Just the thought of using linear motors makes some design engineers cringe. Linear motors can be more difficult to work with than rotary motors because the stator and rotor are separate pieces. Consequently, engineers have in the past encountered difficulties when attempting to incorporate them into products. Linear motors also typically cost a little more. However, they can make sense in virtually any application where linear motion is required.

Kollmorgen’s Platinum series of direct-drive linear motors features a brushless permanent-magnet design that replaces traditional mechanical assemblies. Linear motors, or direct linear drives, are typically used for high-accuracy jobs. According to Chris Miyazaki, manager of the Mechatronics Div. of THK America Inc. (Schaumberg, IL), linear motors can be a better choice where:

• Light loads (<2.5 to 5 kg) are accelerated rapidly (>2 to 3 gs) to high speeds (more than 1 to 2 m/sec) over a long distance (1m travel or more), and the load is expected to settle within a few linear encoder counts in a short time (<50 to 150 msec)
• Minimum maintenance is desired. Linear motors do not require periodical maintenance and there is no preset life expectancy based on distance traveled. Cleanroom and vacuum compatibility are required
• Extremely smooth movement (velocity ripple of <5% typically) over the entire travel range at all operational speeds is required. (Applicable to sine-commutated coreless motors only. Trap-commutated iron-core motors could have up to 40% velocity ripple.)
• Quiet operation is important.  Claude Chirignan, technical support manager for linear motors at Kollmorgen (Radford, VA) agrees that applications requiring quiet operation, high acceleration, and high speed are appropriate applications for linear motors. He also adds that linear motors are appropriate for applications requiring:
• High accuracy and/or repeatability
• Smooth operation with low cogging
• Long travel (up to several meters)
• Increased reliability (applications where maintenance cost is prohibitive)

Advantages of linear motors.

In addition to those advantages that linear motors have over rotary motors, there are other advantages. Miyazaki points out that "Most PMBL linear motors operate on Lorentz Law. The motor's force generation characteristic is in linear proportion to the magnetic flux density and the electrical current flowing through the motor coil. This means, in theory, a linear motor can be combined with a fast motion controller to produce infinite axial rigidity at a holding position." This is not feasible with any rotary-to-linear conversion scheme because all moving components of a mechanical translation system exhibit varying modes and degrees of deflection when subjected to externally applied loads. "These individual-component deflection amounts cannot practically be compensated dynamically, since the sum of all component behavior will exhibit non-linear characteristics," says Miyazaki.

Another advantage of linear motors, according to Miyazaki, is that they can be configured to have multiple moving carriages on a common axial plane. All the carriages may be synchronously controlled for a multi-tasking system, or synchronously controlled for force multiplication purposes. The lack of ballscrew or belt and gearbox means the carriage will move freely if the servo function is disabled when back driven by hand. If a linear axis is driven by a 5-mm lead ballscrew and a motor, it would be nearly impossible to manually back drive the carriage even if the servo function is disabled.

Disadvantages.

There are several disadvantages to linear motors. For example, permanent magnets in linear motors may be hazardous to handle due to extremely high magnetic force from the rare earth magnets used in constructing the magnet tracks. Permanent magnets will also attract and hold ferrous alloys on the magnet track surface and may potentially cause system malfunctions.

"Linear motors generally don't lend themselves well to vertical applications," says Marian Cordone of Anorad (Hauppauge, NY). However, she indicates that they can be used in vertical applications with some form of gravity offset, such as a spring, air cylinder, or mass counterbalance. THK's Miyazaki agrees that linear motors are not easily adopted for vertical motion applications without additional engineering for load counterbalancing measures. The load counterbalancing is necessary for several reasons, he explains.

First, Miyazaki says, a linear motor's moving member does not have any mechanical contact with the stationary base. The motor must generate force equal to the load weight against the direction of gravity force if the load is to be held stationary anywhere between the top and bottom hard stops. This will cause the motor RMS current to be very high during such a holding state, and the coil will generate heat that may have destructive effects on the entire system.

Without the counterbalancing mechanism, the load will free fall at 1g acceleration downward if the system loses power or the servo is disabled. The amount of damage the positioning system will sustain depends on how far the falling load has traveled before encountering the bottom hard stop. The longer the travel distance before the crash, the higher the terminal velocity, which increases the kinetic energy of the falling load heading toward the bottom end stop.

