Patent Publication Number: US-2022228905-A1

Title: Suspendable scale for weighing a bundle and a forest machine

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of PCT/FI2020/050362 filed May 29, 2020, which claims benefit of Finnish Patent Application No. FI 20195450, filed May 29, 2019, each of which is incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a suspendable scale for weighing a bundle, which includes
         a first part for suspending the scale from a structure supporting the scale,   a second part for suspending an implement from the scale,   a third part combining the first part and the second part, arranged to change shape from the effect of the implement acting on the first part and the second part and caused by the bundle to be hoisted by it,       

     the first part, second part, and third part being at least partly radially nesting relative to one another,
         at least two, preferably 4-8 strain gauges connected to the third part for forming measurement data on the forces acting on it, on the basis of the deformation of the third part,   a wireless transmitter to send the measurement data to an operator,   a power supply for feeding an electric current to the strain gauges and the wireless transmitter.       

     The invention also relates to a forest machine. 
     BACKGROUND OF THE INVENTION 
     Traditionally in forestry and particularly in timber harvesting and timber sales, volume measurement, which is performed on the timber at the timber processing plant, has formed the established method of measurement. However, particularly from the viewpoint of logistics, it would be preferable to have the measurement performed at the earliest possible stage of the timber purchasing chain, most preferably immediately in connection with timber harvesting or the local transportation of timber. For this reason the measurement of load has moved more to a mass-based rate or fee to be paid, which for its part emphasizes the importance of the accurate weighing of individual bundles. 
     To weigh a bundle, a special scale is most typically fitted to the loader of a forwarder and used to weigh each bundle lifted. In this connection the term bundle refers to the individual transfer of a single or several trees/logs, for example from beside the forwarder to the forwarder&#39;s load space or vice versa, using a grab or other similar gripping element. Typically by summing the masses of the individual bundles the mass of the forwarder&#39;s load is obtained, and correspondingly by summing the masses of the loads the mass of the whole felling area&#39;s timber accumulation by timber grades is obtained. Because generally the measurement made with a scale is the only weighing in the entire operations chain, the result of the weighing must correspond with an acceptable accuracy to the real mass of the felled and transported bundles. The buyer and seller can then trust the correctness of the weighing. Above a scale is described as part of the weighing system, which is fitted to a loader acting as a hoist, particularly in the case of a forwarder or timber truck. The hoist can also be, for example, a set of hoisting booms, by which any bulk or piece goods can be moved, though the advantages of the solution according to the invention are emphasized in a weighing system, in which the special features of the material being weighed and the loading process itself cause deviation and inaccuracy in the weighing result. 
     Generally scales are known from the prior art, to which power is brought through the forest machine&#39;s set of hoisting booms, using the leads of the hoisting booms. Such a solution is, however, liable to faults, as in a forest machine the scale is subject to vibration and possibly also impacts, which gradually stress and damage the leads, finally cutting off the power supply to the scale. Damage to the power supply makes the scale useless. 
     From the prior art publication EP 3409635 A is known, which discloses, for example, a scale intended for forest machines is suspended from a supporting structure, i.e. the hoisting booms of a forest machine, and from which scale a selected implement is suspended, preferably a grab for gripping trees. The scale include three parts located radially relative to each other, which form the body of the scale. The first of the said parts is closest to the centre of the scale to suspend the scale, for example, from a rotation device suspended at the end of the set of hoisting booms, the second part surrounds the first part radially to suspend the implement from the scale, and the third part joins the first and second parts to each other radially. In addition, the scale includes strain gauges to measure the deformation of the third part to measure the forces travelling through the scale and thus to determine the bundle&#39;s mass, a wireless transmitter to send the measurement data produced by the strain gauges to a receiver, and a power supply to send electricity to the strain gauge and the wireless transmitter. 
     The measurement of the mass is based on the measurement of the elongation in the strain gauges in the aforesaid scale. When elongation occurs, the strain gauges&#39; resistance in the measurement circuit changes, when the strain gauge&#39;s wire changes shape. Strain gauges can be made from different materials. Typically a strain gauge is made from a very thin conductor or membrane and its resistivity is 30-30 000 Ohm. When the strain gauge is loaded with a maximum load, its resistance changes by only a fraction of a percent of its resistivity. In practice, such a small change in resistance cannot be measured, so that in the case of strain gauges a so-called Wheatstone-bridge circuit is needed. 
     The wireless transmitter of publication EP 3409635 A is arranged as a separate unit on top of the scale, so that it does not disturb the deformation taking place in the third part of the scale. On the other hand, the location of the wireless transmitter on top of the scale facilitates the passage of the wireless signal from the wireless transmitter on to the receiver. In addition, it is easy to take electrical leads to a transmitter fitted on top of the scale, to feed current. 
     However, a drawback of the scale of the EP publication cited above is that a transmitter fitted on top of the scale is exposed to impacts and takes up space on top of the scale. 
     SUMMARY OF THE INVENTION 
     The invention is intended to create a suspendable scale for weighing bundles that is more reliable than suspendable scales. The present invention is characterized by a suspendable scale for weighing a bundle comprising a first part for suspending the scale from a structure supporting the scale, a second part for suspending an implement from the scale, a third part joining the first part and second part, arranged to change shape from an effect of the implement acting on the first part and second part and caused by a bundle to be hoisted by the implement, the first part, second part, and third part being at least partly radially nesting relative to one another, at least two strain gauges connected to the third part to form measurement data on forces acting on each strain gauge, on basis of deformation of the third part, a wireless transmitter for sending the measurement data to an operator, a self-powered power supply for feeding an electric current to the strain gauges and the wireless transmitter, a first space formed in the scale to integrate the wireless transmitter inside the scale and a second space to integrate the power supply inside the scale. 
