Patent Grant
6955113
The present invention is generally directed to the field of electro-hydraulic actuators, and more particularly to a method and an apparatus utilizing a highly accurate electro-hydraulic actuator having a force generator to establish closed loop control.
The inventor of the present invention has determined that there are numerous shortcomings with the methods and apparatus of the background art relating specifically to electro-hydraulic actuators.
Electro-hydraulic actuators that are required to maintain a high level of accuracy are typically controlled with servovalves and use position feedback to achieve closed loop control, e.g., with electrical devices. The position feedback may be accomplished by electrical devices such as LVDTs (Linear Variable Differential Transformer), RVDTs (Rotary Variable Differential Transformer), potentiometers, resolvers, Hall effect sensors, or piezo-resistive sensors.
However, there are applications where accurate electro-hydraulic actuators are required and electrical feedback is not available. In these situations, mechanical feedback must be used to maintain closed loop control. In a single-stage type, electro-hydraulic servovalve, a mechanical leaf spring or other spring force from the actuator is normally used to provide the mechanical feedback. As the size or the slew velocity of the actuator increases, the volumetric flow demand eventually exceeds the capacity of the servovalve, e.g., a jet-pipe, and a higher capacity flapper orifice design or a two-stage servovalve is typically required.
The present inventor has determined that it is extremely difficult to attempt to set a higher capacity two-stage servovalve with mechanical feedback from both the second stage as well as the actuator piston (first stage). In addition, the inventor of the present invention has determined that the accuracy of electro-hydraulic servovalves with mechanical feedback is typically limited to only eight percent or higher.
There are several examples of electro-hydraulic servovalves relating to the foregoing discussion of the background art. For example, U.S. Pat. No. 4,335,645 to Leonard, the entirety of which is hereby incorporated by reference, describes a direct drive, two-stage electro hydraulic servo valve incorporating hydro-mechanical position feedback.
However, the inventor of the present invention has determined that this type of complex electro-hydraulic servovalve is relatively expensive and difficult to utilize in practice. As aforementioned, attempting to set a higher capacity two-stage servovalve with mechanical feedback such as that described in the Leonard patent from both the first and second stage is extremely difficult. In the fluidic repeater described by Leonard, the second stage of the servo valve is hydraulically controlled by mechanical feedback from the position piston.
U.S. Pat. No. 4,4450,753 to Basrai et al., the entirety of which is herein incorporated by reference, describes an electro-hydraulic proportional actuator. However, this system requires electrical position feedback. Specifically, a pair of three way solenoid valves is used to position an actuator assembly having a double-acting, linear piston. An electronic control circuit using electrical position feedback nulls out the system and operates the solenoid valves to control fluid flow through the respective ports of the solenoid valves.
U.S. Pat. No. 4,807,517 to Daeschner, the entirety of which is hereby incorporated by reference, describes an electro-hydraulic proportional actuator including at least a piston, a valve for driving the piston into operating positions, a solenoid for producing a driving force for controlling the valve, and a plurality of springs for biasing the piston, solenoid and valve components. In this actuator, compression spring(s) are directly applied to the solenoid plunger and a sliding link rod to produce a force that counters the magnetic pull of the solenoid coil. When an electrical control signal is zero, the compression spring will force the solenoid plunger and the spool rod to the right (as seen and shown in
However, as aforementioned, it has been determined that the electro-hydraulic servovalves with mechanical feedback of the background art suffer from the above-described limitations, including being limited in their accuracy to eight percent or higher error rates.
The present invention overcomes the shortcomings associated with the background art and achieves other advantages not realized by the background art. The present invention is intended to alleviate one or more of the following problems and shortcomings of the background art specifically identified by the inventor with respect to the background art.
The present invention, in part, is a recognition that it will be advantageous to implement a simplified and relatively easily controlled electro-hydraulic actuator utilizing mechanical servo position feedback.
The present invention, in part, is a recognition that an electro-hydraulic actuator using mechanical servo position feedback with a high level of accuracy has heretofore not been achieved by the background art.
The present invention, in part, provides an electro-hydraulic actuator comprising a single stage servomechanism; a current versus load generator, the current versus load generator capable of energizing the single stage servomechanism with an input force, the input force controlling the single stage servomechanism to regulate a regulated servo pressure controlled by the electro-hydraulic actuator.
The electro-hydraulic actuator may further comprise a mechanical feedback device producing a mechanical feedback force for offsetting the input force of the load generator, wherein the mechanical feedback force provides closed loop control of the electro-hydraulic actuator.
The present invention, also in part, provides methods of providing closed loop control for an electro-hydraulic actuator, said method comprising the steps of energizing a single stage servomechanism with a current versus load generator to produce an input force; and offsetting the input force of said load generator with a mechanical feedback force to achieve closed loop control of the actuator, e.g., through a roller assembly.
The present invention will become more fully understood from the detailed description given hereinafter and the accompanying drawings that are given by way of illustration only, and thus do not limit the present invention.
The present invention will now be described in detail with reference to the accompanying drawings.
For many years the Bendix Corporation designed and used computational hydromechanical feedback servomechanism(s) to provide accurate control of subsystems in large gas generator fuel controls. These mechanisms typically sense one or more pneumatic pressures, perform hydromechanical computations, and move an actuator piston to perform some desired function such as positioning a cam or valve in response. The present invention utilizes a similar concept to these types of servomechanisms of the background art. However, the input force that is normally provided by the pneumatic pressure(s) acting upon a bellows is created by a load generator in the present invention, e.g., similar to the magnetic coil of a solenoid or torque motor.
