The concept of a "doser" type of hydraulic actuator has been known in the art for several years. If a measured quantity or "dose" of hydraulic fluid is injected or exhausted from the control chamber of a differential area piston actuator, its output makes a step movement commensurate with the size of the dose. The doses can be administered periodically to achieve a stepping motor type response for digitally administered doses. The dose is controlled by opening a solenoid valve for a discrete time period in response to an electrical pulse from a digital electronic controller. The effective output travel rate of the doser actuator can be varied by varying the pulse frequency and/or the pulse width with the maximum slew rate limited by the flow capacity of the solenoid valve when held continuously open.
Unlike conventional stepper motors, doser actuators do not have inherent digital precision. This is so because, instead of dividing up the stroke of the actuator into precise small fractions for the steps, each step is independently metered so that error is cumulative, and there can be no precise correlation between the number of steps and output positions. Since for most gas turbine control applications geometry is controlled in a closed-loop fashion, the available precision of a true stepping motor exceeds the need, and doser type actuators can serve quite well.
The equilibrium condition for closed-loop operation of a doser or stepper actuator requires either a sensing dead band (for which no position correction is made until the error exceeds the effect of one minimum dose or step) or steady-state limit cycling (where the actuator takes a step, passes the desired position, then steps backward by it, steps forward again, etc.). For either equilibrium condition, precision depends on having a small enough minimum dose or step. Smaller steps require shorter doser solenoid "on" periods and faster stepping motor rates.
While it is true that the size of the dose can be made smaller with progressively shorter energization periods, it is equally true that as the dose is reduced not only does its magnitude become more sensitive to second order effects, but whether it is effected at all becomes more uncertain. For precise actuation, it is highly desirable that a doser actuator be able to administer relatively precise small doses. One way of doing this is by the use of solenoid valves designed for extra fast action and electronic driving circuitry designed to "spike" the solenoid current to help achieve this fast action. Fast solenoid valves and their electronic drive requirements carry penalties in size, weight, electric power and cost.