Inductive and magnetostrictive linear transducers explained

If linear movements are to be recorded, users today often prefer contactless principles, which eliminate mechanical wear. Depending on the demands of the application, the choice is frequently between inductive and magnetostrictive linear transducers. The latest developments in both types of sensors will strengthen this trend even further.

Novotechnik has recently developed two new series of linear transducers that create opportunities for new application areas. The LS1 series of inductive linear transducers, intended for measurement ranges from 25 to 200mm, are appropriate as wear-free alternatives to most standard small linear potentiometers with a square cross-section (illustration 1). Because of their size, these are completely compatible with Novotechnik's T-series potentiometers. The same applies to the new TP1 series of magnetostrictive linear transducers, which are offered for measurement ranges of 50mm to 4500mm (illustration 2). They are mechanically compatible with the previous models of the TLM series. However, this new development offers a very high measurement accuracy of up to 10um and, thanks to many mechanical and electronic improvements, it delivers very stable signals even in hostile environmental conditions.

Inductive linear transducers

The contactless inductive linear transducers have a very compact cross-section of 18 x 18mm. They are available both as instruments with a integrated resetting spring for measurement areas between 25 and 100mm and as linear transducers with a ball coupling without play for pulling and pushing applications intended for measurement ranges of 25 to 200mm. The integrated signal processing makes the measured value available as an absolute current or voltage signal. The integrated teach-in function with LED status display is practical for many application types. A pushbutton on the linear transducer enables the user to define zero and end points of the measurement, invert the measurement output slope or set the desired signal amplitude. Unlike potentiometers, a separate measurement transformer is not necessary; control is therefore simplified and the user does not have to get involved in the control program.

These linear transducers are almost maintenance- and wear-free due to the contactless inductive measurement principle, and offer high repeatability (better than 0.025 per cent of the measured length), high resolution (12 or 13 bits), and high linearity (up to +/-0.05 per cent). The update rate of the output signal is specified to approximately 1kHz, which is more than sufficient for most applications. It is also very advantageous for many applications that the inductive linear transducer is completely unaffected by magnetic fields.

Magnetostrictive linear transducers

Magnetostrictive Linear transducers are offered for long measurement ranges. The new TP1 series is available for measurement ranges of 50 to 4500mm, optionally with movement-free or guided position displays. The linear transducers are very robust and are sealed to IP67 or IP68 over the entire lifetime, even in critical applications. Due to the contactless measurement principle, the average life is, from a mechanical standpoint, virtually unlimited.

The mechanical elements, the measuring elements and the evaluation electronics of the new generation of linear transducers were improved in order to achieve as high a level of interference immunity as possible. Even with surrounding interference fields, machine vibrations or shocks, the measurement receivers deliver stable output signals with linearity values of up to 10um. The resolution is independent of the measurement length and for all digital variants uniformly amounts to 1um.

Special attention was paid to the integrity of the internal signal conditioning and the data output update rate is 16kHz. The electronics calculate the position value from the value of the digitised measurement time and the known speed of the wave guide. The determined position value is checked for plausibility, linearised, and then processed and linearised in accordance with the interface. In addition to analogue current and voltage interfaces, Start-stop, SSI, and DyMoS interfaces are available. The DyMoS interface transfers the current measurement value for the speed as well as the position value. For linear movements, designs with incremental or quadrature interfaces are suitable as substitutes for expensive RPM counters. This product is often a more robust alternative to glass encoder systems, since for many applications the precision of the magnetostrictive linear transducers is sufficient.

The NOVOPAD inductive measurement principle

Covering the whole measurement range, sinusodial and cosinusodial traces forming loops are printed on a circuit board and supplied with alternating currents with a phase-shift of 90 degrees. In this way alternating fields are generated perpendicular to the conducting plate, while their strength over the measurement range is also sinusoidal or cosinusoidal.

The generated signal has a phase shift that is directly proportional to the path. The position indicator, designed as a oscillating circuit, 'floats' over the signal conducting plate; its resonant frequency is tuned to the emitting frequency of the two signal feeds. The oscillating circuit is stimulated by them and then emits its alternating field back to the conducting plate. The rectangular receiving coil that is integrated into the signal conducting plate receives this signal and sends it to the evaluation electronics. There the received signal is compared with the transmitted signals. The resulting phase information is then transformed by the evaluation electronics into an analogue position information, which is linear over the measurement range.

The NOVOSTRICTIVE magnetostrictive measurement principle

The measurement process is triggered by a short impulse current, which creates a circular magnetic field around the wave guide. The field lines of the position indicator run perpendicular to this field, and the position indicator marks the measurement position in the wave guide. An elastic deformation, the magnetostriction, occurs in the wave guide where the two magnetic fields superimpose. The reversible dimension change triggers a mechanical impulse, which is transmitted in the wave guide as a torsional wave with a speed of about 2800m/s. At one end of the wave guide the torsion wave is transformed into an electric signal, while it is damped at the other end so that there are no interferences with subsequent measurements. The time of flight from the origin point to the inductive pickup is directly proportional to the distance between the position indicator and the inductive pickup.

Contact Variohm-EuroSensor for more information about Novotechnik contactless linear transducers.