Pendulum-and-hydrostat control

In control theory the effect of the addition of the pendulum can be explained as turning the simple proportional controller into a proportional-derivative controller since the depth keeping is not controlled by the depth alone anymore but also by the derivative (rate of change) of the depth which is roughly proportional to the angle of the machine.

[1] The relative gain of the proportional and derivative functions could be altered by adjusting the linkages.

It was mainly used to control the depth of torpedoes until the end of the Second World War, and it reduced depth errors from ±40 feet (12 meters) to as little as ±6 inches (0.15 m).

The pendulum and hydrostat control was invented by Robert Whitehead.

It was an important advance in torpedo technology, and it was nicknamed "The Secret".