How Does a Magnetostrictive Position Sensor Work?

The Core Principle: Torsional Acoustic Wave Measurement

The operating principle of a magnetostrictive position sensor is based on the Wiedemann effect — a magneto-mechanical phenomenon in which a ferromagnetic material subjected simultaneously to an axial magnetic field and a circular magnetic field develops a torsional (twisting) stress along its length.

In a magnetostrictive linear position sensor, the sensing element is a thin rod or strip made from a magnetostrictive alloy (typically a nickel-iron or FeNiCo alloy). A brief electrical current pulse is sent along this waveguide. The current pulse generates a circular magnetic field around the rod. Where this field intersects the field of the external position magnet (the float or ring magnet), the Wiedemann effect creates a localized mechanical torsional wave at that precise point.

This torsional wave propagates outward from the generation point in both directions along the waveguide at the acoustic propagation speed of the alloy — approximately 2,830 m/s in nickel-based waveguides. The wave travelling toward the sensing head is detected by a piezoelectric or magnetostrictive pickup at the head. The wave travelling away from the head is absorbed by a damping element at the far end to prevent reflections.

The elapsed time between the emission of the current pulse and the arrival of the torsional wave at the head is measured electronically with nanosecond-class precision. Since the acoustic velocity of the waveguide material is a known, temperature-stable constant, position is calculated as:

Because the measurement depends only on time-of-flight and a fixed material constant — and because no mechanical contact ever occurs between the position magnet and the waveguide — measurement accuracy does not degrade with use, distance, or stroke length. Non-linearity is therefore specified as a percentage of full scale — below ±0.01% FS (min ±50 µm) on SEGMENsensor SGM-PR10 across 25–8,000 mm. For how a % FS figure and its ±50 µm minimum turn into micrometres on your stroke, see what % FS accuracy means on position sensors.

The Key Components of a Magnetostrictive Position Sensor

Understanding how the sensor works requires knowing what each internal and external component does:

Magnetostrictive Waveguide

The waveguide is the core sensing element — a thin rod or flat strip made from a magnetostrictive ferromagnetic alloy. Current pulses travel along it and torsional waves propagate through it. The waveguide is typically 6–10 mm in diameter for rod-type sensors. Its material composition determines the acoustic velocity and is selected for stability over the operating temperature range.

Position Magnet (Float or Ring Magnet)

The external position magnet slides freely along the outside of the sensor housing without touching the waveguide inside. It is the element that "marks" the measurement position. In liquid level sensors, the magnet is built into a float that rises and falls with liquid level. In displacement sensors, the magnet is a ring that mounts to the moving machine part (e.g., the hydraulic cylinder piston).

Pulse Generator Electronics

The pulse generator sends a precisely timed current pulse down the waveguide at a repetition rate of typically 100–5,000 Hz. Each pulse initiates one measurement cycle. The frequency of pulse generation determines the sensor's update rate — high-speed motion control applications may require 5,000 Hz (5 kHz) update rates, while process monitoring applications typically use 10–100 Hz.

Torsional Wave Detector

At the sensing head, a detection coil or piezoelectric element senses the arriving torsional wave. The wave creates a tiny voltage signal in the coil (inverse Wiedemann effect) or a mechanical stress signal in the piezoelectric element. This signal is amplified and fed to the timing circuit.

Signal Processor and Output Electronics

The signal processor measures the elapsed time between pulse emission and wave detection with nanosecond resolution, applies temperature compensation, computes position, and converts it to the specified output format: 4-20 mA analog, 0-10 V DC, SSI digital or RS485 Modbus RTU.

Step-by-Step: What Happens When the Sensor Measures Position

1. Electronics sends current pulse down the waveguide

   The pulse generator electronics fires a short, high-frequency current pulse into one end of the magnetostrictive waveguide. This pulse establishes a circumferential (circular) magnetic field along the entire length of the waveguide simultaneously.

2. Torsional wave is generated at the magnet position (Wiedemann effect)

   At the exact axial location where the position magnet sits alongside the waveguide, the circular field from the current pulse combines with the axial field of the permanent magnet. The superposition of these two perpendicular magnetic fields produces a localized helical magnetization in the waveguide material, generating a torsional (twisting) mechanical wave at that point — the Wiedemann effect in action.

3. Torsional wave propagates back to the head at acoustic speed (~2,830 m/s)

   The generated torsional wave propagates outward from the generation point along the waveguide in both directions. The wave travelling toward the sensing head travels at the characteristic acoustic velocity of the waveguide material — approximately 2,830 m/s for nickel alloy waveguides. The wave travelling away from the head reaches the far-end damper and is absorbed, preventing spurious reflections.

