Controlling auto-cycling of a hydraulic torque wrench system

Abstract

A method for controlling auto-cycling of a hydraulic torque wrench system is provided. The method comprises: receiving a signal indicative of said hydraulic pressure; based on the signal, estimating an exit pressure (Pex1) of a first cycle (TC2) of the tightening, wherein the exit pressure is the pressure corresponding to the torque installed in the joint at the end of the advance stroke of the cycle; and based on the signal, detecting an end of the stroke of the piston in a second cycle (TC3) of the tightening directly following said first cycle. The detection of the end of the stroke in the second cycle is enabled only after the pressure in the second cycle has exceeded the estimated exit pressure of the first cycle. With this method, only the relevant part of the curve, i.e., the part after the fastener has started to turn, is analysed to identify the end of the stroke. Accordingly, more accurate end-of-stroke detections are enabled.

Description

[0001] CONTROLLING AUTO-CYCLING OF A HYDRAULIC TORQUE WRENCH SYSTEM

[0002] Field of the invention

[0003] The present invention generally relates to the field of hydraulic torque wrench systems. In particular, the present invention relates to controlling auto-cycling of such hydraulic torque wrench systems.

[0004] Background of the invention

[0005] Hydraulic torque wrench systems are typically used for high torque applications with large bolts e.g. in wind towers and to join flanges in the oil and gas industry. Such systems comprise a wrench coupled to a hydraulic pump. In the wrench, a piston is arranged in a cylinder to alternatingly advance and retract by the application of hydraulic pressure from the pump on either side of the piston. The piston is in turn mechanically connected to a pawl and ratchet mechanism that translates the advancing movement of the piston to a rotating movement of an output interface (such as a square or socket) of the wrench, whereby the fastener is tightened. The pawl and ratchet mechanism is further arranged to refrain from translating the retracting movement of the piston to the output interface.

[0006] When manually cycling such systems, an operator has to operate one or more actuators on a pendant to manually initiate advancement and retraction of the piston. It may be difficult for the operator to be sure of when the tightening is done, since this is decided by the operator by simply watching when the fastener stops moving. To be sure that the tightening is done, the operator typically cycles over and over again to be sure, which takes a lot of time. Hence, such manual cycling is both cumbersome and time consuming. Therefore, control devices have been developed that automatically controls the cycling of the system, which in the art is referred to as auto-cycling. When auto-cycling, the control device need not just keep track of when the target torque is reached but also when to switch from advancing to retracting the piston. In order to do so, such control devices are typically provided with input from different sensors in the system. First and foremost, the hydraulic pressure that acts on the side of the piston and causes the piston to advance (and tighten the joint) is sensed and used as an input. Sometimes, also flow sensors are used to monitor the hydraulic flow in and out of the cylinder to keep track of what happens in the cycles. For cost reasons, it is however desirable to reduce the number of sensors to a minimum.

[0007] A critical issue in auto-cycling is to keep track of when to switch from advancement to retraction of the piston. In some prior art, the pressure is simply applied in each cycle until the maximum allowed pressure is reached, which is detected, whereupon retraction is initiated. This implies that, for a certain time, as the piston has bottomed out at its’ dead end, the pressure will just build without moving the fastener of the joint. This costs both time and energy.

[0008] In some more sophisticated methods, the control device instead detects when the piston reaches the end of its’ stroke whereupon retraction is initiated. This is typically made by detecting when the gradient of the pressure signal corresponds to a predefined end-of-stroke gradient (typically obtained by calibration). An example of such a control device is disclosed in US 2009000397 Al.

