Watt's Flyball Centrifugal Governor

Watt's flyball governor is an early example of an automatic control system consisting of an error sensor connected by a negative feedback loop to a control device which drives the error to zero thus maintaining a desired operating point.


The drive shaft of Watt's speed sensor is geared to the steam engine's main drive shaft and rotates at a convenient speed in unison with it. As it rotates the two heavy balls are driven outwards by the centrifugal force. As the weights fly outwards, a sliding ring on the drive shaft is pulled downwards by the scissor mechanism supporting the weights. This displacement of the ring along the shaft represents the magnitude and direction of the speed error. A linkage mechanism provides the feedback loop which transfers this movement to the butterfly valve of the throttle control. The pivot in the linkage reverses the direction of the error signal thus providing the negative feedback.

If the engine speed is too high, the centrifugal force on the sensor's weights will cause the actuator rod to be raised, in turn causing the butterfy valve to move so as to restrict the flow of steam into the engine, hence reducing its speed.

Conversely, if the engine speed is too low, the centrifugal force will be lower and the weights will be closer to the drive shaft and the sliding ring will ride higher on the drive shaft. This will force the actuator rod downwards opening up the butterfly valve to admit more steam into the engine thus increasing its speed.

The desired speed of the engine is set by means of a screw thread on the actuator rod which adjusts the rod's length thus enabling the angle of the butterfly valve to be set to the corresponding operating point.