Replies to This Discussion

DeveloperPermalink Reply by Randy on August 15, 2012 at 9:40pm

Lewis,

    The APM_RC library is the one that we use in ArduPilot/ArduCopter to read/write pwm values used by the servos.  It uses low level avr hardware calls (although i suspect the servo lib you're using does as well) which makes it fast.  When you update a pwm value that you want sent out to the servos, our code only needs to update one register in the avr.  The rest is all handled by the hardware.  So I guess your question is why is that servo lib so slow...I'm not sure but we've certainly seen other situations where the arduino libraries are not optimised for speed because, in the beginning at least, people tend to use them for simple things so they're optimised for ease of use and simplicity instead of speed.

     I don't think it's because of the relative speed of the nano vs the 2560.

     Ah...my guess is that it's actually the pulse in that is slow in your code.  I'll bet that it's waiting for the next pulse...if you replace that with a single call to pulseIn and save it to a variable and then use that value to output to both servos i'll bet it speeds up.

Permalink Reply by Lewis W on August 27, 2012 at 5:15pm

Randy,

Thanks for the tips.  I tried what you said, and it's marginally better, but still not ideal... I think that the servo library is also no speed-optimized.  Even if it were, it doesn't help in the long run.  Assuming no speed issues, I was going to write the code something like this:

// Required header files
#include <Servo.h>

// Define input pins
#define inPWM1 0
#define inPWM2 1
#define inPWM3 2
#define inPWM4 3

// Define output pins
#define outServoPin1 4
#define outServoPin2 5
#define outServoPin3 6
#define outServoPin4 7
#define outServoPin5 8
#define outServoPin6 9
#define outServoPin7 10
#define outServoPin8 11

// Magic Numbers
#define SERVO_MIDPOINT 1500

// Create servo control objects
Servo outServo1;
Servo outServo2;
Servo outServo3;
Servo outServo4;
Servo outServo5;
Servo outServo6;
Servo outServo7;
Servo outServo8;

void setup()
{
    // Configure inputs
    pinMode(inPWM1, INPUT);
    pinMode(inPWM2, INPUT);
    pinMode(inPWM3, INPUT);
    pinMode(inPWM4, INPUT);

    // Configure outputs
    outServo1.attach(outServoPin1);
    outServo2.attach(outServoPin2);
    outServo3.attach(outServoPin3);
    outServo4.attach(outServoPin4);
    outServo5.attach(outServoPin5);
    outServo6.attach(outServoPin6);
    outServo7.attach(outServoPin7);
    outServo8.attach(outServoPin8);
}

void loop()
{
    // Get inputs from receiver/autopilot

    // Run algorithms to calculate outputs based on wing flex, airspeed, etc
    /*
    outPWM1 = ...
    outPWM2 = ...
    etc.
    */

    // Output synthesized pwm signals to servos
    outServo1.writeMicroseconds(outPWM1);
    outServo2.writeMicroseconds(outPWM2);
    outServo3.writeMicroseconds(outPWM3);
    outServo4.writeMicroseconds(outPWM4);
    outServo5.writeMicroseconds(outPWM5);
    outServo6.writeMicroseconds(outPWM6);
    outServo7.writeMicroseconds(outPWM7);
    outServo8.writeMicroseconds(outPWM8);
}

/*
Some algorithms and functions would be here
*/

But if I use pulseIn, plus inputs from strain gauges and airspeed and who knows what else, the processor is not going to happily process all that info and output all those signals.  Not to mention that the final product will have 14 outputs.  Yet the Ardupilot seems to be able to do it.  If I load the APM2 (or APM1) code onto an Aruino Mega 2560 board, will that be a good place to start?

Thanks again!

DeveloperPermalink Reply by Randy on August 27, 2012 at 8:52pm

Lewis,

     You might want to try looking at the APM_RC library instead.  It's got some example sketches that might be helpful.  The thing is that there are quite a few differences between the standard Arduino Mega board and the APM1 or APM2...so the sketches might not even work...:-(.

Permalink Reply by Lewis W on August 28, 2012 at 9:16am

I think that's my best bet for now.  I'll take a look at modifying them for the Nano/Mega and see what I can come up with.

Thanks!