Abstract:
A control system for a machine which includes a load handling apparatus, the load is moveable relative to a body of the machine by the load handling apparatus. The machine including a pivot about which a tipping moment is produced by the load. The load handling apparatus including an actuator and is capable of moving the load to a position at which the tipping moment is at a predetermined threshold value. The control system includes a sensor which senses the tipping moment and in use, provides an input to a controller. The controller is responsive to the input to influence operation of the actuator so that in the event that the sensor senses that the value of the tipping moment is approaching the threshold value, the speed of movement of the load is progressively reduced.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Priority is claimed to United Kingdom patent application Serial No. 0216204.8 filed Jul. 12, 2002. This is a division of United States patent application Ser. No. 10/520,499 filed Jul. 18, 2005, which is a national phase of PCT application No. PCT/GB2003/002857 filed Jul. 2, 2003. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not Applicable. 
       TECHNICAL FIELD 
       [0003]    This invention relates to a control system for a machine of the kind which includes a load handling apparatus, the load being moveable relative to a body of the machine by the load handling apparatus. 
       BACKGROUND OF THE INVENTION 
       [0004]    One example of such a machine is a wheeled load handling machine which has a body, a ground engaging structure including a pair of axles each carrying wheels, and the machine including a load handling apparatus which includes a lifting arm. The lifting arm is moveable by one or more actuators to move the load, the load producing a tipping moment about either an axis of rotation of one of the pairs of wheels, or about another pivot where for example, stabilizers are used to stabilize the body relative to the ground during load handling operations. 
         [0005]    In each case, the lifting arm may move the load to a position at which the tipping moment is at a threshold value at which the machine may become unstable. 
         [0006]    Thus it is known to sense the tipping moment, for example by sensing a decreasing load on the pair of wheels remote from the pivot, as the tipping moment reaches the threshold critical value, to operate a safety device which stops further operation of the actuator or actuators. 
         [0007]    Such an arrangement may operate satisfactorily for some lifting arm/load movements, but unless the threshold value is set with a significant safety margin, for some load movements an abrupt cessation of movement can result in machine instability due to the inertia of the load, and of the lifting arm, The problem is particularly pronounced as the lifting arm is lowered after having been loaded at long reach and at height, as lowering of the lifting arm, increases the tipping moment and an abrupt cessation of movement can result in the machine tipping forwards. 
         [0008]    It is known to provide a machine operator with a visual indication of the value of the tipping moment, and therefore a skilled and attentive operator may be able to determine when the tipping moment is approaching the threshold value and the operator may thus take action such as retracting the load, where the lifting arm is capable of such operation, to avoid machine instability. However this relies on operator skill and attentiveness, and moreover such reliance would be inappropriate where the machine does not have an operator, e.g. is robot or remote controlled. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    According to a first aspect of the invention we provide a control system for a machine which includes a load handling apparatus, the load being moveable relative to a body of the machine by the load handling apparatus, the machine including a pivot about which a tipping moment is produced by the load, the load handling apparatus including an actuator and being capable of moving the load to a position at which the tipping moment is at a predetermined threshold value, the control system including a sensor to sense the tipping moment and in use, to provide an input to a controller, the controller being responsive to the input to influence operation of the actuator so that in the event that the sensor senses that the value of the tipping moment is approaching the threshold value, the speed of movement of the load is progressively reduced. 
         [0010]    Thus utilising the present invention, stability of the machine during load movements which may otherwise cause instability, is automatically maintained and does not rely on operator skill. 
         [0011]    In a preferred embodiment, the load handling apparatus is a lifting arm which is moveable about a generally horizontal axis relative to the body of the machine, the arm thus being capable of raising and lowering the load upon operation or a first actuator, the controller influencing operation of the first actuator as the tipping moment approaches the threshold value. The lifting arm may include a plurality of relatively moveable sections, which may for example be telescopic, and the controller may alternatively or additionally influence operation of a second actuator which relatively moves the arm sections as the tipping moment approaches the threshold value. Further, the arm may carry a load handling implement, such as lifting forks, which are movable on the arm by operation of a third actuator and the controller may additionally or alternatively influence operation of the third actuator as the tipping moment approaches the threshold value. 
