Linear motors

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Today, linear motor technology is considered the most innovative drive technology for handling and automation systems. But how old is this technology? The first patent for a linear motor was registered in 1885 by Dr. Th. Burger in Berlin. The first application, an electric hammer with a linear motor, was in fact developed 3 years earlier by the Frenchman Marcel Deprez.
This means that we are talking here about a technology which is already 125 years old and yet counts as one of the most advanced known technologies.

Functional principle

In principle, every linear motor is a standard electric motor, which has been sawn open along its axis of rotation and spread out. This generates a moving magnetic field in the stator, in which a magnetic object, the rotor, can travel.
There are two types of design, depending on the static or moving part of the linear motor. With a fixed coil design, the field coils are static and the magnets are moving; with a moving coil design, the magnets are static and the coils are moving. The moving magnet linear motor has the advantage that the supply cables for the field coils and the measuring system do not have to be moved. With a moving coil linear motor far longer strokes can be achieved, but the cables move, which can generate higher maintenance costs.
The linear motor concept has proven to be very versatile and designs can vary greatly. However, every linear motor axis requires a fixed and linear, moving element, a guiding system and, in most cases, a measuring system which can be integrated together in a single unit.

 

Cartridge motors

With this design, the motor coil is constructed cylindrically, and the magnets can move back and forth within it. The round construction gives this design its performance density. The benefit of encapsulation of the magnets with the motor coil is that the magnetic fields are "contained" within the product and only a very low magnetic field is built up outside the linear motor axis.

DNCE-LAS.png


Example of a cartridge motor: the electric
cylinder DNCE-LAS by Festo, in a
housing suitable for pneumatic applications


Iron-core flatbed linear motors

The bed of an iron-core linear motor consists of a steel track with salient poles separated by an insulator. These poles allow a slide with a moving magnetic field to travel along this bed. No external magnetic fields are built up since the magnetic energy is generated by the moving element and the track is non-magnetic and thus magnetic parts are not attracted. However, the performance density is not high.

ELGL.png


Example of an iron-core linear motor: the linear motor with air bearing
ELGL by Festo


Permanently excited flatbed linear motors

This design type is a variant of the iron-core flatbed linear motor which, instead of having salient poles, has strong permanent magnets. This generates strong magnetic fields which, depending on the application, have to be shielded through suitable measures.
The performance density is far higher than for normal iron-core linear motors. The permanent magnets generate high attractive forces between the slide and bed, which lead to constant loads on the bearing.
They must be dimensioned appropriately.

Ironless linear motors

Ironless linear motors usually consist of two magnetic rails placed opposite each other with a series of coils without an iron core running between them. The moved slide masses are extremely small, so that maximum accelerations can be achieved. The poor heat dissipation caused by this type of design means that only low continuous forces are possible.

Reasons for using linear motor axes

Linear motor axes are used for different reasons. These can be divided into four areas.

1. Compact installation

Complete linear motors consist of a full axis with all the components required for the application within the axis specifications. The linear motor, bearing and displacement encoder are placed in a housing. Positions, acceleration and speeds can be easily programmed into the motor controller and changed if required.
Moreover, linear motor axes have no additional projecting edges, as do spindle or toothed belt axes because of the attached servo motor.

2. Improvement in machine cycle times

With linear motors, machine cycle times can be substantially reduced. A reduction of over 40% of the cycle time in comparison to a standard axis is possible. Depending on the type of linear motor design selected, operation can be low-maintenance. In addition, linear motors are indestructible and are wear resistant in normal operation. 

3. Increased process reliability

Through the integrated displacement encoder, which is directly coupled to the slide, backlash and slip-free detection of the current position and speed is always guaranteed. In comparison to standard spindle and toothed belt axes, on which the displacement encoder is integrated into the servo motor and therefore only indirectly coupled to the load via the spindle and the toothed belt, this means substantial gains in accuracy and reliability. This direct coupling on linear motor axes means that, a fully closed control circuit is always available and external influences are more quickly controlled, or errors are detected more quickly through intelligent motor controllers.
Crash-tolerant operation is possible through the magnetic coupling of the drive. If a drive is blocked, the magnetic fields can be removed without mechanical damage. 