     The scale according to the invention can be implemented without leads being taken from the structure supporting the scale, as the transmitter is wireless and the power supply is self-powered. The wireless transmitter and power supply fitted in the first and second spaces are shielded from impacts and do not cause protrusions in the scale. The scale can then be made lower and narrower than scales of the prior art, as the wireless transmitter does not need space on top of the scale. It is surprising that the wireless transmitter can be placed inside the scale, when the passage of wireless signals to the wireless transmitter is hindered. In connection with the scale according to the invention it has been observed that the structure supporting the scale surprisingly reflects wireless signals, thus permitting the use of a wireless transmitter and its placing inside the scale. In other words, the first space and second space are formed in the scale in such a way that neither the transmitter nor the power supply cause a protrusion in the scale&#39;s support surface. It is also surprising that the first and second spaces, which break the scale&#39;s homogenous structure in the part in which the strain gauges are placed, can be located in the scale, the deflections of which are measured to measure the weight of the bundle. 
     In this context, the term self-powered power supply refers to the fact that the power supply is an energy-storing power unit, such as a battery, or accumulator, or similar (powercell). 
     Preferably the first and second spaces are separate spaces from each other. The spaces can then be located symmetrically on either side of the scale, so that the spaces will not distort the deformation of the third part thus weakening the scale&#39;s measurement accuracy. 
     In this context, the term the self-powered nature of the power supply refers to the external energy not being brought to the power supply along leads, for example, from the forest machine, but instead to the power supply being a battery or accumulator, which stores energy in the form of electricity. 
     Preferably the first part, the second part, and the third part are planar discs, of which the first part is fitted radially inside the second part, while the third part joins the first and second parts inside each other radially. Thanks to the radial construction the scale&#39;s structure is symmetrical, thus improving the scale&#39;s measurement accuracy. In other words the first part, second part, and third part are each planar pieces, in each of which is a centre hole, and the first part is radially innermost in the scale, the second part outermost, with the third radially between them. 
     Preferably the first part, second part, and third part are radially inside each other on essentially the same plane, so that the scale&#39;s structure can be quite low and all forces travel through the strain gauges. 
     The scale can include a total of four or eight strain gauges attached to the third part, which strain gauges are located on a first segment running through the centre point of the first part, symmetrically on both sides of the centre point of the first part, in such a way that on each side of the first part the number of strain gauges is half of the total number. When measuring the elongation on both sides relative to the first segment, the measurement can take account of the forces acting eccentrically on the scale, and which appear, for example, when weighing while the bundle is moving. 
     The strain gauges are preferably attached to the lower surface of the third part in the operating attitude of the scale. The strain gauges will then be protected from impacts and external detrimental factors. 
     The first space and second space are preferably located relative to the first segment symmetrically on both sides of a second segment, which runs through the centre point of the first part and is perpendicular to the first segment. In other words, the strain gauges, the first space, and the second space are located alternately relative to each other, so that each strain gauge is at the end of a 90° sector from the first space and the second space. The strain gauges are then located symmetrically relative to the spaces and the first and second spaces do not distort the forces acting on the strain gauges. 
     Due to the symmetry of the scale, the first space and second space are advantageously both equidistant from the strain-gauge sensors located at the ends of the first segment. Thus, it is easiest to take the first space and second space into account in the scale&#39;s computation. 
     The third part is preferably at least partly thinner than the first and second parts in a direction perpendicular to the plane of the scale, and the third part includes two annular grooves at a distance from each other radially, to increase the deflection of the third part. The use of annular grooves emphasizes the deformation of the third part at the annular grooves, thus facilitating the detection of deformations by the strain gauges. In other words, with the aid of the annular grooving made in the third part the measurement accuracy of the scale can be improved, while scarcely weakening the strength of the structure at all. At the annular grooves, the scale&#39;s thickness in the direction perpendicular to the scale&#39;s plane is less than elsewhere in the third part. 
     In the scale, the strain-gauges are preferably essentially horizontal when the scale is in its operating attitude. The scale can then be implemented as a low structure. 
     In this connection, reference to the direction of the scale&#39;s plane means the scale&#39;s radial direction, or its horizontal direction in the operating attitude. 
     At least one strain gauge is fitted to the third part on the opposite side of the third part relative to the annular grooves, aligned with the annular grooves on both sides of the first part, to measure the elongation at the relevant point in the third part. Measured at each annular groove, the deformation of the third part is greater than elsewhere over the third part and thus the deformation at various parts of the third part can be measured accurately, thus facilitating the measurement of the precise mass of the bundle. 
     The annular grooves are preferably formed in the upper surface of the third part. They then facilitate the deflection of the third part away from the fourth part in the direction of the loading. 
     In addition, in connection with the wireless transmitter there is preferably a 3D acceleration sensor to determine the attitude data of the scale for the compensation of the weighing of the bundle. The use of a 3D acceleration sensor allows an accurate weighing result to be obtained using strain gauges only in the first segment, making the construction of the scale simple. With the aid of a 3D acceleration sensor, it is possible to decide the implement&#39;s attitude and its effect of the weighing. 
     The 3D acceleration sensor can be an inertia measurement unit. With the aid of an inertia measurement unit it is possible not only to determine the accelerations acting on the scale but also the scale&#39;s state of motion, thus improving the accuracy of the weighing. 
     According to one embodiment, the strain gauges are located only on the first segment. The scale&#39;s construction can then be very simple. 
