Electro-hydraulic actuator

Abstract: An electro-hydraulic actuator has an electric motor (25) disposed in a hydraulic fluid reservoir (17) and connected to drive a hydraulic fluid pump (23). The actuator includes an actuator rod (15) which extends or retracts as a piston (20) is hydraulically driven in a cylinder (19). An actuator housing (13) forms the reservoir (17) and the cylinder (19) and contains hydraulic passages (27, 29, 31, 67) connecting the pump (23), the reservoir (17) and the cylinder (19). A one-way filter can be provided to filter the hydraulic fluid. The hydraulic pump (23) is preferably of the rotating piston kind and includes a port plate which allows the pump to drive an actuator piston the retract and extend chambers of which have different or unbalanced fluid drive ratios. A load limiting valve (70) can protect the system from excessive hydraulic pressure and a position sensor (83) can detect the position of the actuator rod (15).

Description: The invention relates to electro-hydraulic actuators. It has long been recognized that hydraulic, as opposed to purely mechanical or electromechanical actuation is more desirable for some applications. One reason for this is that hydraulic systems have been found more practical in applications requiring high reliability and large force/velocity capability combined with rapid response. For example, a majority of commercial and military aircraft today use hydraulic actuation for their primary flight control surfaces. However, hydraulic servoactuation has limitations, foremost of which is the need for a central hydraulic supply system. A hydraulic pump is required, together with a prime mover to drive the pump, a reservoir, an accumulator and piping to convey the hydraulic pressure to each remotely located servoactuator. There is considerable cost and installation expense, potential maintenance problems due to leakage from the piping, substantial energy losses at the pump, undesirable noise, and for aircraft installations considerable weight and bulk of hardware. There have been many attempts to replace hydraulic servoactuation systems with electromechanical servoactuation systems, thereby eliminating the central hydraulic supply system. These attempts have accelerated, due to recent development in servomotors using rare earth permanent magnets, and recent developments in the electronic control hardware that such motors require. However, the necessary gearing (and often clutches) between such improved electric motors and the load have emerged as the weak link, and have not improved to the degree necessary to replace hydraulic servoactuation in many applications. Patent Specification DE-A-3 635 694 discloses an electro-hydraulic actuator, according to the preamble of claim 1, with an electric motor and pump immersed in a hydraulic fluid reservoir and connected to a piston cylinder arrangement, an actuator rod of which provides a mechanical output of the actuator. A spring cushion is provided between the cylinder and an outer housing. An article in Machine Design Vol 59 No 18 of 6 August 1987 at page 52 describes an electro-hydrostatic actuator with bi-directional motor pump assembly with attached reservoir, hydraulic actuator position sensor and electronic assembly. According to the invention there is provided an electro-hydraulic actuator, comprising: a housing having a hydraulic reservoir chamber with a reservoir of hydraulic fluid therein, and having a cylinder chamber therein; an actuator piston and an actuator rod connected thereto, the piston being movable in the cylinder chamber, and the actuator piston dividing the cylinder chamber into a "retract" chamber at the forward end of the cylinder chamber and an "extend" chamber at the rearward end of the cylinder chamber; a hydraulic pump disposed in the hydraulic reservoir to pump hydraulic fluid to the cylinder chamber to move the actuator rod by hydraulic pressure; an electric motor mounted in the hydraulic reservoir chamber and mechanically connected to the hydraulic pump to drive the hydraulic pump to pump the hydraulic fluid; and hydraulic passages disposed in the housing and connecting the pump to the hydraulic reservoir chamber and the cylinder chamber to move the actuator piston in the cylinder chamber; characterised by a bellows reservoir disposed in the hydraulic reservoir chamber and containing a pressurised gas so as to have a variable displacement of hydraulic fluid in the hydraulic reservoir chamber to compensate for any volumetric changes in the hydraulic fluid therein; a temperature sensor disposed to measure temperature changes of gas in the bellows reservoir; and a temperature sensor disposed to measure temperature changes in hydraulic fluid in the hydraulic reservoir chamber. Thus, the invention uses electric motor actuation rather than a central hydraulic supply, but substitutes a self-contained hydraulic transmission for a mechanical transmission. This avoids many of the problems which arise in the use of mechanical clutches and gears. For example, the invention can provide an effective gear ratio of 2,000 to 1 or higher between the