Innovative High-Bandwidth Hexapod Concept for Nanopositioning Applications

Parallel kinematic machines (hexapods) with sub-micrometer precision and six degrees of freedom (6-DoF) are gaining traction in photonics, micro-optics, and wafer-level alignment. To meet industrial demands for reliability, precision, and high dynamics, PI developed a compact, direct-drive 6-DoF hexapod using lever-based actuators to reduce complexity, improve performance, and minimize wear. This article outlines the system design and evaluates its precision and dynamic capabilities for industrial-scale use.

However, traditional compact, screw-driven 6-DoF PKMs are limited in performing high-frequency, high-amplitude motion. As shown by Rudolf et al.^[2], these systems can also be subject to accelerated wear during small-amplitude, high-frequency operation, requiring special design solutions, such as implemented in the H-815 ruggedized hexapod to run them in 24/7 demanding industrial applications. In contrast, direct-drive PKMs offer higher accelerations and velocities with reduced mechanical wear, thanks to their simplified design.

While flexure-based systems like PI’s H-860^[2] or the T-Flex hexapod^[3] offer excellent precision, their size and workspace limitations make them less suitable for industrial production lines. To address this, PI has developed a compact, direct-drive 6-DoF PKM comparable in size to existing screw-driven hexapods, combining high dynamics with precision to support increased throughput and reduced downtime in industrial environments.

The newly developed lever-actuated 6‑DoF positioning system (see Figure 3) employs a parallel-kinematic design. It comprises six identical actuators arranged between a fixed base platform and a movable top platform. This configuration enables the top platform to perform precise lateral and rotational motions along all six axes: X, Y, Z, pitch, roll, and yaw.

The system’s actuators are integrated into the base platform and consist of brushless DC (BLDC) motors with ceramic bearings, each fitted with a lever attached to the motor shaft. This design eliminates moving sensor- or power cables, preventing parasitic forces from degrading system performance. Each lever connects to the top platform via passive struts offering five degrees of freedom — enabled by two universal joints and one rotational joint. Rotating a lever moves the strut’s lower endpoint along a circular path, thereby driving the motion of the top platform. Lever position is measured using a scale mounted opposite the lever (Figures 4a & 4b).

To counteract the weight of the top platform and struts in the neutral position, counterweights are mounted on the levers. The universal joints were newly developed for this model. They must be compact, lightweight, backlash-free, and capable of large angular travel. While ball joints allow high angular motion in a compact form, their friction and hysteresis make them unsuitable for precision applications. Flexure joints, though free of hysteresis, offer limited angular range. To overcome these trade-offs, a sub-compact cardanic joint with crossed axes and four ball bearings was developed (Figure 4), providing backlash-free performance and large angular motion. In addition to the universal joints, each passive strut includes a high-precision rotational bearing, allowing rotation about its longitudinal axis.

The passive strut and lever lengths were carefully selected to match the workspace of PI’s screw-driven H‑811 hexapod. As shown in Table 1, the functional model delivers slightly reduced X/Y travel, increased Z travel, identical rotations around X/Y, but approximately half the Z-axis rotation. Despite a lower payload due to the absence of drive-screws, it supports many photonics alignment tasks, achieving peak speeds of ~65 mm/s with counterweights — more than 3 times faster than the H‑811’s 20 mm/s. A summary of system specifications is provided in Table 1.

The hexapod’s theoretical resolution depends on the lever position (with pure Z-motion extremes detailed in Table 1); however, along the sensor scale it consistently achieves about 20 nm precision.