DN Staff

December 8, 2010

7 Min Read
Linear Motor Benefits in Systems Design

A majority of automated systems using linearmotors today rely on a rotary servo motor coupled to a ball screw,belt-drive, rack-and-pinion, or even a cam-follower mechanism which convert therotary motion of the motor to its linear translation. Increased demand forhigher throughput, a need for millions of operating cycles, and increasedpositioning accuracy requirements, however, have exposed shortcomings in thesetraditional linear motion systems.

To meet these new demands, direct drive linear motor drivensystems are often considered the best alternative. By virtue of being connecteddirectly to the load, the need for conversion mechanisms is eliminated, therebyclearing the way for increased system performance. The absence of theserotary-to-linear conversion mechanisms also simplifies the overall drive systemand is the main advantage of linear motors over servo motors and conversionmechanisms.

Design Advantages

The advantages of a direct drive linear motor are inherentto its design. A linear motor is essentially a rotary servo motor cut open andunrolled. It consists of two main elements: the forcer and the magnetic way.

Linear Motor Benefits in Systems Design

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The forcer is usually the moving member and is comprised ofcoil windings encapsulated in a thermally conductive resin body. The magneticway is typically stationary and consists of rare earth magnets placed on asteel carrier plate.

The same basic technology used to produce torque in a rotarymotor produces force in a linear motor. Thrust is generated by the interactionof the forcer's electrically energized windings and the magnetic fieldgenerated by the magnetic way. The forcer's direct connection to the payloadincreases system stiffness and eliminates any potential compliance, backlashand friction issues associated with ball screw, rack-and-pinion, belt-drivenand cam-follower systems. This increased stiffness in a linear motor systemallows for an improved level of dynamic response.

A typical ball screw drive system with reasonable designparameters has a maximum dynamic bandwidth of 5 to 15 Hz. This means that anunloaded ball screw can reverse its rotation up to five to 15 times per secondin a controlled manner.

The limited bandwidth is a result of the relatively lowtorsional rigidity of the screw shaft and ball nut, as well as the rotarymotor's inertia combined with the inertia of the screw shaft. Furthermore, thelinear stiffness of a ball screw is not constant and is dependent on theposition of the ball nut, further minimizing controllability in dynamicapplications.

In contrast, a typical linear motor has a bandwidth of 40 Hzor more. This superior dynamic response is largely due to the lack ofconversion mechanisms and the linear nature of the motor's forcer. Theincreased bandwidth allows for faster machine cycle times and increasedthroughput. The linear motor's mechanical time constant is also much smallerthan a ball screw system, contributing to its dynamic response.

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Precision Comparisons

With all linear motors, position feedback is provideddirectly at the forcer, supplying an accurate reading of the position of theload. Unless a secondary linear feedback device is added, rotary motors rely ona rotary encoder coupled directly to the motor. The rotary encoder cannotaccount for any backlash or lost motion in the conversion mechanism, so thetrue position of the load is not known.

Although backlash in ball screws can be minimized bypreloading the ball nut, this adds to friction and wear. Backlash in arack-and-pinion system can be eliminated, but only with costly electronicpreloading. On the other hand, linear motor systems can achieve a high level ofabsolute positioning accuracy and positioning repeatability, limited only bythe resolution and accuracy of the linear encoder.

It is principally for this reason that laser cutting andlaser scribing machines benefit from linear motor characteristics such aspositioning accuracy, smooth travel and virtually limitless travel length. Withball screws, limited travel length can be an issue, depending on theapplication. Ball screws are supported at the ends, and as the unsupportedlength increases with longer travel lengths, screw sag becomes an issue thatcan affect performance and speed.

Belt-driven and rack-and-pinion systems can provide longtravel lengths, but not necessarily the precision and accuracy required. Alinear motor provides submicron repeatability and accuracy, and its non-contactoperation makes it friction-free.

A motor's maximum acceleration is inversely proportional tothe moving mass or rotating inertia. The rotary-to-linear conversion mechanismsfound in traditional servo linear motion systems add inertia. Linear motorsdon't have to contend with this inertia, and a typical linear motor design hasrelatively low mass compared to its rotary counterpart. These characteristicsallow linear motors to achieve up to 20 Gs of acceleration and 5 m/s maximumvelocity.

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Flexibility Considerations

Another advantage of linear motors is increased flexibilityin machine design. With a linear motor, multiple carriages can be runindependently on a single track sharing the same support members, linearbearing rails and linear encoder scale - thereby guaranteeing parallelismbetween moving forcers while minimizing space requirements. Multiple forcerscan also be joined for applications requiring larger thrust forces than can beprovided by a single forcer.

An example of the increased flexibility in machine designoffered by linear motors can be illustrated in an application example where arotary cam is replaced by a linear motor. Rotary cams convert rotary to linearmotion by means of a cam and a follower or a linkage system. In this example,machine flexibility is increased by replacing multiple cams driven from onedrive shaft with independent linear axes. Each axis can now run independentlywith separate move profiles, allowing the machine to produce a wider range ofproducts.

Application Design and Cost Issues

Of course, not all applications can benefit from linearmotors - and cost is one of the main factors. While the total life cycle costof a linear motor system can be low compared to a rotary servo motor and lineartranslation motion system, many designers are reluctant to make the higherinitial investment.

Another issue with linear motors is that their higheracceleration rates can cause disturbances unless sufficient stiffness isdesigned into the machine frame. Although every motor produces heat, a linearmotor's direct connection to the load can be a disadvantage when it comes toproviding thermal dissipation as heat is transferred directly to the load.Heat-sensitive applications will require external air or liquid cooling, or thelinear motor can be oversized to minimize the temperature rise in the motorcoil.

Other linear motor design considerations include magneticattraction forces between the motor coil and magnet track. A strong magneticattraction force exists between iron-core linear motors and magnet tracks,typically in the range of two to four times peak force. This magnetic forcemust be accounted for in the selection of the linear bearings; however, it isimportant to note that this magnetic preload can also help in increasing thesystem's frequency response and improving deceleration and settling times.

Designers should also be aware that exposed linear magnetscan attract ferrous particulates, and therefore can present safety concernsassociated with magnetic forces (for example, if the machine operator has anelectronic pacemaker). For these reasons, magnetic shielding or protectivecovers must be designed into the system.

Cable management is another issue of concern for systemsdesigners. In most linear motors, especially those with long travel lengths,the forcer moves while the magnets are stationary. A moving cable managementsystem is therefore required, and it must be designed to prevent prematurefailure of the power and encoder cables. This means using cable carriers withan adequate bend radius and high flexing cables. For short travels, it may bepossible to fix the coil and move the magnet track, thereby removing the needfor any cable management.

Paul Zajac is a product engineer at YaskawaAmerica Inc.

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