A Linear Variable Differential Transducer (LVDT), is used to measure the speed or position of an object. Here at Sentech, our LVDTs facilitate contactless operation, with long life, no loss in performance, infinite resolution, unlimited mechanical life, single-axis sensitivity, damage resistance, fast dynamic response, and absolute output. Keep reading to learn more about LVDT construction, its working principle, and how this relates to it's application in industry.
A LVDT is a sensor that can convert input vibrations or mechanical motion into variable electric signals. The LVDT acts as a secondary transducer, which is when physical quantities like force, tension, and pressure are first converted to displacement by a primary transducer, after which an LVDT measures it in terms of the corresponding electrical signal.
A LVDT is also an AC-controlled device; hence there are no electronic components inside it. As a result, it’s able to work at very extreme temperatures and in harsh conditions.
LVDT construction involves mounting a primary winding, P, and two secondary windings, S1 and S2, on a cylindrical former. The secondary windings have the same number of turns and are placed identically on either side of the primary winding.
Then, a movable soft iron core is placed inside the cylindrical former. This movable core is usually made of nickel-iron with hydrogen annealing which helps to reduce harmonics and residual voltage of the core, consequently increasing LVDT sensitivity. The movable core is also laminated to reduce eddy current losses. During usage, the unknown displacement is attached to this movable soft iron core. To complete the setup, the assembled components are placed in a cylindrical steel housing with end lids to provide electromagnetic and electrostatic shielding.
The working principle of LVDT as a displacement transducer is based on mutual induction. When an AC excitation of 5-15 V at a frequency of 50-400Hz is applied to the primary winding, P, a magnetic field is produced. This magnetic field induces a mutual current in the secondary windings, S1 and S2. As a result, the secondary windings have induced voltages of E1 & E2, respectively.
However, both secondary windings are connected in series. Hence, the net output voltage becomes the difference between the induced voltages in the secondary windings. That is, the differential output voltage of the LVDT will be E0 = E1 – E2.
Because the core can assume various positions, three cases arise in regards to an LVDT sensor's working principle:
When the LVDT core moves towards the S1 secondary winding, the flux linkage with S1 is greater than that of S2. As a result, the EMF induced in S1 is larger and E1 > E2. Hence, the net differential output voltage (E0 = E1 – E2) is positive and will be in phase with the primary voltage.
When the LVDT core is at the null position, the flux linkage with both secondary windings is equal. As a result, the EMF induced in both the windings will be the same. Hence, the net differential output voltage (E0 = E1 – E2) is zero, indicating that there is no displacement of the core.
When the LVDT core moves towards the S2 secondary winding, the flux linkage with S2 is greater than that of S1. As a result, the EMF induced in S2 is larger and E2 > E1. Hence, the net differential output voltage (E0 = E1 – E2) is negative and will be in phase opposition (180° out of phase) with the primary voltage.
LVDTs, from their construction materials and techniques and fundamental physical principles of operation, have significant uses and benefits in mission-critical applications like Power Generation, Gas and Steam Turbines, Nuclear Turbines, and Aerospace. What’s more, disruption of power electronic modules allows for an easy LVDT calibration process, and, when used properly, they have an endless life cycle.
At Sentech, we’re a global leader in both designing and manufacturing position sensors, which includes Linear Variable Displacement Transducers (LVDT), HYDRASTAR sensors, Rotary Variable Differential Transducers (RVDT), FASTAR sensors, solenoids, and signal conditioners. With more than 30 years of experienced engineering, we offer high-quality service and support, with the industry-leading manufacturing you need. Contact us today and find out how we can help you achieve excellence!