Performance of Direct-Drive Linear Motor Stages in Precision Positioning Applications

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Brushless direct drive linear motors provide advantages over leadscrew and ballscrew driven positioning stages when it comes parameters such as acceleration, maximum speed, accuracy, constancy of velocity, and avoidance of vibration. This article explains types of motors, encoders and provides test data of PI’s new V-551 linear motor stage.

Linear motor stages, come in different sizes, and are available with absolute and incremental encoders. (Left: V-551 linear stages, Right: MCS Planar Integrated XY stage)
Linear motor stages, come in different sizes, and are available with absolute and incremental encoders.
Left: V-551 linear stages
Right: MCS Planar Integrated XY stage

 

Linear Motor Stage with Three Phase Motor: How Does It Work?
Linear Motor Stage with Three Phase Motor: How Does It Work?

Iron Core and Ironless Motors

Typically, two types of brushless linear motors are used in magnetic direct drive technology: ironless linear motors and iron core linear motors.

With iron core motors, the active part (primary part) consists of a sheet metal stack (ferromagnetic steel) with “teeth” and the coils wound around those teeth.

Basically iron core motors provide high force and low cost, but the cyclical variation of moving force within the magnetic field (cogging) has negative side effects on the precision of the positioning device due to limited bearing stiffness. The high attraction forces between the sheet metal stack and the magnets can cause wear of the linear guides and the (moving) mass of the iron core can limit the dynamic performance of the positioner.

Ironless linear motor (PI-type V-115)
Ironless linear motor (PI-type V-115)

Ironless motors are the top choice in precision motion control applications. Here, the two-sided magnetic yoke usually is the fixed part and the lightweight, moving forcer typically consists of air coils which significantly reduce moving mass / inertia compared to moving iron cores. While ironless motors provide lower forces compared to iron core motors and are more expensive due to twice the amount of magnetic material used, they have advantages in dynamics and accuracy. There are no attraction forces between motor and magnets and no cogging (variation in forces along the stroke).

Linear Motor Stages work well as range extenders for compact 6-axis positioners.
Linear Motor Stages work well as range extenders for compact 6-axis positioners.

The V-551 linear motor stage uses ironless motors, but instead of moving the coils, the magnetic track is attached to the platform. The reason for this design is that typically, with linear motor stages, the parts with the highest wear are moving cables, limiting the lifetime of such a device.

As with the motor, also the for the absolute position encoder, the scale is moving instead of the sensor head, eliminating all moving cables from the design. Now lifetime is only limited by the linear crossed-roller bearings. The V-115 integrated linear motor integrated is “oversized” running in a low temperature window even under dynamic applications.

Voice Coil Linear Motors use only one phase, they are more simple than 3-phase motors, and work well for displacements, typically in the 1” range and below.
Voice Coil Linear Motors use only one phase, they are more simple than 3-phase motors, and work well for displacements, typically in the 1” range and below.

Absolute Encoder vs. Incremental Encoders

The basic version is equipped with an absolute encoder which provides advantages especially in automation applications: No more reference moves required and no more phase finding at power up. Absolute encoders have been improved continuously over the last decade in terms of resolution and cost, however, when it comes to highest possible resolution, incremental encoders still have the edge.

For sub-nanometer resolution performance, PI uses an in-house designed incremental encoder (PIOne).

10 nanometer steps performed repeatedly by a V-551.4B linear motor stage, operated with the C-891 motion controller. This stage is equipped with an absolute measuring encoder (no referencing required).
10 nanometer steps performed repeatedly by a V-551.4B linear motor stage, operated with the C-891 motion controller. This stage is equipped with an absolute measuring encoder (no referencing required).

 

2 nanometer steps performed repeatedly by a V-551.4B stage with absolute encoder (BISS), driven by the C-891 motion controller, measured with Zygo ZMI interferometer. Steps down to 1 nanometer can be resolved with the absolute encoder. For even smaller steps, the PIOne incremental encoder is available.
2 nanometer steps performed repeatedly by a V-551.4B stage with absolute encoder (BISS), driven by the C-891 motion controller, measured with Zygo ZMI interferometer. Steps down to 1 nanometer can be resolved with the absolute encoder. For even smaller steps, the PIOne incremental encoder is available.

Mechanical Precision

When it comes to nanopositioning applications, smallest details in the design and assembly of the positioning devices matter. These in include, stress relieved materials, precision machining of essential surfaces / mounting interfaces after the final heat treatment (i.e. anodizing process), assembly and alignment of all mechanical components using state-of-the-art metrology equipment and last but not least, performance testing in a metrology lab with designed to provide the stability and accuracy required in the nano-world.

Dynamic straightness measurement at a velocity of 60mm/sec. The crosstalk is significantly below 1μm per 100mm travel. The results for flatness looks similar. Measurement parameters: 10kHz data sample rate
Dynamic straightness measurement at a velocity of 60mm/sec. The crosstalk is significantly below 1μm per 100mm travel. The results for flatness looks similar. Measurement parameters: 10kHz data sample rate

 

Tracking error (position error) at a velocity of 1 mm/sec. The position error is significantly below 1μm. This is an amazing performance. Measurement parameters: 20kHz data sample rate
Tracking error (position error) at a velocity of 1 mm/sec. The position error is significantly below 1μm. This is an amazing performance. Measurement parameters: 20kHz data sample rate

 

Tracking error (position error) at a velocity of 1 mm/sec with a stage employing the PIOne incremental encoder. The position error is significantly below 0.1μm. Measurement parameters: 20 kHz data sample rate
Tracking error (position error) at a velocity of 1 mm/sec with a stage employing the PIOne incremental encoder. The position error is significantly below 0.1μm. Measurement parameters: 20 kHz data sample rate

 

Noise level of a PI linear motor stage employing the PIOne incremental encoder in closed-loop. The position stability is in the single nanometer range. Measurement parameters: 20 kHz data sample rate
Noise level of a PI linear motor stage employing the PIOne incremental encoder in closed-loop. The position stability is in the single nanometer range. Measurement parameters: 20 kHz data sample rate

 

Application overview of different electro-magnetic linear motors and direct drives.
WATCH: Application overview of different electro-magnetic linear motors and direct drives >

 
 

Related blog posts:

> Air Bearing Stages

> LEARN more: Linear Motor Product Overview

> WATCH Videos

> READ more

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About PI

PI (Physik Instrumente) is a leading manufacturer of precision motion control equipment, piezo motors, air bearing stages and hexapod parallel-kinematics for semiconductor applications, photonics, bio-nano-technology and medical engineering. PI has been developing and manufacturing standard & custom precision products with piezoceramic and electromagnetic drives for 4 decades. The company has been ISO 9001 certified since 1994 and provides innovative, high-quality solutions for OEM and research. PI is present worldwide with fifteen subsidiaries, R&D / engineering on 3 continents and total staff of more than 1,000.

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