April, 2011
www.us-
tech.com New Generation of Hexapods Continued from previous page
fits by just looking at several basic designs.
Elimination of Cumulative Er - ror. In serial kinematic positioning systems, wobble and guiding errors in the bearings of each axis accumu- late; the bottom stage supports its own moving platform plus all stages above it. Each actuator is assigned to one degree of freedom. Integrated position sensors assigned to each drive, measure only the motion caused by that drive and in its direc- tion of motion. All undesired motion (guiding error) in the other five degrees of freedom are not seen and therefore cannot be corrected in the servo-loop, which leads to cumulative error.
Less Deviation. In a hexapod, all actuators act directly on the same moving platform, with the same dynamic behavior occurring for the six axes. Because each actuator basi- cally only has one degree of freedom — unlike a serial stack of stages, where the bearings in each axis also contribute to crosstalk and runout — runout and accumulation of off-axis errors is much less of a concern for
New features support
sub-micron ultra-precision with simplified HMI and simulation, streamlining applications in dozens of manufacturing, testing,
bioresearch and medical environments.
hexapod designs. A hexapod can also be designed to be much stiffer, so load-shift-induced flex is far less of an issue. This results in significantly reduced deviation and improved repeatability.
Automatic Strut Compensation for Velocity and Vector. As differ- ent from serial multi-axis position- ing, hexapods require that all six struts alter their lengths if a change in only one axis is intended to occur. However, this is completely trans- parent to the user. The hexapod con- troller automatically runs the required coordinate transformations, directing individual velocity and vec- tor adjustments and transmits new positions to each of the six actuators hundreds of times per second.
One-Third the Parts for Lighter Weight, Lower Friction and Higher Reliability. Compared with serial multi-axes systems, hexapods have one-third the number of parts, which means lighter weight and lower friction. Friction and torque in serial systems, caused by as many as five moving cables (cable management), limit accuracy and repeatability.
Reduced Settling Times. Because of the low mass of the moving plat- form on hexapods, positioning opera- tions can be performed with far lower settling times than with convention- al, stacked multi-axes systems.
High Nominal Load-to-Weight Ratio. The high nominal load-to- weight ratio is a very important advantage of the hexapod over stacked systems. The weight of a load in the hexapod platform is approximately equally distributed on
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the six parallel legs. This means each link carries only one-sixth of the total weight. In addition, under certain loads, the legs on the hexapod act longitudinally exerting either ten- sion or compression on the struts, reducing axial forces.
Free Access to the Work Zone. Hexapods allow free access to the work zone. There is nothing to impede the motion of the machining tool in the workplace.
Large, Central Aperture. This is critical for optical applications, thru- light applications or access to the back of a workpiece. The latest high-resolution hexa-
Non-magnetic high-load piezo ceramic motor hexapod.
pod systems are being designed with high-efficiency precision-controlled servomotors and piezoelectric actua-
Continued on page 62
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