Granite-Based Motion Platforms: A Mechanical-Bearing Case Study

By Brian Fink
Aerotech, Inc.

The selection of the most suitable granite-based linear motion platform for a given application depends on a host of factors and variables. It is crucial to recognize that each and every application has its own unique set of requirements that must be understood and prioritized in order to pursue an effective solution in terms of a motion platform.

One of the more ubiquitous solutions involves mounting discrete positioning stages onto a granite structure. Another common solution integrates the components that comprise the axes of motion directly into the granite itself. Choosing between a stage-on-granite and an integrated-granite motion (IGM) platform is one of the earlier decisions to be made in the selection process. There are clear distinctions between both solution types, and of course each has its own merits — and caveats — that need to be carefully understood and considered.

To offer better insight into this decision-making process, we evaluate two fundamental linear motion platform designs — a traditional stage-on-granite solution, and an IGM solution — from both technical and financial perspectives.

Background

To explore the similarities and differences between IGM systems and traditional stage-on-granite systems, we generated two test-case designs:
  1. Mechanical bearing, stage-on-granite
  2. Mechanical bearing, IGM
In both cases, each system consists of three axes of motion. The Y axis offers 1000 mm of travel and is located on the base of the granite structure. The X axis, located on the bridge of the assembly with 400 mm of travel, carries the vertical Z-axis with 100 mm of travel. This arrangement is represented pictographically in Figure 1.

Integrated Granite White Paper Figure 1
Figure 1. Pictographic representation of motion platform and arrangement of axes.

For the stage-on-granite design, we selected a PRO560LM wide-body stage for the Y axis because of its larger load-carrying capacity, common for many motion applications utilizing this “Y/X-Z split-bridge” arrangement. For the X axis, we chose a PRO280LM, which is commonly used as a bridge axis in many applications. The PRO280LM offers a practical balance between its footprint and its ability to carry a Z axis with a customer payload.

For the IGM designs, we closely replicated the fundamental design concepts and layouts of the above axes, with the primary difference being that the IGM axes are built directly into the granite structure, and therefore lack the machined-component bases present in the stage-on-granite designs. 

Common in both design cases is the Z axis, which was chosen to be a PRO190SL ball-screw-driven stage. This is a very popular axis to use in the vertical orientation on a bridge because of its generous payload capacity and relatively compact form factor.

Figure 2 illustrates the specific stage-on-granite and IGM systems studied.

IGM-Figure -2a ,b
Figure 2. Mechanical-bearing motion platforms used for this case-study: (a) Stage-on-granite solution and (b) IGM solution.

Technical Comparison

IGM systems are designed using a variety of techniques and components that are similar to those found in traditional stage-on-granite designs. As a result, there are numerous technical properties in common between IGM systems and stage-on-granite systems. Conversely, integrating the axes of motion directly into the granite structure offers several distinguishing characteristics that differentiate IGM systems from stage-on-granite systems.

Form Factor: Perhaps the most obvious similarity begins with the machine’s foundation — the granite. Although there are differences in the features and tolerances between stage-on-granite and IGM designs, the overall dimensions of the granite base, risers, and bridge are equivalent. This is primarily because the nominal and limit travels are identical between stage-on-granite and IGM.

Construction: The lack of machined-component axis bases in the IGM design provides certain advantages over stage-on-granite solutions. In particular, the reduction of components in the IGM’s structural loop helps to increase the overall axis stiffness. It also allows for a shorter distance between the granite base and the top surface of the carriage. In this particular case study, the IGM design offers a 33% lower work surface height (80 mm compared to 120 mm). Not only does this smaller working height allow for a more compact design, but also it reduces the machine offsets from the motor and encoder to the workpoint, resulting in reduced Abbe errors and therefore enhanced workpoint positioning performance.

Axis Components: Looking deeper into the design, the stage-on-granite and IGM solutions share some key components, such as linear motors and position encoders. Common forcer and magnet track selection leads to equivalent force-output capabilities. Likewise, using the same encoders in both designs provides identically fine resolution for positioning feedback. As a result, the linear accuracy and repeatability performance is not significantly different between stage-on-granite and IGM solutions. Similar component layout, including bearing separation and tolerancing, leads to comparable performance in terms of geometric error motions (i.e., horizontal and vertical straightness, pitch, roll, and yaw). Finally, both designs’ supporting elements, including cable management, electrical limits, and hardstops, are fundamentally identical in function, although they may vary somewhat in physical appearance.

