PNEUMATIC LOAD BALANCING SYSTEM AND METHOD

A pneumatic load balancing system comprises a pressure sensor for determining the pressure in a chamber of the actuating cylinder, and a controller for controlling the air pressure in the chamber of the actuating cylinder via at least one air supply valve. The controller is configured to during a load balancing sequence continuously or periodically obtain a current air pressure in the chamber from the pressure sensor when supplying air to the chamber via said at least one air supply and to determine a balancing air pressure in the chamber when, if air fed to the chamber, the air pressure stops increasing or when the gradient of the pressure increase is below a pre-determined threshold value; or if the air pressure is let out from the chamber, the air pressure starts to decrease. The balancing air pressure thus determined is then used as the balancing air pressure for the actuating cylinder of the pneumatic load balancing system. Hereby an automatic setting of the air pressure required for load balancing can be achieved. The user does then not need to manually feed the required air-pressure and the system will use the correct air-pressure and mistakes in setting of the air pressure can be avoided.

TECHNICAL FIELD

The invention relates to a pneumatic system for balancing a weight and related devices and methods.

BACKGROUND

Lifting operations in industries often requires balancing of the lifted load. Conventional balancing devices also referred to as hoists, are generally characterized by a pressure fluid operated motor including a piston disposed in an expansible chamber and suitably connected to a rotatable cable drum. A load attached to the hoist cable is raised, lowered, or held in balance by controlling the pressure of the fluid admitted to the hoist chamber and acting on the piston. Such hoists are operable to provide for manual raising and lowering of a balanced load with minimum effort by regulating fluid pressure acting on the piston to a value sufficient to offset the weight of the load. The hoist operator is thereby able to manipulate the load, including raising and lowering it, with a force which is a mere fraction of the actual weight of the load.

Existing systems for providing a balancing of a load/weight includes U.S. Pat. No. 3,758,079 which describes a control system for a fluid operated balancing hoist which is operable to provide for raising, lowering, and automatically balancing a load of any weight up to the capacity of the hoist proper. The system has a control circuit having a pressure regulating valve which is operable to sense the pressure in the hoist motor chamber required to balance the load and automatically adjust itself to maintain the balance pressure value loads of varying weights. The control circuit includes a pressure regulator which automatically senses the balance pressure and is operable to be positively locked at the balance pressure setting.

Another example is U.S. Pat. No. 4,500,074 which also describes a system for balancing a load.

There is a constant need to improve load balancing systems in respect of accuracy, speed, and user-friendliness. Hence, there exists a need for an improved load balancing system and a method for controlling such a load balancing system.

SUMMARY

It is an object of the present invention to provide an improved control and or functioning of a pneumatic load balancing system.

This object is obtained by the devices and methods as set out in the appended claims.

In accordance with the invention, a pneumatic load balancing system comprising an actuating cylinder for balancing a load is provided. The pneumatic load balancing system comprises a pressure sensor for determining the pressure in a chamber of the actuating cylinder, and a controller for controlling the air pressure in the chamber of the actuating cylinder via at least one air supply valve. The controller is configured to during a load balancing sequence continuously or periodically obtain a current air pressure in the chamber from the pressure sensor when supplying air to the chamber via said at least one air supply and to determine a balancing air pressure in the chamber when, if air fed to the chamber, the air pressure stops increasing or when the gradient of the pressure increase is below a pre-determined threshold value; or if the air pressure is let out from the chamber, the air pressure starts to decrease (or at a rate above some threshold value). The balancing air pressure thus determined is then used as the balancing air pressure for the actuating cylinder of the pneumatic load balancing system. Hereby an automatic setting of the air pressure required for load balancing can be achieved. The user does then not need to manually feed the required air-pressure and the system will use the correct air-pressure and mistakes in setting of the air pressure can be avoided. The term supply can refer to both feeding air or letting air out from the chamber.

In accordance with one embodiment, the pressure in the chamber is first set to an initial value before starting to continuously or periodically obtain a current air pressure in the chamber. In particular the initial value can be set to the ambient pressure when air is fed to the chamber or a system pressure (maximum system pressure) when air is let out from the chamber. Hereby a robust automatic setting sequence for the balancing air pressure is obtained.

In accordance with one embodiment, the pneumatic load balancing system is configured to supply air at a constant rate when supplying air to the chamber. Hereby the determination of when the increase rate stops/reduces is easy to find and can be determined more precisely.

