LVDT Linear Displacement Sensor
"The linear variable differential transformer (LVDT) (also called linear variable displacement transformer, linear variable displacement transducer,  or simply differential transformer) is a type of electrical transformer used for measuring linear displacement (position). A counterpart to this device that is used for measuring rotary displacement is called a rotary variable differential transformer (RVDT)." Source: Linear variable differential transformer by Wikipedia Contributors is licensed under CC BY-SA 3.0.
What is a Linear Variable Differential Transformer?
An LVDT is an electromechanical sensor used to transform mechanical motion into a variable electrical signal (current or voltage) and is used for measurement of displacement. They function as actuators for automatic control systems or as mechanical motion sensors in measurement technologies.
LVDT is an acronym for Linear Variable Differential Transformer. LVDT linear position sensors can measure displacements ranging from few microns to centimeters, but are also capable of measuring positions up to ±1 meter.
In short, a LVDT sensor provides output quantity in voltage or current, related to the displacement or movement being measured. LVDT Transducers are prone to external electromagnetic interference.
A linear displacement transducer requires three to four connection wires for power supply and output signal delivery.
Physically, the LVDT Transducer construction is a hollow metallic cylinder in which a shaft of smaller diameter moves freely back and forth along the cylinder’s long axis. The shaft, or pushrod, ends in a magnetically conductive core which must be within the cylinder, or coil assembly, when the device is operating.
In common practice, the pushrod is physically attached to the moveable object whose position is to be determined (the measurand), while the coil assembly is attached to a fixed reference point. Movement of the measurand moves the core within the coil assembly; this motion is measured electrically.
Figure 1: LVDT Linear Displacement Sensor.
A displacement sensor (displacement gauge) is utilized to measure distance between an object and a reference position. In addition to distance range, displacement sensors can be used for dimension measurement to determine one of the three object’s dimension: height, thickness, and width. Selecting the most appropriate sensor according to the application, accuracy, and application environment is paramount. Measurements with displacement sensors can be categorized classified into two large types: non-contact measurement using light or magnetic fields or sound waves, or contact measurement performed in direct contact with the object.
What are the Types of Displacement Sensors?
Amongst several types of displacement transducers, we will highlight four main categories:
Eddy current sensors
“Eddy current sensors uses the principle of eddy current formation to sense displacement. Eddy currents are formed when a moving or changing magnetic field intersects a conductor or vice versa. The relative motion causes a circulating flow of electrons, or currents, within the conductor. These circulating eddies of current create electromagnets with magnet fields that oppose the effect of applied magnetic field. The stronger the applied magnetic field, or greater the electrical conductivity of the conductor, or greater the relative velocity of motion, the greater the currents developed and greater the opposing field. Eddy current probes senses this formation of secondary fields to find out the distance between the probe and target material.” Source: Wikipedia. Source: Eddy Current Sensors by Wikipedia Contributors is licensed under CC BY-SA 3.0.
Eddy current sensors are designed for non-contact measurement of displacement, distance, position, oscillation and vibrations. They are particularly suitable when high precision is required in harsh industrial environments (pressure, dirt, temperature).
Capacitive Displacement sensors
“Capacitive displacement sensors "are non-contact devices capable of high-resolution measurement of the position and/or change of position of any conductive target". They are also able to measure the thickness or density of non-conductive materials. Capacitive displacement sensors are used in a wide variety of applications including semiconductor processing, assembly of precision equipment such as disk drives, precision thickness measurements, machine tool metrology and assembly line testing. These types of sensors can be found in machining and manufacturing facilities around the world.” Source: Capacitive Displacement Sensors by Wikipedia Contributors is licensed under CC BY-SA 3.0.
Capacitive sensors perform non-contact measurements of displacement, distance and position of electrically conductive targets with high precision.
“A laser displacement sensor is a scientific instrument that is used to make non-contact vibration measurements of a surface. The laser beam from the LDV is directed at the surface of interest, and the vibration amplitude and frequency are extracted from the Doppler shift of the reflected laser beam frequency due to the motion of the surface. The output of an LDV is generally a continuous analog voltage that is directly proportional to the target velocity component along the direction of the laser beam.” Source: Laser Dopler Vibrometer by Wikipedia Contributors is licensed under CC BY-SA 3.0.
Laser sensors have set milestones for industrial laser displacement measurement. Whether for displacement, distance or thickness measurement, laser sensors are considered one of the best in their class. These laser sensors are used e.g. in measurement and monitoring tasks in factory automation, electronics production, robotics and vehicle construction.
“An inductive sensor is a device that uses the principle of electromagnetic induction to detect or measure objects. An inductor develops a magnetic field when a current flows through it; alternatively, a current will flow through a circuit containing an inductor when the magnetic field through it changes. This effect can be used to detect metallic objects that interact with a magnetic field. Non-metallic substances such as liquids or some kinds of dirt do not interact with the magnetic field, so an inductive sensor can operate in wet or dirty conditions.” Source: Inductive Sensor by Wikipedia Contributors is licensed under CC BY-SA 3.0.
Inductive sensors for displacement and position measurement from conventional LVDT sensors and inductive sensors. Inductive displacement sensors are used in automated processes, quality assurance, test rigs, hydraulics, pneumatic cylinders and automotive engineering.
FUTEK Advanced Sensor Technology (FUTEK) is a leading manufacturer producing a huge selection of load cell, torque sensors, pressure sensors and electronics, utilizing one of the most advanced technologies in the Sensor Industry: Metal foil strain gauge technology. A Force Transducer is defined as a sensor that converts an input mechanical load, weight, tension, compression or pressure into an electrical output signal (load cell definition). Force sensors are also commonly known as Load Sensors. There are several types of load cells based on size, geometry and capacity.