     The scale preferably includes, in addition, a discoid fourth part fitted between the second part and the implement arranged to be attached to the scale, to receive the torsional moments caused by the bundle hoisted by it. Thanks to the fourth part, the torsions acting on the implement are not able to transfer directly to the scale&#39;s second part supporting the implement. This in turn improves the scale&#39;s measurement accuracy. 
     The fourth part&#39;s diameter can be essentially the same as the second part&#39;s diameter. The fourth part then supports the second part optimally over the same diameter and does not cause an excess protrusion or weight in the scale. 
     A first thinning is preferably formed in the third part to increase the deflection of the third part. Thus, the deflection is emphasized and through it the detection by the strain gauges of deviations is facilitated. In addition, the thinning lightens the scale&#39;s total weight. 
     The scale preferably includes a second thinning formed in the fourth part in alignment with the first thinning to form a third space inside the scale for the strain gauges and their leads. 
     According to one embodiment, the strain gauges are full-bridge connected to compensate for temperature variations. Temperature variations do not then cause disturbances in the measurement and the measurement is therefore more accurate. The term full-bridge connection refers to a Wheatstone-bridge circuit, in which four strain gauges form a bridge circuit, i.e. there is no fixed resistance in the circuit, as there is in a half or quarter-bridge circuit. 
     The wireless transmitter is preferably a Wi-Fi transmitter or more preferably a Bluetooth transmitter. It is surprising that wireless transmission technology can be used in a scale located under a rotation device or directly under the set of hoisting booms, the rotation device being at least a partial obstacle to the passage of the signal. In this connection it has been surprisingly observed that, for example, a forest machine&#39;s hoisting boom and rotation device actually effectively reflect a signal sent by a wireless transmitter. The use of a Bluetooth transmitter is especially advantageous, as its power consumption is small, as is its range. The use of Bluetooth has the additional advantage that it is easily compatible with the devices of many different manufacturers and the scale&#39;s data can be read or checked with the aid of, for example, a smart phone or separate reading device. 
     The scale preferably includes an upper surface facing towards the structure supporting the scale in the scale&#39;s operating attitude and a lower surface facing towards the implement suspended from the scale, in which the first space and second space are formed in the scale&#39;s upper surface. Located in the upper surface, the wireless transmitter&#39;s signal has a nearly unobstructed route forward from the wireless transmitter, whereas the fourth part is preferably attached to the scale&#39;s lower surface to receive the torsions of the implement, which would be an obstacle to a wireless signal. 
     A first cover of the first space and a second cover of the second space are preferably parallel to the scale&#39;s upper surface. This facilitates the opening of the covers and also permits, in the case of space, a low structure, suitable for use in a low scale according to the invention. 
     The first space and second space can be formed in the scale&#39;s second part or third part. The scale&#39;s first part suspended from the structure supporting it remains under, for example, the rotation device, whereas the upper surfaces of the second and third parts remain free, permitting free access to the first space and second space, for example for maintenance operations. 
     The strain gauges are preferably located only in the first segment and the scale includes a 3D acceleration sensor to form the scale&#39;s attitude data to compensate for the scale&#39;s attitude when weighing a bundle. Thus, the scale according to the invention can be implemented quite simply, as the strain gauges measure elongation only on a single measurement line. 
     There is preferably an opening in the centre of the first part, forming a route for taking leads to the implement through the scale from the structure supporting the scale. Thanks to the opening, the scale can be used together with completely rotating rotation devices. The leads taken through the opening are also inside the scale and safe from impacts. 
     According to one embodiment, the scale includes an outlet opening with a moisture-penetrable plug fitted to it for equalizing moisture and pressure with the environment. Thus, for example, excess pressure, which could damage the sealing of the scale, cannot form inside the scale. 
     The scale&#39;s 10 thickness is preferably 40-90 mm, preferably 50-70 mm. The scale is then low in structure, which facilitates, for example, placing the implement in the transport attitude in the load spaces of forest machines, as the height of the totality formed by the implement, scale, and rotation device does not become exceptionally great. 
     The scale&#39;s thickness can be 10-20%, preferably 17-25% of the scale&#39;s diameter. 
     The longitudinal direction of the strain gauges is preferably in the scale&#39;s radial direction. The strain gauges are then easy to implement and the scale&#39;s structure can be made low. 
     The first space and second space are preferably casing structures. A casing structure is easy to insulate. 
     The first space and second space are preferably identical in their plane-direction surface area and their shape, so that their effect on the scale&#39;s deflection is symmetrical. 
     The forest machine according to the invention is characterized by a forest machine comprising a set of hoisting booms, a rotation device suspended from an end of the set of hoisting booms, an implement suspended from the rotation device to hoist a bundle, as well as a scale to weigh the bundle, the scale comprising a first part for suspending the scale from the rotation device supporting the scale, a second part for suspending the implement from the scale, a third part joining the first part and second part, arranged to deflect from effect of the implement acting on the first part and second part and caused by the bundle to be hoisted by the implement, the first part, second part, and third part being at least partly radially nesting relative to one another, at least two strain gauges connected to the third part to form measurement data on the deflections of the third part, a wireless transmitter for sending the measurement data to an operator, a self-powered power supply for feeding an electric current to the strain gauges and to the wireless transmitter, a first space formed in the scale to integrate the wireless transmitter inside the scale and a second space to integrate the power supply inside the scale. 
     Good reliability is achieved with the aid of the arrangement according to the invention, as a wired connection, which is exposed to impacts and breaks in use, need not be formed to the scale. In addition, fitting the wireless transmitter and the self-powered power supply in the first and second spaces protects the said components from external impacts and leaves the scale&#39;s upper surface free. 
