GB2383575A

GB2383575A - A lifting apparatus with load sensing means

Info

Publication number: GB2383575A
Application number: GB0131028A
Authority: GB
Inventors: Steven Caffall Finch
Original assignee: Individual
Current assignee: Individual
Priority date: 2001-12-28
Filing date: 2001-12-28
Publication date: 2003-07-02
Also published as: GB0131028D0

Abstract

A lifting apparatus such as a hydraulic tail lift 1 of a vehicle 2, is equipped with load sensing means such as a hydraulic fluid pressure sensor 10. As an object is raised or lowered by the apparatus, the initial sensed load is stored as a reference value, and this value is continuously compared with the sensed load during the lifting/lowering cycle using an electronic processor 11. If the lifting apparatus encounters an obstruction, such as a trapped object between the tail gate and the vehicle, the sensed load will vary. When the sensed load departs from the reference value by more than a predetermined threshold amount, the operation of the lift is interrupted using stop means.

Description

Safety stop for lifting apparatus, and method of operation This invention relates to methods and devices for interrupting the motion of lifting apparatus, in the event that an object becomes trapped or accidentally engaged by the apparatus or its burden.

Care must be taken when using lifting apparatus to ensure that its moving parts and the burden which it carries do not entrap or accidentally engage nearby objects. For example, if a person or object becomes trapped between the moving platform of a passenger or goods lift and the fixed structure which supports it, injury or damage may result. If the load carried by a crane is raised too high and strikes the jib, the crane may be damaged and the load may be dropped. If the load carried by a lorry mounted crane or forklift truck becomes lodged on a tree or building, the load may be damaged and the vehicle may become unstable.

This problem is particularly acute with platform lifts fixed to vehicles. Wheelchair lifts are commonly fitted to buses to allow disabled access; goods lifts such as tail lifts are used to transfer goods to and from lorries. Both types of lift include a platform, which may fold for transport, on which the passengers or goods are carried. The platform may be located in vertical guides fixed to the vehicle body, and raised by means of steel ropes, or alternatively it may be raised and lowered by means of pivoting or sliding arms. Commonly, the lift is hydraulically powered.

A particular danger exists that when travelling on the platform of this type of lift, the operator's feet may become trapped between the inside edge of the rising platform and the edge of the vehicle body. Since tail lifts may raise loads of a ton or more in weight, serious injury may result.

In response to this problem, US 4,556, 129 to Martin provides a flexible skirt which is arranged below the rear edge of the bodywork of a lorry and deactivates the tail lift when it senses the presence of an operator's foot. Similarly, US 5,653, 568 to Josephs provides a strip switch at the same point. However, this approach offers protection only at the inside edge of the platform.

A different approach is exemplified by DE 4,430, 568 to Rolf, which discloses a combination of foot and hand controls which ensure that the operator remains in a safe position while the platform is in motion. However, this approach is not effective to prevent the entrapment of another person or object.

It is the object of the present invention to provide an improved method and apparatus for interrupting the motion of lifting apparatus, in the event that an object becomes trapped or accidentally engaged by the apparatus or its burden.

According to a first aspect of the present invention there is provided an apparatus for raising or lowering a burden, including support means supporting the burden, such that the burden and the support means together impose a load upon the apparatus; means for raising or lowering the support means between an upper position and a lower position, each instance of said raising or lowering comprising a cycle of operation; and stop means for interrupting the motion of the support means; characterised in that there is provided load sensing means, the load sensing means sensing the load throughout each cycle of operation, and cooperating with the stop means, such that, if the load varies during a cycle of operation, the load sensing means activates the stop means so as to interrupt the motion of the support means.

Preferably, the load sensing means includes reference value storage and comparison means, whereby, substantially at the beginning of each cycle of operation, the load sensed by the load sensing means is stored as a reference value, and thereafter during that cycle of operation, the load sensed by the load sensing means is compared with the said reference value ; such that, if the load sensed by the load sensing means varies from the said reference value during that cycle of operation, the load sensing means activates the stop means so as to interrupt the motion of the support means.

According to a second aspect of the invention there is provided a method of controlling an apparatus for raising or lowering a burden, the apparatus including

support means supporting the burden, such that the burden and the support means together impose a load upon the apparatus ; means for raising or lowering the support means between an upper position and a lower position, each instance of said raising or lowering comprising a cycle of operation ; and stop means for interrupting the motion of the support means ; characterised in the steps of : a) sensing the load throughout each cycle of operation, and b) if the load varies during a cycle of operation, activating the stop means so as to interrupt the motion of the support means.

