Linear motors as a viable alternative to conventional technologies

Paul Boughton

In any application for linear motion there are design considerations relating to speed, acceleration, positioning accuracy, size, complexity, maintenance requirements and, of course, cost. Selecting the right technology for the task is often far from simple. However, newer breeds of linear motors and actuators mean that there are often products available that meet more of the application requirements, with fewer compromises necessary.

The arguments for and against different technologies have often seen engineers moving away from hydraulic or pneumatic actuators in favour of electromechanical alternatives. The days when electromechanical options could not compete with hydraulic actuators in terms of force, or pneumatic actuators in terms of speed, are largely behind us. Furthermore, suppliers of electromechanical products argue that their offerings are simpler to implement and easier to control, with none of the mess or fuss associated with fluid power.

But what of the options and variations within the field of electromechanical linear motion? Engineers are generally faced with a choice of screw drives, belt drives or linear motors. Again, traditionally there have been compromises and tradeoffs to consider in any given application. Conventionally, engineers have turned to toothed belt drives for fast-moving, medium loads, screw drives for lower speed, higher-precision positioning of medium to high loads, and linear motors for high-speed, highly dynamic positioning. Belt and screw drives have traditionally offered the advantages of inherent simplicity, ease of implementation and generally lower cost, but with compromises in terms of speed, acceleration and positioning accuracy. Speeds of around 4m/s are typical, with millimetre precision for belt drives and sub-millimetre precision for screw drives. In contrast, linear motors have offered extremely high speeds and accelerations, together with sub-micron accuracies, but have been unable to generate or sustain high forces.

Redefining linear motion expectations

The latest innovations, however, are redefining expectations for linear motion. While screw drives and belt drives rely on converting the output of a rotary motor into linear motion, linear motors offer direct linear actuation. The linear motor, in essence, is a conventional rotary electric motor unwrapped. Linear motors are often viewed as relatively new technology, but their history can be traced back to at least the 1840s. Given this lengthy history, it seems surprising that linear motors enjoyed minimal commercial success until the late 1980s and early 1990s - and, even then, they were largely restricted to niche applications. One problem was the difficulty in controlling these motors, as this required dedicated hardware.

Today's market for linear motors is due, in part, to the success of the tubular linear motors that can be traced to British engineer Hugh-Peter Kelly, who, in the 1980s, designed a tubular motor with permanent magnets enclosed in a sealed tube. This concept was brought to market by Linear Drives (now Copley Motion Systems). The patented permanent magnet arrangement induces a sinusoidal response in the coils that are enclosed in a square-profiled body. Importantly, machine builders could control these linear motors using standard sinusoidal servo drives as found in conventional rotary motor-based motion control systems.

Tubular linear motors are more rugged than conventional linear motors, which enables them to be used in industrial environments including food packaging and machine tools. The tubular construction protects the permanent magnets from the external environment and automatically balances attractive forces so that the motor is easier to integrate into machine designs. These motors operate at speeds of 5-9m/s with high rates of acceleration for dynamic motion control.

Copley Motion Systems took tubular motor design a further stage in 2005 with the Servotube, which integrated the position sensing electronics within the motor body. Development of the Servotube has continued, with the current generation of motors combining compact dimensions with high speed and accelerations of up to 25 'g.' The company says that its STB11 motors, for example, can provide ten times the speed and ten times the life of ball screw actuators, making them suitable for boosting productivity in high-speed point-to-point positioning applications.

Most recently, the company has introduced a stainless steel version of the Servotube, aimed at hygienic applications requiring frequent wash down. Electrically, the Servotube Hygienic operates from a three-phase 600V supply and incorporates a digital incremental encoder and commutation output for use with most standard digital servo drives. It is available in two models that both offer +/-400 micron position accuracy and 25micron repeatability. The XHB3804 and XHB3810 provide 744 and 1860N peak force capability and are capable of speeds of 4.7 and 2.6m/s, respectively. Both models can be water-cooled, providing up to 70 per cent extra continuous force for applications requiring high work rates or those with high ambient temperatures.

Alternative to pneumatics

Further innovations in tubular linear motors have come from Festo, which is rapidly establishing a broad range of electrical and electromechanical motion options to complement its better known pneumatics products. With a heavily patented technology and a form factor that offers a drop-in alternative to conventional pneumatic actuators, Festo believes its DNCE-LAS electric cylinders are attractive to design engineers seeking a performance advantage.

The DNCE-LAS electric cylinders offer fully programmable, closed-loop position, acceleration and speed control, and are suitable for applications that demand extremely fast and accurate positioning capabilities. The patented linear motors are based on an innovative tubular design, using high-flux annular magnets on the actuator rod, surrounded by a series of specialised windings on a long stator coil. This design is claimed to offer a number of significant advantages, including a low moving mass and no external magnetic field, thereby making the cylinders suitable for use in environments subject to swarf - such as in machine tools.

