GB2159638A - Linear hydraulic actuator system - Google Patents

Abstract

A method of operating a hybrid Linear hydraulic actuator system is by coupling an input electro- mechanical system with a hydraulic system so that the two systems perform together to obtain a linear transfer function. An input system utilises a screw to move a screw jack to actuate the load and is provided with a load sensor at the input which gives a signal for a loop, arranged to control a pilot valve to supply fluid pressure to the piston of the jack. When the input is applied the screw is rotated and a signal from the sensor proportional to the load is fed into the loop which causes the valve to control fluid pressure to substantially reduce the load on the screw. This hybrid system can be arranged so that should one system fail the other system will function as a linear system.

Description

SPECIFICATION Linear hydraulic actuator system Hydraulic systems suffer inherent faults that prevent close following of the input by the output. Where a high performance system is required to give a linear response which has a severe inertia loading at the output many problems beset a designer to obtain an acceptable transfer function. Some examples of the inherent effects which influence output of actuators are as follows: Phase lag, friction loading, fluctuating peak pressures on area of ram, leakage, pressure drop across the valve port, cavitation before valve closure, stiction, overshoot caused by internal and external loading, compressibil ity of fluid, ripple in ram displacement caused by dither fluctuation pressures, tem perature of fluid, aeration of oil, effects on oil column stiffness.

Speaking in general the higher the quality of the linear output that is required for an application the more it is necessary to incorporate precision components with additional equipment to obtain an even fluid pressure such as for example make up pumps, accumulators, heating and cooling means to conserve a stable viscosity of the oil etc. Cold starts are notoriously troublesome in hydraulic systems.

In particular, pilot valves which have to be precision made and have ports which have to be aligned with accuracy are difficult and expensive items to manufacture. Wear of pilot valves may cause instability or hunting problems etc. A screw actuator on the other hand can provide a linear output and avoid problems which hydraulic and pneumatic systems suffer but is subject to the penalty of wear on all but light loads.

It is a step and object of this invention to provide a hybrid linear system and method of operating in which an electromechanical system of screw and loop provided with a load sensing means is combined with a hydraulic actuator system arranged to obtain a linear transfer function which avoids undesirable effects which influence the output of conventional hydraulic and pneumatic systems.

It is a secondary object and step to increase reliability by providing a hybrid actuator in both linear and rotary form whereby the fluid pressure can be substantially reduced, thereby reducing wear and failure rate of components and leakage problems, and yet maintain a linear transfer function of input to output. As will be appreciated derating of fluid pressures provides more scope of use of materials with improved weight ratios of systems, also wear of expensive pilot valves which can cause instability are major problems, seals are less prone to rupture and size of motors and pumps etc. are derated.

A further object is to provide a hydraulic dual system actuator which can maintain a linear output utilizing a non-inflammable fluid as a means to actuate the system. As will be appreciated, if fluctuation of fluid pressure is not a critical factor in maintaining linearity, a non-hydrocarbon non-inflammable liquid may be utilized for some applicatons in which fire risks are involved.

It is intended to provde an actuator system which has a linear input/output transfer function. This is normally achieved conventionally by simply employing say a ram and pilot valve actuator whereby a mechanical lever is arranged to link the output as a loop to the spool of the pilot valve so as to compensate for some of the mentioned undesirable nonlinear effects. However, difficulties arise because a ram is held in a fluid which is compressible, and is subject to cavitation, aeration, friction, fluid pressure fluctuation etc. In particular, any variation of a heavy load imposed at the output such as severe inertia loading for example, has considerable effects to the response and the linear characteristic of the system because of the springyness of the ram in the fluid.Various ways have been tried to overcome the difficulty by increasing fluid pressures, improving the quality and accuracy of components to increase stiffness and linear performance.

However, as has been previously indicated, increasing pressure of the fluid is undesirable and problems of hunting and instability may still exist.

In a hybrid electro-mechanical /hydraulic actuator system the method of operating is by: and comprises the steps of, combining and operating an input electro mechanical system with a hydraulic system whereby the said input electro-mechanical system is coupled and is disposed to be locked with the output to obtain a linear transfer function; arranging the said hydrau lic system to supply fluid pressure to assist and support the said input electro-mechani cal system to actuate the work load; provide means of sensing the effort or load required to operate the said input electro-mechanical system and linking it as a loop to operate and control a valve which supplies and controls the said fluid pressure.