According to a third aspect of the invention there is provided a method of controlling an apparatus for raising or lowering a burden, the apparatus including support means supporting the burden, such that the burden and the support means together impose a load upon the apparatus ; means for raising or lowering the support means between an upper position and a lower position, each instance of said raising or lowering comprising a cycle of operation ; and stop means for interrupting the motion of the support means ; characterised in the steps of : a) substantially at the beginning of each cycle of operation, sensing the load and storing the load as a reference value ; b) throughout that cycle of operation thereafter, sensing the load and comparing the sensed load with the reference value ; and c) if the sensed load varies from the reference value, activating the stop means so as to interrupt the motion of the support means.

Various embodiments of the invention will now be described by way of example and with reference to the accompanying drawings, in which : Fig. 1 shows a tail lift according to the invention installed on a lorry ; Figs. 2-5 show the tail lift of Fig. 1 in use ; Fig. 6 shows a crane according to the invention lifting a burden ; and Fig. 7 is a graph representing the load sensed by the load sensing means according to the invention.

Referring to Fig. 1, a tail lift 1 for loading and unloading goods is installed on a lorry 2, and includes a platform 3 on which a burden may be disposed. The platform may be folded up when travelling, and is shown in its unfolded position ready for use. The platform 3 together with struts 4 and hydraulic rams 5 (of which one set only can be seen) forms support means for raising and lowering the burden between the upper position shown in Fig. 1 and the lower position shown in Fig. 3.

The platform is raised and lowered between the upper and lower positions by means of the rams 5, which are powered by means of hydraulic fluid supplied by a hydraulic pump 6. The hydraulic pump and associated valves are actuated by means of a switch 7, which includes buttons 8 for raising the platform and 9 for lowering the platform. The pump 6 includes an electrical relay in series with the power supply. This relay acts as stop means for interrupting the motion of the platform; when the relay is de-energised, the power supply to the pump is interrupted and the platform stops moving.

The details described so far are conventional.

When a burden is placed on the platform 3, the weight of the burden, together with the weight of the support means, imposes a load on the tail lift which is supported by the hydraulic fluid within the rams 5. The pressure of the hydraulic fluid is continuously sensed by a pressure sensor 10. Since the pressure of the fluid within the rams 5 reflects the load imposed on the tail lift, this sensor forms part of load sensing means which continuously senses the load imposed by the support means and the burden which is being raised or lowered.

The load sensing means includes an electronic processor 11, which monitors the output from the pressure sensor 10 and acts as reference value storage and comparison means. At the beginning of each cycle of operation, as the platform 3 begins to descend from the upper position to the lower position, or to ascend

from the lower position to the upper position, the processor 11 stores the output from the sensor 10 as a reference value. As the cycle of operation continues and the platform continues its travel, the processor continuously compares the output from the pressure sensor 10 with the reference value. If at any point during that cycle of operation the pressure of the hydraulic fluid varies from the reference value, the processor de-energises the relay to stop the pump 6, bringing the platform to a halt. A new reference value is stored in the same way as the next cycle of operation begins.

The exact point in the cycle of operation at which the reference value is stored may conveniently be determined by providing a position switch, for example integrally with one of the rams 5. The position switch senses when the platform 3 has been raised through a short distance, just enough to clear the ground, and signals the processor 11 to store the pressure reading of the sensor 10 as the reference value. This ensures that when the reference value is stored, no part of the platform or burden remains supported by the ground, and so prevents the stop means being actuated as the platform clears the ground. When the platform is being lowered, the reference value may conveniently be stored as soon as the platform leaves the upper position.

If an object becomes entrapped or engaged with the platform or the load during a cycle of operation, the load sensed by the sensor 10 will change, and the processor 11 will actuate the stop means, halting the motion of the platform.

Preferably, the button governing the last direction of motion is also disabled by the processor 11.

A reset button 12 is provided on the switch 7; alternatively it may be located etsewhere on the vehicle body. The reset button prompts the processor 11 to delete the stored reference value and store the output from the sensor 10 as a new reference value. The relay is then re-energised and the lifting or lowering operation may be resumed by pressing buttons 8 or 9.

