New synchronous linear motor technology from Siemens that eliminates the magnets on the secondary track is opening the door to a new class of cost-effective, longer travel distance applications. Ideal target applications are systems with both long travel distances and demanding environments such as transportation systems, material handling, and laser or water cutting CNC machines.
The newly patented 1FN6 design offers the technical advantages of both a synchronous linear motor (higher dynamics, accuracies and force densities) along with a primary advantage of asynchronous linear motors — a non-magnetic secondary track.
The combination of no magnets on the secondary track, plus a 30 percent wider air gap, creates a robust design that is effective in dusty and dirty environments. But compared to asynchronous motors, the new design also offers 40–70 percent higher force densities and much higher dynamic response.
"With 1FN6 technology, users can get the advantages of a synchronous motor and the advantages of an induction motor without the magnets being on the secondary track. So it is really a combination of both designs, and combines the advantages of both," says Ralph Baran, product marketing manager for servo motors and cables for Siemens Industry Inc.
The key design innovation is a synchronous linear motor where the magnets are integrated directly into the lamination of the primary part along with the individual windings for each phase. The result is that the design of the secondary part is much simpler, and consists of milled steel with teeth that are used to control magnetic flow between the primary and secondary sections.
By commutating the current going into the primary section, the permanent magnet's magnetic field can be either strengthened or weakened to effectively guide the magnet flow so that it is always most concentrated on top of the teeth. Advanced commutation techniques are used to move the magnet flow up and down in the primary part, and the field generated by the current in the windings creates, in essence, a standing magnet field on the secondary part.
The traditional design of a permanent magnet or synchronous linear motor, such as Siemens' 1FN3 product, uses an electrically powered primary section that travels on a secondary magnet track. The secondary part consists of a steel plate with a series of permanent magnets mounted next to each other (north pole to south pole). The drive system provides current to the phases to generate a magnetic field, and implements sophisticated commutation algorithms to generate forces in either direction. The basic concept of a permanent magnet motor in a round design is a servomotor or, used in a linear design, you have a linear motor.
The key to lowering system costs for the 1FN6 is the limited use of magnet material which is very costly when laid out in a long magnet track. With secondary sections cut into lengths of about a foot and the travel distance laid out with a linear track where each segment has its own individual magnets, the longer you travel the more expensive the overall motor design gets.
With the 1FN6, because the secondary part is just a milled piece of metal, users can realize very long travel distances with a cost-optimized design because there are no magnets or other expensive materials on the secondary part. Baran says that roughly about two meters is the break-even point in terms of cost effectiveness.
The design also addresses another critical issue for synchronous linear motors: dirt and dust. With the magnets on the secondary part in a traditional system, one disadvantage is that the permanent magnetic attraction forces created by the secondary part attracts metal dust and/or metal chips, and generally must be protected from any magnetic contamination. With the 1FN6, only the primary part (which is completely encapsulated) is generating permanent magnet forces and there aren't magnetic forces coming from the track.
Comparing the 1FN6 design to asynchronous linear motors, the two have completely different physical principles. With an asynchronous design, since the magnet field is induced into the secondary part, losses in the secondary part are created by heat and eddy currents, and overall system efficiency is lower.
Since the 1FN6 is not inducing current into the secondary part, force density is greater because the design uses both the magnet field generated from the magnets in the primary part and the magnet field generated by the windings. Without the magnets on the secondary part in an asynchronous design, the only magnet field available is generated with current.
With the 1FN6 design, commutation algorithms can be used to either weaken or strengthen the magnet field. If it is strengthened, the higher energies in the smaller volume can generate 40–70 percent higher force densities.
The 1FN6 design also offers more reliable operation because it can use an air gap that is 30 percent wider than a traditional synchronous linear motor with a nominal air gap for 1.4 mm. Asynchronous motors usually have an even smaller air gap (1–1.1 mm).
The air gap, which is the distance between the lower surface of the primary part and the upper surface of the secondary part, is critical because chips or dirt in between the parts could damage the surfaces.
Under development is an option for the 1FN6 design that features a basic encoder integrated into the motor which would utilize the secondary teeth to measure position. Usually with a linear motor, there is a high-resolution encoder system mounted next to it. But for most 1FN6 applications there is only a need to travel within an accuracy of 1–2 mm, so it's enough to have a small, cost-effective feedback system to communicate with the drive system unless the specific application would benefit from the higher resolution linear feedback.
The 1FN6 offers a natural cooled design which helps reduce costs, but a water cooling option is also in the works. Because no water needs to be traveling with the motor, natural cooling helps increase the robustness and rigidity of the system while keeping costs low.
Water cooling is an advantage when an application requires very high force densities because any motor losses can be handled in the most efficient way. Higher forces generated using a very lightweight design and system provides the highest dynamic values. With a lighter motor, the overall mechanical system that is moving is also lighter. And with the ability to generate higher forces, system response is more dynamic when accelerating and decelerating. These capabilities are very important for 1FN3 motors used in CNC applications.
"We don't see 1FN6 replacing the 1FN3 because, especially when we integrate the magnets into the primary section, the overall design of the primary section is heavier than with the 1FN3," says Baran. He says that many traditional 1FN3 users have applications where dynamic performance and weight are key. Customers specifically want the water-cooled, light design because it offers the most power dense and lightest design solution. And even when water cooling is available with the 1FN6, its primary section will always be heavier.
"Since the primary section is usually the one which is traveling, you are losing dynamics because you might have the same force but more weight to accelerate and decelerate," Baran says. "It is a different design for a different market."
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