Draft force sensor apparatus

0001The present invention relates generally to an agricultural machine having a draft force sensor apparatus for an implement towed behind an agricultural machine, such as a tractor, which forms part of the tractor draft control system.

0002Devices of this general type are known which allow the height of the implement to be automatically controlled on the basis of the draft force detected in the connection between the implement and the agricultural machine. The power applied by the agricultural machine to draw the implement is thereby regulated to a desired value.

0003Certain embodiments of such devices have a flexure bar mounted on the agricultural machine, at least one connection arm between the flexure bar and the implement to apply the draft force for towing the implement, a positioning member for raising and lowering the implement and an electrical processing circuit for controlling the positioning member by way of a control which is usually of the hydraulic type. Under the action of the force applied by the implement in use, the flexure bar deforms and means are provided to detect the deformation of the bar and to apply a signal to the processing circuit. The signal indicates to the processing circuit the size of the draft force being applied to the implement through the flexure bar.

0004It is important that the means to sense the flexure of the flexure bar should be capable of providing an accurate indication of the relatively small deflections of the flexure bar under load, be capable of manufacture at an acceptable cost, and remain serviceable despite the arduous operating conditions that are to be found in agricultural machines. In the prior art it has been proposed to use a variety of sensor systems including electrical resistance strain gauges, such as depicted in EP-A-0.432.548, and hydromechanical transformer control circuits, such as exemplified in EP-B-0.088.915, but each of the prior systems suffers from one or more defects in regard to cost, a low signal to noise ratio thereby necessitating an electronic amplifier, sensitivity to oil temperature and difficulty of manufacture or installation.

0005In EP-B-0.376.093 some of the above drawbacks have been attenuated by providing a flexure bar in which a cantilevered sensor pin is operable to deflect together with the flexure bar upon the creation of external forces thereon. The sensor pin carries a Hall Effect sensor for sensing variations in a magnetic field which are proportional to the forces experienced on the flexure bar. However, as the sensor pin is connected to the flexure bar at the point of introduction of the external forces, it is prone to vibrations resulting from implement movements. It furthermore presents difficulties to electrically connect the Hall Effect sensor to a control circuit considering that the Hall Effect sensor moves relative to the chassis of the vehicle. The introduction of the wiring moreover has to occur at the outer end of the flexure bar which is a hostile environment as it is exposed to dirt and humidity.

0006It is therefore the objective of the present invention to overcome the above mentioned disadvantages of prior art arrangements by providing a draft force sensor apparatus which is durable in construction, inexpensive of manufacture, carefree of maintenance, facile in assemblage and simple and effective in use.

0007According to the present invention, an agricultural machine having a draft force sensor apparatus for an implement towed behind an agricultural machine is provided, comprising : <ul id="ul0001" list-style="dash" compact="compact"><li>a flexure bar mounted on the chassis of the agricultural machine;</li><li>at least one connection arm between the implement and the flexure bar;</li><li>a control system including a member for raising and lowering the implement;</li><li>an electrical processing circuit for controlling the said member and including a Hall Effect sensor to detect movement of the flexure bar relative to the chassis as a result of tractive or pressure forces on the implement causing deflections of the flexure bar; and</li><li>a sensor pin housed within the flexure bar and comprising a first end portion free standing therefrom.</li></ul>

0008The arrangement is characterized in that : <ul id="ul0002" list-style="dash" compact="compact"><li>the sensor pin further comprises a second end portion immovably fixed relative to the chassis; the arrangement being such that relative movement is allowed between said flexure bar and said first end portion upon deflection of said flexure bar; and</li><li>said Hall Effect sensor is disposed at said first end portion of the sensor pin to detect movement of the flexure bar relative to said first end portion and to generate an output signal proportional to the forces experienced on the flexure bar.</li></ul>

