Parallel-Kinematic Precision Positioning Systems

Six Axes of Motion with Hexapods and SpaceFAB

  • Hexapod platforms are used for moving and precision positioning, aligning and displacing loads in all six degrees of freedom, i.e., three linear and three rotational axes. Hexapods have a parallel-kinematic structure, i.e., the six drives act together on a single moving platform.
  • The length of the single drives is variable, allowing the system to move in all six degrees of freedom in space. This special hexapod design optimizes the overall system stiffness and allows a large central clear aperture.
  • Depending on their design, hexapods can position loads from several kg to a few hundred kg or even several tons in any spatial orientation, that is, independently of the mounting orientation and with high precision.

Operating Principle

Motors and Drives

PI hexapods are based on >> Piezoelectric or >> Electromechanical drives and are much more accurate than the hydraulic hexapods known from flight or driving simulators. Precision leadscrew drives or magnetic or piezo-based linear motors are used. Most systems are self-locking. Direct-drive hexapods ensure higher velocities; for industrial use, brushless motors (BLDC) are particularly suitable.

The application determines the drive technologies: Hexapods with piezoelectric >> PiezoWalk® Stepping Drives for ultrahigh vacuum applications and can also be operated in very strong magnetic fields.

Ball and socket joints
Ball-and-socket joints

For particulary high precision and low linear deflections and angles, >> Flexure Joints can be used. They work friction-free, play-free, and without the need of lubricants.
Universal joints of the H-840 Hexapod model

Joints

Hexapods for precision positioning often have cardanic joints with two orthogonally arranged axes. This is the optimum combination of two degrees of freedom and the stiffness of the structure. Ball-and-socket joints offer more degrees of freedom in a relatively simple design. However, the overall stiffness and precision can suffer in case of external loads and torque. A compensating preload is recommended but requires drives with high output forces such as >> NEXLINE® Piezo Motor Drives.

Work Space

The entirety of all combinations of translations and rotations that a hexapod can approach from any given position is called the workspace; it is given in reference to the origin of the coordinate system used. The workspace can be limited by external factors such as obstacles or the dimensions and position of the load.

Simulations for various hexapod workspaces

PI offers >> Software Tools that simulate the workspace and that can check eventual collisions with objects in the environment.

Advanced Motion Control

Positioning accuracy of a precision H-824
The positioning accuracy of a precision H-824 Hexapod in Z direction over the complete travel range of 25 mm is a few micrometers, and the repeatability is considerably lower than ±0.1 µm

The individual drives of a hexapod do not necessarily point in the direction of motion, which is why a powerful controller that can handle the required coordinate transformations in real time is needed.

PI uses advanced digital controllers along with user-friendly software. All motion commands are specified in Cartesian coordinates, and all transformations to the individual actuators take place inside the controller.

An important hexapod property is the freely definable pivot point. The possibility to rotate around any point in space opens up new application possibilities, and the hexapod platform can be integrated in the overall process.

SpaceFAB

SpaceFAB are alternative parallel-kinematic set-ups with constant strut lengths. The six degrees of freedom are achieved by moving a passive strut in two or more axes: In SpaceFAB, an XY arrangement of linear stages moves the individual struts.

HexapodSpaceFAB
Design6 struts with variable length or moving joints act on a moving platform3 XY act in parallel on a moving platform
GeometryCompact surfaceLow height
Linear travel ranges in a planeLimitedFlexibly by selecting XY stages
Linear vertical travel rangeLongLimited
Rotatory travel rangesLarge tilt around X and Y axes possibleLimited
StiffnessHigh vertical stiffnessHigh stiffness in X and Y

Characteristics

Advantages of Parallel Kinematics Compared to Serial Set-Ups
Directions of motion of a parallel kinematics (left) and comparable serial set-up (right)

In case of serially stacked systems, the lowest axis not only moves the mass of the payload but also the mass all other following drives.

Parallel-kinematicsSerial stacked design
DimensionsCompact set-upRelatively high set-up, stabilization at base required
Moved massPlatform plus loadEach stage carries the ones positioned on top plus the load and has to be designed accordingly
Dynamics / stiffnessDue to the low moved mass, relatively high, the same for all motion axesDynamic axes should be places as far on top as possible
AccuracyThe same for all motion axesErrors add up from "bottom" to "top", high parallel error due to set-up height
Cable managementNo moving cables, no effect on accuracyRequired
CommandsPI motion controllers can be used to save commands in Cartesian coordinates; they transform these commands into the motion of the individual strutsCartesian, individually for each axis
Pivot pointAny point in space can be selected by means of PI softwareDefined by selection of stage geometries
Work spaceGenerally limitedDefined by selection of stage geometries

Hexapods in Automation: Control and Interfaces for Easy Integration

PI H-850 Coordinate System
Any coordinate system used as a reference for target values of the hexapod maybe defined

Precision path control with jerk limiting

The hexapod controller makes motion possible on the basis of complex trajectories with defined velocity and acceleration. Like this, >> Hexapods in Mechanical Engineering can, for example, move a workpiece or tool jerk controlled and with high precision during machining without the mechanical system starting to vibrate.

 

User-Defined Coordinate Systems

To adapt the trajectory perfectly to the requirements of the application, it is possible to define various coordinate systems which refer, for example, to the position of workpiece or tool. This offers great advantages for applications in industrial automation, but also for fiber alignment.

 

Interfaces

EtherCAT interface: Standardized >> Fieldbus Interfaces guarantee an easy connection to parent PLC or CNC controls so that hexapods can work synchronously with other components in one automation line.

Products

6-Axis Hexapods / Parallel Positioners

Hexapod positioners are often referred to as Stewart Platforms. A hexapod is based on a 6-axis (XYZ, Pitch, Roll, Yaw) actuator system arranged in parallel between a top and bottom platform. PI parallel kinematics (PKM) precision positioning systems have many advantages over serial kinematics stages, such as lower inertia, improved dynamics, smaller package size and higher stiffness. In addition...
Learn more

Downloads

Parallel-Kinematic Precision Positioning Systems

Version / Date
TEC73 10/2017
Document language English
Download
pdf - 407 KB