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Beyond Bits Motor Control Edition Encoder Signal Processing Devices for industrial applications Introduction Quadrature encoders are widely used in industrial motor control applications as precise rotor position sensors mounted directly on the motor shaft. This article will provide information on the principle of the encoder operation and the quadrature decoder MCU module used for decoding of the position information. Quadrature Incremental Encoder processed by the on-chip hardware of the controlling MCU. The quadrature incremental encoder is a position feedback device that provides incremental counts. The incremental encoder provides relative information, where the feedback signal is always referenced to a start position. The signals are directly The quadrature encoder typyically has three output signals. The “phase A” and “phase B” signals consist of a series of pulses, the phase shifted by 90° (therefore the term “quadrature” is used). The third signal, called “index,” provides the absolute position information, in motion Figure 1: Quadrature Encoder Output Signals and Position Encoding Modes Phase A Phase B Counter Increment/ Decrement +1 +1 +1 +1 +1 +1 +1 +1 -1 -1 -1 -1 -1 -1 -1 +1 +1 +1 +1 +1 +1 +1 +1 -1 -1 -1 -1 -1 -1 -1 +1 +1 +1 +1 +1 +1 +1 Counter Value Time Phase A Phase B Counter Increment/ Decrement +1 +1 +1 +1 +1 +1 +1 +1 -1 -1 -1 -1 -1 Counter Value Time Beyond Bits control used to check the pulse counting consistency. That means, after each revolution, the value of the counted pulses is captured and compared against the defined value. If a difference is detected, the control algorithm then must perform the position offset compensation. Quadrature Decoder The quadrature decoder is the MCU peripheral module used for hardware decoding of the encoder signals and is a standard component of Freescale motor control dedicated chips. Though there are differences in the implementation of the quadrature decoders in particular MCU families, the principle is the same: it counts rising and falling edges of both phase signals. The internal logic of the quadrature decoder evaluates the direction of the rotation by increasing or decreasing the counter value. There are also other arrangements of the output encoder signals. For example, the phase A signal is a chain of the pulses while the polarity of phase B determines the counting direction, as shown in the lower time diagram in figure 1. Implementation of the Quadrature Decoder on DSCs The special purpose enhanced quadrature encoder/decoder module is part of the newly introduced MC56F84xx family of DSCs. It provides interfacing capability to position/speed sensors used in industrial motor control applications and has four input signals: phase A, phase B, index and home. This module is used to decode shaft position, revolution count and speed. It also supports different sources of interrupt events. Implementation of the Quadrature Decoder on Kinetis MCUs The quadrature decoder is one of several features of the FlexTimer module that is part of the Kinetis family of MCUs. It is not as rich in features as the previously mentioned module and only has two input signals, phase A and phase B, so only the position can be evaluated directly. If required, processing of the index signal has to be made by another on-chip timer. Motor Control Edition More information on the initialization and configuration of the quadrature decoders can be found in the following application notes, design reference manuals and device reference manuals: •A N4381 (Configuring the FlexTimer for Position and Speed Measurement with an Encoder) •D RM128 (PMSM Vector Control with Quadrature Encoder on Kinetis) RM102 (PMSM Vector Control •D with Single-Shunt Current-Sensing Using MC56F8013/23) •M C56F844XXRM, K40P100M72SF1RM These documents are available at freescale.com/motorcontrol. Beyond Bits Motor Control Edition How to Reach Us: Home Page: freescale.com Motor Control Portfolio Information: freescale.com/motorcontrol e-mail: [email protected] USA/Europe or Locations Not Listed: Freescale Semiconductor Technical Information Center, CH370 1300 N. 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