A hybrid-linear actuator will now be described by way of example only. To hold close linearity of say a simple linear actuator of ram and pilot valve throughout the length of travel of the ram, the precision and quality of the pilot valve together with a requirement of control of flow of the fluid is essential to obtain good response the precise positional control. However, if means of holding or mechanically locking the input with the output are introduced into the system the fluctuation of fluid pressure which causes the mentioned undesirable effects becomes less critical to the linear performance of the system.If, say the ram shaft for example, were to be extended through the end wall of the actuator, and teeth were cut on it and they were meshed with a servo motor and pinion arranged to govern the system's input - if the shaft were long enough and suitably sealed, fluid pressure, say air for instance, could be arranged to exert a force on the ram to actuate this somewhat crude rack/ram arrangement to provide practically all of the power to move the ram and work load. By this method of operating the two systems together however the duty of the servo motor in maintaining the linearity of the system need only be marginai.

However, in this instance of example the means and method of mechanically locking the input with the output is by coupling a screw and nut (preferably an off the shelf ball screw and nut) so that the axis of the screw is located on the cylinder axis, whereby a plain portion of the screw extends through the end wall of the cylinder. The other end of the screw is screwed into the nut which is housed and fixed into a hollow portion of shaft upon which the ram is mounted. The plain portion of screw shaft is suitably sealed in the end wall of the cylinder and a gear wheel is keyed and fixed on a protruding end piece. When the gear is meshed with a driving pinion of a servo motor it comprises the electro mechanical means to apply the input.It can be seen that as the input is now directly coupled to the output the actuator could operate even without fluid to obtain a linear output and position. However, it is intended to combine two systems in this example to work together so that the fluid pressure side provides the muscle power for the actuator, thereby reducing the load on the screw to that of only the minimum load required to enable the linearity of the actuator to be easily held. As will be appreciated reducing the load on the screw will not only reduce wear and preserve the precision of the screw, but will allow close following of the input throughout the length of travel of the ram/output.

It should be noted that in conventional linear hydraulic arrangements an error signal is usually taken from the output and fed into a loop. In this arrangement of hybrid actuator, the load at the input is sensed and is utilised to operate the pilot valve.

There are several ways in which this can be achieved. The load at the input can be sensed by mechanical means, and be arranged to operate a pilot valve to provide fluid pressure.

However, in a preferred example and method of operating a transducer, in this instance a strain gauge, is disposed to detect minute axial displacement of the screw, and provides an electrical signal which is fed to an amplifier and then to a synchro which operates a pilot valve. The transducer in this instance is fixed to a metal bridge which spans and houses the end portion of screw and gear wheel, and is suitably fixed to the cylinder. The transducer is disposed to provide a signal for the loop which is proportioned to the screw thrust imposed by the load in either direction. As will be appreciated the signal in the loop can be amplified to control the pilot valve to supply fluid pressue to substantially reduce the load on the screw despite variation of the work load.Also it should be understood that any forward pull of the ram shaft and screw will change the signal from positive to negative and cause the valve to supply fluid to buffer the system against the effects of the inertia of the work load.

In a modification of this linear form of hybrid actuator the axis of the said screw is located outside the cylinder and is parallel with the axis of at least two actuators suitably mounted on a base plate. The ram output shafts of the actuators are linked by an end plate which has a hollow central shaft which extends to about half the length of the actuators and is provided with a nut fixed and located at its end hollow portion which engages the screw housed in the base plate.

Axial displacement of the screw is detected by means of a transducer fixed on the base plate and provides the signal for the loop. A gear wheel mounted and fixed on the screw is meshed with a servo motor and serves as the input for this multi-actuator arrangement. As will be appreciated readily available "off the shelf" non-linear actuators can be used in this arrangement; the signal from the loop operates a common valve which supplies fluid of equal pressure to each actuator. Thus this modified arrangement operates as one so to speak to provide a linear output.

An example of a hybrid linear actuator will be described in rotary form. To convert linear motion of the ram/output so far described into angular or rotary motion it may be seen that a worm could replace the pinion on the servo motor and be meshed with a worm wheel located and fixed on the shaft of a rotary actuator to act as a means of input to maintain linearity. A transducer as before could be arranged to detect minute axial displacement of the worm to provide a signal for the loop to operate the valve which supplies hydraulic fluid to actuate the rotor member.