In accordance with one embodiment, a position sensor is provided to determine the position of the cylinder, and the controller is configured to use the position sensor output signal to determine the balancing pressure. By also using the position of the cylinder or more accurate determination of the balancing pressure can be obtained. Also, malfunction of the automatic balancing pressure can be determined.

In accordance with one embodiment, the pneumatic load balancing system is configured to initiate the load balancing sequence based in an initiation signal. Hereby the system can be set to trigger the automatic sequence for determining the balancing pressure at any suitable time. In particular the initiation signal can be a user command signal.

In accordance with another aspect of the invention, pneumatic load balancing system comprising an actuating cylinder for balancing a load connected to the actuating cylinder is provided. The pneumatic load balancing system comprises a pressure sensor for determining the pressure in a chamber of the actuating cylinder, and a controller for controlling the air pressure in the chamber of the actuating cylinder via at least one air supply valve. The controller is configured to continuously or periodically obtain a current air pressure in the chamber from the pressure sensor, and to determine a decrease in pressure to a pressure below balancing pressure. Based on the determined pressure decrease the controller can-determine that an accident event has occurred. Hereby it is enabled to take appropriate action required when an accident has occurred such as issuing a warning signal.

In accordance with one embodiment, activity of the at least one air supply valve is continuously or periodically monitored and the controller is configured to determine that an accident event has occurred based on the valve activity. Hereby a more robust determination of an accident event is enabled since the valve activity can be used to provide additional information that is helpful in determining that an accident event is present.

In accordance with one embodiment, the pneumatic load balancing system comprises a position sensor to determine the position of the cylinder, and the controller is configured to use the position sensor output signal to determine that an accident event has occurred. This also enables a more robust determination of an accident event is enabled since the position can be used to provide additional information that is helpful in determining that an accident event is present.

In accordance with one embodiment, the accident event can be determined to be a load being detached from the actuating cylinder.

In accordance with one embodiment, the controller is configured to apply a predetermined setting for at least one air supply valve when an accident event has been determined. Hereby negative results of the accident event can be prevented or at least reduced. For example, the predetermined setting can comprise to close a valve used for increasing the balancing pressure. Hereby the pressure in the actuating cylinder will not increase and the actuating cylinder can be prevented from providing an increased force that could cause damage to the load balancing system of the surroundings thereof. In another embodiment the predetermined setting comprises to open a valve used for decreasing the balancing pressure.

In accordance with one embodiment, the controller is configured to determine a decrease in pressure to a pressure below balancing pressure when the pressure is below a preset threshold value. Hereby a robust limit is provided.

In accordance with one embodiment, the controller is configured to apply a predetermined setting based on a position obtained from a position sensor. The controller is configured to store the position of the actuating cylinder, when the accident event is detected, and then control the position of the actuating cylinder in to the stored position. Hereby, when an accident event is detected the pneumatic load balancing system will strive to keep the actuating cylinder at a steady position when an accident event is detected. Hereby the risk for damage can be reduced in that the actuating cylinder can be prevented from moving. The control can be made in a closed loop.

In accordance with one embodiment, when a position sensor for determining the position of the actuating cylinder is provided, wherein controller is configured to continuously or periodically obtain the position of the actuating cylinder using output from the position sensor and to determine if the actuating cylinder is outside an allowed range. When it is determined that the actuating cylinder is outside the allowed range the controller is configured to drive the actuating cylinder towards the allowed range. Hereby additional safety can be added to the pneumatic load balancing system that prevents the pneumatic load balancing system from operating the actuating cylinder outside a safe range.

The invention also extends to methods for operating a pneumatic load balancing system in accordance with the above.

DETAILED DESCRIPTION

InFIG.1, a pneumatically driven cylinder14is controlled by a load balancing system1is illustrated. The pneumatic cylinder14can be configured to operate as an actuator (an actuating cylinder) to lift a weight22of some kind. It is also envisaged that some other load than a weight22is connected to the load balancing system1. The load balancing system1is fed with air from an air-supply2and the cylinder14is controlled by the load balancing system1as will now be described in conjunction with the exemplary load balancing system1ofFIG.1.