Bearings: For this particular design, one of the most notable differences is the selection of linear guide bearings. Although recirculating ball bearings are used in both stage-on-granite and IGM systems, the IGM system makes it possible to incorporate larger, stiffer bearings into the design without increasing the axis working height. Because the IGM design relies on the granite as its base, as opposed to a separate machined-component base, it is possible to reclaim some of the vertical real-estate that would otherwise be consumed by a machined base, and essentially fill this space with larger bearings while still reducing the overall carriage height above the granite.

Stiffness: The use of larger bearings in the IGM design has a profound impact on angular stiffness. In the case of the wide-body lower axis (Y), the IGM solution offers over 40% greater roll stiffness, 30% greater pitch stiffness, and 20% greater yaw stiffness than a corresponding stage-on-granite design. Similarly, the IGM’s bridge offers a fourfold increase in roll stiffness, double the pitch stiffness, and over 30% greater yaw stiffness than its stage-on-granite counterpart. Higher angular stiffness is advantageous because it directly contributes to improved dynamic performance, which is key to enabling higher machine throughput.

Load Capacity: The IGM solution’s larger bearings allow for a substantially higher payload capacity than a stage-on-granite solution. Although the PRO560LM base-axis of the stage-on-granite solution has a load capacity of 150 kg, the corresponding IGM solution can accommodate a 300 kg payload. Similarly, the stage-on-granite’s PRO280LM bridge axis supports 150 kg, whereas the IGM solution’s bridge axis can carry up to 200 kg.

Moving Mass: While the larger bearings in the mechanical-bearing IGM axes offer better angular performance attributes and greater load-carrying capacity, they also come with larger, heavier trucks. Additionally, the IGM carriages are designed such that certain machined features necessary to a stage-on-granite axis (but not required by an IGM axis) are removed in order to increase part stiffness and simplify manufacturing. These factors mean that the IGM axis has a greater moving mass than a corresponding stage-on-granite axis. An indisputable downside is that the IGM's maximum acceleration is lower, assuming that the motor force output is unchanged. Yet, in certain situations, a larger moving mass may be advantageous from the perspective that its larger inertia can provide greater resistance to disturbances, which can correlate to increased in-position stability.

Structural Dynamics: The IGM system’s higher bearing stiffness and more rigid carriage provide additional benefits that are apparent after using a finite-element analysis (FEA) software package to perform a modal analysis. In this study, we examined the first resonance of the moving carriage because of its effect on servo bandwidth. The PRO560LM carriage encounters a resonance at 400 Hz, while the corresponding IGM carriage experiences the same mode at 430 Hz. Figure 3 illustrates this result.

IGM_Figure -3a ,b -final
Figure 3. FEA output showing first carriage mode of vibration for base-axis of mechanical bearing system: (a) stage-on-granite Y-axis at 400 Hz, and (b) IGM Y-axis at 430 Hz.

The higher resonance of the IGM solution can be attributed in part to the stiffer carriage and bearing design. A higher carriage resonance makes it possible to have a greater servo bandwidth and therefore improved dynamic performance.

Operating Environment: Axis sealability is almost always compulsory when contaminants are present, whether generated through the user's process or otherwise existing in the machine's environment. Stage-on-granite solutions are particularly suitable in these situations because of the inherently closed-off nature of the axis. PRO-series linear stages, for instance, come equipped with hardcovers and side seals that protect the internal stage components from contamination to a reasonable extent. These stages may also be configured with optional tabletop wipers to sweep debris off of the top hardcover as the stage traverses. On the other hand, IGM motion platforms are inherently open in nature, with the bearings, motors, and encoders exposed. Although not an issue in cleaner environments, this can be problematic when contamination is present. It is possible to address this issue by incorporating a special bellows-style way-cover into an IGM axis design to provide protection from debris. But if not implemented correctly, the bellows can negatively influence the axis' motion by imparting external forces on the carriage as it moves through its full range of travel.

Maintenance: Serviceability is a differentiator between stage-on-granite and IGM motion platforms. Linear-motor axes are well known for their robustness, but sometimes it does become necessary to perform maintenance. Certain maintenance operations are relatively simple and can be accomplished without removing or disassembling the axis in question, but sometimes a more thorough teardown is required. When the motion platform consists of discrete stages mounted on granite, servicing is a reasonably straightforward task: dismount the stage from the granite, perform the necessary maintenance work, and remount it — or simply replace it with a new stage.

