System and method for calibrating an acceleration sensor using a payload system

Abstract

The present invention relates to a system and method for calibrating an acceleration sensor using a payload system. A method for calibrating an acceleration sensor coupled to a machine may include: obtaining a first acceleration measurement value from the acceleration sensor at a first position of a connecting rod of the machine, and obtaining a first force value indicating a first force on the connecting rod from an input; obtaining a second acceleration measurement value from the acceleration sensor at a second position of the connecting rod of the machine, and obtaining a second force value indicating a second force on the connecting rod from an input; determining a rotational offset calibration value of the acceleration sensor based on the first acceleration measurement value, the first force value, the second acceleration measurement value, and the second force value; and calibrating the acceleration sensor using the rotational offset calibration value.

Description

Technical Field

[0001] The present invention relates generally to mobile machines and, more particularly, to systems and methods for calibrating sensors of such machines. Background Art

[0002] Industrial machinery, such as heavy equipment. Heavy equipment may have various sensors coupled thereto to measure various parameters associated with the operation of the heavy equipment. For example, heavy equipment may utilize an inertial measurement unit ("IMU"), which may include sensors such as accelerometers, gyroscopes, and magnetometers. An IMU coupled to a machine may measure characteristics such as acceleration and angular velocity of the machine. During machine manufacturing, the IMU or other sensor may be coupled to the machine using, for example, bolts or other mechanisms. Although the IMU and equipment may include pre-drilled holes for accommodating bolts or other mechanisms that are intended to produce a consistent installation of the IMU on the machine, an IMU mounted within manufacturing tolerances may have slightly different alignments. For example, the IMU may be slightly tilted relative to the machine. In particular, the IMU may be misaligned about the z-axis because the z-axis (pitch) orientation of the IMU may not be well controlled during the manufacturing process.

[0003] Even a small deflection of an IMU may affect the usefulness of measurements obtained by the IMU. In particular, if the IMU is misaligned, the gravity vector measured by the IMU may be slightly angled from the actual gravity direction. For example, if the IMU is misaligned about the z-axis, the gravity vector may be rotationally offset from the actual gravity direction.

[0004] International Patent Publication No. WO2006098654 A1, filed by Actronic Limited and published on September 21, 2006, describes a weighing device for measuring the payload of a load lifting (excavator) machine. The disclosure describes the use of an accelerometer on the boom of the machine. As described in the disclosure, the excavator operates on level ground and the boom is operated through at least six lifts with a known load in the bucket of the machine. Information from the accelerometer and other sensors is used to calibrate the weighing device. The disclosure describes the use of the vertical acceleration measured by the accelerometer and the forward acceleration of the accelerometer to calibrate the vertical vector. However, the Actronic disclosure does not describe taking into account any offset in the accelerometer installation. Instead, the vertical acceleration measured by the accelerometer is used without any such calibration. The system of the present invention can solve one or more of the above-mentioned problems and / or other problems in the art. However, the scope of the present invention is defined by the appended claims, rather than by the ability to solve any specific problem. Summary of the invention

[0005] In one aspect, a method for calibrating an accelerometer coupled to a machine may include: obtaining, at a first position of a link of the machine, a first acceleration measurement from the accelerometer and obtaining, from an input, a first force value indicating a first force on the link; obtaining, at a second position of the link of the machine, a second acceleration measurement from the accelerometer and obtaining, from the input, a second force value indicating a second force on the link; determining a rotational offset calibration value of the accelerometer based on the first acceleration measurement, the first force value, the second acceleration measurement, and the second force value; and calibrating the accelerometer using the rotational offset calibration value.

[0006] On the other hand, a method for calibrating an acceleration sensor connected to a machine may include: obtaining a first measurement value from at least one sensor at a first position of a link of the machine, wherein the link supports a load having a first fixed mass, and wherein the first measurement value is related to the mass of the load; obtaining a second measurement value from at least one sensor at a second position of the link of the machine, wherein the link supports the load, and wherein the second measurement value is related to the mass of the load; and determining a rotational offset value of the acceleration sensor using the first measurement value and the second measurement value.

