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Linear Positioning Technology for Tomorrow’s Automation...Today
Introduction
Driven by semiconductor industry requirements, linear motor manufacturers have steadily increased precision, reduced prices, developed multiple motor types and simplified integration into automation equipment. Modern linear motors provide 20-g peak acceleration and 10-meters/second velocity, deliver unmatched dynamic agility, minimize maintenance and multiply uptime. They have moved beyond specialized semiconductor industry usage to provide advanced performance in a host of applications. In fact, with ten times the speed and ten times the operating life of ballscrews, linear direct drive technology is often the only solution for modern productivity-enhancing automation.
Three Little Motors
Different manufacturing groups have specialized in one or another of three basic linear motor configurations—flat bed, U-channel and tubular. Each motor has its intrinsic benefits and limitations, but 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 the 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.
With its ironless core, the U-channel motor has low inertia and thus maximum agility. However, the Forcer’s load carrying magnetic coils travel deep within the U-Channel frame, restricting heat removal.
The simplest to install, tubular linear motors are rugged and thermally efficient. Furthermore, they provide drop-in replacements for ballscrew 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 Trust Rod support, load travel is limited to 2 to 3 meters, depending on Thrust Rod diameter.
About Tubular Linear Motors
Tubular linear motors have benefited from a fundamental engineering innovation. Copley’s linear motors replace the traditional external linear encoder with integral Hall sensors. Eliminating the external encoder delivers benefits to laboratories on many fronts. For instance, linear encoders can cost almost as much as the linear motor itself, so an immediate advantage of encoder elimination is a major cost reduction.
Dispensing with the external encoder also simplifies linear motor integration into automation systems: there’s no finicky encoder to support and align. Other advantages include ruggedness, dependability and freedom from an encoder’s need for protected environments.
Additionally important, linear motors comprise two basic parts and may be configured as complete positioning stages to provide drop-in upgrades for ballscrew mechanisms. With roughly ten times the speed and a tenfold increase in dependable operating life, the linear stage offers a superior productivity gain over the ballscrew mechanism.
Offering easy integration, tubular linear motors are built with a large air gap between the Forcer and the Thrust Rod. This clearance, besides preventing wear, simplifies motor integration. Replacing the traditional linear encoder with integral position sensors brings further installation ease by eliminating alignment hassles. Motor loads mount directly to Forcer T-slots without added brackets.
Furthermore, these motors lend themselves to productivity-doubling applications with two independent Forcers operating on a single Thrust Rod. Each Forcer has its own servo drive and can travel fully independent of the other. For example, one Forcer can load while the other unloads. The technique can double throughput by lifting items two at a time.
Similarly, multiple Forcers operating on a single Thrust Rod can double, triple or even quadruple drive force. The Forcers can be operated by a single controller.
Linear Actuator Incarnation
Tubular linear motors may be transformed into powerful, versatile direct drive linear actuators. In an actuator incarnation, the Forcer remains stationary (bolted to the machine frame) while the load positioning Thrust Rod travels on low friction, lubrication-free bearings mounted within the Forcer. The linear actuator not only provides performance advantages over ballscrews and belt drives but also creates a potent alternative to programmable servo-pneumatic positioning systems.
Regarding mechanical benefits, the linear actuator’s Thrust Rod glides on long-life, lubrication-free bearings mounted in the Forcer. This intrinsic simplicity enables the actuator to deliver 10 million dependable operating cycles. Actuator bearings are self-aligning, which adds to installation ease. The actuator drive force is applied directly to the Thrust Rod, ensuring unmatched acceleration and responsiveness.
Introducing the STA11 and STB11
Copley has recently introduced the STA11 and STB11, a micro actuator and linear motor, which bring automation to a new level of intricacy. Both provide travel and drive force surpassing piezo electric or voice coil positioners. An assembly deploying ten independently controlled actuators occupies less than 12 inches.
The STB11’s compactness—forcer width measures only 26 mm—provides benefits in applications where space is critical and costly: desktop, benchtop and laboratory applications. With compact form-factor and fast repetitive motion, the STA11 actuators are natural candidates for productivity-enhancing innovations in multi-axis automation.
| The STA11 micro actuator (left) and STB11 linear motor (right) provide travel and drive force surpassing piezo electric or voice coil positioners. |
Less Than Obvious Benefits
Comprising just two parts, direct drive linear motors and actuators are inherently simple, which leads to superior dynamic agility. But there’s more to these benefits than meets the eye.
With the traditional external encoder replaced by a solid state sensor integrated into the Forcer, direct drive motors and actuators become very simple, two-component devices. Forcer and Thrust Rod are both inherently robust components, which enables both the motor and actuator to conform to international IP67 washdown ratings.
Furthermore, the motors and actuators are abrasion-free and foodsafe. Conventional positioning mechanisms involve fast-spinning lead screws and gear train, which can produce oil spray and abraded particles. This potential for contamination can prevent their use in food preparation and other critical applications. The linear actuator runs on lubrication-free slide bearing and qualifies for foodsafe and other contamination-sensitive uses. In some applications, the linear motor may also be operated with lubrication-free bearing to meet the needs of cleanrooms and other critical environments.
Every lab benefits from silent equipment operation, and the absence of grinding gears and whirring lead screws gives linear motors and actuators this vital feature. 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.
Dynamic Superiority
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 produce servo control complexity. Linear motors and actuators shed the mass and inertia of the conventional positioners and, freed from these fundamental limitations, provide unequalled dynamic stiffness.
Linear motors and actuators also excel in making millimeter-distance moves that operate in the static friction zone. Their low mass and minimal static friction minimize 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 artifacts only millimeters apart.
Matched Pair
A linear motor or actuator, matched with an advanced servo drive, makes a powerful plug-and-play positioning sub system. DSP-based, today’s drives permit multi-axis and standalone operation and provide a wide range of application versatility. Operating modes include point-to-point moves, indexing, contouring, velocity and torque control. The drives are available with CANopen, EtherCat, DevNet and MACRO network interfaces.
Highly intuitive software tools speed the system designer’s task. A Java-based drive configuration environment provides auto tuning and makes system setup and commissioning fast and simple.
Conclusion
Linear motors and actuators are now cost competitive with ballscrews and belt drives and offer superior agility. They provide cost competitive drop-in ballscrew replacements for bandwidth upgrade or to exploit direct drive simplicity and MTBF advantage.
New micro motor and actuator models will spark creative automation of tasks not previously feasible. Direct linear drives will increasingly replace servo-controlled pneumatic cylinders, contributing reliability and controllability, free from the cost, noise and upkeep of air compressors.
For more information, contact Jim Woodward, application engineering manager with Copley Controls Corp., at jwoodward@copleycontrols.com or by phone at 781-828-8090 x261.
AT A GLANCE
• The three basic linear motor configurations are flat bed, U-channel and tubular
• Linear motors replace the traditional external linear encoder with integral Hall sensors
• In actuator incarnation, the Forcer remains stationary, bolted to the machine frame
• Comprising just two parts, direct drive linear motors and actuators are inherently simple, which leads to dynamic agility
ONLINE
For additional information on the technologies discussed in this article, see Laboratory Equipment magazine online at www.LaboratoryEquipment.com or the following Web site:
• www.copleycontrols.com
Laboratory Equipment
Advantage Business Media
Rockaway, NJ, 07866
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