Pendulum-and-hydrostat control

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Pendulum-and-hydrostat control is a method developed to keep a Whitehead torpedo at a chosen depth. A hydrostat measures depth from water pressure, so depth is proportional to pressure. But with depth control from the hydrostat alone, the torpedo tended to oscillate around the target depth. The addition of a pendulum senses the torpedo’s pitch and provides damping. The pitch information is combined with depth data to set the depth-control fins, reducing oscillations. In control terms, this turns a simple proportional controller into a proportional-derivative controller, since the depth rate of change (related to the pendulum angle) also influences the fins. The relative strength of the proportional and derivative parts could be adjusted with different linkages. This approach was mainly used to control torpedo depth through World War II, lowering depth errors from about ±40 feet (12 m) to as little as ±6 inches (0.15 m). Invented by Robert Whitehead, it was a major advance and was nicknamed “The Secret.”