4. Arrival time is measured with nanosecond precision

   The arriving torsional wave induces a small voltage in the detection coil at the sensing head via the inverse Wiedemann (Matteucci) effect. High-speed timing circuits measure the time elapsed between pulse emission and wave detection with nanosecond resolution — typically using a precision oscillator-based counter running at 100 MHz or higher.

5. Position = (time × acoustic velocity)

   The signal processor multiplies the measured time-of-flight by the known acoustic velocity of the waveguide. Temperature compensation corrections are applied based on the temperature sensor embedded in the electronics head. The resulting position value is output with the non-linearity and repeatability given in the series datasheet.

6. Analog or digital output signal is generated

   The computed position value is converted to the sensor's configured output format: a 4-20 mA current proportional to stroke position, a 0-10 V DC voltage, an SSI (Synchronous Serial Interface) digital word, or an RS485 Modbus RTU register value. The output updates on every measurement cycle (every current pulse).

Accuracy Advantages Over Other Position Sensors

The time-of-flight measurement principle gives magnetostrictive sensors a combination of accuracy characteristics that no other linear position technology can match in the sub-millimetre, long-stroke category. The table below compares the key technologies (for how resolution, repeatability and non-linearity differ, and how to convert % FS to micrometres for your stroke, see resolution vs accuracy vs repeatability):

The magnetostrictive sensor's primary structural advantage over encoders is that it provides absolute position — no reference-seeking or homing cycle is needed after a power cycle. The sensor reports the exact magnet position immediately on power-up. This is critical for hydraulic safety applications (press brakes, injection molding) where homing cycles are a production and safety liability.

Why Magnetostrictive Sensors Are Preferred for Hydraulic Cylinders

The rod-type magnetostrictive sensor was originally developed specifically to solve the problem of real-time position feedback inside hydraulic cylinders — and it remains the dominant technology for this application worldwide.

Built-In Cylinder Integration

The rod-type sensor installs directly inside the hollow bore of a hydraulic cylinder piston rod. The sensor rod protrudes from a port in the cylinder rear cap, and the position magnet ring mounts inside the hollow piston. As the piston strokes, the magnet moves along the sensor rod inside the sealed cylinder — completely protected from the external environment. The sensor head remains outside the cylinder, connected to the machine control system.

No External Failure Points

Because the sensor is installed inside the cylinder, there are no external position feedback cables, external linear scales, or mechanical linkages that can be damaged by contamination, mechanical impact, or vibration. This is a significant reliability advantage in harsh industrial environments such as presses, forming machines, and mobile hydraulic equipment.

Full Stroke, Real-Time Absolute Position

The sensor provides continuous absolute position feedback across the full cylinder stroke — from 25 mm to 8,000 mm — at update rates up to 5 kHz. The machine controller always knows the exact piston position without any position initialization. This enables precise force-position control, end-of-stroke speed reduction, and synchronization of multiple cylinders.

OEM-Ready Dimensions

SEGMENsensor rod-type magnetostrictive sensors are available in standard rod diameters of φ10 mm, φ12 mm, φ16 mm, and φ18 mm to match standard hydraulic cylinder hollow piston bore dimensions. Custom rod diameters and thread connections are available for OEM programs. The sensors are rated IP68 and compatible with hydraulic oils, cutting fluids, and phosphate-ester fluids.

Magnetostrictive Sensor Output Interfaces

SEGMENsensor magnetostrictive linear position sensors are available with five standard output interfaces, selectable at the time of order. The right choice depends on your control system architecture, cable run length, and required update rate.

4-20 mA Analog Output

The simplest and most universally compatible output. The 4-20 mA current loop signal is proportional to position across the stroke range. 4 mA represents the zero (start) position; 20 mA represents the full stroke. Current loop signals are immune to voltage drop along cable runs, making 4-20 mA reliable at cable runs up to 1,000 m. Resolution is limited by the resolution of the receiving analog input module (typically 12-bit or 16-bit).

0-10 V DC Analog Output

Voltage analog output for short-distance applications where the control system has a high-impedance voltage input. Suitable for cable runs up to approximately 10 m. More susceptible to noise and voltage drop on longer runs than 4-20 mA.

SSI (Synchronous Serial Interface) Digital Output

SSI is a point-to-point synchronous serial protocol used by industrial motion controllers, servo drives, and high-end PLCs. The controller sends clock pulses; the sensor shifts out an absolute position word synchronously; word length, Gray/binary code and resolution are set in the ordering code (e.g. SS25G005F on SGM-PR10 / SGM-PE10). This is the preferred output for high-speed servo press and injection molding position control. Timing, Gray code decoding, cable length and wiring are explained in what SSI output is on a position sensor.

