Linear Motors 101

Gaining in popularity, linear motors are ideal for applications that require high speed, accuracy, and repeatability. By using linear motors, design engineers can also gain more travel in the same envelope-they can even have two different motors go to the same area.

Although the market is expanding, one of the factors limiting growth is lack of familiarity with the technology. There are many different types of linear motors, and there are special considerations, such as heat dissipation and cabling.

This special report is intended as a basic overview of linear motors for engineers who are looking for an introduction or may need a quick refresher course. Here, we cover the basic operating principles, the different types of linear motors, the linear motor system, and some of the unique application requirements. Check out our website at www.designnews.com for additional technical information and a complete list of linear motor suppliers.

Operating principles. Going back to review first semester physics is a good starting point to understand the basics behind electric motor motion. The physics involved are simple-two like magnetic poles repel each other, and two opposite poles attract each other. It is even easier to understand that linear motors generally work on the same principles as rotary motors. In fact, it's fair to say that at least in concept a linear motor is simply a rotary motor that is laid out flat.

Almost all motors are made of only two parts-a copper coil assembly and a row of permanent magnets. When a current passes through the coil assembly, a magnetic field is created in the surrounding space. As a result, the row of permanent magnets reacts by exerting a force on the copper coil assembly.

Simply turning on an electric current to move the coil assembly will not create continuous motion, so the current is commutated. Commutation means switching the current on and off in the coil windings. By changing the current direction, at a key point and time along the path, you can control the magnetic flux to create continuous motion.

Compared to ball screws or timing belt technology, linear motors can provide higher acceleration, velocity, and accuracy. Furthermore, due to higher repeatability and reduced settling time, throughput can also be significantly increased.

All motors have two critical components-the primary and secondary. The primary is made of a copper coil assembly in which the electric current is applied to create an electromagnetic force. In all motor designs, the number and length of the windings determine the motor force. The secondary is the component that reacts to the electromotive force, and is typically made up of a series of permanent magnets. Either the primary or the secondary can be the stationary element; the other moving element.

If the core in the copper coil assembly is iron, the total force created by the motor is the summation of the magnetic attraction between the iron and the permanent magnets and the magnetic force created by the current in the copper windings. Since an iron-core adds to the magnetic attraction, iron core motors typically produce higher overall force, speed, and acceleration. However, the magnetic attraction between the iron core and the permanent magnets creates cogging. Which, can cause the system to shake. To overcome this problem, manufacturers have designed an ironless core motor made of copper windings that are wrapped around laminated steel and embedded in epoxy resin. Ironless core motors create finer motion, but produce about 250 lbs of continuous force-only half that of an iron core motor.

A linear motor can be flat, U-channel, or tubular in shape. The moving part of a flat motor, which is typically an iron core design, rides close to the surface of the secondary. A U-channel motor's moving part rides within two rows of permanent magnets. In all cases, the moving copper coil assembly is ironless, and thus has a lower inertia, which can produce higher accelerations for an equivalent force density.

The configuration that is most appropriate for a particular application depends on the specifications and operating environment. Flat motors are often cheaper because they require fewer machined parts and magnets. However, since the surface of the magnets is exposed, one limitation is that they cannot be used in environments that will be affected by magnetism that will "flood" out of the system. In U-channel designs, this problem is significantly reduced since the magnets are contained within the motor casing.

Tubular motors are based on the same principles as flat and U-channel types, but the moving part is composed of a stack of magnetic rings that move back and forth inside a column of coils. They share the same advantage as the U-channel motor of containing the magnetic field.

Basic motor types.Several types of linear motors, including dc brush, ac brushless, stepper, and induction motors are available.

DC brush motors provide high force at a reasonable cost. The motor contains carbon brushes in the secondary that carry the current through the commutator bars and the coils of stationary copper windings. A brushed motor is often cheaper and the control system simpler because the speed of the motor is directly proportional to the magnitude of the voltage. However, brushed motors contain moving parts that rub one another and can wear out or fail.

AC brushless motors are often called contactless motors because they do not need brushes for commutation. "One advantage to ac brushless motors is that they are commutated with non-contact switches, thereby increasing overall life of the motor," says Roger Bullock, manager of servo and drive applications at Normag.

Current is commutated through the coils with a 3-phase sinusoidal or trapezoidal signal in a closed-loop feedback system that tracks the motion of the copper coil assembly in real time. "An advantage to ac brushless motors is that the servo systems provide real time feedback on the position, force and velocity of the moving load," says Brian Miller, president of Flexible Technologies. "In an open loop system you may not know if the moving load arrived at its position." AC brushless technology delivers the highest accuracy and repeatability of any linear motor in the industry, but the required electronics result in a higher overall system cost.

