Absolute encoder definition – An absolute encoder identifies the placement of an axis by reading a digital code from an encoder face. The designs of absolute encoders and also incremental encoders are virtually the exact same, the only distinction remains in the method of measurement. An absolute encoder produces a code signal at its output. Each angular setting of an axis represents a details code value at the output. Two types of coding are used right here: binary code and also Gray code. Absolute encoders are split into singleturn as well as multiturn encoders. Singleturn encoders identify settings just within the framework of one turning, so the result of turning the shaft of such an encoder by precisely 360 ° will certainly coincide signal at the output. As it has actually been stated previously, incremental encoders aim to accomplish the greatest feasible variety of divisions per revolution – while preserving tiny size of the gadget, whereas absolute encoders make every effort to boost resolution as well as most of all data transmission rate. An absolute encoder has the capability to keep in mind the current placement even after turning off the supply voltage (e.g. for measuring the number of transformations made by a rolling mill).
Most importantly, the variety of applications for absolute encoders is boosting, including variations with commercial network interfaces. The miniaturization of such encoders is likewise really important, thanks to which they can be used in small gadgets which are not makers – e.g. speed electronic cameras.
Absolute encoders convert their current setting right into a digital value in binary, BCD or Gray code. The crucial thing is that each encoder placement represents a single, one-of-a-kind digital value – therefore the name absolute encoders. The resolution of an absolute encoder is the variety of settings per change or unit of length that the encoder is capable of converting into a digital value.
The functioning concept of an absolute encoder is shown in the figure below (utilizing a rotary encoder as an example):.
The most important attribute of absolute versus incremental encoders is that they supply absolute placement info. This kind of encoder produces a special digital code for every setting at its output thanks to a built-in code matrix. The position of the maker can always be determined specifically, even if the power supply to the machine is turned off. If the encoder axis is moved when the device power is turned off, the absolute encoder will exactly identify the current placement when the machine is activated once more, which was not possible with incremental encoders. For that reason, it is not needed to accomplish the standard procedure and also identify the zero point every time the equipment is powered up again. An absolute encoder outputs a digital signal saved in the form of a code, which can be written in binary, Grey’s or BCD code. These encoders are geared up with parallel or serial output circuits (SSI, BISS, Modbus, and so on). Encoders with absolute measurement are divided right into single-turn or multi-turn encoders.
The placement details is output by the encoder in the form of e.g. an 8-bit binary word. In encoders with parallel output, this is merely an appropriate zero-one combination. Nonetheless, this kind of signal requires lots of digital inputs in the host device (e.g. 8 bits = 8 inputs). This is why other user interfaces are generally used in absolute encoders. The basic one is SSI, which is a serial synchronous interface. In this sort of encoder, a digital word is transferred on the data bus bit by bit at the clock rate of the clock input.
Furthermore, an entire range of communication interfaces are offered, which can be separated into 2 main groups:
- field-type networks: Modbus, CC-Link, CANopen, Devicenet, Profibus,
- Ethernet type networks: Powerlink, EtherCAT, Ethernet/IP, Modbus TCP/IP, Profinet
In addition to digital interfaces, absolute encoders with analog outputs of 0-10V, 0-20mA or 4-20mA are also readily available.
The department into absolute solitary or multi-turn encoders makes good sense for rotating encoders. Due to the reality that each encoder placement has its very own special digital code after transforming the encoder shaft by 360 ° we would have the same value. This is actually the instance with single-turn encoders. When a larger number of changes has actually to be identified, a multi-turn absolute encoder comes to be needed. The number of changes is identified by means of a gear system as well as is specifically specified for each and every encoder. Acknowledging more than one transformation boosts the total resolution many times over, which is the product of the variety of positions per transformation as well as the number of transformations.
Incremental encoder basics
An incremental encoder contains numerous standard components. On the mechanical side, it is a shaft with a placed coded disk, two bearings situated at a maximum distance from each various other, a scanning system and also an electronic system that forms the output signal.
