GB2495497A - Cutting apparatus having means for measuring load - Google Patents

Abstract

This invention relates to an apparatus for cutting vegetation, in particular for cutting verges. The apparatus comprises an elongate arm 14 having a first end and a second end, mounting means 16 connected to the first end of the arm for attaching the apparatus to a vehicle, a cutting head 20 including means for cutting vegetation connected to the second end of the arm, means for measuring a load 60 exerted on the cutting head, and means for controlling a position of the cutting head relative to the mounting means in response to said measured load. The means for measuring a load may be load cell or strain gauge. The load cell may comprise a shaft and the cutting head is attached at a first end of the shaft and the arm is attached at a second opposite end of the shaft. Also disclosed is a method of controlling an apparatus for cutting vegetation.

Description

Cutting Apparatus

BACKGROUND

a. Fieid of the Invention This invention relates to an apparatus for cutting vegetation. In particular, this invention relates to an apparatus for attachment to a vehicle for cutting verges.

b. Related Art It is known to use a cutting head on the end of an extendable arm mounted to a tractor or similar vehicle to cut hedges and verges alongside roads and tracks.

Typically the arm comprises at least two pivotally connected members that are additionally pivotally mounted to the back of the vehicle. Manipulation of the arm is actuated by a series of hydraulic rams connected to and between each section of the arm. The cutting head typically includes cutting means or flails and a roller.

The roller is located behind the flails and, when the cutting head is being used for verge cutting, the roller is positioned to roll along the ground. Adjustment of the position of the roller relative to the flails can then be used to adjust the height at which vegetation is cut.

In use, the arm is rotated so that it extends from the side of the vehicle and the cutting head is lowered into contact with the hedge or verge. The pivots along the length of the arm allow the arm to be extended to cut vegetation further from the vehicle. When cutting is completed, the cutting head is lifted, and the arm is retracted and rotated into position behind the vehicle for transportation. Typically, an arm will comprise two sections and two independent hydraulic systems. The first hydraulic system includes a first ram (the lift/lower ram) which controls the up and down movement of the arm. The second hydraulic system which includes a second ram controls the horizontal extent of the arm.

The cutting apparatus, or trimmer, described above is generally used to cut a variety of different vegetation, at varying distances from the vehicle and at varying angles to the road or track along which the vehicle is moving during cutting. For example, the cutting apparatus may be used to cut the sides or tops of hedges, or vegetation in ditches, dykes, grass banks and verges.

During cutting of a verge, the roller is lowered relative to the flail in the cutting head, and secured in position to achieve a desired cut height of the vegetation.

The arm is then moved so that the roller just contacts the ground in the area to be cut. It is common, however, to have large variations or fluctuations in the height of the verge relative to the road, or obstructions and depressions in the road itself, which cause the height of the cutting head relative to the vegetation being cut to fluctuate. In order to maintain a constant cutting height and to reduce the forces that are exerted by the ground on the cutting head, and that are transmitted to the rest of the trimmer and vehicle, it is, therefore, necessary to keep adjusting the position of the arm and cutting head relative to the vehicle and to the verge being cut.

The height of the cutting head may be manipulated by an operator, through activation of the necessary hydraulics to move the arm up and down. However, moving the arm in this way is generally relatively slow and, therefore, it can become difficult for the operator of the trimmer, who is also driving the vehicle, to maintain sufficient forward speed whilst also maintaining a constant cut length of vegetation due to undulations in the ground. This can lead to longer grass being left in dips in the ground, but of more concern, can lead to shock loads being transferred to the trimmer and vehicle when the cutting head hits higher regions of the verge. These shock loads lead to fatigue of the pivots and other joints in the arm and cutting head, and can ultimately lead to failure of the apparatus.

There are known devices that are designed to at least partially address this problem, however, these devices still have disadvantages.

One known device is a lift float, using a hydraulic accumulator, which acts as a damper, stopping large shock loads being transferred through the trimmer. In this system, a chamber is provided in the hydraulic system of the arm, which in turn contains a gas-filled elastomeric bag. The presence of the compressible and extendible bag allows for some movement of the arm which would not be possible with a purely oil filled, and therefore incompressible, hydraulic system. The lift float, therefore, provides limited height movement of the cutting head (typically only a few inches) due to small changes in the ground level, to prevent the cutting head digging into the ground. However, the degree of compensation is limited by the size of the elastomeric bag, and this system, therefore, still requires significant operator input and control which is undesirable.

