Arduino PWM output

[Greg McCoy]

Originally Posted by sand500

I dont know why this is happening but the duty cycle frequency was wrong, then why does it stop completely at 255?

The motor controller is behaving erratically because the PWM frequency and the duty cycle are wrong. The analogWrite() function is generating this PWM signal:


(from Arduino.cc)

...but your motor controller is expecting this:




The frequency of the signal in Hz is the number of pulses per second. The reciprocal of the frequency is the period, which is the time between the rising edges of the PWM signal. For your motor controller, this should be 50 Hz, or a 20 ms (millisecond) period. According to the Arduino documentation, the analogWrite() function produces a 490 Hz PWM signal, or a 2 ms period.


The duty cycle of the signal is the ratio between the pulse width and the period. For your motor controller, the pulse width is "mapped" as follows:

Full Forward: 2 ms pulse / 20 ms period = 10% duty cycle.
Neutral: 1.5 ms pulse / 20 ms period = 7.5% duty cycle.
Full Reverse: 1 ms pulse width / 20 ms period = 5% duty cycle.


As others have suggested, the Arduino servo library should be able to produce the correct signal. 180 degrees on a servo is the equivalent of "full forward" on a motor attached to the motor controller (0 degrees is full reverse, 90 is neutral, etc.)

Originally Posted by sand500

When I hook up the pwm and ground on the arduino to a regular dc motor that you would find in an electronics kit, the motor works just like how pwm should.

The motor controller does just this - it generates a 0-100% duty cycle PWM signal to the motor, using a circuit called an "H-bridge" to reverse the voltage when changing directions.

Hope this helps...

[slijin]

Originally Posted by Greg McCoy

The motor controller is behaving erratically because the PWM frequency and the duty cycle are wrong

Can't believe I overlooked that. Should've been the first thing I realized.

[Greg Needel]

I have a custom victor library for the arduino that was written for one of my projects. This takes care of the timing issues, and duty cycle issues. While the the stock servo library will work, there are things that will cause hiccups.

Edit, I have attached the files.


revictorlibsforarduino.zip

Greg

[sand500]

Originally Posted by Greg McCoy

The motor controller is behaving erratically because the PWM frequency and the duty cycle are wrong. The analogWrite() function is generating this PWM signal:


(from Arduino.cc)

...but your motor controller is expecting this:




The frequency of the signal in Hz is the number of pulses per second. The reciprocal of the frequency is the period, which is the time between the rising edges of the PWM signal. For your motor controller, this should be 50 Hz, or a 20 ms (millisecond) period. According to the Arduino documentation, the analogWrite() function produces a 490 Hz PWM signal, or a 2 ms period.


The duty cycle of the signal is the ratio between the pulse width and the period. For your motor controller, the pulse width is "mapped" as follows:

Full Forward: 2 ms pulse / 20 ms period = 10% duty cycle.
Neutral: 1.5 ms pulse / 20 ms period = 7.5% duty cycle.
Full Reverse: 1 ms pulse width / 20 ms period = 5% duty cycle.


As others have suggested, the Arduino servo library should be able to produce the correct signal. 180 degrees on a servo is the equivalent of "full forward" on a motor attached to the motor controller (0 degrees is full reverse, 90 is neutral, etc.)



The motor controller does just this - it generates a 0-100% duty cycle PWM signal to the motor, using a circuit called an "H-bridge" to reverse the voltage when changing directions.

Hope this helps...

Thanks a lot, its a lot clearer to me now. I'll try Greg Needel's library and the servo library.

[markgryzwa]

Here is some servo code that I put together to try to run some basic robot functions. I've not tested in out for polarity yet but the ideas are correct.

#include <Servo.h>

// global variables

// ------------- Servo setup ------------------------------
Servo myservo_left;
Servo myservo_right;

int left_pwm = 10; // set up the servo connections for the Jaguar controllers
int right_pwm = 9;
int rate = 0;


// ------- Spike relay control signals -------------------
int FWD = 1;
int REV = 2;
int OFF = 0;
int spike_white = 0; // pin 0 for spike signal wire
int spike_red = 1; // pin 1 for spike red wire


void setup() {
// put your setup code here, to run once:

myservo_left.attach(left_pwm);
myservo_right.attach(right_pwm);

pinMode(spike_white, OUTPUT);
pinMode(spike_red, OUTPUT);

}

void loop() {

// Set up the default
Spike(OFF);


// Make stuff happen
Forward(100, 1000); // move forward as a % of full speed for ms Foward(%*full speed,ms)
delay(1000);

Spike(FWD);
Reverse(100, 1000);
delay(1000);

Spike(REV);
Right(50, 100);
delay(1000);

Left(50,200);
delay(1000);

Spike(OFF);
Right(50, 100);
delay(6000);


}



//------------------ Subroutines ---------------------------

//-----------------------------------------------------------
void Forward(int rate, int duration) {

int spd = rate * 500; // set speed as a % of rate

myservo_left.writeMicroseconds(1500+spd);
myservo_right.writeMicroseconds(1500-spd);
delay(duration);
myservo_left.writeMicroseconds(1500);
myservo_right.writeMicroseconds(1500);
}

//-----------------------------------------------------------
void Reverse(int rate, int duration) {

int spd = rate * 500; // set speed as a % of rate

myservo_left.writeMicroseconds(1500-spd);
myservo_right.writeMicroseconds(1500+spd);
delay(duration);
myservo_left.writeMicroseconds(1500);
myservo_right.writeMicroseconds(1500);

}

//------------------------------------------------------------
void Left(int rate, int duration) {

int spd = rate * 500; // set speed as a % of rate

myservo_left.writeMicroseconds(1500+spd);
myservo_right.writeMicroseconds(1500+spd);
delay(duration);
myservo_left.writeMicroseconds(1500);
myservo_right.writeMicroseconds(1500);

}


//------------------------------------------------------------
void Right(int rate, int duration) {

int spd = rate * 500; // set speed as a % of rate

myservo_left.writeMicroseconds(1500-spd);
myservo_right.writeMicroseconds(1500-spd);
delay(duration);
myservo_left.writeMicroseconds(1500);
myservo_right.writeMicroseconds(1500);

}



//---------------------------------------------------------------
void Spike(int state) { //states can be 0=Off, 1 = FWD, 2=REV

switch (state) {
case 0:
digitalWrite(spike_red, LOW);
digitalWrite(spike_white, LOW);
break;

case 1:
digitalWrite(spike_red, HIGH);
digitalWrite(spike_white, LOW);
break;

case 2:
digitalWrite(spike_red, LOW);
digitalWrite(spike_white, HIGH);
break;

default:
digitalWrite(spike_red, LOW);
digitalWrite(spike_white, LOW);
break;
}

}