Designing a two-axis positioning stage can be a challenging affair. Simplifying the design makes assembly easier and increases reliability by reducing the number of components that can eventually fail. One way to simplify the design is to use a linear stepper motor with only one moving part. It provides an alternative to mechanically complex assemblies that require regular maintenance and develop inaccuracies over time.
Dual-axis linear stepper motors are relatively simple consisting of a motor primary called the forcer, and a motor secondary called the platen. The forcer consists of four single-axis coil assemblies, two of which are mounted at 90° to the other in a common housing. One set of coils drives the forcer in the X direction and the other in the Y direction. The surface of the platen is a steel square tooth pattern with the spaces between the teeth filled with epoxy. The platen surface is then lapped to a precision flatness. Tooth pitch determines the resolution when either full stepping or microstepping the motor. Increasing the number of motor phases also increases motor resolution.
The forcer rides on top of the platen surface via a pneumatic bearing system. The air bearing consumes roughly 2 cfm of air at 50 to 60 psi. The air flows into a 0.25-in. fitting on the side of the moving forcer and is evenly distributed through precision orifices on the bottom surface of the forcer between it and the platen. The forcer is constructed with rare-earth magnets which preload the air bearings to the steel platen with up to 350 lb of attractive force. This allows the motor to be inverted without the forcer separating from the platen.
The motors can be operated either open or closed loop. Open-loop accuracy is 0.0004 in./6 in. with unidirectional repeatability of 0.00001 in. absolute. It is also a common practice to profile the motor forcer and platen producing a map for software error compensation when running open loop. Mapping increases resolution and accuracy and offers an alternative to closed-loop operation.
Closed-loop accuracy and repeatability will depend on the type of position feedback selected. However, resolutions and accuracies of less than one micron are attainable. Compact laser interferometers and vision systems are often used for position feedback.
The dual axis motors are capable of creating forces to 30 lb and carrying payloads up to 100 lb. Light payloads can be accelerated to 1.5 g's. Because of the rare-earth permanent magnets in the forcer that preload the pneumatic bearing, the motor can be inverted allowing the payload to hang below the motor. In addition, multiple forcers can share a common platen providing a compact multiaxis assembly with overlapping trajectories.
Information for this article was provided by Ted Scheumann, Baldor Linear Motion Products, 25026 Anza Dr., San Clarita, CA, 91355, fax: (661) 257-0216, www.baldor.com