0009A draft force sensor apparatus in accordance with the present invention will now be described in greater detail, by way of example, with reference to the accompanying drawings, in which: <ul id="ul0003" list-style="none" compact="compact"><li>Figure 1 is a perspective view of the rear portion of a tractor incorporating a draft force sensor apparatus according to the present invention;</li><li>Figure 2 is a partial sectional view of the draft force sensor apparatus of Figure 1;</li><li>Figure 3 is a detailed view of a sensor incorporated into the apparatus of Figure 2;</li><li>Figures 4 and 5 show graphs relating to the operation of the apparatus of the preceding Figures; and</li><li>Figure 6 is a partial sectional view of an alternative draft sensor apparatus according to the present invention.</li></ul>

0010As shown in Figure 1, a device for controlling the attitude of a tool (not shown) adapted to be drawn by an agricultural machine, such as a tractor 2, is generally indicated with the reference numeral 1. The device 1 comprises two cylindrical flexure bars 3 housed within respective hollow cylindrical elements 4 fixed at one end, for example by screws, onto the chassis 5 of the tractor 2. More specifically, the chassis 5 has two parallel side walls 6 from which the elements 4 project coaxially. The flexure bars 3 may be formed together in one piece to constitute a single flexure bar but it is preferred to provide two separate but coaxial bars on respective sides of the tractor chassis 5. The device 1 further is provided with two lower arms 7 for connecting the flexure bars 3 to said tool or implement to be towed by the tractor 2, a control system (not illustrated) operable to control the rotation of two upper arms 8 mounted above the arms 7 about a pivot axis located within the chassis 5 parallel to and above the bars 3 and two connecting levers 11 pivotally connecting the lower arms 7 to the upper arms 8.

0011Referring now to Figure 2, one of the elements 4 is shown in cross section and has a central portion 12 projecting from the chassis 5 and an end portion 13 of smaller diameter than that of the central portion 12. The cavity formed within the interior of the element 4 has a section 14 formed in correspondence with the central portion 12, a section 15 of smaller diameter than that of the section 14 and formed in correspondence with the end portion 13, and a section 16 connecting between the sections 14 and 15. The section 16 is formed in correspondence with the end of the central portion 12 connected to the portion 13. Each flexure bar 3 has an end portion 17 housed in cavity sections 14 and 16, a central portion 18 of greater diameter than that of the portion 17 and lodged in the section 15, and an end portion 21 of smaller diameter than the portion 18 and projecting from the element 4. Between the central portion 18 and the end portion 21, the bar 3 has an intermediate portion 22 the diameter of which is intermediate between those of the portions 18 and 21. A cylindrical cover 24 is fixed by screws 23 to the end portion 13 of the element 4. The cover 24 has a central through hole of a diameter slightly greater than the intermediate portion 22 to allow a predetermined clearance for deformation of the flexure bar 3 without obstruction.

0012It will be appreciated that the provision of the cover 24 and the specific shape of the flexure bar 3 and the element 4 enable the flexure bar 3 to be mounted in an expedient but nonetheless effective and firm manner.

0013Each lower arm 7 has a through hole 26 for the connection to an implement to be towed and a central through hole 27 on which is pivotally mounted a lower end of the corresponding lever 11. The inner end of each arm 7 is fitted onto the corresponding flexure bar 3 by means of a ball joint 31 and bush 32. A fixing ring 33 forms a shoulder against the ball joint 31 and is held in place by a bolt 34 passing through the portion 21 of the flexure bar 3. A spacer ring 38 is mounted on the end portion of the bar 3 between the ball joint 31 and the cover 24. The direction of the forces transmitted from the implement to the flexure bar 3 are represented by a double-headed arrow F taking account of the occurrence of both pulling and pressing forces.

0014Each of the flexure bars 3 is hollowed to form an axial cavity 41 extending for the whole length of the bar 3. It will be seen that the cavity 41 has a first section 42 which leads to a second section 43 of smaller diameter. The second section 43 has a region 44 of a further reduced diameter. At the outward end of the portion 21 of the flexure bar 3, the cavity 41 is sealed off from the environment with a plug member 39 which closely fits into said cavity 41 and is pierced by the bolt 34.