[0007] In yet another aspect, a system for calibrating a machine may include at least one acceleration sensor coupled to the machine; a force sensor configured to measure at least one force on a link of the machine; and a controller configured to: obtain a first acceleration measurement from the acceleration sensor at a first position of the link and obtain a first value indicating a first force on the link from an input; obtain a second acceleration measurement from the acceleration sensor at a second position of the link and obtain a second value indicating a second force on the link from the input; determine a rotational offset value of the acceleration sensor based on the first acceleration measurement, the first value, the second acceleration measurement, and the second value; and calibrate the acceleration sensor using the rotational offset value. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 is a side view of an exemplary machine in a first configuration and a second configuration.

[0009] Figure 2 is used for Figure 1 A block diagram of an exemplary control system for a machine.

[0010] Figure 3 It is shown that the use Figure 2 An exemplary control system is used to determine Figure 1 Flowchart of an exemplary control sequence for a rotational offset of a sensor of a machine. DETAILED DESCRIPTION

[0011] Both the foregoing general description and the following detailed description are exemplary and explanatory only and are not limiting of the features claimed. As used herein, the terms "comprises," "comprising," "having," "including," or other variations thereof are intended to cover a non-exclusive inclusion such that a process, method, article, or apparatus that includes a list of elements includes not only those elements but may also include other elements not expressly listed or inherent to such process, method, article, or apparatus.

[0012] In the present invention, relative terms, such as "about", "substantially", and "approximately" are used to indicate a possible variation of ±10% in the stated value. Although the present invention is described with reference to a wheel loader, this is exemplary only. In general, the present invention is applicable to any type of heavy equipment / machine, such as any type of truck, loader, load dumper, haul bulldozer, etc. Although the present invention refers to exemplary placement of sensors, such sensors may be placed in other appropriate locations consistent with the present invention. Although the present invention specifically refers to measuring the gravity vector, it should be understood that the disclosed embodiments may also be applied to other measurements, including values ​​that are sensitive to the z-axis alignment of the sensor.

[0013] Figure 1 An exemplary machine 10 is shown. Figure 1 The machine shown is a wheel loader, but the machine 10 may be any of the types of machines described above. Figure 1 As shown, the positive x-direction may be toward the front of machine 10 relative to an operator in the cab of machine 10. The positive y-direction may be toward the top of machine 10. Figure 1 As shown), the positive z-direction may be toward the right side of the machine 10. The machine 10 may include a machine body 12, which may include components such as an operator station, an engine housing, and an engine. The machine 10 may also include an arm 14 or another moving component. A bucket 16 may be coupled to the end of the arm 14. The bucket 16 may also be a different work implement, such as a fork, a grapple, etc.; the bucket is merely exemplary. The moving components of the machine 10 may include a link 17. The link 17 may have two degrees of freedom. The machine 10 may include a ground engaging device, such as a wheel 18, to support the machine 10. Although a wheeled machine is shown and described, those skilled in the art will appreciate that other machines, including tracked machines, may also be used. Figure 1 The machine 10 is shown with the linkage 17 in a first, lowered configuration (solid lines) and with the linkage 17 in a second, raised configuration (dashed lines).