motor and the load, without using any gears. This eliminates gear tooth fatigue problems encountered in electromechanical servoactuators. The need for clutches in redundant mechanical systems is eliminated since a failed servoactuator according to the invention can be backdriven by other parallel servoactuators. Leakage can generally be eliminated in an actuator according to the invention since the design can provide only one likely leakpoint, rather than the many such potential leakpoints of previous constructions thereby greatly reducing maintenance expense. Advantageously the electric motor drives the hydraulic pump on a demand basis, generating only the required pressure and flow. This can conserve energy, reduce electrical power costs, and also generate less noise which can be important in industrial applications. The actuator can provide self contained failure detection capabilities to reduce maintenance costs. The pump is preferably a fixed displacement bi-directional hydraulic pump provided for pumping hydraulic fluid to move the actuator rod and the motor is preferably a reversible brushless DC electric motor with integral feed back tachometer and motor winding temperature sensor mechanically connected to and driving the hydraulic pump with the housing containing the pump and the reservoir of hydraulic fluid in which the electric motor is submerged. The housing also includes the actuator cylinder which contains the piston and the hydraulic passages connecting the pump to the hydraulic reservoir and the cylinder as required for moving the actuator rod. If the front or retract chamber of the cylinder has a different volume to the rear chamber, movement of the piston will cause an imbalance of hydraulic fluid which is preferably compensated for by providing an asymmetrical port plate for the pump. A first port of this plate has a different radial extent from a second port which provides different sizes for the first and second ports. These sizes are matched to the volume/rod movement ratio of the chamber to which each of the ports is open when the pump rotates. A third port allows the pump to drive the differential volume of hydraulic fluid to and from a variable volume chamber. The pump and motor are preferably reversible variable speed devices, to allow variable speed movement of the actuator rod in either direction by means of electrical signals to the motor. In addition to the temperature measurement the sensors can detect, by the rate of temperature change, the presence of gas in the hydraulic fluid or the presence of oil in the gas chamber. Also preferably a position sensor is connected to the actuator rod which is driven by the piston of the hydraulic cylinder. In addition, the hydraulic circuit can be provided with a load limiter/relief valve, which limits the actuator force output to a preset value. The invention is diagrammatically illustrated by way of example in the accompanying drawings, in which:- Figure 1 is a schematic cross sectional view of an electro-hydraulic actuator according to the invention; Figure 2 is a plan view of the actuator of Figure 1; Figure 3 is a cross sectional view of a portion of the actuator of Figure 1; Figure 4 is a schematic view of a portion of the actuator of Figure 1; and Figure 5 is a schematic view similar to Figure 4 showing an alternate embodiment of an electro-hydraulic actuator according to the invention. Referring to Figures 1 and 2, an electro-hydraulic actuator 11 is of the kind used to control flight surfaces in an aircraft. Although the actuator 11 is designed specifically for an aircraft, those skilled in the art will recognize that this electro-hydraulic actuator can be used in many other applications. The actuator 11 includes a trunnion 12 which is formed at one end of a housing 13 to allow the actuator 11 to be attached to the structure of an aircraft. A rod end 16 of an actuator rod 15 can be attached to a flight surface to be moved by the actuator 11. The housing 13 comprises a single piece which extends from an hydraulic fluid reservoir 17 to a cylinder chamber 19 in which a piston 20 moves. The piston 20 is attached to the actuator rod 15 and divides the cylinder chamber 19 into a front chamber 22 and a rear chamber 24. Disposed within the reservoir 17, and immersed in the hydraulic fluid which fills the reservoir 17, is a hydraulic pump 23 driven by an electric motor 25. The electric motor 25 drives the pump 23 to move hydraulic fluid between the chambers 22 and 24 to extend or retract the actuator rod 15. Hydraulic fluid passages 27 are machined in the housing 13 to port the fluid between the pump 23 and the chambers 22 and 24. The pump 23 and the electric motor 25 are reversible and operate so that as fluid is being supplied to one of the chambers 22 and 24 it is being drawn from the other of the chambers 22 and 24. In this way, the extension and retraction of the actuator rod 15 is positively driven by the pressure of the hydraulic fluid in both of the chambers 22 and 24. The pump 23 is bolted to the housing 13 and connected to the motor 25 by a shaft coupling 37. A pin 33 indexes the motor 25 