Provide controlled, precise positioning along a linear axis.

Solutions to provide controlled and precise positioning greatly depend on the application. Are there any cost restraints? What are the technical requirements?

“The best way to provide controlled, precise positioning along a linear axis is with a linear motor driven stage,” adds Paul Zajac, product engineer, Yaskawa Electric America. “Precise and accurate motion is possible due to the advantages of a direct driven payload and linear feedback device.”

According to Mark McCann, system design engineer, Myostat Motion Control, a linear motor with high speed, accuracy and repeatability may not suit an application that a ballscrew actuator would. “Understanding your application, and the options available, is the first step in attaining the motion and positioning you require,” says McCann.

The mechanical compliances associated with the components required to convert rotational motion to linear motion are eliminated with a direct driven payload, according to Zajac. The elimination increases machine stiffness and positioning response. 

“The linear position feedback device is also directly coupled to the motor, and the most representative of the actual payload position,” adds Zajac. The higher linear encoder resolutions also have the ability to improve positioning repeatability and accuracy.

The linear motor system also features a non-contact and presumably maintenance-free design.

What To Spec

Besides simply selecting a linear slide that is capable of the application’s motion requirements, the positioning and mechanical accuracies should also be considered.

“It is important to understand the machine requirements before selecting a linear stage,” adds Zajac. “Many applications require just point-to-point moves, so repeatability is an important specification.”

As long as the machine is repeatable, positioning inaccuracies can be compensated at each specific point under identical conditions. Zajac also notes that it is important to select a stage accordingly if positioning straightness and flatness are important to machine performance, since the inaccuracies typically can’t be compensated.

“The first specifications often looked at are required force, speed, accuracy, repeatability and backlash,” reports McCann. “Additional forces are often overlooked and can affect the system immediately, or further down the line.”

Are any side loads applied? Could you expect any moments on any of the carriages? According to McCann, “with multiple stages, the [internal] play can greatly affect the required accuracy.”

Cable life is another important consideration, especially for linear motor drive stages. “Linear motors are common in high speed, high cyclical applications, and some low profile stage designs don’t allow the proper cable bend radius for long life,” says Zajac.

A typical guide mechanism for linear slides and stages are precision ground profile rails with recirculating ball bearings.

According to Zajac, this design allows for high load-carrying capacities, high speed and acceleration and high running accuracies. Different levels of accuracy are available, but as Zajac notes, the increase in cost may not be justified for the application requirements.

Absolute Increase

In the past year, the industry has experienced an increase in linear motor driven stages. “More encoder manufacturers are also introducing absolute linear encoders,” advises Zajac. “Absolute encoders eliminate the need for homing and motor phase detection, thereby eliminating the need for hall sensor and reference marks and simplifying linear motor stage designs.”