Quadrature Decoder

Compute position of quadrature encoder

Since R2020a

• 

Libraries:
Motor Control Blockset / Sensor Decoders
Motor Control Blockset HDL Support / Sensor Decoders

Description

The Quadrature Decoder block computes the position of the quadrature encoder.

To calculate the angular position of the quadrature encoder (and the rotor) in either degrees, radians, or per-unit, the block uses one of the following methods.

• Difference between current encoder counter value and encoder counter value at the previous index pulse (when index pulse is available).

• Current encoder counter value (when index pulse is not available).

This figure shows a quadrature encoder disk with two channels (QEPA and QEPB) and an index pulse (QEPI):

In this example, the timer driven by the QEP increments by four for each slit:

Equations

The block computes the angular position (in counts) of the quadrature encoder using the Cnt and Idx block inputs:

When Cnt ≥ Idx:

$Position count=\left(Cnt-Idx\right)$

When Cnt < Idx:

$Position count=Counts per revolution+\left(Cnt-Idx\right)$

When Cnt (an unsigned integer) exceeds the maximum value of the selected counter size, the block adds the necessary compensation internally.

When you clear the External index count checkbox, only Cnt input is available, therefore:

$Position count=Cnt$

where:

• $Position count$ is the angular position of the quadrature encoder in counts.

• $Counts per revolution$ is the number of counts in one rotation cycle of the quadrature encoder (Counts per revolution = Encoder slits ⨯ Encoder counts per slit).

The block computes the output θ_m as:

$${\theta }_m=MaxPosition\times \left(\frac{Position count}{Encoder slits\times Encoder counts per slit}\right)=MaxPosition\times \left(\frac{Position count}{Countsperrevolution}\right)$$

Where, MaxPosition = 360 (degrees) or 2π (radians) or 1 (per-unit), based on the selected value of the Position unit parameter.

Examples

Quadrature Encoder Offset Calibration for PMSM

Calculates the offset between the d-axis of the rotor and encoder index pulse position as detected by the quadrature encoder sensor. The control algorithm (available in the field-oriented control and parameter estimation examples) uses this offset value to compute an accurate and precise position of the d-axis of rotor. The controller needs this position to implement the field-oriented control (FOC) correctly in the rotor flux reference frame (d-q reference frame), and therefore, run the permanent magnet synchronous motor (PMSM) correctly.

• Open Model

Field-Oriented Control of PMSM Using Quadrature Encoder

Implements the field-oriented control (FOC) technique to control the speed of a three-phase permanent magnet synchronous motor (PMSM). The FOC algorithm requires rotor position feedback, which is obtained by a quadrature encoder sensor. For details about FOC, see Field-Oriented Control (FOC).

• Open Model

Ports

Input

Cnt — Quadrature encoder counter value

scalar

Value that the quadrature encoder counter generates with respect to the slit-position. The port only accepts a scalar unsigned integer based on the Counter size parameter. For example, if you select 8 bits for Counter size, the input data type must be uint8.

Data Types: uint8 | uint16 | uint32

Idx — Quadrature encoder counter value at last index pulse

scalar

Value that the quadrature encoder counter generated with respect to the slit-position at the time of the last index pulse. The port only accepts a scalar unsigned integer based on the Counter size parameter. For example, if you select 8 bits for Counter size, the input data type must be uint8.

Dependencies

To enable this port, select the External index count parameter.

Data Types: uint8 | uint16 | uint32

CountsPerRev — Number of counts generated for every revolution

scalar

Number of counts the quadrature encoder generates for each revolution.

CountsPerRev = Encoder slits ⨯ Encoder counts per slit

Dependencies

To enable this port, select the Use input port for computed parameters (run-time motor swap) parameter.

Data Types: uint8 | uint16 | uint32 | single | double | fixed point

1/CountsPerRev — Inverse of the number of counts generated for every revolution

scalar

Specify the inverse of the number of counts the quadrature encoder generates for each revolution at this port. The Data type for 1/CountsPerRev input parameter defines the data type of this input.