so that the motor 25 is held fixed with respect to the housing 13. The region surrounding the pump 23 and the interior of the motor 25 are at the reservoir pressure. Consequently, leakage from the pump does not cause leakage of hydraulic fluid from the system; the leakage simply returns to the reservoir, where the fluid is re-used. Similarly, no pressure seals are required between the pump 23 and the motor 25 interior, eliminating a source of wear and failure present in previous designs. The portion of the hydraulic reservoir 17 which extends around the motor 25 is provided with heat exchanger fins 35. Because the reservoir 17 is filled with hydraulic fluid, heat from the motor 25 can be rapidly transferred to the housing 13 and dissipated by the fins 35. This advantage results from immersing the motor 25 in hydraulic fluid. Another advantage of this arrangement of parts is the relatively low weight of hydraulic fluid required to operate the actuator. Relatively little volume of hydraulic fluid is required other than the amount necessary to fill the front and rear chambers 22 and 24. Figure 3 shows the pump 23 in more detail and that the pump 23 is a piston type device. The pump shaft 37 is supported by bearings 43 and rotates in a pump housing 45. Pistons, including pistons 49 and 51, are located in an array around the shaft 37 and connected to rotate therewith. The pistons 49 and 51 are moved in a reciprocating motion as they rotate by means of a swash plate 47 which is designed at a sufficient angle from a perpendicular to the shaft 37 to cause the desired amount of fluid displacement by the pistons 49 and 51. The pistons 49 and 51 are reciprocated in a piston manifold 48. As the pistons 49 and 51 reciprocate they move hydraulic fluid into and out of the pump 23 through openings 50 and 52 in the manifold 48. A pump port plate 53 at the end of the pump 23 has shaped ports 55, 57, 59 (see Figure 5) located adjacent the openings 50 and 52 as the pistons rotate, which direct the fluid to and from passages 29 and 31. As the shaft 37 rotates, hydraulic fluid is driven to and from the passages 29 and 31. Reversal of the motor and shaft rotation reverses the flow. Thus, the rate of hydraulic flow is directly proportional to the speed of rotation of the pump shaft 37. Pumps of the kind shown as pump 23 are well known to those skilled in the art. Although such pumps are especially advantageous in an actuator according to the invention, it is believed that other reversible hydraulic pumps could be used. Operation of the motor 25 and the pump 23 can result in the generation of heat. It is, therefore, desirable to monitor the temperature in the hydraulic fluid and a temperature sensor 61 is attached to the upper end of the reservoir 17 for this purpose. In addition, however, the sensor 61 has a resistance heating device which can be pulsed so that the temperature change caused by the heat from the pulsed heating device can be measured. If the decay characteristics of the temperature change following the pulsing of the heating device is too slow, this indicates that gas is present in the hydraulic fluid and maintenance of the actuator is required. To allow for changes in the amount of the hydraulic fluid in the reservoir 17, a gas filled metal bellows 60 is sealingly connected to the top of the reservoir. The bellow 60 is filled with an inert gas such as nitrogen and, therefore, can expand or contract with the amount of hydraulic fluid in the reservoir 17. A fill port 62 is attached to the housing 13 to allow filling of the bellows 60. A temperature sensor 63 is attached to the housing 13 at the upper end of the bellows 60 to allow the temperature of the gas to be measured. As with the sensor 61, the sensor 63 is provided with a thermocouple to allow the temperature decay characteristics of the gas to be monitored. This allows the presence of liquid in the bellows to be detected. Fluid passages and cavities 67 are provided in the housing 13 to allow hydraulic fluid to be conveyed between various auxiliary components and to protect the system. For example, the passages and cavities 67 extend to the blind end of the housing, past a seal of the rod 15, to prevent a build-up of hydraulic fluid at the end of the rod. The passages 67 also connect with a quick-disconnect fitting 66 to allow the actuator to be filled with hydraulic fluid. The passages 67 also extend from the reservoir 17 to a pressure transducer 70. The pressure transducer 70 allows remote electrical monitoring of the static hydraulic pressure in the reservoir 17. Pressure variations in the reservoir 17 may occur due to the thermal expansion or contraction of the fluid or due to depletion of the fluid caused by mechanical, structural or seal failure. The pressure transducer 70 allows remote electrical monitoring of the fluid pressure so that maintenance can be scheduled prior to failures and so that failures can be detected. The passages 67 also connect the reservoir 17 to a load-limiter relief valve 68 which is connected to the passages 29 and 31 to limit the hydraulic fluid loads in the front and rear chambers 22 and 