A hydraulic actuator is a mechanical actuator that is used to impart a unidirectional force through a unidirectional stroke. The detailed study of various control actuations and design of typical hydraulic actuation system is need as it plays a major role in the working of aerospace vehicle. This study involves the design of hydraulic actuation system for the flex nozzle of a solid motor as part of thrust vector control actuation system. This actuation system is going to control the vehicle in pitch and yaw direction during powered phase. The design, fabrication and testing of the hydraulic actuator are considered in this work. After designing the hydraulic actuator according to the specifications, testing is done manually, to check the performance of the vehicle by using different techniques. The actuator which is designed, fabricated and tested successfully is ready to be fitted in the aerospace vehicle.
Aircraft actuators are devices that can transmit and redirect one form of motion energy to another. The various types of transmitting elements are employed to drive the mechanical links to a desired orientation. They are Hydraulic, Pneumatic, Electrical AC and DC Motors. Hydraulic system uses ‘oil’ under pressure and use electro-hydraulic valve. Hydraulic actuators are used in converting the energy of the working fluid into mechanical energy related to the reciprocating motion. The pressure of the working fluid acts on the piston and creates a force causing the piston assembly to move. As a result, the piston rod can perform useful work. Hydraulic actuators are executive elements in power hydraulic systems. These structures have several advantages, which include the possibility of obtaining large working forces and low operating speeds. Hydraulic power may be reconverted to mechanical power by means of the aircraft.
A. Hydraulic Actuator
An actuator where in hydraulic energy is used to impart motion is called hydraulic actuator. Actuator produces physical changes such as linear and angular displacements. They also modulate the rate and power associated with these changes. A fluid power hydraulic cylinder is a linear actuator which is most useful and effective in converting fluid energy to an output force in linear direction for performing desired work. Hydraulic cylinders are broadly classified according to function performed and construction.
B. Main Components of Actuator
Actuator consists of a servo jack and a servo valve. Servo valve gives the required flow to move the linear actuator at specified velocity. Jack consists of piston& cylinder and a linear potentiometer to give position feedback. Servo valve (flow control valve) plays a major role in electro-hydraulic actuation system. It consists of a torque motor stage and a hydraulic amplifier stage. Output flow rate is proportional to the input current.
LVDT: An inductive sensor (LVDT) is integrated in the hydraulic cylinder. It provides information about the exact stroke and supplies the signal to the machine control to regulate the oscillation stroke.
II. WORKING OF HYDRAULIC ACTUATION SYSTEM
A typical Aerospace vehicle hydraulic actuation system consists of thrust vector control (TVC) system and aerodynamic control (ADC) system. TVC system consists of Boot strap hydraulic reservoir, a pressure relief valve, a non-return valve, a pump motor package, 4 actuators, 11 hose assemblies (both pressure and return) stainless steel pipe assemblies, hydraulic connectors and TVC linkage systems. ADC system consists of 4 actuators, 10 hose assemblies, an accumulator, 2 QC/DC nipples, SS pipe assemblies, ADC linkage system, and a charging valve.
Hydraulic actuators achieve the gimballing of the engine in TVC and movement of the control surface in ADC. Each phase of the control scheme i.e., TVC and ADC is provided with a set of actuators. In the TVC phase, each engine is mounted with two actuators in mutually perpendicular directions (planes) for pitch and yaw. In ADC phase, each of the four control surfaces is connected with an actuator. Total number of actuators in the vehicle is eight. The oil is stored in a bootstrap hydraulic reservoir, which supplies oil to the suction of the pump. The reservoir is self-pressurized piston type. It takes system pressure to develop a suction pressure. A variable delivery axial piston type pump driven by a DC compound motor, pumps the oil from reservoir at a rated system pressure. The electrical supply to the motor is taken from a battery. High-pressure fluid is pumped through a nonreturn valve and a high-pressure filter. A high-pressure filter relief valve is provided in the pressure line. The outlet of the relief valve is connected to the return line. Two accumulators are connected in line, up-stream to the actuators.
Accumulators supply demand flow over the pump flow and dampen the pressure surges. The accumulators are charged with ultra-pure nitrogen. From each accumulator the supply is distributed to four actuators. Electro-hydraulic actuators convert fluid pressure into motion in response to a signal. Return oil from all the eight actuators are passed through two low-pressure filters and fed-back to the reservoir. Two quick connect couplings are provided in the pressure and return line for interfacing external rig.
IV. TESTING AND RESULTS
A. Testing Of Actuator
The following tests are performed to ascertain the performance of all the units. All the units shall be subjected to the following tests prior to installation in the system.
Examination of the product
Operation and leakage test
Maximum displacement test:
Null off set test and leakage measurement test
Gain and linearity test
Frequency response test
We express our sincere thanks to Sri. RJK Chari, Scientist Research Center Imarat for extending their cooperation in completing this project.
This project aims at study of the actuation system, its types and working of the hydraulic actuator in an aerospace vehicle. The selection of the actuator is accomplished according to the requirements of the vehicle. The design and fabrication of the hydraulic actuator is completed by using solid woks and different methods of manufacturing. To know the performance of the actuator tests like, 100 cycles test, leakage test, linearity test, null shift, gain, polarity, threshold test and frequency response test are conducted. In all the above-mentioned tests the actuator exhibited satisfactory results. Therefore, the design, fabrication and testing of hydraulic actuator is performed successfully.