LVDT Sensor Application Guide
LVDT Sensor Application Guide
The Linear Variable Differential Transformer (LVDT), also referred to as a linear variable displacement transformer, linear displacement differential converter, or simply a differential transformer, is an electromagnetic transformer-structured device used to measure linear displacement (positional change along a specific axis). The LVDT constitutes the core working principle and foundational structure of LVDT-type displacement sensors. The corresponding device used to measure angular displacement is called a Rotary Variable Differential Transformer (RVDT).
A driving current is applied to the primary coil at A, which thereby generates an induced current (or induced voltage signal) via electromagnetic induction in the secondary coil at B.
The LVDT is a highly reliable absolute linear displacement/position sensor. Its structure is intrinsically frictionless and free of mechanical contact, granting it a virtually infinite operating life under reasonable usage conditions. Because the AC-excited LVDT body contains no active electronic components, it can be engineered for cryogenic environments or high-temperature conditions up to approximately 1200 °F (around 650 °C), and can operate stably under intense vibration, high shock, and harsh industrial conditions. LVDTs are widely deployed in applications requiring high reliability and safety integrity, such as power generation and gas turbines, hydraulic systems, industrial automation, aerospace vehicles, satellites, and nuclear reactors. This class of sensors typically exhibits exceptionally low hysteresis error and excellent repeatability.
The basic function of an LVDT: It converts a linear displacement relative to a mechanical reference point (null or null position) into an electrical signal proportional to that displacement. This electrical signal contains both amplitude information (corresponding to the magnitude of the displacement) and phase information (corresponding to the direction of the displacement). The operational process of an LVDT requires no electrical or mechanical contact between the moving part (the pushrod or core assembly) and the coil assembly; signal conversion relies entirely on electromagnetic coupling.
Structure and Working Principle
A typical LVDT consists of three solenoidal coils aligned coaxially and wound on a single non-magnetic tube:
As the core position changes, the degree of magnetic coupling between the primary coil and the two secondary coils changes, causing the induced voltages in the two secondary coils to vary inversely. The two secondary coils are typically connected in a series-opposing configuration, making the output voltage equal to the difference between the two secondary voltages (i.e., a "differential" output).
When the core is located at the geometric center (the null position), equidistant from both secondary coils, the induced voltage amplitudes in the two secondary coils are theoretically equal and opposite in polarity, resulting in a zero output voltage. However, in practical systems, due to manufacturing variations in winding and slight magnetic circuit asymmetries, a very small residual output voltage will still be present at the null position.
Quadrature Error and Signal Demodulation
This residual voltage primarily originates from a phase shift and is termed Quadrature Error. In closed-loop control systems, this error can induce oscillation around the null point and is often unacceptable in high-precision measurement applications. Quadrature error is typically associated with direct AC differential demodulation methods.
In modern LVDT measurement and control systems (especially safety-critical systems), online fault detection of the LVDT is usually required. A common approach is:
Digital Processing and Ratiometric Measurement
When the system incorporates digital processing capabilities, such as microprocessors or FPGAs, fault detection is typically handled by the digital domain. Concurrently, to enhance measurement accuracy, a ratiometric measurement method is frequently employed. This involves:
Displacement Direction and Linearity: When the core moves in one direction (e.g., upwards), the corresponding secondary coil voltage increases while the other decreases; the differential output voltage increases from zero, and the output signal is in-phase with the primary excitation signal. When the core moves in the opposite direction, the output voltage similarly increases, but its phase is 180 degrees out-of-phase with the primary excitation signal.
Therefore, the phase of the output signal is used to determine the direction of displacement, while the amplitude of the output signal represents the magnitude of the displacement. By utilizing a synchronous detector, a signed DC output voltage directly corresponding to the displacement amount can be obtained. The elongated coil structure design allows the LVDT to maintain highly linear output characteristics over a large stroke range (from fractions of an inch to hundreds of millimeters).
Advantages as an Absolute Position Sensor
The LVDT is a true absolute position sensor. Even in the event of a system power loss, it can directly output the current position upon power restoration without any loss of position data. Its most prominent advantages include:
LVDTs are widely utilized in:
The Linear Variable Differential Transformer (LVDT), also referred to as a linear variable displacement transformer, linear displacement differential converter, or simply a differential transformer, is an electromagnetic transformer-structured device used to measure linear displacement (positional change along a specific axis). The LVDT constitutes the core working principle and foundational structure of LVDT-type displacement sensors. The corresponding device used to measure angular displacement is called a Rotary Variable Differential Transformer (RVDT).
LVDT Cross-Sectional Diagram
A driving current is applied to the primary coil at A, which thereby generates an induced current (or induced voltage signal) via electromagnetic induction in the secondary coil at B.