In
An increase in power piston 20 pressure Px results in the power piston 20 moving away, e.g., to the left as seen in FIG. 1 and to the right in
The power piston 20 includes a cam 22 having a cam surface 23 with a predetermined slope S. The cam 22 and cam surface 22 is engaged with a cam follower 25 and lever assembly 24 that provides mechanical feedback to the load generator 5 via a roller assembly 40 operatively connected via the cam follower 25 and lever assembly 24. The cam follower 25 is arranged to follow the cam surface 22 throughout the power piston's 20 travel.
When the power piston 20 has moved to the desired linear position, e.g., the power piston 20 pressure Px and servo pressure Pr are at their desired values, the lever assembly transfers a mechanical feedback force via the roller assembly 40 that offsets or nullifies the initial load generator force Fsol. When the solenoid force Fsol, e.g., the input force, is nullified, the mechanical feedback via the roller assembly 40 is completed and thereby provides closed loop mechanical feedback to the electro-hydraulic actuator 1. A trim spring 41 is provided that spring biases the mechanical feedback force of the roller assembly 40 in a preferred embodiment.
One of skill in the art will appreciate that a single shaft engine (gas turbine) normally drives a load via a reduction gearbox. This reduction gearbox may then be used to also drive engine accessories, e.g., such as fuel and oil pumps. A typical load is normally an electrical generator, mechanical pump or in some cases a second air compressor. However, a single shaft engine cannot normally accept any kind of load until it has started and accelerated to operating speed. Therefore, it is normally the case that all mechanical load should be removed from an operating gas turbine before it is shut down. For example, many aircraft APUs are single shaft designs with the aforementioned characteristics.
Alternatively, twin shaft gas turbines have the advantage that they can be started with a mechanical load applied. The compressor part of the engine or “Gas generator” is started and accelerated up to speed. The exhaust from the gas generator spins a power turbine driving the load. This type of small gas turbine is especially useful for starting larger engines and is known as a gas turbine starter (GTS) or jet fuel starter.
The power turbine in a twin shaft gas turbine must either drive a load or be connected to a mechanical governor so that the gas generator speed can be controlled to prevent the power turbine from over-speeding. GTS units do not always employ a governor, instead a speed sensing device shuts the GTS down when the load reaches a pre-determined speed. In addition, some GTS units are fitted with power turbine governing systems and can also drive loads such as AC generators and operate as APUs.
In the embodiment shown in
TABLE I and TABLE II include experimental values for a current versus load generator utilizing mechanical feedback as described hereinabove. As seen in TABLE I, the slope S of the cam surface can be represented in degrees, e.g., 14 degrees, and/or in terms of length versus current, e.g., inches of cam follower 25 travel along the cam surface per mA of solenoid current. This linear relationship between length and current permits accurate mechanical feedback in response to an input force from an electrical input device, e.g., a load generator with a solenoid. The mechanical feedback force provided by the spring-biased roller assembly 40 is accordingly proportional to the servoposition feedback, e.g., the servoposition or cam position obtained and related by the cam follower 25 and lever assembly. In TABLE II, the relationships between mA of solenoid current, Fsol and piston travel are shown.
One of skill in the art will also appreciate that mechanical feedback may be achieved by alternative sources not shown by the spring-biased, roller assembly of the preferred embodiments shown in the accompanying figures. For example, the present inventor has determined that it may be possible to also provide mechanical feedback by using a combination(s) of a hydraulically loaded piston or bellows assembly that receives a pressure signal that is biased as a function of the piston being positioned by the load solenoid. It may also possible to use a series of pivoted levers and springs manipulated by the piston to derive a position feedback signal.
| Number | Name | Date | Kind |
|---|---|---|---|
| 383901 | Neu | Jun 1888 | A |
| 686942 | Hegarat | Nov 1901 | A |
| 1518894 | Bliss | Dec 1924 | A |
| 2969808 | Horlacher | Jan 1961 | A |
| 3039988 | Trinkler | Jun 1962 | A |
| 3942327 | Noe | Mar 1976 | A |
| 3967533 | Tyler | Jul 1976 | A |
| 4011891 | Knutson et al. | Mar 1977 | A |
| 4033233 | Toi | Jul 1977 | A |
| 4177713 | Lewis et al. | Dec 1979 | A |
| 4335645 | Leonard | Jun 1982 | A |
| 4365770 | Mard et al. | Dec 1982 | A |
| 4450753 | Basrai et al. | May 1984 | A |
| 4807517 | Daeschner | Feb 1989 | A |
| 5014667 | Meyer | May 1991 | A |
| 5179888 | Schendel et al. | Jan 1993 | A |
| 5346360 | Cooper | Sep 1994 | A |
| 5735122 | Gibbons | Apr 1998 | A |
| 5735503 | Hietkamp | Apr 1998 | A |
| 5899064 | Cheung | May 1999 | A |
| 5938124 | Lowi, Jr. | Aug 1999 | A |
| 5997431 | Vukovich et al. | Dec 1999 | A |
| Number | Date | Country |
|---|---|---|
| 1 054 161 | Nov 2000 | EP |
| 1 445 680 | Aug 1976 | GB |
| Number | Date | Country | |
|---|---|---|---|
| 20040089144 A1 | May 2004 | US |