         [0012]    In each case, the speed of movement of the load is progressively reduced and desirably is stopped altogether when the tipping moment is at the threshold value, which preferably is set so that instability of the machine is avoided. 
         [0013]    The machine may include a ground engaging structure by which the machine is supported on the ground. The structure may include a pair of supports, the tipping moment being produced about a pivot axis established by one of the supports. The tipping moment may be sensed by the sensor sensing loading of one of the supports. 
         [0014]    In one example the machine is a so called wheeled load handling machine having a ground engaging structure including a pair of supports provided by axles which each carry wheels. Thus the tipping moment may be produced about a rotational axis of one of the pairs of wheels whilst the sensor may sense the loading on the other pair of wheels. 
         [0015]    As the value of the tipping moment approaches the threshold value, the loading on the other pair of wheels will reduce which reduction in loading will be sensed by the sensor. 
         [0016]    The actuator, the operation of which is influenced, may be a fluid operated actuator such as a double acting linear hydraulic ram. The controller may influence operation of the actuator by reducing a flow of fluid to or from the actuator, regardless of any control input e.g. from a machine operator, so that the controller responds to the input from the sensor sensing the tipping moment by overriding any such control signal. 
         [0017]    Thus the system may include a main control valve for providing fluid to the actuator under operator or robot/remote control, and a valve which is independent of the control valve, but responsive to the controller to reduce the flow of fluid to or from the actuator as the sensed tipping moment approaches the threshold value. 
         [0018]    The sensor may be a transducer which provides an electrical input signal to the controller, whilst a control signal to influence actuator operation may be an electrical or fluid signal. 
         [0019]    Where the load handling apparatus includes a plurality of actuators, for example where the load handing apparatus is a raisable and lowerable lifting arm which may be telescopic and/or may include a load handling implement mounted on the arm, each operated by respective fluid operated actuators, the controller may influence the operation of one of the actuators as the value of the tipping moment approaches the threshold value, for example by reducing the permitted flow of fluid from the actuator, and may prevent the flow of fluid to or from the remaining actuator or at least one of the remaining actuators if the tipping moment value reaches the threshold value, whilst permitting only further actuator correctional operation which will result in a reduction in the tipping moment. 
         [0020]    However, for example where the load handling implement is a lifting forks, during any permitted correctional actuator operation, the attitude of the lifting forks relative to the ground may be maintained. 
         [0021]    For example the machine may include a displacement actuator which is operated as the lifting arm is raised and lowered to exchange fluid with the third actuator which controls the attitude of the load handling implement relative to the ground, and during correctional actuator operation, when the third actuator may be isolated, fluid pressure in a circuit containing the third and displacement actuators may be maintained. 
         [0022]    The controller may operate according to an algorithm which enables the controller to ignore transient changes of loading sensed by the sensor as a result of changing machine dynamics or of reaction to initial lift arm movements. 
         [0023]    According to a second aspect of the invention we provide a machine having a control system according to the first aspect of the invention. 
         [0024]    According to a third aspect of the invention we provide a load handling apparatus controlled by a control system according to the first aspect of the invention. 
         [0025]    According to a fourth aspect of the invention we provide a method of operating a load handling system according to the third aspect of the invention including progressively reducing to speed of lowering of the load in response to increasing machine instability. 
         [0026]    Embodiments of the invention will now be described with the aid of the accompanying drawings in which: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]      FIG. 1  is a side illustrative view of a machine embodying the invention; 
           [0028]      FIG. 2  is a rear view of the machine shown in  FIG. 1 ; 
           [0029]      FIG. 3  is an illustrative hydraulic circuit diagram of the machine of  FIGS. 1 and 2 , which incorporates features of the control system of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0030]    Referring to the drawings a load handling machine  10  includes a body  11  which includes in this example an operator&#39;s cab  12 , at one side longitudinally of the body  12 , and a mounting  13  for a lifting arm  14  at an opposite side of the body  12 , the mounting  13  being provided in this example towards a rear of the body  12 , such that the lifting arm  14  extends forwardly from a pivot axis B alongside the cab  12 . 