24. When hydraulic pressure in either of these two chambers exceeds a predetermined force level set at the load-limiter relief valve 68, fluid is relieved to the reservoir 17 through the passages 67. The predetermined force level of the relief valve 68 can be adjusted by means of a spring which bears on a valve piston of the valve 68. Check valves are provided to prevent flow from the chamber 22 to the chamber 24 and vice versa, even though both are connected to the relief valve 68. A rotary position encoder 83 is attached to the housing 13 adjacent the rod 15. The position encoder 83 operates by reading movement of a rack and pinion mechanism which forms a part of the encoder 83. The rack portion of the encoder is disposed parallel to and moves with the rod 15. The rotation of the pinion is electrically detected and can be electrically remotely read so that the position of the rod 15 is determined. In other words, the encoder 83 produces electrical signals which indicate the amount of extension or retraction of the actuator rod 15. This allows a confirmation of the extend or retract commands given to the motor 25. It also provides a more direct reading of the location of the rod 15. Porting in the pump port plate 53 can compensate for the kind of actuator rod shown in Figure 5. As shown in Figure 5, the rod 15 does not extend right through the piston 20 so that the front chamber 22 has a different volume to rod movement ratio to that of the rear chamber 24. In a conventional rotating piston pump, the ports 55 and 57 are symmetrical and, therefore, an equal amount of fluid is driven through each port. For an unbalanced piston as shown in Figure 5, this requires some of the fluid to be pumped to or from a variable volume excess fluid reservoir. The extra port 59 in the port plate 53 balances the flow to or from a variable volume chamber 69. By controlling the size of the port 59, a precise flow to and from the chamber 69 will balance the flows to the chambers 22 and 24. This produces a much more efficient movement of fluid by providing a positive displacement of the fluid to and from the chamber 69. Check valves 71 and 73 can be provided to correct any slight differences in the flow to the chamber 69. Referring now to Figure 4, filtration of the fluid conveyed to and from the actuator "extend" and "retract" chambers 24 and 22 is provided . The "retract" passage 31 has a one way filter 80 comprising a first passage 82 including a filter 84 and a check valve 79 which allows fluid to pass through the filter 84 only in the direction from the pump 23 towards the "retract" chamber 22. A bypass passage 85 with a check valve 81 allows fluid to flow only in the direction opposite the flow allowed by the check valve 79. Similarly, the "extend" passage 29 has a one way filter 70 comprising a first passage 72 including a filter 78 and a check valve 77 which allows flow from the pump 23 towards the chamber 24. Flow from the chamber 24 towards the pump 23 passes through a bypass passage 74 including a check valve 75 and around the filter 78.

Claims: An electro-hydraulic actuator, comprising: a housing (13) having a hydraulic reservoir chamber (17) with a reservoir of hydraulic fluid therein, and having a cylinder chamber (19) therein; an actuator piston (20) and an actuator rod (15) connected thereto, the piston (20) being movable in the cylinder chamber (19), and the actuator piston (20) dividing the cylinder chamber (19) into a "retract" chamber (22) at the forward end of the cylinder chamber (19) and an "extend" chamber (24) at the rearward end of the cylinder chamber (19); a hydraulic pump (23) disposed in the hydraulic reservoir (17) to pump hydraulic fluid to the cylinder chamber (19) to move the actuator rod (15) by hydraulic pressure; an electric motor (25) mounted in the hydraulic reservoir chamber (17) and mechanically connected to the hydraulic pump (23) to drive the hydraulic pump (23) to pump the hydraulic fluid; and hydraulic passages (27) disposed in the housing and connecting the pump (23) to the hydraulic reservoir chamber (17) and the cylinder chamber (19) to move the actuator piston (20) in the cylinder chamber (19); characterised by a bellows reservoir (60) disposed in the hydraulic reservoir chamber (17) and containing a pressurised gas so as to have a variable displacement of hydraulic fluid in the hydraulic reservoir chamber to compensate for any volumetric changes in the hydraulic fluid therein; a temperature sensor (63) disposed to measure temperature changes of gas in the bellows reservoir (60); and a temperature sensor (61) disposed to measure temperature changes in hydraulic fluid in the hydraulic reservoir chamber (17). An electro-hydraulic actuator according to claim 1, in which the electric motor (25) is a reversible brushless DC electric motor.

Inventors: KENYON RICHARD L, NOLAN MICHAEL E, SCANDERBEG DINO, WILKERSON WILLIAM D, KENYON, RICHARD L., NOLAN, MICHAEL E., SCANDERBEG, DINO, WILKERSON, WILLIAM D.
Applicants: PARKER HANNIFIN CORP, PARKER HANNIFIN CORPORATION
Application year: 1990
Main classification: F15B15
IPC codes: F15B11, F15B15, B64C13