Feedback is an intuitive concept. Each time we reach for an object, we slow our hand as it approaches the object so that it is captured with minimal impact. There is visual feedback that our brain uses to control the arm and ensure that it reaches the target and stops.
An equipment operator, for example a backhoe operator, watches the bucket and controls the flow of oil to the cylinder. Experienced operators develop such a feel for the controls that they can control the bucket almost as they control their own arms. Let’s review what happens when the operator is controlling cylinder speed. The pump is a fixed displacement pump, so it puts out a constant flow of oil. The operator cracks open the DCV to create an orifice, and resultant pressure drop causes the relief valve to open and bypass part of the pump flow back to tank. The orifice is continuously varied to obtain the desired cylinder speed. The operator provides the feedback, and the DCV functions like a servo valve. It is helpful to think of the servo valve as a programmable orifice. Feedback continuously programs the size of the orifice (required pressure drop) to achieve a given actuator speed or position.