EP0756574B1

EP0756574B1 - A device for damping undesirable oscillations in a crane member

Info

Publication number: EP0756574B1
Application number: EP95917527A
Authority: EP
Inventors: Peter ASTRÖM; Gösta SVENSSON
Original assignee: Hiab AB
Current assignee: Hiab AB
Priority date: 1994-04-22
Filing date: 1995-04-07
Publication date: 2001-06-20
Anticipated expiration: 2015-04-07
Also published as: SE9401369L; DE69521423T2; ATE202324T1; SE505698C2; EP0756574A1; WO1995029118A1; AU2354395A; SE9401369D0; DE69521423D1

Abstract

The present invention relates to a device for damping undesirable oscillations of a crane member (1) which is pivotally movable by means of at least one hydraulic motor (5, 5'). The hydraulic circuit (8) has a restriction (13) co-operating with a circuit-closing device (14; 18) the purpose of the restriction being to convert energy inherent in the system to heat in the hydraulic oil. The device includes means (14, 15, 16, 16') for controlling the closing device (14; 18) according to instantaneous conditions and/or changes or conditions, such as pressure and/or flow, in the hydraulic circuit (8). The energy-converting action of the restriction (13) is instantaneously activated as soon as a certain condition and/or a certain change of condition occurs. The conduit (12) in which the restriction (13) lies is completely shut off, rendering flow of oil through the restriction impossible, as long as the energy-converting action of the restriction is not called for.

Description

Technical field of the invention

This invention relates to a device for damping undesirable oscillations in an arm or other member of a crane that is pivotally movable by hydraulic means. It is concerned with a system having at least one hydraulic motor in a hydraulic circuit in which there is a restriction. When a circuit-closing device or function is operated to stop the main motion, this restriction converts energy inherent in the system (deformation energy and kinetic energy) to heat in the hydraulic fluid that still flows as a result of oscillation. The heat dissipates that energy to damp or suppress any oscillations of said member.

Background of the invention

In cranes, e.g. of the outer-boom type, there can be a plurality of movements which are carried out by means of hydraulic motors, usually linearly operating motors in the form of cylinder actuators. One such movement is the horizontal slewing of the crane arm between various azimuth positions. This is often accomplished by means of two integrated hydraulic cylinders, which by way of a common toothed rack actuate a gear wheel coaxially carried by the post supporting the crane arm. Such a crane movement is schematically illustrated in the appended drawing where a crane arm seen from above is designated 1. This crane arm 1 is mounted on a post 2 having a coaxial gear wheel 3, which engages a toothed rack 4 connected at opposite ends to motors in the form of two linear hydraulic actuators. These have cylinders 5, 5' in which pistons 6, 6' define two operating chambers 7, 7'. These two hydraulic actuators are part of a hydraulic circuit, generally designated 8, in which is also included a directional valve 9. This is operable to direct oil from a pump 10 to either one of the cylinders 5, 5' through main conduits 8a, 8b, while exhaust oil flows to a tank 11. By moving an adjustable lever 9' of the directional valve 9 from a neutral position to either one of two operative positions the crane arm 1 can be slewed either clockwise or anti-clockwise about the axis of the wheel 3.
A problem, particularly in cranes having a great reach (today there are cranes having reaches of the order of 14 metres or more), arises when the crane arm has finished slewing, i.e. after the adjustment lever of the directional valve 9 has been returned to the neutral position. Both the arm and the load it carries are subject to oscillatory or reciprocating motions, and the duration of these may be long. They are caused by the fact that deformation energy is easily absorbed in the crane arm, and potential energy in a freely hanging load, when the crane arm is slewed. Assume that the crane arm 1 at a certain moment during a cycle of oscillation tends to slew anti-clockwise about the centre of the gear wheel 3. Then the toothed rack 4 is moved to the right in figure 1, the oil pressure in the chamber 7' increasing at the same time as the oil pressure in the chamber 7 decreases. The directional valve 9 is shut off and will not permit flow. With such conditions, the arm 1 will act as a spring at the same time as the free-hanging load acts as a pendulum. That will accelerate in the opposite direction when it reaches the end of its swing and then the arm will be set in motion clockwise. The oil pressure in chamber 7 is thus increased. In this manner the crane arm will oscillate. In a 14-metres crane for instance the tip of the crane arm swing or deflect between positions of the order of 2 metres apart and it can be 15 to 20 seconds before the tip and accordingly the load are stopped. Such oscillations are especially troublesome where the load is at the end of a lengthy rope hanging down from the tip of the crane arm. In such cases the crane arm tip oscillates to and fro at the same time as the load oscillates relative to the tip through the rope. Such oscillations may also occur during the slewing of the crane arm tip between two desired positions. They may happen, for example, if the hydraulic flows to the respective cylinders 5, 5' are disrupted, as a consequence of sharp lever movements at the directional valve 9 perhaps or as a consequence of the load or the crane arm hitting obstacles.