Furthermore, when the arm is extended and the cutting head is held further from the vehicle the hydraulic pressure in the system is higher and the elastomeric bag is generally more compressed. Likewise, when the arm is retracted and the cutting head held closer to the vehicle, the hydraulic pressure is less and the elastomeric bag is able to expand. The degree of damping provided by the system, therefore, changes depending on the position of the arm and, in particular, there is a compromise in the damping at the extremes of position of the arm.

Another known device uses sensors to determine the exact extension of the arm relative to the vehicle. Data from these sensors, and also from a pressure transducer in the lift hydraulic line, is then used to operate the lift/lower hydraulic ram on the arm to control the height of the cutting head. This system, therefore, acts to compensate for the increase in moment when the cutting head is extended away from the vehicle. As such, the system helps to maintain a constant height of the cutting head relative to the ground when the arm is extended to different lengths.

Further cutting apparatuses are known which include a plurality of pivots and/or linkages between the roller and the rest of the cutting head. These pivots and/or linkages act as a suspension system and are arranged to permit a limited degree of movement of the head relative to the ground to accommodate small undulations in the ground. Larger undulations cause micro-switches, located at the extreme ends of the linkage movement, to be activated which in turn controls the lifting and lowering of the hydraulic ram. However, the pivots and linkages in this system are susceptible to fatigue and failure.

There is, therefore, a need for a cutting apparatus for cutting vegetation along a verge that can compensate for significant changes in ground contours while allowing the vehicle to travel at an appropriate speed along the road.

SUMMARY OF THE INVENTION

According to the invention there is provided an apparatus for cutting vegetation comprising; -an elongate arm having a first end and a second end; -mounting means connected to the first end of the arm for attaching the apparatus to a vehicle; -a cutting head including means for cutting vegetation connected to the second end of the arm; -means for measuring a load exerted on the cutting head; and -means for controlling a position of the cutting head relative to the mounting means in response to said measured load.

As the apparatus is used to cut vegetation over an area of ground, the load exerted on the cutting head will, typically, be a generally vertical load exerted in either an upwards direction by the ground through the cutting head or a downwards direction due to the weight of the cutting head when spaced from the ground.

The means for controlling a position of the cutting head will cause the position of the cutting head relative to the ground to be adjusted in response to the measured load. This allows the position of the cutting head to be adjusted to accommodate changes in ground height permitting a constant cut height of the vegetation to be achieved.

Preferably the means for measuring a load comprises a ioad cell.

The load cell is typically mounted between the cutting head and the arm, so that the load cell experiences a bending force due to the loads exerted on the cutting head.

In a preferred embodiment the load cell comprises a shaft and the cutting head is attached at a first end of the shaft and the arm is attached at a second, opposite, end of the shaft, such that the load cell measures a bending strain due to a force applied to the cutting head.

Generally it is desirable if the apparatus further comprises a pivot pin connected to an end of the arm permitting rotation of the cutting head relative to the arm. The shaft of the load cell may then be attached at the first end of the shaft to the cutting head and at the second end of the shaft to the pivot pin.

Typically the arm comprises a first portion and a second portion, the first and second portions being pivotally connected to each other.

Preferably the apparatus further comprises at least one hydraulic ram for moving the arm relative to the mounting means, and preferably the means for measuring a load comprises a load cell and the means for controlling a position of the cutting head comprises a control unit for receiving an output from the load cell and actuating the at least one hydraulic ram to move the arm and control the position of the cutting head relative to the mounting means.

Preferably the load cell comprises one or more strain gauges.

The apparatus may comprise more than one load cell.

Preferably the apparatus comprises means for determining an angle of the cutting head relative to horizontal. Preferably the means for determining an angle of the cutting head includes an inclinometer.

The invention further provides an assembly comprising an apparatus according to the invention and a vehicle.

Typically the vehicle is a tractor, and generally the mounting means of the apparatus is attached to a rear of the vehicle. However, in other embodiments the mounting means may be attached to a front part of the vehicle, or may be mid mounted on the vehicle.