[0014] In the example where the machine 10 is a wheel loader, the lifting of the bucket 16 and / or the linkage 17 can be powered and controlled by a lifting actuator 22. The lifting actuator 22 can include, for example, a hydraulic fluid cylinder actuator or any other type of actuator, which is obvious to those skilled in the art. One or more lifting pressure sensors 24 can be configured to measure the force in the cylinder end of the lifting actuator 22, or on another component of the lifting actuator 22, and can be a force sensor. The tilting of the bucket 16 can be powered and controlled by a tilt actuator 26. The tilt actuator 26 can include, for example, a hydraulic fluid cylinder actuator or any other type of actuator, which is obvious to those skilled in the art. One or more tilt pressure sensors 28 can be configured to measure the force in the cylinder end of the tilt actuator 26, or on another component of the tilt actuator 26, and can be a force sensor. For example, the lifting pressure sensor 24 and the tilt pressure sensor 28 can be respectively disposed in the head end of the cylinder of the lifting actuator 22 and the tilt actuator 26 / on the head end. Alternatively, the lift pressure sensor 24 and the tilt pressure sensor 28 may be arranged at other locations relative to the actuator, such as within a hydraulic circuit associated with the actuator.Force or pressure information may also be obtained from other sources, including other sensors.

[0015] The forces acting on the lift and / or tilt cylinders 22, 26 may include head-end pressure and / or rod-end pressure on each side of the piston of the actuator. The lift pressure sensor 24 and the tilt pressure sensor 28 may be configured to measure one or both of the head-end and rod-end pressures of the lift and tilt cylinders, respectively. Alternatively, the lift pressure sensor 24 and the tilt pressure sensor 28 may be configured to measure the net force acting on the lift or tilt cylinders, respectively. The lift pressure sensor 24 and the tilt pressure sensor 28 may detect the fluid pressure within their respective actuators.

[0016] The arm 14 may include an arm position sensor 32. The arm position sensor 32 may collect data indicating the position of the arm 14, including, for example, the angle, height, or extension of the arm 14. The bucket 16 may include a bucket position sensor 34. The bucket position sensor 34 may collect data indicating the position of the bucket 16, including, for example, the height, lateral position, and / or tilt of the bucket 16. The bucket position sensor 34 may sense directly or indirectly. For example, the bucket position sensor 34 may include one or more sensors (such as a rotation sensor) located on a portion of the linkage 17 and may indirectly measure the position of the bucket 16. For example, the bucket position sensor 34 may provide information about the extension length of the linkage actuator, the angle between the components of the linkage 17, and / or the tip of the bucket 16 relative to the ground or another reference. The arm position sensor 32 and / or the bucket position sensor 34 may provide information about the linear displacement of the cylinder of the tilt actuator 26 and / or the angular displacement of the arm 14. The arm position sensor 32 and the bucket position sensor 34 may include a rotary sensor, a magnetic pickup type sensor, a magnetostrictive type sensor associated with a waveguide, a cable type sensor associated with a cable, an internally or externally mounted optical sensor, a lidar, a radar, a SONAR, a camera type sensor, or any other type of height detection / position detection sensor known in the art. The arm position sensor 32 and the bucket position sensor 34 may also include information from other sources.

[0017] An inertial motion unit (“IMU”) 40 may also be coupled to machine 10. Figure 1 As shown, IMU 40 can be coupled to the rear of machine 10, opposite linkage 17. IMU 40 can send acceleration and / or angular velocity signals. Alternatively, IMU 40 can be mounted at any other suitable location on machine 10. IMU 40 can be positioned on any suitable surface of machine 10. For example, IMU 40 can be located on the bottom surface of machine 10, on the front or rear surface of machine 10, or on the top surface of machine 10. IMU 40 can be enclosed in a housing of machine 10 or can be on an exterior surface of machine 10.

