Absolute position encoder vs. other transducers

Absolute position encoder vs. other transducers

From elevators to high-end robotics—the encoder can be found in various applications of different complexity. This electromechanical device is key to flawless functioning of machinery, whether it is used to monitor conveyor movements or to control shaft motor positions in robotics.


Encoders read positions of the component to which they are attached and convert them into signals. Rotary devices do this for rotating elements, whereas the linear type is utilized with machine constituent moving along a straight line. In any case, the measuring instruments play the role of a mediator linking the mechanical parts of a mechanism with a controller or another monitoring and control unit.

In robotics, an absolute encoder is one of the most efficient transducers for communicating position data within a motion system. What makes the device stand out?

absolute encoder

Rozum Robotics has created a closed-loop motion control system with two position encoders integrated into RDrive servo motors.

Measurement systems

There are two basic measurement types employed for the encoding process—absolute and incremental. A measurement system determines the method of acquiring data in a transducer. More specifically, it is the coded disc or scale type and the way those are scanned.

absolute position binary-code encoderAbsolute measurement

Absolute measurement involves precise tracking of rotation. It produces a digital or analog position that can be used to derive other motion information, such as velocity or acceleration. Each position correlates with a unique binary output—i.e., it is determined absolutely.

Another distinguishing trait of absolute measurements is reading exact positions upon motion start without referencing to a home setting.

Why absolute encoder is much in demand 

  • allows determining angular displacements with pinpoint accuracy
  • retains position data despite power disruptions
  • enables improving speed control by enabling faster motion response
  • has multiple interface options, such as BiSS, Profibus, CANOpen, etc.
  • has high resolution

Incremental measurement

An incremental encoder defines relative positions— differences between two points in a rotation trajectory—and requires a reference point to be set.

Output type

Since the encoder measurement system can be incremental or absolute, there is a conceptual difference between produced outputs as well. Two common output types are a data word to designate an absolute position and an incremental pulse to let users know a relative position.

Data words

An absolute position encoder basically works the following way: for each position of the shaft, there is a corresponding unique code. For this purpose, the rotating disc of the device includes a sophisticated slotted pattern.

In absolute encoders, major coded wheel patterns are the binary format or Grey code. The binary format is associated with inaccuracies that occur frequently at high-velocity measurements. As a result, the Grey code is the preferred pattern for a wider speed range.

absolute position binary-code encoder

Incremental pulses

In incremental encoders, series pulses are counted. From the two output signal streams of the transducer, it is possible to infer the extent of displacement (the pulse quantity) and movement direction (which pulse flow takes the lead). When a third signal stream is available, it enables setting a reference for pulse counting.

Encoder sensing method

Sensing inside an encoder—i.e., reading the codes or pulses from the coded disk—can either be contact or non-contact. This division defines whether a mechanical contact is needed to track position, or tracking relies on non-contact physical effects.

Contact

Contact reading is based on a contact between a coded disc and a sensor pin, or a brush, or another fixture. A coded disc has a series of tracks (or rings). When the disc is rotating, the sensors contact the tracks, producing signals.

There are significant downsides of the contact sensing method making it less attractive for users. The most hard-hitting ones are segmenting restraints and contact wear.

Non-contact

Non-contact encoders employ a particular physical phenomena to read a coded disc without contact. The main advantage of the non-contact method is improved wear resistance. On the flip side, the method is rather expensive, sensitive to dirty environments, and demands a complex infrastructure.

Operating principle

Whether contact or non-contact sensing is employed, manufacturers design encoders using either the magnetic or optical operation principle.

Magnetic

magnetic absolute encoder

The magnetic encoder comprises a sensor, a coded disk with magnetized strips, and a conditioning circuit. The purpose of the sensor is to detect a change in voltage or magnetic field and convert the data into a signal. The conditioning circuit processes the sensor signals to generate the desired encoder output—absolute or incremental positions.

Optical

An optical encoder of both the absolute and incremental types is perhaps the most extensively used transducer, which is due to its top-notch measurement accuracy. Whereas magnetic devices use the magnetic attraction phenomena, an optical encoder uses light.

However, optical encoders have significant downsides, such as poor immunity to dirt and moisture, and demand costly operating environment.

optical linear encoder operation

When do you need absolute position encoder

You should choose the type of encoder:

  1. If your application needs position data immediately at equipment startup
  2. To avoid machinery damage or staff injuries by making motion control more efficient
  3. To reduce wear caused by dust and electrical noise
  4. If your application requires real-time positioning and has sophisticated digital control

Rozum Robotics has two high-end encoders built into RDrive servos

In Rozum Robotics servo motors, the absolute position encoder reads motor shaft positions. As part of the solution, it is used multiple applications of intricate design delivering the following benefits :

  • excellent reliability
  • 19-bit resolution
  • resistant to impurities, heat, shock, and vibration
  • magnetic operating principle
  • compact and robust

absolute position encoder ensures precise performance


 

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