Are Industrial Encoders Considered Inputs or Outputs in PLC Automation Systems?

I understand encoders to a certain point. I know encoders tells it's precise position. There are two kind of encoders which are incremental and absolute. Now the question that i have is when it comes to industrial automation is a encoder an input or output? Can it be used as a timer kind of in a way? I was thinking an input because once the set value is reached it'll tell the PLC "hey I'm here, give it to me."  Is my way of thought correct or completely off?

Christopherwhen it comes to industrial automation is a encoder an input or output? I have never seen an encoder used as an output- it is always an input although it is frequently affixed to a mechanical device driven by an output to provide a feedback as to the position and/or speed of the device driven by the output.Can it be used as a timer kind of in a way?I can't think of an instance where this has been done or is even possible. Maybe attached to some sort of mechanism like a mechanical egg timer? Perhaps someone else has an idea. I was thinking an input because once the set value is reached it'll tell the PLC "hey I'm here, give it to me."An encoder will provide information on position and/or speed (linear or angular). Detection of limits is normally done in software after reading that input- there may be encoders with limit sensors on them especially at the limits of movement, but it would seem to me that they would be separate inputs to the PLC.One of the manufacturers of encoders is a company called BEI, although I am sure a Google search will show up many others. 

Hi Christoper-The most common encoders are incremental "quadrature" encoders.  As they are rotated they generate two square wave outputs at are 90 degrees out pf phase with each other.  These two outputs are usually fed into a microcontroller which allows it to determine the speed and direction of rotation. These encoders work on one of three principles; mechanical, optical, and magnetic. The mechanical encoders are most often found in lower-cost consumer items. They consist of two sets of contacts and a rotating disc.  They tend to wear out faster than the other two types. Optical encoders are very common. They use one or two light sources (usually LED's), and two photodetectors, usually phototransistors. The third type is magnetic.  The magnetic type most often uses a  pair of Hall effect sensors.  These sensors can either be linear (analog) or digital.  The linear type can be used as an absolute position sensor, and the digital type is almost always incremental.  Both the optical and magnetic types are often more expensive than the mechanical type, but have a much longer life expectancy.-Rick

Hi ChristopherThe previous two replies have given a lot of information but maybe an example of use might help you as well.Lets take something that needs precise and controlled linear motion, like a print head on an inkjet or 3D printer. If you have an encoder on the drive mechanism then you (or in this case the microcontroller driving the printer) will be able to deduce exactly where the printhead is (from the number of pulses from the encoder.  In most cases though the mechanism will have a microswitch or an optical interrupt sensor to tell it when the carriage is in the home position.Or look on any modern oscilloscope.  The knobs usually turn right round, not just 3/4 of a turn like the old potentiometers.  This is a clue that they are using a rotary encoder.  These enable the user to step through menus or through the values in a menu.  A scope might have many settings for vertical sensitivity, from 5mV per division (or less) up to even 100v per division.  To do this with a rotary switch would require one with many positions - you can get them above 12 positions but they are expensive.  Use an encoder and you can sense the direction and number of steps that the knob is being turned, and select your setting using software.Many encoders have a push switch on them as well, you can use this to select a setting or cycle though different menus,  So this makes them a very versatile component.You also get absolute encoders which output a code giving the position of the encoder.  They would be used in such things as weather stations (to indicate the wind direction).  To avoid wrong outputs they use Gray codes where only one bit of the output changes at a time.There's a good article on rotation sensors here, and the wikipedia article is also worth a look - it covers both rotary encoders and absolute encoders .And Max our esteemed editor has written a good series of articles on Gray codes:Gray Code Fundamentals – Part 1 (Introducing Gray Codes)Gray Code Fundamentals – Part 2 (Generating Gray Codes)Gray Code Fundamentals – Part 3 (Sub-2^n Sequences)Gray Code Fundamentals – Part 4 (More on Sub-2^n Sequences)Gray Code Fundamentals – Part 5 (Non-Boolean Gray Codes)
There's enough in this subject to keep you busy for days.... but hope this has added to your understanding.

Thank you all for the responses. When talking about encoders there are two types and those are absolute and incremental, but are the "disks" for both types the same or different. Now my question is are all incremental encoder disk look this way? I seen on google images that there is also a disk with square markings. Is the disk with the square markings only for absolute encoders or can it be found in both absolute and incremental encoders?

          

Christopher...there is a really good article on encoders just been posted here, I thought of you when I saw it.http://www.machinedesign.com/sensors/basics-rotary-encoders-overview-and-new-technologies-0
Enjoy!

Christopher.... an incremental encoder will have a disk with just "spokes" radiating outwards from the centre (though you may only see them at the edge).  This enables the encoder to provide sufficient information about the speed and direction of rotation only.  There would be one sensor only for speed, or two sensors for speed and direction (the phase difference of the signals from the two sensors gives direction information).An absolute encoder will have a pattern of light and dark areas and these will look like squares in some parts.  They have many sensors, one for each band of the disk, and they provide a code corresponding to the shaft position in its rotation.  They'd look like thishttps://cdn.thingiverse.com/assets/db/cb/53/bd/92/gray_precision.png
So an encoder with 7 bands will give 27 codes = 128 codes.  I actually bought an encoder like this when a supplier had some on special not long ago.  it looks like a potentiometer (volume control) except it has 8 legs (7 for the data bits and one common).