There is currently a choice of three 32mm profile models with stroke lengths of 100, 200 and 320mm, and four 40mm profile models with stroke lengths of 100, 200, 320 and 400mm. The cylinders offer a variety of acceleration, speed and thrust ratings, with the largest model accelerating at up to 90m/s2, achieving a maximum velocity of 3m/s, and being capable of producing a thrust of 202N peak and 55N continuous. All seven models provide a positioning repeatability to within +/-0.02 mm. Pressurised air cooling and pneumatic clamping options are available.

A further innovation is what Festo describes as an entirely new class of electric actuators employing a magnetically preloaded air cushion bearing to ensure very precise positioning and excellent linearity. The ELGL-LAS linear motor uses a conventional linear synchronous design in which 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. Up to three carriages can be used on the same bed simultaneously and controlled independently.

In addition, 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. Furthermore, the use of an air cushion bearing minimises friction in the guide components, making lubrication and routine maintenance unnecessary.

There are currently six models in the ELGL-LAS linear motor range; one has a standard stroke length of 1 metre, while the other five models provide double the stroke length and offer a variety of different acceleration, speed and thrust ratings. The most powerful model can handle acceleration rates of up to 50m/s2 and speeds as high as 4m/s, and can produce a continuous thrust of 349N, with a peak of 463N. All six models have a repeatability of +/-0.01 mm.

Short strokes

For short stroke lengths, however, there are two additional technologies to consider: piezo actuators and voice coil actuators. Piezo actuators rely on the piezoelectric effect in which a crystal changes its dimensions when subjected to a voltage. Even very high voltages correspond to only tiny expansions, so the technology is only suitable where short strokes are required, but the actuators can achieve extremely fine positioning accuracies.

Typical of the innovations in this area is the Nexact range from PI (Physik Instrumente), which the company says replaces classical lead screw actuators thanks to their combination of high forces, long travels and sub-nanometre resolution - all in a compact package. Note, however, that for PI a 'long' stroke is one of around 30 mm - although that is still far in advance of what is achieved by traditional piezo actuators. Other features of these actuators include very high accelerations, resolutions measured in picometres and thrusts of up to 10N. Where applications require it, PI can also supply non-magnetic and vacuum-compatible versions of these actuators.

In contrast to actuators based on dc servo or stepper motors, the Nexact units use 'piezo walk' drive technology to provide linear motion. In operation, piezoceramic bending elements act on a ceramic runner, which is connected to the moving limb of the actuator. For long-distance motion, the stepping mode is used. Typical applications for the actuator include micromanipulation and nanomanipulation, nanotechnology, microscopy and optics, nano-imprint, laser tuning, semiconductor test and production equipment, bio-technology, and medical technology.

A further option for design engineers is moving coil linear actuators, in which the principle of operation is not dissimilar to the motion of a permanent magnet loudspeaker: the voice coil sits in a strong magnetic field and, by passing a current through the coil, an axial force is generated. Although stroke is limited to around 100mm, the technology offers very high speeds and extremely precise positioning.

Innovations in moving coil linear actuators have come recently from SMAC and BEI Kimco. For example, SMAC's CAL36 series actuators have a 36 mm diameter body and stroke lengths of 15, 25 and 50mm. Basing the design on a round-centred coil and guide results in no internal moment, and low internal friction enables very light forces to be applied to the workpiece. Due to the innovative design and construction, the units can achieve up to 50 'g' acceleration and can offer a long life expectancy due to over-guiding and internal permanent lubrication.

The CAL36 series of electric cylinders also benefits from independent control of the position, speed and force. Further to this, SMAC's patented 'softlanding' capability is incorporated to enable the actuator to land on parts gently and then apply the required force - which can be useful when handling fragile components and materials. The actuator also has the ability to do work and provide feedback simultaneously.

Most recently, SMAC introduced a vacuum-through-shaft version of its linear actuator that the company says eliminates dust problems that are common in pick-and-place applications. As a result, the new LAR31 series is claimed to boost reliability significantly in areas such as assembly of printed circuit boards.

Small but powerful

BEI Kimco, meanwhile, recently unveiled the small but powerful LA100-90-001 cylindrical voice coil linear actuator. With a stroke of 50 mm, this offers a continuous stall force of 1.29kN and a peak force rating of 2.45kN. It is described as a two-terminal, hysteresis-free, cog-free, direct-drive linear servo with infinite position sensitivity and a linear force constant characteristic that simplifies control. Unloaded acceleration capability is 12.8 'g' continuous and 24 'g' for ten seconds. It also features a 71mm central bore for linear bearing installation, forced ventilation or other requirements such as cable runs, optical paths or sensor installation.

In some cases the engineer will wish to specify a linear motor that can be incorporated within an overall design, but sometimes it is preferable to specify an all-in-one unit that includes guidance bearings and other functions, as this avoids the need to specify the components separately then run the risk of incompatibilities. In late 2009 Yaskawa launched a linear actuator that consists of a linear motor, profile rails, linear guides, sliders, encoder and plug-in power electronics. Furthermore, this unit is said to be maintenance-free and, thanks to the application of a special surface coating, it may be cleaned using aggressive detergents and pressure washers. Standard versions are available in four installation sizes with instantaneous peak thrusts of up to 5400N and stroke lengths of up to 6m. If required, Yaskawa can also supply customised linear actuators, gantries and multi-axis systems.


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