[0009] A problem with such systems is that the control device may erroneously detect an end of stroke in the beginning of a cycle since the pressure may typically rise rather fast in the beginning before the fastener starts to turn, in a similar manner as in the end of the stroke. To avoid this, a time factor may be used, so that a certain time (from the start of the cycle) need to have lapsed before end-of- stroke detection is enabled. However, the time between the start of the cycle and when the piston reaches the end of its’ stroke may vary in different applications and also between different cycles within the same tightening operation depending on e.g. joint stiffness and the temperature of the hydraulic oil, which may result in erroneous end-of-stroke detection. Summary of the invention

[0010] It would be advantageous to achieve a method and a control device overcoming, or at least alleviating, the above-mentioned drawbacks. In particular, it would be desirable to enable a method and a control device able to do more accurate end- of-stroke detections.

[0011] To better address one or more of these concerns, a method and a control device having the features defined in the independent claims are provided. Preferable embodiments are defined in the dependent claims.

[0012] Hence, according to a first aspect, a method for controlling auto-cycling of a hydraulic torque wrench system is provided. The auto-cycling comprises regulating a hydraulic pressure of the system to alternatingly advance and retract a piston of the system for tightening a joint. The method comprises: receiving a signal indicative of the hydraulic pressure (over time); based on the signal, estimating an exit pressure of a first cycle of the tightening, wherein the exit pressure is the pressure corresponding to the torque installed in the joint at the end of the advance stroke of the cycle; and based on the signal, detecting an end of the stroke of the piston in a second cycle of the tightening directly following said first cycle. The detection of the end of the stroke in the second cycle is enabled only after the pressure in the second cycle has exceeded the estimated exit pressure of the first cycle.

[0013] According to a second aspect, a control device is provided. The control device is configured to perform the method according to the first aspect.

[0014] By awaiting the pressure to build up to the same level as the estimated exit pressure of the previous cycle before enabling end-of-stroke detection, the very first part of the signal / curve of the second cycle is never examined / monitored for the purpose of detecting the end of the stroke. That first part of the advance pressure signal / curve typically contains at least one steep increase in pressure caused by the pressure building up to a level able to start advancing the piston, and then eventually to turn the threaded fastener of the joint. That steep pressure increase may be erroneously recognized as an end-of-stroke gradient. With the present method, the risk of such erroneous detection of the end of the stroke is reduced as the detection of the end of the stroke is enabled only in response to the pressure in the second cycle reaching the estimated / determined value of the exit pressure of the first cycle. End-of-stroke detection may thus be disabled as long as the pressure of the second cycle is below the exit pressure of the previous cycle. Consequently, the need for using a time factor to delay the enabling of the end-of-stroke detection is reduced, or even eliminated. With the present method, only the relevant part of the curve, i.e., the part after the fastener of the joint has started to turn, is analysed in order to identify the end of the stroke. Accordingly, more accurate end-of-stroke detections are enabled.

[0015] In the present specification, the term "first" cycle does not necessarily mean the very first cycle of the tightening, but rather any cycle preceding another cycle.

[0016] In the present specification, the pressure in the second (i.e. following) cycle (roughly corresponding to the exit pressure of the first cycle) may sometimes be referred to as the entry pressure of the second cycle. Hence, the entry pressure of one cycle is the pressure when the fastener starts to turn and the tightening start to progress, whereas the exit pressure of a cycle is the pressure installed in the joint at the end of the advance stroke / phase of the cycle, such as when the piston reaches the very end of its’ stroke, whereby the progression of the tightening is paused until the entry pressure of the next cycle is reached.

[0017] In the present specification, if not otherwise defined, when referring to “pressure” it is meant the pressure on the advance side of the hydraulic system. In other words, the pressure acting on the side of the piston that can push the piston in the advancing direction.

[0018] According to an embodiment, the method may further comprise initiating retraction of the piston in response to the end of the stroke being detected.

[0019] According to an embodiment, the method may further comprise, for each one of a plurality of data points of the signal: for a first interval ranging forward from the data point, estimating (calculating / determining) a first average gradient; and for a second interval ranging backwards from the data point, estimating (calculating / determining) a second average gradient.

[0020] Further, the estimation of the exit pressure may be based on the estimated first and second average gradients.