     Preferably the strain gauges are located relative to a segment running through the centre point of the first part, symmetrically on both sides of the centre point of the first part on the first segment joined to the third part, and the scale is attached to the implement, in such a way that pivot shafts of arms, belonging to the implement gripping an object to be hoisted, are parallel to the first segment. Then, for example, the longitudinal directions of the trees to be hoisted lie parallel to the first segment, so that the weighing of the bundle is accurate even when hoisting elongated bundles, for example when hoisting bundles of logs with the aid of a grab acting as the implement. 
     Preferably the arrangement is fitted in connection with a forest machine and the implement is a harvester head, or more preferably a grab for handling trees. The use of the arrangement according to the invention is particularly advantageous, as in forest machines various external loadings act on the scale to a considerably greater extent than, for example, in gantry cranes in industrial plants. 
     Preferably an opening is formed in the centre of the first part, forming a route for taking hoses through the scale from the structure supporting the scale to the implement. Thanks to the opening, the scale can be used together with completely rotating rotation devices. 
     Preferably a completely rotating rotation device is used in the arrangement according to the invention, and in the arrangement at least one set of leads is taken through the rotation device and the scale to take electricity or a compressed medium from the hoisting booms to the implement. 
     In addition, the arrangement preferably includes a wireless transmitter to send measurement data to be received. Thus, measurement data can be received, for example, in a forest machine, where it can be transferred to be processed in the control computer of the forest machine. 
     The wireless receiver preferably comprises a link to a data-transfer bus. Thus, the wireless receiver can be easily connected, for example, to existing systems in the forest machine. The link is preferably a CAN bus connection. 
     According to one embodiment the arrangement also includes a computation unit to calculate the mass of the bundle on the basis of the strain gauges&#39; measurement data, which computation unit is preferably located separately from the scale. Thus, calculation can be performed at a place where power is easily available along cables and the power consumption of the computation unit is not a problem. 
     The arrangement preferably includes first attachment means to attach the scale from the first part to the structure supporting it, second attachment means to attach the fourth part to the second part, and third attachment means to attach the implement to the fourth part. 
     Preferably in the arrangement a computation unit, which is preferably at least part of the forest machine&#39;s control computer, is used for the final calculation of the bundle&#39;s mass. The computation unit can be implemented by a device configuration that includes a microcontroller and its memories. The computation unit can also be integrated in the receiver. 
     According to one embodiment the arrangement includes a rotation sensor of the hoisting booms to determine the position of the hoisting booms. With the aid of the rotation sensor it can be seen when it is best to perform weighing, i.e. is the bundle to be weighed, for example a bundle of trees, in the air, is the tree bundle dragging on the ground, or when the number of factors distorting the weighing result is at a minimum. With the aid of the rotation sensor the moment when weighing is performed can be automatically detected. The operator need not then always enter the moment of weighing, instead it is detected automatically without slowing the operator&#39;s work. The rotation sensor is preferably wireless. 
     Thanks to low-energy electronics and Bluetooth technology the battery&#39;s service life can exceed even  10  years if the weighing or measuring state is not used. This facilitates, for instance, the storage of the scale and reduces the detaching of the self-powered power source. The scale sends data on itself at regular intervals, for example, data on the battery&#39;s voltage or individuation data on the scale, though measurement is not used. This facilitates system diagnostics and permits, for instance, the use of a smart phone equipped with Bluetooth as a diagnostics tool or computation unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following, the invention is described in detail with reference to the accompanying drawings showing some embodiments of the invention, in which 
         FIG. 1  shows a schematic image of a forest machine, which is fitted with the arrangement according to the invention comprising a scale according to the invention, 
         FIG. 2  shows an axonometric view of the scale according to the invention attached to a rotation device and an implement, 
         FIG. 3  shows a cross-section of the scale according to the invention, 
         FIG. 4 a    shows a cross-section of the scale according to the invention and a rotation device supporting it and an implement suspended from the scale, 
         FIG. 4 b    shows an enlarged cross-section of the scale according to the invention and a rotation device supporting it and an implement suspended from the scale, 
         FIG. 4 c    shows a cross-section of the scale according to the invention sectioned from a different direction to  FIGS. 4 a    and  4   b,    
         FIG. 5  shows a partly exploded axonometric view of the scale according to the invention, when the transmitter and power supply have been lifted out of the first and second spaces, 
         FIG. 6 a    shows an axonometric bottom view of the scale, seen without the scale&#39;s fourth part, 
         FIG. 6 b    shows the scale seen from directly below seen without the scale&#39;s fourth part, 
         FIG. 7  shows a schematic circuit diagram of the scale according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows the arrangement  100  according to the invention implemented in connection with a forest machine  106 . It should, however, be understood that the arrangement according to the invention can also be implemented in other environments than forest machines. Such other implementation environments of the arrangement according to the invention can be, for example, various overhead cranes in production plants, trucks&#39; hoists, harbour cranes, and similar. At its most advantageous, the arrangement and scale according to the invention are used, however, in connection with a forest machine, in which when hoisting a bundle the scale is also acted on by other than gravity acceleration components and in that way the weighing of the bundle is more demanding than usual. The implement suspended from the scale can be, for example, a forest machine&#39;s harvester head, but most preferably a grab intended to grip objects. 
