Interesting facts about displacement sensors with inductive technology
Guide for LVDT position sensors
Inductive linear sensors are robust, absolute measuring systems with a resolution down to 20 µm. They are suitable for applications with small strokes and especially for applications in which oscillating movements with high dynamics or high accelerations must be detected. Applications that require pressure-tight versions up to 30bar can also be realized.
Our LVDT linear sensors detect mechanical measuring distances from 1.3 mm to 940 mm. In addition, the sensor life is unlimited because the measured values are recorded without contact. For a variety of applications, variants are available as probes with spring return, with loose core or ball joints to compensate lateral misalignment.
Despite the wide range of available variants, some demanding applications require customized sensor adaptation. MEGATRON is your partner for this adaptation process. For the implementation of special measuring tasks, our many years of experience and our broad application know-how are at your disposal for the optimal design-in; and this already from relatively small quantities.
The measured value acquisition is contactless. Our portfolio contains exclusively LVDT displacement transducers. For the sake of completeness we distinguish between two types of inductive displacement sensors:
Half bridge circuit
(differential throttle principle)
(Linear Variable Differential Transformer)
A LVDT displacement sensor consists of a corrosion protected, magnetically shielded hollow body, a primary coil (primary winding), two secondary coils (secondary windings) arranged one behind the other and a push rod with a soft iron core at the end (highly permeable iron-nickel alloy). The push rod can be moved axially in the hollow body over the coils without contact and uses the inductive measuring principle as Linear Variable Differential Transformer: LVDT
The primary winding is supplied with an alternating voltage (aka excitation voltage or primary voltage): usually with a constant frequency in the range of 1...10kHz. This AC voltage is induced in the secondary coils into the two secondary windings depending on the position of the soft iron core, whereby the secondary coils are connected in series in antiphase.
LVDT has the advantage over the half-bridge circuit that the LVDT coils are wound in such a way that mechanical length changes of the coils, caused by temperature changes, are largely compensated.
If the soft iron core is located exactly in the middle between the secondary coils, the output voltage is 0 V, as the magnetic fields in the secondary coils neutralize each other. If the soft iron core is displaced axially, the induced voltages change depending on the direction: the voltage in one secondary coil increases steadily while it decreases in the other secondary coil.
State 1: Move the soft iron core to the right so that the coupling is uneven and the voltage increases. (1)
State 2: If the soft iron core is in the middle of the two secondary coils, the output voltage is 0 V (2), because the magnetic fields in both secondary coils cancel each other out.
State 3: Move the soft iron core to the left so that the coupling is unequal and the voltage increases. (3)
The output signal results according to the circuit as the difference between these two voltages. This means that the output signal drops / rises to the middle position when the core moves, depending on the direction, and rises / drops again with a phase shift of 180° when the core continues to move.
- Small measuring paths very well realizable
- Wear and maintenance-free
- Absolute measuring principle - no offset
- Very well suited for oscillating movements
- Suitable for very high temperatures
- Largely insensitive to temperature changes thanks to LVDT technology
- Pressure tight versions for high atmospheric pressure
- Suitable for oscillating applications
- Suitable for high adjustment speeds
- EMC-resistant with suitable material selection
An LVDT is a differential transformer that is supplied with an alternating voltage (AC) and also generates an alternating voltage (AC) as an output signal.
In order to obtain a versatile output signal, inductive displacement transducers are offered with suitable further processing electronics such as IMA2LVDT or with integrated measuring amplifier electronics. The latter consists of an oscillator for generating the excitation voltage, a demodulator and a differential builder, as well as an output amplifier with filter. Due to different realizable output voltage ranges, the displacement transducers can be easily adapted to the most different measuring and control devices.
The ratio of the voltages is differentially evaluated by the electronics and usually converted into a standardized output signal (0...10 V, 4...20 mA, etc.). Within the specified measuring range, LVDT sensors have a very good linearity.
Supply voltage - output voltage: For inductive systems with external amplifier module an input voltage of 10 VRMS between 500 Hz - 5 kHz is required. The DC-DC types are usually specified to ±15 Volt or 24 VDC.
However, if, as in the case of a potentiometric displacement transducer, a DC voltage supply and further processing electronics for DC voltage measurement signals are available, an inductive displacement transducer can be equipped with DC electronics which generate the AC voltage internally in the transducer (oscillator) and convert the measurement signal back into a DC voltage (demodulator), as shown schematically in sketch 2. Thus the DG-displacement transducer is simply supplied with ±15V or 24V DC voltage and delivers 0..5 V / ±5 V / 0..10 V / ±10 V / 0..20 mA / 4..20 mA as measured output value.
The measurement certificate with all the necessary data about linearity tolerance, sensitivity etc. is included.