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Direct Drive Linear Motors

Author : M Babb

05 September 2009

This distinct motion technology eliminates all rotary-to-linear conversion devices between motor and load—ball screws, gear boxes, rack-and-pinions, and belts—to obtain high-dynamic performance in a growing number of applications. By Frank J. Bartos, P.E., Control Engineering

Hegla GmbH's glass cutting machine uses Bosch Rexroth's IndraDyn L linear motors
Hegla GmbH's glass cutting machine uses Bosch Rexroth's IndraDyn L linear motors

Compared to conventional linear motion systems, direct-drive linear (DDL) motors deliver superior speed, acceleration, load-positioning accuracy, rapid cycling, and more. However, DDL motion systems have higher initial cost and require tighter design integration with a specific machine or overall system.

The core of a typical DDL system is the linear motor. In simplest terms a linear motor consists of a primary and a secondary element. Respectively, these correspond to the rotor and stator of a rotary motor, with the stator rolled flat. The primary (or slider), moves linearly relative to the secondary, separated by an air gap. Several linear motor design variants exist.

It started with machine tools
Direct-drive linear (DDL) motion technology had its start in the machine tool industries—in applications such as high-quality machining, honing, grinding, punching, and laser cutting. Linear motor systems then expanded into gantries/material handling, flying cut-off equipment, metal forming, assembly shuttles/conveyors, and food-processing machinery.

‘Initially, mostly high-end machines contained linear motors but today more and more mainstream and ‘economy machines’ take advantage of DDL motor benefits,’ says Karl Rapp, at Bosch Rexroth.

He regards DDL motor systems to be a mature technology, yet one that continues to make incremental advancements. The company cites examples, such as improved coolant jacket designs that minimise temperature differential to less than 2 K between stator and machine bed, optimised magnet shapes to reduce force ripple and material cost, and magnet tracks protected by stainless steel covers supplied pre-assembled in various lengths, to ease system installation and operation.

‘Direct-drive linear motor systems combine high speed and acceleration with high accuracy in a quite simple machine construction,’ says Mr. Rapp. ‘This enables machine builders to develop new designs that were hard or impossible to perform before.’

For example, in an industrial glass-cutting application, DDL linear motion improved production time by around 20% compared to a conventional servo motor and rack system. The Galactic plate-glass cutting machine from Hegla (see photo) achieves up to 3.5 m/s x-y axis speed and 12 m/s2 acceleration using Bosch Rexroth IndraDyn L linear motors, while holding tolerances of around ±0.05 mm over a 20 m2 glass area. This compares to only ±0.2 mm accuracy with the previous motion system.

Another example of process improvement due to DDL motion is in orbital grinding of crankshaft pins. Previously, each pin had to be centered and ground individually, but in the new process, the crankshaft rotates around its center, explains Mr. Rapp.

The x-axis linear motor follows the pin using its high dynamic and static stiffness and maintains roundness accuracy of below 1 µm. Moreover, two grinding wheels can easily work on two pins simultaneously. The latest success of this process is grinding f huge crankshafts with very large throw—dedicated for ship propulsion.

Quick and simple
Today’s intelligent digital drives combine with DDL motors to achieve very fast control-loop closures in the drive, rather than in the controller. Functions in IndraDrive, such as force/torque ripple compensation, nanometre-level interpolation, and ultra-high resolution sinusoidal feedback, are said to result in superior dynamic/static stiffness and motion accuracy.

‘Additionally, automatic commutation functions can eliminate the need for absolute feedback or Hall-effect sensor boxes, although they’re supported as well,’ states Mr. Rapp.

Yaskawa Electric emphasises the importance of quick and simple DDL motion system setup. Advanced features in its new Sigma-5 Series servo amplifiers make that possible, explains Paul Zajac, product engineer at Yaskawa. These features include a tuning-less function, said to provide ‘instant performance,’ and an advanced autotuning function for higher performance.

Another feature is enhanced vibration suppression, which reduces machine resonance and settling time. It also eliminates vibration due to machine resonance and load disturbances.

