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spom
Joined: 12 Jul 2007
Posts: 32
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| Problem with stepper motor |
 Posted: Wed Apr 02, 2008 5:02 am |
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Hi,
I have a problem with my stepper motor.
I use a PIC 18F2480, L293D as a driver and a motor which uses 4.6V and 0.8 A/Phase.
I use the same power supply both for the PIC and for the motor.
The problem is that the motor makes three or four steps and then the programm starts at the begin.
Why? Have I forgotten any component?
Thanks!
| Code: | #include <18f2480.h>
#device *=16
#device ADC=10
#use delay(clock=20000000)
#fuses HS,NOWDT,NOPROTECT,NOLVP
#use rs232(baud=9600,xmit=PIN_C6,rcv=PIN_C7,BRGH1OK,stream=debug)
#include <input.c>
int16 seconds; // A running seconds counter
int8 int_count; // Number of interrupts left before a second has elapsed
#define INTS_PER_SECOND 76 // (20000000/(4*256*256))
#int_rtcc // This function is called every time
void clock_isr() { // the RTCC (timer0) overflows (255->0).
// For this program this is apx 61 times
if(--int_count==0) { // per second.
++seconds;
int_count=INTS_PER_SECOND;
}
}
#BYTE port_b = 0xf81
BYTE const POSITIONS[8] = {0b0101,
0b0001,
0b1001,
0b1000,
0b1010,
0b0010,
0b0110,
0b0100};
drive_stepper(BYTE speed, char dir, int16 steps) {
static BYTE stepper_state = 0;
BYTE i;
for(i=0; i<steps; ++i) {
delay_ms(speed);
fprintf(debug,"i: %u;\n",i);
set_tris_b(0xf0);
port_b = POSITIONS[ stepper_state ];
if(dir!='R')
stepper_state=(stepper_state+1)&(sizeof(POSITIONS)-1);
else
stepper_state=(stepper_state-1)&(sizeof(POSITIONS)-1);
}
}
main() {
BYTE speed;
int16 steps;
char dir;
while (TRUE)
{
speed = 1;
dir = 'F';
printf("\n\rSteps (hex): ");
steps = gethex();
drive_stepper(speed,dir,steps);
delay_ms(1000);
seconds = 0;
while(seconds < 60)
{
;
}
}
} |
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RLScott
Joined: 10 Jul 2007
Posts: 465
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| Re: Problem with stepper motor |
| Posted: Wed Apr 02, 2008 6:10 am |
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Maybe the motor spikes are not sufficiently decoupled and are getting back into the PIC and resetting it. An easy way to find out is to disconnect the motor and driver and run the PIC all by itself. You have an RS-232 debug of the stepping function that will tell you if it is working or being reset.
Robert Scott
Real-Time Specialties |
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Ttelmah
Guest
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| Posted: Wed Apr 02, 2008 7:15 am |
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One other comment. Have you got the trap diodes installed?. The L293, doesn't include it's own trap diodes, and these must be added (they are shown on the example circuits).
Best Wishes |
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spom
Joined: 12 Jul 2007
Posts: 32
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| Posted: Wed Apr 02, 2008 7:34 am |
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I use L293D. So I thought I don't need any more diodes as the diodes are installed in the driver, isn't it?
How can I better decoupling the motor spikes? |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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Ttelmah
Guest
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| Posted: Wed Apr 02, 2008 10:24 am |
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PCM programmer has pointed to a good page about the general connection layout.
The big question I have, is how is the Vs of the driver chip connected and decoupled, relative to the PIC?. The problem is that as you step, the energy in the coils, when disconnected, has to go somewhere. the effect will be to put significant spikes on the supply rail at the driver. If the capacitance on the rail is not enough to store this without the supply rail rising significantly, the result will be significant noise on this rail. Also, though the size of the capacitors matters for this, separately you have the question of their ESR (which is why it is common to see several slightly smaller capacitors, in some places where one large one would seem more obvious), and also how they react to high frequencies (most electrolytic capacitors behave quite poorly at HF).
So:
(not very good at ASCII art - fixed pitch font required...)
| Code: |
Vin ---+----------()()()----+----+---
| ()() | |
--- --- ---
___ At PIC ___ ___ To Motor
| | | |
| --- |
| | |
0V -+--+ +----+---
| |
------------------------
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Decouple the supply at the motor, with both a reasonably large electrolytic, and a small ceramic or polyester cap in parallel.
Make sure the ground connection for the PIC, is not 'picked off' along the run to the motor.
Decouple right close to the PIC with a ceramic or polyester cap close to the chip (I am assuming the supply already has decoupling present).
You might consider adding an inline inductor on the supply to the motor as shown.
Best Wishes |
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rnielsen
Joined: 23 Sep 2003
Posts: 852
Location: Utah
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| Posted: Wed Apr 02, 2008 12:37 pm |
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One thing that I could think of is that the motor might not be in the same 'physical' position as the 'electrical' position of the PIC. Upon start up, the PIC has it's outputs in a default state. The motor will move to this state. Then, the PIC will (I assume) set the outputs to it's initialized state. The motor will need to move to this state.
Not knowing the exact details of your project, this is what comes to mind first.
Ronald |
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