         [0031]    The body  12  is supported on and may be driven over the ground on a ground engaging structure which includes a pair of front wheels  16  carried on a front axle which usually is fixed relative to the body  12 , but may be suspended therefrom as desired, and a rear pair of wheels  17  also carried on an axle  19 , the rear axle  19  being in this example, coupled to the body  12  by a pivot  20  which permits oscillating rear axle  19  movement about a pivot axis A, relative to the body  12 . 
         [0032]    The lifting arm  14  in this example includes two relatively telescopic sections  22 ,  23 , an inner of the sections  22  being mounted by the mounting  13 , and the outer  23  of the sections carrying a load handling implement  26  which in this example is a pair of lifting forks. In another example the arm  14  may include more than two telescopic or otherwise relatively extendible sections, or a single section only. 
         [0033]    The arm  14  is raisable and lowerable by operation of a lifting actuator  24 , which is a double acting hydraulic linear actuator. The outer section  23  of the arm  14  may be extended/retracted relative to the inner section  22  by a further double acting hydraulic linear extension actuator  25  which is shown mounted exteriorly of the arm  14  although practically may be mounted interiorly of the arm  14 . The load handling implement  26  is moveable about the pivot axis D by a yet further double acting linear hydraulic fork actuator  27 . 
         [0034]    The actuators  24 ,  25  and  26  are all controlled in this example by an operator in cab  12  operating controls to operate a main control valve  44 , which is indicated in  FIG. 3 , but in another example the actuators may be remotely controlled by a computer i.e. may be robot controlled. 
         [0035]    It will be appreciated that a load L carried by the arm  14  will produce a tipping moment about a pivot axis C. In this example of a wheeled load handling machine  10  with the lifting arm  14  being rearwardly mounted and extending forwardly, the pivot C will be coincident with the axis of rotation of the front wheels  16 . However, where for example stabilizers  32  are provided which can be lowered into contact with the ground during some load handling operations, perhaps to raise the front wheels  16  off the ground, the pivot axis may otherwise be located. 
         [0036]    Even though the weight of the load L is counterbalanced by the mass of the machine  10  and in particular in this example by the machine engine E which may be positioned at the rear of the body  12  as indicated, or elsewhere, if the load L is moved forwardly of the tipping axis C beyond a certain position, dependent upon the magnitude of the load, it will be appreciated that the stability of the machine  10  will decrease as the machine  10  will tend to tip about the tipping axis C. Such load L movement may occur for example as the lifting arm  14  is extended, or as is pertinent to the present invention, upon lowering of a load L from a high position, e.g. as indicated in dotted lines to a lowered position shown in dotted lines. 
         [0037]    The resultant increase in the tipping moment about tipping axis C is conventionally determined by sensing a reduction in loading on the rear axle  19  on which the body  12  is supported. 
         [0038]    Thus a tipping moment sensor  30  is provided, such as a load cell or other transducer to sense the loading on the axle  19 , in this example at the pivot  20  connection of the rear axle  19  to the body  12 . The sensor  30  is operative to provide an input to a controller  32  indicative of rear axle  19  loading and thus of the tipping moment about the tipping axis C. 
         [0039]    In known arrangements, when the input to the controller  32  indicates that the tipping moment is about to increase to such an extent that the machine  10  is about to tip forwardly about the tipping axis C, the controller  32  acts to prevent further forward movement of the load L relative to the body  12 . For example the extension actuator  25  may be prevented from extending further and/or the lifting actuator  24  may be prevented from further lowering the lifting arm  14 . 
         [0040]    In the latter case, because the inertia of a loaded lifting arm and load L may be massive, an abrupt cessation of the downward movement of the arm  14  can result in the machine  10  tipping about the tipping axis C unless the threshold value of the tipping moment permitted is set to an impracticably acceptable safety limit. 
