A LOAD CONTROL SYSTEM PREFERABLY FOR HYDRAULIC CRANES

Abstract: A load control system preferably for hydraulic cranes comprising at least one relevant signalling device connected to a control unit which, by loading of the crane beyond its nominal load rate, operates a dump valve for deactivation of such functions in the manually operable control-valve assembly, which may be actuated to cause an increase of the load on the crane. The special feature of this load control system is that it operates according to an educational principle of collective "punishment", comprising a total inactivation of the functions of the crane in a penalty period in case of attempts to exceed the nominal load rate of the crane, within a safety margin. By an increasing load on the crane, the control system calculates new penalty periods and load rates beyond the nominal load rate within a safety margin, and the control system will induce a constantly increasing penalty period in response to the extent and number of excesses. The educational principle consists in that the risk of the release of a penalty will cause the operator to adopt a better feeling of how much the crane can be loaded within its nominal load rate.

Description: A load control system preferably for hydraulic cranes. The present invention concerns a load control sys- tem, preferably for hydraulic boom cranes of the kind com¬ prising a boom pivotable relative to a base about one end in a vertical plane by a hydraulic cylinder and comprising at least one relevant signal emitter connected to a surveillance unit which in the case of loading the crane in excess of its no imal load deactivates the functions in the control valve block that may cause an increase of the load of the crane. The nominal load of cranes is determined with a certain safety margin depending on how great a moment may be absorbed by the turning point of the crane, i.e. the bearing supporting the crane, and which then is transmitted to the support of the bearing, e.g. the chassis of a lorry, and in that connection to possibly extended supporting legs without the crane tipping over. The known load control systems for use in hydrauli- cally driven cranes are of the kind mounted as accesories on e.g. a lorry or definite mobile cranes. The known load con¬ trol systems may comprise at least one or more pressure transducers provided on the hydraulic cylinders of the crane. The pressure transducers are connected to a control unit connected with controlable valves in connection with the con¬ trol valve block of the crane. A such load control system is known from EP 0 708 053 Al, where the control unit is ar¬ ranged in such a way that it by a registrated maximum allow¬ able pressure registered by a sensor placed on the main cyl- inder of the crane blocks for all further maneuvering of the crane by its control valve block. However, the blocking may subsequently be deactivated in a short period (3-5 sec.) by a manually controlled liberating element, so that it is possi¬ ble to get out of an otherwise "impossible lifting situa- tion". An "impossible lifting situation" may be defined in that any movement of the crane boom allowed by the control system with the intention of reducing the load will be impos¬ sible in practice because of the physical circumstances in the ongoing lift so that the only possibility of getting out of this lifting situation is a deactivation of the load con¬ trol system in a short pre-determined period, wherein a forced increase of the load is possible. The said load con- trol system also comprise acoustic and visual signal emitters coming into action at different load levels about the maximum allowable load level for the crane. Other load control systems may furthermore comprise position indicators on the boom sections to be pivoted in a vertical plane, besides the pressure transducers on the hy¬ draulic cylinders, and which are also connected to the con¬ trol unit. The signals from the position indicators are used in the control unit for indicating whether a movement of the pivotable parts of the boom in a desired direction will cause an increased load on the crane. The control of the load with position indicators works in the way that the control unit at a registered maximum allowable load of the crane (the nominal load) ensures that the operator of the crane cannot perform load increasing operations as the control unit at the activa- tion of the previously mentioned controlable valves is hin¬ dering such movements of the boom. However, the control unit allows all movements of the boom that by experience cause an immediate reduction of the moment load in proportion to the maximum allowable moment load of the crane, e.ge. a reduction of the distance between the load and the turning point of the crane in an operation wherein the length of the boom is re¬ duced. Still, experience shows that crane operators almost always and with succes try to circumvent the known load con- trol systems. An example of this is that an operation allowed by the load control system that usually cause a load reduc¬ tion is utilized for increasing the moment load of the crane beyond the allowed level. This will by example be possible by performing a reduction of the boom length in connection with an "inallowable" lift whereby the load is dragged across the ground over at the beginning and then is lifted off the ground. Such a lift will also cause a very disadvantageous and destructive loading of the support bearing of the crane. The load control systems described above are rela¬ tively costly since they often comprise up to several pres¬ sure transducers and one or more position indicators that by wired connections are connected to the control unit, which again are connected to the control valves in the control valve block. Looking at service procedures the large number of components and wire connections become expensive. Common to the known load control system is, how¬ ever, that they in their own individual way in a rather abrupt