Selecting a Stage and Motion Controller for Your Automation Application

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 about 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 very 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, rather it rotates around some 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 very 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. This is truly remarkable considering this is a fraction of the wavelength of visible light. 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 technology, exemplified by a company called Nanomotion, 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 servomotor-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.

To help you find the right stage for your application and minimize the amount of time for configuring your system, NI has partnered with many different stage companies to provide easy connectivity between the NI 734x and 735x Series motion controllers and third-party stages. You can compare specifications and connectivity options for the different stages from the partnership companies at the Stage Advisor on NI's motion control Web site.

See Also:
Nanomotion Ltd.
NI Motion Control Information
NI 7344 Motion Controller Information
Bayside Motion Group
Primatics

Another stage attribute to consider when choosing a stage for your precision motion system is backlash. Backlash is the lag occurring when one gear in a system changes direction and moves a small distance before making contact with a companion gear. Backlash can lead to significant inaccuracies especially in systems using gears in the drive train. When moving on a nanometer scale, even a small amount of backlash can introduce major inaccuracy in the system. To deal with this problem, controllers often have methods for compensating. For example, the NI 7344 motion controller has dual loop feedback that you can use to help compensate for backlash. Dual-loop feedback means the controller accepts feedback from two different sources for a single axis. To understand the circumstances in which you would want to use this feature, consider a stage. If you monitor the stage position directly (as opposed to the position of the motor driving the stage), you can tell if a move you have commanded has reached the target location. However, because the motion controller is providing input signals to the motor and not to the stage, the primary source of feedback, the difference in the expected output relative to the input could cause the system to become unstable. To make the fine adjustments necessary to help the system stay stable, you can also monitor the feedback directly from the encoder on the motor as a secondary feedback source. Using this method, you can monitor the real position of your stage and account for the inaccuracies in the drive train.

Engineers use NI motion control products with measurements and inspection equipment in a variety of applications, including semiconductor and fiber-optic alignment manufacturing, as well as biotech counting applications, where speed is very important. Many of these applications previously used RS-232 or GPIB to integrate measurements from an instrument and a stage. With this method, however, you lose a tremendous amount of time waiting on an instrument to stabilize and then waiting for the data to transmit over a data bus.

For example, when using RS-232 or GPIB, you must command the stage to move, then wait for the motion to stop, and then command the instrument to take a measurement at each location. Using National Instruments Real Time System Integration (RTSI) bus removes these performance gaps. National Instruments data acquisition, motion control, and image acquisition products use the Real Time System Integration bus or RTSI bus. The RTSI connects two or more National Instruments boards using a ribbon cable connected across the top of PCI plug-in boards or built into the backplane of PXI to dramatically improve performance. For example, using RTSI you can acquire a measurement through a data acquisition board while simultaneously recording the position of the stage. In addition, you can use machine vision to guide your motion task. National Instruments driver software and LabVIEW application software deliver the integration.

Customers report that using RTSI and plug-in devices rather than RS-232 or GPIB devices results in as much as 10 to 40 times faster performance.

See Also:
NI PXI Information
NI Biotech Information
NI LabVIEW
NI Machine Vision
NI Motion Control

After selecting the appropriate stage and connecting it with your motion controller, the next step is to program the stage. NI Motion Assistant is a new development and code generation tool that helps you develop your stage applications quickly and easily. Motion Assistant is ideal for developing stage applications because of the easy-to-configure 2D and 3D moves. There is also a preview window that you can use to prototype your stage applications before actually running them. Within the preview window, you can set up a virtual workspace that you use to define the travel limits of your stage and ensure you do not go outside of those limits while you program. When you have completed your prototype, you can easily create a LabVIEW VI based on the prototype using the automatic code generation feature available with NI Motion Assistant. Using this feature, you can quickly create integrated motion control programs to solve your automation applications.

Fiber optics is an area where integrated, PC-based motion and stages come together in key precision alignment applications. Motion control products help align the fiber, or move the different fiber components from one sample to the next. Timing and synchronization are two critical areas in the fiber-optic alignment process. Real-time synchronization between motion and measurements helps align the fiber faster and with greater precision, enhancing production overall.
In semiconductor alignment, motion control products and stages work together to play an important role. For instance, a company can select a motion controller and a stage to work in tandem for thin film characterization. Motion control products can measure the film in different positions. While an optical head remains stationary, the stage moves; the controller tells it where it should go. The accuracy of the stage is measured in microns, so accuracy is another important factor.

Within the biotechnology field, finding a means of accelerating drug testing is an important task for scientists. By using a combination of motion controllers and stages, you can precisely position plates that contain minute quantities of drug samples. Aided with the precision of motion control and stages, biotechnicians can test more samples more quickly, resulting in new pharmaceutical solutions faster.

Learn more about integrating your measurements with motion and vision by clicking on the links below.

See Also:
Integrating Measurements with Vision and Motion
NI Motion Control