Problem with setPos() and setRotation on QGraphicsItem

Afterwork

Hi everyone ! I have problem with function setPos() on QGraphicsItem . I want the rect can rotate only,rotate and move (moving on a cycle) and only move. Here is class where I use it :

#include <QPainter>
#include <QGraphicsItem>
#include <QGraphicsScene>

class robot : public QObject , public QGraphicsItem
{
public:
    robot();
    QRectF boundingRect() const;
    void paint (QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget);

    void setSpeed (int);
    void setSize(int ,int );
    void setAngle(int);
    void setStep(int);
    int  CurrentAngle();

protected:
    void advance(int phase);

private:
    qreal a;
    QRect size;
    qreal angle,speed,stepAngle;
    void DoCollision();
};

I have the rect on QGraphicsScene and i want it rotate around it's center. I use

robot::robot()
{
    speed = 0;
    angle = 0;
    stepAngle = 0;
    a = 0;
    size = QRect(0,0,20,20);
    setTransformOriginPoint(10,10); // center of a rect
    setPos(mapToParent(300,300));
}

and in advance:

void robot::advance(int phase)
{
    if(!phase) return;
    setRotation(rotation()+stepAngle);
}

and it is working : the rect is rotating all time around it's center . But when i write function

void robot::advance(int phase)
{
    if(!phase) return;
    setRotation(rotation()+stepAngle);
    **setPos(mapToParent(0,-speed));**   // speed = 0 , stepAngle > 0
}

the rect is rotating and moving ( but speed = 0 ) , and its moving trajectory seems like cycloid and rect goes beyond qgraphicswiev. I notice when i write

setTransformOriginPoint(0,0);  // not 10,10 ( not center)

instead of

setTransformOriginPoint(10,10);

it is working fine ( is rotating without moving ) even with

void robot::advance(int phase)
{
    if(!phase) return;
    setRotation(rotation()+stepAngle);
    **setPos(mapToParent(0,-speed));**   
}

I want rect to rotate(stepAngle != 0 ) , rotate and move ( moving on a cycle) and move ( on line,stepAngle = 0 ) with setTransformOriginPoint(rect.center()) . Thanks for help !