0015Within the axial cavity 41 is mounted a cantilever sensor pin 46 which has a root end 47 fixed securely within the section 43 of the axial cavity and is locked in place within the cavity 41 by means of an abutment member (not shown) inserted in the cavity 41 and shouldering the root end 47 of the pin 46 against the reduced diameter region 44 of the section 43 of the cavity 41. The sensor pin 46 has a reduced diameter to pass through the reduced diameter region 44 and projects in cantilever fashion through section 18 of the flexure bar and into the end portion 21 thereof. As seen in Figure 2, a radial clearance is provided between the portion of the pin 46 extending beyond the narrowed region 44 and the wall of the flexure bar 3. This clearance ensures that the tractive and pressure forces on an implement being towed by the tractor 2 and applied to the flexure bar 3 by way of the connecting arm 7 allows the flexure bar 3 to deflect relative to the free end of the sensor pin 46. The sensor pin is fixed in the flexure bar 3 at a point where the bar 3 is itself held fixedly by the element 4 and therefore the free end of the sensor pin 46 does not move in reaction to the forces that cause deflection of the flexure bar 3.

0016Referring now to Figure 3, the free end of the sensor pin 46 comprises a projecting end portion 66 carrying, at the interior surface thereof, a polarised magnet 48 within a bore 49. The magnet 48 is fixed in position so as to be precisely located, in a predetermined manner, in relation to the end portion 66 of the sensor pin 46. The plug member 39 of the flexure bar 3 has a projection 50 which overlies the bore 49 in the sensor pin 46. The projection 50 carries a further polarised magnet 51 that is registered with the first magnet 48 and disposed so that a gap separates the two magnets 48, 51. The gap has a width C when no forces are exerted on the flexure bar 3. In practice, the width C is chosen in the range of 5 mm. Both the magnet 48 and 51 have a south pole S facing said gap. The magnetic fields of the two magnets 48, 51 are therefore disposed in opposition and cancel each other centrally of the gap.

0017Mounted on the end portion 66 of the sensor pin 46, a Hall Effect sensor 52 is arranged within the magnet gap. The sensor has signal leads 53 by means of which the Hall Effect voltage in the sensor 52 can be detected. The leads 53 are connected into an electric processing circuit 54, shown schematically in Figure 3, which controls the positioning of the arms 8 to raise and lower the lower arms 7 and thereby position the implement being towed by the tractor 2. The Hall Effect sensor 52 is imbedded in a synthetic or like material, indicated with reference numeral 68, which unaffectedly allows magnetic currents to flow therethrough. The material 68 is secured to the end portion 66 on top of the magnet 48 and the Hall Effect sensor 52 is arranged therein in such a manner that, in the rest position of the sensor pin 46, i.e. when no forces are exerted on the flexure bar 3, the Hall Effect sensor 52 is located perfectly centrally of the gap inbetween the magnets 48 and 51. Consequently, since the magnetic fields cancel each other centrally of the gap, as already mentioned, no output is generated by the Hall Effect sensor 52 when the sensor pin 46 is in the rest position relative to the flexure bar 3.

0018In a preferred embodiment, the position of the Hall Effect sensor 52 is chosen such that it coincides with the central axis 70 of the sensor pin 46. In other words, in the rest position, both magnets 48 and 41 are equidistantly spaced from said central axis 70.

0019In operation, the draft force exerted by the tractor in towing an implement connected to the lower arms 7 results in a deflection of the flexure bars 3. Within each flexure bar, the end portion 21 deflects so as to carry the polarised magnet 51 into a deflected position whereby the Hall Effect sensor 52 no longer is located centrally of the two magnets 48 and 51 and thus is out of a position in which the magnetic fields of the polarised magnets 48 and 51 cancel one another. Consequently, a resultant magnetic field is detected by the Hall Effect sensor 52. Referring to Figure 4, a graph is shown which relates the magnetic flux density (in Gauss) at the Hall sensor 52 to the travel (in mm) of the magnets 48 and 51 relative to one another. Figure 5 is a graph which relates the draft force applied to the flexure bar 3 to the output voltage from the Hall sensor 52.

