An LVDT is a robust, frictionless electromechanical transducer that converts the rectilinear motion of an object into a corresponding electrical signal.
1. The LVDT Signal
The LVDT does not produce a simple DC voltage. Instead, it operates on the principle of magnetic induction.
Signal Generation
- Excitation: A primary coil is energized with an AC reference signal (typically 1–10 kHz).
- Induction: Two secondary coils, placed on either side of the primary, pick up the induced voltage via a movable ferromagnetic core.
- Differential Output: The secondaries are wired in "series-opposition." The output voltage is the difference between them: $$V_{out} = V_{S1} - V_{S2}$$
Signal States
- Null Position: When the core is centered, $V_{S1} = V_{S2}$, resulting in 0V output.
- Positive Displacement: Moving one way increases $V_{S1}$ and decreases $V_{S2}$. The output is in-phase with the excitation.
- Negative Displacement: Moving the other way increases $V_{S2}$. The output is 180° out-of-phase with the excitation.
AD698
Universal LVDT Signal Conditioner
The AD698 is a complete, monolithic Linear Variable Differential Transformer (LVDT) signal conditioning subsystem.
It is used in conjunction with LVDTs to convert transducer mechanical position to a unipolar or bipolar dc voltage with a high degree of accuracy and repeatability.
All circuit functions are included on the chip.
With the addition of a few external passive components to set frequency and gain, the AD698 converts the raw LVDT output to a scaled dc signal. The device will operate with half-bridge LVDTs, LVDTs connected in the series opposed configuration (4-wire), and RVDTs.
2. Common Applications
LVDTs are chosen for "mission-critical" environments where failure is not an option and precision is mandatory.
Aerospace & Aviation
- Fly-by-Wire Systems: Feedback for flight control surfaces (ailerons, elevators).
- Cockpit Controls: Sensing pilot input on joysticks and pedals.
- Engine Management: Measuring fuel flow valve positions.
Industrial Automation
- Roll Gap Measurement: Controlling the thickness of steel, paper, or plastic during manufacturing.
- CNC Metrology: Highly accurate dimensional inspection of machined parts.
- Hydraulic Cylinders: Integrated into actuators to provide precise position feedback.
Power Generation
- Turbine Control: Monitoring the position of steam inlet valves in power plants.
- Structural Health: Measuring minute shifts in dam walls or bridge supports over decades.