‘This function automatically detects and suppresses oscillation frequencies under 1 kHz. A notch filter is also available to control frequencies of 1 kHz and above,’ he says.

Parker Hannifin connects increased performance and reduced motion errors in DDL motor systems to the power of today’s controllers and drives. ‘Controllers increase performance because they can pre-emptively solve servo errors associated with the first and second derivatives of speed (acceleration and jerk, respectively), which are typically caused by friction, stiction, and other repeatable disturbances,’ states Jay W. Schultz, product manager at Parker.

Siemens' 1FN6 Series brushless synchronous linear motors
Siemens' 1FN6 Series brushless synchronous linear motors

‘Error reduction and performance benefits are especially noticed in DDL systems because the load is coupled directly to the motor and thus to the feedback device.’

No magnets
At Siemens, expansion of direct-drive linear motion technology is seen to have reached well beyond machine tool applications. Jeff Gerlach, a Siemens consultant, summarises the newer usage areas as packaging and automated sorting machines, as well as high-speed conveyor systems.

DDL motion can be especially cost-effective for conveyors due to much higher speeds developed than with belt- or roller-type arrangements. The company is pursuing these applications with its latest-generation brushless PM synchronous linear motor, the self-cooled 1FN6 Series.

What is unusual about the 1FN6 is that it has a magnet-free secondary section track, which makes it very inexpensive, especially in the case of long traversing distances. It is positioned as an alternative to classic drive solutions with mechanical transmission elements such as gear racks or ball screws, as well as an alternative to other types of motor such as asynchronous linear or reluctance motors.

In many applications in the machine tool area, long traversing distances have to be travelled quickly and precisely. Due to the costs of permanent magnets or in applications where it is difficult to protect the secondary section track against dirt, previous linear motors were used to a limited extent. Applications with very large air gaps can also be implemented with the 1FN6.

In the case of machine tools, the 1FN6 is especially suitable for applications where water-jet or laser-beam cutting is used. As far as the traversing distance is concerned, any number of secondary sections can be mounted next to each other. In addition, several primary sections can be operated on a secondary section track.

The first delivery stage of the 1FN6 linear motor is designed as a self-cooling version for the power range with a maximum force of 880 N to 7920 N.

No more ‘black art’
Long known—and famous—for its pneumatic actuators, of which it builds 8 million units every year, Festo has decided to launch its own brand of high performance electric actuators, based on linear motors. The first product is the ELGL-LAS linear motor axis which employs a magnetically preloaded air cushion bearing to ensure precise positioning and linearity.

According to Nigel Dawson, Festo's Drives Manager in the U.K., his company’s entry ‘will hopefully cause people to re-evaluate their opinions about this type of technology.

‘Until now, the use of linear motors for highly dynamic positioning applications, especially those requiring accurate repeatability, has proved something of a black art. We intend to change this, so that system designers eventually come to regard our linear motors as just another actuation technology choice. To bring this about, we effectively had to reinvent the linear motor, to come up with a tubular design that can be integrated in actuators with the same form factors as our pneumatic products (see box, Tubular Motors).

But, back to the ELGL-LAS. The motor is a conventional linear synchronous design; the stator takes the form of a fixed bed, and the windings in the moving carriage are fed with drive signals from the motor controller. The carriage features an integrated displacement transducer to provide digital position feedback to the controller, and up to three carriages can be used on the same bed and controlled independently.

Here is what is really novel about the Festo motor: The carriage contains a series of embedded permanent magnets to provide an integrated locking brake function The carriage is only free to move when the air cushion overcomes the magnets' attraction to the fixed bed. To provide the air bearing, compressed air must be fed into the motor to maintain a 4 bar pressure.

The use of an air cushion bearing minimises friction in the guide components, making lubrication and routine maintenance unnecessary. If either the compressed air or the electricity is shut off to the motor, it immediately locks it into position so that it cannot more. This is especially important for vertical applications, where holding a part in a vertical position may be critical for safety. It is a built-in fail-safe brake.