The invention further provides a method of controlling an apparatus for cutting vegetation, the apparatus comprising a cutting head on an end of a moveable arm, the method comprising the steps of: -measuring a load exerted on the cutting head; -setting a desired load range corresponding to acceptable loads exerted on the cutting head during normal operation of the apparatus; and -in response to the measured load falling outside of the desired range, moving the arm in a given direction so as to restore the measured load to within the desired load range.

Preferably the step of measuring a load comprises providing a load cell attached to the cutting head for providing an output corresponding to a measured load exerted on the cutting head.

In preferred embodiments the method comprises the steps of: -measuring an exerted load corresponding to the normal operating position of the cutting head and setting this as a target load value; -establishing a target load range corresponding to a range of acceptable load values above and below the target load value; and -moving the arm in a first direction when the measured load drops below the target load range and moving the arm in a second direction when the measured load rises above the target load range.

Typically the first direction is opposite to the second direction.

Preferably the speed of movement of the arm is faster the greater the deviation of the measured load from the target load range.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be further described, by way of example only, and with reference to the accompanying drawings, in which: Figure 1 is a schematic drawing of a cutting apparatus attached to the back of a tractor for cutting hedges and verges; Figure 2 is a side view of part of the cutting apparatus according to a preferred embodiment of the present invention showing the location of a load cell between a cutting head and an arm of the apparatus; Figure 3 is a plan view from above of the part of the cutting apparatus of Figure 2.

DETAILED DESCRIPTION

Figure 1 shows a cutting apparatus 10 attached, in a conventional manner, to the rear of a tractor 12. It will be understood that although the cutting apparatus 10 has been shown attached to a tractor 12, the apparatus may be attached to any

suitable vehicle.

The cutting apparatus 10 comprises an elongate arm 14, a mounting portion 16 at a first, proximal end of the arm 14 and a cutting head or flail head 20 at a second, distal end of the arm 14. The mounting portion 16 is secured to the rear of the tractor 12 in a conventional manner and includes control means and drive means for controlling the movement of the arm 14 and the flail head 20.

The arm 14 is connected to the mounting portion 16 by a fixed length support post 24 and a first hydraulic ram 26. The support post 24 and hydraulic ram 26 are pivotally connected to a first side 28 of a pivot plate, or rocker assembly 30, and the arm 14 extends from a second side 38 of the pivot plate 30. The arm 14 comprises a first arm member or first boom 32 and a second arm member or second boom 34. In this embodiment, the first boom 32 comprises two struts 32a, 32b and a first, proximal end 36a, 36b of each of the struts of the first boom 32 is pivotally attached to the second side 38 of the pivot plate 30. By extending and retracting the first hydraulic ram 26, a second, distal end 40 of the first boom 32 may be raised and lowered with respect to the mounting portion 16 through rotation of the pivot plate 30, which in turn causes the cutting head 20 to be raised and lowered with respect to the tractor 12.

The distal end 40a, 40b of each of the struts 32a, 32b of the first boom 32 is pivotally attached to a first, proximal end 42 of the second boom 34. A second hydraulic ram 44 is connected between the pivot plate 30 and one of the struts 32a, and extending and retracting this ram 44 causes the distal end 40 of the first boom 32 and the second boom 34 to extend outwards from and retract inwards towards the tractor 12.

The mechanisms, described above, for raising and lowering, and extending and retracting, the arm 14 of the cutting apparatus 10 are well known in the art. It will be understood by a skilled person that alternative arm geometries and mechanisms may be used to enable a cutting head 20 to be positioned at a distance from a tractor 12 or similar vehicle. In particular, the second boom 34 may be telescopic or varying in forward reach to provide additional movement, as is known in the art.

Returning to the present embodiment, the cutting head 20 is connected to a second, distal end 46 of the second boom 34 by means of a linkage mechanism 48 that enables the cutting head 20 to pivot with respect to the second boom 34, as will be described in more detail later. The angle at which the cutting head 20 is positioned relative to the second boom 34 is controlled by the operation of a third hydraulic ram 50 connected between the second boom 34 and the linkage mechanism 48.

