dsPIC30F Family Reference Manual
DS70063C-page 16-2 © 2004 Microchip Technology Inc.
16.1 Module Introduction
16.1.1 Features Overview
Quadrature encoders (a.k.a. Incremental encoders or Optical encoders) are used in position
and speed detection of rotating motion systems. Quadrature encoders enable closed loop
control of many motor control applications, such as Switched Reluctance (SR) motor and AC
Induction Motor (ACIM).
A typical incremental encoder includes a slotted wheel attached to the shaft of the motor and an
emitter/detector module sensing the slots in the wheel. Typically, three outputs, termed: Phase
A, Phase B and INDEX, provide information that can be decoded to provide information on the
movement of the motor shaft including distance and direction.
The two channels, Phase A (QEA) and Phase B (QEB), have a unique relationship. If Phase A
leads Phase B, then the direction (of the motor) is deemed positive or forward. If Phase A lags
Phase B then the direction (of the motor) is deemed negative or reverse. A third channel,
termed index pulse, occurs once per revolution and is used as a reference to establish an
absolute position. See Figure 16-1 for a relative timing diagram of these three signals.
The quadrature signals produced by the encoder can have four unique states. These states are
indicated for one count cycle in Figure 16-1. Note that the order of the states are reversed when
the direction of travel is changed.
A Quadrature Decoder captures the phase signals and index pulse and converts the information
into a numeric count of the position pulses. Generally, the count will increment when the shaft is
rotating one direction and decrement when the shaft is rotating in the other direction.
Figure 16-1: Quadrature Encoder Interface Signals
QEA
QEB
INDX
QEA
QEB
INDX
1 Cycle
01
00
10
11
11
10
00
01
Forward Travel
Reverse Travel
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