     The forest machine  106  shown in  FIG. 1  can be, for example, a normal forwarder, which includes a load space  114  fitted on top of a chassis and a set of hoisting booms  102  pivoted to the chassis at one end. With the aid of the set of hoisting booms  102  the trunks of already felled trees can be lifted into the load space  114 , at the same time weighing the mass of each hoisted bundle. At one end  54  of the set of hoisting booms  102  there is preferably suspended a rotation device  104  while a scale  10  according to the invention is attached by its upper surface to the under surface of the rotation device  104 . The implement  18  attaches to the under surface  46  of the scale  10 . In this case the set of hoisting booms  102  acts as the structure  14  supporting the scale. The rotation device  104  is intended to permit the implement  18  to be rotated around an essentially vertical axis relative to one end  54  of the set of hoisting booms  102 . In the embodiment of  FIG. 1 , a grab  112  for lifting trees is used as the implement  18 . 
     Though in  FIG. 1  the scale  10  is attached between the rotation device  104  and the implement  18 , it should be understood that the scale according to the invention can also be situated between the set of hoisting booms and the rotation device. However, the scale is then a suspension scale, on which the weight of the rotation device also acts. 
     According to  FIG. 2 , the scale  10  includes a first part  12 , a second part  16 , and a third part  20 , which are preferably planar discs  32  on the same plane and radially nesting relative to each other. In addition, the scale  10  includes at least two strain gauges  22  shown in  FIGS. 6 a  and 6 b   , a wireless transmitter  24  shown in  FIG. 5 , a self-powered power supply  26 , as well as a first space  28  for the wireless transmitter  24  and a second space  30  for the power supply  26 . The scale&#39;s operation is based on measurement of the strain gauges, in which the preferably four or eight strain gauges situated in the third part form advantageously a full (or two full) Wheatstone-bridge circuits. The elongation of the strain gauges changes the bridge circuit&#39;s output voltage, from which change the force acting on the scale can be determined and through it the mass of the bundle calculated. The first part  12  is arranged to be attached to the structure  14  supporting the scale  10 , which is the rotation device  104  in the embodiments shown in  FIGS. 1-6   b . The first part  12  is surrounded by the second part  16  and these are joined together with the aid of the third part  20 . The first part  12 , second part  16 , and third part  20  form the scale&#39;s  10  body  33 . In the embodiment of  FIGS. 1 and 3-4   c  the implement  18  is attached to the scale&#39;s  10  second part  16  using a separate fourth part  42 . 
     The fourth part  42  acts preferably as an intermediate piece between the second part  16  and the implement  18  when attaching the implement  18  to the scale  10  according to  FIGS. 1 and 3-4   c . If deformations take place in the implement  18  when loading, the fourth part  42  receives these deformations, so that they do not pass directly to the scale&#39;s  10  body  33  and distort the measurement. For this purpose the fourth part  42  can be made a more rigid structure than the scale&#39;s  10  body  33 . Preferably the first part  12 , second part  16 , third part  20 , and fourth part  42  are all planar discs  32 , i.e. discoid and circular metal plates, in which there is preferably a centre hole. The circular structure conditions the forces acting on the scale to become symmetrical, so that in the operating attitude the scale&#39;s transverse forces do not create an effect of different magnitude on the measurement due to the scale&#39;s geometry. As an alternative to the circular structure the first part can also have some other symmetrical shape, such as a cross, when the second and third parts too will be shaped to correspond to the first part, surrounding the first part radially. The radially nesting disc formed by the first part, second part, and third part can also be called the scale&#39;s upper part and the fourth part the scale&#39;s under part. Similarly, the first part can be called the upper part&#39;s inner circle, the second part the upper part&#39;s outer circle, and the third part its intermediate circle. 
     Preferably there is a centre hole  48  according to  FIG. 4 c    in the centre of the scale&#39;s  10  body&#39;s  33  first part  12 , and also in the centre of the fourth part  42 , permitting the conductors, i.e. leads  96  and the pressure-medium lines to be fed through the scale from the rotation device to the implement. Thus, for example hydraulic oil can be moved from the forest machine along the set of hoisting booms, through the completely rotating rotation device and scale, as far as the implement, which utilizes the hydraulic oil to operate hydraulic operating devices. Being led through the scale the leads are better protected from impacts than when led externally over the scale. 
       FIGS. 3 and 4   a - 4   c  show the scale&#39;s  10  attachment in greater detail. Though  FIG. 4 a    does not show a section line, it should be understood that  FIG. 4 a   , like  FIG. 4 b   , shows a cross-section.  FIG. 4 c    does not show the third attachment means that attach the implement  18  to the fourth part  42 . 
     The scale  10  is attached from the body&#39;s  33  first part  12  to the structure  14  supporting the scale  10 , which is either the set of hoisting booms  102  of  FIG. 1  or preferably the rotation device  104  according to  FIGS. 4 a -4 c   . The attachment takes place with the aid of first attachment means. The first attachment means can include counter-threads  61  formed in the first part  12 , into which grub screws  60  equipped with threads are screwed and locked, for example with an adhesive. The screws  60  protrude from the upper surface  44  of the first part  12 . Correspondingly, the rotation device  104  can include as a part of the first attachment means openings  118  in its under part, through which the screws  60  are fitted. The scale  10  is preferably locked using the screws  60  to the openings  118  in the rotation device  104  (in  FIG. 4 c   ). 
     The scale&#39;s  10  body  33  attaches to the fourth part  42  from the second part  16  with the aid of second bolts  76  seen in  FIGS. 4 c , 6 a , and 6 b   . The second bolts  76  are part of the second attachment means. The second part  16  can include, in addition, second counter-threads  78  as second attachment means. The fourth part  42  includes in turn third openings  86  belonging to the second attachment means, through which the second bolts  76  are fitted and tightened in the second openings  78 , locking the fourth part  42  to the scale&#39;s  10  body&#39;s  33  second part  16 . 
