# Linear Position Sensor Selection: A Comprehensive Guide for Industrial Automation and Process Control
Introduction
Selecting the right linear position sensor is critical for the efficiency and reliability of industrial automation and process control systems. Linear position sensors convert the physical position of an object into an electrical signal, enabling precise monitoring and control in various applications, from hydraulic cylinders to CNC machines. The choice of sensor impacts not only the accuracy and resolution of the measurement but also the overall system performance and maintenance requirements.
This article delves into the technical aspects of linear position sensor selection, focusing on three primary types: Linear Variable Differential Transformers (LVDTs), magnetostrictive sensors, and wire-draw (string pot) sensors. We will explore their operating principles, stroke lengths, resolution, linearity, output signal configurations, mounting options, and environmental ratings. By the end of this guide, you will be equipped to make an informed decision when choosing a linear position sensor for your specific application.
Operating Principles of Linear Position Sensors
1. LVDT (Linear Variable Differential Transformer)
The LVDT operates on the principle of electromagnetic induction. It consists of a primary coil excited by an AC carrier signal (typically 2-10 kHz) and two secondary coils wound in series opposition. A ferromagnetic core moves through the coil assembly, inducing voltages in the secondary coils. The differential voltage output is proportional to the core’s position, providing an analog signal with theoretically infinite resolution.
Key Features:
- Non-contact operation: Eliminates mechanical wear, ensuring long-term reliability.
- High linearity: Typically ±0.01% to ±0.25% of full stroke.
- Stroke range: Standard ranges from 1mm to 500mm, with extended options up to 1000mm.
- Environmental robustness: Excellent resistance to vibration and shock.
2. Magnetostrictive Sensors
Magnetostrictive sensors, such as the MTS RHS0500MD561E101, utilize the magnetostrictive effect, where a magnetic field induces a mechanical strain in a ferromagnetic material. The sensor consists of a waveguide wire inside a tube and a position magnet that moves along the tube’s exterior. An interrogating pulse travels down the waveguide, and the interaction with the magnetic field creates a torsional strain pulse that returns to the sensor head at the speed of sound in the material (~2830 m/s). The time-of-flight measurement provides an absolute position with high accuracy.
Key Features:
- Non-contact operation: Ensures durability and longevity.
- High linearity: Typically ±0.01% or better.
- Stroke range: From 25mm to 5000mm.
- Environmental resistance: Standard IP67 rating, suitable for harsh conditions.
3. Wire-Draw (String Pot) Sensors
Wire-draw sensors, like the Linear Potentiometer MIDORI LP-150F-C, use a spring-loaded cable that extends from the sensor body. The cable’s position is measured by a precision potentiometer or encoder. This design allows for a compact sensor body regardless of the stroke length, which can range from 50mm to 20000mm.
Key Features:
- Mechanical contact: Provides a cost-effective solution but with limited life (10-100 million cycles).
- Linearity: Typically ±0.025% to ±0.05%.
- Stroke range: Extensive, up to 20000mm.
- Ease of retrofit: Suitable for hydraulic cylinder applications.
Comparison of Linear Position Sensor Types
| Feature | LVDT | Magnetostrictive | Wire-Draw (String Pot) |
|---|---|---|---|
| Operating Principle | Electromagnetic induction | Magnetostrictive effect | Mechanical displacement |
| Stroke Range | 1mm – 500mm (standard), up to 1000mm | 25mm – 5000mm | 50mm – 20000mm |
| Linearity | ±0.01% to ±0.25% | ±0.01% or better | ±0.025% to ±0.05% |
| Resolution | Theoretical infinite, practical 14-16 bit | 1-16 micron depending on stroke | 0.01-0.05% of full stroke |
| Contact/Non-contact | Non-contact | Non-contact | Mechanical contact |
| Environmental Resistance | Excellent | Very good (IP67 standard) | Limited by spring mechanism |
| Cost | Moderate to high | Moderate to high | Low to moderate |
| Typical Applications | High-precision applications, laboratory use | Hydraulic cylinders, industrial automation | Hydraulic cylinder retrofit, long-stroke applications |
Output Signal Configuration
The output signal is a crucial aspect of linear position sensor selection. The choice impacts the system’s compatibility, noise immunity, and power requirements.
1. Analog Voltage Output
Analog voltage outputs, such as 0-5V, 0-10V, ±5V, and ±10V, are commonly used in applications where the sensor is close to the control system. They offer high resolution and are straightforward to interface with most analog-to-digital converters (ADCs).
Key Considerations:
- Load impedance: Typically >10kOhm to ensure accurate readings.
- Voltage drop: Longer cable runs can lead to voltage drop, affecting accuracy.
- Noise susceptibility: More susceptible to electromagnetic interference (EMI) compared to current outputs.
2. Analog Current Output
Analog current outputs, such as 4-20mA and 0-20mA, are preferred in industrial environments due to their inherent noise immunity and ability to transmit over long distances without significant signal loss.
Key Features:
- Loop-powered: Eliminates the need for a separate power source.
- Immunity to voltage drop: Current output remains constant regardless of cable length.
- Robustness: Better suited for harsh environments with high EMI.
3. Digital Output
Digital outputs, such as SSI (Synchronous Serial Interface) and IO-Link, offer high resolution and advanced features like parameter access and diagnostics.
SSI (Synchronous Serial Interface):
- Resolution: 12-24 bits.
- Advantages: High-speed data transmission, low latency.
- Applications: Magnetostrictive sensors in high-precision applications.
IO-Link:
- Resolution: 16-bit process data.
