Position sensing is a critical function with a wide range of applications, from robot drive chains and conveyor belts in supply chain operations to the swaying of wind turbine towers. It can take many forms, including linear, rotational, angular, absolute, incremental, contact, and non-contact sensors. Specialized sensors have been developed to determine three-dimensional position. Position sensing technologies include potential, inductive, eddy current, capacitive, magnetostrictive, Hall effect, fiber optic, optical, and ultrasonic sensors.
This FAQ briefly introduces the various forms of location awareness, and then reviews a range of technologies that designers can choose from when implementing location-aware solutions.
There are many factors to consider when selecting a position sensor. Some examples include:
• Position measurement can be linear, rotational, or angular, and can be static or dynamic (measuring velocity and/or acceleration).
• Linear sensors are typically limited to a specific measurement range, while rotational sensors typically provide measurements in revolutions or degrees.
These sensors can be based on contact or non-contact technologies. Contact sensors are often cheaper, while non-contact sensors are more reliable.
• The resolver is a dedicated non-contact rotation sensor that can provide position and velocity feedback.
Some sensors provide only incremental measurements from one point to another, while others provide absolute position information relative to a specific reference point.
Potential position sensor
Potential position sensors are resistance-based devices that combine a fixed resistor track with a windshield wiper attached to an object to be measured. Movement of the object causes the wiper to move along the track. The object's position is measured using a fixed DC voltage, with the track and wiper forming a voltage divider network to measure linear or rotational motion. Potential sensors are inexpensive but typically have low accuracy and repeatability.
Position sensor
Inductive position sensors utilize changes in the characteristics of a magnetic field induced in a sensor coil. Depending on their structure, they can measure linear or rotational position. Linear variable differential transformer (LVDT) position sensors use three coils wound around a hollow tube: one primary coil and two secondary coils. The primary and secondary coils are connected in series with a correlation coefficient relative to the primary coil, 180 degrees apart. A ferromagnetic core, called a heater, is placed inside the tube and connected to the position of the object being measured. Applying an excitation voltage to the primary coil causes the secondary coils to generate an electromagnetic force. By measuring the voltage difference between the secondary coils, the object to which it is attached can be determined. Rotating voltage differential transformers (RVDTs) use the same technology to track rotational position. LVDT and RVDT sensors offer good accuracy, linearity, resolution, and high sensitivity. They are frictionless and can be sealed for use in harsh environments.
Eddy current position sensor
Eddy current position sensors work in conjunction with conductive objects. Eddy currents are induced currents that occur in conductive materials under changing magnetic fields. These currents flow in a closed loop, generating a secondary magnetic field. An eddy current sensor consists of a coil and a linearization circuit. Alternating current enables the coil to generate a primary magnetic field. As an object moves closer to or further from the coil, its position is sensed by the interaction of the secondary magnetic field generated by the eddy currents; this interaction affects the coil's impedance. When an object approaches the coil, eddy current losses increase, and the oscillation voltage decreases. The oscillation voltage is corrected and processed by the linearization circuit to produce a linear DC output proportional to the target distance.
Eddy current devices are robust, non-contact sensors and are frequently used as proximity sensors. They are omnidirectional and can determine relative distance to an object, rather than its direction or absolute distance.
Capacitive position sensor
As the name suggests, capacitive position sensors measure changes in capacitance to determine the position of an object. These non-contact sensors can be used to measure linear or rotational positions. They consist of two plates separated by a dielectric material and use one of two methods to detect the object's position:
• Change the dielectric constant of the capacitor
• Change the overlap area of the capacitor plates
To induce a change in dielectric constant, an object whose position is to be detected is attached to the dielectric material. As the dielectric material moves, the effective dielectric constant of the capacitor changes due to variations in the dielectric material area and the dielectric constant of air. Alternatively, the object can be attached to one of the capacitor plates. As the object moves, the plates move closer or further away, and the change in capacitance is used to determine the relative position.
Sensors can measure the displacement, distance, position, and thickness of objects. Due to their signal stability and high resolution, capacitive displacement sensors are used in laboratory and industrial environments. For example, capacitive sensors are used to measure film thickness and adhesive applications in automated processes. In industrial machines, they are used to monitor displacement and tool position.