Typically, the linkage mechanism 48 includes a pivot pin 49 which enables the cutting head 20 to pivot about an axis parallel to a direction of travel of the cutting head 20 in use. This enables the cutting head 20 to be rotated to cut, for example, the sides of hedges or the sides of verges sloping upwards or downwards away from the vehicle 12.

Control lines (not shown) including hydraulic lines extend from the cutting head 20 and the hydraulic rams 44, 50 to the mounting portion 16 along the first and second booms 32, 34 to permit control of the operation of the apparatus 10, in particular the movement of the arm 14 and cutting head 20, from the tractor 12.

The cutting head 20 may be of any suitable design known in the art, and typically comprises a main body 52, cutting means or flails 54 rotatably mounted within the main body 52 and a roller 56 attached to a lower rear portion of the main body 52, as shown most clearly in Figure 2.

When the cutting apparatus 10 is being used to cut vegetation, for example along a verge, the cutting head 20 is positioned so that the roller 56 is in contact with the -10-ground. By adjusting the relative positions of the roller 56 and the cutting means 54 within the cutting head 20, the height at which the vegetation is cut can be adjusted. The cutting head 20, therefore, typically also includes adjustment means 58 for raising and lowering the roller 56 with respect to the rest of the cutting head 20. Various systems for raising and lowering the roller in this way are known in the art and wiil not be described further.

In the present invention, a load cell 60 is located between the cutting head 20 and the linkage mechanism 48 of the arm 14. In particular, a first end 62 of the load cell 60 is attached to a part of the pivot pin 49, and a second end 64 of the load cell 60 is attached to an upper rear portion of the main body 52 of the cutting head 20. In this way, the cutting head 20 is only attached to the arm 14 of the cutting apparatus 10 via the load cell 60, such that the load cell 60 acts as a cantilever, extending from the end 46 of the second boom 34 and supporting the weight of the cutting head 20.

In this embodiment, the load cell 60 comprises an elongate and substantially cylindrical shaft portion 66, and a pair of attachment flanges 68, 70. Each attachment flange 68, 70 extends outwardly around each of the first and second ends 62, 64 of the shaft portion 66 respectively. Each of the flanges 68, 70 include a plurality of holes (not shown) spaced equidistantly around the flange 68, for attachment of the load cell 60 between the cutting head 20 and the pivot pin 49. In particular, the load cell 60 is attached at its first end 62 to a part of the pivot pin 49 by a series of bolts (not shown) that pass through the holes in the flange 68.

Similarly, the second end 64 of the load cell 60 is fixed to an attachment plate 78 in the main body 52 of the cutting head 20 by a series of bolts (not shown) that pass through the holes in the flange 70.

In other embodiments, the load cell 60 may be attached to the cutting head 20 and second boom 34 using any suitable attachment means such that the load cell 60 always remains in a fixed orientation with respect to the cutting head 20 during use.

The load cell 60 is designed to measure bending loads exerted on the shaft 66.

The load cell 60 is oriented and fixed with respect to the cutting head 20 so that the loading direction is perpendicular to a longitudinal axis of the roller 56 and vertical bending loads are measured.

As such, in one embodiment of the invention, the load cell 60 includes two strain gauges (not shown) mounted within the shaft portion 66. The strain gauges are mounted midway along the length of the shaft portion 66 and first and second strain gauges are located diametrically opposite each other within the shaft 66. In some embodiments the strain gauges may be located within a central bore (not shown) extending longitudinally through the shaft portion 66. In other embodiments, the strain gauges may be located proximate an outer surface 80 of the shaft 66. The load cell 60 is oriented with respect to the cutting head 20 50 that the strain gauges are in a vertical plane, with the first strain gauge proximate an upper-most part of the shaft 66 and the second strain gauge proximate a lower-most part of the shaft 66. With the load cell 60 connected in this way, the strain gauges are located in the regions of maximum strain when a vertical force is applied to the cutting head 20 of the apparatus 10. In other embodiments, the strain gauges may lie in the same horizontal plane, so that when a vertical load is applied to one end of the load cell, the strain gauges are arranged to measure the same strain on opposite sides of the load cell.

