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Ttelmah
Joined: 11 Mar 2010
Posts: 19540
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 Posted: Wed Jul 08, 2015 4:34 am |
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BLDC, and steppers don't have much in common.
With a stepper you drive each pole (or pair) in turn, and PWM, is only used to adjust the current. With BLDC, you are continuously modulating the PWM, to synthesise sinusoidal drives (like a synthesised synchronous motor), _but_ the key thing is that you adjust this to lead/lag the current position of the motor, to give the required drive. Now the problem is how you know 'where' the motor is?. One form is 'sensorless' drive, and on this you sense the position of the rotor using the BEMF on the one undriven phase (you only drive two phases at a time out of three. This works fine once the motor is spinning, but not when you start. The other way is to have separate sensors. Now the sensorless drive 'starts' by a process similar to a stepper motor, feeding a low power pattern at a low speed that the motor 'ought' to be able to handle, until it starts. Problem is that this only works if there is only a very low load at the start (so works fine for things like fans, but not for things like pumps). the sensored drive instead knows where the motor is, and can sense the position right from the start. Hence any application needing high starting torque must be sensored. So as I've already said the first thing he is going to have to crack is generating the three sensors in the right positions.
Just because it 'looks' a bit like a stepper, with 'poles' that need to be driven, don't think there is actually any real similarity, when it comes to driving them. there isn't....
As a 'quickie' for anyone wanting to play with BLDC, this is AN857, translated to run on CCS:
| Code: |
//Code based on AN857 for BLDC
#include <16F877.h>
#device adc=16 //Left justify the ADC
#FUSES NOWDT, HS, NOBROWNOUT, NOLVP
#use delay(clock=16MHz)
#define ManThresh (0x3F) //Change to 0xFF for manual mode - 0x3F else
#define AutoThresh (0x100-ManThresh)
int8 flags=0;
#bit DriveOnFlag=flags.0
#bit AutoRPM=flags.1
#bit FullOnFlag=flags.4
#bit Tmr0Ovf=flags.5
#bit Tmr0Sync=flags.6
int8 PhaseIndx=6,Drive,RPMIndex;
int8 ADCRPM;
int8 PWMThresh=0;
int8 ADCOffset;
int8 Vsupply;
int8 DeltaV1,DeltaV2;
int16 CCPSave,CCPT2;
int8 RampTimer;
#define AccelDelay 100
#define DecelDelay 10
#bit BEMF1Low=DeltaV1.7
#bit BEMF2Low=DeltaV2.7
#bit ADCneg=ADCOffset.7
enum {RPMSetup,RPMRead,OffsetSetup,OffsetRead,Vsetup,Vidle,Vread,BEMFSetup,BEMFIdle,BEMFRead,BEMF2Idle,BEMF2Read,Inval} STATE=RPMsetup;
#use fast_io(C)
/*
ADC channels are wired as:
AN0 = RPM I/P voltage
AN1 = Offset
AN3 = sense on OP/A BEMF etc..
*/
#define RPM (0)
#define OFFSET (1)
#define BEMF (3)
//Drive word definitions
#define OffMask (0B11010101)
#define Invalid (0B00000000)
#define Phase1 (0B00100001)
#define Phase2 (0B00100100)
#define Phase3 (0B00000110)
#define Phase4 (0B00010010)
#define Phase5 (0B00011000)
#define Phase6 (0B00001001)
//ADC switching definitions
//Each assumes the ADC is in a defined previous state, and changes it to the next
#define ADC0to1 (0B00001000)
#define ADC1to3 (0B00010000)
#define ADC3to0 (0B00011000)
#byte ADCON0=getenv("SFR:ADCON0") //0xFC2 //("SFR:ADCON0")
#byte ADRESH=getenv("SFR:ADRESH") //0xFC4 //("SFR:ADRESH")
#bit GO=ADCON0.1
#byte STAT=getenv("SFR:STATUS")
#bit CARRY=STAT.0
#bit ZERO=STAT.2
#byte TMR0=getenv("SFR:TMR0")
#byte CCP1CON=getenv("SFR:CCP1CON")
#bit SPECIAL_EVENT=CCP1CON.0
#include "T1Tableold.h" //You will need to use the code as shown and build this for your motor
//format is const int16 Timer1Table[256] = {xxx,xxx...};
#INT_GLOBAL
void timer_0_int(void) {
Tmr0Ovf=TRUE;
Tmr0Sync=TRUE;
clear_interrupt(INT_TIMER0); //fastest possible int handler - relies on instructions not changing flags etc..
