Hydraulic actuation control in propellers

1. A propeller pitch change hydraulic actuation system, comprising: a double-acting dual chamber hydraulic pitch change actuator comprising:

a first pressure circuit having first fluid supply lines and a first hydraulic chamber;

a second pressure circuit having second fluid supply lines and a second hydraulic chamber;

a piston separating the first and second chambers;

at least one pressure sensor for obtaining pressure measurements from which a load differential applied to the piston by the circuits can be calculated; and

a closed loop controller arranged to control the fluid supplied to the first and second pressure circuits, wherein the closed loop controller includes:

an actuator position loop arranged to utilise feedback on the actuator position to control the actuator position; and

an inner load loop within the actuator position loop created utilising the load differential calculated from the pressure measurements obtained by the at least one pressure sensor, the inner load loop arranged to feed a current correction to the actuator position loop in order to stabilise control of the actuator position loop on the actuator position.

2. A system as claimed in claim 1 , wherein the at least one pressure sensor is a differential pressure sensor arranged to measure the pressure differential between the first and second pressure circuits.

3. A system as claimed in claim 1 , comprising two pressure sensors, one located in each pressure circuit, each pressure sensor being arranged to measure the pressure in the circuit in which it is located.

4. A system as claimed in claim 3 , wherein the pressure sensors are located in the fluid supply lines or hydraulic chambers.

5. A system as claimed in claim 3 , wherein the closed loop controller is arranged to determine the pressure differential between the first and second pressure circuits from the pressure measurements in each circuit.

6. A system as claimed in claim 1 , wherein the closed loop controller is arranged to calculate the load differential applied to the piston, utilising the pressure measurement(s) and the working area of the piston.

7. A system as claimed in claim 1 , further comprising a flow metering valve controlled by the closed loop controller, wherein the flow metering valve is arranged to supply fluid pressure and flow to the first and second fluid circuits according to the control of the closed loop controller; and wherein optionally the flow metering valve is an electrohydraulic servo valve or a direct drive servo valve.

8. A system as claimed in claim 7 , wherein the first fluid supply lines comprise supply lines between the flow metering valve and the first chamber, and the second fluid supply lines comprise supply lines between the flow metering valve and the second chamber.

9. A system as claimed in claim 1 , wherein the first pressure circuit is an increase pitch pressure circuit that acts to increase the pitch of blades of the propeller; and wherein the second pressure circuit is a decrease pitch pressure circuit that acts to decrease the pitch of blades of the propeller.

10. A method of controlling the pitch of a variable pitch propeller, comprising:

selectively applying fluid pressure and flow to first and second pressure circuits in a double-acting dual chamber hydraulic pitch change actuator so as to control the position of a piston separating first and second chambers of the first and second pressure circuits respectively; wherein the fluid pressure and flow applied to the first and second pressure circuits is controlled by a closed loop controller that includes an actuator position loop;

obtaining pressure measurement(s) from which a load differential applied to the piston by the pressure circuits can be calculated;

calculating a load differential applied to the piston by the pressure circuits;

creating an inner load loop within the actuator position control loop utilising the calculated load differential; and

stabilising the control of the actuator position loop on the fluid pressure and flow applied to the first and second pressure circuits by feeding a current correction to the actuation position loop from the inner load loop.

11. A method as claimed in claim 10 , wherein the pressure measurement(s) are obtained utilising a differential pressure sensor that measures the pressure differential between the first and second pressure circuits.

12. A method as claimed in claim 10 , wherein the pressure measurements are obtained utilising a pressure sensor located in each circuit.

13. A method as claimed in claim 12 , wherein each pressure circuit comprises fluid supply lines and a respective chamber, and wherein the pressure sensors are located in the fluid supply lines or chambers.

14. A method as claimed in claim 10 , further comprising supplying fluid pressure and flow to the first and second pressure circuits utilising a flow metering valve, optionally an electro-hydraulic servo valve or direct drive valve, which is controlled by the closed loop controller.

15. A method as claimed in claim 10 , wherein the first pressure circuit acts to increase the pitch of blades of the propeller; and wherein the second pressure circuit acts to decrease the pitch of blades of the propeller.