XYZ three-axis arrangements can be found in machine tools, measuring machines and laser processing systems, among others. They are also used in a variety of devices such as 2D and 3D printers, copiers, assembly systems and, in very large versions, overhead cranes. If a three-axis system is to be set up, the vertical axis is usually mounted on a supporting structure above the XY axes. Typically, the linear tables with the longer travels are positioned horizontally. In principle, three basic concepts can be described: stack, gantry and gantry. Which architecture is best suited to the respective application depends on the specific task. Several aspects are taken into consideration: In addition to installation space, natural frequency, dynamics and accuracy, the requirements for the controller and the overall costs are decisive.

Three basic architectures

Whether stack, gantry or gantry - no architecture achieves the best values in all areas, and every construction method has its advantages and disadvantages. However, a few simple correlations can be made: The higher the natural frequency, the more precisely and quickly the positioning system can be adjusted and the easier it is to shield against disruptive vibrations from outside (caused by machines, vehicles, fans, pumps or people walking, for example). The natural frequency is primarily determined by the mechanical structure and in the drive direction - particularly in systems with a linear motor - by the control rigidity. A high natural frequency due to the appropriate basic architecture is also a prerequisite for fully exploiting the potential of linear motors for high accelerations and therefore high speeds. The achievable speed is the measure of the process time and is particularly critical for automated production lines. However, the higher the speed, the higher the demands on the rigidity of the system and the performance of the controller. The controller requirements also increase with the increase in accuracy.

Finally, the accuracy is mainly determined by the pitch errors of the individual axes. Together with the influence of temperature, this error component is almost always one dimension higher than the errors of the feedback systems used. For example, the individual axes can bend when moving under load, which has a negative effect on the desired values. Misalignments and large distances between the individual axes also lead to systematic errors, which can be reduced by selecting the optimum architecture.

The stack

Whether stack, gantry, gantry or horizontal gantry - the optimum architecture depends on the task in hand.

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In the stacking architecture, two linear tables are mounted on top of each other, crossed in the direction of travel. As the workpiece is moved on both the X and Y axes, a lot of installation space is required - twice the surface area of the workpiece. The charm of the stack design lies in its simplicity and cost-effectiveness, and the requirements for the controller are also minimal. Another advantage is that there is no overdetermination between the individual tables. However, stacking the two tables on top of each other also results in reduced rigidity and therefore reduced natural frequency of the overall system.

The high moving mass also means that the dynamics quickly reach their limits. The overhanging of the loaded individual tables leads to high pitch deviations and thus to limited accuracy. In addition, all cables have to be moved in two directions. This makes the stack particularly suitable for short travel distances, lightweight parts and applications in which the objects are smaller than the travel distances. This architecture can be found, for example, in devices such as microscopes.

The portal

The advantages and disadvantages of the most common multi-axis systems on

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Unlike the stack, the axes in the portal architecture are not connected to each other, but are mounted individually at right angles to the base and to a crossbeam in the direction of travel. As both tables are connected to the structure, the individual systems achieve high values for rigidity and natural frequency. The result is a high natural frequency of the overall system. The portal design also scores highly in terms of dynamics, flatness and accuracy. In addition, there are no overdeterminations between the individual tables and, compared to the stack, cable routing is simpler in just one direction.
Portals are particularly suitable when large areas need to be traversed and both the object to be measured and the sensor can be moved. Applications with heavy measurement objects also benefit from the gantry design. As the workpiece is only moved in the X direction, the drive can be smaller than with a stack. Even heavy parts can be positioned quickly and precisely with gantries.

Gantries for stationary measurement objects

Elger Matthes is Head of Product Management at Steinmeyer Mechatronik in Dresden.

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With the stack and gantry architecture, the object to be measured is also moved. However, if the workpiece must not be moved during measurement, the gantry is the right choice. Here, one or both horizontal directions of movement are formed by two synchronously moving linear tables. Similar to a gantry crane, for example, a laser or a sensor is moved across the surface. The gantry architecture is characterized by high rigidity and natural frequency.

Decoupling measures avoid the design-related overdetermination between the individual tables. As at least one pair of drives must be electronically synchronized, the demands on the controller are correspondingly high. On the other hand, the pair of axes ensures high dynamics as well as good flatness and low pitch and yaw deviations, as the transverse axis is supported on both sides. This results in accuracy in the higher range. A particular strength of gantries is their minimal space requirement - they require the least installation space of all basic architectures.

Horizontal gantry with air base

Horizontal gantries with an air base, such as those found in wafer steppers, are a special form. They are considered the 'premier class of positioning technology'. In the horizontal version, the Y-axis is in the same plane as the two X-axes and forms the letter 'H' with them. The highlight: air bearings support the load on the base plate. This enables the best possible values to be achieved in terms of accuracy, rigidity, dynamics and flatness, as decoupling measures mean that only the geometry of the high-precision base plate is decisive. Straightness, pitch, yaw and roll are therefore in the top league. However, a lot of installation space is required, as at least one pair of drive axes surrounds the movement space and the workpiece is also moved.