I was experimenting with microcontroller based stepper motor control. I couldN'T find any decent resources on the Internet that were directly related to acceleration, deceleration, etc., so I created a very simple proof of concept program for a PIC18F452. This program has no real use other than to show how a simple pulse engine can be created. As is, the program only spins a stepper back and forth using different speeds and acceleration rates. This was dumbed down from a more advanced 2 axis version with external control.
It basically works like this. You setup a timer to trigger an interrupt at a rate that is higher than any anticipated step rate. You don't want to get too greedy here, remember this is just an 8-bit microcontroller. I chose 25MHz.
Before each move, you pre-calculate a bunch of things to keep the math very simple inside the engine. You will then have values of acceleration and maximum speed per engine tick. Inside the engine you have an accumulator that counts the distance to move at each engine tick. You just add the acceleration to the current speed and add the current speed to the distance. The move per tick is always less that one, so it will take several ticks before you add up to a full step. At that point you take the step. To avoid using floating point values which will be too slow to calculate, these numbers are all multiplied by an offset. It chose 2^24 for this. Therefore the accumulator must get to 2^24 before a step is taken. The offset is then subtracted from the accumulator and the process repeats until you reach the target.
The only two other issues the engine must deal with are max speed and deceleration. Before it add acceleration to the current speed it checks to see if it is at the maximum. It also check to see if we got to a pre calculated deceleration point and subtracts speed rather than adding it at each engine tick.
The video below shows the program running. It is using my EasyPIC4 development board from MicroElectronika connected to a very old Xylotex 3 axis stepper board, connected to a small stepper. The stepper is a 1.8 Deg stepper with the Xylotex in 1/8 step mode. The first move is 1600 steps and 1000 steps/sec/sec acceleration and 2000 steps/sec maximum speed. The second goes back to 0 at 50,000 step/sec/sec acceleration and 20,000 steps/sec max speed.
/*
Progam: Stepper Motion Test
Author: B. Dring
Email: bdring@buildlog.net
Date: 11/20/09
Device: 18F452
Osc: 8MHz xtal with 4x PLL
Compiler: CCS PCWH 4.042 from Customer Computer Services Inc
www.ccsinfo.com
Motion Control Test Program
This program controls a stepper motor by outputting step and direction signals. It allows
varying the move distance, acceleration and max speed. All units are in steps. An external
program needs to convert from real world units to steps. We maintain a 32bit
current location. So all moves can be absolute (ie move to 1000...then move back to 500)
This uses a 25Khz Pulsing Engine in the Timer2 interrupt.
On every interrupt (engine tick) an amount gets added to a distance accumulator. Since the engine
rate is higher than any step rate we would ever see, this amount is always less than a
step. To avoid floating point math, this accumulator is shifted up by 24 bits.
Therefore, when the accumulator is greater than 2^24 we take a step and reduce the
accumulator by 2^24.
The amount added to the accumulator is based on the speed and acceleration. Acceleration
and max speed numbers are calculated before the move in accumulator units per tick. This
makes the math very simple in the engine. On each tick, add the acceleration value
to the current speed and add the current speed to the accumulator. The accululator is
always positive. It does not care about direction, just time to next step
The only other things you need to do in the engine is to stop accelerating when
you reach the max speed and decelerate at the right time.
The engine must be kept very simple so it finishes it's work WELL within it's period,
which is 1/25kHz or 40uS.
*/
#include <18F452.h>
#device adc=8
#FUSES NOWDT //No Watch Dog Timer
#FUSES WDT128 //Watch Dog Timer uses 1:128 Postscale
#FUSES H4 //High speed osc with HW enabled 4X PLL
#FUSES NOPROTECT //Code not protected from reading
#FUSES NOOSCSEN //Oscillator switching is disabled, main oscillator is source
#FUSES BROWNOUT //Reset when brownout detected
#FUSES BORV20 //Brownout reset at 2.0V
#FUSES PUT //Power Up Timer
#FUSES STVREN //Stack full/underflow will cause reset
#FUSES NODEBUG //No Debug mode for ICD
#FUSES LVP //Low Voltage Programming on B3(PIC16) or B5(PIC18)
#FUSES NOWRT //Program memory not write protected
#FUSES NOWRTD //Data EEPROM not write protected
#FUSES NOWRTB //Boot block not write protected