A very vital component in optical encoders is the code disc. SICK was the first to introduce F-Technology based upon a nickel coded target for activity control sensors. This is what you will discover in encoders. The steel codewall is suitable for applications with shock or vibration. Encoders with F-technology have a lot longer life span, along with they are resistant to any type of sort of unchecked influences in comparison to their counterparts with code discs constructed from glass or plastic.
In addition, the metal code dial guarantees a dramatically higher repeatability of dimension. Increments positioned on the code disc are nothing else than laser-cut spaces in the metal disc, where the light of the transmitter passes. The variety of increments defines the encoder resolution parameter. A dozen approximately years ago encoder discs were constructed from glass with sprinkled increments. Glass, because of its properties, tarnishes after some time, which significantly decreases the repeatability of measurement.
A very essential element are the bearings, as well as much more particularly their spacing. Encoders of the DFS60 or AFM60 families have optimal bearing spacing. This makes it feasible to operate with higher stress forces on the device shaft.
The scanning system consists of:
- a light source,
- a lens that routes the light beam, which falls on the code disc with the increments,
- a mask, which is put behind the target as well as is used to develop two, but offset rays from a beam gone through one increment,
- a photosensitive aspect which catches the light rays.
By using a mask, two standard channels An and also B of the encoder output signal are formed from a single beam, shifted in phase by 90 ° electrical. Utilizing channels An and B simultaneously, we can recognize the instructions of rotation of the shaft. The controller contrasts which signal records first. If channel An after that the encoder turns clockwise, if channel B is first it means that the instructions of rotation has been altered to counterclockwise. Additionally, the mask creates the third channel of the encoder – the Z network, which is produced just once when the code disc is completely rotated. The extra channel is utilized to base or integrate the motion system of our maker or gadget.
A photoelement at the end of the scanning system transforms light rays gone through the mask as well as code disc into electrical pulses – in this case, rectangular pulses.
Encoders with a sinusoidal output use Hall sensors rather than an optical code disc, to name a few points. Any kind of sinusoidal signal can be interpolated and also converted into a rectangular signal. This relationship is used in magnetic encoders.
An extremely essential factor is that an incremental encoder generates pulses just when attached to power and only at the moment of activity. It does not have the performance of saving its setting. In case of distance or variation measurement at the moment of switching off the power supply and after changing it on once again, it is constantly necessary to base the system. In case of rate measurement this element is not so vital.
Certainly, the current state of innovation as well as signal conversion permits to remove several aspects of the scanning systems offered above. Nowadays it is hard to find common increments on a code disc or a Hall sensor in the sense of a textbook on digital modern technology. Nowadays, the standard component of any kind of encoder is a printed circuit board with an incorporated circuit, which relying on the needs today is an incremental encoder and tomorrow might be an absolute one. The main idea of the operating concept has stayed.
Incremental vs absolute encoder
Encoders can be divided right into incremental and also absolute. The difference between them is composed in the reality that the first ones shed their position after disconnecting supply voltage from them. This makes that the device needs basing as well as just hereafter activity can we start proper measurements. The second type, on the other hand, is able to show its placement even if the power supply is disconnected and then linked once more. At this point it deserves stating Posital encoders, which have a distinct function – the opportunity of remembering the placement in absolute multi-turn encoders without making use of a battery to maintain the memory or a challenging mechanism with equipments. Despite the speed of the encoder shaft, its rotation generates a short voltage spike in a little circuit that powers the transformation counting module. This runs separately of the external sensor supply. This is enabled by a cutting-edge RPM counting system based upon the Wiegand impact.
|Incremental Encoders||Absolute Encoders|
|Output signal||The output signal is rectangular pulses||The output signal is a code signal|
|Position memory||When the power is turned off, it does not save the position||When the power is turned off, it remembers the position|
|Direction of rotation||the shape of the generated output signal is the same regardless of the direction of rotation, only the order of the generated signals changes||-position regardless of the direction, the information is always about a specific position -velocity depending on the direction of rotation, the generated speed value can be with “-” or “+” sign|