[0018] The IMU 40 may be mounted or otherwise coupled to the machine 10 so that a plane fixed in the coordinate system of the IMU 40 is substantially parallel to the XY plane of the machine 10. It is noteworthy that the front frame and the rear frame of the machine 10 (for example, when the machine 10 is a wheel loader) may be offset from each other by a steering angle or an articulation angle. Angles measured relative to the IMU on the rear of the machine 10 may have to be transformed in order to be applied to the front frame of the machine 10. Such transformations are within the skill of those skilled in the art. The IMU 40 may have a mounting position on the machine 10 so that the measurement can be converted into an equivalent representation in the coordinate system of the linkage 17. For example, such a transformation may be achieved by a rotation matrix representing the angle between the rear frame of the machine 10 and the linkage 17. The IMU 40 may be coupled to the machine 10 using any suitable mechanism, such as bolts or other coupling mechanisms. The IMU 40 and / or the machine 10 may be configured to facilitate placement of the IMU 40 on the machine 10. However, manufacturing tolerances may cause the IMU 40 to be skewed or oriented relative to the machine 10. Errors in parallelism of the plane of the IMU 40 relative to the XY plane of the link 17 do not significantly affect a system measuring with the IMU 40. Thus, it can be assumed that the IMU 40 is sufficiently parallel to the XY plane of the link 17 that any errors in parallelism and their associated measurement errors are negligible for a system measuring with the IMU 40. However, errors in angular orientation about the Z axis may result in unacceptable errors in the IMU measurements and require compensation and / or calibration, such as described below.

[0019] IMU 40 may be any suitable type of conventional inertial sensor, and may be or may include an acceleration sensor, an angular velocity sensor, a slope and / or a magnetic field sensor. For example, IMU 40 may be an electronic device that measures and reports information about machine 10, including, for example, acceleration (such as appropriate acceleration), angular velocity, slope and / or magnetic field. IMU 40 may include, for example, one or more accelerometers, one or more gyroscopes and / or one or more magnetometers. IMU 40 may be configured to measure a specific acceleration of machine 10. For example, IMU 40 may be configured to provide a gravity vector, an appropriate acceleration vector, an angular velocity vector, a slope vector and / or a magnetic field vector. However, when IMU 40 is not calibrated for a specific placement of IMU 40 on machine 10, any vector measured may not be accurately aligned with an actual physical vector. In particular, such a difference may occur when IMU 40 is not aligned with respect to the z-axis.

[0020] Figure 2An exemplary calibration system 100 that can be used with the machine 10 is shown. The system 100 can include one or more inputs 102, a controller 104, and a rotation offset output 106. The lift pressure sensor 24 can provide values ​​such as lift head end pressure and lift rod end pressure. For convenience, the lift pressure sensor 24 is shown with one reference numeral in the figures herein, but in fact it can include two sensors, one connected to the head end and one connected to the rod end. The tilt pressure sensor 28 can provide values ​​such as tilt head end pressure and tilt rod end pressure. For convenience, the tilt pressure sensor 28 is represented by one reference numeral in the figures herein, but in fact it can include two sensors, one connected to the head end and one connected to the rod end. The IMU 40 can provide a raw output vector before calibration (as performed by the process 200 described in detail below). As described above, the raw output vector can be a raw gravity and / or appropriate acceleration vector relative to the local coordinate system of the IMU 40. The vector provided by the IMU 40 can include x, y and z direction components. The arm position sensor 32 and / or the bucket position sensor 34 may provide data such as the linear displacement of the cylinder of the tilt actuator 26 and / or the angular displacement of the arm 14 .

[0021] The controller 104 may be embodied as a single microprocessor or multiple microprocessors, which may include hardware and software for monitoring the operation of the machine 10 and issuing instructions to the components of the machine 10. For example, the controller 104 may include memory, auxiliary storage devices, clocks, and processors, such as central processing units or any other devices for completing tasks according to the present invention. The memory or auxiliary storage devices associated with the controller 104 may store data and / or software routines that can help the controller 104 perform its functions. In addition, the memory or storage devices associated with the controller 104 may also store data received from various inputs 102 associated with the machine 10. Many commercially available microprocessors can be configured to perform the functions of the controller 104. It should be understood that the controller 104 can be easily embodied as a general machine controller capable of controlling many other machine functions. Various other known circuits may be associated with the controller 104, including signal conditioning circuits, communication circuits, hydraulic or other actuation circuits, and other appropriate circuits. The controller 104 can be programmed with various characteristics of the machine 10, including, for example, the mass and size of components of the connecting rod 17.