#FUSES NOWRTC //configuration not registers write protected
#FUSES NOCPD //No EE protection
#FUSES NOCPB //No Boot Block code protection
#FUSES NOEBTR //Memory not protected from table reads
#FUSES NOEBTRB //Boot block not protected from table reads
#use delay(clock=32000000)
#use rs232(baud=9600,parity=N,xmit=PIN_C6,rcv=PIN_C7,bits=8,errors)
#define ENGINE_RATE 25000 // be sure timer interrupt matches this
#define SPEED_OFFSET 16777216 // 2^24 The is the offset of the accumulator 2^24 units = 1 step
#define PIN_X_STEP PIN_A1 // pin corrector to step of stepper board
#define PIN_X_DIR PIN_A2 // pin connected to direction pin of stepper board
#define DIRECTION_FORWARD 1
#define DIRECTION_REVERSE -1
signed int32 currentLocation; // store current location in steps so we can use absolute moves
int32 decelLocation; // pre-calculated point we start to decelerated
signed int8 motionDirection; // what direction do we move 1 = fwd -1 = rev
short bEnableMotion; // this turns on/off the 25kHz Pulse Engine
short bDecel; // in decel mode
int32 currentSpeed; // current speed in accum units per engine tick
int32 accel; // accel rate in accum units per engine tick
int32 maxSpeed; // max speed in accum units per tick
int32 accumulator;
signed int32 targetLocation; // target position in steps
long moveAccelSteps; // accel rate in steps / sec /sec
long moveMaxSpeedSteps; // max speed in steps /sec
// this is the pulsing engine...the the file header for more info
#int_TIMER2
void TIMER2_isr(void)
{
// we only do anything if this motion is enabled and we are not at the target position
if (bEnableMotion && (currentLocation != targetLocation) )
{
// set the current speed
// If we are before the decel point we accelerate or coast
// else we decel
if (currentLocation == decelLocation)
bDecel = true;
if (!bDecel)
{
// if we are not at max speed and accel to the speed
if (currentSpeed < maxSpeed)
{
currentSpeed += accel;
}
}
else // decel
{
currentSpeed -= accel;
if (currentSpeed <= 0) // never let it get to zero otherwise we could get stuck
{
currentSpeed = accel;
}
}
accumulator += currentSpeed;
//see if we have accumulated enough for a step step
If (accumulator > SPEED_OFFSET)
{
currentLocation += motionDirection;
output_high(PIN_X_STEP); // turn on step pin
delay_us(4); // wait a while
output_low(PIN_X_STEP); // turn it off
accumulator -= SPEED_OFFSET;
}
}
else
{
bEnableMotion = false;
}
}
/* this is the simple "motion planner". It does all the intesive calulations
It calculated the acceleration per engine tick
It calculated the max speed per engine tick
It determines the motion direction
It resets the current speed to 0
It turns on the Pulse Engine
*/
void doMotion()
{
float tempAccel;
float tempMaxSpeed;
float t;
int32 distToTarget;
int32 accelDist;
distToTarget = abs(targetLocation - currentLocation); // figure out the distance we will travel on this move
tempAccel = moveAccelSteps;// * stepsInch; // in steps per sec
tempAccel = tempAccel / (ENGINE_RATE * ENGINE_RATE);
tempAccel = tempAccel * SPEED_OFFSET;
accel = (int32)tempAccel;
tempMaxSpeed = moveMaxSpeedSteps;
tempMaxSpeed = (tempMaxSpeed / ENGINE_RATE) * SPEED_OFFSET;
maxSpeed = (int32)tempMaxSpeed;
//determine Accel distance
// d = 1/2 a t^2
t = (float)moveMaxSpeedSteps / (float)moveAccelSteps;
accelDist = (moveAccelSteps / 2) * t * t;
// determine the direction
if (targetLocation > currentLocation)
{
motionDirection = DIRECTION_FORWARD;
decelLocation = currentLocation + decelLocation;
output_low(PIN_X_DIR);
if (accelDist >= distToTarget / 2)
decelLocation = targetLocation - distToTarget / 2;
else
decelLocation = targetLocation - accelDist;
}
else
{
motionDirection = DIRECTION_REVERSE;
decelLocation = currentLocation - decelLocation;
output_high(PIN_X_DIR);
if (accelDist >= distToTarget / 2)
decelLocation = targetLocation + distToTarget / 2;
else
decelLocation = targetLocation + accelDist;
}
delay_us(4); // time for direction to "take" before steps start
bDecel = false;
currentSpeed = 0; // initialize this
bEnableMotion = true; // all set time to move
}
void main()
{
setup_adc_ports(NO_ANALOGS);
setup_adc(ADC_OFF);
setup_psp(PSP_DISABLED);
setup_spi(SPI_SS_DISABLED);
setup_wdt(WDT_OFF);
setup_timer_0(RTCC_INTERNAL);
setup_timer_1(T1_DISABLED);
setup_timer_2(T2_DIV_BY_1,159,2);
setup_timer_3(T3_INTERNAL|T3_DIV_BY_1);
currentLocation = 0;
bEnableMotion = false;
enable_interrupts(INT_TIMER2);
//enable_interrupts(INT_RDA);
enable_interrupts(GLOBAL);
output_low(PIN_X_STEP);
output_high(PIN_X_DIR);
while(true)
{
moveAccelSteps = 1000;
moveMaxSpeedSteps = 2000;
targetLocation = 200 * 8;
doMotion();
delay_ms(2000);
moveAccelSteps = 50000;
moveMaxSpeedSteps = 20000;
targetLocation = 0;
doMotion();
delay_ms(1500);
}
}
