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Motion Control Feedback Devicequadrature encoder Motion control Motion control can be simply defined as the precise control of anything that moves. The system consists of advance motion controllers, wiring and connectivity devices, motor drive units, software tools and interface to third party devices. Controllers generate trajectories, which the motor follows. Drives then take the signals sent by the controller and change them into signals that will actually move the motor. Feedback devices are used to close the control loop in closed-loop systems.Components of a Motion Control System:

Mechanical elements -Motors are designed to provide torque to some mechanics. These include linear slides, robotic arms, and special actuators.

Feedback device or position sensor -A position feedback device is not required for some motion control applications (such as controlling stepper motors), but is vital for servo motors. The feedback device, usually a quadrature encoder, senses the motor position and reports the result to the controller, thereby closing the loop to the motion controller.

OverviewIncremental encoders provide a specific number of equally spaced pulses per revolution (PPR) or per inch or millimeter of linear motion. A quadrature encoder is a type of incremental encoder used in many general automation applications where sensing the direction of movement is required.How does a Quadrature Encoder work?The code disk inside a quadrature encoder contains two tracks usually denoted Channel A and Channel B. These tracks or channels are coded ninety electrical degrees out of phase, as indicated in the image below, and this is the key design element that will provide the quadrature encoder its functionality. In applications where direction sensing is required, a controller can determine direction of movement based on the phase relationship between Channels A and B. As illustrated in the figure below, when the quadrature encoder is rotating in a clockwise direction its signal will show Channel A leading Channel B, and the reverse will happen when the quadrature encoder rotates counter clockwise.

Apart from direction, position can also be monitored with aquadrature encoderby producing another signal known as the marker, index or Z channel. This Z signal, produced once per complete revolution of the quadrature encoder, is often used to locate a specific position during a 360 revolutionWhen to use Quadrature Encoders?Quadrature encoders are used in bidirectional position sensing and length measuring applications. However, in some unidirectional start-stop applications, it is important to have bidirectional information (Channel A & B) even if reverse rotation of the shaft is not anticipated. An error in count could occur with a single-channel encoder due to machine vibration inherent in the system. For example, an error in count may occur with a single-channel encoder in a start/stop application if it mechanically stops rotating when the output waveform is in transition. As subsequent mechanical shaft vibration forces the output back and forth across the edge the counter will up-count with each transition, even though the system is virtually stopped. By utilizing a quadrature encoder, the counter monitors the transition in its relationship to the state of the opposite channel, and can generate reliable position information.Achieving higher resolution with Quadrature EncodersWhen more resolution is needed, it is possible for the counter to count the leading and trailing edges of the quadrature encoders pulse train from one channel, which doubles (x2) the number of pulses per revolution. Counting both leading and trailing edges of both channels of a quadrature encoder will quadruple (x4) the number of pulses per revolution. As a result, 10,000 pulses per turn can be generated from a 2,500 PPR quadrature encoder. Typically with a Dynapar encoder, this 4x signal will be accurate to better than 1 count.

Absolute rotary encoderAn "absolute" encoder maintains position information when power is removed from the system. The position of the encoder is available immediately on applying power. The relationship between the encoder value and the physical position of the controlled machinery is set at assembly; the system does not need to return to a calibration point to maintain position accuracy. An "incremental" encoder accurately records changes in position, but does not power up with a fixed relation between encoder state and physical position. Provides a unique value for every shaft or linear position

Absolute encoders retain their position after a power cycle

Signals typically use SSI, parallel, or field businterfaces (Ethernet/IP, EtherCAT, Profinet, Devicenet, CANopen, Profibus, etc)

Absolute encoders are used in applications were position information is necessary

Code disc for absoluteencoders

LEDLensScanning maskCode discPhoto-ElementsAbsoluteConstruction of absolute encoderIt have in two basic types: optical and mechanical.Mechanical absolute encodersA metal disc containing a set of concentric rings of openings is fixed to an insulating disc, which is rigidly fixed to the shaft.A row of sliding contacts is fixed to a stationary object so that each contact wipes against the metal disc at a different distance from the shaft. As the disc rotates with the shaft, some of the contacts touch metal, while others fall in the gaps where the metal has been cut out. The metal sheet is connected to a source of electric current, and each contact is connected to a separate electrical sensor.The metal pattern is designed so that each possible position of the axle creates a unique binary code in which some of the contacts are connected to the current source (i.e. switched on) and others are not (i.e. switched off).Optical absolute encodersThe optical encoder's disc is made of glass or plastic with transparent and opaque areas. A light source and photo detector array reads the optical pattern that results from the disc's position at any one timeThis code can be read by a controlling device, such as a microprocessor or microcontroller to determine the angle of the shaft. The absolute analogue type produces a unique dual analogue code that can be translated into an absolute angle of the shaft.SingleturnMeasures the Absolute position within 1 revolution/turn

MultiturnMeasures the Absolute position within 1 revolution/turnIn Addition, measures the number of revolutions as well.Multi-turn encoders are recommended for applications involving multiple revolutions of the encoder shaft.HOW ABSOLUTE ENCODER WORK?

AbsoluteElectronic InterfaceSSIParallelBUSAbsolute Encoder InterfacesParallel OutputFirst form of communication for absolute encoders

Connection Point-to-point communication where each output wirerepresents a different data bit

BenefitsDirect output to digital inputsFast (60us typically)

DrawbacksComplex cabling due to separate bit wiresHigh cost

Parallel Output

Synchronous Serial Interface (SSI)Very common serial interface standard for industrial applicationsDeveloped by Stegmann in 1984 for absolute encoders now in many products

Connection Point-to-point connection from a master (PLC, microcontroller) to a slave (encoder)

BenefitsDrawbacksSimple cabling, especially compared to Point-to-point connection, topology restrictionsparallel outputsFast communication speedsLow cost

Synchronous Serial Interface (SSI)Advantages of Absolute Non-Volatile Memory. Absolute encoders are non-volatile position verification devices. True position is not lost if the power fails. Continuous reading of position is not required.Safety. In some applications, a loss of position could result in damage to the machinery or injury to the operator. An absolute encoder provides position verification the moment power is applied without requiring movement to a reference position.Noise Immunity. Absolute encoders determine position by continually reading a coded signal. Stray pulses will not accumulate and accurate position is available again on the next reading