The LVDT is a highly reliable absolute linear displacement/position sensor. Its structure is intrinsically frictionless and free of mechanical contact, granting it a virtually infinite operating life under reasonable usage conditions. Because the AC-excited LVDT body contains no active electronic components, it can be engineered for cryogenic environments or high-temperature conditions up to approximately 1200 °F (around 650 °C), and can operate stably under intense vibration, high shock, and harsh industrial conditions. LVDTs are widely deployed in applications requiring high reliability and safety integrity, such as power generation and gas turbines, hydraulic systems, industrial automation, aerospace vehicles, satellites, and nuclear reactors. This class of sensors typically exhibits exceptionally low hysteresis error and excellent repeatability.
The basic function of an LVDT: It converts a linear displacement relative to a mechanical reference point (null or null position) into an electrical signal proportional to that displacement. This electrical signal contains both amplitude information (corresponding to the magnitude of the displacement) and phase information (corresponding to the direction of the displacement). The operational process of an LVDT requires no electrical or mechanical contact between the moving part (the pushrod or core assembly) and the coil assembly; signal conversion relies entirely on electromagnetic coupling.
Structure and Working Principle
A typical LVDT consists of three solenoidal coils aligned coaxially and wound on a single non-magnetic tube:
- The center coil is the primary coil (Primary).
- The two end coils are the two secondary coils (Secondary, top/bottom or front/back).
As the core position changes, the degree of magnetic coupling between the primary coil and the two secondary coils changes, causing the induced voltages in the two secondary coils to vary inversely. The two secondary coils are typically connected in a series-opposing configuration, making the output voltage equal to the difference between the two secondary voltages (i.e., a "differential" output).
When the core is located at the geometric center (the null position), equidistant from both secondary coils, the induced voltage amplitudes in the two secondary coils are theoretically equal and opposite in polarity, resulting in a zero output voltage. However, in practical systems, due to manufacturing variations in winding and slight magnetic circuit asymmetries, a very small residual output voltage will still be present at the null position.
Quadrature Error and Signal Demodulation
This residual voltage primarily originates from a phase shift and is termed Quadrature Error. In closed-loop control systems, this error can induce oscillation around the null point and is often unacceptable in high-precision measurement applications. Quadrature error is typically associated with direct AC differential demodulation methods.
In modern LVDT measurement and control systems (especially safety-critical systems), online fault detection of the LVDT is usually required. A common approach is:
- Perform synchronous demodulation on the signals from the two secondary coils individually (half-wave or full-wave precision rectification, typically implemented using operational amplifiers).
- Perform a differential operation (subtraction) on the demodulated DC signals to obtain the displacement output.
Digital Processing and Ratiometric Measurement
When the system incorporates digital processing capabilities, such as microprocessors or FPGAs, fault detection is typically handled by the digital domain. Concurrently, to enhance measurement accuracy, a ratiometric measurement method is frequently employed. This involves:
- Dividing the difference between the two secondary coil voltages by the sum of the two secondary coil voltages, thereby rendering the measurement result insensitive to variations in the excitation signal amplitude.
Displacement Direction and Linearity: When the core moves in one direction (e.g., upwards), the corresponding secondary coil voltage increases while the other decreases; the differential output voltage increases from zero, and the output signal is in-phase with the primary excitation signal. When the core moves in the opposite direction, the output voltage similarly increases, but its phase is 180 degrees out-of-phase with the primary excitation signal.
Therefore, the phase of the output signal is used to determine the direction of displacement, while the amplitude of the output signal represents the magnitude of the displacement. By utilizing a synchronous detector, a signed DC output voltage directly corresponding to the displacement amount can be obtained. The elongated coil structure design allows the LVDT to maintain highly linear output characteristics over a large stroke range (from fractions of an inch to hundreds of millimeters).
Advantages as an Absolute Position Sensor
The LVDT is a true absolute position sensor. Even in the event of a system power loss, it can directly output the current position upon power restoration without any loss of position data. Its most prominent advantages include:
- Exceptional Repeatability and Reproducibility: Aside from axial linear motion, rotation or slight radial deviation of the core does not affect the measurement results.
- High Long-Term Reliability: Because the core does not contact the tube wall during movement, there is virtually zero friction or wear.
- Environmental Robustness: The absence of sliding or rotating electrical contacts allows the LVDT to be fully hermetically sealed, making it suitable for the most severe environments.
LVDTs are widely utilized in:
- Position feedback in servo control systems;
- Automated gauging and closed-loop control in machine tools;
- Numerous industrial and scientific measurement scenarios.