To achieve accurate measurements from the strain gauges, the shaft portion 66 of the load cell 60 may be shaped so that a central section of the shaft portion 66 is narrowed compared to the end sections. The central portion is then the weakest part of the load cell and this results in the maximum stress in this region when the load cell is deflected.

When the cutting head 20 is not in contact with the ground, the force measured by the load cell corresponds to the weight of the suspended cutting head 20 acting in a downwards direction at the second end 64 of the load cell 60. As the cutting -12-head 20 is lowered into contact with the ground, this measured force decreases as the weight of the cutting head 20 is, at least partially, supported by contact of the roller 56 with the ground.

In use, as the cutting head 20 moves along the verge and over the vegetation to be cut, vertical unduiations in the ground's surface wiil cause the force exerted on the cutting head and, therefore, on the second end 64 of the load cell 60 to change.

In particular, as the level of the ground rises, an upwards force is exerted on the cutting head, and a resultant decrease in load is measured by the load cell 60. In contrast, when the level of the ground drops, less of the weight of the cutting head is supported by the roller 56, an increased downwards force is exerted on the end of the load cell 60 and a resultant increase in load is measured.

The load cell 60 is arranged to output an electrical signal corresponding to the measured load to a control unit (not shown), which is preferably located in the mounting portion 16. The control unit is configured to control movement of the arm 14 in response to the received load cell output signal, and in particular to actuate the first hydraulic ram 26 to raise or lower the arm 14 in response to a decrease or increase in load on the cutting head 20 respectively.

In order to prevent constant small adjustments being made to the position of the arm 14, which uses unnecessary energy and leads to increased fatigue of the pivot joints, the control unit is arranged to operate the arm 14 only when the output signal exceeds predetermined limits. In a preferred embodiment, the load measured by the load cell 60 when the cutting head 20 is in an optimum cutting position relative to the ground is set as a target value of the output signal. A range of output signal values, referred to as a working range, is then established that brackets the target value. This working range defines the limits of the acceptable loads that may be experienced by the cutting head 20 during operation of the apparatus 10. -13-

Typically, the target value and working range will be set during production of the apparatus 10. However, it is desirable to provide at least limited means to enable a user of the apparatus 10 to adjust the target value or the limits of the working range, or both. This ability to adjust the set values may be limited, and may involve, for exampie, choosing one of three preset operational modes, with each operational mode corresponding to a particular target value and working range.

In use, as the cutting head 20 moves over relatively flat or gently undulating ground, the load exerted on the cutting head 20 may vary slightly, but the output signal remains within the working range. In this state, the arm 14 of the apparatus is held in a fixed position.

If the ground level rises significantly, a large upwards force is measured by the load cell 60 and the output signal decreases to a value below the working range.

The control unit then actuates the first hydraulic ram 26 to raise the arm 14 until the output signal increases to a value within the working range. Preferably, the arm 14 continues to be raised until the output signal reaches the target value.

Similarly, if the ground level drops away significantly, a large downwards force is measured by the load cell 60 and the output signal increases to a value above the working range. The control unit actuates the first hydraulic ram 26 to lower the arm 14 until the output signal decreases to a value within the working range.

Preferably, the arm 14 continues to be lowered until the output signal reaches the target value.

In a preferred embodiment, the further the output signal deviates from the working range the faster the arm 14 is lifted or lowered to return the signal to within the working range. This means that the arm 14 will be lifted faster if the ground rises more steeply, or will be lowered faster if the ground drops away more steeply.

This allows a more constant cutting height to be achieved as any lag in the movement of the cutting head 20 relative to the contours of the ground is -14-minimised.

The limits of the working range may be adjusted and set depending on the particular application for which the cutting apparatus 10 is being used, for example, the type or condition of vegetation being cut.

In a preferred embodiment, the cutting apparatus 10 also comprises a hydraulic accumulator lift float system (not shown), which provides a limited degree of movement of the arm 14 when the load exerted on the cutting head 20 is within the working range. Typically the lift float system comprises a chamber, formed from a gas-filled elastomeric bag, provided in the first hydraulic ram 26. In use, small rises in ground level exert an upwards force on the cutting head 20.

