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All you Need to Know about Encoders

Introduction

Computer peripherals are a wide array of plans that let computers to interrelate with the outside world. They come in dissimilar shapes and sizes, from mice and keyboards to scanners and printers. Encoders are parts inside computer peripherals that convert input data into digital signals, an input-output device. There are many types of encoders:

  • Angle Encoder
  • Linear Encoder
  • Rotary Encoder
  • Quadrature Encoder

These two types of encoders work alike but have different features which can help you decide which one meets your needs more than anything else.

Linear Encoders

There are many sensitive transducers in a linear encoder, which is used to convert rotary motion into a series of digital signals that the computer can understand. The head is a magnetic or optical transducer that can be placed at an angle, usually around 30-45 degrees from the axis of rotation. As this head moves, it will push against something which will move at a constant speed. In other words, the encoder has an attached to it, which moves as quickly as the encoder does. This connected device moves through a scale with holes in it so that each hole can block only one sensor from the attached device. Therefore, a set of holes with a single hole stopped will equal one revolution.

To use a linear encoder to control a motor, its transducers must have the same number of teeth as the gear’s tooth count. If the tooth count is ten and the transducers have ten holes (one-hole per tooth), then ten motor revolutions will correspond to one rotation of the encoder head.

The most significant advantage of using this type of encoder is that it can be used with any gear and not just gears with this particular number of teeth and holes.

Rotary Encoders

A rotary encoder is a mechanical device with a spindle with a series of gears. When the encoder shaft rotates, it will push against a unique magnetic ring separate from the shafts. For the encoder to turn, it must rotate around its axis in one direction. The system’s rotation must identify what type of motion occurred by comparing its distance from this unique magnetic ring’s location about itself. This mechanism is not dependent on speed and can only tell if there was a change in distance between itself and this specific magnetic ring.

This type of encoder is much like a rotary switch, but instead of measuring the number of clicks, it measures the cap’s location compared to the shaft. It is also similar to an optical encoder in this way, except that an optical encoder can track fast motion that a rotary encoder cannot.

Quadrature Encoders

A quadrature encoder usually comes in two parts called quadrature wheels or discs. These two discs are placed on either side of the axis, which they are trying to measure motion for. The output signal consists of both signals from these two transducers, which will give full information about motion direction and speed. This type of encoder has two signals, 180 degrees out of phase. Therefore, when both signals are combined, it can determine which direction the shaft is spinning. When the shaft is not rotating, the two signals will line up.

This type of encoder can be designed to measure around 4 million counts per revolution of the shaft that it is attached to. It uses two signals that are out of phase with one another to determine direction and speed of motion. This type of encoder is good for applications where precise measurement is needed, and its use allows for systems to be easily scaled or designed around their features.

Angle Encoders

In an angle encoder, the encoder disc is placed perpendicular to the axis of rotation. This allows for the total measurement of a single degree of motion each time it is turned by one complete turn. It is often used in systems that need to measure tiny movements, and its features allow it to be easily scaled. These angles can also be recorded and easily converted to linear distances, easily added or subtracted from on the field. The output comes as a pair of quadrature signals arranged as 90-degree out-of-phase sine-cosine pairs, then processed into a six-bit digital word by an Analog-to-digital converter (ADC).

Absolute and Incremental Encoders

An absolute encoder outputs a unique signal so that the input will always return the same position. This kind of encoder is used when you need to know precisely where something is, such as how many rotations have occurred or how far something has travelled. This type of encoder cannot be easily scaled or reversed.

Incremental encoders encode movement and output a total signal (the difference between the current and last position), making it easy to determine distance and direction. It is also possible to reverse this encoding type since incremental can be treated as absolute signals but with a limited output range.

This encoder is used in many applications, such as automotive and construction equipment. Its features make it ideal for crane and hoist controls and combinations, direction indicators, and speed limiters. There are several types of these:

This sensor outputs a pulse when it senses a change in a magnetic field. Its output is proportional to the rate of change of the magnetic field strength, making it easy to measure its position based on magnetic fields such as those from large magnets. The output is an analog signal that can drive stepper motors or brushless DC motors. It can also generate time pulses that could then be used for synchronization purposes.

Encoder Sensing Technologies

This kind of encoder can be used in any mechanical system that requires a position sensor. It differs from a traditional encoder because it does not contain an axis for rotation or a shaft. The sensor uses minimal magnetic means, such as a fluxgate magnetometer chip, to change shape and detect the magnetic field. In this case, there is no require for gears or drive mechanisms because the magnetic field changes with position instead of providing motion.

To sense, the direction of rotation, this type of sensor uses two degrees of freedom that rotate at different rates while still detecting changes in their relative positions.

Several different sensing technologies may be used to detect motion or position within an encoder. The most ordinary sensing technologies that are used in encoder designs include:

Optical Encoders

Optical encoders rely on light-emitting diodes (LEDs) and a photoresistor to determine the amount of rotation that has occurred. A pair of LEDs are separated by a known distance and can emit a beam of light that illuminates the photoresistor. Since the beam will always be at a set distance from the photoresistor, as it rotates, it will pass by different zones that change its relative position concerning the photoresistor. To measure this relative position, an analog circuit must compare the voltage at both ends of the photoresistor to one another.

Magnetic Encoders

Magnetic encoders work by measuring the magnetic field between a pair of magnets. One magnet will pass near the other and induce a voltage within it as they turn. By comparing the voltage from both magnets, it is possible to determine how much rotation has occurred. The difference between the two voltages has been proportional to the motor’s angle since the last measurement. A digital filter can be used after each sample is taken to eliminate any offset voltages that may occur due to ambient magnetic fields.

Capacitive Encoders

Capacitive encoders change the strength of the capacitive field through which it detects motion. Only when this analog field changes will it detect motion. This type of sensor uses a pair of coils to measure relative position by comparing the current in each coil. Because current flows in opposite directions, the direction of rotation will be detected by using an Analog-to-Digital Converter (ADC) which takes two different samples at different points. The ADC only needs to be able to take one sample per revolution, so its output can be easily scaled or reversed without too much effort.

Summary

An encoder is a device that allows for precise measurement of position or motion. Depending on the type of mechanism that it is used in, an encoder may provide for both linear movement and rotation or distinguish between linear and angular rotation.

Several technologies can be used within an encoder to sense position or motion. The best type is typically selected based on the application and its requirements. An encoder often uses sensors with different means to detect both rotation and linear movement. A magnetic sensing technology may provide all of the functionality required with little effort while sensing magnetic fields, with other technologies requiring minimal circuitry.

 

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