Page 279 of catalogue Position sensors and object detection systems of ifm Electronic GmbH

Encoders

Incremental encoders, absolute encoders

System description

Profibus encoder Due to the implementation of an interface for PROFIBUS-DP this encoder can be directly connected to the bus. The advantage: there is no further cost for the user for additional controller hardware. He simply needs to set the slave address and the encoder is a PROFIBUS-DP participant. The encoders operate to the requirement profile of the Profibus user organisation and support the device class 2. Thus immediate and decentralised processing of many controller functions can be performed in the encoder: the resolution of 25 bits is freely scaleable and the direction of rotation for rising code values can be programmed. This is useful for Machines with different directions of rotation of the Axes. Further controller features are the limit switch function, the detection of movements and the preset value function. Thus the zero point of the encoder is conveniently adapted to the mechanical zero point of the installation. In case of a fault the units transfer diagnostic messages and warnings. Thus machine downtimes and related cost are reduced to a minimum. Code The rotating coded disc of absolute encoders is encoded in the so-called “Gray” code. This code proceeds by one step, i.e. when going from one number to the following only one single bit changes. This means an additional reliability factor. As opposed to the binary code where each bit combination is allotted a certain decimal number the individual bits in the Gray code do not correspond to a numerical value. The bit combination read is converted into a binary code by a suitable code converter (in the PLC) and can then be further processed. The genuine Gray code, i.e. the one fully using the number of bits, counts from zero to ... 2n –1 (n is the number of bits). Sometimes, however, resolutions are requested which cannot be represented by a power to two such as the value 360. To represent this number in the Gray code on the coded disc a trick is used: For a required resolution the Gray code for the next highest number (which must be a power of 2) is taken as a basis, but only the mid-section of the code is used, i.e. the excess codes at the beginning and at the end are deleted. This ensures that for each increment no more than one step will change even when counting past zero. This type of code is called Gray excess code.

Controller integrated: Multiturn encoder with integrated PROFIBUS-DP interface.

DC 8 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 4

BA 2 1

DC none

BA

DC none 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

BA

0 1 2 3 4 5 6 7 8 9

binary code

10 Gray excess code 3

Gray code

Gray excess code: These bit combinations allow the use of all resolutions which can be divided by two.

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