The pneumatic cylinder14comprises a first chamber15with pressure A and a second chamber16with pressure B. The two chambers15and16are located on different sides of the cylinder14(typically on different sides of the cylinder head of the cylinder14). In the exemplary embodiment ofFIG.1, the chamber16is pressurized (and depressurized) by an arrangement comprising a set of valves9,10. The valves can for example be valves similar to the valve described in U.S. Pat. No. 10,641,397 B2, but the valves can be any kind of valve suitable to be used in a pneumatic load balancing system. Also, while the valves9,10are shown as two valves in the exemplary embodiment ofFIG.1, any number of valves are envisaged. For example, the chamber15can also be pressurized (and depressurized) by a supplementary valve arrangement.

The cylinder actuation is controlled by pressurizing (and depressurizing) the cylinder chamber16. In other words, air is supplied to the chamber16by feeding air to the chamber16or letting air out from the chamber16. Hereby a balancing force is achieved that can balance the weight of the weight22connected to the actuating cylinder14.

In the load balancing system ofFIG.1, the force applied by the cylinder14is controlled by a controller5. The controller controls the airflow via the valves9,10. The valves9,10can be supplied with air via the air supply2. The air supply2can be referred to as the system air pressure and sets an upper limit for the maximum air pressure that can be reached in the chamber16. By controlling the pressure in the cylinder chamber16, the force applied to the weight22can be controlled to balance the weight of the weight22. The control can typically be performed by increasing the pressure in the chamber16by opening the valve9to let air into the chamber16. Similarly, the valve10can be used to decrease the pressure in the chamber16by letting air out from the chamber16.

The load balancing system1can in accordance with some embodiments be provided with a pressure sensor21that outputs the current pressure in the cylinder. In particular the pressure in chamber16that is pressurized/depressurized can be sensed by the pressure sensor21. Also, a position sensor13can be provided to output a current position of the cylinder14. The output from the pressure sensor21and/or the output from the position sensor13can be supplied to the controller5.

The controller5can also receive a control input signal8from some external source, such as an operator operating the load balancing system. The control input signal can for example be a signal to lift or lower the weight22.

For example, assuming that the weight22is balanced by the pressure in chamber16, if a lift signal is received as the control input signal8, the controller can activate valve9to increase the pressure in the chamber16. Hereby the force applied on the weight22by the cylinder14will increase and the weight22will start moving upwards in the system ofFIG.1. If on the other hand a lowering signal is received as the control input signal8, the controller can activate valve10to decrease the pressure in the chamber16. Hereby the force applied on the weight22by the cylinder14will decrease and the weight22will start moving downwards in the system ofFIG.1.

The controller5can use the position sensor13to control the position of actuating cylinder14in a closed loop. A desired position of actuating cylinder14can then be stored by the controller5. If a current position from position sensor13is determined to be outside the stored desired value (or outside a range around the stored desired value), the controller5can activate valve9or10to adjust the pressure B. Using this principle, the controller5can control the actuating cylinder14to hold a stored position.

During balancing of the weight22, pressure B of the cylinder14(i.e., the pressure in chamber16) is adjusted to correspond to the weight of weight22. Pressure B will then apply a force that is equal to the gravitational force on weight22. This pressure level for pressure B can be referred to as a balancing pressure. In practice, this can be obtained by storing the required pressure to balance weight22in controller5. Controller5can compare the required pressure to balance weight22with the pressure B obtained from pressure sensor21. If pressure B is lower than the required pressure to balance weight22, valve9is activated to increase pressure. If pressure B is lower than the required pressure to balance weight22, valve10is activated to reduce pressure.

If, still assuming that the weight22is balanced by the pressure B in chamber16, an operator applies a force in the upward direction on the weight22, the pressure in chamber16will decrease. The pressure decrease in chamber16will be detected by the pressure sensor21. The controller will activate valve9that compensates the pressure decrease. The weight22will move upwards. This function enables the weight22to be manipulated upward and downward by an operator, using only a fraction of the force needed to lift the weight22.

InFIG.2a simple example of a lift operation is illustrated. First, at position a) ofFIG.2, a lifting device controlled by the pneumatic cylinder14is moved to the weight22. The lifting device is here represented by a hook connected to the actuating cylinder14. The weight22to be moved is then connected to the hook that is connected to the actuating cylinder14as is illustrated at b) ofFIG.2. A balancing pressure is then fed to the pneumatic actuating cylinder14so that the weight22is balanced. The weight22can then be moved up/down by an operator in an easy manner as is illustrated at c) ofFIG.2.

However, for the operation illustrated inFIG.2to work properly, the balancing force corresponding to the weight of the weight22must by applied by the actuating cylinder14. Thus, a correct balancing force must be entered into the balancing system controlling the pneumatic cylinder14. This force can for example be entered as a weight by the operator and the system then converts the weight into a pressure in the cylinder that gives the correct balancing force.

