Tripod on XY cross table The advantages of stacked and parallel kinematic systems in a positioning solution

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Steinmeyer Mechatronics has developed a combination of a tripod and an XY cross-table. The result: highest accuracy, robustness, and reliability. The hybrid is tailored by the Dresden company exactly to the application-specific customer requirements and thus offers solutions for the assembly and measurement of optics and miniaturized lens systems.

In the measurement, assembly, and inspection of lenses, prisms, wafers, or semiconductor chips, maximum precision in the micrometer and submicrometer range is required. However, the transfer of components to the carrier is usually performed by systems optimized for long distances, maximum cycle times, and a robust, cost-effective design. Extreme accuracies? Not at all. Also, the number of degrees of freedom is often limited. For example, tilting in two directions is impossible if transport is carried out using an XYZ system. To meet the high accuracy requirements, the components must still be aligned with a separate alignment device after feed-in. In many cases, this is done with a hexapod. But is this always the best possible solution? Far from it. Innovative approaches such as the tripod on an XY cross table from Steinmeyer Mechatronics are highly precise, robust, easy to operate, and have proven to be a user-friendly and economical alternative in numerous alignment applications in the semiconductor industry.

Universality comes at a price

Hexapods are parallel kinematic systems and generate any spatial movements by controlled overlay of individual linear movements. Several drives support the component in parallel from different directions, which theoretically offers an advantage in stiffness. Virtual centers of rotation, i.e., rotation around axes outside the movement system, can also be accomplished. Hexapods are extremely popular and are considered the ultimate for high-precision alignment processes. But, as with all universally designed systems, compromises are inevitable. The sticking points lie especially in the complex mechanics, the high control effort, the occurrence of non-linearities, as well as the limited industrial suitability.

With hexapods, the correlation between positioning accuracy and displacement is strongly non-linear. In the area of slight deflection around the mid-position, the desired accuracies and stiffness are well maintained, but they deteriorate rapidly with increasing deflection. Now, up to six drives must work simultaneously in a hexapod to realize the desired movement - and each contributes to the error. In the worst-case scenario, the sum of errors is therefore six times higher. To compensate for these non-linearities, each individual actuator must be manufactured with extremely high precision. Accordingly, the price is high.

Speaking of price: Unlike in the laboratory sector, where controlled conditions and forces prevail, automated systems in production environments require a maximum degree of robustness and reliability. Hexapods quickly reach their limits in this regard. Industrially viable concepts that can hold their own in a rough environment can only be realized at high costs and by very few providers. Additionally, the complexity of hexapods, despite all progress and efforts, stands in the way of their wide adoption in machine applications and automation. Also, software modules that calculate the necessary transformations from the Cartesian input coordinates are rarely compatible with PLC controls (Siemens, Beckhoff, Bosch Rexroth, etc.). This further limits the possibilities for industrial use. Adjustments of the centers of rotation can only be performed within the supplied software modules. Integrating these into the machine control software, which is based on G-code, represents another significant hurdle.

Tripod on XY cross table instead of hexapod

"When using all six degrees of freedom in space and taking into account the significantly restricted accuracy at large deflections, hexapods are quite useful in quality assurance and assembly tasks with low annual throughput," says Elger Matthes, development and product management at Steinmeyer Mechatronics. "However, if higher accuracies, robustness, and reliability are required, then a different design is recommended - namely a tripod on a classic XY cross table." The combination of Cartesian stack and parallel kinematics positions with high precision over the entire travel range, requires little installation space, is easy to control, and thanks to its robust structure, it can handle millions of cycles under industrial conditions. In many applications, it clearly surpasses the hexapod.

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Up to five degrees of freedom can be realized with a tripod on an XY cross table: movements in the X and Y direction, height adjustment along the Z-axis, and two tilts in Phi and Delta. Thanks to the Cartesian basic framework, the travel paths for the X and Y movements can be carried out linearly and thus with high precision. The number of drives to be moved simultaneously and synchronized is reduced to three, and in numerous cases, there are even only two. This helps to keep the complexity of the control manageable. "The majority of parts to be aligned are rotationally symmetrical, as in the case of lenses. This eliminates rotation around the optical axis when aligning," explains Elger Matthes and adds: "Parallel kinematic systems are not suitable for the highly accurate 360-degree measuring rotation to check the quality of rotational symmetry before assembly - they are too inaccurate and too inflexible. A freely rotatable, air-bearing precision rotary table usually performs this task."

The design of the positioning system is very stiff in the vertical direction due to the action of three geometrically parallel drives - ideal for heavy loads. In the other directions, the required stiffness and force are also unproblematic, as one can rely on existing, appropriately sized standard tables. With a tripod on XY, any travel ranges in the translative degrees of freedom can be realized. As far as tilts are concerned, the correction requirement in optical applications and microassembly is typically under +/- 1 degree or +/- 2 degrees. This allows the use of solid-state joints to correct parasitic side movements. As a result, the accuracy of the three vertical drives is transferred to the test object free of play, wear-free, and almost completely linear. Virtual pivot points are feasible.

Highest precision, compact design, easy control.

"Parallel kinematic systems like the tripod are significantly more compact and stiffer than classic stacked constructions and offer advantages in terms of accuracy, especially in applications with small travel ranges," says Elger Matthes and explains: "By using only the nearly linear range of the travel paths in parallel kinematics, the repeatability of the individual drives is fully transferred to the test object, particularly when using solid-state joints. Integration into the familiar PLC machine control is significantly simplified by the partially orthogonal architecture and can be taken over and influenced by the customer themselves."

The tripod on XY design is used in the following positioning systems from Steinmeyer Mechatronics: KDT235 (scanning table with large aperture) with MP200-3 (miniature Z-Phi-Delta positioning system) or also MP500-6 with additionally integrated rotating unit.

Additional hexapod alternatives.

If larger tilts of up to +/- 10 degrees are required, a combination of a very stiff gimbal mechanism and a cross table is advisable - especially if an adjustment in the vertical direction, such as a focus adjustment in the sensor, is already available in the application or a classic lifting table is used. Steinmeyer Mechatronics has implemented this architecture in the MP250-4. A combination of MP70-3 (rotative adjustment unit with three degrees of freedom) on an MP130-3 (high-precision 3-axis adjustment system) works similarly, where even all six degrees of freedom can be adjusted again using a central ball joint. However, due to the Cartesian basis and the quasi-linear transfer function with small movements, the non-linearities are less pronounced than with hexapods. As a result, higher accuracies are possible.

There's no question: hexapods are excellent positioners. However, users should not make the mistake of glorifying them as a panacea. "Hexapods are sometimes used almost inflationarily. Yet for many applications there are technically and economically more attractive solutions. Here, a closer look at the specific application requirements helps - especially in terms of accuracy, industrial suitability, lifespan, handling, and degrees of freedom," says Elger Matthes, making it clear: "With the tripod on an XY cross table, the customer gets a user-friendly, highly precise, and robust positioning system, that is clearly advantageous over hexapods, especially when it comes to assembly and measurement of optics and miniaturized lens systems." As a specialist for customer-specific innovations, Steinmeyer Mechatronics has a high level of engineering expertise and adjusts the solutions to the respective application in terms of load, accuracy, space, motorization, and adjustment range.

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