In this case, a rod-position demand signal is fed into a control-rod position controller. The controller moves the rod until the difference between the desired position and the measured position is zero. Figure 7.1b shows the closed-loop rod control system. This is a very convenient method for dynamic testing, but it is not the way control rods are normally positioned in nuclear reactors. Usually, it would be necessary to install a special-purpose controller for dynamic testing. This is generally a simple technical problem, but may require considerable effort to satisfy operational and licensing requirements.