Microchip Technology MCP6 series Manual de usuario descargar pdf (Pagina 9)

 2004 Microchip Technology Inc. DS00900A-page 9

AN900

INITIALIZING THE QUADRATURE ENCODER

INTERFACE

When implementing closed-loop V/f control, a form of

velocity feedback is required. The Quadrature Encoder

Interface can be used in conjunction with Timer5 to

provide very accurate velocity feedback and direction

information. In Velocity Measurement mode, velocity

event pulses are generated on each edge of the QEA

signal. Timer5 counts upward and its value is captured

on each velocity pulse when it is reset to zero. The

captured Timer5 value is stored in the Velocity registers

(VELRH:VELRL), which is used by V/f control with

Velocity Feedback mode.

To enable velocity measurement:

1. Configure the QEI module for Velocity mode

measurement:

a) Enable Velocity mode (clear the VELM

bit).

b) Set the QEI module mode to one of the 2x

capture configurations.

c) Set the pulse reduction ratio to 1:1.

2. Configure Timer5 as the QEI time base:

a) Disable the Timer5 special event Reset.

b) Enable Continuous Count mode.

c) Set the input clock prescaler to 1:1.

d) Enable Synchronous Timer mode.

e) Enable the timer.

3. Enable the Timer5 interrupt and set as low priority

V/f CONTROL FIRMWARE

The heart of the ACIM control is accomplished with the

PCPWM peripheral operated in Complimentary mode.

The duty cycle of the three PWM channels are changed

in a regular manner using a Timer0 interrupt to

synthesize the three-phase waveforms that drive the

motor.

A sine table is stored in program memory. It is trans-

ferred to data memory during initialization for faster

access. Three registers are used as offsets to the table

through indirect addressing. Each of the offset values

points to one of the values in the table, such that there

is always a 120-degree phase shift between the

phases. (Each of the waveforms in Figure 3 is created

by an offset register associated with its PWM.) The

code sample in Example 1 shows how the table is read

using the indirect addressing registers.

In this application, the potentiometer determines the

target motor speed reference signal. Waveform synthe-

sis is identical for open-loop V/f and V/f with velocity

feedback. The difference lies in how the motor drive

frequency is generated from the target frequency.

Depending upon the control strategy, the reference

signal from AN1 is used in one of two ways:

DRIVE FREQUENCY CALCULATION FOR

OPEN-LOOP V/f CONTROL

In this method, the motor drive frequency is directly

calculated from the potentiometer input. Specifically, the

upper byte of the A/D Result register is divided by four to

give the target drive frequency f in Hz. In this application,

the target speed has a lower boundary of 12 Hz and an

upper boundary of 60 Hz. The V/f function determines

the drive amplitude corresponding to that frequency.

Since the synchronous speed (in RPM) for an induction

motor is 120 f/p, where p is the number of stator poles,

the target drive speed (in RPM) can be directly calcu-

lated as 30 times the value of ADRESH (120 divided by

times ADRESH divided by 4), divided by p. For the

current application, a motor with two stator poles is

assumed. This reduces to a motor speed equal to 15

times ADRESH.

EQUATION 1: CALCULATING DRIVE

FREQUENCY AND SPEED

EXAMPLE 1: ACCESSING THE SINE TABLE THROUGH INDIRECT ADDRESSING

target

= (ADRESH/4)

(5 ≤ f

target

≤ 60)

Drive Frequency:

= (120/f • p) × (ADRESH/4) = (30 • ADRESH)/p

Calculated Speed:

UPDATE_PWM_DUTYCYCLES ;first update PWM1

MOVF TABLE_OFFSET1,W ;place offset for first sine value inWREG

MOVF PLUSW0,W ;use the value in WREG as an offset to FSR0

;FSR0 points to beginning of sine table

;value at sine_table + table_offset1

;is copied to WREG

BZ PWM1_IS_0 ;check to see if value is 0

MULWF FREQUENCY ;multiply the table value times the frequency

MOVFF PRODH,PDC0H_TEMP ;copy the result to the duty cycle register

MOVFF PRODL,PDC0L_TEMP

BRA UPDATE_PWM2 ;continue on to update PWM2

PWM1_IS_0 ;If table value is zero,

MOVLW 0x02 ; make the duty-cycle a small non-zero value

MOVWF PDC0L_TEMP

;continue on to update PWM2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 ... 23 24

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