A hydraulic servomotor comprising one or two controlled motor spaces. An accumulator is connected to each of the motor spaces, a flexible diaphragm in each accumulator dividing the accumulator space into a compartment which the hydraulic medium can flow into or out of via a restriction, and a compartment in which a gas under pressure is contained.
What is claimed is
1. A hydraulic servomotor comprising a controlled motor space, means for conducting hydraulic medium to said space, and means for damping oscillations of pressure of the hydraulic medium, said damping means comprising an accumulator having first and second compartments separated by a flexible diaphragm, said accumulator comprising a disk member and a block member, said disk member being a wall of said first compartment, and a viscous flow restriction means for connecting said first compartment directly to said motor spaces, said viscous flow restriction means being at least one restrictive passage through said disk member, said second compartment containing a gas under pressure.
2. A hydraulic servomotor as claimed in claim 1, wherein said diaphragm comprises a ring near the circumference of the diaphragm, said accumulator has a recess between the disk member and the block member having a smaller cross sectional area than the cross sectional area of the ring, said ring being clamped in said recess between said members.
3. A hydraulic servomotor as claimed in claim 1, wherein said diaphragm has a tubular construction enclosing a variable volume, said second compartment being the volume enclosed within the tubular diaphragm.
The invention relates to a hydraulic servomotor comprising one or two controlled motor spaces and a valve for controlling hydraulic medium flow from and to each of the motor spaces.
A controlled motor space is to be understood to mean herein a motor space in which the pressure of the hydraulic medium is adjustable so as to exert a force of a desired value. The servomotor may be of the linear type in which either the pressure on either side of a piston is adjustable (two motor spaces), or the pressure on one side of a plunger is controllable and is constant on the other side (one motor space). The servomotor may also be constructed as a rotating motor.
Such hydraulic servomotors may be used in a variety of fields of technology. Generally, hydraulic power amplifiers have poor dynamic properties (very small damping). The effects of low damping are particularly noticeable, for example, in chipping operations or in rapid positioning. The resulting residual vibration keeps a member connected to the piston rod from assuming the desired position accurately and without vibration, within a desired period of time. This drawback is very obvious particularly when the hydraulic motor is used as a drive of a slide or another member in microtechnology, such as the manufacture of masks for semiconductor manufacture.
The object of the invention is to provide a hydraulic servomotor which has a sufficiently large damping and in which a detrimental oscillation of the piston is prevented so that a high adjusting rate can be obtained. In order to achieve this end, according to the invention an accumulator is connected to each of the motor spaces, a flexible diaphragm being provided in each accumulator and dividing the accumulator space into a compartment which the hydraulic medium enters or leaves via a restriction and a compartment in which a gas under pressure is present.
A differential pressure across, for example, the piston of a linear servomotor arising from forces of acceleration or operation is smoothed by the accumulator so that oscillation of the piston is prevented by the increased damping. An additional advantage of a servomotor according to the invention is its great simplicity and reliability, while the increased damping is not associated with energy consumption.
The accumulators may be connected to the motor spaces via a short wide pipe. In a preferred embodiment, however, the accumulator is arranged in the motor space. This is possible because the accumulator requires only a small volume for good performance. This location also provides the possibility of optimum damping.
In a further embodiment the restriction consists of one or more capillary resistances which produce laminar flow. In this case a truly linear variation of the flow resistance of the restriction is ensured.
The accumulators may be constructed in various manners. In a structurally advantageous embodiment, each of the accumulators comprises a disk-shaped member and a block-shaped member, the disk-shaped member comprising a number of restrictions communicating with the motor space and the accumulator space being provided in the block-shaped member. The flexible diaphragm may be provided near its circumference with a ring which is clamped between the disk-shaped member and the block-shaped member in a recess having a smaller volume than the ring. The diaphragm may also be constructed to be tubular, and be located in the accumulator space.
The invention will be described in greater detail with reference to the drawing.
FIG. 1 is a diagram showing the principle of a hydraulic servomotor according to the invention.