Because the arm is hydraulically actuated, without a lift float system, the arm 14 would remain rigid and these forces would be transmitted along the arm 14 of the cutting apparatus 10. The addition of the lift float system provides a means of damping the forces by allowing limited compression or extension of the hydraulic ram 26 via compression or expansion of the gas within the elastomeric chamber.

In many situations, the verge being cut by the apparatus 10 is not flat and may be banked or sloped upwards or downwards away from the road's surface. The cutting apparatus 10 may, therefore, additionally comprise a head float system (not shown). The head float system allows the cutting head 20 to rotate relative to the second boom 34, about an axis parallel to a direction of travel of the cutting head 20 in use, to accommodate changes in the lateral slope of the verge being cut. Typically the head float system comprises means for joining hydraulic lines from both ends of the third hydraulic ram 50, used to adjust the angle of the cutting head 20 relative to the second boom 34. As the slope of the verge changes, the ground will exert a greater force on one end of the roller 56 compared to the other end. This causes the cutting head 20 to rotate about the pivot pin 49 and the hydraulic ram 50 to either extend or retract.

Because the load cell 60 remains in a fixed orientation with respect to the cutting -15-head 20, forces exerted on the cutting head 20 by bumps and dips in the ground will still be measured in a similar way by deflection of the strain gauges within the load cell 60. However, because the cutting head 20 is at an angle relative to true horizontal, the loads measured by the load cell 60 will differ from those measured due to true vertical loads when the cutting head 20 is horizontal. This difference in measured load is dependent on the angie of the cutting head 20 meaning that the measured load is not as accurate as in the case when the cutting head 20 is horizontal.

It is, therefore, desirable to include one or more inclinometers (not shown) on or in the cutting head 20 to output a signal corresponding to the angle of the cutting head 20, about an axis parallel to a direction of travel of the cutting head 20 in use, relative to a true horizontal ground line. This inclinometer output signal is sent to the control unit which resolves the forces acting on the cutting head 20 to determine a corrected measured load and, therefore, the resultant necessary actuation of the first hydraulic ram 26 to provide the required movement of the cutting head 20 in response to the output signal from the load cell 60.

In other embodiments it may be desirable to have more strain gauges mounted within the load cell 60, arranged to form a multi-axis load cell, permitting a measurement of the load due to bending in more than one plane. This would allow for adjustments in the angle of the cutting head 20 from true horizontal, while still obtaining accurate measurements of the exerted load without the need for inclinometers.

In alternative embodiments, it may be preterable to have more than one load cell attached to the cutting head 20. For example, a first load cell may be located proximate a left-hand end of the cutting head 20 and a second load cell may be located proximate a right-hand end of the cutting head 20. The difference in the loads measured by each of the first and second load cells can then be used to control the angle of the cutting head 20 relative to the ground, about an axis parallel to a direction of travel of the cutting head 20 in use. This system could, -16-therefore, be used to replace the head float system described previously.

Furthermore, this system could also be used to replace the lift float system, If, and when, the two load cells each measure approximately the same load, then the substantially identical output signal from both load cells could be used to control the lifting and lowering of the arm 14 in response to those signals.

A switch or button may be provided that enables a user of the apparatus 10 to activate or de-activate all of the load cell control system, lift float system and head float system. An additional button or switch may be provided that allows only the head oat system to be switched on or off. These two buttons, therefore, allow the user of the apparatus 10 to control whether each of the systems is operational simply by pressing one or two buttons. As such, the user may choose to activate or de-activate all of the systems, may choose to switch off the head float system so that only the load cell control system and lift float system remain in operation, or alternatively may only switch on the head float system. Typically it is desirable to keep the cutting head in a fixed position when using the head to trim hedges, when little or no force is exerted by the hedge onto the cutting head 20.