Desirably, the reset button 12 is disabled until the platform has been moved a short distance in the opposite direction. This ensures that if an object becomes trapped, the operator cannot ignore it and proceed with the cycle of operation, increasing the pressure on the object. By briefly reversing the platform, any trapped object is released, so that when the new reference value is stored no pressure is exerted against the object. If the object remains trapped, the lift will again cut out as the cycle is resumed and the support means contacts the object again.

Thus if the lift cuts out on raising a burden, the order of operation of the buttons will be"lower" (9) followed by "reset" (12). If the lift cuts out while lowering a burden, the order of operation will be"raise" (8) followed by"reset" (12).

Similarly, if the operator jumps onto the platform while it is moving, or places additional goods on the platform while it is moving, the stop means will be actuated and the button governing the last direction of motion of the lift will be disabled. The operator may then briefly reverse the platform then press the reset button 12 to resume the cycle of operation. When the lift restarts, the new reference value will reflect the additional weight of the operator or goods which have been added to the platform.

Desirably, the processor 11 is adjustable to set a threshold amount by which the output from the sensor may vary from the reference value before the stop means is actuated. This threshold amount may be determined by the user, so as to adjust the sensitivity of the apparatus and reduce the possibility of false stops, and an adjustment dial may be provided for this purpose.

Referring to Fig. 2, the tail lift 1 is being used to lift a burden comprising a trolley 13 and the operator 14. The reference value stored in the processor 11 is the load sensed by the hydraulic fluid pressure sensor 10 at the beginning of the lifting cycle, and reflects the weight of the trolley 13 and operator 14. The operator has inadvertently placed his foot 15 in the pinch point between the inner edge of the platform 3 and the rear edge 16 of the lorry floor, and as the platform

continues to rise, his foot will become trapped. As this happens, the load sensed by the sensor 10 will rise, reflecting the pressure exerted by the platform 3 against the operator's foot and the lorry bodywork. When the sensed load exceeds the reference value by the threshold amount, the processor 11 actuates the stop means and the platform stops moving. The operator may then press the "down"button 9 to move the platform downwards and free his foot, followed by the"reset"button 12 to resume lifting.

Referring now to Fig. 3, the tail lift is in the lower position ready to lift the trolley 13 for loading into the lorry 2. A second lorry has parked behind the lorry 2 so that its overhanging load 17 is positioned above the trolley 13 and platform 3. If the"up"button 8 is now pressed, the trolley 13 will be lifted towards the obstruction 17. When the trolley contacts the obstruction, the pressure exerted on the obstruction will be reflected by the increased pressure of the hydraulic fluid in the rams 5. The sensor 10 will sense this increased load and pass the information to the processor 11, which compares the sensed load with the reference value which was stored as soon as the platform 3 cleared the ground.

When the difference between the sensed load and the reference value exceeds the predetermined threshold amount, the processor 11 will actuate the stop means and the platform 3 will come to a stop.

The foregoing examples illustrate the use of the apparatus to activate the stop means if the load increases as the burden is being raised. The apparatus may alternatively or additionally be arranged to activate the stop means if the load decreases as the burden is being lowered, as illustrated by the following examples with reference to Figs. 4 and 5. If desired, the processor 11 may be configured to actuate the stop means if the load increases or decreases while the platform is moving in either direction.

It will be appreciated therefore that the present invention offers a means of interrupting the motion of lifting apparatus, in the event that an object becomes trapped or accidentally engaged by any part of the apparatus or its burden. The present safety stop is therefore responsive to a wide variety of hazards, and

offers a significant safety advantage over those prior art designs which can sense a trapped object only at one point. This is particularly advantageous when applied to vehicle mounted lifts, such as tail lifts, wheelchair lifts, lorry mounted cranes and hoists, forklift trucks and the like, since the operating environment for such lifts is unpredictable and depends on where the vehicle is parked. However, it will be appreciated that the invention may equally well be applied to any lifting apparatus, whether fixed or mobile, where the load can be sensed.

Referring now to Fig. 4, the lorry 2 is being unloaded and the tail lift is in use to lower the trolley 13 and user 14 to the ground. As the platform left the upper position, the load sensed by the sensor 10 was stored by the processor 11 as the reference value. The operator has accidentally parked next to a bollard 18, and as the platform 3 descends it contacts the bollard. As the weight of the burden 13,14 and the support means is transferred to the bollard, the load sensed by the sensor 10 reduces. When the sensed load falls below the reference value by more than the predetermined threshold amount, the processor 11 actuates the stop means and the platform 3 comes to a halt. The operator must then press the"up"button 8, followed by the"reset"button 12, in order to resume lowering the platform. If he fails to observe the obstruction, the stop means will be actuated again in the same fashion.

