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evsource
Joined: 21 Nov 2006
Posts: 129
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| PWM with 16F88, spike on power down |
 Posted: Wed Dec 24, 2008 1:55 am |
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I'm using a 16F88 chip to control motor speed with PWM. 0% duty cycle is full off, and 100% duty cycle full on, as you would expect. The PWM output of the PIC is connected to a NPN transistor to provide inverting and an open collector output that the motor control circuitry expects.
I've noticed that when I pull power from the circuit, there's a spike on the PWM output, which causes the motor to get a brief pulse of power (command duty cycle is at 0% when this happens). I can think of ways in hardware to prevent this, but I don't understand why the PIC would output this spike on the PWM channel. Any ideas? The code:
| Code: |
#include <16f88.h>
#byte WDTCON=0x105 // the WDT configuration register on the 16F88, has to be manually set due to a CCS compiler error in setup_counters and setup_timer0
#DEVICE ADC=10
#device *=16
#fuses NOWDT,NOPROTECT,NOMCLR,NOLVP,HS,CCPB3
#use delay(clock=20000000)
float get_throttle_reading();
void main() {
float throttle_reading;
long PWM_duty_cycle;
setup_ccp1(CCP_PWM); // Configure CCP1 as a PWM
setup_timer_2(T2_DIV_BY_1, 249, 1); // about 20kHz. In set_pwm1_duty, 0 is fully off, and 1000 fully on.
// It will also be ensured to start out off.
// The 249 value was chosen instead of 250 to ensure a full-on stream with a value of 1000 for the PWM.
set_pwm1_duty(0);
setup_adc( ADC_CLOCK_INTERNAL );
setup_adc_ports( sAN0 | sAN1 | sAN3 );
set_adc_channel(3);
// setup_timer_0(RTCC_INTERNAL | RTCC_DIV_256); // not using right now
WDTCON = 0x11; // 0x15; // 0x14 turns WDT off, set for approximately 1s WDT w/ 0x15, 1/16th of a second with 0x11
throttle_reading = get_throttle_reading();
// high pedal disable, the pedal outputs 0.8V when fully off
while( throttle_reading > 0.80) {
restart_wdt();
throttle_reading = get_throttle_reading();
delay_ms(20);
}
while( TRUE ) {
restart_wdt();
throttle_reading = get_throttle_reading();
if(throttle_reading < 0.85) PWM_duty_cycle = 0;
else if(throttle_reading > 4.0) PWM_duty_cycle = 1000;
else {
PWM_duty_cycle = (long)(((HEPA_reading - 0.85) / 3.15 ) * 1000.0);
}
set_pwm1_duty(PWM_duty_cycle);
}
}
float get_throttle_reading() {
long value;
int i;
value = 0;
for(i=0;i<10;i++) {
value+=read_adc();
restart_wdt();
}
return (5.0 * ((float)value / 10230.0)); // the 10230.0 takes into account the divide by 10 for the accumulation of 10 readings
}
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FvM
Joined: 27 Aug 2008
Posts: 2337
Location: Germany
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| Posted: Wed Dec 24, 2008 2:35 am |
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| Does the pulse also appear on reset? If not, activation of brown-out reset may help. |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Wed Dec 24, 2008 2:39 am |
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It's possibly caused by back-EMF. Do you have a diode connected
across the motor, as shown in this schematic ? If not, add one.
http://www.cpemma.co.uk/555pwm.html |
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PICoHolic
Joined: 04 Jan 2005
Posts: 224
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| Posted: Wed Dec 24, 2008 3:06 am |
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| You may also consider adding a decoupling capacitor 0.1uF in // with the diode. |
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Ttelmah
Guest
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| Posted: Wed Dec 24, 2008 3:29 am |
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It won't be back-emf, if the duty is at 0% when he switches off (as he says).
My thoughts would be:
Supply sequencing. How is the PIC supply generated, relative to the power rail feeding the motor?. What sort of size motor is involved (current/voltage). One thing that could happen, is that if the motor supply drops before the PIC supply, you could be generating a reverse voltage across your voltage regulator (potentially damaging), and depending what internal diode structures are 'around' the circuit, currents may be flowing to places you don't expect.
How is the NPN transistor driven?. Is there anything to ensure the transistor switches 'off', if connected to an open circuit?. If you have brownout reset selected, the chip will reset, before the supply disappears.
What does the circuit do, when power is initially applied?. Would it enable the PWM then?. Thought here is that as the supply is dropping, the circuit is resetting, then restarting, then resetting again.
Best Wishes |
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asmboy
Joined: 20 Nov 2007
Posts: 2128
Location: albany ny
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| seconding tlemah |
| Posted: Thu Dec 25, 2008 1:54 pm |
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| be sure that in the event of TRIS being wrong ( as it will be at power on OR after reset ) - even momentarily - that there is some biasing resistor to insure the switch element is biased OFF - this is especially critical with mosfet switches - even 10K ohms is usually plenty - a PWM duty of 0% is nOT enuf by itself. |
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evsource
Joined: 21 Nov 2006
Posts: 129
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| Re: seconding tlemah |
| Posted: Sat Dec 27, 2008 8:40 pm |
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| asmboy wrote: | | be sure that in the event of TRIS being wrong ( as it will be at power on OR after reset ) - even momentarily - that there is some biasing resistor to insure the switch element is biased OFF - this is especially critical with mosfet switches - even 10K ohms is usually plenty - a PWM duty of 0% is nOT enuf by itself. |
This is all good information - I left on vacation right after posting the original question, so will try some of this out when I get back.
A little additional info, the motor is being driven with a IGBT module from a bank of batteries isolated from the PIC power supply. The IGBT module has an isolated gate driver circuit. The PIC is responsible for sending the PWM signal. The NPN transistor being used to achieve an open collector output and inverting to the gate driver circuit did not have a resistor biasing the input to the transistor to ground, so this might be the problem. However, the momentary "on" pulse I was talking about does not occur on startup.
Max motor current is 1000A, at 170V, with 100-200A continuous being pulled, so noise could certainly be an issue! |
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