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Absolute Rotary Encoders Information

Figure 1: Shafted absolute rotary encoder. Source: IP83/CC BY-SA 3.0

An absolute rotary encoder determines the angular position of a shaft on the basis of a static reference point. They are sensors that generate digital signals that correspond to the position in response to movement. The method of determining position varies based on the type of encoder. The primary advantage of this type of encoder over others is that absolute rotary encoders do not lose their position data upon power-down. They can provide the absolute position when powered up without any shaft rotation or a home cycle.

Absolute rotary encoders are used in a variety of applications where mechanical systems are controlled or monitored. They are often installed on motor shafts or any driven shaft that requires positional monitoring and control. They are used in surgical robotics, microelectronics, industrial robotics, automation, and CNC machines. Their use spans across many industries such as aerospace, military, industrial, oil and mining, telecommunications, consumer goods, and the medical industry.

Figure 2: Airplane radial engine. Source: Eduardo Buscariollo/Unsplash

Technology

Encoders can be linear or rotary. Linear encoders measure linear movement while rotary encoders measure radial movement. Rotary encoders can be further divided into two categories: absolute and incremental. While incremental rotary encoders can measure distance, position, and speed, absolute rotary encoders can measure the specific angular position. If the absolute rotary encoder is digital, the exact position of the shaft correlates with a unique digital code.

The difference between rotary and absolute encoders can be illustrated by comparing a stopwatch and a clock that tells the time. Stopwatches measure the incremental time between start and stop similar to how an incremental encoder will tell the difference in position. In comparison, an absolute encoder provides a specific position similar to how a clock shows the current time.

What is the Difference Between Absolute and Incremental Encoders?

Video: The difference between absolute and incremental encoders. Source: RealPars

Absolute rotary encoders use a variety of technologies. Each technology has advantages and disadvantages.

Optical

An optical rotary encoder has a disc made of plastic or glass with opaque and transparent areas that mark the disc’s position. The disc can also be composed of metal and contain holes in it to mark the disc’s position. A light source, typically in the form of an LED, along with a photodetector array or photoreceivers can interpret the resulting optical pattern from the disc’s position and convert it to data bits. The code is then sent to a controller to determine the angle of the shaft.

Gray Code is commonly used for disc positions in optical rotary encoders. This code differs from binary code as successive values differ by only one bit. For example, the decimal value of ‘1’ in binary would be ‘001’ with the successive decimal value of ‘2’ being ‘010’. However, using Gray Code, ‘1’ would translate to ‘001’, and ‘2’ would be ‘011’. The primary advantage of Gray Code is that the transition between states does not correlate to other states, this helps to prevent errors that result from transitory states in binary code.

These types of absolute rotary encoders consider of three major components, housing, optical block, and an electronics block. The housing protects the encoder from the environment. The optical block consists of an optical coding and detection system while the electronics block allows for amplification and signal processing. 

Optical absolute rotary encoders provide high operating speeds, high resolutions, and are reliable in many operating environments. 

Fiber optic

Fiber optic rotary encoders will transmit signals along a fiber optic cable. Fiber optic absolute encoders are typically used when conventional electronics-based encoders are insufficient and cannot perform effectively. Fiber optic encoders can perform well in environments with electromagnetic interference (EMI) and radio frequency interference (RFI). They are preferred in areas where sparks could create explosions. They can also be used to transmit signals quickly over long distances.

Mechanical

Mechanical absolute encoders will typically use a metal disc. As brush-type contacts are susceptible to wear with use, they are typically used in low-speed applications such as manual volume knobs or tuning controls.

Magnetic

Magnetic absolute rotary encoders use magnetic fields to determine the angular position of the shaft. The permanent magnet or electromagnet generates a magnetic field that is not homogeneous around the disc. A circular disc with multiple north and south pole called a multipolar ring or a bipolar magnetic with only a north and south pole may be used.

The differences in the magnetic field are used to represent the disc’s position. Depending on the position of the magnetic source and sensor cells, the magnitude of the field changes around the encoders. The specific geometry is used to analyze the specific position of the encoder.