It would however be beneficial if the balancing system could automatically apply the correct balancing force.

In accordance with one embodiment, such an automatic setting of the balancing force can be obtained by monitoring the pressure in the cylinder chamber.

The sequence in the flow chart ofFIG.4describes how the balancing force can be obtained by ether monitoring the pressure B or the position of actuator14. Typically, before the weight is connected to the actuator, the pressure in chamber16is set to an in initial value. The initial value is typically very low and can for example be zero or at least lower than a pressure corresponding to a lowest weight to be balanced by the actuating cylinder14. In step401, the pressure B is set at this initial value and the weight22is connected to the actuator14.

The pressure B is then set to gradually increase in a step403. The increase can in accordance with some embodiments be performed at a set rate. The gradual increase of pressure B at a set rate can be achieved by opening valve9at a constant flow rate. This step can be initiated by an operator using external signal8to indicate that it is desired to lift the weight22.

The pressure is then continuously, or at least periodically, monitored by the load balancing system1via the output from the pressure sensor21in a step405while pressure is still set to gradually increase at a set rate.

In a typical scenario, operating pressure, i.e., the air supply2can be set to 0.5 MPa. The weight of weight22can correspond to a pressure of 0.2 MPa in chamber16. The gradual increase rate of pressure B can be set to 0 to 0.5 MPa/s. The increase rate of pressure B can be obtained by setting the flow of valve9at a constant flow rate. As long as pressure B is lower than the pressure required to lift weight22, the flow from valve9will give a gradual increase of pressure B as can be seen in the diagram ofFIG.3.

When the pressure in chamber16reaches the pressure required to lift weight22, the weight22will move upward. As a result, chamber16will expand in volume and the pressure gradient will significantly change.FIG.3illustrates how the pressure gradient significantly changes when pressure in chamber16reaches the pressure required to lift weight. Thus, when the air supplied to the chamber16via the valve shows that the pressure increase stops or at least significantly reduces in increase rate.

In accordance with another embodiment data from the position sensor13is used as an alternative or as a supplement to data from the pressure sensor21to automatically determine the balancing pressure required to obtain a balancing force in the actuating cylinder14. In such an embodiment, when the pressure in chamber16reaches the pressure required to lift weight22, and the weight22moves upward, the movement can be detected by position sensor13. When a lift action is detected, the pressure is recorded and used as the balancing pressure.

In step405, the controller5detects a (significant) change in pressure increase rate from pressure sensor21and or a movement from the output of position sensor13. When such a pressure increase rate change and or a position change is detected, the pressure B is recorded in a step407and used as the pressure to balance the weight22. The operator can then start using the load balancing system1and start moving the weight as in pre-existing systems with the balancing pressure set accordingly. Hereby an automatic balancing pressure setting can be obtained.

In an alternative embodiment, the sequence to find the balancing pressure is started at a high pressure and air is then let out from to chamber16. In such a scenario the weight22will be lifted up by the high pressure and when the balancing pressure is reached as air is let out from the chamber16, the pressure in the chamber will start to drop at a significant rate above some threshold value. This sequence can also be supplemented by recording a position from the position sensor13in accordance with the above. The initial pressure used when starting from a high pressure can for example be the air supply pressure2(system pressure) or some other high pressure above some pre-set value that ensures that the pressure in the chamber is above the balancing pressure when the sequence to find the balancing pressure starts.

Once, the weight22has been balanced and the operator has started to move the weight, there is a risk that an accident can occur if the weight22is accidentally dropped and the actuating cylinder is not stopped. In order to handle such an emergency situation, the load balancing system1as described herein can in accordance with some embodiments be adapted to continually or periodically record the pressure in the cylinder14and or the position of the cylinder14using data from the pressure sensor21and or position sensor13, respectively.

An accident can for example occur if the weight22is dropped. The weight will then no longer pull against the pressure B in chamber16. The arm of actuator14will move upward and chamber16will expand. This will give a reduction in pressure. During balancing, the reduction in pressure will be counter acted. This will give further fluid flow into chamber16. The arm of actuator14will move upward and can then cause an accident. This is one example of an accident event that can occur when using the balancing system1.

