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Quadrature Encoder Fundamentals

Quadrature Encoder Fundamentals

Quadrature encoders are one of the most common types of encoders. Quadrature encoders are incremental, meaning that they can accurately measure changes in position, but cannot provide an absolute position.

Quadrature encoders are often used on motors. They are also commonly used to provide a reference input, such as the position of a belt, or the position of a board on a flying cut-off application.

Quadrature feedback is supported by the following modules:

The RMCs require an RS-422 quadrature signal. This is also commonly referred to as a 5V differential signal. The RMCs do not support single-ended quadrature signals.


A and B Signals

Quadrature encoders use two output channels (A and B) to indicate position. Using two code tracks with sectors positioned 90 degrees out of phase, the two output channels of the quadrature encoder indicate both position and direction of rotation. If A leads B, for example, the disk is rotating in a clockwise direction. If B leads A, then the disk is rotating in a counter-clockwise direction.

By monitoring both the number of pulses and the relative phase of signals A and B, both the position and direction of rotation can be tracked.

The resolution of quadrature encoders is typically given in Pulses per Revolution (PPR), also called Lines per Revolution. The RMC feedback sees each rising or falling edge of the A or B signal as one count. Therefore, each pulse or line of the encoder will result in four counts on the RMC feedback. For example, a 1000 PPR encoder will give the RMC 4000 counts per revolution.

The RMC increments the counts register when A leads B. It decrements the counts register when B leads A.

Index Pulse

Some quadrature encoders also include a third output channel, called a zero or index or reference signal, which supplies a single pulse per revolution. This single pulse is used for precise determination of a reference position. The RMC70 QA module and the RMC150 Quad module have an Index (Z) pulse input; the RMC70 Q1 module does not.

Homing Quadrature Axes

Quadrature encoders do not provide absolute position. To use a quadrature encoder in a position application, a known reference position must be established. This position is referred to as the Home position. The process of moving the axis to find the home position is called Homing. Once the Home position is established, the encoder increments or decrements the position from the initial home position as it rotates.

There are several methods of homing. Typically, it involves moving the axis to a proximity switch that then sends a signal to the RMC. The RMC sets the position at that precise point to whatever value the user requested. The index pulse can also be used for homing.

For details on homing, see the Homing topic.


See Also

Quadrature Scaling | QA Module (RMC70) | Q1 Module (RMC70) | Quadrature (Q) Module (RMC150)

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