Tubular linear motor
A tubular linear motor or polysolenoid linear motor is understood to be an electrical direct drive in which the linear movement is generated directly on the basis of the electromagnetic development of force. This means that the translational movement is not generated by a mechanical conversion via a spindle, belt or cam disk, but is based directly on the electromagnetic forces. In principle, such motors can be constructed according to the Lorenz force principle as well as according to the Maxwell force principle and belong to the class of linear motors. Tubular linear motors belong to the group of linear motors, the difference to the flat or U-shaped linear motors being that the excitation winding of the stator encompasses the magnets in the rod-shaped rotor in a circular (tubular) manner. From a structural point of view, this creates a drive element that is similar to a pneumatic or hydraulic cylinder.
history
Charles Wheatstone presented the first linear motor to the public as early as 1845 . This was designed as a flat linear motor and primarily served to research the electromagnetic relationships. Prof. Jacobi (St. Petersburg) also presented an early linear motor, which was used as a replacement for steam cylinders and generated a rotary movement via a mechanism. In contrast to rotary electric motors, which can rotate in one direction for any length of time, linear motors always have a limited stroke, with the exception of the railway area . It was only with the advent of transistor technology that it became possible to control electric motors and, in particular, linear motors in a reliable manner, that is, to control their movement. This technology, which was introduced under the name of servo motors, gradually helped linear motors to make their breakthrough from around 1970. In the beginning, linear motors were primarily used in the so-called high-end area such as semiconductor production or machine tools, especially since the flat linear motors used in the process caused additional design effort in the form of bearings and sensors that should not be underestimated. In the 1980s, the first tubular linear motors with external sensors and bearings appeared on the market. The industrial tubular linear motors ( industrial linear motors ) with integrated bearings and position sensors, which have been available since 1996 , set another milestone .
technology
In modern servo motor technology, the use of rare earth magnets for generating the flux has become generally accepted. In a tubular linear motor, the magnets (mostly neodymium magnets ) are located in a non-magnetic steel tube. The individual magnets (disc or ring magnets) are inserted into this precision steel tube with opposite poles, so that a north-south-north-south field arrangement is created on the outside of the steel tube . The stator consists of an iron tube that serves as a return path for the magnetic flux. Inside the iron pipe are the windings , which are typically designed as two- or three-phase windings . The windings themselves are applied to a winding support, which also functions as a plain bearing. This means that in many cases an additional guide can be dispensed with, or at least the complex alignment between the rotor and stator is eliminated. Since the magnetic forces of attraction between the rotor and the stator or the iron yoke compensate each other radially, the bearing load is much lower than that of flat or U-shaped linear motors. Compared to a simple mounting of the rotor at both ends of the stator, the integrated mounting over the entire length of the stator has the advantage that the rotor can be chosen to be shorter and the load on the plain bearing is significantly lower due to the large contact surface, so that it is a correspondingly longer one Lifetime results. Modern industrial tubular linear motors can perform several billion strokes, depending on the application. In the middle of the stator there are so-called Hall effect sensors that detect the magnetic field lines of the magnets in the rotor. With a so-called sine-cosine evaluation, the relative position between the rotor and stator can be determined on the basis of this field detection. An additional external sensor system in the form of a glass scale or a magnetic tape is only required if extreme accuracy requirements below approx. 0.05 mm are required. In addition to the windings and the position sensors, modern tubular linear motors have a microcontroller in the stator. This is used for serial communication with the control electronics, so that further relevant data such as the temperature in different areas of the stator or general information belonging to condition monitoring can be exchanged during operation. Linear motors are controlled by controllers with PWM modulation and position control loops, as they are known from rotary servo motors. However, the special topology of linear motors requires some additional functions on the controller, provided that optimum performance is to be achieved. Such controllers for linear motors are typically connected to the higher-level control (PLC, PC) using fieldbus interfaces such as Profibus, DeviceNet, CanOpen or ETHERNET.
Applications
Due to their simple and wear-free principle, (tubular) linear motors are ideal for use in harsh industrial environments. In addition, there are comparatively very expensive but extremely efficient and high- performance variants with high-temperature superconductor technology.
Because of the comparatively far less complex, conventional linear technology , in which simple pneumatic and hydraulic cylinders still dominate, tubular linear motors are a niche product .
Typical applications are increasingly found in packaging machines, textile machines, test and inspection facilities, the handling area and in special machine construction of all kinds.
Around 1990, the technology of tubular linear motors as a damping element was further developed to replace the hydraulic shock absorbers of motion simulators in the entertainment industry. In 2009, this application variant was further developed in a cooperation between Tufts University and the Argonne National Laboratory into a regenerative electricity-generating shock absorber or an electromagnetic linear generator until it was ready for series production. As a rule, a cylindrical, brushless 3-phase permanent magnet electric motor, which is also known as a 'ServoRam', is used.
Individual evidence
- ↑ Laithwaite Eric : A History of Linear Motors , ISBN 0-333-39928-5 - A detailed description of the history of the linear motor.