Microchip Technology dsPIC30F User manual

dsPIC30F Quadrature Encoder Interface Module

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dsPIC30F Quadrature Encoder

Interface Module

DS

Digital Signal Controller

Welcome to the dsPIC30F Quadrature Encoder Interface Module web seminar.

dsPIC30F Quadrature Encoder Interface Module

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Session Agenda

O What is a Quadrature Encoder?

O General Features Overview

O Programmable digital noise filters

O Quadrature Decoder

O The QEI as a Timer/Counter

These are the main topics we will address during this seminar.

First of all, we will see the purpose of the QEI module is. Then we will go

through all the main functional blocks, the digital noise filters, the decoder, and

the position counter. Finally we will see that this peripheral, if not used as an

encoder, can behave as an up/down counter/timer.

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What is a Quadrature Encoder?

slotted wheel

light source

sensing device

logic

A Quadrature Encoder (or incremental encoder, or optical encoder) is used to

detect the position and speed of rotors, enabling closed loop control in many

motor control applications like switched reluctance and induction motors.

Typically, an encoder includes a slotted wheel attached to the motor shaft, a

light source, a light sensing device and some logic. During the rotation of the

shaft, the light passes through the slots and hits the sensing element,

generating electric signals.

The three digital outputs are called Phase A, Phabe B and Index. With some

decoding performed by the dsPIC quadrature encoder interface, these signals

can tell us the speed and position of the rotor.

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What is a Quadrature Encoder?

Phase A leads Phase B

Phase B leads Phase A

An example of the encoder output waveform is illustrated here.

In the upper plot we see the Phase A and Phase B signals coming in on the

QEA and QEB pins respectively. In this case, Phase A leads Phase B: the shaft

is spinning forward. If we reverse the rotor rotation, like in the lower part of the

figure, Phase B will lead Phase A: so by detecting which rising edge comes

first, we are able to detect the direction of rotation.

The encoder outputs can only have four different states as indicated in the

figure: 01, 00, 10, 11. Note that if we reverse the direction the sequence is

reversed as well.

The index occurs only once per revolution and is used to establish an absolute

position.

The quadrature decoder, which is part of the interface circuitry, will read these

signals and converts them into a numeric count of the position pulses. The

count will increment when the shaft is rotating in one direction and will

decrement when the rotation is reversed.

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dsPIC QEI Features

O QEI Features

O QEI decodes signals and accumulates count

O Logically swap A and B inputs

O Programmable noise filters on inputs

O x2 and x4 counting modes

O 16-bit Position count register

O Reset on index pulse (if enabled)

O Reset on rollover/underflow

O Count error status bit

The QEI will perform all the operations needed to effectively use the information

coming from the encoder.

Since these signals are heavily affected by noise, a digital filter is available on

each input. The filtered phase edges are counted by a dedicated 16 bit up/down

counter, also referred to as the Position Counter. To establish a reference point

for position and speed measurements, the counter can be reset either by the

index signal or by a counter period match. The hardware can also perform

some error checking on on the accumulated count.

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Block Diagram

Clock

Divider

INDEX

1

0

TQCS

Tcy

Digital Filter

Logic

Digital Filter

Logic

Digital Filter

Logic

QEB

QEA

Prescaler and

Sync. Logic

Tcy

UPDN

16-Bit Up/Down

Counter

DIR

Quadrature

Decoder

Logic

Clock

Reset

Max. Count

Register

Timer Mode

Comparator

This block diagram depicts the internal architecture of the QEI modules. We

can see the input pins and the associated digital filters. There is also an

up/down input pin that is mainly used when the unit operates as a counter. The

quadratrute decoder logic is responsible for analysing which edge comes first

and the counter accumulates the edge count and is compared with the internal

Max Count Register.

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Digital Filters

O Multiple clock options to digital filter

O Tcy, 2Tcy, 4Tcy, 8Tcy, 16Tcy, …, 256Tcy

O Signal must be stable for 3 clock cycles

O Adjust clock divide bits to change noise

filtering characteristics

O Use of digital filter generates latency

The digital filters are responsible for rejecting noise from the three inputs. The

instruction cycle clock can be divided down by 2 , 4, 16, 32, 64, 128, 256 before

being used in the filter. The lower the clock frequency the lower frequencies are

rejected by the filter.

The prescaled clock is used to sample the input signal: if and only if three

consecutive samples have the same value the input is considered stable and

the value is output from the filter. One of the effects of this sampling is an

added latency, because there is a “propagation delay” of the input signal

through the filter.

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Digital Filters

T

CY

QEA/B

Filter

Here we can see that the input signal at the QEA or QEB pin is sampled using

the selected clock, in this case the instruction cycle period Tcy. If at least three

samples having the same value are detected the output is updated, otherwise

the input signal changes are disregarded. Glitches and spikes can be efficiently

filtered out by the digital filters.