     The locking of the fourth part  42  to the scale&#39;s  10  body  33  takes place only to the second part  16  according to  FIG. 4 c   , so that when the scale  10  is loaded the first part  12  can move in a direction perpendicular to the scale&#39;s  10  plane, i.e. in the operating attitude mainly in a vertical direction when the third part  20  deflects. The effect of the number of the second bolts on the structure&#39;s operation has been studied by FEM computation and it has been shown that there must be 14-32, preferably 18-24 second bolts on the ring formed by the second part to achieve sufficiently stable locking. Using a larger number of bolts will not bring significant additional value, but if there are less than 18 second bolts, the scale&#39;s operation is weakened, allowing in uneven loading the implement&#39;s torsions to be partly transferred to the scale&#39;s body through the structural elasticity of the fourth part. 
     The implement  18  is attached to the scale&#39;s  10  fourth part  42  with the aid of third bolts  72  acting as third attachment means, according to  FIG. 4 b   . The fourth part  42  also includes, as third attachment means, drill holes  62 , in which are third counter-threads for the third bolts  72 . The implement&#39;s  18  upper part includes at attachment flange  68 , in which are fourth openings  88  to fit the third bolts  72  through the attachment flange  68  and to lock the third bolts to the fourth part&#39;s  42  drill holes  62 . The attachment flange  68  also has preferably a second centre hole  74  for leads. 
     The scale&#39;s body is preferably formed by machining from a single cast piece. A first thinning  64 , visible in  FIGS. 2 and 4   a - 4   c , is preferably formed in the scale&#39;s  10  body  33  in its surface facing the fourth part  42 . With the aid of the first thinning  64  the deformation of the third part  20  when loading the scale can be emphasized. The fourth part  42  too preferably includes a second thinning  66 , which is intended to form together with the first thinning  64  a third space  80  inside the scale  10 , more specifically circumferentially between the scale&#39;s  10  body  33  and the fourth part  42 . The third space  80  is used to fit the transmitter  24  and power supply  26  inside the scale  10  and protected from impacts. In addition, the third space  80  acts as a space for the leads between the strain gauges  22 , wireless transmitter  24 , and power supply  26 , which are shown by the broken lines  87  in  FIG. 6 a   . The third space  80  is preferably sealed. The fourth part  42  can include, according to  FIG. 4 c   , an outlet opening  52  equipped with a plug  50  breathing in two directions, allowing moisture to exit from the third space  80 , but at the same time preventing water from entering the third space  80  from the outlet opening  52 . Such a moisture-permeable plug  50  can preferably be a plug utilizing a Goretex or similar membrane. The moisture-permeable plug seeks to keep the third space  80  at the same relative humidity as the third space&#39;s  80  external spaces. The plug also preferably acts as a pressure-equalization valve, keeping the third space  80  at the same pressure as that of the surrounding space. Large pressure differences, which could cause leaks, do not then act on the seals used in the scale. Moisture removal from the third space is important in order to avoid corrosion of the strain gauges, which destroys the strain gauges. The outlet opening  52  and plug  50  are preferably situated in a protected place, as far as possible from oil sprays due to pipe breaks and mechanical stresses. 
     The third part  20  preferably includes two annular grooves  38  at a distance d from each other radially, according to  FIG. 5 . The annular grooves  38  are like rings when the scale&#39;s  10  body  33  is a circular structure. The annular grooves  38  are preferably situated radially in the area of the third part  20 , close to the innermost and outermost edges of the first thinning  64 , according to  FIG. 2 , so that at the annular grooves  38  the third part  20  is vertically thinner than in the case of the web  70  remaining between the annular grooves  38 . Thanks to the first thinning  64 , the third part  20  is preferably thinner over its entire radial width than the first part  12  and the second part  16 , when the forces acting through the first part  12  and second part  16  through the third part  20  bend the third part  20  but not the first part  12  and second part  16 , which are essentially rigid. In this connection, the term essential rigid refers to the fact that the deflection of the first and second parts is less than 1% of the third part&#39;s deflection. When loaded, the third part&#39;s deflection can be in the order of 0.01-0.2 mm. The third part&#39;s  20  annular grooves  38  cause the third part&#39;s deflection to be focussed on a small distance in the third part  20  precisely at the annular grooves  38 . 
     Alternatively, instead of two grooves a single wider groove can also be used, but then a smaller portion of the third part&#39;s elongation comes at the groove and in that way detection of the elongation is more difficult. 
     The strain gauges  22  are preferably attached to the third part  20  and more specifically to the first thinning  64  according to  FIG. 6 a    aligned with the annular grooves, so that the strain gauges  22  measure the deformation in the third part  20  in the radial direction of the third part  20 . Thus, the strain gauges can be essentially the width of the annular grooves in the radial length of the third part. The distance of the annular grooves from each other, i.e. the distance d, can be 10-60 mm, preferably 20-40 mm. The width of the annular grooves can be 5-25 mm, preferably 15-20 mm and their depth 5-15 mm, preferably 8-12 mm. 
     The strain gauges  22  are preferably attached to the under surface of the third part  20  in the scale&#39;s  10  operating attitude, radially to the first thinning  64 , according to  FIG. 6 b   . More specifically, the strain gauges  22  are preferably glued to the surface of the third part  20 . So that the strain gauges can measure the deformation of the third part reliably, the under surface of the third part must be machined to a sufficient smoothness, for example by face grinding. To improve the adhesion of the strain gauges, the face-ground surface is further roughened, for example manually, using 150-400 sandpaper to improve the adhesive&#39;s adhesion. 
