Wikipedia:VideoWiki/Incremental Encoder

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Introduction

An, incremental encoder, is an electromechanical motion detector. It has two output signals, A and B, which issue pulses when the device moves. Together, these signals indicate the direction of movement and distance traveled.

Types

Incremental encoders are generally classified as either rotary or linear.

Linear type

Linear encoders move along a linear path. Each output pulse corresponds to a constant, linear distance.

Rotary type

In rotary encoders, movement consists of a shaft rotating about a central axis. Each output pulse corresponds to a constant angular change of the shaft.

Quadrature encoding

The pulses are quadrature-encoded, meaning that when the encoder is moving at constant speed, the signals are square waves. with equal on and off times,

Phase difference

and there is a 90 degree phase difference between the, A, and B signals. The phase difference is positive or negative, depending on the direction of movement. For example,

Positive phase difference

In this diagram, A, rises before B, so the phase difference is positive. However,

Negative phase difference

when the encoder moves in the opposite direction, B rises before A, so the phase difference is negative. The direction of movement can be determined by measuring this phase difference.

Sensing method

Incremental encoders employ various techniques to sense movement, and to generate pulses when movement is detected.

Optical encoders

In optical encoders, light is shined on an optical detector such as these. As the encoder moves, the light is alternately blocked and allowed to pass through to the detector.

Example mechanism

In the device shown here, the edge of a transparent disc passes between a light source and detector. As the disc rotates, the black bands on the disc block the light, whereas the gaps between the bands allow light to pass.

Pulse generation

The optical detector will output pulses as the disc alternately blocks and passes light. The frequency of the optical interruptions is proportional to encoder speed. Consequently, encoder speed can be determined by measuring the pulse frequency.

Quadrature generation

An incremental encoder has two pulse generators, which are mechanically coupled to each other to produce synchronized, quadrature outputs.

Rotary encoder mechanism

Rotary encoders typically use a single rotating disc which has two concentric optical interrupters, one for the A output and another for the B output. The patterns of the two interrupters are staggered to produce quadrature-encoded signals.

Linear encoder mechanism

Linear incremental encoders employ a similar technique, but use a linear scale to produce the output pulses.

Interface

Incremental encoders do not indicate the location or position of a mechanical system; they only report incremental movements.

Interface example

To determine position, it is necessary to send the encoder signals to an electronic circuit known as an incremental encoder interface, such as the one shown here.

Counter

The interface keeps track of position by counting encoder pulses. It counts up when the quadrature phase difference is positive and down when the difference is negative, or vice versa. To do this, interfaces employ a quadrature decoder to convert the A and B pulses into direction and count enable signals, which in turn control an up/down counter.

Applications

Incremental encoders report position changes in real-time, making them useful for monitoring and controlling motion in numerous applications. They are commonly used to control automated machining equipment, such as CNC mills.

Escalators

Rotary incremental encoders are used for closed-loop speed control of conveyor belts and other conveyor systems, including escalators and moving sidewalks.

Trackballs

In human input devices such as trackballs,

Mice

and mechanical mice,

Mouse innards

two rotary encoders are used, to simultaneously monitor position on two different axes.

Radar speed

A radar uses a single rotary encoder in two different ways. The encoder's pulse frequency is measured and used to control the rate of antenna rotation.

Radar bearing

The pulses are also counted. To keep track of the antenna angle. When the radar detects an object, the target bearing is indicated by the pulse count.

Pipeline inspection

Rotary encoders are used to monitor underground pipeline inspection tractors. This is made possible by a cable which is towed behind the tractor. The incremental encoder monitors the length of cable that pays out as the tractor drives through a pipe. The exact location of the tractor can be determined by counting encoder output pulses.

Motion platforms

Incremental encoders are used to control motion platforms in aircraft simulators and amusement rides.

Robotics

Incremental encoders are extensively used in robotics, to control the position and speed of mechanical components.

PCB mount

Some, rotary encoders are designed to be mounted on circuit boards. These typically have a knob attached to the shaft, and are used as hand-operated controls in electronic equipment.

CMM

Linear encoders are used when extremely high accuracy is required, because they eliminate errors caused by mechanical backlash. Common applications include position monitoring in coordinate measuring machines.

Steppers

and precise, high-resolution position control in steppers, used in semiconductor fabrication.