There is a choice of six ELGL-LAS linear motor axes; one model has a standard stroke length of 1 metre, while the other five models provide 2 metre stroke lengths and offer a variety of different acceleration, speed and thrust ratings. The most powerful 2-metre axis can handle acceleration rates of up to 50 metres/second/second and speeds as high as 4 metres/second, and can produce a continuous force of 349 N, with a peak rating of 463 N. All six models have a very high repetition accuracy of ±0.01 mm. The 2-metre models are available ex-stock; Festo will also produce models with stroke lengths up to 6 metres, to special order.

Motor types (see also "Linear Motor Types and Terminology" at the end of this article)

DDL motors fall into two main categories: brushless PM (permanent magnet) synchronous and LIM (linear induction motor). The first type dominates, mostly because they have higher force density and efficiency and faster response due to the existing magnetic field. They also allow less heat loss transferred into the machine structure and are generally lower in stack height.

On the market for five years, Bosch-Rexroth's IndraDyn L synchronous linear motor
On the market for five years, Bosch-Rexroth's IndraDyn L synchronous linear motor

Siemens’ first linear motors were LIM type, but the technology has migrated to brushless PM synchronous motors. The older asynchronous LIM had 30% of the force density of our new 1FN6 Series,’ says Mr. Gerlach.

LIMs have market appeal in transportation, warehousing, and some entertainment applications, but most general automation vendors offer only brushless PM synchronous linear motor systems.

Vital feedback devices
Until recently, glass-scale optical encoders were the only feedback devices suitable for DDL motion systems. Because of this—a combination of encoder cost and sensitivity—DDL motion systems were limited to clean environment electronics and semiconductor applications. (An exception is in machine tools where optical feedback, including metal scale, is successfully applied.)

Advancements such as low-cost magnetic technology now allow automation control suppliers to apply magnetic encoders, and this brings two main benefits: First, they’re more rugged for industrial usage because they are less sensitive to dust, dirt, and debris than glass-scale encoders. Secondly, they’ve helped lower DDL motion-system cost.

Newer magnetic encoder devices can be integrated for one tenth the cost of glass-scale encoders. The optical encoders are more precise, but for some industries the magnetic encoder resolutions of 1 and 5 microns are adequate.

Yaskawa’s linear motor systems are compatible with absolute linear encoders, specifically the electromagnetic induction type that is resistant to oil and water contamination. They are available in 0.1 or 0.5 micron resolution and lengths up to 6 m. The non-contact design is optimal for high-speed, high-acceleration linear motor applications. Using an absolute encoder eliminates the need for homing as well as for magnetic pole detection.

Total cost of ownership
The initial cost of direct-drive linear motion systems (the cost is mainly in the motor) can be higher than the alternative. However, ‘many OEMs admit that once linear motor technology is embraced and properly designed into a machine, cost of the total machine is lower,’ says Bosch Rexroth’s Mr. Rapp. It’s due to simpler construction, reduced assembly and startup time, and better machine performance for product quality.

Siemens’ Mr. Gerlach cites DDL motion systems’ low parts count in contrast to ‘all the parts that can fail in a servo motor/ball-screw system.’

He adds, ‘Using ball screws also involves more steps, like mapping torque and pitch variations into the control calculations. With a direct-drive system, potential component failures are limited to the linear motor’s primary section and feedback device.’

Parker Hannifin connects DDL motion systems’ cost-effectiveness to complementary benefits of precision, speed, and flexibility. Customers trading precision for speed when using belt-and-pulley transmissions, can now switch to DDL motors and increase precision by an order of magnitude without sacrificing speed, explains Mr. Schultz.

Also, it’s possible to run multiple motor coils on the same secondary—the most costly part of a system with a magnet track—because each coil is physically decoupled from the force transmission. ‘This increases flexibility, reduces cost per axis, and can result in more throughput and space savings in many applications,’ he says.

Market status
While a comparatively smaller market than general motion control, industry marketers estimate DDL technology currently is at $725 million worldwide, and growing annually at 10-12%—about twice the growth rate of general motion control solutions.