[0021] By analysing the first and second average gradients for a particular data point, it may be possible to understand how the pressure changes in an area around that data point. For example, it may be possible to identify that, around a specific data point, the pressure gradient suddenly gets steeper. That may be an indication that the exit pressure of the cycle is reached.

[0022] For example, the first and second intervals may each range a predetermined number of data points (forwards and backwards, respectively) from the data point.

[0023] The average gradients may not necessarily be an arithmetic mean gradient of the interval, but rather an approximation / representation thereof.

[0024] For example, linear regression may be applied to the data points in the interval, whereby the gradient (such as a k-value) of the resulting linear function may be determined as the average gradient. Other analysing techniques for deriving the first and second average gradients may also be envisaged.

[0025] According to an embodiment, the method may further comprise, for each one of the plurality of data points, determining a score indicative of a difference between the first average gradient and the second average gradient. The estimation of the exit pressure may then be based on that score.

[0026] The score may e.g. be a calculated difference between the first and second average gradients itself, a ratio between the first and second average gradients or any other parameter / value indicative thereof. The score may thus indicate how the first and second gradients are related to each other to thereby give input in order to identify the point in the curve where the piston reaches the end of the stroke, whereby the exit pressure is reached.

[0027] According to an embodiment, the exit pressure may be estimated (and accordingly set in the software of the control device) as the pressure of the one data point in a set of at least some of the plurality of data points (such as a subset of the plurality of data points) having a determined score indicative of the largest difference between the first average gradient and the second average gradient. This set of at least some of the plurality of data points may comprise only data points having positive first and second average gradients. In other words, the set may be exclusive from data points having at least one of the first and second average gradients being negative.

[0028] The present embodiments are advantageous in that the exit pressure can be estimated with a high degree of accuracy since each one of the data points is evaluated with respect to its’ first and second average gradients. It may therefore be possible to find the one data point where the difference between the gradient before and after is the largest, which is a clear indication of that the exit pressure has been reached at that point. Inaccurate estimation of the exit pressure may cause higher risk of erroneous detection of the end of the stroke in the upcoming cycle in certain cases, such as when the end of the stroke of the upcoming cycle is very close to the entry pressure of that cycle.

[0029] Sometimes, small jumps / notches may occur in the pressure signal. In order not to mistake data points in such jumps / notches for a data point at the exit pressure, the data points having at least one of the first and second gradients being negative may be discarded from the set. The data point at the true exit pressure will have both first and second average gradients being positive.

[0030] According to an embodiment, the estimation of the first and second average gradients for the data points may be made with start from a data point subsequent to the termination of advancement of the piston of the first cycle (such as from when an end of stroke is detected or from a peak pressure of the cycle) and then for data point by data point backwards.

[0031] The present embodiment is advantageous in that computing resources are saved, as the exit pressure as a rule is found in the end of the pressure build-up phase of the cycle. So, by starting from behind, only a relatively small part of the signal needs to be examined to estimate the exit pressure.

[0032] According to an embodiment, evaluating the signal in order to estimate the exit pressure may be made retrospectively, after the pressure has peaked in the first cycle. Since the estimated exit pressure is not needed until the next cycle starts, the analysis of the signal to derive the exit pressure can be made after the torque build up phase, such as during the retraction phase, or even after the retraction phase, but before the next advance phase of the piston starts.

[0033] According to an embodiment, evaluating the signal in order to detect the end of the stroke may be made continuously or intermittently during the second cycle (as from when the exit pressure of the first cycle is exceeded). This is to discover, as the second cycle proceeds, the moment in time when the piston reaches the end of stroke to be able to then, as soon as possible, start retraction of the piston in order to save time and energy.

[0034] According to an embodiment, the detecting of the end of the stroke of the second cycle may comprise comparing a gradient of the signal with a reference end-of- stroke gradient.