However, in a preferred instance of this rotary example the servo motor pinion is engaged with a gear wheel which is mounted on one of the shafts of a double ended rotary actuator and serves as the electromechanical means input. The torque of the servo motor is in this instance measured electrically and the signal obtained is suitably applied in the loop via an amplifier to a synchro which operates the valve of the actuator. It should be well understood that in both the linear and rotary forms of this invention the ratings of the mentioned systems can be designed and arranged so that should one side of the system fail, say through loss of the hydraulic fluid for example, the other side can operate or continue to actuate the output.

Claims (4)

1. A method of operating a hybrid linear hydraulic actuator system to obtain a linear transfer function comprises the steps of: (a) coupling an input of an electro-mechanical system with the output of a hydraulic system to actuate the work load, (b) arranging the said hydraulic system to supply fluid pressure to assist and support the said elecro-mechanical system to actuate the work load, (c) provide means of sensing the effort or load required to operate the said electro-mechanical system and linking it as a loop to operate and control a valve which supplies the said fluid pressure.

2. A method of operating a linear hydraulic actuator hybrid system to obtain a linear transfer function as definded in Claim 1 includes the additional step of: (a) providing the said input electro-mechanical system with a screw and coupling it with a nut located in a hollow portion of ram shaft, (b) include a transducer, amplifier and servo mechanism in the said loop to control the said valve.

3. A method of operating a linear hydraulic actuator hybrid system to obtain a linear transfer function as defined in Claim 1 includes the additional modification step of: (a) coupling an input of electro-mechanical system with the output of a rotary hydraulic system to actuate the work load, (b) providing the said input of electro-mechanical system with a pinion to mesh or couple with a gear wheel mounted on the shaft of a rotary hydraulic system, (c) utilising the said loop which operates and controls the said valve to supply fluid pressure to assist and support a rotary hydraulic system to actuate the work load.

4. Measuring the said load electrically of the said rotary input system as defined in Claims 1 to 3 and feeding it into the said loop to control the said valve to supply fluid pressure.

4. Measuring the torque electrically of the electro-mechanical system as defined in Claim 3 and feeding it into the said loop to control the said valve to supply fluid pressure.

5. Sensing the effort or load of the input as claimed in Claims 1 and 3 by at least one means of sensing and utilizing a loop to apply the signal obtained to operate and control the said valve to supply fluid pressure.

6. Sensing the effort or load on a screw as claimed in Claim 2 and feeding the said signal obtained into a loop to operate and control the said valve to supply fluid pressure.

7. A method of operating a linear hybrid actuator system as claimed in any preceding claim in which air comprises the said fluid pressure.

8. A method of operating a linear hybrid actuator system as claimed in Claim 1 whereby should one side of the system fail the other system will function as a linear system.

9. A method of operating a linear hybrid actuator system substantially as described herein with reference to Figs. 1 and 2 of the accompanying drawing.

CLAIMS Amendments to the claims have been filed, and have the following effect: Claims 1-4 above have been deleted.

New claims have been filed as follows:

1. A method of operating a hybrid linear hydraulic actuator system comprises the steps of: (a) combining a means of rotary input system with a hydraulic system disposed to include the step of producing a linear transfer function between the said means of rotary input system, and the output of the said hydraulic system, (b) arranging the said hydraulic system to supply fluid pressure to assist and support the said rotary input system means to produce the said linear transfer function, (c) provide means of sensing the load of the said rotary input system means and linking it as a loop to operate and control a valve which supplies the said fluid pressure.

2. A method of operating a linear hydraulic actuator system as defined in Claim 1 includes the additional steps of: (a) providing the said means of rotary input system with a screw and coupling it with a nut located within a hollow portion of ram shaft, (b) include a transducer, amplifier, and servo mechanism in the said loop to control the said valve.

3. A method of operating a linear hydraulic hybrid system as defined in Claim 1 includes the additional modification step of: (a) coupling means of rotary input system with the output of a rotary hydraulic system to actuate the work load, (b) providing the said rotary input system means with a pinion to mesh with a gear wheel mounted on the shaft of a rotary hydraulic system, (c) utilising the said loop which operates and controls the said valve to supply fluid pressure to assist and support the said rotary hydraulic system to actuate the work load.