Referring to Fig. 5, the lorry 2 is parked in another location and the operator has begun to lower the trolley 13, which has inadvertently been placed only partly on the platform 3. As the platform 3 left the upper position, the load sensed by the sensor 10 was stored by the processor 11 as the reference value. As the platform descends, the weight of the trolley 13 is transferred to the lorry floor, reducing the load on the platform 3. When the sensed load falls below the reference value by more that the predetermined threshold amount, the processor 11 de-energises the relay and the platform stops moving before the trolley topples over.

It will be appreciated that the invention is not confined to use with hydraulically operated lifts, and other means of sensing the load, whether on hydraulic lifts or

non-hydraulic lifts, will be readily conceived by those skilled in the art. For example, where the support means includes a resilient member, the load may be sensed by measuring the deflection of that member. Instead of measuring the pressure of the hydraulic fluid, a dedicated load measuring cell or equivalent device may be arranged as part of the support means, for example by mounting it at either end of one or both rams 5 in Fig. 1. The load may also be sensed in discrete steps. It will also be appreciated that the reference value storage and comparison means may be mechanical rather than electronic. For example, pneumatic or hydraulic valve means may be configured to store the initial sensed load as a variable setting and compare it with the subsequently sensed load, and close or open valve means to interrupt the lifting operation if the two values diverge.

Referring to Fig. 6, a load 20 is suspended from the support means 21, 21'of a crane 22. The crane has lifted the load too high so that it has hit the boom 21.

The additional load imposed by the tension in the wire rope 21'as the load 20 is urged against the boom is detected by a load cell which is arranged within the boom to support the axle 23 of the pulley 24 over which the wire rope travels.

The load sensed by the load cell is passed to an electronic processor, where it is compared with the reference value which was established as soon as the load 20 cleared the ground. The crane is equipped with stop means, and when the sensed load exceeds the reference value, the processor actuates the stop means to interrupt the lifting operation.

It will be appreciated that for some lifting apparati, the load sensed by the load sensing means will vary according to the position of the support means in a predictable pattern determined by the configuration of the support means. For example, for the crane shown in Fig. 6, the load will reduce in a linear fashion as the wire rope is retracted, at a rate depending on the weight of the rope. Similarly for a hydraulic platform lift, as the platform is lifted or lowered, the travel of the ram may be in a non-linear relationship with the travel of the platform.

Preferably therefore position sensing means are provided for sensing the position of the support means, together with adjusting means for adjusting the reference value as the burden is raised or lowered to reflect the position of the support means. In the crane of Fig. 6, the position sensing means may include a revolution counter on the winding drum on which the wire rope is carried. In the apparatus of Fig. 1, position sensing means may be integrated into one of the rams 5, as a plurality of magnetic switches, a continuous ultrasonic distance sensor, or any other suitable means for determining the position of the platform 3. Other arrangements will easily be conceived.

The processor includes adjusting means which cooperates with the position sensing means to adjust the reference value according to the position of the support means, so as to reflect the said predictable pattern as illustrated in Fig.

7.

Referring to Fig. 7, as the support means of a lifting apparatus moves between the lower and upper positions, represented by the axis marked"position", the load sensed by the load sensing means varies in a predictable pattern depending on the position of the support means. This pattern is illustrated for a given lifting apparatus and a given burden by line A, which shows the load sensed by the load sensing means as the burden is raised or lowered through a cycle of operation.

As the support means moves through each cycle of operation, the processor of the load sensing means adjusts the reference value to conform to this pattern.

Thus for example for the load illustrated, the reference value is adjusted continuously as the support means is raised or lowered so that it always lies along line A. When a heavier burden is carried by the support means of the same lifting apparatus, the reference value lies for example along line E.

Lines B and C show respectively the upper and lower predetermined threshold amounts by which the sensed load may depart from the reference value before the stop means is actuated. In the example shown, an obstruction is

encountered while raising the burden, so that the sensed load rises along line 0 as the burden is lifted. As soon as the sensed load exceeds the upper predetermined threshold amount B, the processor actuates the stop means and the support means is halted in position X.

Many other advantages and applications of the invention will readily be conceived by one skilled in the art on studying the foregoing description and drawings. It is to be understood therefore that the invention is not limited to the embodiments described, but may be applied to any apparatus for raising or lowering a burden where the load imposed by the burden may be sensed by any means.

