Gantry robots, X-Y tables, split bridges, and stages all serve specific functions.
There are many ways to build linear systems for motion in the X, Y, and/or Z directions — also called Cartesian coordinates. Industry terms for these systems depend on how the axes are assembled, where the load is positioned, and (to some extent) the type of use for which the system was designed. In many industrial applications, Cartesian and gantry-style robots are prevalent … but in precision applications, XY tables are often the better choice due to their compact rigid structure and very high travel and positioning accuracies.
Cartesian systems consist of two or three axes — X-Y or X-Z or X-Y-Z. They often incorporate an end effector with a rotational component for orienting the load or workpiece, but they always provide linear motion in at least two of the three Cartesian coordinates.
Cartesian systems can include two axes (X and Y) or three axes (X, Y, and Z). When a Cartesian system is used, the load is usually cantilevered from the outermost axis (Y or Z). For example, in an X-Y gantry the load is mounted to the Y axis either to the end of the axis or at a distance from the axis … creating a moment arm on the Y axis. This can limit load capacity, particularly when the outermost axis has a very long stroke, creating a large moment on the lower, supporting axes. They’re used in a wide range of applications with maximum strokes on each axis typically one meter or less. The most common of these applications include pick-and-place, dispensing, and assembly.
To address the issue of outer axes causing a moment load on the inner axes, gantry systems use two X axes, and in some cases, two Y and two Z axes. Gantries almost always have three axes — X, Y and Z. The load on a gantry system is located within the gantry’s footprint and the gantry is mounted over the working area. However, for parts that cannot be handled from above, gantries can be configured to work from below.
Gantry systems are used in applications with long strokes (greater than one meter) and can transport very heavy payloads that are not suitable for a cantilevered design. One of the most common uses for gantry systems is overhead transport, such as moving large automotive components from one station to another in an assembly operation.
XY tables are like XY Cartesian systems in that they have two axes (X and Y, as their name implies) mounted on top of each other, and typically have strokes of one meter or less. But the key difference between XY Cartesian systems and XY tables lies in how the load is positioned. Instead of being cantilevered, as in a Cartesian system, the load on an XY table is almost always centered on the Y axis, with no significant moment created on the Y axis by the load.
This is where the principle of how the system is used helps distinguish between the various types of multi-axis systems. XY tables generally work only within their own footprint, meaning the load does not extend beyond the Y axis. This makes them best suited for applications where a load needs to be positioned in the horizontal plane (X-Y). A typical example is a semiconductor wafer being positioned for inspection, or a part being positioned for a machining operation to take place. Designs called open-frame or open aperture have a clear opening through the center of the table. This allows them to be used in applications where light or objects need to pass through, such as back-lit inspection applications and insertion processes.
Because XY tables are primarily used for very high precision applications, the guideway of choice is crossed roller slides, which provide extremely smooth and flat travel. Drive mechanisms are typically ballscrew or linear motor, although very fine pitch lead screws are also common.
The special case of stages
Positioning stages and rotary tables are leading the migration to integrated designs in motion applications. Just consider how fiber optic, test and measurement, and semiconductor applications such as assembly setups all use precision stages and tables to boost throughput and quality. More specifically, the manufacture of flat-panel displays has spurred ever-larger motion-stage formats with linear-motor actuation and air-bearing load carrying. Elsewhere, belt-driven stages satisfy the need for long strokes but avoid ballscrew support challenges and the cost of linear motors.
Another growth industry using positioning stages is additive manufacturing. Here, ever-improving materials and layup techniques demand new stages at all performance levels. Many basic maker-level machines use stages with synchronous belt-driven axes. More demanding applications (such as research and prototyping, medical, and small-batch manufacturing) commonly use positioning stages with motor-driven ballscrews to coordinate motion. In the same way, medical and life-research applications make use of ultra-precision stages that deliver performance motion profiles impossible a decade ago. Here, piezomotors, miniature linear supports, and coarse-and-fine tandem actuators are increasingly common options.
Pre-engineered positioning stages dominate packaging, as this industry often forces integrators to satisfy design schedules that bar design and set up of multi-axis functions in house. Likewise, the machine-tool industry is making more use of custom-built positioning stages—in laser-cutting and similar machines, for example. These and the stages for CNC applications are increasingly customized to specific motion tasks. That helps them deliver on dynamic parameters at lower cost and without the hassle of reformatting or retrofitting stock setups. Enabling this newer approach are proliferating software tools that let OEMs and end users manipulate initial design iterations within virtual environments that have accurate models of real-world stage components.
Assembly is different. Here, semi-custom Cartesian stages excel for pick-and-place and inspection via machine vision. More typical in these setups are traditional rotary motors paired with rotary-to-linear devices (ballscrews, for example) and controllers that compensate for system dynamics to get accuracy to a few micrometers or better.
Automotive applications widely vary. For example, the large scale of sheet metal and body-assembly tasks present unique challenges that, in some cases, only overhead stages (or those with rack-and-pinion sets) satisfy. At the opposite end of the spectrum, stages that carry inspection instrumentation to detect part features on a nanometer scale often take the form of direct-drive axis assemblies driven by a precision controller that even corrects for environmental fluctuations.