         [0041]    Referring particularly to  FIG. 3 , a control system  40  is shown partially integrated within a hydraulic system for operating and controlling the actuators  24 ,  25 ,  27 . 
         [0042]    When the control system  40  is actuated, for example in anticipation of handling a heavy load, a solenoid valve  41  is closed e.g. by a machine  10  operator operating a switch in the cab  12 , so that fluid to a rod side  24   a  of the lifting actuator  24  from main control valve  44  as the lifting arm  14  is lowered, is constrained to flow through a proportional valve  42 , via a restrictor  43 . The restrictor  43  reduces permitted flow from that which would be permitted when the control system  40  is not active. Thus the lowering speed of the lifting arm  14  will be constrained in any event. 
         [0043]    However the flow of fluid to the rod side  24   a  of the lifting actuator  24  may be further restricted by the proportional valve  42  as hereinafter explained, to maintain the value of the tipping moment of the machine about axis C below a threshold value. 
         [0044]    In parallel with the proportional valve  42  there is a counterbalance valve  45  which permits fluid from the main control valve  44  to be directed to the rod side  24   a  of the actuator  24  when it is desired to lower the lifting arm  14  when the control system of the invention is inactive. 
         [0045]    In the event that from the input from the sensor  30 , the controller  32  determines that the value of the tipping moment about pivot C is approaching a predetermined threshold value, for example is about 65% of the permitted tipping moment threshold value, the controller  32  acts to prevent the value of the tipping moment exceeding the threshold value. 
         [0046]    If the lifting arm  14  is being lowered, the controller  32  signals the proportional valve  42  to reduce the permitted flow of fluid to the rod side  24   a  of the actuator  24  progressively as the lifting arm  14  is continued to be lowered, until further lowering of the lifting arm  14  is prevented altogether when the value of the tipping moment reaches the threshold value, as all fluid flow to the rod side  24   a  of the actuator  24  is prevented by the proportional valve  42  closing completely or substantially completely. 
         [0047]    It can be seen that the proportional valve  42  is in this example solenoid operated, so that the controller  32  provides an electrical command signal to the proportional valve  42  although in another example a fluid pressure signal may be provided by the controller  32 . 
         [0048]    The machine operator in the cab  12  may reverse operation of the lifting actuator  24  by operating the main valve  44  to direct fluid to a cylinder side  24   b  of the actuator  24  to raise the lifting arm  14  and thus reduce the tipping moment about axis C, and/or may retract the extension actuator  25  to move the load L closer to the tipping axis C, by operating the main control valve  44  to direct fluid to a rod side  25   a  of the extension actuator  25 . 
         [0049]    Upon the threshold tipping value being reached, when further lowering of the lifting arm  14  will be prevented, the controller  32  also acts to open a further solenoid operated valve  48  in the circuit to prevent any operation of the extension actuator  25  which would move the load L further away from the tipping axis, and to isolate altogether the actuator  27  which is otherwise operative to move the lifting forks  26 . 
         [0050]    This is achieved as the further solenoid operated valve  48  when opened provides a by-pass to tank T. Thus in the event that the main control valve  44  is operated such as would otherwise extend the lifting arm  14 , fluid in line  50  which would otherwise pass to cylinder side  25   b  of the extension actuator  25  to extend the extension actuator  25 , will be relieved to tank T, via a non return valve  51  and the valve  48 , via line  52 . 
         [0051]    Moreover in the event that the operator operates the main valve  44  such as otherwise to operate the actuator  27  to move the lifting forks  26  about axis D on the arm  14 , again fluid in either of lines  55 ,  56  which would otherwise act to operate the actuator  27 , will be relieved to tank T, via one or other of the non-return valves indicated at  59 ,  60  and the valve  48 , via line  52 . 
         [0052]    If desired, where the machine  10  has stabilizers S which may be lowered into engagement with the ground during some working operations, a relief valve as indicated at  62  may be provided which restricts the angle to which the lifting arm  14  may be raised when the stabilizers S are not lowered. For example, when the machine  10  is performing working operations with the stabilizers S raised, such that there is greater potential for machine  10  instability, when the arm  14  is raised at an angle of 45.degree., the relief valve  62  may be opened e.g. by operation of the controller  32 , so that further fluid directed from the main control valve  44  to the rod side  24   a  of the lifting actuator  24  is relieved to tank T. 