     The strain gauges can be according to the prior art, such as, for example, the strain gauges marketed by the German manufacturer Hottinger Baldwin Messtechnik GmbH:n (HBM) under the product name HBM 1-LY41-10/120-1000. The strain gauges measure the elongation of the third part with the aid of a lattice inside each strain gauge. The length of the strain gauges&#39; lattice can be 4-20 mm, preferably 6-10 mm. When the lattice stretches, its resistance changes, when the output voltage in the bridge circuit changes. The value of the elongation experienced by the strain gauge is the mean value of the elongation under its measurement lattice. On the other hand, strain gauges can also be used to measure the compression of the lattice at another point in the third part. Maximizing the elongation of the area under the lattice also maximizes the change in its resistance. 
     By machining the annular grooves at optimized places in the third part, the elongation is maximized under the strain gauges without weakening the scale&#39;s structural strength. By placing the strain gauges at the annular grooves the greatest elongation is aligned with the strain gauges, thus improving the measurement accuracy. In a scale, in which there are no annular grooves in the third part, the elongation of the third part is distributed evenly over the entire distance of the third part and is then smaller at the strain gauges and more difficult to detect. 
     According to  FIGS. 6 a  and 6 b   , the strain gauges  22  are preferably situated in the first thinning  64  in such a way that the strain gauges are distributed on both sides of the centre point on a first segment  34  drawn through the centre point of the scale  10 . The strain gauges  22  then measure the deformations of the third part  20  symmetrically on both sides of the centre point of the first segment  34 . The strain gauges are connected according to the prior art by a Wheatstone-bridge circuit, a change in voltage is determined from a change in resistance and through it the deformation of the third part and then the bundle&#39;s mass. When using four strain gauges the strain gauges can be full-bridge connected, when a temperature-compensated circuit will be achieved which is not affected by temperature changes. When using two strain gauges a half-bridge circuit can be used, but then the temperature must be compensated otherwise. In the embodiment of the scale according to the invention of  FIGS. 6 a  and 6 b    there are only two strain gauges  22 , but preferably there are two strain gauges on each side of the first segment relative to the centre point, each on one annular groove. 
     Thanks to the annular third part  20  the third part&#39;s  20  deflection is quite symmetrical over the entire third part  20  at any point at all in the scale&#39;s sector. So that the deflection would be as symmetrical as possible, the first space  28  and second space  30  are preferably formed on the second segment  36  according to  FIG. 6 b   , when they are a 90° sector away from the strain gauges  22  symmetrically on both sides of the second segment  36  relative to the centre point according to  FIGS. 6 a  and 6 b   . The first space  28  and second space  30  are preferably also identical in plane-direction surface area and shape, when their effect on the deflection of the third part  20  will be symmetrical. The first space  28  is preferably shallower in depth, in the direction perpendicular to the plane of the scale&#39;s  10  body  33 , than the second space  30 . This is due to the wireless transmitter  24  situated in the first space  28  being lower in height than the self-powered power supply  26  in the second space  30 . 
     According to  FIG. 5 , the first space  28  and second space  30  preferably form a first opening  90  for the first space  28  and a second opening  92  for the second space  30  in the second part  16  or third part  20  or both. In this case, the first opening  90  and second opening  92  penetrate the first part, extending to the third space  80  formed between the scale&#39;s body  33  and the fourth part  42 , which is shown in  FIG. 2 . The first space  28  is not only a first opening  90  but also a first casing  82  shown in  FIGS. 6 a  and 6 b   , inside which the wireless transmitter  24  is fitted. The first casing  82  encloses the wireless transmitter  24  thus preventing water and moisture from coming in contact with the wireless transmitter  24 . Further, the first space  28  includes a first cover  56 , shown in  FIGS. 5 , which closes the first space  28  against the first casing  82  to form a hermetic space. The cover  56  closing the first space  28  is preferably of plastic, when it will hardly neither disturb nor attenuate the wireless transmitter&#39;s  24  transmitted signals. The first cover is preferably made from POM plastic, which is penetrated extremely well by the wireless transmitter&#39;s signal. In addition, the wireless transmitter itself is located close to the scale&#39;s upper surface, which also improves the passage of the wireless transmitter&#39;s signal. 
     If the wireless transmitter is extremely low, in some cases the first space can be formed entirely onside the scale&#39;s body, when an opening penetrating the scale&#39;s body is not formed. In this case the first casing is not needed, but only a recess made in the scale&#39;s body. 
     Correspondingly the second space  30  includes, in addition to the second opening  92 , a second casing  84  shown in  FIGS. 6 a    inside which the power supply  26  is fitted. The second casing  84  encloses the power supply  26 , thus preventing water and moisture from coming in contact with the power supply  26 . Further, the second space  30  includes a second cover  58  seen in  FIG. 5 , which closes the second space  30  against the second casing  84  to form a hermetic space. Both the first cover  56  and the second cover  58  can be detachably locked in place with the aid of separate screws or bolts  59 . The use of detachable covers permits the servicing when needed of the wireless transmitter and the electronics in it, and of the power supply. The second cover  58  too is preferably made from POM plastic. The first cover and second cover are made from plastic also for the reason that metallic covers are relatively rigid, thus affecting the scale&#39;s reading. A plastic cover&#39;s effect on rigidity is small, so that the cover&#39;s effect on the scale&#39;s measurement result is negligible. The first and second casings too are preferably made from plastic, so that they bend when the third part bends and do not interfere with the measurement. 
