behaviour for the case of a drill string two sensors are needed to measure the torsion angle and or the angular velocity of the string directly on the drive as well as a small distance below the drive e g 2 meters cf detail in fig1 the two measurement points are located above the ground area and are therefore readily accessible the idea of the control method is based on the fact that the rate of propagation of torsional waves is infinite in addition the rate of propagation is independent of the frequency of the wave in question the torsional vibrations in a rod are described by the wave equation where u03c6 x t is the torsion angle as a function of the length coordinate x parameter c is the wave propagation velocity in the material it holds that c 2 g p where g is the shear modulus and u03c1 is the density of the material let the length of the structure in question be le and the short section 0 lt x lt 1 of the structure shall be considered below and in addition le gt 1 it is assumed that there are no externally acting torques within the section in question in addition the measurement of the rotational rate u03c9 x 0 u03c90 should be at the point x 0 and the measurement of the rotational speed u03c9 x 1 u03c91 should be at the point x 1 the sensor spacing d is selected here to be 1 however through appropriate scaling all other spacings d are also possible the measurements are assumed to be available continuously and free of noise these measurements may be interpreted.