in one embodiment a novel spherical joint (although other suitable couplings may he employed). At least six of the total number ofjoints are active. Multi-axis positioning of the moving platform is achieved by coordinating the extension, till and swivel of the limbs. These and other features of the invention will become more fully understood from the Ibilowing description of certain preferred embodiments taken together with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. I is a drawing of a position mechanism formed in accordance with the invention.

DETAILED DESCRIPTION

Referring to FIG. 1 in detail, numeral 0 generally indicates a fixed base frame and numeral 20 a moving platform. There arc three limbs, in this embodiment identical, (I, 2, 3) connecting the moving platform 20 to the fixed base 0. The moving platlbrm 20 of a position mechanism is called the translation platform. Each limb assembly consists of a "shoulder" joint (4, 5, 6) at the base, an upper arm link (7,8,9), an "elbow"joint (10, 11, 12) and a forearm (13, 14, 15) Each forearm is connected to the translation platform at a wrist joint (I 6. I 7, 1 8).

The positions of the shoulder joints (4, 5, 6) are lixed with respect to the base frame (0). In the preferred embodiment, the axes of rotation of the shoulder joint actuators are coplanar, the plane in which the axes lie being perpendicular to an origin axis (not shown). In the pictured embodiment, this is achieved by providing three fixed protrusions extending radially from the base frame to form a tripod-like structure around the origin axis. The protrusions are preferably, but not necessarily, separated by 120 degrees. The distal end of each protrusion supports a shoulder joint. Each shoulder joint 4, 5, and 6, comprises a rotary actuator whose housing is mounted to the base frame. The axis of rotation of each shoulder actuator are preferably, hut not necessarily equidistant from the origin axis, and as discussed preferably, but not necessarily in the same plane.

Upper arm members (7,8, and 9) are attached to the rotating element of each rotary actuator via a revolute joint whose axis of rotation is perpendicular to the rotational axis of the actuator.

Each upper arni member in turn has at its distal end a second rotary actuator (1 0, 11, 12) arranged with its rotational axis parallel to the axis of rotation of the revolute joint at its proximal end.

Forearm niembers (13, 14, 1 5) are attached to thc rotary element of each second rotary actuator( 1 0, II, 12). The distal end of each forearni is provided with a spherical coupling 16, 17, and 18 which is connected to the moving platform 20.

In a practical preferred embodiment of the device, although each firearm member is driven about an axis which lies at or near the distal end of its respective upper arm member, the actual driving means is an actuating device (typically a motor gearbox or other means) positioned at or near the proximal end of the said upper arm member, distance driving means (e.g. a toothed timing belt) being used to apply the drive at the distal end. This allows the weight of the motor to he optimally located.

The spherical coupling 16, I 7, and 18 consists ofa spring loaded three pronged claw which is rotatahly mounted to the forearm, hut whose axial movement is restricted along the fi)rearm axis.

The spring loaded claw grips a spherical fixture attached to the moving platform (20). This arrangement allows the spherical coupling more compliance than conventional spherical couplings.

Angular position sensors can he mounted at any of the revolute joints. Angular position 5CflSO5 are preferably, but not necessarily mounted to monitor the angle extended by each rotating element.

The angle extended by any of the revolute joints is comniiinicated to a controlling means so that the spatial positioii and orientation of the moving platform can be calculated.

Note that when two revolute joint axes intersect at a common point, ii is kineniatically equivalent to a universal joint.

The three limbs are preferably, hut not necessarily, separated by 120 degrees at the points of connection with the translation platform 20 and with the fixed base 0 as illustrated in F1(I. 3.

In operation in a practical preferred embodiment of the device the angle of elevation (tilt) of each limb at each shoulder are driven, the angle of extension of each limb at each elbow are driven, and lateral rotation (swivel) of each limb is constrained at the shoulder by geometry. By controlling the extension and tilt of each limb multi-axis positioning of the moving p1atfom is achieved.

The preferred embodiment of present invention enables manipulating the position and the orientation of the moving platform utilising three limbs, to achieve light weight, high stiffness and a large workspace.

Due to the nature of the link arrangement, the useful workspace of the present invention is generally larger than that of manipulators discussed in the "Background Art." I. It has closed-form direct and inverse kinematics solutions.

2. lt.s workspace is substantially larger than the prior art.

3. Revolute joints can be precisely made at low cost.

ALTERNATIVE EMBODIMENTS OF THE INVENTION

In order to operate at least six of the total number of joints need lobe active. For the alternative limb constntction, not illustrated, the tilt and swivel of each limb are driven and the extension constrained by geometry.

Although the position mechanism illustrated in FIG. I has three limbs, theoretically any number of limbs that is greater than one can he employed to achieve an useful purpose. It should he noted that the placement ofactualors can be arbitrarily SO long as they comply with the mobility criteria of the mechanism. Both rotary and linear actuators can he incorporated into the mechanisms. For example, a linear ball screw or hydraulic actuator can he used to actuate the driven joints.

While the invention has been described by reference to certain specific embodiments and configurations, it should he understood that various changes could he made without departing from the spirit and scope of the inventive concepts described.

Claims

I. A device for controlling the translation and orientation of a moveable platform with six-degree-of-freedom comprising three or more jointed limbs between a fixed base and the moveable platform where the elevation, lateral rotation and extension, of each limb is constrained using revolute joints with at least six of the revolute joints being active and the distal end of each limb being connected to the moving platform by joints with three degrees of freedom, and where the axis of rotation of the shoulder actuator and the axis of rotation of the revolute joint at the proximal end of the upper arm are perpendicular, and where the axis of rotation of the revolute joint at the proximal end of the upper arm member and the axis of rotation of the revolute joint at the distal end of the upper arm member are parallel.
2. The device of claim I wherein angular position sensors are mounted at each of the revolute joints to monitor the angle extended by any of the revolute joints and that these angles arc communicated to a controlling means so that the spatial position and orientation of the moveable platform can be calculated.
3. The device of claim I wherein the axes of rotation of the revolute joints at the proximal end of each upper arm member are coplanar with the origin axis of the fixed base frame. Co

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