The prior art

In order to cope with the long duration of these oscillations, attempts have been made to damp them. By SE 9101544-6 a damping unit is known which is installed in a branch or intermediate conduit bridging the main conduits between the directional valve and the hydraulic cylinders. The damping unit is in the form of a restriction associated with a co-operating shut-off device. In SE 9101544-6 this shut-off device consists of a piston captive in a cylinder and freely movable between opposite end positions, the piston shutting off the oil flow through the restriction when it reaches either end position. When the oil passes the restriction a turbulent flow in the oil is generated which converts the energy inherent in the system to heat. In this way of the energy remaining in the system is dissipated, and the duration of the oscillations of the crane arm is reduced to a significant extent. Assuming the crane arm is suddenly stopped from the full velocity of slewing, the duration of oscillatory motions (to when the arm is finally still) can be reduced from 15 to 20 seconds to 3 to 5 seconds.
The solution of the problem disclosed in SE 9101544-6 is, however, unsatisfactory for several reasons. One reason is that the piston of the damping cylinder may assume more than one distinct position when the need for damping has ceased. This means that hydraulic oil may flow in limited amounts through the restriction and accordingly to one of the two hydraulic actuators. Therefore, the crane arm will not with certainty assume any positively stationary position. Quite the reverse; the crane arm will have a freedom to swing within a limited range. In certain states of operation, e.g. during loading on the brow of a hill or on inclined ground, this is perilous. Another reason for the known solution being unsatisfactory is that the restricting or choking function becomes irregular as a consequence of the shut-off piston in the damping cylinder moving freely all the way to the end position. There it abruptly interrupts further flow and so briefly stops further damping. This results in a bad damping and in a great statical play in the slewing function of the crane dependent on what is prioritized when adjusting the damping unit.

Objects and features of the invention

The present invention aims to alleviate the problem still left by SE 9101544-6 and provide a damping device which, during the damping operation, promotes regular or even movements of the crane arm and at the same time makes sure that the arm will stop in a reliable manner and in a desired position after slewing. Accordingly, a basic object of the invention is to provide a damping device which is capable of being activated whenever an oscillation tends to occur due to an expected or unexpected external influence on the hydraulic system, such as intentional or unintentional lever movements or collisions against obstacles. Another object is to provide a damping device which, after the arm has stopped moving, will ensure that any flow of hydraulic fluid through the damping restriction will cease and so prevent uncontrolled movements of the crane arm.
According to the invention the above-mentioned objects are achieved by means of the features defined in the characterizing clause of claim 1. Preferred embodiments of the invention are further defined in the dependent claims.

Further elucidation of the prior art

DE 4 216 241 discloses a further developed embodiment of the damping device according to SE 9101544-6. In this embodiment a circuit-closing device in the form of two shut-off valves is arranged in the intermediate circuit in order to keep the damping unit closed until a predetermined pressure occurs in said circuit. The threshold value of the pressure is, however, given once and for all and cannot be adjusted to varying conditions.

Brief description of the appended drawing

In the drawing Fig 1 is a schematic illustration of a hydraulic system of the type previously mentioned, in which a schematically shown damping device according to the invention is included. More particularly figure 1 shows a first, preferred embodiment having control means including a microcomputer or similar control unit.