         [0053]    Referring again to  FIG. 1  it can be seen that the machine  10  includes a displacement actuator  64  between the lifting arm  14  and the body  12  of the machine The displacement actuator  64  is a double acting hydraulic actuator, a piston  64   a  of the actuator  64  being extended relative to a cylinder  64   b  thereof, as the lifting arm  14  is raised, and being retracted into the cylinder  64   b  as the arm  14  is lowered. 
         [0054]    As indicated in  FIG. 3 , in normal operation, the displacement actuator  64  is provided in parallel to the actuator  27  which moves the lifting forks  26  about the axis D, and so as the arm  14  is raised and lowered, the attitude of the forks  26  or other load handling device  26  relative to the ground, may be maintained without intervention of the operator operating the main control valve  44  to operate the forks actuator  27 . 
         [0055]    Such an arrangement is known, but it will be appreciated that in the event that, with the control system of the invention, the relief valve  48  is opened to relieve fluid in that part of the circuit containing the fork actuator  27 , such automatic attitude maintenance will be lost. So in the event that the operator operates the lifting actuator  24  to correct machine  10  imbalance by raising the lift arm  14 , until the relief valve  48  again is closed by the controller  32 , the attitude of the forks  26  relative to the ground will not be maintained. 
         [0056]    However, to accommodate this, there is provided in each of the fluid lines  55  and  56  from the fork actuator  27  and displacement actuator  64 , a counterbalance valve  70 ,  71  respectively, which closes automatically upon loss of pressure in the lines  55 ,  56  as the relief valve  48  is opened, whilst permitting the transfer of fluid between the fork actuator  27  and the displacement ram  64  trapped in that part of the fluid circuit upstream of the counterbalance valves  70 ,  71 . 
         [0057]    Other features of the control circuit  40  are as follows. In the lines  55 ,  56  to and from the fork actuator  27  and displacement actuator  64 , there are provided solenoid operated restrictor valves  80 ,  81  which when operated e.g. by the controller  32  when the control system is actuated, may restrict operational speed of the fork actuator  27 , by restricting fluid flow to and from the actuators  27 ,  64  in proportion to the degree of instability of the machine  10  as sensed by the load sensor  30 . 
         [0058]    Other check valves and the like, e.g. as indicated at  85 ,  86  and  87  may be provided to ensure proper operation of the circuit. 
         [0059]    It has been found that in some conditions, when commencing lowering of the load L e.g. from a high position, there is an initial reaction which is transmitted through the machine  10  to the load sensor  30  which indicates a sudden increase in loading on the rear axle  19 . To prevent the control system reacting to such transient conditions, preferably the controller  32  is adapted to operate according to an algorithm which ignores such transient conditions. For example upon initiating lowering of the lifting arm  14 , the controller  32  may be arranged not to respond to the sensor  30  input for say, one or two seconds, by which time steady state conditions will ensue. 
         [0060]    Also, it will be appreciated that a false indication may be received from the sensor  30  of impending machine  10  instability as a result of changing machine  10  dynamics during some load handling operations, for example during loading/unloading of the lifting forks  26 . The controller  32  may be programmed to recognize such irregular indications, for example by responding only to a smoothly progressively changing tipping moment, rather than sudden changes in loading. 
         [0061]    Preferably, the controller  32  provides a visual indication on an indicator  33  in the operator&#39;s cab  12  to the operator of the stability of the machine  10  so that a skilled operator may still exercise his skill in avoiding unstable conditions with reference to the indicator  33 . For example such an indicator may include an array of lights, e.g. LED lights, the array being increasingly lit up as instability of the machine  10  increases. 
         [0062]    Various other modifications may be made without departing from the scope of the invention as will be apparent to the person skilled in the art. 
         [0063]    The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilized for realizing the invention in diverse forms thereof