}
const int8 OnTable[]={Invalid,Phase6,Phase5,Phase4,Phase3,Phase2,Phase1,Invalid};
void DriveMotor(void);
void LockTest(void);
void commutate(void) {
//Commutation triggered by CCP1IF
if (SPECIAL_EVENT) {
//Here special event trigger is set
clear_interrupt(INT_CCP1);
if (--PhaseIndx==0) PhaseIndx=6; //Handle 'bottom of table'
Drive=OnTable[PhaseIndx];
DriveMotor();
}
}
void DriveOff(void) {
set_adc_channel(RPM); //Changing to start with my current reading
STATE=RPMSetup;
}
void main(void)
{
int8 temp;
output_c(0); //Motor drivers off to start....
set_tris_c(0);
//setup_adc_ports(sAN0|sAN1|sAN3);
setup_adc(ADC_CLOCK_DIV_32);
set_adc_channel(RPM);
setup_spi(FALSE);
setup_timer_0(T0_DIV_2);
enable_interrupts(INT_TIMER0);
setup_timer_1(T1_INTERNAL|T1_DIV_BY_4);
setup_ccp1(CCP_COMPARE_RESET_TIMER);
clear_interrupt(INT_TIMER0);
enable_interrupts(GLOBAL);
do {
if (interrupt_active(INT_CCP1)) commutate();
DriveOnFlag=TRUE;
if (!FullOnFlag) {
temp=(int8)PWMThresh+TMR0;
if (!CARRY) DriveOnFlag=FALSE;
DriveMotor();
}
LockTest();
switch (STATE) {
case RPMSetup:
if (Drive==Phase1) {
output_high(PIN_A4);
read_adc(ADC_START_ONLY);
set_adc_channel(OFFSET); //Offset default
STATE++;
Tmr0Sync=FALSE;
output_low(PIN_A4);
}
break;
case RPMRead:
if (adc_done()) {
ADCRPM=read_adc(ADC_READ_ONLY);
}
break;
case OffsetSetup: //Wait for Phase2 ADC Go RA3->ADC
if (Drive==Phase2) {
read_adc(ADC_START_ONLY); //Trigger ADC
set_adc_channel(BEMF);
STATE++;
}
break;
case OffsetRead:
//Wait for ADC Read ADC->ADC Offset
if (adc_done()){
ADCOffset=ADRESH ^ 0x80; //Generate +/- offset
PWMThresh=ADCRPM+ADCOffset;
if (!bit_test(ADCOffset,7)) { //Offset is +ve
if (CARRY) PWMThresh=0xFF;
}
else {
if(!CARRY) PWMThresh=0;
}
if (ZERO) {
DriveOff();
break;
}
FullOnFlag=FALSE;
if (PWMThresh>0xFD) FullOnFlag=TRUE; //Fixed full on threshold
STATE++;
}
break;
case Vsetup:
//wait for phase4
if (Drive==Phase4) {
CCP_1=Timer1table[RPMIndex]; //Table from spreadsheet
STATE++;
}
break;
case Vidle:
//Wait for Drive ON, wait Tacq, set ADC go
if (DriveOnFlag) {
delay_us(8); //Tacq
read_adc(ADC_START_ONLY);
STATE++;
}
break;
case Vread:
if(adc_done()) {
//ADC has finished
Vsupply=ADRESH;
STATE++;
Tmr0Sync=FALSE;
}
break;
case BEMFSetup:
if(Drive==Phase5) {
if (Tmr0Sync) {
if (bit_test(PWMThresh,7)!=0) {
if (DriveOnFlag==FALSE) break;
}
//BEMFS1
SPECIAL_EVENT=FALSE; //Turn off event on compare
CCPSave=CCPT2=CCP_1; //Save two copies of CCP
CCPT2/=2; //1/2 Phase time
CCP_1=CCPT2/2; //1/4 phase time into CCP_1
STATE++;
