MTS Sensors - APPLICATION NEWS
Linear Drive utilizes magnetostrictive Position Sensor.
Innovative linear technology inspires development and design engineers. Highly dynamic linear motors solve drive and transport tasks with unequaled accuracy, utilizing magnetostrictive technology in the measurement system.
Linear motors are used for a wide range of applications, because they offer a multitude of advantages based on their functional principle. Due to high dynamics, positioning accuracy and reduced maintenance requirements, the linear drive improves the quality and efficiency of the production.
Flat long stator system
An example of innovative linear technology is the direct drive developed by EAAT GmbH in Chemnitz (Fig. 1). The winding is fitted on a firmly mounted stator in the form of a long stator system. The magnetic conductor plate is considerably shorter and slides guided with rollers over the horizontal inductor. The speed curve of the traveling magnetic field and of the magnet plat is synchronous over the segmented displacement of maximum 1,800 mm length. With an overall height of only 20 mm, the flat motor can be integrated easily into the user's machine concept.
A frequency inverter connected to the supervisory computer and a magnetostrictive measurement system complete the linear motor into a precision-controlled drive. The inverter developed by EAAT corresponds to a standard controller and energizes the motor windings with a frequency of 300 - 800 Hz and a voltage of 0 - 28 Vac at an input voltage of 60 Vdc. For position feedback, a rod-shaped sensor is mounted firmly with the sensor tube in parallel to the long stator (Fig. 2). The only mobile part of the measurement system is the mobile carrier to which a small permanent magnet is fitted. It transmits the position of the conductor plate through the wall of the tube. System-specific apertures in the measurement system are completely unnecessary.
Before, during and after the displacement, the magnetostrictive measurement system continuously provides the SSI-compliant position feedback signal of the carrier to the electronics. Due to the synchronous measurement, a real-time measured value which can be read by the computer is available at all times. Based on the actual position value and the position setpoint determined by the computer, the inverter energizes the relevant motor windings, in order to initiate the traveling magnetic field and to position the conductor plate as required.
Precise, dynamic and without jitter
Designed for small loads, the linear motor as presented in this description can move loads between 0.05 kg and 1 kg. Whether the application is in the fields of microelectronics, nanotechnology, photovoltaics or assembly robotics and packaging of various industrial branches, the linear motor dynamically solves all drive tasks - even under high-vacuum conditions - with a shear force of 10 N. Suitability for operation in a vacuum is ensured by an insulating resin, which is subjected to a special heat treatment during hardening. The sensor - with its measuring tube immerged in the vacuum - is also suitable for use in the vacuum. Thus, in one particular case of application, components can be driven and positioned accurately at a pressure of 10-7bar in a metallizing plant.
Apart from the position of the carrier, its velocity and acceleration of 10 m/s2 are detected by a Temposonics® sensor (Fig. 3). At velocities within 0.5 - 1000 mm/s, the movement of the conductor plate is completely without jitter and of highly constant speed. Prerequisite for obtaining the low speed range is the high accuracy of the measurement system. The Temposonics® R-Series with SSI output used for this application offers a resolution as low as one micrometer. With its powerful integrated electronics, the R-series is the showpiece of MTS. Quick and safe connection to the control system and interference-proof data transfer offer many advantages. With possible measuring rates up to 4 kHz and Baudrates up to 1.0 MBd dependent on transfer distance, R-Series SSI sensors of the R-Series are a warranty of utmost dynamics in the linear motor. Precise position control of defined positions is ensured by a linearity of 60 µm dependent on the maximum stroke.
Another advantage of all MTS sensors is in the absolute position measurement. After switching on the linear drive, in case of mechanical collisions or after power failure, no re-calibration is necessary, i.e. the drive is always ready for operation.
No wear parts
The secret of the unique functionality of Temposonics® sensors is the magnetostrictive measurement technology. For determining a position, a current pulse from the sensor electronics travels through the sensing element - the waveguide. A partial twist in the area of the position magnet produces a torsional wave. This ultrasonic wave travels to the ends of the waveguide and is processed by a special signal converter. The position is determined exactly by measuring the pulse runtime, with the magnet position being calculated from the time between the start of the current pulse and the arrival of the electrical response signal. As the measurement principle makes use of magnetomechanical effects, direct contact between the position determining magnet and the sensing element can be omitted. This explains why the measurement system is completely contactless and free of wear. A receiver system responding to torsional pulses offers the advantage of being insensitive to vibration, because torsional waves cannot be generated by external mechanical effects.
Due to the use of a sensor that works on the magnetostrictive principle, no cable drag chain - as needed e.g. with incremental measurement - is required. The position magnet moves along the sensor tube quickly, without contact and without mechanical resistance. Position feedback from the sensor to the inverter is with a typical cycle time of = 1 ms.
Large variety of applications
MTS offer a range of designs, stroke lengths and interfaces, magnetostrictive position sensors are extremely versatile and suitable for use in a multitude of applications. For instance, the RF sensor with a flexible sensing rod permits linear measurement up to 20 m. Since positioning tasks often require determination of several positions, a single Temposonics® sensor with CAN, Profibus or EtherCAT interface options can use up to 20 magnets for multiple position measurements. And in case a position magnet is missing or a measuring range is exceeded, a diagnostics LED on the sensor head signals that the user has to take remedial measures.
MTS out-performs the competitors
Prior to selecting the ideal measurement system for the linear drive, EAAT has subjected the Temposonics® sensor to a comparison with other manufacturers of magnetostrictive sensor technology. Tests revealed that the MTS position sensor is by far the best, especially in resistance to noise and reliability (Fig. 4). The time curve of the velocity signal is low in fluctuations, while the system of one competitor shows peak deviations from the actual signal of more than 50 mm/s even after correction by the electronics. As noise, jitter, long reaction times and measurement errors impair the operating characteristics of the motor decisively, a precise linear drive cannot be implemented if fluctuations of this magnitude occur. Consequently, the scientific industrial company decided to replace the previously selected product immediately by a Temposonics® sensor. According to the comment given by EAAT, selection of this sensor finally warranted the successful operation of the linear motor. The excellent support provided by the responsible sales engineer ensured smooth project execution.
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