Dependencies

To enable this port, select the Use input port for computed parameters (run-time motor swap) parameter.

Data Types: single | double | fixed point

Note

The data type of the input at the Cnt and Idx ports must be identical.

Output

θ_m — Angular position of quadrature encoder

scalar

Angular position that the block computes based on the Cnt and Idx inputs.

Data Types: single | double | fixed point

Parameters

Encoder slits — Number of slits per phase

1000 (default) | scalar

The number of slits available in each phase of the quadrature encoder.

Dependencies

To enable this parameter, clear the Use input port for computed parameters (run-time motor swap) parameter.

Encoder counts per slit — Number of counts generated for every slit

4 (default) | 1 | 2

The number of counts that the quadrature encoder generates for every slit. A count indicates a slit position. For example, select 4 (quadrature mode) if you want the encoder to generate four counts corresponding to 00, 10, 11, and 01 slit positions or values. If you select the quadrature mode:

Encoder counts per revolution (post-quadrature) = Encoder counts per slit ✖ Encoder slits

Dependencies

To enable this parameter, clear the Use input port for computed parameters (run-time motor swap) parameter.

Counter size — Size of quadrature encoder counter

16 bits (default) | 8 bits | 32 bits

Counter size signifies the register size of the counter used by the processor to count the quadrature encoder pulses. For example, when using TI’s C2000™ based hardware, it is the data type of the counter output of the eQEP block from C2000™ Microcontroller Blockset.

External index count — Enable Idx input port

on (default) | off

The block enables the Idx input port only if you select this parameter. The Idx input port expects a value equal to Cnt at the time of index pulse.

Position unit — Unit of angular position output

Radians (default) | Degrees | Per unit

Unit of the angular position output.

Position data type — Data type of angular position output

single (default) | double | fixed point

The data type for the angular position output.

Note

The Quadrature Decoder block can display the warning message 'Wrap on overflow detected' when needed.

Advanced Options

Use input port for computed parameters (run-time motor swap) — Enable CountsPerRev and 1/CountsPerRev input ports

off (default) | on

The block enables the CountsPerRev and 1/CountsPerRev input ports only if you select this parameter. When you select this parameter, the block also disables the Encoder slits and Encoder counts per slits parameters.

Data type for 1/CountsPerRev input — Data type of 1/CountsPerRev input

single (default) | double | fixed point

Specify the data type for the 1/CountsPerRev input. If you specify the fixed-point data type, you can use this parameter to specify the precision of the 1/CountsPerRev input.

Dependencies

To enable this parameter, select the Use input port for computed parameters (run-time motor swap) parameter.

Extended Capabilities

C/C++ Code Generation
Generate C and C++ code using Simulink® Coder™.

HDL Code Generation

Generate VHDL, Verilog and SystemVerilog code for FPGA and ASIC designs using HDL Coder™.

HDL Coder™ provides additional configuration options that affect HDL implementation and synthesized logic.

HDL Architecture

This block has one default HDL architecture.

HDL Block Properties

ConstrainedOutputPipeline	Number of registers to place at the outputs by moving existing delays within your design. Distributed pipelining does not redistribute these registers. The default is 0. For more details, see ConstrainedOutputPipeline (HDL Coder).
InputPipeline	Number of input pipeline stages to insert in the generated code. Distributed pipelining and constrained output pipelining can move these registers. The default is 0. For more details, see InputPipeline (HDL Coder).
OutputPipeline	Number of output pipeline stages to insert in the generated code. Distributed pipelining and constrained output pipelining can move these registers. The default is 0. For more details, see OutputPipeline (HDL Coder).

Restrictions

The block supports HDL code generation when you set Position data type as well as the inputs CountsPerRev and 1/CountsPerRev to use identical data type (either floating point or fixed point).

Fixed-Point Conversion
Design and simulate fixed-point systems using Fixed-Point Designer™.

Version History

Introduced in R2020a