Claims

CLAIMS 1. An apparatus for raising or lowering a burden, including support means supporting the burden, such that the burden and the support means together impose a load upon the apparatus; means for raising or lowering the support means between an upper position and a lower position, each instance of said raising or lowering comprising a cycle of operation; and stop means for interrupting the motion of the support means; characterised in that there is provided load sensing means, the load sensing means sensing the load throughout each cycle of operation, and cooperating with the stop means, such that, if the load varies during a cycle of operation, the load sensing means activates the stop means so as to interrupt the motion of the support means.
2. An apparatus according to claim 1, characterised in that the load sensing means includes reference value storage and comparison means, whereby, substantially at the beginning of each cycle of operation, the load sensed by the load sensing means is stored as a reference value, and thereafter during that cycle of operation, the load sensed by the load sensing means is compared with the said reference value ;

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such that, if the load sensed by the load sensing means varies from the said reference value during that cycle of operation, the load sensing means activates the stop means so as to interrupt the motion of the support means.
3. An apparatus according to claim 2, characterised in that, as the support means moves between the upper position and the lower position, the load sensed by the load sensing means varies according to the position of the support means in a predictable pattern determined by the configuration of the apparatus; and in that there are provided position sensing means for sensing the position of the support means, and adjusting means for adjusting the reference value, whereby, as the support means moves between the upper position and the lower position, the adjusting means cooperates with the position sensing means to adjust the reference value according to the position of the support means, so as to reflect the said predictable pattern.
4. An apparatus according to any of claims 1-3, characterised in that the load sensing means activates the stop means if the load varies by more than a predetermined threshold amount.
5.-An apparatus according to claim 4, characterised in that the predetermined threshold amount may be determined by a user.

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6. An apparatus according to any of claims 1-3, characterised in that the load sensing means activates the stop means if the load increases as the burden is being raised.
7. An apparatus according to any of claims 1-3, characterised in that the load sensing means activates the stop means if the load decreases as the burden is being lowered.
8. An apparatus according to any of claims 1-3, characterised in that the support means includes a platform and the burden is disposed on the platform.
9. An apparatus according to claim 8, characterised in that the platform is mounted on a vehicle, for loading or unloading the burden to or from the vehicle.
10. An apparatus according to any of claims 1-3, characterised in that the burden is suspended from the support means.
11. An apparatus according to either of claims 2 or 3, characterised in that the reference value is determined after the support means has travelled through a predetermined short distance.
12. An apparatus according to either of claims 2 or 3, characterised in that reset means is provided, whereby, if at any point during a cycle of operation the stop means is activated, a user may reset the load sensing means, such that the load then sensed by the load sensing means is stored as the reference value,

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so as to recommence the cycle of operation.
13. An apparatus according to any preceding claim, characterised in that the apparatus is powered by means of hydraulic fluid, and the load sensing means senses the pressure of the hydraulic fluid.
14. A method of controlling an apparatus for raising or lowering a burden, the apparatus including support means supporting the burden, such that the burden and the support means together impose a load upon the apparatus; means for raising or lowering the support means between an upper position and a lower position, each instance of said raising or lowering comprising a cycle of operation; and stop means for interrupting the motion of the support means; characterised in the steps of a) sensing the load throughout each cycle of operation, and b) if the load varies during a cycle of operation, activating the stop means so as to interrupt the motion of the support means.
15. A method of controlling an apparatus for raising or lowering a burden, the apparatus including support means supporting the burden, such that the burden and the support means together impose a load upon the apparatus;

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means for raising or lowering the support means between an upper position and a lower position, each instance of said raising or lowering comprising a cycle of operation; and stop means for interrupting the motion of the support means; characterised in the steps of a) substantially at the beginning of each cycle of operation, sensing the load and storing the load as a reference value ; b) throughout that cycle of operation thereafter, sensing the load and comparing the sensed load with the reference value ; and c) if the sensed load varies from the reference value, activating the stop means so as to interrupt the motion of the support means.
16. A method according to claim 15, characterised in that, as the support means moves between the upper position and the lower position, the sensed load varies according to the position of the support means in a predictable pattern determined by the configuration of the apparatus; and in that, as the support means moves between the upper position and the lower position, the reference value is adjusted according to the position of the support means, so as to reflect the said predictable pattern.
17. An apparatus substantially as herein described with reference to the accompanying description and drawings.

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18. A method substantially as herein described with reference to the accompanying description and drawings.