Robot.prototype.onAnimate = function() {

  if (this.movement == 'raise left arm') {

    var T = -Math.PI;
    this.left_upperarm.quaternion.slerp( new THREE.Quaternion(Math.sin(T/2),   // x
                                                              0,               // y
                                                              0,               // z
                                                              Math.cos(T/2)),  // w
                                        0.1 );

  } else if (this.movement == 'kick') {

    // check if slerp reached almost the end
    if (this.right_upperleg.quaternion.w < 0.72) {

      // signal that the kick is done and the leg should move back
      this.movement = 'kick done';

    } else {

      var T = -Math.PI/2;
      this.right_upperleg.quaternion.slerp( new THREE.Quaternion( Math.sin( T / 2 ),   // x
                                                                  0,                   // y
                                                                  0,                   // z
                                                                  Math.cos( T / 2 ) ), // w
                                            0.1 );
                                      
    }

  } else if (this.movement == 'kick done') {

    // reset leg back to identity
    this.right_upperleg.quaternion.slerp( new THREE.Quaternion(0,0,0,1), 0.1 );

  }

};