The vertical load counterbalancing to negate the earth's 1g gravity will help the vertical linear motion system's performance. Imagine a 10-kg load that must travel vertically at 2g acceleration in both directions. In order to accelerate upward, the motor must generate an acceleration force of 10 kg x 2gs (according to the motion requirement) + 10 kg x 1g (since the load is being pulled down at 1g by earth) = 30kg (total needed). On the inverse, when the load is traveling downward, the motor is required to generate 10 kg less force to go 2gs downward for the same reason. However, during the deceleration phase of the profile, the motor must generate 10 kg extra force in the upward direction (2g deceleration = 20 kg force + 10 kg more force upwards to negate earth's 1g, in order to maintain the 2g deceleration required by the motion profile).

Linear motors also may not function well in applications where load requirements vary largely, Miyazaki asserts. Because the linear motors do not have any gearing mechanism between the load and the prime mover, any change in the load inertia will be reflected back to the prime mover at a 1:1 ratio. If the motor is sized and tuned to move a 5-kg load per a specific motion profile, a system that worked well with a 5-kg load will not operate equally well if the load is increased to 20 kg. This new 20-kg load will reflect four times the inertia of a 5-kg load back to the prime mover. The prime mover must manage a four-times-larger inertia in generating the same motion profile. The same motor sized for a 5-kg load may not generate enough force for this new load.

In a belt and 5:1 ratio gearbox-driven system, the load inertia reflected back to the prime mover is reduced by the inverse square of the gearbox reduction ratio. The 5:1 gearbox will reduce the reflected load inertia 25 times less than the original inertia. This convenient mechanism is not built into a linear motor system, so the task of motor sizing becomes extremely critical. It is relatively easy for an underestimated load value to be carried all the way through motor sizing calculations. Although such miscalculation may not become critical for a conventional mechanical drive system, it could have a devastating impact on the results for a linear motor system design.

Specifying concerns for end-users.

Linear direct-driven motor systems are dynamically superior due to a lack of complexity in construction, says Miyazaki. Any dynamic changes of the moving part can be accurately detected and the required compensation can be applied to the moving part directly through a shorter mechanical path. This increases the system bandwidth providing for a highly accurate and responsive system. In order to take full advantage of such a dynamically superior system, a motion controller must be equally upgraded. A motion controller that worked sufficiently for a less responsive system may not be sufficient for a linear motor-based system, particularly if the system is intended for a high-acceleration and high-speed application. The motion controller for such a system should have the highest servo loop update rate, encoder count capture rate, and output DAC resolution available.

Equally critical is how mechanical end stops are designed and implemented on the linear motor-driven axes. If the servo function is disabled or electrical power is interrupted while the carriage is traveling at its peak velocity and carrying a substantial load, the carriage will continue to travel freely until it collides with whatever object is at the end of its travel. If that is not a well-designed shock-absorbing end-stop mechanism, the carriage will transfer a considerable amount of kinetic energy into the object it meets and possibly damage it. A bumper system designed based on the amount of total energy it must safely absorb during the worst case crash is a high priority.

In general, linear motor systems will result in high initial implementation costs because linear motors (coil assembly and magnet tracks) are typically more expensive. The linear motor coils should not cost any more than with rotary motors. However, they can cost more because the technology both in the motor designs and the production methods is still in the developmental phase. Linear motors are just not yet on a larger scale of production economy as are conventional rotary motors.

Amplifiers also contribute to the expense of linear motors. "For most applications, sinusoidal commutation of the motor is necessary," says Jared George, an application engineer for Parker Hannifin's Daedal Div. (Harrison City, PA). "The amplifier is more expensive and complex than trapezoidal amplifiers."

According to California Linear's (Carlsbad, CA) Graham Jones, "Geometry is typically an issue in trying to fit a new configuration when going from a rotary to linear conversion approach or from a hydraulic/pneumatic system to a flat motor.” Additional factors to consider before specifying linear motors include frequency response, velocity, bearing life, space envelope, ability to withstand harsh environments, and UL or EC certification.