Pendulum-and-hydrostat control

Pendulum-and-hydrostat control is a control mechanism developed originally for depth control of the Whitehead Torpedo. It is an early example of what is now known as Proportional and derivative control

Operation

A pendulum-and-hydrostat control
  1. Pendulum pivot
  2. Elevator and fixed pivot
  3. Pendulum weight
  4. Hydrostat pressure valve

The hydrostat is a simple bellows which senses the depth. However, with only a hydrostat the system tends to oscillate around the desired depth. The pendulum, mounted at right angles within the machine, senses the fore and aft pitch of the torpedo and this angle information is combined with the depth information in order to provide control signals to the machine's vertical-plane steering mechanism. The angle information is key in that it provides a damping term to the oscillation, now known as derivative 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 reduced depth errors from ±40 feet (12 meters) to as little as ±6 inches (0.15 m).

It's importance was so fundamental to the invention of the torpedo that Robert Whitehead nicknamed it The Secret to both show its importance and hide its operation.

References

  1. ↑ Bennett, "A History of control engineering: 1800-1930"
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