Linear steppers are popular because they provide high repeatability at a relatively low cost. The linear stepper motor is constructed of a forcer (primary) with windings that are inserted in a laminated core assembly. The platen, which is a stationary secondary, is a photochemically etched steel plate filled with epoxy. When power is applied to a winding, a magnetic force is created between the forcer and the platen aligning the teeth. When power is applied onto another winding, and the current is reversed at key points, the magnetic teeth continuously align themselves along the motor stroke to create linear motion.

Since the amount of current applied is directly proportional to the magnitude of the step, a stepper can function as an open loop system that does not require servo tuning. The number of pulses to create motion determines the resolution of the movement. If the pulse frequency increases, the increment of movement becomes smaller, and as a result, repeatability is improved. "Linear stepper motors are popular in the semiconductor industry because they provide high repeatability at a low cost," says Bullock. "Since motion occurs at a discrete number of steps, the system can be moved a certain distance to ensure the payload is reaching its target with a desired repeatability."

AC induction motors are a good choice when high accuracy is not critical. When ac power is supplied into the stationary windings, a traveling magnetic wave is created that induces current in the secondary. As a result, the secondary creates its own magnetic field and a reactionary force that causes linear motion. AC induction motors do not require a costly closed loop feedback system. A vector controller is all that is needed.

Special considerations. Linear motors cannot produce motion within an application without being part of system, composed of a stationary base, a cable carrier, linear bearings, an encoder and switches for a control system. Since the motor contains heated wires and magnets that can affect the surrounding environment, a number of technical issues including heat transfer, cabling, and structural problems must be addressed to ensure proper performance.

All linear motors produce heat that can affect system performance. At the very least, poor heat transfer can cause a linear motor system to periodically shut down. At the very worst, it can cause irreparable system damage. Copper windings play a vital role in determining the upper limit of the motor force. "The number of cubic inches available in the motor defines a physical envelope in which the motor can operate," says Jack Marsh, senior staff engineer at Trilogy. "All engineers would like to have high coil density to maximize the motor force, but this increases the internal resistance and heat generation." Marsh recommends understanding the relationship between the force produced and the copper coils in the assembly to estimate the amount of heat generated, by using this correlation:

Power = (# of Windings)(Length of Winding)(Magnetic Flux)(Current)

Once the force is calculated, a motor constant defines the efficiency:

Motor Constant = (Force Produced)/ (Input Amperage)

One way to improve performance is to reduce the heat loss by designing a cooling system into the stage. "Effective cooling can increase the continuous torque output of a motor by 40% or more" says Robert Novotnak, chief engineer, Components Group, Aerotech Inc. Air and liquid cooling are popular, but the choice depends on the working environment. "I prefer to use air over liquid cooling systems because if catastrophic failure in the machine occurs, liquid can cause a lot of damage, says Mike Everman, cofounder of Bell-Everman, a linear motor stage design company."

Another common issue with linear motors-often not realized until after weeks or months of service-results from insufficient cabling of power and encoder leads to the moving part. "Traditional cabling will simply not survive the constant high speed cycling in linear motor applications," says Mark Sabine, vice president of Automation Solutions. There are several ways to overcome this problem.

"Primatics, a manufacturer of precision positioning stages, uses a high-flex, flat cable that contains multiple conductive lines to handle the high current demands," says Sabine. "The flat geometry of the cable allows it to easily survive the extreme flexes of the high-speed stages. Many cable manufacturers now offer high-flex cables specifically designed for linear motor applications. This solution may solve the fatigue problem, but leaves the management of these high flex cables unresolved."

At this point, the problem of choosing the proper cable is shifted to the "cable carrier" vendors. Carrier manufacturers, like KabelSchlepp, have encountered and solved a number of motor applications. Stephan Achs of KabelSchlepp cautions his customers to call the factory applications engineers to ensure that the proper carrier is selected due to the extremely high speeds of the linear motor.

Structural challenges also abound in the design of linear motors says Sabine. "Flatness of the mounting service, and ensuring that the motor is effectively bolted down must be considered. Iron-core motors can create tremendous attractive forces between the stationary component and the windings.

"When using iron core brushless linear motors, engineers must be aware that extremely high attractive forces have bent more than one structural frame out of commission when it was simply mounted to the machine," says Sabine. "When linear motors, incredible amounts of kinetic energy can be developed quickly," says Aerotech's Novotnak. "A combination of electrical safety functions and competent mechanical design at both ends of travel can ensure safe operation."

In the end, linear motors offer tremendous advantages in terms of speed, accuracy, and repeatability. Understanding their capabilities, limitations, and the trade-offs involved is a good first step toward using them successfully in design applications.