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.

 

[alphy]

I can provide some insight into how the order of the A and B signals can indicate direction in an incremental encoder. The key concept here is the principle of quadrature encoding, which refers to the use of two signals that are 90 degrees out of phase to determine position and direction.

In an incremental encoder, the A and B signals are generated by two detectors that are positioned at 90 degrees from each other on the encoder shaft. As the shaft rotates, the detectors produce pulses that are out of phase with each other. The order in which these pulses occur can indicate both the direction and the amount of rotation.

When the shaft rotates in a clockwise direction, the A signal will lead the B signal, meaning that the A pulse will occur before the B pulse. Conversely, when the shaft rotates counterclockwise, the B signal will lead the A signal. This difference in the order of the pulses is what allows the encoder to determine the direction of rotation.

To understand this concept further, imagine a wheel with spokes that is being rotated. If you were to trace the motion of one of the spokes, you would see that it moves in a circular path. However, if you also trace the motion of a spoke that is 90 degrees away, you would see that it moves in a sinusoidal pattern. This is essentially what is happening with the A and B signals in an incremental encoder. By analyzing the order and timing of these pulses, the encoder can determine the direction and amount of rotation.

I hope this explanation helps clarify the relationship between the order of the A and B signals and direction in an incremental encoder. Keep in mind that there may be variations in the specific design and implementation of incremental encoders, but the underlying principle of quadrature encoding remains the same.