Control Engineering Online Update for November 12, 2004

11/12/2004


Control Engineering Online Update - Technology

November 12, 2004

Highlights


Sponsored by BEI

Motion control design and implementation presents an array of options to the engineer. To select the most optimal approach, an understanding of stages and associated drive technologies is important.





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Selecting a stage for motion control

One of the most common applications of motion control involves moving an object from one position to another. Suppliers design motion control boards to interface to motors to achieve such applications; however, rotary motors are not very useful for moving objects from one place to another. One common way to change the rotary motion of a rotary motor to useful motion is by connecting the motor to a stage.

Stages are mechanical devices that provide linear or rotary motion that is useful in positioning and moving objects. These devices come in a variety of different types and sizes for use in many different applications. To find the correct stage for your application, you need to be familiar with the common terminology used when describing stages. Some of the key terms to understand when selecting a stage include:

  • Accuracy : How closely does the length of a commanded move compare to a standard length?

  • Resolution : The smallest length of travel of which a system is capable (can be as small as a few nanometers).

  • Travel distance : Maximum length the system is capable of moving in one direction.

  • Bi-directional repeatability : How close can a stage get to a commanded target position from one direction?

  • Uni-directional repeatability : How close can a stage get to a target position when arriving at it from both directions?

  • Maximum load : The greatest amount of weight the stage is designed to carry.

  • Prime mover : Designated when the actuator or motor is actually performing the motion.

  • Drive train : Mechanisms attached to the motor transferring the movement from the motor shaft to something useful.


Which stage is right?

Because such a variety of stages exist, it is often difficult to figure out which stage is right for your application. However, you can ask yourself some key questions to figure this out. Two main types of stages exist—linear and rotary.

Linear stages move in a straight line and are often stacked on each other to provide travel in multiple directions. A three-axis system with a X, Y, and Z component is a common setup used to position an object anywhere in 3D space.

A rotary stage is a stage that rotates on an axis (usually at the center). While linear stages are used to position an object in space, rotary stages are used to orient objects in space and adjust the roll, pitch, and yaw of an object. Many applications, such as very high-precision alignment, require both position and orientation to perform accurate alignment. The resolution for a rotary stage is often measured in degrees or arc minutes (1 degree equals 60 arc minutes).

One special type of rotary stage, called a goniometer, does not rotate around the center of the stage—it rotates around a point in space. You would use a goniometers in an application in which you need the radius of rotation to be very large, but you do not need the stage to travel through a full rotation. Goniometers look like linear stages that move in an arc rather than a straight line.

Another special type of stage is the hexapod. A hexapod is a parallel mechanism that gives you movement in six axes to control—X, Y, Z, roll, pitch, and yaw. Suppliers design parallel mechanisms so that each axis is connected in parallel to the platform. To move in any direction, each of the six axes must move together in parallel. To achieve the same number of axes using linear and rotary stages, you have to stack them together. However, stacking linear and rotary stages together introduces extra error for each stage you add. With a hexapod, you can define a virtual point in space about which the stage can rotate. While that is a benefit, the disadvantage is that hexapods are parallel. The kinematics involved are much more complex than those for simple, stacked stages. For example, if you wanted to move a stacked stage up two millimeters, you simply command the Z-axis to move two millimeters in the up direction. For a parallel mechanism such as a hexapod, you need to synchronize each of the six axes to move up two millimeters at the same time. X, Y, roll, pitch, and yaw moves are even more complex and must be carefully calculated based on the design of the hexapod. However, once you solve the kinematics for the parallel mechanisms, they can be powerful tools for a variety of high-precision applications.

The combination of the motor or actuator type and the drive train type ultimately determines the accuracy and performance of the stage. For example, some stages use piezos for controlling the movement of the stage. Piezo stages use piezoelectric crystals that change shape when a voltage is applied to them. When used as actuators, piezos offer high precision in the range of just a few nanometers. Piezos can also handle large loads and move very fast. However, you must still consider that many piezo-driven stages often have limited travel distances. Some newer technologies offer piezo-based motors with longer travel distances but smaller loads. Also, because of the effects that environmental factors—such as temperature—have on them, piezo-driven stages are not usually designed for industrial applications that run constantly.

Often, engineers use stages of different types together to provide the required performance. For example, you can stack a piezo on a typical brushed servo motor-driven lead screw stage to provide the right combination of speed, resolution, and travel distance.

The chart below offers a brief comparison of stage functionality.

Stage Drive Technology

Resolution

Speed

Maximum Load

Travel Distance

Repeatability

Relative Complexity

Relative Cost

Linear brushless motor

High

High

High

High

High

High

High

Stepper

Medium

Medium

Medium

High

Medium

Low

Low

Piezo

High

High

High

Low

High

High

High

Brushed servo

Medium

High

High

High

Medium

Low

Low


This article was submitted by National Instruments (NI). For more information on NI's motion control solutions, click here .





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