     The scale  10  according to the invention preferably also includes the 3D acceleration sensor  40  shown in  FIGS. 5 and 7  for forming the scale&#39;s  10  attitude data and to compensate the scale&#39;s weight measurement on the basis of the attitude data. The 3D acceleration sensor  40  is preferably situated in the first space  28  together with the wireless transmitter  24 , on the electronics card  52  shown in  FIG. 5 . The 3D acceleration sensor can measure the scale&#39;s attitude relative to a gravity vector. Such measurement does not, however, allow for the scale&#39;s state of motion. More preferably the 3D acceleration sensor is an IMU (Inertia Measurement Unit) sensor, i.e. an inertia measurement unit, which measures both 3D acceleration and angular acceleration and thus also detects the scale&#39;s state of motion. The tilt data obtained with an IMU sensor is thus more comprehensive than the data obtained with a 3D acceleration sensor. 
     The wireless transmitter  24  and the 3D acceleration sensor  40  are preferably fitted to the same circuit card, which is situated in the first space  28 . Preferably the first space&#39;s  28  first casing  82  is also filled with a protective mass after installation of the electronics card, thus protecting the electronics card from moisture. When using a 3D acceleration sensor, the scale&#39;s measurement with strain gauges  22  can be performed using strain gauges  22  placed on a single straight line. The third part&#39;s deformation is then measured using strain gauges in a single measurement bridge and the possible effect of the scale&#39;s attitude on the third part&#39;s deformation is taken into account computationally. Thus, the scale can be implemented reliably with fewer strain gauges than without a 3D acceleration sensor. In implementation without a 3D acceleration sensor strain gauges would have to be installed on at least two straight lines, if the bundle is weighed while moving. In calculation, the acceleration measured by the 3D acceleration sensor can be taken into account for simply using the basic physical formula F=ma. 
     According to  FIG. 7 , the electronics card  52  preferably includes a microcontroller  126 , a wireless transmitter  24  for data transfer, a 24-bit AD converter  128  to convert the strain gauges&#39;  22  analog signal into digital form, a 3D acceleration sensor  40  to detect the scale&#39;s tilt and preferably also state of motion, and a current-control circuit  130  to feed current to the electronics card&#39;s  52  components and to the strain gauges  22 . 
     The arrangement according to the invention preferably includes, in addition, the wireless receiver  120  shown in  FIG. 7 , which is located, for example, in connection with the cab, where it can connect with the forest machine&#39;s data-transfer bus  122 . The scale can preferably operate in such a way that measurement data are sent to the wireless receiver  120  from the scale using, for example, a Bluetooth connection with the aid of the wireless transmitter  24 . The values are not calculated by the scale  10  using the self-powered power supply  26 , but preferably by the forest machine&#39;s control computer  124 , to which the measurement data are transferred from the wireless receiver  120 , for example using the forest machine&#39;s data bus  122 . This reduces the scale&#39;s power consumption. The data bus is preferably a CAN bus. 
     The voltage of the strain gauges&#39;  22  measurement bridge has its own voltage regulator, i.e. current-control circuit  130 , but the same measurement bridge voltage goes to the 24-bit AD converter  128 . Precise measurement requires data for the AD converter  128  on the measurement bridge&#39;s voltage or its change. Therefore in the Wheatstone circuit the measurement bridge&#39;s voltage connects as a reference to the AD converter. The same also applies to the ground connection marked by GND in  FIG. 7 . 
     The measurement data to be sent can be all the values to be measured in the scale, for example, the battery voltage, the AD converter&#39;s values or the strain-gauge circuits&#39; values or both, temperature, the tilt sensors values in the X, Y, and Z directions, and in addition time-stamp data and “field-strength data” RSSI. Using the identifier data sent with the measurement data, a protected connection is formed between the wireless transmitter and the wireless receiver. The computation unit used to calculate the bundle&#39;s final mass, which is preferably at least part of the forest machine&#39;s control computer, can be implemented with a device configuration that includes a 16-124-bit processor, a 128-1024-kB Flash memory, and a 16-128-kB RAM central memory. The computation unit can also be integrated in the wireless receiver. 
     The wireless transmitter used is preferably a low-energy Bluetooth module, for example a Bluetooth 4.2 module or a newer version, which has an internal antenna and which used the common Bluetooth 2.4-GHz frequency. The low-energy wireless transmitter permits, for its part, low energy consumption when transmitting the measurement data. The scale&#39;s self-powered power supply can then be, for example, a Saft LS 3.6-V 17-Ah Li-SoCl 2  battery, which has a service life of at least 6 months, but possibly more than a year. The battery can be easily changed by opening the second space&#39;s second cover. The wireless transmitter&#39;s transmission range is preferably 10-20 m, preferably 10-15 m. By placing the scale under the rotation device it has been observed surprisingly that the rotation device&#39;s structure acts as an element reflecting the wireless transmitter&#39;s signal. 
     The scale&#39;s outer diameter can be 200-500 mm, preferably 300-400 mm and its thickness 40-90 mm, preferably 50-70 mm. The centre hole&#39;s diameter can be, for example, 100-150 mm or 25-40% of the scale&#39;s diameter. The material used in the scale&#39;s body and fourth part is preferably stainless steel, which can be for example Hardox 900 grade. The scale&#39;s body in the third part can be 12-mm thick. 
     In terms of the operation of the scale and arrangement according to the invention it is important that the scale is installed in connection with the grab acting as the implement in such a way that the strain gauges are in the first segment essentially parallel to the trees lifted by the grab. In other words, the radial direction of the strain gauges is essentially parallel to the pivot arms of the grab&#39;s arms.