[0022] The rotation offset output 106 may indicate the amount by which the IMU 40 is offset from a planned or ideal position of the IMU 40. Additionally or alternatively, the rotation offset 106 may reflect the amount by which a vector (e.g., an acceleration or gravity vector, as appropriate) measured by the IMU 40 deviates from an actual gravity vector caused by gravity. The rotation offset 106 may be used to calibrate the IMU 40 and may be considered during future calculations involving output from the IMU 40. For example, the rotation offset 106 may be considered when the machine 10 is in use and detects a payload being carried by the machine 10.

[0023] Figure 3 is a flow chart describing an exemplary process 200 for operating the calibration system 100. Prior to starting the process 200, the bucket 16 may be loaded with a load that remains constant during the completion of the process 200. The load placed in the bucket 16 does not need to be a known mass. The load placed in the bucket 16 may be a load with a known or unknown mass. As described below, the calibration process 200 includes moving the linkage 17 (and the bucket 16) to different positions. If the raw gravity vector from the IMU 40 is used to measure the payload of the bucket 16 without calibration, the constant payload may be measured as a different payload at different positions of the linkage 17. However, since the constant load is in the bucket 16, such measured payloads should be the same. The difference in payload measurement may be due to small differences between the raw gravity vector measured by the IMU 40 and the actual gravity vector. The calibration process 200 uses these differences to determine the rotational offset of the IMU 40 in order to correct its measurement. It should be noted that although the gravity vector measured from the IMU 40 may be used for calibration, the resulting calibration may correct subsequent measurements that the IMU 40 may perform.

[0024] In step 202, a value from input 102 may be obtained at a first position of the link 17. The first position of the link 17 may be a predetermined position at a particular height and / or inclination of the bucket 16. Alternatively, the first position of the link 17 may not be predetermined and may be measured using, for example, the arm position sensor 32 and / or the bucket position sensor 34. Even if the position of the link 17 is predetermined, the arm position sensor and / or the bucket position sensor 34 may be used to determine the position of the link 17. The calculation of the payload carried by the bucket 16 may be an actual gravity vector g (particularly the gravity components in the x and y directions, g x and g y ), known values, and data from one or more inputs 102. The function may incorporate known fixed values, such as machine and linkage properties, which may be stored, for example, on the controller 104. The gravity vector may be a function of the raw IMU sensor output vector rotated by the rotation offset 106. For example, the following equation may apply:

[0025] m payload =f payload (g x , g y , F tilt , F lift , d tilt ,θ arm , m tool , d tool )

[0026] g=R z (θ cal )s

[0027] In the above equation, m payload It can be the mass of the payload; f payload may be a function of the payload applied to the particular linkage 17 of the machine 10 and may be an equation or calculation known in the art and / or derived with the aid of a computational algebra system; F tilt It may be the force of the tilt cylinder of the tilt actuator 26 measured by the tilt sensor 28. lift may be the force of the lift cylinder of the lift actuator 22 measured by the lift sensor 24; d tilt θ may be the linear displacement of the cylinder of the tilt actuator 26 measured or calculated from sensors such as the arm position sensor 32, the bucket position sensor 34, and / or other inputs 102; θ arm may be the angular displacement of the arm 14, as measured by a sensor such as the arm position sensor 32 and / or other input 102; m tool This may be the mass of a tool such as bucket 16 (eg, a known mass); d tool may be the known distance between the two link pin connection points on a tool such as bucket 16; g may be the actual gravity vector relative to the coordinate system of machine 10, including the component g x , g y and g z ; R z (θ) can be applied θ cal The rotation matrix about the z-axis of the machine 10 (by assuming that the XY plane of the IMU 40 and the machine 10 is parallel to the z-axis of the IMU 40); θ cal may be a rotation offset calibration value; and s may be a raw output vector from the IMU 40 relative to the local coordinate system of the IMU and may include coordinates s x 、s y and z .