Detailed description of preferred embodiments of the invention

In a manner that is per se known by SE 9101544-6, a restriction, symbolically indicated at 13, is arranged in a branch or intermediate conduit 12 between the main conduits 8a, 8b. In practice this restriction may consist of a fixed restriction in the form of an annular insert or shoulder of reduced diameter located in the conduit 12. The conduit may be a tube or a drilled bore in a block. A shut-off valve, generally designated 14, and in the form of a magnetic valve of the on-off-type for example, co-operates with this fixed restriction 13. By means of the shut-off valve the conduit 12 may either be completely blocked, whereby oil flow through the restriction 13 is rendered impossible, or opened in order to allow oil to flow through the restriction. This opening and closing of the valve is controlled by a control unit 15 which in practice advantageously, but not necessarily, may consist of a microcomputer. In the embodiment shown, input data to this control unit are obtained by means of two sensors 16, 16' in the form of pressure or flow sensors. These may be connected as shown to the intermediate conduit 12, or they may be connected to the conduits 8a, 8b, on either side of the restriction 13 and the magnetic valve 14. The purpose of these sensors 16, 16' is to detect significant conditions and/or changes of conditions in the hydraulic oil in the circuit. It is preferred in practice to use sensors which are capable of detecting significant pressures and/or changes of pressure which occur in the two actuators 5, 5' when the crane arm 1 starts to oscillate. When the control unit 15 receives information from the sensors 16, 16' that pressures and/or pressure changes of a certain magnitude have arisen in the system, the unit will provide a signal to the magnetic valve 14 to open, whereby the valve will allow oil to flow through the restriction 13. In other words the restriction is activated, and a turbulent flow is created in the oil passing to and fro through it. The varying deformation energy and the kinetic energy in the system is converted, in a manner known per se, to heat in the oil, and the oscillatory motions of the crane arm are damped. As soon as these oscillatory motions have subsided to a level at which the sensors no longer detect significant conditions and/or changes of conditions (and that may happen within the course of a few seconds) the control unit 15 provides a signal to the magnetic valve 14 to close. Any further flow of oil through the restriction 8 is thus blocked. It should be noted that the directional valve 9 is kept closed all the time during this damping operation. Therefore, when the valve 14 is shut off even a minimum flow of oil between the two cylinders 5, 5' is rendered impossible. This ensures that the crane arm 1 is reliably maintained in its given, desired position.
Unlike the solution of the problem disclosed in SE 9101544-6, the damping device of figure 1 makes possible a completely, reliably and positively controlled closure of the intermediate conduit 12 when the crane arm is arrested. Thus, a basic advantage of the invention is that it provides a reliable and positive adjustment of the crane arm to the desired position while the duration of the resultant oscillations is substantially reduced in comparison with such cranes that lack damping devices. Another advantage in comparison with SE 9101544-6 is that the restriction - and accordingly the damping - can be controlled in a positive and efficacious manner. The control unit 15, when it is a microcomputer, may be programmed in such a manner that the magnetic valve 14, which is normally closed, will be opened not only when the crane arm has finished slewing (when the lever of the directional valve 9 is moved to the neutral position) but also at any moment during slewing. A sequence of oscillations, irrespective of whether it occurs after the basic crane arm movement has stopped or during slewing, manifests itself in a harmonic function the condition of which is read by the microcomputer by means of the sensors. On a given condition existing the computer may send a signal to the magnetic valve to either open or close. In contrast, the freely movable or floating piston in the damping cylinder according to SE 9101544-6 cannot be acted on at all from outside in any controllable manner.

Conceivable modifications of the invention

The invention is not merely limited to the embodiment disclosed above and in the drawing. The embodiment of figure 1 includes a fixed restriction and a magnetic valve separate from the restriction. In practice it is, however, conceivable to use, instead of these separate components, a check valve serving both as a variable restriction, and for completely shutting-off the hydraulic conduit. Instead of one single fixed restriction (in practice in the form of an insert or a shoulder having a small percolation aperture) it is possible to use two or more restrictions having wider apertures. Instead of pressure sensors it is also possible to use either flow sensors in the hydraulic circuit or position sensors which instantaneously detect the position of the crane member to be damped. Neither is the invention limited to the use of just two sensors, since the necessary information regarding the flow or pressure conditions in the system may also be obtained by means of one single sensor in combination with the characteristics of the restriction. The principle of the invention is in no way limited to the horizontal slewing of a crane about a vertical axis. Thus, the damping device of the invention may also be used for other members forming parts of a crane, such as the inner boom and the outer boom, in order to damp oscillations in other planes, such as vertical.

Claims

A device for damping undesirable oscillations in a member (1) of a crane that is pivotally movable by means of at least one hydraulic motor (5, 5') in a hydraulic circuit (8) in which there is a restriction (13) co-operating with a circuit-closing device (14), said restriction serving to convert energy inherent in the system to heat in the hydraulic medium in order to damp or suppress oscillations of said member caused by said energy, characterized by sensor means (16, 16') responsive to thresholds in conditions relating to the member when oscillating and arranged to operate the circuit closing device (14) to permit and block flow through the restriction (13) when the thresholds are respectively exceeded and not exceeded, and by means (15) for electronically processing information from the sensor means (16, 16') to a form capable of operating said circuit closing device (14).
A device according to claim 1, characterized in that the circuit closing device is a shut-off valve (14).
A device according to claim 2, characterized in that the shut-off valve is a magnetic valve of the on-off type.
A device according to claim 1, characterized in that the circuit closing device is a valve capable of complete and intermediate closure to give variable throttling of the hydraulic medium flowing in the hydraulic circuit.