IGM solutions can at times be more challenging when performing maintenance. Although replacing a single magnet track of the linear motor is very simple in this case, more complicated maintenance and repairs often involve completely disassembling many or all of the components comprising the axis, which is more time-consuming when components are mounted directly to granite. It is also more difficult to re-align the granite-based axes to one another after performing maintenance — a task that is considerably more straightforward with discrete stages.

A summary of the fundamental technical differences between mechanical-bearing stage-on-granite and IGM solutions is provided in Table 1.

Table 1: Technical Comparison of Mechanical-Bearing Solutions

     Stage-on-Granite System, Mechanical Bearing IGM System, Mechanical Bearing
Base Axis (Y) Bridge Axis (X) Base Axis (Y) Bridge Axis (X)
Normalized Stiffness  Vertical  1  1  1.2  1.1
 Lateral  1  1  1.2  1.5
 Pitch  1  1  1.3  2.0
 Roll  1  1  1.4  4.1
 Yaw  1  1  1.2  1.3
Payload Capacity (kg)  150 150 300 200
Moving Mass (kg)  25 14 33 19
Tabletop Height (mm)  120 120 80 80
Sealability  Hardcover and side-seals offer protection from debris entering the axis IGM is usually an open design; sealing requires the addition of a bellows way-cover or similar
Serviceability  Component stages can be removed and easily serviced or replaced Axes are inherently built into the granite structure making servicing more difficult

Economic Comparison

While the absolute cost of any motion system will vary based on several factors including travel length, axis precision, load capacity, and dynamic capabilities, the relative comparisons of analogous IGM and stage-on-granite motion systems conducted in this study suggest that IGM solutions are capable of offering medium- to high-precision motion at moderately lower costs than their stage-on-granite counterparts.

Our economic study consists of three fundamental cost components: machine parts (including both manufactured parts and purchased components), the granite assembly, and labor and overhead.

Machine Parts: An IGM solution offers noteworthy savings over a stage-on-granite solution in terms of machine parts. This is primarily due to the IGM’s lack of intricately-machined stage bases on the Y and X axes, which add complexity and cost to the stage-on-granite solutions. Further cost savings can be attributed to the relative simplification of other machined parts on the IGM solution, such as the moving carriages, which can have simpler features and somewhat more relaxed tolerances when designed for use in an IGM system.

Granite Assemblies: Although the granite base-riser-bridge assemblies in both the IGM and stage-on-granite systems appear to have a similar form factor and appearance, the IGM granite assembly is marginally more expensive. This is because the granite in the IGM solution takes the place of the machined stage bases in the stage-on-granite solution, which requires the granite to have generally tighter tolerances in critical regions, and even additional features, such as extruded cuts and/or threaded steel inserts, for instance. However, in our case study, the added complexity of the granite structure is more than offset by the simplification in machine parts.

Labor and Overhead: Because of the many similarities in assembling and testing both the IGM and stage-on-granite systems, there is not a significant difference in labor and overhead costs.

Once all of these cost factors are combined, the specific mechanical-bearing IGM solution examined in this study is approximately 15% less costly than the mechanical-bearing stage-on-granite solution.

Of course, the results of the economic analysis depend not only on attributes such as travel length, precision, and load capacity, but also on factors such as the selection of the granite supplier. Additionally, it is prudent to consider the shipping and logistics costs associated with procuring a granite structure. Especially helpful for very large granite systems, although true for all sizes, choosing a qualified granite supplier in closer proximity to the location of the final system assembly can help to minimize costs as well.

It should also be noted that this analysis does not consider post-implementation costs. For instance, suppose it becomes necessary to service the motion system by repairing or replacing an axis of motion. A stage-on-granite system can be serviced by simply removing and repairing/replacing the affected axis. Because of the more modular stage-style design, this can be done with relative ease and speed, despite the higher initial system cost. Although IGM systems can generally be obtained at a lower cost than their stage-on-granite counterparts, they can be more challenging to disassemble and service because of the integrated nature of construction.

Conclusion

Clearly each type of motion platform design — stage-on-granite and IGM — can offer distinct benefits. However, it is not always obvious which is the most ideal choice for a particular motion application. Therefore, it is greatly beneficial to partner with an experienced motion and automation systems supplier, such as Aerotech, that offers a distinctly application-focused, consultative approach to explore and provide valuable insight into solution alternatives to challenging motion control and automation applications. Understanding not only the pros and cons of multiple varieties of automation solutions, but also the fundamental aspects of the problems they are required to solve, is the underlying key to success in choosing a motion system that addresses both the technical and financial objectives of the project.