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Quadrature Decoder

O Four basic modes

O x 2 mode with Index Pulse reset

O X 2 mode with reset by match

O X 4 mode Index Pulse reset

O X 4 mode with reset by match

As we have already seen, the quadrature decoder must determine the direction

of rotation looking at the two incoming phase signals, and generate the clock

that will be used by the position counter.

We can select between two modes: in the first one (x2) the decoder only

generates a clock impulse at the rising and falling edges of Phase A signal; in

the other mode (x4), the clock pulses are generate at each edge of phase A

and Phase B. The position counter can be reset either by the index pulse

coming from the encoder or by the matching of the current position counter

value with the number in the Maximum Count Register.

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Timing Diagram

+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

COUNT

PHASE A

PHASE B

UPDN

This is an example of how the decoder works. We are using the x4 mode,

where the clock pulse is at each edge of both phases. In the first part of the

timing diagram Phase A leads Phase B, so that the counter is counting up.

Then, in the second half, the rotation of the rotor is reversed, Phase B now

leads Phase A and the counter is counting down. This is why an up/down

counter is required in this application.

The count direction can be determined by reading the UPDN bit in the QEI

control register, but the UPDN pin can also be used to indicate the count

direction status.

With the x4 mode we can get a very high angular resolution, but we also get a

relatively high output clock frequency. With the x2 mode, the resolution is twice

as fine, but the frequncy is lower.

The maximum allowed quadrature frequency is one-third of the instruction cycle

frequency Fcy.

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Position Counter

O Up/down counter

O Counts pulses generated by the decoder

O Count is accumulated in POSCNT register

O POSCNT can be accessed, both for read

and write

O Its value can be compared to MAXCNT

register

The position counter can be used either for position or speed measurement.

To measure motor position, we must know the relationship between the

displacement and the number of phase pulses we get from the encoder. This

relation can be known in advance, or can be measured during initialization by

accumulating the total count for the maximum allowed displacement. We can

set a constant value in the Maximum Count register, which is typically the

number of pulse edges generated by one encoder revolution. As soon as we

have a match between the Position Count and the Maximum Count, an interrupt

is generated. In the Interrupt Service Routine, the user software can increment

or decrerment a software counter containing the most significant bits of the

position count.

For speed measurement application, the time interval between two index pulses

or count match events gives a measure of the angular velocity.

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QEI as a 16 bit Timer/Counter

If the quadrature decoder functionality is not needed, the QEI peripheral module

can be configured as an additional 16 bit counter/timer.

In this mode, the Position Counter register has the same functionality as the

Timer registers in general-purpose timers, while the Maximum Count register

serves as a period register. An additional feature compared to the general-

purpose timers is that the QEI counter is able to both increment and decrement

its count, thus providing up-down counter functionality.

The timer clock can be either internal or external; in the latter case the input pin

is the QEA pin and the input clock, after digital filtering, will be synchronized

with the instruction cycle. As in the general-purpose timers, gated time

accumulation is also possible.

The counting direction can be selected either with the UPDN bit in register

QEICON, or with the QEB pin.

An interrupt is generated when the value in the Position Counter register

maches the value in the Maximum Count register.

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Key Support Documents

Device Selection Reference Document #

General Purpose and Sensor Family Data Sheet DS70083

Motor Control and Power Conv. Data Sheet DS70082

dsPIC30F Family Overview DS70043

Base Design Reference Document #

dsPIC30F Family Reference Manual DS70046

dsPIC30F Programmer’s Reference Manual DS70030

MPLAB MPLAB® C30 C Compiler User User’s Guide DS51284

MPLAB ASM30, LINK30 & Utilities User DS51317

dsPIC® Language Tools Libraries DS51456

For more information, here are references to some important documents that

contain a lot of information about the dsPIC30F family of devices.

The Family Reference Manual contains detailed information about the

architecture and peripherals, whereas the Programmer’s Reference Manual

contains a thorough description of the instruction set.

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Key Support Documents

Device Specific Reference Document #

O dsPIC30F2010 Data Sheet DS70118

O dsPIC30F3010/3011 Data Sheet DS70141

O dsPIC30F4011/4012 Data Sheet DS70135

O dsPIC30F6010 Data Sheet DS70119

Microchip Web Site: www.microchip.com

For device-specific information such as pinout diagrams, packaging and

electrical characteristics, the device datasheets listed here are the best source

of information.

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Related Material

O Apps Notes on Motor Control

AN901 Using the dsPIC30F for Sensorless BLDC Control

AN908 Using the dsPIC30F for Vector Control of an ACIM

AN957 Sensored BLDC Motor Control Using dsPIC30F2010

We also have some application notes on motor control, in which the peripheral

is used.

All these documents can be obtained from the Microchip web site, by clicking

on the “dsPIC® Digital Signal Controllers” or “Technical Documentation” link.

This wraps up the seminar on dsPIC30F QEI. Thank you for your interest in the

dsPIC30F Family of Digital Signal Controllers.

Microchip Technology dsPIC30F User manual

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