Medical Device Link.

Originally Published MEM Spring 2004

MOTION CONTROL

A manufacturer's component specifications may be determined under conditions not found in the real world of an application.

Kyle Tomson

Selecting motion system components to meet the demanding requirements of medical equipment applications involves more than consideration of trade-offs between cost and performance. Medical system designers need to be aware of so-called specsmanship issues that can make direct apples-to-apples comparison of components offered by different manufacturers difficult. If they do not comprehend certain factors that lie behind the specifications, designers can easily find themselves making expensive late-stage design changes, struggling to meet certification requirements, or dealing with long-term-reliability and repair issues.

Medical equipment such as dispensers and scanners typically requires motion control components with tight specifications. Those applications often involve multiaxis positioning, extremely precise tolerances, high-speed operation, and high duty cycles. When analyzing the specifications of motion system elements, designers should be careful to account for positioning requirements at the machine level and allow for sufficient margin in the components so as to ensure that the machine will perform at an acceptable level.

Motion control components are always tested under ideal conditions. Duty cycles are low, loads are small and centered, mechanical components are mounted on massive granite structures with lapped surfaces, and environmental conditions such as temperature variation, humidity, and vibration are tightly controlled. As a real-world machine deviates from such abstract conditions, large discrepancies between the specifications measured in the metrology laboratory and those measured on the production floor can materialize.

Understanding the conditions under which specified components were originally tested, how the testing was performed, and how the resulting data were interpreted will help the machine designer determine which sort of specifications really need to be obtained from each motion component supplier so that true apples-to-apples comparisons can be conducted (see the sidebar "Key Specification Questions to Be Answered").

Key issues to address in selecting motion control components for medical equipment systems are:

• Test procedures and parameters.
• Measured versus calculated specifications.
• System constraints.
• Environmental constraints.
• Maintenance and repair.
• Customer support.

Successfully designing around these areas of concern involves understanding how specifications for motion components are developed and how these specs relate to the requirements of a given application.

Test Procedures and Parameters

The stakes are high in critical medical equipment applications. How key motion component characteristics are measured is thus very important in determining whether a particular product is appropriate for incorporation into a system.

Figure 1. A positioning stage with significant angular errors (stage curvature is exaggerated to aid visualization). Q = angular error of the stage, X = position of the POM if no angular errors were present, and X+ d = position of POM with the angular errors. (click to enlarge)

Several test techniques and assumptions can affect the reported performance of a positioning stage. For example, if the point of measurement (POM)—the location in the equipment at which the customer cares about performance—is far away from the surface of the stage, then the angular errors of the stage will tend to dominate system performance regardless of the performance rating of the feedback transducer. The curvature of the stage will affect the accuracy to an extent dependent on the POM (see Figure 1). If the POM is centered on the carriage, the effect of the curvature will be small. But if the POM is far to the side of the stage, the errors will be increased dramatically. This type of error is called Abbe error. As the number of axes increases, the effects of Abbe errors become worse because the distance from the bottom stage to the POM continues to grow, as does the error induced by the widening arc of the angular errors.

The stiffness of the system also can have a dramatic effect on system performance, especially when multiple stages are combined. A typical medical equipment application may involve three axes of linear motion. For example, a dispensing application will have x- and y-axes to position a substrate under the dispensing head and a z-axis to position one of a group of alternative dispensing heads at the dispensing position. In systems of this type, the designer must take into account the way the stages will behave in conjunction. The y-axis will deflect under gravity to a varying degree, depending on its cross section, the way it is oriented, the mass of the load, and the position of the carriage bearing the load. This deflection can manifest itself in inaccuracy. It can also create resonance and servo instability, which will affect the repeatability, velocity stability, and resolution of the system.

In addition, the characteristics of the surface that the stage is mounted on can affect performance. For example, motion stage performance specifications often are measured by the manufacturer when the stage is mounted on a laboratory-grade granite surface plate, yet many medical devices are sold and used as transportable benchtop systems. Medical equipment designers should allow for variation between optimal performance and actual performance. And, in some cases, they should provide for recalibration of systems as necessary. In the most accuracy- and stability-sensitive applications, designers should consider using granite platforms as recommended by the stage manufacturer.

Mechanical motion devices are always tested by their manufacturers under ideal circumstances. But many issues can affect nearly all performance test results. These include velocity and dwell times, positioning-stage geometry during the test, type of measuring device, test duty cycles, number of test runs, and increments between data points. It is important that designers discuss with potential vendors the details of their test procedures. Some manufacturers are willing to conduct custom testing in order to give customers a more accurate picture of how their motion devices will perform in a particular application. Further, because the setup and data collection methodology for certain testing is critical, it is a good idea for customers to request that they be physically present to observe the initial testing. It is important to actually see that the products meet stringent requirements.

Measured versus Calculated Specifications

Gilson's Constellation 1200 uses both an x- and a y-axis to position a substrate under the dispensing head. (click to enlarge)

Many published specifications can be calculated rather than measured. Although the calculations may lead to specs that mirror actual test results closely, the initial conditions on which the calculations are based can vary wildly, producing dramatic differences in calculated results. For example, as the resolution of mechanical devices approaches 1 µm and less, the difference between the theoretical resolution and the obtainable resolution becomes an important consideration. Obtainable resolution is affected by friction, stiction, backlash, placement of the feedback, and the capabilities of the motion controller.

Designers must know whether the vendor's specifications are calculated or actually measured. If a critical spec is calculated, then the medical system designer should allow some margin to account for natural variability in the materials that compose the motion system. Once again, some suppliers are willing to verify a calculated specification by means of special testing. Design engineers should always ask whether this service is available.