As set out above, the controller5can be configured to record the pressure B of chamber16, Typically, pressure B will be adjusted to balancing pressure by controller5. An operator can apply a force on weight22, this will result in a deviation from balancing pressure for pressure B. The deviation counter acted by controller5. This will result in weight22moving in the direction of the force applied by the operator. The force applied by the operator is much smaller than force required to lift weight22. Therefore, the deviation of pressure B from the balancing pressure is much smaller than the balancing pressure.FIG.5illustrates how the setting of the valves9and10can be used to control the balancing pressure B. Thus, opening of the valve9will act to increase the pressure B and opening of the valve10will act to decrease the pressure B.

FIG.6illustrates pressure B as a function of time if weight22is balanced. In the example, weight22is successfully balanced between time0and T1. At time T1, the weight22is dropped. x axis shows time, y axis shows pressure B. The line illustrates the balancing pressure of weight22. Between time0and T1, when weight22is balanced by pressure B.

Between time0and time T1, pressure B is controlled to balancing pressure. Pressure B is monitored by controller5using signal from pressure sensor21. If pressure B is lower than balancing pressure, valve9is activated by controller5giving an increase in pressure B. And if pressure B is higher than balancing pressure, valve10is activated by controller5giving a decrease in pressure B.

Typically, if no external force is applied to weight22pressure B will be stable. In this case, the difference between pressure B and balancing pressure will be very small. In this case, the recorded values in controller5will show, no valves active, no change in pressure B and no change in position of actuator14.

An operator can apply a force in upward direction on weight22, as a result pressure B will decrease. The decrease in pressure B will be detected by controller5. Controller5will then activate valve9, as a result pressure B will increase. The position of actuator14can be monitored by position sensor13, then a position change in upward direction will be detected. In this case, the recorded values in controller5will show, valve9is active, a positive pressure gradient of pressure B and a position change of actuator14in upward direction.

An operator can apply a force in downward direction on weight22, as a result pressure B will increase. The increase in pressure B will be detected by controller5. Controller5will then activate valve10, as a result pressure B will decrease. The position of actuator14can be monitored by position sensor13, then a position change in downward direction will be detected. In this case, the recorded values in controller5will show, valve10is active, a negative pressure gradient of pressure B and a position change of actuator14in downward direction.

At time T1, when weight22is dropped. The weight of weight22will no longer pull actuator14in downward direction giving a rapid movement in upward direction of actuator14. This will result in a decrease in pressure B. As a result, valve9will be activated, since weight22is no longer compressing chamber16, pressure will not increase. In this case, the recorded values of controller5will show, decrease in pressure B considerably higher than typical conditions, valve9is active, negative pressure gradient of pressure B and position change of actuator14in upward direction.

To detect if the weight is dropped, a detection limit can be set as illustrated inFIG.6. Actuator (cylinder14) can be chosen so that the balancing pressure of weight22is set to a balancing pressure such as 0.2 MPa. Typically, during balancing the deviation of pressure B from balancing pressure is smaller than 0.0005 MPa. A detection limit is set that is higher than a normal deviation from the balancing pressure. In this example the detection limit can be set to 0.005 MPa.

In accordance with some embodiments, the controller5can detect negative pressure gradient of pressure B coupled with valve9active to detect if weight22is dropped. Hereby a more robust detection mechanism can be obtained.

In accordance with some embodiments, the controller5can detect pressure B lower than balancing pressure coupled with position change in upward detection. Hereby a more robust detection mechanism can be obtained.

Once the controller5has detected an accident event, for example due to that the actuator has dropped the weight22, the information can be used to limit the movement of the system to prevent additional hazardous events. For example, the load balancing system can then be controlled to prevent an actuator piston to move. This can be done by deactivating (closing) valve9and or valve10. This prevents movement of cylinder, as air is closed off in chamber16. In accordance with another embodiment, the controller5can prevent movement of the actuator piston by deactivating valve9and activating valve10. This will decrease pressure B, preventing upward movement of actuator14. Thus, by controlling the valves9,10to a pre-determined setting in response to a to a detected accident event, accidents can be avoided or limited.

InFIG.7a flowchart illustrating steps performed when detecting an accident event is shown. First in a step701the system continuously or periodically obtains a current air pressure in the chamber from a pressure sensor. Next in a step703a decrease in pressure to a pressure below balancing pressure is determined. When a decrease in pressure to a pressure below balancing pressure is determined in step703. For example, if the pressure is below a predetermined threshold value a decrease in pressure can be determined. Hereby noise and normal operation of the system will not trigger a determination that the pressure is decreased. The system determines that an accident event has occurred based on the determined decrease in pressure in a step705. In addition to base the decision that an accident event has occurred on a decrease in pressure, the system can in accordance with some embodiments also base the decision that an accident event has occurred on valve activity and or on a position signal. More detailed examples illustrating how such signals can be used are illustrated and described in conjunction withFIGS.8-11below. In response to determining an accident event the controller can be configured to apply a predetermined setting for at least one air supply valve in a step707.