Optimizing precision for specific applications
When evaluating the accuracy of a linear motion system, the area of focus is often the positioning accuracy and repeatability of the drive mechanism. But there are many factors that contribute to the accuracy (or inaccuracy) of a linear system, including linear errors, angular errors, and Abbé errors. Of these three types, Abbé errors are probably the most difficult to measure, quantify, and prevent, but they can be the most significant cause of undesirable results in machining, measuring, and high-precision positioning applications.
Abbé errors begin as angular errors: Abbé errors are caused by the combination of angular errors in the motion system and the offset between the point of interest — for tooling or the transport of load — and the origin of the error such as the linear-axis screw or guideway. Angular errors — commonly called roll, pitch, and yaw — are unwanted motions due to the rotation of a linear system around its three axes.
If a system is moving horizontally along the X axis, as shown below, pitch is defined as rotation around the Y axis, yaw is rotation around the Z axis, and roll is rotation around the X axis.
Errors in roll, pitch, and yaw typically result from inaccuracies in the guide system, but mounting surfaces and methods can also be sources of angular errors. For example, mounting surfaces that are not precisely machined, components that are not sufficiently fastened, or even varying rates of thermal expansion between the system and its mounting surface can all contribute to angular errors greater than those inherent in the linear guides themselves.
Note that crossed-roller linear bearings are particularly sensitive to mounting errors. Their rigidity and accuracy make them less forgiving than other options of mounting inaccuracies. That’s why many linear-bearing manufacturers recommend only mounting crossed-roller variations to honed surfaces expected to exhibit no more than a few micrometers of deflection.
Abbé errors are especially problematic because they amplify what, in most cases, are very small angular errors, increasing in magnitude as the distance from the error-causing component (called the Abbé offset) increases.
For overhung loads, the farther the load is from the cause of the angular error the higher the Abbé error will be. The cause of the angular error is typically the guideway or a point on the mounting surface. What’s more, Abbé errors for multi-axis configurations are even more complex because they’re compounded by the presence of angular errors in each axis.
The best methods for minimizing Abbé errors are to use high-precision guides and to ensure that mounting surfaces are sufficiently machined — so they don’t introduce additional inaccuracies to the system. Reducing the Abbé offset by moving the load as close as possible to the center of the system will also minimize Abbé errors.
Abbé errors are most accurately measured with a laser interferometer or other optical device that is completely independent of the system. But laser interferometers aren’t practical for most setups, so linear encoders are used in many applications where Abbé error is a concern. In this case, the most accurate measurements of Abbé error are achieved when the encoder read head is mounted on the point of interest such as the tooling or the load.
XY tables are less susceptible to Abbé errors than other types of multi-axis systems such as Cartesian robots. That’s primarily because they minimize the amount of cantilevered travel and typically operate with the load located at the center of the Y axis carriage.
In an ideal world, a linear motion system would exhibit perfectly flat, straight motion and reach the intended position with zero error every time. But even the highest precision linear guides and drives (screws, rack and pinions, belts, linear motors) have some errors due to machining tolerances, handling, mounting, and even the way they’re applied.
There are three types of errors found in linear motion systems — linear errors, angular errors, and planar errors — and each type has a different effect on the system and the application. To avoid paying for high-precision components where they’re not needed or ending up with a system that doesn’t meet the application requirements, it’s important to understand the differences between these three types of linear motion errors and their causes.
Linear errors in linear systems: Linear errors include positioning accuracy and repeatability. These errors are sometimes referred to as positioning errors because they specify the system’s ability to reach the desired position. In the context of linear systems, the term “accuracy” typically refers to positioning accuracy, which is the deviation between the target position and the position the system achieved. Repeatability refers to how well a system returns to the same position over multiple attempts. The main contributor to linear errors is the drive mechanism (screw, rack and pinion, or linear motor, for example), but the system’s tuning can also affect its ability to reach the target position accurately and repeatably.
Angular errors in linear systems: As mentioned earlier, angular errors are errors in which the point of interest rotates around an axis. These are typically referred to as roll, pitch, and yaw errors, denoting rotation around the X, Y, or Z axis, respectively. If the point of interest is the center of the table, or slide, angular errors may not have a significant effect on the application. But when the point of interest is some distance away from the table or slide, Abbé errors, which are angular errors amplified by distance, can produce undesirable results, especially in machining, measuring, and assembly applications. The primary causes of angular errors, and by extension, Abbé errors, are inaccuracies in the linear guides and poorly machined mounting surfaces.
Planar errors in linear systems: Planar errors — often called straightness and flatness issues — occur during the system’s travel, but rather than rotation around an axis, planar errors are deviations from an ideal, straight reference plane. Straightness defines the extent of motion along the Y axis as the system travels along the X axis. Similarly, flatness defines the extent of motion along the Z axis as the system travels along the X axis.
Note here that the point of reference is the axis of travel (typically the X axis), so there are only two types of planar errors, involving motion along the remaining two axes.
Planar errors are detrimental to applications such as dispensing, machining, or measuring, where the system’s behavior during motion is critical. In multi-axis systems, planar errors in one axis affect the adjacent axis (or axes) especially when the axes are stacked such as in X-Y tables, planar tables, and some Cartesian systems.
Filed Under: Linear Motion Tips