[0035] The present embodiment is advantageous in that it requires relatively small computing recourses, which is beneficial as the evaluation of the signal for detecting the end of stroke preferably is made continuously (or intermittently) during a large part of the second cycle.

[0036] According to an alternative embodiment, the detecting of the end of the stroke of the second cycle may be made in the same manner as the estimation of the exit pressure by means of estimating first and second average gradients for each data point (and optionally determining a score based on those). However, that method requires more computational resources and may therefore be less beneficial to do continuously (or intermittently) during a large part of the second cycle as preferred when detecting the end of the stroke.

[0037] According to an embodiment, the method may further comprise: before starting the tightening, and in response to an actuation input by a user, controlling the system to perform an idle stroke (that is, a stroke without applying torque to the joint); during the idle stroke, detecting a gradient of the signal when the piston has reached the end of its’ stroke; and determining the reference end-of-stroke gradient based on the gradient detected during the idle stroke.

[0038] Hence, the user / operator may initiate a learning cycle, in which the end-of- stroke gradient is detected and used as a reference in the upcoming tightening. Optionally, several learning cycles may be made in order to make a more accurate determination of the reference end of stroke gradient. For example, this kind of learning cycle may be made each time the system is started up and / or each time the wrench of the system is exchanged.

[0039] According to an embodiment, the estimation of the exit pressure and / or the detecting of the end of the stroke is made based on a derivative of the signal.

[0040] The present embodiment is advantageous in that the derivative of the signal may be easier / more distinct to analyse as compared to the original signal.

[0041] In the present specification, “the derivative” of the signal means any representation of the rate of change of the pressure signal. The derivative of the signal may be provided by calculation and / or filtering.

[0042] According to an embodiment, a hydraulic torque wrench system is provided comprising a control device according to the second aspect.

[0043] According to an embodiment, a computer program is provided comprising instructions which, when the program is executed by a computer (such as the control device), cause the computer to carry out the method as defined according to the first aspect or any one of the above-described embodiments. Computer-readable storage medium comprising instructions which, when executed by a computer (such as the control device), cause the computer to carry out the method as defined according to the first aspect or any one of the above-described embodiments.

[0044] Brief description of the drawings

[0045] These and other aspects will now be described in more detail in the following illustrative and non-limiting detailed description of embodiments, with reference to the appended drawings.

[0046] Figure 1 shows a hydraulic torque wrench system according to an embodiment.

[0047] Figure 2 shows a time-pressure curve of three learning cycles according to an embodiment.

[0048] Figure 3 shows a time-pressure curve of a tightening operation according to an embodiment.

[0049] Figure 4 shows an example of a rundown trial cycle according to an example.

[0050] Figure 5 shows an example of a rehit trial cycle according to an example.

[0051] Figure 6 shows an example of a partial rehit trial cycle according to an example.

[0052] Figure 7 shows an enlargement of the curve of two of the cycles of the tightening operation of Figure 3.

[0053] Figure 8 shows a further enlargement of the curve of Figure 7.

[0054] Figure 9 shows an enlargement of the curve of three of the cycles of the tightening operation of Figure 3.

[0055] Like reference numerals refer to like elements throughout the description.

[0056] Detailed description of embodiments

[0057] A hydraulic torque wrench system 1 according to an embodiment will be described with reference to Figure 1. The system 1 comprises a wrench 2 hydraulically connected to a pump unit 4. The wrench 2 may comprise a piston 5 arranged to reciprocate inside a cylinder 8 as hydraulic pressure is applied via hydraulic connections 6, 7 on either side of the piston 5. The linear movement of the piston 5 may be translated via a pawl and ratchet mechanism (not shown) to a rotational movement of an output interface 9, such as a square or socket, of the wrench 2. The piston 2 advancing and retracting one time may be referred to as a cycle.

