Attack the Stack

Hexapods advance motion capabilities beyond convention

Multi-axis motion has conventionally been achieved by bolting-together multiple linear and rotary stages. And (speaking as a leading manufacturer of linear and rotary stages) this can certainly be an effective approach we thoroughly endorse.

But hexapods take multi-axis motion control to another level entirely:

  • PI hexapods' highly triangulated configuration and proprietary joint design results in greater stiffness than any stack of stages can provide. For example, the resonant frequency of our M-850 hexapod--our original model--with a substantial, 10kg load exceeds 90Hz transversely and 500Hz axially-- meaning this six-degree-of-freedom positioner when significantly loaded has a higher resonant frequency than many single-axis stages, unloaded!

  • Parallel kinematics means a PI hexapod’s six stiff linear actuators share the load of the moving platform, and their loading is purely axial, maximizing their stiffness. By comparison, with a stack of stages, only the top stage supports the load… the next stage supports the load and the top stage; the next stage supports the load and the top stage and the next stage… and so on, down to the bottom stage in the stack, which supports the load and all the other stages. This is a key reason for the superior stiffness and higher resonant frequency of hexapods, with direct impact on system responsiveness.

  • You need never tune a PI hexapod. On the other hand, each of the axes in a stack of servo stages will have its own tuning requirements, and optimizing for your load can be a significant chore for stacked configurations and can even risk damage.

  • Well-designed hexapods have no moving/sweeping cables to rub, wear and foul. After all, cabling tends to be the most unreliable part of conventional motion systems, so this advantage goes directly to reliability and MTBF. In addition, cable-borne vibration, rubbing and tugging will inevitably reduce the minimum incremental motion, repeatability and stability of stage stacks.

    However, not all hexapods are created equal-- you will find some manufacturers whose cables drape and dangle, forming an untidy mess all the way to the controller. Besides raising reliability and stability issues, this is an invitation to electromagnetic interference. PI’s popular hexapods have exactly two cables going from high-quality connectors on the hexapod's base to the controller: one for the hexapod’s integrated, high-efficiency drive amplifier (no extra box required), and the other for control signals. Each cable has one connector on each end, for simple and reliable connections and painless setup.

  • The center-of-rotation for rotation stages and goniometers are fixed in space. By comparison, the center-of-rotation for PI hexapods may be placed anywhere in space with a single, simple software command. And, PI hexapods speak in human units: millimeters for the X, Y and Z axes (with resolution to 0.1 micron) and degrees for the pitch, yaw and roll axes (with resolution to 0.1 millidegree).  This is all kept easy-to-use by the sophisticated digital controller which transparently handles all the coordinate transformations.

  • PI hexapod controllers offer advanced microrobotic capabilities like automatic vectoring and our General Command Set, which is both inherently multi-axis and easy-to-use. Powerful macro capabilities are built-in. and well-documented LabVIEW libraries, Windows .dll and Linux shared object libraries come standard and support the instrument’s high-level multi-axis capabilities, yet the sophisticated mnemonic command set can be utilized directly if desired.  Up to two additional axes of servo-controlled motion can be optionally provided by the controller. Two channels of optical or analog data acquisition can similarly be provisioned internal to the controller. RS-232 and TCP/IP interfaces are standard, with GPIB a cost-effective option. And PI hexapods provide a repeatable absolute coordinate system that is consistent from power-up to power-up; beware alternative configurations which rely on springs to support the load or address hidden backlash issues in the drivetrain.
While PI hexapods offer many advantages over stacks of stages, one thing in common is a great diversity of available configurations. Specialized PI hexapods are available, ranging from vacuum-compatible models (for which the naturally low-surface-area hexapod configuration offers additional benefits compared to stage stacks), to ultra-stable models designed to carry high-bandwidth active-optic packages in professional telescopes, to the popular F-206, optimized for high-throughput photonic packaging automation.

PI has many years of experience in designing and manufacturing hexapods for the world's most demanding applications.  As a consequence, PI makes more hexapods than all competitors combined.  Our experience benefits your application with superior performance, reliability, software and global support.  PI hexapods range in size from smaller than a coffee can to the size of a small car. If you don’t see what you need for your application, count on responsive support from a PI applications engineer at any of our worldwide offices.

Read more articles about Hexapod applications

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