RS485 Modbus RTU

Modbus RTU over RS485 is the dominant protocol in industrial SCADA systems, tank gauging networks, and process control installations. Up to 32 sensors share a two-wire RS485 bus (up to 247 with repeaters). Position is read from Modbus holding registers using standard function codes. This is the preferred output for liquid level gauging networks and process instrumentation.

Applications Where Magnetostrictive Sensors Excel

Hydraulic Presses, Metal Forming, and Injection Molding

These are the highest-volume applications for rod-type magnetostrictive sensors. Position-controlled hydraulic presses use the sensor for: accurate depth control; automatic ram speed ramping at end-of-stroke (to reduce impact and noise); multi-point pressure-position profiling; and die protection (stopping the ram if contact is detected at the wrong position). Injection molding machines use the sensor to control screw position during injection, pack, and hold phases. In both applications, SSI output to a servo press controller or PLC motion module is standard.

Liquid Level Measurement in Storage Tanks

Rod-type and flexible-cable magnetostrictive sensors are used for precise liquid level measurement in process and storage tanks, including interface level detection where two immiscible liquids (e.g., water and oil) are present. A dual-float configuration (SEGMENsensor SE-LX20 / SE-LX30) uses two floats on a single waveguide: one float rides the upper surface and one the interface, providing simultaneous level and interface readings. This is significantly more accurate than radar or guided wave radar for short-range level measurements in small tanks (<3 m diameter).

Medical Equipment and Surgical Robotics

Magnetostrictive sensors are used in medical imaging positioning tables, surgical robot linear axes, and rehabilitation equipment where precise, repeatable, non-contact position feedback is required in a clean environment. The non-contact principle and IP67 rating allow the sensor to operate in sterilization environments.

Semiconductor Wafer Handling

In semiconductor wafer handlers, the combination of sub-millimetre absolute position accuracy, non-contact operation (no particle generation), and SSI digital output makes magnetostrictive sensors suitable for linear positioning of wafer carriers and end-effectors in clean room environments.

SEGMENsensor Magnetostrictive Linear Position Sensors

SEGMENsensor designs and manufactures magnetostrictive linear position sensors for industrial OEM customers across hydraulic, process, and automation industries. Our SGM / SE range covers in-cylinder rod (SGM-PR10, PF10, PF20, PR40, PR50), profile (SGM-PE10, SGM-PE20), IP68 stainless (SGM-PR20), intrinsically safe (SE-PR30) and level (SGM-LS10, LC10, LK10, SE-LX10/20/30) versions.

Standard specifications:

• Stroke range: 25 mm – 8,000 mm (standard); custom lengths on request
• Non-linearity: <±0.01% FS, min ±50 µm (SGM-PR10, PE10, PR20, PE20, PF10)
• Resolution: 16-bit D/A (analog output)
• Repeatability: <±0.001% FS, min ±5 µm (SGM-PR10 family); ±0.005 mm (SGM-PF20, PR40, PR50, LS10, LK10)
• Output: 4-20 mA, 0-20 mA, 0-10 V, 0-5 V, RS485 Modbus RTU, SSI (per series)
• Operating temperature: -40°C to +85°C
• Protection: IP67 or IP68 per series
• Hydraulic pressure (in-cylinder): 35 MPa continuous, 70 MPa peak
• Certifications: ISO 9001, CE, RoHS; flameproof SE-LX and intrinsically safe SE-PR30 versions
• Samples: 1–5 pcs in 5–7 working days; production lead time and MOQ are confirmed with the quotation

If you are comparing magnetostrictive and LVDT technology for your application, see our detailed comparison: LVDT vs Magnetostrictive Sensor — Which Should You Specify?

To download product datasheets and dimensional drawings for SEGMENsensor magnetostrictive sensors, visit the magnetostrictive sensor downloads page.

Which SEGMENsensor Magnetostrictive Model Fits Each Application?

Every application described in this article maps to a published SGM or SE series. Choose the mounting first (inside a hydraulic cylinder, outside on the machine, or in a tank), then the stroke and the output signal. Ranges, outputs and ratings below are taken from each series page and datasheet; the exact output options for one part number are confirmed in the ordering code.

All series above operate from −40 to +85 °C; the SE-LX rod has a −40 to +250 °C high-temperature version. The standard supply is 24 V DC, with 12 V and 5 V ranges depending on output and model. To get a quotation, send the stroke or tank height, mounting (in-cylinder, external or tank flange), output signal, supply voltage, medium, operating pressure and temperature, hazardous-area requirement, application and estimated annual volume. Qualification samples of 1–5 pcs ship in 5–7 working days; production lead time and MOQ are confirmed with the quotation.

Frequently Asked Questions