But the growth is not uniform: it may be 15% in some areas, while in others, such as semiconductor fabrication, it is in decline.

Helping the increased adoption of DDL are developments like magnetic encoder feedback and less costly magnet materials that have significantly lowered overall cost.

Some say that traditional rotary servo technology is reaching its limits, which is forcing machine builders to look for alternative technologies to boost performance to the next level.

Siemens notes more inquiries daily for DDL motion systems, particularly from custom-machinery builders. ‘We also see increased interest in converting linear (and rotary) axes to direct-drive motors on existing machine designs as OEMs update their product lines,’ Mr. Gerlach says.

Details of the THK actuator GLM20 with its integrated linear motor
Details of the THK actuator GLM20 with its integrated linear motor

The next logical step is to replace the servo motor, ball screw, and associated components with a linear motor primary and sufficient secondary sections to cover the traverse length.

Yaskawa’s Mr. Zajac says he has seen a ten-fold increase in DDL motion sales, specifically to semiconductor and solar-panel industries.

Parker claims its linear motors are the easiest to integrate into applications, largely due to their ‘generous air gap’ design. Mr. Schultz emphasises the point by reportedly having assembled a DDL motor system using everyday tools, even though he is ‘only a marketing guy.’
Direct-drive linear motion systems have reached maturity, but have not lost the ability to innovate.

lINEAR MOTOR TYPES AND TERMINOLOGY

the permanent magnet (PM) brushless synchronous motor dominates today’s direct-drive linear (DDL) motion systems. this linear motor type has several variations and subclasses.

Moving magnet type contains permanent magnets in the primary and coils in the secondary. This allow for simpler design with stationary power cabling and easier integration of the feedback device. Moving coil reverses the linear motor structure. Motor coils are in the primary section and the secondary has the magnet track. It means that more permanent magnets are required (especially for long traverse) and the need for power cables to move with the primary and feedback integration tend to be more complex.

Ironcore refers to adding steel laminations to the magnet track for increased flux to develop higher thrust forces per frame size. Further, single magnet track and dual magnet track variations exist. The latter has the advantages of balancing the high magnetic forces developed between the primary coil and the magnet track.

Ironless refers to a primary containing only copper coils (and epoxy encapsulation). Smooth "cog-free" motion is produced since no attractive force exists between coil and magnet—but at the cost of lower force output

Slotless refers to a special design of steel laminations where the windings go through holes in the stator rather than slots. The result is a smoother surface facing the magnets. This design also reduces cogging by eliminating variation in attractive force.

Tubular linear motors roll up the flat linear structure about an axis parallel to its length. In one style, an outer thrust block carrying the motor coils envelops and moves along a stationary thrust rod that houses magnets. Another style incorporates magnets in a central rod that moves relative to an outer stator member. Travel is limited since the thrust rod must be supported at both ends—or at one end for the moving-rod version.

lINEAR MOTOR BASED ACTUATORS

Known primarily for its linear motion (LM) and caged ball LM guides, THK also has developed four models of linear-motor powered actuators.

Most applications, it says, can be accommodated with either the GLM and KLM series units, but there are also several coreless models for special needs such as higher precision, reduction in low speed ripple or very limited space.

The universal GLM is based on an aluminium profile with a compact rectangular section and is available in four sizes. The thrust forces of this unit range from 30 to 3100 N, and the maximum stroke available in standard configuration is 4000 mm. Multiple sliders can be incorporated and controlled either independently or synchronised with high positioning precision on a single axis.

The design of the KLM type integrates the motor system and linear motion guides with a highly rigid steel profile, similar to the ballscrew driven THK actuators KR and SKR. This design offers a combination of an extremely compact design and with high rigidity.

The maximum travel speed of these linear motor actuators is specified as 5 m/s and the highest permitted acceleration equates to 10 g. Optical linear encoders are incorporated as standard to provide a positioning repeatability of up to ± 0.3 µm. The highest thrust force of more than 3000 N is available with the GLM series product due to its wide cross sectional design.


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