Image Credits: Allen-Bradley/Rockwell Automation | Capacitec, Inc. | Columbia Research Labs, Inc.| Measurement Specialties, Inc.

 

Linear position sensors use contact or non-contact methods to measure the speed and/or position of an object.

 

How Linear Position Sensors Work

Linear position sensors use a variety of technologies to detect and measure an object's position and movement. The general mechanism for most sensors involves emittance which is interfered with by the target. The interference is recorded and translated into a measure of distance or velocity. Types of linear position sensors are distinguished based on their specific functionality.

 

Selection

 

When selecting linear position sensors, it is most important to distinguish between the different types of sensing technology. Beyond this, industrial buyers should consider performance specifications, output types, and various features.

 

Types of Linear Position Sensors

There are a variety of different linear position sensor technologies for industrial buyers to distinguish between during selection.

  • Capacitance sensors are noncontact sensors which function by measuring the voltage difference applied between the sensor and its target. They can be used on conductive and nonconductive target materials, but can be sensitive to environmental parameters that change the dielectric constant of the medium between the sensor and the target (usually air).
  • Eddy current sensors are noncontact sensors which contain two coils, one active coil that is influenced by the presence of a conducting target, and a second coil that completes a bridge circuit and provides temperature compensation. As the target comes closer to the probe, the eddy currents become stronger, which changes the impedance of the active coil and causes a bridge unbalance related to the target position.
  • Photoelectric sensors, including fiber optic, optical triangulation, and optical time of flight, function by using the projection and detection of light. Reflective properties of the target and environment are important considerations in the choice and use of photoelectric sensors.
  • Ultrasonic sensors function by using the projection and detection of sound. The distance between the sensor and the target is calculated from an acoustic signal's return time and the propagation velocity of the measurement medium.
  • Inductive, Hall Effect, magnetoresistive, magnetostrictive, and variable reluctance sensors measure the disruption of magnetic fields.
  • Linear potentiometers produce a resistance output proportional to an object's displacement or position. The resistance element is excited by either DC or AC voltage, and the output voltage is ideally a linear function of the input displacement.

Output Types

Linear position sensors are available in a variety of different output signal types to suit different application requirements and control architectures.

  • Analog sensors provide an output in the form of either a DC voltage or DC current. These sensors make up the overwhelming majority of sensors used in industrial automation.
  • Time-based digital sensors generate digital pulses, separated by time. The time between pulses is proportional to linear position. They are primarily implemented in magnetostrictive linear position sensors. Output types include start/stop, pulse-width modulated (PWM), and recalculated PWM.
  • Serial digital sensors provide discrete, single stream outputs using numbers via digital protocols including Synchronous Serial Interface (SSI), DeviceNet, Ethernet, and Profibus.
Output Type Advantages Disadvantages
Analog Ease of implementation - analog inputs are common and less expensive on industrial controllers Electrical interference (noise) susceptibility - noise generating devices such as motors, drives, and solenoids can limit effective resolution of the sensor
Ease of troubleshooting - testing and verification done using simple multimeters, requiring no specialized equipment
Time-based Digital Immunity from interference - use of differential line drivers for sending and receiving results in near-immunity to electrical interference Somewhat specialized interfact - generally require a dedicated, purpose-built interface module in a PLC or motion controller
Lower cost - due to lack of need for additional processing electronics, sensors are often less expensive than comparable analog sensors Difficult troubleshooting - some cases require specialized equipment (e.g. oscilloscopes) to test these sensor signals
Serial Digital Built in diagnostics - protocols may allow for real-time status monitoring of sensing. Dedicated bits can be used to indicate operation failure Expense - the additional internal processing required makes sensors more costly than analog or time-based digital counterparts
More information - sensors can provide linear velocity feedback in addition to position, freeing up processing by eliminating need for velocity calculations by controller

Table Information Credit: Sensortech 

Specifications

Proper sensor selection requires the industrial buyer to consider the performance specifications of products.

  • Measurement range describes the range of distances the sensor is designed or able to measure.
  • Accuracy describes the percent deviation from the actual/real value of the measurement.
  • Resolution describes the smallest order of magnitude that a sensor can detect. A device with a higher resolution can make smaller, more detailed measurements.
  • Frequency range is the range of frequencies at which the device is designed to operate, typically described in kHz.

Design Features

When selecting between linear position sensor products, certain design features may also be important to consider.

  • Sensors may be contact or noncontact depending on whether they require physical contact to measure. Contact sensors tend to wear or degrade more over time.
  • Sensors have different body styles which determine mounting characteristics and physical compatibility. These styles include threaded barrel, cylindrical, switch, rectangular, slot, ring, and window.
  • Sensors can be packaged as a raw element or a housed transducer. Self-contained instruments or meters display output at or near the device. Gauges/indicators have an analog display and no electronic output.
  • Devices may have operating temperature ranges (e.g., 32 to 130°F) which specify the range of temperatures in which the device can operate safely and reliably without failure.
  • Sensors can be shielded (protected against EMI and RFI), weld field immune, short circuit protected, and/or intrinsically safe (cannot cause ignition of atmospheric mixtures).

References

 

Durham Instruments - Linear Position Sensor Sub-Categories

Euclid Research - Motion Sensor Reference

Machine Design - Finding the Right Sensor for Linear Displacement

Sensorland.com - How They Work

Sensortech - Linear Position Sensor Output Types