4. Controlling production of highly diverse products

More and more variants of individual products are being made available, so that production systems are having to become more and more flexible. Sometimes different products are manufactured in one production plant in a changeable process, placing high demands on the flexibility of the equipment. Linear motors are ideally suited for such production plants, as theoretically they have no mechanical limits. All linear motor axes are freely programmable via the motor controller, so that retrofitting is possible through a simple software change.

Differentiation according to application

Due to its numerous advantages, the linear motor could be the solution for all applications in handling and automatic technology. However, every drive solution has specific strengths and weaknesses and can provide unbeatable advantages in certain applications.

Drive
technology
Toothed belt drive Lead screw  Ball screw Linear motor
Stroke up to 10 m up to 2 m up to 2 m up to 10 m
Speed 5…10 m/s  0.5 m/s 3…5 m/s  5…10 m/s
Acceleration 100 m/s² 30 m/s² 50 m/s² …250 m/s²
Precision 100 µm 50 µm 20 µm 1 µm
Rigidity medium very high
(Reverse backlash)
high high
Costs  low medium  high high to very high

Toothed belt drive

The transmission element on a toothed belt drive is a belt. This is a flexible component which tends to vibrate during high accelerations and decelerations. The moving weight is low, so that high accelerations can also be achieved. However, because of the flexible toothed belt, high accuracies cannot be obtained. It is ideal for all positioning applications in which high accurancy across the total service life is not required. 

Lead screws

The most important advantage of lead screws is that by their nature they can be self-locking, which is particularly important for vertical applications and format setting applications. These axes cannot change position without a drive (safety function) and can simply be switched off during lengthy downtimes (increase in energy efficiency). However, the speed and service life are limited by the large mechanical contact area. 

 Ball screws

Ball screws are a state-of-the-art technology for positioning applications with high demands on precision. Due to the resonant frequencies occurring at higher speeds, i.e. mechanical whip of the screw, the speed and strokes can be limited.

Linear motors

In terms of the most important technical data, linear motors can achieve top values, though these are often limited by the mechanical parameters. It must be noted that the higher the required performance, the higher the costs may be. If the demands for accuracy increase, we must switch from an inexpensive magnetic displacement encoder to more costly optical measuring systems and glass scales. This is similar for the guides required. Simple systems can be created using plain-bearing guides, but the higher the demands on load, speed and precision, the higher the demands become on the guiding system. 

 Design guidelines when using linear motors

Linear motors are known for their capacity to achieve very high speeds and accelerations. The high acceleration and decelaration values create large forces which have an impact on the mounting surface.
Using linear motors, forces can be built up within a fraction of a second, so that large force impacts affect the machine frame.
The force impact or pulse change is the product of changes in force and time. The faster a load is accelerated or decelerated, the stronger the force impacts become. On many applications in which linear motor axes are used for the first time, these force impacts are not observed and the machine frame is not rigid enough. This leads to major machine movement and vibrations.
The linear motor's displacement encoder has resolutions of around 1 µm or better. It is frequently expected that this resolution is achieved. However, it is often forgotten that factors such as the mounting surface, the type of installation, fluctuations in the ambient temperature and heat generated by the linear motor have a substantial influence on the positioning accuracy. Linear motor axes with air bearings can achieve positioning accuracies in the range of 1 µm or better; but to do this correspondingly flat mounting surfaces are required.
The temperature influence is not to be underestimated in linear motors.
Linear motors are frequently dimensioned to peak values, so they can get very hot during operation. The heat expansion coefficients of aluminium and steel lie at 23, 8⋅10-6/K or at 10⋅10-6/K. This means that a temperature increase of 10 K can result in a length difference of 0.14 mm. This leads to stresses and inaccuracies. If this is not observed when selecting the mounting surface, the mounting method, the linear motor axis, the cooling system and during commissioning, high accuracies cannot be achieved.
In comparison to other drive technologies, linear motors provide clear advantages in the reduction of process costs through shorter cycle times and improvement of process reliability through increased precision. However, when used, certain parameters must be observed, making selection more difficult, but worth it in the end.