POSITION TRANSDUCERS

INTRODCTION  A position transducer is a device that permits position measurement.  Position Sensors detect the position of an object, which means that they are referenced either to or from some fixed point or position.  These types of sensors provide a “positional” feedback.

POSITION TRANSDUCERS  Position Sensors can detect the movement of an object in a straight line (linear movement) using Linear Sensors or by its angular movement using Rotational Sensors.

POSITION TRANSDUCERS Some position sensors available are:  Potentiometer  LVDT (Linear variable differential transformer)  Strain gauge  Capacitance  Encoders

POTENTIOMETER  The most commonly used of all the “Position Sensors”, is the potentiometer because it is an inexpensive and easy to use position sensor.  A potentiometer is a three- terminal resistor with a sliding or rotating contact that forms an adjustable voltage divider.

POTENTIOMETER OPERATION  Potentiometers comprise a resistive element, a sliding contact (wiper) that moves along the element, a mechanical shaft which is linked to the wiper, two electrical terminals at each end of the element.

POTENTIOMETER OPERATION  The wiper contact is linked to a mechanical shaft that can be either angular (rotational) or linear (slider type) in its movement, and which causes the resistance value between the wiper and the two end connections to change giving an electrical signal output.  The electrical output signal has a proportional relationship between the actual wiper position on the resistive track and its resistance value. In other words, resistance is proportional to position.

LVDT(Linear variable differential transformer)  LVDT is a type of electrical transformer used for measuring linear displacement (position).  This is an inductive type position sensor which works on the same principle as the AC transformer that is used to measure movement.

LVDT OPERATION

 The LVDT's primary winding, P, is energized by a constant amplitude AC source. The magnetic flux thus developed is coupled by the core to the adjacent secondary windings, S1 and S2.  If the core is located midway between S1 and S2, equal flux is coupled to each secondary so the voltages, E1 and E2, induced in windings S1 and S2 respectively, are equal.  At this reference midway core position, known as the null point, the differential voltage output, (E1 - E2), is essentially zero.

LVDT OPERATION  If the core is moved closer to S1 than to S2, more flux is coupled to S1and less to S2, so the induced voltage E1 is increased while E2 is decreased, resulting in the differential voltage (E1 - E2).  Conversely, if the core is moved closer to S2, more flux is coupled to S2 and less to S1, so E2 is increased as E1 is decreased, resulting in the differential voltage (E2 - E1).

LVDT OPERATION

STRAIN GUAGE  A Strain gage (sometimes referred to as a Strain Gauge) is a sensor whose resistance varies with applied force.  It converts force, pressure, tension, weight, etc., into a change in electrical resistance which can then be measured.  A strain gauge takes advantage of the physical property of electrical conductance

STRAIN GUAGE  When an electrical conductor is stretched within the limits of its elasticity such that it does not break, it will become narrower and longer, changes that increase its electrical resistance end-to- end.  Also, when a conductor is compressed such that it does not buckle, it will broaden and shorten, changes that decrease its electrical resistance end-to-end.  From the measured elastic resistance of the strain gauge, the amount of applied force may be deduced.

STRAIN GUAGE OPERATION  The majority of strain gauges are foil types, They consist of a pattern of resistive foil which is mounted on a backing material.  They operate on the principle that as the foil is subjected to stress, the resistance of the foil changes in a defined way.

CAPACITANCE TRANSDUCERS  Capacitive displacement sensors are non-contact devices of high resolution measurement of the position of any conductive target.  Non-contact capacitive sensors measure changes in capacitance.  Capacitance describes how two objects with a space between them respond to a voltage difference applied to them.

BASIC CAPACITIVE THEORY  Capacitance is an electrical property which is created by applying an electrical charge to two conductive objects with a gap between them.  A simple demonstration is two parallel conductive plates of the same profile with a gap between them and a charge applied to them.

BASIC CAPACITIVE THEORY

ENCODERS  Encoders are sensors that generate digital signals in response to movement.  An encoder is a device (transducer) that is used to convert rotary or linear motion into useful information. The primary parameters determined are speed, rate, velocity, distance, position, or direction.

CLASSIFICATION OF ENCODERS  Classification Based On Motion Rotary Encoders and Linear Encoders  Classification Based On Output Incremental Encoders and Absolute Encoders  Classification Based On Sensing Technology Optical and magnetic Encoders

CLASSIFICATION BASED ON MOTION  Rotary Encoders: which respond to rotation  Linear Encoders: which respond to motion in a line

ROTARY ENCODERS  Rotary Encoders are non-contact optical devices used for converting the angular position of a rotating shaft into digital data code.  They convert mechanical movement into an electrical signal (preferably digital).

LINEAR ENCODERS  A linear encoder is a sensor paired with a scale that encodes position.  The sensor reads the scale in order to convert the encoded position into a digital signal which can then be decoded into position by a digital readout (DRO) or motion controller.

CLASSIFICATION BASED ON OUTPUT  Incremental Encoders:  They are a measure of the angular or linear distance moved  Incremental encoders generate a precisely defined number of pulses per revolution  Absolute Encoders:  Absolute encoders generate multi-bit digital words that indicate actual position directly.  provide an absolute numerical value for each angular position even over several revolutions

CLASSIFICATION BASED ON SENSING TECHNOLOGY  Optical Encoders:  Optical encoders use a glass disk with a pattern of lines deposited on it, a metal or plastic disk with slots (in a rotary encoder), or a glass or metal strip (in a linear encoder).  Light from an LED shines through the disk or strip onto one or more photo detectors, which produce the encoder’s output.  An incremental encoder has one or more of these tracks, while an absolute encoder has as many tracks as it has output bits.

CLASSIFICATION BASED ON SENSING TECHNOLOGY  Magnetic Encoders:  Variable reluctance sensors detect changes in the magnetic field caused by the presence or movement of a ferromagnetic object.  The simplest variable-reluctance rotary sensor, often called a magnetic pickup, consists of a coil wound around a permanent magnet. This generates a voltage pulse when a gear tooth moves past it.

CLASSIFICATION BASED ON SENSING TECHNOLOGY  Another type of sensor uses a permanent magnet and a Hall effect or magneto-resistive device to produce a change in either voltage or electrical resistance in the presence of ferromagnetic material.  It can be in the form of a gear tooth (in a rotary encoder) or a metal band with slots (in a linear encoder).

CLASSIFICATION BASED ON SENSING TECHNOLOGY  Magnetic sensing, often used in such rugged applications as steel and paper mills, provides good resolution, high operating speeds, and maximum resistance to dust, moisture, and thermal and mechanical shock.  It is very resistant to dust, grease, moisture, and other contaminants common in industrial environments, and to shock and vibration.

APPLICATION OF ENCODERS  Robotics  Computers Systems e.g. Mouse  Aerospace  Testing Machines  Medical Equipment

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