way deactivates the functions of the crane completely or partly at the exceeding of the maximum allowable load on the crane, though after a preceeding warning from acoustic or visual signal emitters also activated by the control system. However, the known load control systems easily permit the op- erator to exceed/override the deactivation of the maneuver¬ ability of the crane in connection with overload situations which inevitably leads to some indifference towards the load control system, since exceeding the the maximum allowable load of the crane do not have any consequence as long as op- erations take place with more or less virtuosity in the alarm range and with an intrepid use of the said deactivation pe¬ riod preceeding a total blocking of the control system. The deeper meaning with the alarm is to call the operatoris at¬ tention to the fact that operation takes place in a poten- tially dangerous area and that measures for a return to harm¬ less conditions immediately has to be taken, e.g. that the lorry is driven to a position closer to the burden to be lifted. The operator may then decide to try anyway from the consideration that in the said deactivating period he may come in time to discontinue the alarm creating act if the transgression is so great that it downright may give rise to an ultimative blocking signal. This practice results in appreciable problems that may be read from different accident statistics. The crane manufacturers may decide to place the alarm range in a safer distance from the ultimate border, but this will inevitably raise the cost of the crane in proportion to its capacity. An optimum relationship between price and capacity highly presupposes that the operators respect the alarm range ac¬ cording to its purpose and do not make experiments instead by pressing the crane extremely, causing danger for themselves, the machines and other persons in the close surroundings. The problem in question is thus not of technical nature, but rather educationally and morally dependent since the total solution to the problem already made possible is a question about operator discipline. It has to be acknowledged as a regrettable fact that time has not yet come for a rea- sonable expectation of such self-discipline as several other reasons make it attractive for the operators to press the crane to the limit exist, even in already extreme situations and in spite of the hazards belonging to them. By the present invention it has been recognized that it is possible by means of "technique" to support a more correct practice of operation for avoiding practically super¬ fluous overdimensioning or capacity limits. According to the invention we are speaking of a kind of psychological counter¬ action on the part of the crane against an operator trying to exceed the capacity of the crane into its hazard area. Such a counteraction may be established according to the invention by letting the crane equipment itself produce a capacity re¬ duction of a first or lower safety limit, that is to say by a not very technical but psychological and educationally effec- tive "penalty" to the operator in the form of a more or less total blocking of the crane functions during a certain period of time, e.g. if the operator does not react in an expectedly suitable way after the alarm has been activated the first time, namely by returning the system to a not alarmgiving state. By giving the operator such a time "penalty" with respect to the use of the crane two important aspects will appear, namely that the operator has to reckon with the im¬ possibility of desired capacity gain by using a slightly or heavily overloaded crane, for example because the time was better used in driving the lorry closer to the burden to be lifted, and that the same operator has to face the surround¬ ings ' observation that his heroic struggle with the crane to press it to the outermost simply result in stoppage while maybe an alarm signal audible for the whole congregation is sounded. Hereby the operator has to accept a loss of both crane capacity and personal prestige, and both will act in the direction that the operator will keep to the instructions of the crane manufacturer concerning a temporary operative overloading of the crane. Still, there will exist a few operators who want to test the limits of such a system with the anticipation of ac- cepting a short-term "penalty" at a moderate overload, if it is possible then to continue the work with a greater over¬ load. According to the invention this may be counteracted by producing increasing blocking periods for all crane functions for a correspondingly increasing degree of such overload, whereby the operator soon will experience that any attempt to a further increase of the crane capacity will result in that the same capacity is lessened appreciably because of increas¬ ing periods of total blockage of the crane functions. In this way the operator has nothing to gain by his attempts to cir- cumvent the security system, while he at the same time by such ineffective circumventing attempts becomes exposed to scorn from the surroundings eo ipso that the crane is stand¬ ing still after the sounding of an alarm signal. Several signal levels may then be introduced by which there is produced increasing "penalty periods", so that the operator will experience a continuously increasing pas¬ sivation of the crane the more he tries to overload it. At the same time, however, consideration is given to the opera¬ tor so that the final total blocking of the crane only takes place in quite extreme cases, where the operator in the course of time has had to accept a series of increasing pen¬ alty periods and therefore has worked in a quite inefficient way. By the invention it has further been realized that for the surveillance of the moment load of the crane it is hereby in reality only necessary to watch the pressure on the main cylinder of the crane supporting the main boom of the crane, since this pressure is an objective measure