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Direct drive linear actuators

Direct drive linear actuators Russell Cleasby of Dunkermotoren Linear Systems UK discusses tubular motor design, focusing on installation ease, flexibility and performance.

Modern linear motors provide better than 50g peak acceleration and 10m/sec velocity, deliver unmatched dynamic agility, minimise maintenance, and multiply uptime. With ten times the speed and ten times the operating life of ball screws, linear direct drive technology is often the most effective solution for modern productivity enhancing automation.

There are three basic linear motor configurations: flat bed, U-channel, and tubular. Each has its own intrinsic benefits and limitations. Drawbacks specific to one motor type can often be sidestepped by using either of the two alternatives. Flat bed motors, while offering unlimited travel and highest drive force, exert considerable and undesirable magnetic attraction between the load carrying forcer and the motor's permanent magnet track. This attraction force requires bearings that support the extra load. The U-Channel motor, with its ironless core, has low inertia and hence maximum agility. However, the forcer's load carrying magnetic coils travel deep within the U-Channel frame, restricting heat removal.

Tubular linear motors are rugged, thermally efficient, the simplest to install and provide drop in replacements for ball screw and pneumatic positioners. The tubular motor's permanent magnets are encased in a stainless steel tube (thrust rod), which is supported at both ends. Without additional thrust rod support, load travel is limited to 2-3m depending on thrust rod diameter. They can be transformed into powerful and very versatile direct drive linear actuators. In an actuator incarnation, the forcer remains stationary (bolted to machine frame), while the load positioning thrust rod travels on low friction lubrication free bearings mounted within the forcer. Not only does the linear actuator provide major performance advantages over ball screws and belt drives, it also creates a potent alternative to programmable servo-pneumatic positioning systems.

Of the three motor types, tubular motors are best equipped for mainstream industrial usage. They have profound benefits as a result of a novel and patented magnetic circuit innovation that enables Hall effect sensors to achieve almost tenfold improvement in resolution and repeatability. Thanks to this critical magnetic circuit invention, tubular linear motors replace the traditional external linear encoder with integral Hall sensors. The result is repeatability from 8-20µm.

Eliminating the external encoder brings a number of benefits. Linear encoders can cost almost as much as the motor itself, so an immediate benefit of encoder elimination is cost reduction. Also, dispensing with the external systems means there are no fiddly encoders to support and align. Other benefits include ruggedness, dependability, and freedom from an encoder's need for protected environments.

Tubular linear motors are built with a large air gap between forcer and thrust rod. This clearance, besides preventing wear, simplifies motor integration into industrial equipment. Motor loads mount directly to Forcer T-slots without added brackets. The linear motor load-carrying forcer travels on long-life single rail bearings. This intrinsic simplicity enables the actuator to deliver 10 million dependable operating cycles. Actuator bearings are self-aligning, which adds to installation ease and the actuator drive force is applied directly to the thrust rod, ensuring unmatched acceleration and responsiveness.

Inherently robust design
With traditional external encoder replaced by a solid state sensor integrated into the forcer, direct drive motors and actuators become very simple two-component devices. The forcer and thrust rod are both inherently very robust components, which enables motor and actuator to conform to IP67 ratings. In addition, conventional positioning mechanisms involve fast-spinning lead screws and gear trains, which can produce oil spray and abraded particles. This potential for contamination can prevent their use in the food industry. The tubular linear actuator, by contrast, runs on a lubrication-free slide bearing and qualifies for food safe and other contamination-sensitive uses.

Absence of grinding gears and whirring lead screw gives linear motors and actuators an increasingly vital qualification: low noise operation. OSHA is following close on the heels of European industrial codes, which place increasingly stringent rules on workplace noise. Quiet operation is already critical in laboratory and hospital environments; this concern will become increasing widespread as OSHA extends its ruling to other production environments.

The forcer of tubular motors and actuators is surrounded on all sides with free air and is inherently self cooling. The forcer is also equipped with cooling fins that further facilitate heat removal. Rarely do either motor or actuator applications require additional cooling. Vertical operation is not a barrier in linear actuator applications, either, as an electromechanical brake option will hold load position when drive power is removed.

Dynamic performance of conventional positioning mechanisms is limited by leads screws, gear trains, belt drives, and flexible couplings, which produce hysteresis, backlash and wear. Similarly, pneumatic actuators suffer from piston mass and piston-cylinder friction, as well as air compressibility, which lead to servo control complexity. Linear motors and actuators shed the mass and inertia of the conventional positioners, so are freed from these fundamental limitations, providing unequalled dynamic stiffness.

Direct creation of drive force enables linear motors and actuators to achieve closed loop bandwidths unavailable with alternative positioning mechanisms. Absence of cascaded mechanical linkages obviates positioning uncertainty and mechanical resonances. Motor and actuator are able to take full advantage of modern controller performance. The controller is tuned for high loop gain operation, achieving wide bandwidth control, fast settling, and rapid recovery from transient disturbances.

Linear motors and actuators excel in making millimetre distance moves that operate in the static friction zone. Their low mass and minimal static friction minimise the drive force necessary to start travel, and simplify the control system's task in preventing overshoot when stopping. These attributes enable direct drive motors and actuators to scan microscope slides, for instance, and chart the X-Y locations of artefacts only millimetres apart.

Tubular linear motors and actuators are now cost competitive with ball screws and belt drives and offer distinctly superior agility and bandwidth. They provide cost competitive drop-in ball screw replacements for bandwidth upgrade, or to exploit direct drive simplicity and MTBF advantage. Direct linear drives will increasingly replace servo-controlled pneumatic cylinders, contributing reliability and controllability, free from the cost, noise, and upkeep of air compressors.
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