variable displacement electro-hydrostatic actuator
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VARIABLE DISPLACEMENT ELECTRO-HYDROSTATIC ACTUATOR John A. Anderson Project Engineer
Dynamic Controls, Inc. Dayton, Ohio
ABSTRACT The electro-hydrostatic actuator (EHA) is a power- by-wire actuator (PBW). PBW actuation is an attractive concept for flight control systems in that the need for a central hydraulic system is eliminated. This simplifies the secondary power generation and allows the hydraulic lines to be replaced with electric power cables. Replacing current flight control actuators with EHAs raises concerns about actuator stiffness, package size, and heating. Most EHAs have been developed with fixed displacement pumps. This paper discusses the effects of pump displacement on actuator performance characteristics and the possible advantages of using a variable displacement pump in an EHA. BACKGROUND INFORMATION The work discussed here was performed under Air Force Contract F33615-86-C-3606, sponsored by Wright Laboratories, FIGL; Mr. Gregory J. Cecere, Program Manager. A PBW flight control system would eliminate the need for a central hydraulic system on-board aircraft. Requiring a single type of power generation (electric) simplifies the secondary power generation system. The simplified system should lead to greater reliability and reduced design and maintenance costs. PBW would also replace the hydraulic lines running to and from the actuators with electric power cables. This is advantageous because the cables allow more flexibility in routing and are less vulnerable to damage. Another advantage to PBW is that a failure in one actuator is more readily isolated from the other actuators. Breakers and fuses can interrupt a short circuit before the power generator is damaged. With a central hydraulic system, a leak in a line or an actuator will cause hydraulic fluid to be lost which degrades the entire system. Valves can be installed to isolate leaks but the location of leaks is often difficult to determine. Fluid lost is gone from the system and cannot be replaced until after the flight. The lost fluid also is a fire hazard. The EHA contains hydraulic fluid but the volume is small, therefore the associated fire hazard is also comparatively small.
The advantages mentioned above are some of the reasons PBW is an attractive concept for flight control actuation. The EHA uses a hydraulic pump to couple the rotational motion of the electric motor to the actuator output. The hydraulic coupling a!lows a great deal of flexibility in the design of the actuator package. The location and orientation of the motor and pump relative to the output cylinder can be readily varied to meet the package dimensional requirements. This gives the EHA an advantage over the electromechanical actuator (another PBW candidate). The relative location and orientation of the motor, gears, and ball screw of an electro- mechanical actuator cannot be readily varied. The EHA also offers the possibility of varying the coupling ratio by varying the pump displacement. The variable coupling ratio could be used to increase actuator stiffness, decrease package size, and to decrease package heating. These are some of the primary concerns in replacing the currently used electro- hydraulic valve (EHV) controlled actuators with EHAs in flight control systems.
HEATING Motor heating can be a significant problem for PBW actuators. With a central pumping system, the rejected heat is carried from the actuator, by the hydraulic fluid, to a central heat exchanger. The EHA does not have this exchange of fluids so if cooling were required, the heat exchangers would have to be mounted on the individual actuators or another fluid system would be added to the aircraft. This would make the actuators significantly more difficult to package in the limited space near the control surface.
529 U.S. Government work not protected by U.S. copyright The source of the rejected heat is the actuator's inefficiency and the nature of aerodynamic loads. Servovalve-controlled actuators are very inefficient because flow to and from the actuator is subject to a pressure drop across the valve. With no load or an aiding load, the sum of these pressure drops is equal to or greater than the system pressure. The hydraulic energy lost in these pressure drops is converted into heat. An aerodynamic load will generally be proportional to the degree the control surface is raised into the airstream. The actuator does work on the airstream when the surface is raised into the airstream and the airstream does an equal amount of work on the actuator when the surface is lowered. In a servovalve actuator this energy is seen as a pressure drop across the valve and heats the working fluid. In this manner, all the energy put into the actuator becomes rejected heat and is carried away by the fluid. The EHA is more efficient than the valve-controlled actuators because there are no unbalanced pressures except to support the load. When the EHA lowers a load, the pump is back drives the motor as a generator. In this way, the work done on the actuator is converted into electric energy and removed from the system as electric current. This is important for an EHA since there is no exchange of fluid to remove the energy from work done on the actuator if it were converted to heat. Other sources of heating of the EHA are the component inefficiencies. The most significant of these is the 12R losses of the motor. The torque the motor produces is proportional to the armature current. The torque required to hold a stall load is proportional to pump displacement. The pump displacement must be matched to the motor speed and cylinder area so that the actuator slew rates are met. The variable displacement pump allows the displacement to be reduced under high load and slow rate conditions. This limits the 12R heating losses of the motor. SIZE Another advantage of using a variable displacement pump in the EHA package is that the motor size can be more closely matched to the actuator's power requirements. Motors have a power curve (a plot of torque versus velocity) for a given amount of heat dissipation. The power curve is current limited at low velocities. The 12R losses make up most of the heating load. As velocity increases, the viscous and non-viscous damping generates some heating so the total current must be reduced. The damping will also consume some of the torque which also reduces the total output torque as velocity increases. This trend continues to a point where the motor power output becomes voltage limited. This is the point where the motor velocity is high enough so that the supply voltage is not large enough to overcome the back emf and create enough current to overheat the motor. This curve is approximated by two straight lines in Figure 1. The point at which the lines intercept would be the peak power. In reality, the curves are not straight and may be as indicated by the dashed line in Figure 1. The straight-line approximation to this curve is a conservative estimation of motor power.