for the moment acting on support of the crane when using it. Thereby it is posssible to indicate a new and more simply arranged load control system that is cheaper in acquisition as well as in service as compared to the known load control systems, and which in addition gives the operator of the crane more de¬ grees of freedom during special lifting situations and a bet¬ ter feeling of the maximum load of the crane. By the invention there is thus indicated a new load control system comprising a pressure transducer situated on the main lifting cylinder of the crane and connected to a control unit with the output connected to a by-pass valve, also called "dump valve", controlling the oil supply to the maneuvering valve block of the crane, which control unit is arranged in such a way that the transgression of the nominal load capability of the crane in question will result in acti¬ vation of the said "dump valve", so that all the functions of the crane are made inactive in a period of time calculated by the control unit, a so-called penalty period, whereafter the dump valve is deactivated. Evidently, the load control system according to the invention does not give any immediate possibility for the crane operator to rectify a transgression of the nominal load of the crane since it is not possible to relieve the load as long as the functions of the control valve block of the crane are deactivated during the penalty period. After the expiration of the penalty period that may be rather short at the first transgression the control unit deactivates the dump valve and the crane becomes aneuverable again. If the load on the crane is reduced to a level below the nominal load nothing further will happen, i.e. the work may continue freely. With the purpose of giving the operator a possibil¬ ity of remedying any arised impossible load situations the control unit is arranged in such a way that it allow a fur- ther load on the crane to a calculated level (within the safety margin) in excess of the nominal load after the expi¬ ration of the penalty period. If this calculated level for the load is exceeded the dump valve is activated again by the control unit, this time in an perceptibly longer period of time than the first activation. After the expiration of this so-called penalty period the dump valve is deactivated again. If the load on the crane is reduced the control unit will continuously calculate a new, lower value for the maximum allowable load and penalty period in case of a trans¬ gression concurrently with the relieving of load until the nominal load is reached. However, if the operator of the crane tries to increase the load during the relieving the control unit will immediately release a new penalty period. The load control according to the invention may be programmed with several limits of load levels between the nominal load for the crane and an absolute acceptable maximum within the safety margin. Transgression of the individual load levels/limits will release an increasing penalty period concurrently with the magnitude and number of transgressions. In order to give the operator the possibility of following how much the crane is loaded in relation to the nominal load the cabinet of the control unit may be provided with a percent graduated scale with light diodes that are switched on and off concurrently with a load increase and de¬ crease, respectively. In addition, for warning the operator of the crane as well as persons staying within the hazard area of the crane about the nominal load being reached or exceeded, the control unit may also be provided with an acoustic alarm emitting a periodic alarm signal at an existing load between 95% and 100%. During an imposed penalty period at the trans¬ gression of the nominal load (100%) a continuous signal is given. Therefore, the operator has to accept that his over¬ loading of the crane is made widely known. Furthermore, the control unit may be provided with a logging unit that is registering data in a ring buffer mem¬ ory for all load transgressing actions with the crane, so that data concerning the latest load transgressing events af¬ terwards may be read from the ring buffer memory on e.g. a hand terminal, and that is registering data concerning the largest registered overloads in another data memory, the lat¬ ter also being readable on a hand terminal. The load control system described in the above be- ing more simple in arrangement as compared to the known load control systems contains thus some increased degrees of free¬ dom and important educational principles. The increased degrees of freedom consist in that the operator at the transgression of the nominal load up to an absolute allowable maximum limit within the safety margin and after the expiration of penalty period has the possibil¬ ity of getting out of an "impossible" lifting situation by utilizing all of the maneuvering functions of the crane, even with a short-term, smaller load increase allowed after the expiration of a penalty period. The educational element consist in that the opera¬ tor of the crane will be inclined to learn a better percep¬ tion of the load situations implying transgression of the nominal load of the crane, because the penalty period con- nected with a transgression of the nominal load will slow down the work procedures in connection with the use of the crane and thereby for the operator as well as for the person or persons cooperating in the work within the hazard area of the crane. The invention is explained in more detail in the following with reference to the drawing, where: Fig. 1 is a perspective view of a hydraulic crane provided with a load control system according to the inven¬ tion, and Fig. 2 is a functional example of the load control system. In Fig. 1 there is shown a hydraulic crane 2 in¬ tended for mounting on a vehicle, for example a lorry. The crane comprise a basic frame 4 for mounting on the chassis of the lorry. The basic frame has also laterally extendable girders 5 at the ends of