Arduino: Class with Servo corrupts servo behavior if this->object.attach(pin) is used in constructor

Question

I have an issue with a solution, but I don't understand the solution. It may be similar to Arduino: initialise custom object in constructor but I don't think this is the same issue.

The context is the following. I am controlling a small legged robot with a Arduino board. Each leg is an object "MChLeg" that includes two servos. The servos are controlled using the library.

The issue is the following: if I assign the servos in the constructor, the program compiles fine but the servos behave in a crazy manner (I suppose that I break the servo scheduling); if I assign the servos outside of the constructor, everything is fine.

I list only part of the C++ Class below is it is the only one that is changed between the working version and the non-working version.

Initial program (servo assigned in constructor), code has issues:

#include <Arduino.h>
#include <Servo.h>

class MChLeg{
    int kneeIndex, hipIndex;
    int kneeAngle, hipAngle;
    Servo kneeServo; 
    Servo hipServo;
public:
    MChLeg(int, int); // (knee servo index, hip servo index); use 0 if no servo
    ~MChLeg();
    void setLeg(int, int); // (knee angle, hip angle); in degrees, 0-180, 90 is straight
    void rollLeg();
};

MChLeg::MChLeg(int x, int y){
    this->kneeIndex = x;
    this->hipIndex = y;
    if (this->kneeIndex != 0) {
        this->kneeServo.attach(this->kneeIndex);
        this->kneeServo.write(90);   // this line can be removed with no impact on issue
    }
    if (this->hipIndex != 0) {
        this->hipServo.attach(this->hipIndex);
        this->hipServo.write(90);    // this line can be removed with no impact on issue
    }
}

New code (servo assigned using a method), code operates fine:

#include <Arduino.h>
#include <Servo.h>

class MChLeg{
    int kneeIndex, hipIndex;
    int kneeAngle, hipAngle;
    Servo kneeServo; 
    Servo hipServo;
public:
    MChLeg();
    ~MChLeg();
    void assignLeg(int, int); // (knee servo index, hip servo index); use 0 if no servo
    void setLeg(int, int); // (knee angle, hip angle); in degrees, 0-180, 90 is straight
    void rollLeg();
};

MChLeg::MChLeg(){
    this->kneeIndex = 0;
    this->hipIndex = 0;
}

void MChLeg::assignLeg(int x, int y){
    if (this->kneeIndex != 0) {
        this->kneeServo.detach();
    }
    if (this->hipIndex != 0) {
        this->hipServo.detach();
    }
    this->kneeIndex = x;
    this->hipIndex = y;
    if (this->kneeIndex != 0) {
        this->kneeServo.attach(this->kneeIndex);
    }
    if (this->hipIndex != 0) {
        this->hipServo.attach(this->hipIndex);
    }
}

Why would the second code be better than the first one ?

Calling code:

// Include application, user and local libraries
#include <Servo.h>
#include "MChButton.h"
#include "MChLeg.h"


// pin connections
const int buttonIndex = 2; //pin for button
const int greenLed = 3;//pin for green Led
const int yellowLed = 4;//pin for yellow Led
const int redLed = 5;//pin for red Led
const int servoPin1 = 9;//pin for servo
const int servoPin2 = 10;//pin for servo
const int servoPin3 = 11;//pin for servo


//define persitent objects
MChButton theButton(buttonIndex); // declare the switch pin as an input
MChLeg rightLeg; // declare right leg - was 'MChLeg rightLeg(0, servoPin1);' in first version
MChLeg leftLeg;  // declare left leg - was 'MChLeg leftLeg(servoPin2, servoPin3);' in first version


// Setup phase
void setup(){

    // declare the LED pins as outputs
    pinMode(greenLed,OUTPUT);
    pinMode(yellowLed,OUTPUT);
    pinMode(redLed,OUTPUT);

    leftLeg.assignLeg(0, servoPin1); //right knee - hip - did not exist in first version
    rightLeg.assignLeg(servoPin2, servoPin3); //right knee - hip - did not exist in first version

}

// Loop phase
void loop(){

    //state machine counter (persistant)
    static int machinestate = 0;
    const int machinestateMax = 4; //max statemachine
    const int machinestateMax1 = machinestateMax+1; //max statemachine for modulo counting

    //check if there is an event, if there is one increment state machine counter and mark event done
    //statemachine counter is limited for machinestateMac (modulus)
    if (theButton.buttonEventGet()==true) {
        machinestate = ++machinestate % machinestateMax1;
    }

    // set LEDs and Legs according to state
    switch (machinestate) {
        case 0:{
            digitalWrite(greenLed, HIGH); // turn the green LED on pin 3 on
            digitalWrite(yellowLed, LOW);  // turn the red LED on pin 4 off
            digitalWrite(redLed, LOW);  // turn the red LED on pin 5 off
            leftLeg.setLeg(10, 10); // set leg to 0°
            rightLeg.setLeg(10, 10); //set leg to 0°
            break;
        }
        case 1:{
            digitalWrite(greenLed, HIGH);
            digitalWrite(yellowLed, HIGH);
            digitalWrite(redLed, LOW);
            leftLeg.setLeg(45, 45);
            rightLeg.setLeg(45, 45);
            break;
        }
        default:{ // detect if I did it wrong :-)
            digitalWrite(greenLed, HIGH); 
            digitalWrite(yellowLed, HIGH); 
            digitalWrite(redLed, HIGH);  
        }
    }
    delay(10);
}

Thanks for your help.

Answer 1

I don't have an Arduino with me right now, but I remember running into the same issue.

As far as I recall, I was using a for loop in my constructor for an array which did not work. So I ended just assigning values one by one.

I assume the issue in your case if with the if statement.

I'll try different things later this evening when I get home.

EDIT - 2015/01/08

I made some tests to try to reproduce the issues I was talking about, but did not succeed. I'll keep my answer as what the solution might not be.