}
}
break;
case BEMFIdle:
if (interrupt_active(INT_CCP1)) {
DriveOnFlag=TRUE;
DriveMotor();
Delay_us(8);
read_adc(ADC_START_ONLY);
CCP_1+=CCPT2; //add 1/2 phase time to give 3/4 phase time
clear_interrupt(INT_CCP1);
STATE++;
}
break;
case BEMFRead:
if (adc_done()) {
DeltaV1=ADRESH-(Vsupply/2);
STATE++;
}
break;
case BEMF2Idle:
if (interrupt_active(INT_CCP1)) {
DriveOnFlag=TRUE;
DriveMotor();
delay_us(8);
read_adc(ADC_START_ONLY);
set_adc_channel(RPM);
CCP_1=CCPSave; //Full time
clear_interrupt(INT_CCP1);
SPECIAL_EVENT=TRUE; //enable event on compare
STATE++;
}
break;
case BEMF2Read:
if (adc_done()) {
DeltaV2=ADRESH-(Vsupply/2);
STATE=RPMSetup; //Changing to start with current reading
}
break;
case Inval:
STATE=RPMSetup;
//Status=0;
set_adc_channel(RPM);
break;
}
} while(TRUE);
}
void DriveMotor(void) {
if (DriveOnFlag) {
output_c(Drive | 0x80);
}
else {
output_c(Drive & OffMask);
}
}
void LockTest(void) {
if(Tmr0Ovf==TRUE) {
if (bit_test(PWMThresh,7)){
//On longer than off
if (DriveOnFlag==FALSE)
return;
}
//Here drive is either on, and on>off, or drive off, and off>on - LT05
Tmr0Ovf=FALSE;
if (--RampTimer!=0)
return;
AutoRPM=TRUE;
if (ADCRPM<=ManThresh) {
AutoRPM=FALSE;
}
if (BEMF1Low) {
//LT10
output_low(PIN_B6);
output_high(PIN_B7); //ahead of lock
RampTimer=AccelDelay;
if (AutoRPM==FALSE) {
//Manual code
RPMIndex=ADCRPM;
return;
}
if (RPMIndex==0xFF) {
return;
}
RPMIndex++;
return;
}
//LT20 - else
RampTimer=DecelDelay;
if (!BEMF2Low) {
output_low(pin_B6);
output_low(PIN_B7); //behind lock
if (AutoRPM==FALSE) {
//Manual code again
RPMIndex=ADCRPM;
return;
}
if (RPMIndex>0) --RPMIndex;
return;
}
//Showlocked
output_high(PIN_B6);
if(AutoRPM==FALSE) {
//Manual code
RPMIndex=ADCRPM;
}
}
}
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The table has to be built using Excel or something similar as outlined in the AN. Wiring is as for the AN. |
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snoop911
Joined: 24 Aug 2007
Posts: 4
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| Posted: Tue Sep 08, 2015 11:33 pm |
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due to the plethora of cheap l298n boards on ebay, I was doing a similar search for l298n and bldc.
in case it helps, there's a very nice write-up of someone doing a bldc project with a an l298n:
http://www.electronics-homemade.com/BLDC.html
I hope to use a low-voltage bldc motor from a cdrom, with sensorless 6-step commutation. |
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