which vertically extendable legs 6 are provided. On the basic frame 4 the column 10 of the crane is supported by the bearing 8. In addition, the bearing car- ries the support for the main cylinder 12 of the crane that is connected to the main arm 14 of the crane. In continuation of the main arm 14 the bending arm 16 of the crane is pro¬ vided and in this a number of telescopic arms 18. On the main cylinder 12 of the crane there is pro¬ vided a pressure transducer 20 which by a wire connection 21 transmits an oil pressure signal to a control unit 22. As a consequence of the very quickly varying oil pressures appear¬ ing in the main cylinder 12 of the crane and which at the transmission to the control unit will cause "signal confu¬ sion", the control unit 22 is provided with a not shown sig¬ nal receiving block performing filtering, stabilization and compensation of the received oil pressure signal from the pressure transducer 20 before the signal is transmitted to the control unit for further processing. The control unit is connected to a by-pass valve 24, in the following called dump valve that is placed on the oil supply pipe for the maneuvering valve block 25 of the crane, through a wire connection 23, so that the dump valve in its active position will cause that the oil in the supply pipe is returned directly to the hydraulic oil tank through a not shown by-pass pipe and around the maneuvering valve block 25 whereby maneuvering of the crane is not possible. In the inactive position of the dump valve the hydraulic oil is sup- plied to the valve of the maneuvering valve block whereby it is possible to maneuver the crane. The control unit 22 comprise a cabinet the front of which is furnished with a number of bulbs 28 placed in con¬ junction with a percentage scale, one bulb 30 placed by the word "run", an acoustic signal emitter 32, and a stop switch 34. The shown scaling indicate the existing load on the crane proportional to the nominal load of the crane in the range between 80 and 100%. At a load below 80% of the nominal load only one bulb 30 opposite the word "run" is lit. At a load on the crane within the range 80-100% the bulbs are lit concurrently with the increased load so that the operator has a possiblility of following the development of the existing load of the crane. At a load over 95% of the nominal load the control unit activates the acoustic signal emitter 32, which emits a periodic signal. At a load on the crane corresponding to 100% or more the control unit cause a continuous signal to be sounded from the emitter 32 during the said penalty pe¬ riod. The mode of operation of the load control system is explained in the following with basis in the functional exam¬ ple for the load control system shown in Fig. 2, which shows a coordinate system with abscissa indicating time and ordi- nate indicating the load of the crane proportional to the nominal load of the crane. At a load in the range between 0 and 85% of the nominal load the load control system do not react. At a load between 85 and 95% the control unit per¬ form a gradual activation of the bulbs 28 opposite the per¬ centage indications on the cabinet of the control unit. At the trangression of 95% of the nominal load the control unit activate the acoustic signal emitter 32 so that it gives a periodic signal which at 100% nominal load changes to a continuous signal in connection with the running of a penalty period. At the expiration of a penalty period the signal emitter changes to the periodic signal indicating that the crane functions are active again, but the existing load of the crane is more than 95% of the nominal load. ' If the crane operator attempts to increase the load above 100% the control unit 22 activates the dump valve 24 in the short period of time calculated by the control unit, a so-called penalty period A, cf Fig. 2, wherein the functions of the crane are paralyzed. After that the load on the crane should be reduced below nominal load capability. The load on the crane may be increased at the expiration of the penalty period A in order to give the operator of the crane the pos¬ sibility to get out of an adverse lifting situation which is possible up to 102% of nominal load in the given example. If the load is increased in excess of the level determined by the control unit, the control unit activates the dump valve in at penalty period B that is longer than the first period. During this penalty period the control unit calculates a new stop level, e.g. 102.5% of nominal load, and a new and longer penalty period C coming into use if the crane load is in¬ creased more than the new stop level . In the example shown in Fig. 2 the principle in the development of the length of the penalty period is illus¬ trated as a function of the current load on the crane. The control unit calculates the development in the length of the penalty period in relation to the current load of the crane and the current number of attempts of additional overloading. The control unit 22 further comprises an "override" button 36 which by the operator may be activated in case of an "impossible" lifting situation that has to be rectified immediately. By activating the "override" button it will be possible within the subsequent 5 seconds to perform a smaller load increasing operation with the crane within an interval determined by the load control system, the exceeding of which causes a penalty period of 30 seconds. The "override func¬ tion" is usable up to the absolutely maximum allowable load level of the crane, whereafter the load control system to¬ tally deactivates all of the functions of the crane. In that situation there may possibly be a case of a final adjusting possibility, which however require the breaking of a seal or calling of a security inspector. It shall be emphasized that by the invention it has been possible to reduce all of the sensor equipment on the crane to the pressure transducer 20 only, but naturally it cannot be precluded that more sensors can be used.