[0028] As mentioned above, the rotation matrix R on the sensor vector s can be used z The calibration value θ calApplied as a rotation about the z-axis of the machine 10 / IMU 40, the corrected gravity vector g representing the actual gravity is obtained as shown in the following equation:

[0029]

[0030] By replacing g x and g y To combine the above equations:

[0031]

[0032] Even when the linkage 17 is moved to other positions in subsequent steps of the process 200, the actual payload will remain constant because the bucket will be carrying the same load. Significant differences in the measured payload will be due to misalignment of the IMU 40, which can be expressed as the rotational offset (θ) in the above equation. cal )106 to correct. In addition, the rotation offset (θ cal ) 106 will also remain the same since this is a result of the mounting position of the IMU 40, which does not change with the position of the link 17. Therefore, the payload at the first position (the position of the link 17 in step 202) can be calculated by the following equation:

[0033]

[0034] In step 204, bucket 16 may be moved to a second position. For example, lift actuator 22 may be actuated to raise or lower linkage 17 and / or tilt actuator 26 may change the tilt of bucket 16. As in step 202, the second position of linkage 17 may be predetermined and / or may be determined using sensors such as arm position sensor 32 and / or bucket position sensor 34. As in step 202, data from input 102 may be obtained. The same equations for payload and gravity vector as described with respect to step 202 may be applied in step 204, substituting the values ​​measured at position 2, as shown below. Thus, the payload at the second position may be calculated using the following equation:

[0035]

[0036] For the above equation for calculating the rotational offset 106, only two link positions are mathematically required. However, in practice, information from more positions may be used. For example, to suppress noise present in the measurements, information from additional positions may be used in an overdetermined system of equations and solved using fitting techniques to select a single "optimal" rotational offset 106. These techniques are well known to those skilled in the art. The number of positions at which measurements are taken may depend on the desired accuracy, the signal noise involved, etc. If further measurements are desired, step 204 may be repeated until all desired measurements are obtained. The following equation may be used to calculate the payload at the kth position:

[0037]

[0038] After the sensor values ​​have been measured at all desired positions, as determined in step 206, the rotational offset 106 can be calculated. As described above, the payload is the same at all positions of the linkage 17 because the load of the bucket does not change. Therefore, the above payload equations for positions 1 to k can all be balanced:

[0039]

[0040] If only two positions are used, the system has one equation and one unknown. The unknown is the rotational offset (θ cal ) 106. Thus, using the above equation, the rotational offset 106 may be solved to use only two positions of the connecting rod 17 (e.g., the first and second positions as described above and shown in the above equation as subscripts 1 and 2). In practice, noise in the measurement of the input 102 and / or noise in other sensor measurements may warrant solving for the rotational offset 106 using techniques such as regression, fitting, etc. In this case, more than two positions of the connecting rod 17 may be used to determine the rotational offset (θ cal ) 106. These techniques are within the knowledge of those skilled in the art. In addition, techniques such as measuring data from input 102 during the lifting and lowering of the connecting rod 17 can be used to reduce the friction effect.

[0041] In step 210, the rotational offset calibration value 106 may be stored. Subsequently, when the machine 10 is engaged in productive work, the gravity equation provided above and the gravity equation reproduced below may be used by the controller 104 or another component of the machine 10 for convenience in order to generate an active gravity vector:

[0042] g=R z (θ cal )s

[0043] Therefore, the rotation offset calibration value (θ cal) can be used together with the raw IMU sensor output vector (s) to determine the gravity vector (g). The gravity vector g calculated using this method will not be affected by the orientation error of the IMU 40 in the z-axis direction. As mentioned above, any of the above steps may also include a transformation between the rear frame and the front frame of the machine 10 to account for steering or articulation angles. Such transformations are within the skill of those skilled in the art.