The present invention, therefore, provides a cutting apparatus for cutting vegetation along a verge that can compensate for significant changes in ground contours while allowing the vehicle to travel at an appropriate speed along the road. -17-

Claims (1)

1. <claim-text>CLAIMS1. An apparatus for cutting vegetation comprising; -an elongate arm having a first end and a second end; -mounting means connected to the first end of said arm for attaching the apparatus to a vehicle; -a cutting head including means for cutting vegetation connected to the second end of said arm; -means for measuring a load exerted on the cutting head; and -means for controlling a position of the cutting head relative to the mounting means in response to said measured load.</claim-text> <claim-text>2. An apparatus as claimed in Claim 1, wherein the means for measuring a load comprises a load cell.</claim-text> <claim-text>3. An apparatus as claimed in Claim 2, wherein the load cell is mounted between the cutting head and the arm.</claim-text> <claim-text>4. An apparatus as claimed in Claim 2 or Claim 3, wherein the load cell comprises a shaft and the cutting head is attached at a first end of the shaft and the arm is attached at a second, opposite, end of the shaft, such that the load cell measures a bending strain due to a force applied to the cutting head.</claim-text> <claim-text>5. An apparatus as claimed in Claim 4, wherein the apparatus further comprises a pivot pin connected to an end of the arm permitting rotation of the cutting head relative to the arm, and wherein the shaft of the load cell is attached at the first end of the shaft to the cutting head and at the second end of the shaft to the pivot pin.</claim-text> <claim-text>6. An apparatus as claimed in any preceding claim, wherein the arm comprises a first portion and a second portion, the first and second portions being pivotally connected to each other. -18-</claim-text> <claim-text>7. An apparatus as claimed in any preceding claim, wherein the apparatus further comprises at least one hydraulic ram for moving the arm relative to the mounting means.</claim-text> <claim-text>8. An apparatus as claimed in Claim 7, wherein the means for measuring a load comprises a load cell and the means for controlling a position of the cutting head comprises a control unit for receiving an output from said load cell and actuating said at least one hydraulic ram to move the arm and control the position of the cutting head relative to the mounting means.</claim-text> <claim-text>9. An apparatus as claimed in Claim 2, wherein the load cell comprises one or more strain gauges.</claim-text> <claim-text>10. An apparatus as claimed in Claim 2, wherein the apparatus comprises more than one load cell.</claim-text> <claim-text>11. An apparatus as claimed in any preceding claim, wherein the apparatus comprises means for determining an angle of the cutting head relative to horizontal.</claim-text> <claim-text>12. An apparatus as claimed in Claim 11, wherein the means for determining an angle of the cutting head includes an inclinometer.</claim-text> <claim-text>13. An assembly comprising an apparatus as claimed in any of Claims 1 to 12 and a vehicle.</claim-text> <claim-text>14. An assembly as claimed in Claim 13, wherein the vehicle is a tractor.</claim-text> <claim-text>15. An assembly as claimed in Claim 13 or Claim 14, wherein the mounting means of the apparatus is attached to a rear of the vehicle. -19-</claim-text> <claim-text>16. A method of controlling an apparatus for cutting vegetation, the apparatus comprising a cutting head on an end of a moveable arm, the method comprising the steps of: -measuring a load exerted on the cutting head; -setting a desired load range corresponding to acceptable loads exerted on the cutting head during normal operation of the apparatus; and -in response to the measured load falling outside of the desired range, moving the arm in a given direction so as to restore the measured load to within the desired load range.</claim-text> <claim-text>17. A method as claimed in Claim 16, wherein the step of measuring a load comprises providing a load cell attached to the cutting head for providing an output corresponding to a measured load exerted on the cutting head.</claim-text> <claim-text>18. A method as claimed in Claim 16 or Claim 17, wherein the method comprises the steps of: -measuring an exerted load corresponding to the normal operating position of the cutting head and setting this as a target load value; -establishing a target load range corresponding to a range of acceptable load values above and below the target load value; and -moving the arm in a first direction when the measured load drops below the target load range and moving the arm in a second direction when the measured load rises above the target load range.</claim-text> <claim-text>19. A method as claimed in Claim 18, wherein the first direction is opposite to the second direction.</claim-text> <claim-text>20. A method as claimed in any of Claims 18 or 19, wherein the speed of movement of the arm is faster the greater the deviation of the measured load from the target load range.</claim-text> <claim-text>21. An apparatus for cutting vegetation substantially as herein described, with -20 -reference to and as shown in the accompanying drawings.</claim-text> <claim-text>22. An assembly substantially as herein described, with reference to and as shown in the accompanying drawings.</claim-text> <claim-text>23. A method of controlhng an apparatus for cutting vegetation substantially as herein described with reference to the accompanying drawings.</claim-text>