Magnetostrictive position sensor
Magnetostriction is a property of ferromagnetic materials that allows them to change size or shape under the influence of a magnetic field. In a magnetostrictive position sensor, a movable position magnet is attached to the object being measured. It includes a waveguide consisting of a wire through which current pulses are transmitted and connected to a sensor located at the end of the waveguide. When the current pulse is emitted along the waveguide, a magnetic field is generated on the wire, which interacts with the axial magnetic field of the permanent magnet. This magnetic field interaction is caused by torsion (the Wiedermann effect), which induces strain in the wire, generating acoustic pulses that propagate along the waveguide and are detected by the sensor at the end of the waveguide. By measuring the time between the initiation of the current pulse and the detection of the acoustic pulse, the relative position of the position magnet and the object can be measured.
Magnetostrictive position sensors are non-contact sensors used to detect linear position. Waveguides are typically housed within stainless steel or aluminum tubes, allowing these sensors to be used in dirty or humid environments.
Hall effect position sensor
When a thin, flat conductor is placed in a magnetic field, any flowing current will pool on one side of the conductor, creating a potential difference called the Hall voltage. If the current in the conductor is constant, the magnitude of the Hall voltage will reflect the strength of the magnetic field. In a Hall effect position sensor, an object is connected to a magnet in the sensor shaft. When the object moves, the position of the magnet changes relative to the Hall element, producing a changing Hall voltage. By measuring the Hall voltage, the position of the object can be determined. Specialized Hall effect position sensors are available to determine three-dimensional position. Hall effect position sensors are non-contact devices that provide high reliability and fast sensing and can operate over a wide temperature range. They are widely used in consumer, industrial, automotive, and medical fields.
Fiber optic position sensor
There are two basic types of fiber optic sensors. In fiber optic sensors, the optical fiber serves as the sensing element. In external fiber optic sensors, the optical fiber is combined with another sensor technology to relay the signal to a remote electronic device for processing. In the case of fiber optic position measurement, devices such as optical time-domain reflectometers can be used to determine the time delay. Wavelength displacement can be calculated using an optical frequency-domain reflectometer. Fiber optic sensors are immune to electromagnetic interference, can be designed to operate at high temperatures, and are non-conductive, allowing them to be used near high voltages or flammable materials.
Another fiber optic sensing technology based on fiber Bragg gratings can also be used for position measurement. A fiber Bragg grating acts as a notch filter, reflecting a small portion of light (B) around the Bragg wavelength when illuminated by broad-spectrum light. It is made of microstructures embedded in the core of an optical fiber. FBGS can be fabricated to measure various parameters such as temperature, strain, pressure, tilt, displacement, acceleration, and load.
Optical position sensor
There are two types of optical position sensors, also known as optical encoders. In one type, light is sent to the sensor at the other end of a receiver. In the second type, the emitted light signal is reflected from the monitored object and then returns to the light source. Depending on the sensor's design, changes in light characteristics, such as wavelength, intensity, phase, or polarization, are used to determine the object's position. Encoding-based optical position sensors can be used for both linear and rotational motion. These sensors are divided into three main categories: transmissive optical encoders, reflective optical encoders, and interference optical encoders.
Ultrasonic position sensor
Ultrasonic position sensors use piezoelectric crystal sensors to emit high-frequency ultrasonic waves. The sensor measures the reflected sound. Ultrasonic sensors can be used as simple proximity sensors, or in more complex designs to provide ranging information. Ultrasonic position sensors work with target objects of various materials and surface properties, and can detect small objects at greater distances than many other types of position sensors. They are resistant to vibration, ambient noise, infrared radiation, and electromagnetic interference. Applications using ultrasonic position sensors include liquid level detection, high-speed object counting, robot navigation systems, and automotive sensing. A typical automotive ultrasonic sensor consists of a plastic housing, a piezoelectric transducer with a membrane, and a printed circuit board with electronic circuitry and a microcontroller for transmitting, receiving, and processing signals.
Position sensors can measure the absolute or relative linear, rotational, and angular motion of objects. They can also measure the motion of devices such as actuators or electric motors. They are also used in mobile platforms such as robots and automobiles. A wide variety of technologies are used in position sensors, offering various combinations of environmental robustness, cost, accuracy, repeatability, and other properties.
