It is important to use the simplest available method to induce the desired control-rod motion for a test. Safety considerations also influence the selec­tion of a procedure. In general, there are three possibilities.

(a) Open-Loop Rod Motion Control

In this case, the rod moves as long as the drive signal is supplied. Figure 7.1a shows the open-loop control system. The motion may be continuous or step-wise. A binary pulse chain (or a finite-rise-time approximation to it) can be introduced by actuating the insert and withdraw commands for a predetermined length of time. Also, it may be necessary to use different command durations for insert and withdraw signals because of gravitational effects on rod coasting characteristics.

(c)