Figure 10.10 shows a mechanical hydraulic servo system with automotive power steering, the sequential operation of which occurs as follows:
• The input or command signal is the turning of the steering wheel.
• This results in movement of the valve sleeve, which ports oil to the actuator (steering cylinder).
• The piston rod moves the wheels through the steering linkage.
• The valve spool is attached to the linkage, thereby moving it.
When the valve spool has moved far enough, it cuts off the oil flow through the cylinder. This stops the motion of the actuator.
It is therefore clear that mechanical feedback re-centers the valve (servo valve) in order to stop motion at the desired point which in turn is determined by the position of the steering wheel. Additional motion of the steering wheel is required to cause further motion of the output wheels.