Inductive sensors are the favoured choice for short lengths in fluids-based drive technology. These are passive sensors based on the principle of self-induction. If an alternating current flows through a coil, this will generate a constantly changing magnetic field, which in turn induces a voltage in the coil as per the law of induction. A position-dependent change of induction is used to measure the position. In most cases, the sensors used operate in accordance with the differential transformer or differential choke system.
Displacement transducers in a differential transformer circuit (Figure I 7 a) act like a transformer, from an electrical perspective, with one primary winding and two loosely connected secondary windings. In spatial terms, the secondary windings are arranged one behind the other along a shared axis and, in electrical terms, connected parallel to each other so the output voltage is zero when the core (plunger) is in the centre position.
If the plunger is moved, the AC voltage induced in the secondary coils will become unequal. At the output of the transducer, an AC voltage U will then be present in proportion to how far the plunger has been moved. In most cases, coils are operated at a carrier frequency of 5 kHz or higher. Following demodulation and filtering, a DC voltage signal will then be present.
Thanks to their design, the transducers are particularly well suited for use in difficult environmental conditions such as those where water, aggressive gases or oil are present.
Displacement transducers in a differential choke circuit act, in electrical terms, like a Wheatstone half bridge with adjustable and complex resistances and supplemented by an ohmic half bridge in the amplifier with carrier frequency (Figure I 7 b). The output voltage at the bridge rectifier is proportional to how far the iron core of a solenoid has been moved. The measured signal then needs to be demodulated and smoothed again in the measuring amplifier.
These transducers, which are also contact-free, are particularly suitable for short distances (0.1 to 3 mm). A major area of application is measurement of the distance shifted for continuously adjustable valves.
Where larger distances are involved, the open displacement transducers based on the oscillator damping process are more suitable. With this process, approaching an electrically conductive measured object causes a change in terms of damping and therefore a change in the amplitude of an LC resonant circuit. Above a relatively large distance, the amplitude of the output voltage is proportional to the position of the measured object. Following rectification and filtering of the output signal, a DC voltage signal will then be present. At high oscillator frequencies (50 to 500 kHz), it is possible to achieve highly dynamic measuring systems.
Figure I 7: Inductive position measurement; a) differential transformer principle, b) displacement transducer based on the differential process