Programming a double pendulum

Erdem · wrote on 26 Dec 2018, 20:17

Hi guys,

I try to programm a double pendulum using Qt. So far I have written a Pendel class which contains all properties needed.

Pendel.h

#ifndef PENDEL_H
#define PENDEL_H

#include <QGraphicsObject>

class Pendel : public QGraphicsObject
{
    Q_OBJECT
    Q_ENUMS(pendelSetup)

public:
    enum pendelSetup {firstPendel = 1, secondePendel = 2};
    Pendel(QGraphicsItem *parent = nullptr);
    ~Pendel();
    QRectF boundingRect() const;
    void paint(QPainter *painter,
               const QStyleOptionGraphicsItem *,
               QWidget *);

    // public methods
    void setPendelPtr(Pendel *);
    void originsSetup(int, int, int);
    QPointF getOrigins();

    // Length getter and setter
    int getLength();
    void setLength(int);

    // Phi getter and setter
    int getPhi();
    void setPhi(int);

    // Phi_dot getter
    int getPhi_dot();

    // Phi_ddot getter
    int getPhi_ddot();

    // Mass getter and setter
    int getMass();
    void setMass(int);

    // Gravity getter and setter
    int getGravity();
    void setGravity(int);

public slots:
    void adjustAngle(int);
    void calcAcc1();
    void calcAcc2();

signals:
    void angleChanged(int);
    void angleChanged();

private:
    int length = 80;
    int phi = 0;
    int phi_dot = 0;
    int phi_ddot = 0;
    int mass = 1;
    int gravity = 1;

    Pendel *other = nullptr;
    QPointF origins;
};

#endif // PENDEL_H

Pendel.cpp

#include "pendel.h"
#include <QPainter>
#include <QtMath>
#include <QDebug>

Pendel::Pendel(QGraphicsItem *parent) :
    QGraphicsObject(parent)
{
    setTransformOriginPoint(5,0);
}

Pendel::~Pendel()
{
}

QRectF Pendel::boundingRect() const
{
    return QRect(0,0, 10,length);
}

void Pendel::paint(QPainter *painter,
                   const QStyleOptionGraphicsItem *,
                   QWidget *)
{
    painter->setPen(QPen(Qt::black,3));
    painter->drawLine(5,0, 5,length);
    painter->setBrush(Qt::black);
    painter->drawEllipse(0,length-5, 10,10);
}

// public methods
void Pendel::setPendelPtr(Pendel *ptr)
{
    if(other != ptr)
    {
        other = ptr;
    }
}

void Pendel::originsSetup(int setUp = firstPendel, int xv = 0, int yv = 0)
{
    if(setUp == firstPendel)
    {
        origins.setX(xv);
        origins.setY(yv);
    }
    if(setUp == secondePendel)
    {
        origins.setX(xv+qSin(other->getPhi()*M_PI/180)*other->getLength());
        origins.setY(yv+qCos(other->getPhi()*M_PI/180)*other->getLength());
    }
}

QPointF Pendel::getOrigins()
{
    return origins;
}

// Length getter and setter
int Pendel::getLength()
{
    return length;
}
void Pendel::setLength(int l)
{
    if (length != l)
    {
        length = l;
    }
}

// Phi getter and setter
int Pendel::getPhi()
{
    return phi;
}
void Pendel::setPhi(int p)
{
    if(phi != p)
    {
        phi = p;
        emit angleChanged(p);
    }
}

// Phi_dot getter
int Pendel::getPhi_dot()
{
    return phi_dot;
}

// Phi_ddot getter
int Pendel::getPhi_ddot()
{
    return phi_ddot;
}

// Mass getter and setter
int Pendel::getMass()
{
    return mass;
}
void Pendel::setMass(int m)
{
    if(mass != m)
    {
        mass = m;
    }
}

// Gravity getter and setter
int Pendel::getGravity()
{
    return gravity;
}
void Pendel::setGravity(int g)
{
    if(gravity != g)
    {
        gravity = g;
    }
}

//public slots
void Pendel::adjustAngle(int p)
{
    setPhi(p);
    setRotation(p);
    emit angleChanged();
}

void Pendel::calcAcc1()
{
    double val1 = -other->getMass()/(mass + other->getMass())*(other->getLength()/length);
    qDebug() << val1;
    double val2 = other->getPhi_ddot()*qCos((phi-other->getPhi())*M_PI/180);
    double val3 = qPow(other->getPhi_dot(),2)*qSin((phi-other->getPhi())*M_PI/180);
    double val4 = gravity/length*qSin(phi*M_PI/180);
    phi_ddot += val1*(val2+val3)-val4;
    phi_dot += phi_ddot;
    phi += phi_dot;
    setRotation(phi);
}

void Pendel::calcAcc2()
{
    double val1 = -other->getLength()/length;
    double val2 = other->getPhi_ddot()*qCos((other->getPhi()-phi)*M_PI/180);
    double val3 = qPow(other->getPhi_dot(),2)*qSin((other->getPhi()-phi)*M_PI/180);
    double val4 = gravity/length*qSin(phi);
    phi_ddot += val1*(val2-val3)-val4;
    phi_dot += phi_ddot;
    phi += phi_dot;
    setRotation(phi);
}

MainWindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>

class QGraphicsScene;
class Pendel;
class QTimer;

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();

public slots:
    void updatePos();
    void returnPendel1Prop();
    void returnPendel2Prop();

private:
    Ui::MainWindow *ui;
    QGraphicsScene *scene;
    QTimer *timer;
    Pendel *pendel1, *pendel2;
};

#endif // MAINWINDOW_H

MainWindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include "pendel.h"
#include <QGraphicsScene>
#include <QTimer>
#include <QtMath>
#include <QDebug>

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow),
    scene(new QGraphicsScene(this)),
    timer(new QTimer(this)),
    pendel1(new Pendel), pendel2(new Pendel)
{
    // general setUp
    ui->setupUi(this);
    ui->graphicsView->setScene(scene);
    ui->spinBox->setRange(-900,900);
    ui->spinBox_2->setRange(-900,900);
    timer->setInterval(1000);

    // pendel setup
    pendel1->setParent(this);
    pendel1->setLength(50);
    pendel1->setPendelPtr(pendel2);
    pendel1->originsSetup(Pendel::firstPendel, 0, 0);

    pendel2->setParent(this);
    pendel2->setPendelPtr(pendel1);
    pendel2->originsSetup(Pendel::secondePendel, pendel1->getOrigins().x(),
                                                 pendel1->getOrigins().y());
    pendel2->setPos(pendel2->getOrigins().x(),
                    pendel2->getOrigins().y());

    scene->addItem(pendel1);
    scene->addItem(pendel2);

    // connect instruction
    connect(ui->spinBox, SIGNAL(valueChanged(int)),
            pendel1, SLOT(adjustAngle(int)));

    connect(ui->spinBox_2, SIGNAL(valueChanged(int)),
            pendel2, SLOT(adjustAngle(int)));

    connect(pendel1, SIGNAL(angleChanged()),
            this, SLOT(updatePos()));

    connect(ui->pushButton, SIGNAL(clicked()),
            timer, SLOT(start()));

    connect(timer, SIGNAL(timeout()),
            pendel1, SLOT(calcAcc1()));

    connect(timer, SIGNAL(timeout()),
           pendel2, SLOT(calcAcc2()));
}

MainWindow::~MainWindow()
{
    delete ui;
}


// public slots
void MainWindow::updatePos()
{
    int angle = pendel1->getPhi();
    double xKoo = pendel1->getOrigins().x()+qSin(angle*M_PI/180)*pendel1->getLength();
    double yKoo = pendel1->getOrigins().y()+qCos(angle*M_PI/180)*pendel1->getLength();
    pendel2->setPos(-xKoo, yKoo);
}

void MainWindow::returnPendel1Prop()
{
    qDebug() << "Phi1: " << pendel1->getPhi();
    qDebug() << "Phi_dot1: " << pendel1->getPhi_dot();
    qDebug() << "Phi_ddot1: " << pendel1->getPhi_ddot();
    qDebug() << "Mass1: " << pendel1->getMass();
    qDebug() << "length: " << pendel1->getLength() << "\n";
}

void MainWindow::returnPendel2Prop()
{
    qDebug() << "Phi2: " << pendel2->getPhi();
    qDebug() << "Phi_dot2: " << pendel2->getPhi_dot();
    qDebug() << "Phi_ddot2: " << pendel2->getPhi_ddot();
    qDebug() << "Mass2: " << pendel2->getMass();
    qDebug() << "length: " << pendel2->getLength() << "\n";
}

My idea so far is to start the oscillation by clicking the PushButton adjusting the angles of the both pendulums. To achiev the motion of the pendulums I have embedded the equaltion of Motion of pendulum system into the methods
calcAcc1 and calcAcc2.
When I click the PushButton nothing happens. To see what happens within the methods I have i have embedded the instruction
qDebug() << val1;
into pendel.cpp//calcAcc1().
The Result of the calculation is zero and i don't know why.

In the picture above you can see the result of the calculation for val1 in teh command window.

can somebody help ?

mrjj · 26 Dec 2018, 20:54

@Erdem said in Programming a double pendulum:

The Result of the calculation is zero and i don't know why.

double val1 = -other->getMass()/(mass + other->getMass())*(other->getLength()/length);
Its get truncated to int. is my guess.
getMass returns int amount others.

Try
double val1 = -other->getMass()/double((mass + other->getMass()))*(other->getLength()/double(length));

Erdem · 26 Dec 2018, 21:03

@mrjj Thank you for your advice.
But now val2 results zero.
And there I already tried the same but nothing. Can you maybe explaine the reason for this behaviour or link the respective homegape

mrjj · E Erdem 26 Dec 2018, 20:54

@Erdem
http://www.informit.com/articles/article.aspx?p=352857&seqNum=4

"The / operator divides its first operand by the second. For example, 1000 / 5 evaluates to 200. If both operands are integers, the result is the integer portion of the quotient. For example, 17 / 3 is 5, with the fractional part discarded."

Its a standard c++ type thing.
more info here
https://stackoverflow.com/questions/36266370/division-in-c