FIG. 2 is a sectional view of a hydraulic servomotor having accumulators incorporated in the motor spaces.
FIG. 3 is a sectional view showing a connection of the diaphragm of the accumulators.
FIG. 4 is a sectional view of a diaphragm having a tubular construction.
A linear hydraulic servomotor shown in FIG. 1 comprises a cylinder 1 and a piston 2. The piston rod 3 in this embodiment is passed out of the cylinder on two sides, a slide 4 to be adjusted, for example, being secured to the piston rod 3 on one side. Near the two ends of the cylinder 1 a duct 5 for the supply of hydraulic medium is connected, which ducts 5 communicate with a control valve 6. The piston 2 and hence the slide 4 can be placed in a desired position by means of the control valve 6.
A hydraulic servomotor is suitable for exerting great forces. Due to the inherently small damping in such a motor -- the damping factor is approximately 0.001 in a hydrostatically journalled slide to approximately 0.1 in a nonhydrostatically journalled slide -- the desired vibration-free adjustment cannot be obtained in a short period of time. In order to increase the adjusting rate considerably, an accumulator 7 is connected to each of the ducts 5. A diaphragm 8 divides the accumulator space into two compartments 9 and 10. Compartment 10 contains a gas under pressure, for example nitrogen. Compartment 9 communicates with the duct 5 via a restriction 11.
When the accumulators are used, the damping is considerably increased. Damping factors from 0.5 to 0.8 are easy to obtain. A higher damping is usually not desired since the operation of the servomotor may then become too inert. When using the accumulators, the damping is independent of the position of the piston, the construction is simple and the reliability is great. One of the applications of such a hydraulic servomotor is to drive a slide in manufacturing masks for semiconductor production. Very short adjusting times are desired to provide an economically justified process.
FIG. 2 is a cross-sectional view of a preferred embodiment of a hydraulic servomotor having accumulators for damping undesired pressure variations. A piston 16 is present in the cylinder 15; piston rods 17 are passed through the covers 18 of the cylinder (one end of which is shown in the drawing). The control valve as well as the ducts for supplying hydraulic medium to the motor spaces 19, 20 are not shown. The accumulators 21 are incorporated in the covers 18 of the cylinder. Very small volume accumulators will suffice, the volume in the embodiment shown being approximately 2% of the volume of a motor space. Arranging the accumulators immediately adjacent the motor space -- incorporation in the piston is also possible -- is advantageous for satisfactory operation.
Each of the accumulators 21 comprises a block-shaped member 22 in which the accumulator space is accommodated. A flexible diaphragm 23 is clamped between the block-shaped member 22 and a disk-shaped member 24 of the cover 18, dividing the accumulator into compartments 26 and 27. A number of restrictions 25 in the form of capillary resistances through the member 24 connect the compartment 27 to the motor space 19. A gas under pressure, for example, nitrogen, is present in compartment 26, which is sealed by means of a screw 28. Via the restrictions 25, hydraulic medium can fllow into the compartment 27 or flow out of it until the gas pressure and the medium pressure cause the diaphragm 23 to assume an equilibrium position. Pressure variations across the piston are smoothed by the accumulators; due to the strong damping, the piston is rapidly adjusted to the desired position. The overall value of the capillary resistance can be controlled at will, for example, by sealing a number of the restrictions 25 by means of a bolt.
In order to obtain in a simple manner a suitable seal between the block-shaped member 22 and the disk-shaped member 24 of the accumulator, the diaphragm may be provided with an annular thickening 29, see FIG. 3, which is clamped in a recess 30 having a smaller volume than the ring 29. The recess 30 may be present in both members 22 and 24 or in one of the two. It is furthermore possible, as is shown in FIG. 4, to give the diaphragm 23 a tubular construction. The diaphragm has a nipple 31 for supplying the gas under pressure. The three sides of the diaphragm 23 adjoining the block-shaped member 22 are preferably vulcanized on those sides.
The embodiments shown in the figures relate to a symmetrical linear servomotor. However, the invention may also be used in an asymmetric linear servomotor and in a rotating servomotor.