Setting the stage for good machine design

Alan Petersen
President
Primatics Inc.
Corvallis, Oreg.

Motion-control systems consist of three main components: the positioning mechanism, motor drive electronics, and motion controller. Each of these components should be carefully selected but for best system results, plan the positioning mechanism first. If the mechanism is not capable of meeting the requirements, the drives and motion controller cannot make up the difference.

The first step in designing any motion system is to fully describe and understand the process. Make a list of component performance parameters from this description. This list includes first-order parameters such as number of axes, travel length of each axis, precision of the motion (including resolution, repeatability, and accuracy), payload capacity, and physical size of the stages. Less obvious but equally important parameters include environmental constraints or challenges, drive selection, operation in multiple orientations, cable management in multiaxis configurations, lifetime planning, and ease of integration. A quick review of these parameters shows that they all relate to the positioning mechanism and so a thorough evaluation of these components is critical to project success.

The application will define whether the positioning stage is linear, rotary, or incorporates a combination of stages into a multiaxis system. Even in fairly straightforward single-axis applications, there are many considerations. Loads are a vital aspect of this profile, as issues such as payload weight and offset (center of gravity) can dramatically impact motion requirements. Consider typical and maximum load weights as well as the maximum and minimum distance the stage must travel, required travel speeds, and acceleration.

It is important to consider the stage as an integral part of the larger system. How the stage is mounted and the mounting structure, for instance, have a dramatic impact on stage performance and ability to meet specifications. For example, in a high-speed inspection application where samples quickly oscillate back and forth under a camera, a linear-positioning stage should be mounted on a structure that can withstand the "paint-shaker effect" of the moving load. Similarly, a long-travel linear stage selected for high precision in flatness must be mounted on an appropriately flat surface to avoid distortion from the stage conforming to a nonflat surface.

Also consider the lifetime requirements of the system when defining stage specifications. If requirements change over the machine's lifetime, it may put the system outside of positioning-stage tolerance and can degrade machine accuracy, productivity, and reliability. As with any moving component, positioning capabilities may change with extended use. Make sure the stage is rated to meet motion requirements over the intended service life of the machine.

Other influences include the size and environmental constraints of the system. Consider both horizontal and vertical size constraints. Factors that can influence the total footprint of the system include whether drive mechanics are external or internal and how cabling is managed. Environmental constraints can include clean-room applications, in which the moving parts of the machine must generate few particulates, or dirty environments, where ambient particulates can cause excessive friction within the stage and impact reliability and performance. Operating temperature is a key environmental issue that can dramatically affect stage performance. A temperature change of as little as two or three degrees can cause enough expansion to change stage tolerance.

Many applications require multiple-axis motion. In a multiaxis system, stages must be stacked for motion in different directions. A silicon-wafer inspection system, for instance, may need to provide linear X and Y motion as well as rotational theta. In such systems, it is important to consider how geometry effects tolerances in the rest of the system. For example, with two stages stacked on top of each other, the top stage can deflect at the ends of its travel. Deflection of the top stage is a function of the cantilever load on the bottom stage. This deflection must be taken into account or a different configuration should be considered. The stage manufacturer should ensure that the specifications of the stacked stages meet the application requirements.

In multiple-stage systems, cable management can become a logistics and reliability problem. Cables are often overlooked but can affect the life, geometry, and performance of the system. Look to the stage manufacturer for innovative cabling solutions. These might include integrating cables internally to reduce rubbing and drag, or using a single external cable interface rather than external cable connectors for more flexibility.

Deciding on the system drive is a key element. The two most common drive types are ball-screw and linear-motor drives. Ball-screw drives are inexpensive and easy to understand. With natural damping, they are easy to control and a brake can be easily added. On the other hand, mechanical friction can make it difficult to maintain a constant velocity. Under some conditions, such as temperature or humidity extremes, the pitch of the ball screw can change and affect accuracy. If thermal effects are an issue, a linear encoder may be required or a linear-motor stage may be a better choice.

Linear-motor drivetrains consist of a magnetic track and coil assembly. The magnetic track is typically stationary and consists of a series of permanent magnets mounted on a steel substrate. The coil assembly contains all of the copper windings and typically mounts to the sliding stage carriage. Some linear motor stages have the permanent magnets on the sliding carriage assembly as a means of simplifying the cabling, but magnet length limits the travel of these systems.

Linear-motor drives are typically best for light-to-moderate loads in high-speed, constant-velocity, or long-travel applications. Linear-motor drives have much longer travel capability than ball-screw drivetrains because they do not sag as the travel length increases. They can provide better velocity control but the moving coil and linear encoder electronics make cable management more complex. In addition, large linear drives are heavier and can become expensive as the travel length and magnet size increase.

An important consideration in choosing a drive type is stopping ability and mounting orientation. Linear-motor drives are free moving without power, whereas ball-screw drives have friction to dampen motion. This is particularly important in applications where the drive must mount vertically. Because a linear-motor stage is virtually frictionless, a loss of power will let the carriage free fall. In addition, the force of gravity must always be overcome, which puts a large continuous force requirement on the motor. Ball-screw drives are more appropriate for vertical applications, as linear motors can overheat quickly when run vertically or may require a counterbalance.

Selecting a motor may also involve trade-offs. Common rotary motors are the least-expensive option, but they add to drive-system space requirements. Linear motors take up less space but are more expensive because they have more magnets than a rotary motor and require a linear encoder. Ball-screw-driven stages can use linear encoders, but rotary encoders on the motor and ball screw will often work just as well and cost less. There are also trade-offs associated with using stepper motors or servomotors. Steppers are less expensive but servomotors have better high-speed performance.

An option for a ball-screw-driven stage is a frameless motor. A frameless motor is a standard brushless motor built into the stage. The rotor magnets are bonded directly to the ball-screw shaft and the stator windings are integrated into the end of the stage. This configuration eliminates the motor coupler, which saves several inches of space. The absence of the coupler reduces the hysteresis and wind-up of the motor-to-ball-screw connection, which improves performance. Stage manufacturers should provide expertise on motors and encoders to help define the best total solution for the application.

Once the mechanical and electrical aspects of the system motion are well understood and the stages selected, the details of the control system can be resolved. A control system should be compatible with the drive electronics, with special attention to the fact that not all drives provide feedback information on their connectors. Ideally, the controller should interface directly to transducer and actuator signals without additional hardware. The controller should also have enough performance to close the control loops within the natural data rates of the system, or simultaneously coordinate the motion of multiple motion axes as needed.

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