def set_target(self, new_target):
        """
        Sets the new target in mm, the movement starts immediately. If the value is outside
        the interval [0, 250] or the motor is already moving, an error is returned, otherwise
        the new target is returned."""
        try:
            self.device.target = new_target
            return 'T={}'.format(new_target)
        except RuntimeError:
            return 'err: not idle'
        except ValueError:
            return 'err: not 0<=T<=250'