- Advantages: Real-time parameter access, diagnostics, and configuration.
- Displacement Sensor MARPOSS A120 3415159605 Precision utilizes IO-Link for enhanced functionality.
4. Digital Start-Change
This output type is specific to magnetostrictive sensors and provides a pulse train proportional to the position.
Key Features:
- Absolute position: Provides absolute position data.
- Simplicity: Easy to interface with microcontrollers.
Resolution and Linearity
Resolution
Resolution refers to the smallest change in position that the sensor can detect. LVDTs, with their analog principle, theoretically offer infinite resolution, but practical limitations due to the accompanying electronics (typically 14-16 bit ADC) mean that the actual resolution is finite. Magnetostrictive sensors offer resolutions ranging from 1 to 16 microns, depending on the stroke length. Wire-draw sensors have the lowest resolution, typically 0.01-0.05% of the full stroke.
Linearity
Linearity is a measure of the sensor’s accuracy in measuring position. It is usually specified as a percentage of the full-scale output.
Independent Linearity vs. Terminal Linearity:
- Independent Linearity (Best Fit Straight Line): The deviation from a best-fit straight line over the measurement range.
- Terminal Linearity (End-Point to End-Point): The deviation from a straight line connecting the end points of the measurement range.
Key Considerations:
- LVDT: Typically ±0.01% to ±0.25% independent linearity.
- Magnetostrictive: Typically ±0.01% or better independent linearity.
- Wire-Draw: Typically ±0.025% to ±0.05% terminal linearity.
Mounting and Mechanical Interface
LVDT
LVDTs typically have a cylindrical body with threaded mounts or clamp brackets. The core rod extends from the measurement object, and a core guide prevents side loading, ensuring accurate measurements.
Magnetostrictive
Magnetostrictive sensors have a tube body with flange or thread mounts. The position magnet slides on a separate rod or attaches directly to a piston, allowing for flexible installation options.
Wire-Draw
Wire-draw sensors have a fixed body mount with the cable end attaching to the moving object. They can be configured with either a spring return or a fixed cable attachment, depending on the application.
Environmental Ratings
Environmental ratings are critical for ensuring the sensor’s reliability in various operating conditions.
- IP64: General industrial use, protected against dust and splashing water.
- IP65: Dust-tight and protected against water jets.
- IP67: Submersible in 1m of water for up to 30 minutes.
- IP68: Continuous submersion in water.
- Operating Temperature: Industrial sensors typically operate from -40 to +85°C, while standard models operate from -20 to +70°C.
- Vibration Resistance: LVDTs offer excellent vibration resistance due to their non-contact operation. Magnetostrictive sensors also exhibit good vibration resistance. Wire-draw sensors are more limited due to their mechanical components.
- Shock Resistance: Typical shock resistance ranges from 50-100G.
Application Selection
Hydraulic Cylinder Position
For hydraulic cylinder position feedback, the MTS RHS0500MD561E101 magnetostrictive sensor is an excellent choice. Its built-in design and 4-20mA output make it ideal for industrial automation systems.
Press Ram Position
The Displacement Sensor MARPOSS 767X000210 12DP6739 Industrial LVDT is suitable for press ram position applications, offering high accuracy with ±0.01% linearity.
Die Casting Machine
In die casting machines, the Magnetostrictive Sensor is preferred due to its high-temperature resistance and ability to withstand harsh conditions, often requiring water cooling.
Wood Saw Positioning
For wood saw positioning, the Linear Potentiometer MIDORI LP-150F-C wire-draw sensor offers a cost-effective solution with a long stroke of up to 3000mm.
Test Stand Displacement
In test stand applications requiring sub-micron resolution, the Displacement Sensor MARPOSS A120 3415159605 Precision LVDT is the ideal choice, providing laboratory-grade accuracy.
FAQ Section
Q1: What is the primary advantage of LVDTs over other linear position sensors?
A1: The primary advantage of LVDTs is their non-contact operation, which eliminates mechanical wear and ensures long-term reliability. Additionally, they offer high linearity and resolution, making them suitable for precision applications.
Q2: How does the magnetostrictive principle differ from the LVDT principle?
A2: The magnetostrictive principle relies on the interaction between a magnetic field and a waveguide wire to produce a torsional strain pulse, which is measured by time-of-flight to determine position. In contrast, LVDTs use electromagnetic induction to generate a differential voltage output proportional to the core’s position.
Q3: What is the typical stroke range for wire-draw sensors?
A3: Wire-draw sensors typically have a stroke range from 50mm to 20000mm, making them suitable for long-stroke applications.
Q4: What is the difference between independent and terminal linearity?
A4: Independent linearity refers to the deviation from a best-fit straight line over the measurement range, while terminal linearity is the deviation from a straight line connecting the end points of the measurement range.
Q5: What are the environmental ratings for industrial sensors?
A5: Common environmental ratings include IP64, IP65, IP67, and IP68, with IP67 being the most common for industrial applications, indicating submersion in 1m of water for up to 30 minutes.
Conclusion
Choosing the right linear position sensor involves careful consideration of the operating principle, stroke length, resolution, linearity, output signal configuration, mounting options, and environmental ratings. By understanding the strengths and limitations of LVDTs, magnetostrictive, and wire-draw sensors, you can select the most appropriate sensor for your specific application. For further exploration of these sensors, consider exploring the Displacement Sensor MARPOSS 767X000210 12DP6739 Industrial, Linear Position Sensor MTS RHS0500MD561E101, and Displacement Sensor MARPOSS A120 3415159605 Precision for more in-depth insights.
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