Technical guide · GUEMISA
Types of position sensors according to the physical principle
The most commonly used sensors for measuring position or displacement can be linear or angular, and there is a wide variety of both.
| Guy | Operating principle |
|---|---|
| Linear or angular potentiometer | Excited by a DC voltage source; the output voltage varies between positive and negative values of the maximum DC voltage and is proportional to the position relative to a zero reference or displacement. |
| Inductive | Converts a linear displacement into a change in the self-inductance of a winding |
| Capacitive | Variation of a capacitor when the position of its plates or its dielectric changes |
| Greyhounds | Variation in resistance of a conductor or semiconductor subjected to mechanical stress |
| Encoder (linear and angular encoder) | Converts the angular position of a shaft into a digital code; it can be absolute or incremental. |
| LVDT / RVDT | Variation of mutual inductance between a primary and two secondary windings when a ferromagnetic core is displaced |
| Syncros and resolvers | Rotary transformers with variable coupling depending on the rotor; output voltages proportional to the sine or cosine of the angular position |
| Inductosyn | Variation of inductive coupling between printed circuit traces of two elements in relative motion |
| Flight time | Laser and ultrasonic rangefinders, which calculate distance by collecting the reflection of a beam of light or a sound wave |
Potentiometers, inductive, capacitive and strain gauges
Different configurations exist to solve different problems: servo potentiometers measure several complete rotations of a shaft, while precision potentiometers have very high linearity and are designed to operate at high temperatures and with vibrations. Potentiometers are inexpensive, but their lifespan is limited by wear from friction between their fixed and moving parts.
| Transducer | Variants and characteristics |
|---|---|
| Inductive, coupled to the part | A permeable core that moves within a coil and changes the inductance of the winding. |
| Inductive, contactless | The proximity variation of the part increases or decreases the magnetic flux and varies the self-inductance of the winding. |
| Capacitive, mobile dielectric | The dielectric, attached to the moving part, moves between the plates |
| Capacitive, moving plates | The moving element is attached to one electrode that slides relative to the other and over the dielectric material. |
| Capacitive, object as electrode | The object must be conductive; it allows for a contactless design. |
| Greyhounds | For small displacements, around 0.4 mm; static element with one or more strain gauges working in tension and compression; little used because it is only applicable to the ranges measurable by the strain gauge, it stands out for its simplicity |
Encoders
A rotary encoder is an electromechanical device that converts the angular position of a shaft into a digital code. It is one of the most popular transducers for measuring angular positions. They can be absolute or incremental.
| Optical encoder type | Operation |
|---|---|
| Components | Light source, coded disc, and light detector |
| Incremental | The disk has two concentric rings of equidistant lines offset from each other; one ring gives a pulse train whose count is proportional to the angular displacement and the second ring determines the direction of rotation; it needs a pulse counter and a digital-to-analog converter |
| Absolute | The position is encoded in binary, GRAY or BCD code on the disk itself using rings of slots. |
| Incremental resolution | It depends on the number of lines per revolution |
| Resolution, absolute | It depends on the number of concentric rings, which depends primarily on the diameter of the disc |
LVDT, RVDT, syncros, resolvers and Inductosyn
The LVDT consists of three coaxial windings on an insulating cylinder: the central one is the primary, and the other two are secondary. It works by measuring the variation in mutual inductance between the primary and each secondary winding as a ferromagnetic core moves inside the cylinder, driven by a non-ferromagnetic stem attached to the part being measured. Its main advantage is that there is no mechanical wear, which eliminates electrical noise produced by contacts.
| Device | Description |
|---|---|
| ACLVDT | The 3 windings with ferromagnetic core, operated by AC |
| DCLVDT | LVDT with the necessary electronics to generate a sinusoidal carrier signal from an input DC voltage and demodulate the output to obtain a DC voltage proportional to the offset |
| RVDT | LVDT modification for angular positions; laminated stator with four slots, containing the primary coil and the two secondary coils, and a two-pole rotor |
| Syncro transmitter | It converts mechanical rotation into a three-phase voltage system; with AC excitation in the rotor, the amplitudes in the stator are related to the angular position |
| Differential transmitter | Add or subtract your rotor angle from the electrical angle received from the synchro transmitter |
| Pair receiver | It rotates to the angular position transmitted by the voltage amplitudes applied to its stator from a differential transmitter |
| Differential receptor | Its rotor rotates at the sum or difference of the transmitted angles |
| Solve | Two stator windings, 90° out of phase, in which voltages are induced with amplitude proportional to the sine and cosine of the shaft position; it gives two output signals in quadrature |
| Transsolver | It converts the voltage waveforms of a synchro to the waveforms of a resolver and vice versa; rotor and stator can be used interchangeably as inputs |
| Inductosyn | Printed circuit traces in the shape of a square wave on two flat steel substrates, without contact between them and therefore without wear; it is used in linear and angular measurement, for example in the military and aerospace fields. |
In instrumentation, with small magnitudes, torque synchros are used; when high precision is required, control synchros are used.
Presence and time-of-flight sensors
In industry, presence sensors are often used, placed at the points whose position is measured, with or without contact.
| Switch | Principle |
|---|---|
| Electromechanical and limit switches | Pushbuttons that close or open a circuit upon contact with an object |
| Capacitive and inductive | When faced with an object, they modify their capacitance or inductance, respectively. |
| Ultrasonic | They emit an echo that the object returns. |
| Foucault currents | The metallic object absorbs the electromagnetic field of the sensor winding through eddy currents. |
| Hall effect | Difference that appears between opposite faces of a semiconductor prism under a perpendicular magnetic field when current flows |
| Photoelectric | The light source and sensor detect objects that do or do not block the path of the light. |
| Reed | Metal plate that closes the contact in the presence of a magnet or magnetic field. |
Laser rangefinders calculate distance by collecting the reflection of a structured light beam, while ultrasonic rangefinders collect the reflection of a sound wave. Optical sensors also employ triangulation with a camera. Other solutions include differential capacitors and magnetoresistors.
Frequently Asked Questions
What is the simplest position transducer?
A linear or angular potentiometer excited by a DC voltage source, with output voltage proportional to the position relative to a reference.
What limits the lifespan of a potentiometer?
The wear produced by friction between its fixed part and its moving part.
What is the fundamental advantage of an LVDT?
It has no mechanical wear problems, which eliminates the electrical noise produced by the contacts.
What types of encoders exist?
They can be absolute or incremental. Incremental storage uses pulses and requires a counter; in absolute storage, the position is encoded on the disk in binary, Gray code, or BCD.
What is the difference between LVDT and RVDT?
The LVDT measures linear variations with three coaxial windings and a ferromagnetic core; the RVDT is its modification for angular positions, with a laminated stator and a two-pole rotor.
Related
- LVDT: what it is and how the sensor works
- Incremental encoder: operating principle
- Potentiometric, magnetostrictive or LVDT: which one to choose
- Displacement
- All technical information about GUEMISA
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