How Do A and B Signals Determine Motor Direction in Incremental Encoders?

[tomizzo]

So I've been trying to read up a little on incremental encoders and have been looking at the outputted signals typically labeled A and B. These signals are 90 degrees out of phase. Apparently, the purpose of having both of these signals is for identifying motor direction. However, I'm kind of confused as to how the order of the A and B signals are able to show direction.

I've searched the web and physics forums database but was not able to find any discussion in how direction effects the order of signal outputs. Any help?

 

[mfb]

A and B detect the rotation of something?

In one direction, the signal will look like A(short pause)B(long pause)A..., in the other the long pause is after A and the short one is after B.

 

[Bobbywhy]

The incremental rotary encoder employs two outputs called A & B, which are called quadrature outputs, as they are 90 degrees out of phase.
http://en.wikipedia.org/wiki/Rotary_encoder

Quadrature Encoder Overview:
http://www.dynapar.com/Technology/Encoder_Basics/Quadrature_Encoders/

Quadrature Encoder Fundamentals, from National Instruments:
http://www.ni.com/white-paper/4763/en/

 

[meBigGuy]

Basically if you look at the level of the B when A rises you will see it is high in one direction of rotation and low in the other. Connect A to the clock of an up-down counter and B to the up-down control and you get a relative position detector.