There are a few different types of technologies that are used in magnetic rotary encoders. Common technologies are inductive encoders, magneto-resistive encoders and encoders that use the Hall effect.

Magnetic absolute rotary encoders are often used in rugged applications. They are cost-effective while providing excellent resolution, and high operating speeds. Furthermore, they are especially resistant to shock, moisture, dust, and changes in temperature. These types of encoders are reliable and robust and are also resistant to chemicals. To prevent magnetic interference in the positional readings when magnetic encoders are used near external magnetic fields such as next to motors, magnetic shielding can be added to the encoder.

Designs

Figure 3: Absolute rotary encoder. Source: SparkFun Electronics/CC BY 2.0

Absolute rotary encoders come in a variety of different designs:

Ring or Bearingless

Ring encoders are often bearingless, and the rotary encoder is composed of a ring. Bearingless encoders are free from wear due to magnetic non-contact sensors. This rotary encoder design is resistant to harsh environments and sports long component life.

Hollow Shaft

A hollow shaft rotary encoder is designed for the motor shaft to pass through the encoder housing. These encoders are available in a variety of sizes and resolutions. They are directly mounted to a motor shaft and installed using a torque arm or flexible tether. This design is easier to install than shafted encoders as they do not require coupling or motor shaft alignment with the encoder. 

Shaft

Rotary encoders with a shaft are mounted using flexible coupling. Coupling can be changed to match a variety of motors, even older and non-standard motors. Coupling helps to absorb shaft movement and provides electrical isolation from the motor or driven shaft.

Miniature

Absolute rotary encoders may be designed for use in especially tight spaces. Precision instruments may require miniature rotary encoders.

Multi-Turn

Multi-tun absolute rotary encoders can determine the number of turns that the encoder has made. The number of turns is measured with an internal reduction gear connected to the drive shaft.

Kit Encoder

Encoder kits allow for encoders to be configured with ease. These solutions are often cost-effective and allow for customization. They make be quick-assembly to allow for quick maintenance and installation.

Electrical Output and Interface

Absolute rotary encoders support different interfaces such as SSI, BiSS, Fieldbus and Ethernet interfaces, and other relevant controllers.

Parallel

Classically, parallel wiring has been used when interfacing absolute encoders. Latency is diminished as all bits transfer simultaneously but at the cost of an increased number of wires. This can result in increased errors, higher costs, and more complexity, which results in additional points of failure.

Serial

Serial interfaces are often simpler, more reliable, and faster than parallel interfaces. This method of transmitting data from the encoder to the controller aggregates data to minimize wiring.

SSI/FSSI

Synchronous serial interface (SSI) is commonly used in industrial applications and is a common choice for sensor applications. In this point-to-point serial communications protocol, data is transmitted by synchronizing transmission using a common clock signal.

SSI is often the most straightforward communication protocol available for absolute rotary encoders.

BiSS 

BiSS (bidirectional/serial/synchronous) is a point-to-point encoder interface that can send full absolute position data whenever the controller polls the encoder.

Features 

With a compact design, absolute rotary encoders have non-volatility of memory and provide accurate and reliable positioning without shaft rotation. Often these types of encoders provide higher resolution than incremental encoders. They can measure accurately across multiple axes and have several output protocols allowing for integration with many electronic components. Many can be programmed flexibly.

Absolute rotary encoders have high resistance to electrical noise. Absolute encoders read an error-checked code from binary output or digitally over a serial bus, which reduces susceptibility to noise.

Start-up performance is better than incremental rotary encoders due to low homing and time to initial position. This feature allows them to have better recovery in cases of power and system failures or resets.

Absolute encoders are used when the measurement of position is critical and an initializing step cannot be afforded. Overall, absolute rotary encoders are often more expensive than incremental encoders though as production technologies advance the price gap is reducing. When choosing between absolute or incremental rotary encoders it is recommended to consider performance, user experience, as well as the price to choose the right encoder type for the application.

Related Information

Globalspec—Gurley’s high-reliability rotary encoders offer accuracy and precision, solve difficult problems

Globalspec—Linear encoders and the increasing accuracy of optical patterning


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