0020It has been found that the sensor arrangement is capable of providing a good signal to noise ratio which does not require a preamplifier to amplify the output signal on the leads 53 before signal processing. Furthermore the sensor arrangement is insensitive to lateral movements, and provides a linear and sufficiently steep relationship between draft force and output voltage. The sensor pin 46 is fixedly secured at the root and does not bend as part of the flexure bar structure so providing a more easily engineered and assembled structure than hitherto with consequent benefits in regard to cost of manufacture and assembly.

0021Referring to Figure 2, the magnetic flux density T may be calculated approximately by the following formula:<maths id="math0001" num=""><math display="block"><mrow><msup><mrow><mtext>T = kFa</mtext></mrow><mrow><mtext>2</mtext></mrow></msup><mtext>(2a+3b)/6EJ</mtext></mrow></math><img file="EP0713637B1_D0001.tif" /></maths> and the output voltage signal V_out by the formula :<maths id="math0002" num=""><math display="block"><mrow><msub><mrow><mtext>V</mtext></mrow><mrow><mtext>out</mtext></mrow></msub><mtext> = hT</mtext></mrow></math><img file="EP0713637B1_D0002.tif" /></maths> wherein : <dl id="dl0001" compact="compact"><dt>T (Tesla or Gauss) =</dt><dd>magnetic flux density</dd><dt>F (N) =</dt><dd>force</dd><dt>a (mm) =</dt><dd>distance between the fixation point of the flexure bar 3 and the application point of the force F</dd><dt>b (mm) =</dt><dd>distance between the application point of the force F and the centre of the Hall Effect sensor 52</dd><dt>E (N/mm²) =</dt><dd>modulus of elasticity of the flexure bar 3</dd><dt>J (mm⁴) =</dt><dd>modulus of inertia of the flexure bar 3 at the application point of the force F</dd><dt>k, h =</dt><dd>constants</dd><dt>V_out =</dt><dd>voltage signal</dd></dl>

0022Turning now to Figure 6, a modification of the draft sensor apparatus of the previous Figures is shown. In Figure 6, the element 4 for supporting the flexure bar 3 has been adapted to provide a cavity of uniform diameter to receive the flexure bar 3 and has a yoke portion 55 to receive an outer extension 56 of the flexure bar 3. The ball joint 31 and lower arm 7 are thus disposed between the main body of the element 4 and the yoke portion 55. The flexure bar 3 is fixed to the outer face of the yoke portion 55 by means of a cover 57 which engages a waist 58 of the flexure bar 3 and is itself screwed to the yoke portion 55 by means of a screw 59. In this arrangement, the flexure bar 3 is fixedly secured relative to the element 4 as well in the axial as the radial direction in a simple but highly effective manner. A sensor pin 64 has a root portion which is fixedly secured into a cavity 65 of the flexure bar 3 and is of a diameter to fit the cavity. A projecting portion 60 of the sensor pin is of reduced diameter to give the required clearance between the pin and the flexure bar 3 which allows the flexure bar 3 to deflect relative to the sensor pin 64. The Hall Effect sensor 52 is disposed at the tip of the sensor pin 64 in the same general arrangement as previously described with reference to Figures 1 to 5. It will be appreciated that the embodiment of Figure 6 operates in a comparable manner as the arrangement of Figures 1 to 5 i.e. forces F on the flexure bar 3 cause the same to deflect relative to the stationary sensor pin 64 whereby the amount of deflection, and hence the magnitude of the force F, is detected by the Hall Effect sensor 52.