An additional source of accident is movement of weight22upward or downward outside of a safe range. An example is an obstacle in the way of weight22. Movement of weight22into an obstacle can give damage to obstacle, to the weight22or cause other accidents. To prevent this, the controller5can be configured with a range where weight22is allowed to move. In this case, controller5is configured to monitor the position of actuator using position sensor13.

If position of actuator cylinder14is moved above (outside) allowed range, the controller5detects this using position sensor13. The cylinder14is then forced back to be within the allowed range. For example, the valve9is closed and valve10is opened, this gives a reduction of pressure in chamber16. As a result, weight22will move downward back to allowed range if the weight is above an allowed maximum height. In another scenario, If the position of actuator cylinder14is moved below allowed range, the controller5detects this using position sensor13. The valve9can then be opened and the valve10can be closed, this gives an increase of pressure in chamber16. As a result, weight22will move upward back to allowed range.

In accordance with some embodiments, the controller5can be configured to detect a negative pressure gradient of pressure B together with other input signals such as valve activity and or position signals to detect an accident event, such as if the weight22is dropped.FIGS.8,9,10and11illustrate different examples of operation of balancing actuator with weight22attached to actuator14.

FIG.8illustrates an example when weight22is balanced by the pressure B in chamber16. No external force is applied to weight22. As a result, pressure will be at a constant level, valve9and10are deactivated (closed) and position sensor13measures a constant position.

FIG.9illustrates an example when a force in upward direction is applied to weight22by an operator at time0. Initially, pressure B will decrease as a result of the external force expanding chamber16. At time T1, the decrease in pressure B is detected by controller5and valve9is activated to counteract the decrease in pressure B. Between time T1and T2, the controller detects an increase in pressure B as a result of activation of valve9. If position sensor13is connected to controller5, a movement in upward direction is also detected.

FIG.10illustrates when a force in downward direction is applied to weight22by an operator at time0. Here, pressure B will increase as a result of chamber16being compressed by external force. At time T1, the increase in pressure B is detected by controller5and valve10is activated to counteract the increase in picture. Between time T1and T2, the controller detects a decrease in pressure B as a result of activation of valve10. If position sensor13is connected to controller5, a movement in downward direction is also detected.

FIG.11illustrates an example when load22is dropped at time0. As a result of this, described previously, the pressure B will decrease. The decrease is detected by controller5at time T1and valve9is activated to counteract the decrease of pressure B. Since weight22is detached from actuator14, instead of an increase in pressure B chamber16will expand further. After time T1, the controller detects a decrease in pressure B as a result of activation of valve9. If position sensor13is connected to controller5, a movement in upward direction is also detected.

In the example described inFIG.11, a decrease in pressure is detected as a result to activation of valve9. This is distinguishable from examples described inFIGS.9and10, where activation of valve9results in an increase in pressure B and activation of valve10results in a decrease in pressure B Hence, the activity of the valve9can be used to improve the determination of an accident event such as the dropping of a weight.

Further, in example described inFIG.11, a movement in upward direction is detected by position sensor13in combination with pressure B lower than balancing pressure. This is distinguishable from examples inFIGS.9and10, where pressure B higher than balancing pressure gives movement in upward direction and pressure B lower than balancing pressure gives movement in downward direction. Hence, the reading from the position sensor can be used to improve the determination of an accident event such as the dropping of a weight.

Further,FIG.6illustrates a threshold value for pressure B. Detached load is detected when pressure is lower than the threshold value. The threshold value is typically chosen so that noise and normal operation of the system will not trigger a determination that the pressure is decreased. A further advantage of coupling a negative gradient of pressure B together with activation of valve9is that controller5can detect a dropped weight before pressure is lower than threshold value inFIG.6. Thus, if a valve signal is used together with a pressure signal to determine an alarm event such as a dropped weight. The system can be made to react faster to such accidents events. Similarly, by coupling movement in upward direction in combination with pressure B lower than balancing pressure enables controller5to detect a dropped weight22before pressure is lower than threshold value inFIG.6.