[0044] The benefits of orientation correction also extend to other vector outputs of the sensor. In particular, the disclosed embodiments may be applied to other systems that are sensitive to misalignment in the z-axis. For example, the disclosed embodiments may be used to improve the measurement of the slope at which the machine 10 is oriented (e.g., the slope of the ground on which the machine 10 rests). For example, a speed reduction system may provide speed control of the machine over a range of slope conditions. The disclosed embodiments may be used to further improve such systems.

[0045] Industrial Applicability

[0046] The disclosed aspects of the system 100 and method 200 described herein can be used during operation of the machine 10 in various settings. For example, it may be helpful to obtain an accurate gravity vector (g) to measure the payload of the linkage 17 and / or bucket 16. The system 100 and method 200 address potential alignment issues of the IMU 40 relative to the machine 10, particularly rotation about the z-axis. Misalignment of the IMU 40 may have a significant effect on the gravity vector determined by the IMU 40 prior to calibration. Because the orientation of the IMU 40 relative to the machine 10 may be constant over time, the method 200 may only need to be performed once or may be performed infrequently. In the event that the IMU 40 is replaced or changed, the method 200 may be repeated to recalibrate the system 100.

[0047] System 100 and method 200 are particularly useful for providing accurate payload data when machine 10 is used on different terrains having various spacings, as payload measurement methods that do not utilize a gravity vector from an IMU may be sensitive to variations in the spacing of machine 10 as opposed to when machine 10 is calibrated.

[0048] It will be apparent to those skilled in the art that various modifications and variations may be made to the disclosed system without departing from the scope of the invention. Other embodiments of the system will be apparent to those skilled in the art through consideration of the specification and practice of the machine disclosed herein. The specification and embodiments are to be considered exemplary only, with the true scope of the invention being indicated by the appended claims and their equivalents.

Claims

1. A method for calibrating an acceleration sensor coupled to a machine, the method comprising: obtaining, at a first position of a connecting rod of the machine, a first acceleration measurement from the acceleration sensor and a first force value indicative of a first force on the connecting rod from an input; obtaining, at a second location of the connecting rod of the machine, a second acceleration measurement from the acceleration sensor and obtaining, from the input, a second force value indicative of a second force on the connecting rod; determining a rotational offset calibration value of the acceleration sensor based on the first acceleration measurement value, the first force value, the second acceleration measurement value, and the second force value; as well as calibrating the acceleration sensor using the rotation offset calibration value, The first position and the second position of the connecting rod are different rotational positions of the connecting rod around the z-axis of the machine, and the z-axis is along the left-right direction of the machine.

2. The method of claim 1, wherein the input is a first input, and wherein the method further comprises: obtaining a first position value indicative of a first position of the connecting rod from a second input; as well as obtaining from the second input a second position value indicative of a second position of the connecting rod; as well as The determination of the rotation offset calibration value is also a function of the first position value and the second position value. 3 . The method of claim 2 , wherein at least one of the first position value and the second position value is obtained from at least one rotation sensor.

4. The method of any one of the preceding claims, wherein the machine comprises a tool, and wherein the tool carries the same payload in the first position and in the second position.

5. The method of any one of claims 1 to 3, wherein the acceleration sensor is an inertial measurement unit.

6. The method of any one of claims 1 to 3, wherein the first force value and the second force value are obtained from one or more pressure sensors.

7. The method of any one of claims 1 to 3, wherein the first acceleration measurement and the second acceleration measurement belong to a gravity vector measured by the acceleration sensor.

8. The method of any one of claims 1 to 3, wherein the rotational offset calibration value reflects the amount by which the acceleration sensor is offset from a z-axis of the machine.

9. A method as claimed in any one of claims 1 to 3, wherein the determination of the rotational offset calibration value is also a function of one or more of the mass of a tool of the machine or the distance between two points on the tool.

10. The method of claim 1, wherein the determination of the rotational offset calibration value is also a function of one or more of a linear displacement of an actuator of the machine or an angular displacement of a linkage component of the machine.

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