A hexapod, also known as a Stewart or Gough-Stewart platform, is a six-legged parallel mechanism. In its most common form, it consists of two platforms — one fixed and the other movable — connected by six actuator legs (or struts) that expand and contract. Coordinated motion of these struts enables the movable platform, and devices mounted to it, to move in any direction, operating with six degrees of freedom relative to the base. Motion is possible in the lateral, longitudinal, and vertical directions, and in pitch, roll, and yaw. The flexibility enables complex manipulations.
Hexapods were introduced in the late 1800s and first employed in an industrial application in the late 1940s, on tire-testing machines. Six-axis parallel-kinematic positioning hexapods debuted in 1954.
Now, recent advances in actuators and computer programming are making electromechanical and piezoelectric hexapods more efficient and accurate than some serial linkage and hydraulic kinematic positioners, including those on robotic arms. When integrated with servomotors and piezoelectric actuators, freely definable virtual pivot points, and simplified HMI and simulation, the units are suitable for positioning tasks in manufacturing, testing, and medical environments. In fact, the incremental-motion resolution of a hexapod designed for positioning often reaches 30 nm; velocity can exceed 10 mm/sec.
Hexapods versus serial kinematics
Parallel-kinematic mechanisms have a number of advantages over standard motion designs that use serial (stacked) linear stages for positioning.
- Elimination of cumulative error.
Serial kinematic positioning allows accumulation of wobble and guiding errors in the bearings of each axis, because the bottom stage supports its own moving platform plus all stages above it. In addition, each actuator is assigned to one degree of freedom, so integrated position sensors assigned to each drive measure only the motion caused by that drive: Moving errors in the other five degrees of freedom go undetected, so cannot be corrected in the servo loop — which leads to cumulative error.
In contrast, some parallel kinematics are appropriate for sub-nanometer positioners in atomic force microscopy and other novel motion applications. Here, integrated noncontact capacitive sensors allows engineers to track runout errors on sub-nanometer scales. The sensors continually measure actual position against the stationary external reference, and a servo-controller compensates for errors in realtime, for active trajectory control — and resolution and repeatability to 0.1 nm. Integrated servomotors and piezoelectric actuators are typically selected for their ability to maintain this resolution.
- Less deviation.
In a hexapod, all actuators act directly on the same moving platform, for the same dynamic behavior in all six axes. Each actuator has only one degree of freedom — unlike serial stacks, in which bearings in each axis also contribute to crosstalk. Therefore, runout and accumulation of off-axis errors are reduced. A hexapod is also stiffer, so induced flex caused by load shifts is reduced, and repeatability is increased.
- Fewer parts for reduced weight.
Hexapods have one-third the parts of serial multi-axis systems, which lowers weight and friction. In contrast, mechanisms with serial linkages exhibit wear, torque, and cable management issues that limit accuracy and repeatability.
- Reduced settling times.
The moving platform's low mass reduces settling times during positioning. In addition, most modern hexapod struts are equipped with cardanic joints. Those with a Z offset provide the best load characteristics, as they are much stiffer than gimbal-type joints (though do require more sophisticated control algorithms to account for each actuator's complex geometry.) Newer hexapod designs also maintain higher stiffness and rigidity in their bearings and drive screws. High natural frequencies result; 500 Hz at 22 lb of load is typical.
- High nominal load-to-weight ratio.
The load on a hexapod platform is equally distributed on the six parallel legs — so each link carries only one-sixth of the total weight. Under certain loads, hexapod legs also act longitudinally to exert either tension or compression on the struts, and reduce axial forces.
- Free access to the work zone.
Hexapods include no components to impede machine-tool motion.
- Large central aperture.
Critical for optical and through-light applications is the hexapod's allowance of access to the backs of workpieces.
Controls and connectivity
It's true: Even if only one axis must move, hexapods do require that all six struts change lengths. This is why these units require automatic strut compensation for velocity and motion vectors. How is this executed?
Modern hexapod controllers automatically run coordinate transformations, to direct individual strut velocity and vector adjustments, and transmit new positions to all six actuators, all at hundreds of times per second.
Target positions in six dimensions are specified in Cartesian coordinates, and the controller transforms them into the required motion-vectors for the individual actuator drives. In other words, positions and orientations can be entered directly — and software ensures that all target points in space are reached by smooth vector-defined motion. The pivot point remains fixed relative to the platform.
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More specifically, PC-based digital controllers are facilitated by open-interface architecture to allow high-level commands — so within the software, a designer can choose any point in space as the pivot point for the rotation axes.
Because visualizing complex hexapod motion with software is helpful, some simulation tools also show the possible motion range of a given mounted payload — and even predict potential collisions.
When using this software, engineers simply enter the shape and size of the payload inside or outside the hexapod envelope; the software then calculates distances to potential obstacles.
Other software also allows hexapods to utilize virtual programmable pivot points for rotational alignment tasks, allowing motion around any point — not unlike the human hand.
In fact, simulation tools are increasingly being incorporated for graphical configuration and simulation of hexapods to verify workspace requirements and loads. Such software provides full functionality for creation, modeling, simulation, rendering, and playback of hexapod configurations to predict and avoid interference with possible obstacles in the workspace.
For more information, contact Stefan Vorndran of Physik Instrumente at (508) 832-3456 or [email protected]. Otherwise, visit pi-usa.us.
Some new hexapod controllers allow extra axes — to operate rotary stages, linear stages, or linear actuators … plus macro languages for programming and storing command sequences. These feature TCP/IP and other interfaces for remote, network, and Internet connections.