Claims: PATENT CLAIMS: 1. A load control system for hydraulic cranes (2) and of the kind comprising a relevant signal emitter (20) con-nected to a surveillance unit which by loading the crane (2) in excess of its nominal load produces limitations on the functional capability of the crane, charact e ri zed in that a pressure transducer (20) placed on the main lifting cylinder (12) of the crane is connected to a control unit (22) arranged in such a way that it in case of a transgres- sion of the nominal load established for the crane (2) in question will produce a marked or total functional limitation during a period of time determined by the control unit (22) , a so-called penalty period, after which the crane may be fur¬ ther loaded up to an acceptable absolute maximum, the trans- gression of which will cause the control unit to make the crane inoperable. 2. A load control system according to claim 1, charact eri zed in that the control unit (22) at the loading of the crane (2) beyond its nominal load up to an ac- ceptable absolute maximum within a safety margin after the expiration of the penalty period allows transgressions of the load causing the last penalty period in intervals in such a way that the trans-gression of the different levels will ac¬ tivate the functional limitation in a preferably perceptibly increasing penalty period concurrently with the magnitude and the number of the transgressions. 3. A load control system according to claim 1 or 2 , charact eri zed in that the control unit (22) deter¬ mines the intervals so that these become continuously smaller concurrently with the number of the load transgressions. 4. A load control system according to any of the claims 1-3, character i zed in that concurrently with the reducing of load on the crane down to the nominal load the control unit (22) con-tinuously calculates a new lower value for the maximum al-lowable load together with a penalty period in case of a transgression thereof, which penalty pe¬ riod is released immediately at an attempt to increase the load. 5. A load control system according to any of the claims 1-4, characteri zed i n that it comprises an "override" function cancelling an imposed penalty period through e.g. 4-10 seconds, within which period a load in- crease is allowed within a limit determined by the load con¬ trol system, the transgression of which limit causes an in¬ stant passivation of the crane during a separately determined period of e.g. 30 seconds. 6. A load control system according to claim 2 and 5, charact eri zed in that the "override" function is active in each of the load increasing intervals up to the ab¬ solute maximum allowable load for the crane. 7. A load control system according to any of the claims 1-4, characteri zed in that the cabinet of the control unit is provided with a percent graduated scale with bulbs (28) that are switched on and off concurrently with a load increase or decrease, respectively, in an upper load range. 8. A load control system according to any of the claims 1-4, characteri zed in that the control unit in addition comprises an acoustic alarm (32) which during an im¬ posed penalty period at the transgression of the nominal load emit a "penalty signal" audible by the surroundings, and which preferably at an exis-ting load on the crane between 95% and 100% of the nominal load emits an e.g. intermittent alarm signal . 9. A load control system according to any of the claims 1-6, characteri zed in that the penalty period as a margin as well as a stop level at the loading of the crane in excess of the nominal load is returned to the ini¬ tial value when the crane load is reduced to a level below the nominal load. 10. A load control system according to any preced¬ ing claim, charac teri zed in that the relevant sensor equipment of the crane only consist of a pressure transducer for sensing the pres-sure in the main cylinder of the crane, and that the control unit preferably is arranged to activate the dump valve of the crane.

Inventors: BRAENDGAARD GUDMUND, SOERENSEN JAN, BRAENDGAARD, GUDMUND, SOERENSEN, JAN
Applicants: BRAENDGAARD GUDMUND, HOEJBJERG MASKINFABRIK A S, SOERENSEN JAN, BRAENDGAARD, GUDMUND, HOEJBJERG MASKINFABRIK A/S, SOERENSEN, JAN
Application year: 1997
Main classification: B66C23
IPC codes: B66C23