Another specification whose calculated values can be misleading is load capacity. While medical equipment applications typically do not have to accommodate heavy loads, they do sometimes involve high accelerations, which in turn impose large dynamic loads on a positioner. Load capacity is directly related to the service life of the positioner. A 10% increase in the static or dynamic load a positioner handles can produce as much as a 40% reduction in the service life of the device.

So, designers should do two things: (1) work to minimize the load that the positioner must handle and (2) make sure that any positioner chosen is rated adequately for the required lifetime of operation. Load capacity and service life are almost always calculated values. It is critically important that design engineers understand how the calculations were made, including the supplier's starting assumptions. Depending on the initial conditions, the rated load capacity and service life that fall out of the calculations can vary by as much as 1000%.

Load capacity also has a direct tie to motion system specifications. For example, in defining system specifications, designers should use the thrust capacity of the weakest link in the drive train to set load limits.

System Constraints

Figure 2. An x-y-Q positioning system with internal cable management, showing detail of the internal terminated-cable wire harness at top. (click to enlarge)

Constraints in the design or use of the motion system can have a serious effect on the performance capabilities of the application. Cable management, for example, is connected with reliability, performance, and accuracy issues and is an important consideration with regard to system aesthetics. Many medical equipment systems serve in dispensing applications, in which the motion stage must be easily accessible by laboratory personnel. Cables must be laid out in a way that supports this access. At the same time, cables have to be designed and mounted such that they will not shed particles into dispensing or sampling work areas. They must also be designed so as not to impose loads on motion elements and thus affect system performance. Figure 2 shows an internal cable management scheme for a system stacked with linear x- and y-axes and an angular q-axis that is designed to minimize the impact of moving cables on the system.

Other system constraints include space and mounting. Medical equipment is often designed as a desktop or benchtop system. Even when the overall size of a system is minimized, its performance and service life requirements cannot be compromised. System ruggedness, accessibility, and serviceability need to be evaluated continually as design form-factor decisions are made.

While not a performance issue, overall system aesthetics can influence equipment marketability. Unlike many industrial electronic systems that are housed inside cabinets, motion components in medical systems, such as the stage and cable assemblies, often are front and center. Designers should look for attractive industrial designs that can meet the system's performance requirements.

Environmental Constraints

Environmental issues are another critical area of concern for medical equipment. System performance can be affected by both ambient temperature and heat sources within the system itself.

Many medical applications are situated in environments that are not temperature controlled. In a typical laboratory, ambient temperature can range over as much as 10°F depending on the day or season and the location of the lab. The various components of the motion system will expand and contract at different rates when subjected to a temperature gradient, which can cause dramatic changes in the performance of the system. For example, the steel bearings of a 300-mm travel stage will expand by 9 µm with only a 3°C temperature increase, while the aluminum base in the same stage expands 18 µm. This bimetallic expansion can create enormous stresses in the positioning stage that translate into degraded performance specifications. For medical applications that typically require submicron accuracy, engineers should look to stage vendors whose models take into account the desired operational temperature range and then specify their products accordingly.

Even in a temperature-controlled room, the motor or drive train in the motion system can generate heat. The system should be designed to keep the thermal load minimized. For instance, a high-efficiency motor can provide the same torque as a standard motor but with half the temperature rise.

In addition, cooling critical components can scrub heat away from the system before thermal distortion can occur.

Cleanroom requirements also can present a challenge for medical equipment designers. Some medical systems are so sensitive to particulate contamination that they have to operate in a vacuum. System designers may need to work closely with motion component vendors to specify components that will not generate particles that could contaminate the system or make it difficult to establish and maintain a vacuum.

When specifying motion components, designers should consider unusual environmental circumstances for which the manufacturer may not have tested. For example, some fluids used in medical and biological applications are corrosive. Anticipating these types of environmental issues generally requires going beyond typical manufacturers' specifications for components.

Maintenance and Repair

The service life of the system and its components must be taken into consideration when component specifications are evaluated. Often, motion stage mechanisms can hold up under heavy loads or high duty cycles for a short while; however, medical equipment has to function as advertised for a full life cycle, without maintenance personnel to watch over it. The term service life refers to how long the system or component can deliver performance to spec—providing the expected service—not simply how long it can continue operating.

Because medical equipment systems typically are not attended by trained service technicians, they must be designed for minimal, easily performed maintenance. In addition, design engineers should understand how long each component is designed to last under load and duty-cycle conditions similar to those of the application of interest. A positioning stage meant for production applications will require substantially oversized components in order to survive around-the-clock operation for any length of time. Safety margins of 1000% or more are necessary to ensure that the stage will provide years of trouble-free operation in a production environment.

It is also important that positioners are chosen that are easy to service. Ideally, design engineers should look for positioners that can be serviced with no disassembly. Positioners designed for production applications should allow access to critical components that need periodic preventive maintenance.

Conclusion

The last key issue to address in selecting motion control components, customer support, is not covered by component or subsystem specifications, but it can make the difference between a successful or an unsuccessful project. Its importance underscores that of investigating deeper when sourcing components than simply comparing published specifications. The designer should determine whether the supplier is knowledgeable about how the entire machine is to ultimately work or only the portion that relates directly to the supplier's product. Another key consideration is whether the supplier can provide before-the-sale information based on machine-building experience, as well, of course, as delivering after-the-sale support in a timely manner.

By looking beyond the specs of motion control elements, medical equipment designers can minimize design and certification problems, reduce the likelihood of long-term reliability and repair issues, and ensure maximum customer satisfaction with their system designs.

Kyle Tomson is director of marketing and sales for Primatics Inc. (Corvallis, OR). He can be reached at 541-757-9678 or via e-mail at kylet@primatics.com.

Copyright ©2004 Medical Electronics Manufacturing