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 as:

When the encoder rotates in the clockwise direction:

If $I d x \leq C n t$ ,
$P o s i t i o n c o u n t = (C n t - I d x)$
If $I d x > C n t$ and the shaft continues to rotate in the clockwise direction,
$P o s i t i o n c o u n t = (C n t - I d x)$
If $I d x > C n t$ and the shaft starts rotating in the anticlockwise direction,
$P o s i t i o n c o u n t = C o u n t s p e r r e v o l u t i o n - (I d x - C n t)$

When the encoder rotates in the anticlockwise direction:

If $I d x \geq C n t$ ,
$P o s i t i o n c o u n t = C o u n t s p e r r e v o l u t i o n - (I d x - C n t)$
If $I d x < C n t$ and the shaft continues to rotate in the anticlockwise direction,
$P o s i t i o n c o u n t = C o u n t s p e r r e v o l u t i o n - (I d x - C n t)$
If $I d x < C n t$ and the shaft starts rotating in the clockwise direction,
$P o s i t i o n c o u n t = (C n t - I d x)$

When you clear the External index count parameter, the Idx pulse resets Cnt to zero, therefore:

$P o s i t i o n c o u n t = C n t$

where:

$P o s i t i o n c o u n t$ is the angular position of the quadrature encoder in counts.
$C o u n t s p e r r e v o l u t i o n$ is the number of counts in one rotation cycle of the quadrature encoder.

The block computes the output θ_m as:

$P o s i t i o n = 360 \times P o s i t i o n c o u n t / (E n c o d e r s l i t s \times E n c o d e r c o u n t s p e r s l i t)$ (in degrees)

$P o s i t i o n = 2 π \times P o s i t i o n c o u n t / (E n c o d e r s l i t s \times E n c o d e r c o u n t s p e r s l i t)$ (in radians)

$P o s i t i o n = C n t / (E n c o d e r s l i t s \times E n c o d e r c o u n t s p e r s l i t)$ (in per-unit)

Examples

Quadrature Encoder Offset Calibration for PMSM Motor

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 Example

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 Example

Ports

Input

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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

Note

The input data types for both Cnt and Idx must be identical.

Output

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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

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Encoder slits — Number of slits per phase
`1000` (default) | scalar

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

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 density, in counts per revolution (post-quadrature) = Encoder counts per slit (4) ✖ Encoder slits

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. It is the data type of the counter output of eQEP.

External index count — Enable `Idx` input port
`on` (default) | `off`

The block enables the Idx input port only if you select this parameter. The block expects that the Cnt input port value resets at the time of the Idx 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` | `fixdt(1,16,0)` | `fixdt(1,16,2^0,0)` | `<data type expression>`

The data type for the angular position output.

Note

The Quadrature Decoder block may occasionally display the warning message 'Wrap on overflow detected.'

Extended Capabilities

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

HDL Code Generation
Generate Verilog and VHDL 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).

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

Version History

Introduced in R2020a

Quadrature Decoder