OpenGL C++中180到180的运动?

我用一个GLUTTIMERfunc(…)来实现机器人手臂的运动,我的问题是左侧0度到90度很容易完成,

当我尝试0到-90度时,手臂没有停止?我尝试了各种方法,但都失败了,你能提出更好的选择吗?

这是我的计时器功能,

void myTimerFunc(int var)
{
switch(var)
{
case 1: if(shoulder>=90)
            stop=1;

    if(!stop)
    {    
        shoulder = (shoulder + 5);
        glutPostRedisplay();
        glutTimerFunc(100,myTimerFunc,1);
        break;
    }


    if(shoulder<=0)
        stop1=1;

    if(!stop1)
    {
        shoulder = (shoulder - 5);
        glutPostRedisplay();
        glutTimerFunc(100,myTimerFunc,1);
        break;
    }


case 2:    if(shoulder>=360)

        stop2=1;

    if(!stop2)

    {

        shoulder = (shoulder - 5);

        glutPostRedisplay();

        glutTimerFunc(100,myTimerFunc,2);

        break;

    }


// here robot arm is not stopping...........

if(shoulder<=270)
        stop2 = 0;
        stop3 = 1;



    if(!stop3)

    {

        shoulder = (shoulder + 5);

        glutPostRedisplay();

        glutTimerFunc(100,myTimerFunc,2);

        break;

    }


default:break;
}
} 

我是从键盘功能调用这个……

void keyboard (unsigned char key, int x, int y)
{
   switch (key) 
   {
      case 's':             
        glutTimerFunc(100,myTimerFunc,1);
            break;

        glutTimerFunc(100,myTimerFunc,2);
             break;