[0058] The pump unit 4 may comprise a single- or multistage pump. For example, the pump unit 4 may comprise a piston pump. For example, the piston pump may comprise several pistons driven by a motor. In a first stage of the pump, all of the pistons of the pump may be engaged so as to produce an as high hydraulic flow as possible. As the pressure in the system rises, the resistance for the motor increases and one piston at the time may therefore be disengaged so as to enable the motor to continue to drive the engaged piston(s), however at a lower flow rate. This may be referred to as the pump being operated in several stages (each stage corresponding to a unique number of pistons being engaged). Other types of single- or multistage pumps may also be envisaged.

[0059] The hydraulic torque wrench system 1 may be governed by a control device 3. The control device 3 may be configured to control auto-cycling of the system 1. The control device 3 may be integrated in the pump unit 4 (as illustrated in Figure 1) or provided as a separate device. The control device 3 may be a single device or distributed in several devices, such as cloud based. The control device 3 may comprise a memory and processing means as well as input and output interfaces for communicating with the pump unit 4.

[0060] The system 1 may further comprise one or more pressure sensors 11, 12. For example, one sensor 12 may be arranged to sense the pressure acting to advance the piston 5 in order to tighten the fastener. That pressure may be referred to as the advance pressure. Another sensor 11 may be arranged to sense the pressure acting to retract the piston 5. That pressure may be referred to as the retract pressure. Data from the one or more sensors 11, 12 may be communicated to the control device 3. The system 1 may further comprise a user interface, which e.g. may comprise a control pendant 10, enabling a user to actuate the system and / or input different parameters needed for the operation of the system 1.

[0061] Before starting a tightening operation, the user may enter a target torque, which may be received by the control device 3 and translated into a target pressure. Alternatively, the user may enter the target pressure directly. The target pressure is the threshold / limit, that the system aims at installing in the joint. The target pressure is the threshold / limit, that, when reached during advancement of the piston, may cause the control device 3 to end the tightening operation. Optionally, the user may further set a pressure relief valve (not shown) of the system 1 to a pressure slightly higher than the target pressure. While the target pressure may be referred to as a digital limit that the system 1 will aim for, the set pressure of pressure relief valve may be referred to as the physical limit that the pressure can reach.

[0062] Further, the user may initiate, e.g. by the actuation of a button on the pendant, a learning sequence for the system 1. The learning sequence may comprise one or more idling cycles for the purpose of calibrating the system 1. When the learning sequence is performed, the wrench 2 may preferably not be applied to any fastener, but rather just held in the air.

[0063] Figure 2 shows a time-pressure curve for the advance pressure (e.g. based on a signal from the sensor 12) of a learning sequence according to an embodiment. In this example, three learning cycles LC1-LC3 are performed and the target pressure Pmax is set to 400 bar. In the beginning of each cycle LC1-LC3, the pressure rises quickly before the piston starts to move. A bump 21 (small peak) may occur when the static friction between the piston and the cylinder is overcome and as the counter pressure on the other side of the piston is overcome to start advancing the piston. Then, a section 22 follows without any pressure increase as the play in the mechanical connection (such as in the pawl and ratchet mechanism) between the piston and the output interface is closed and as the piston then advances through the cylinder. Then, as the piston encounters the end of the advance stroke and therefore stops, the pressure starts to increase at point 23. A slight decrease in the gradient of the curve may occur at a point 24 if the pump unit shifts to its next stage (e.g. by disengagement of one of the pistons of the pump). The pressure continues to rise until the target pressure Pmax is reached, whereupon the control device may command retraction of the piston and the pressure therefore quicky falls down to (at least close to) zero again.

[0064] A gradient after the end-of stroke is reached, preferably for each cycle L1-L3, may be detected (and saved) by the control device 3. Then, the control device 3 may determine a reference end-of-stroke gradient based on the detected gradient(s). The reference end-of-stroke gradient may alternatively be referred to as a predetermined gradient threshold. The reference end-of-stroke gradient may e.g. correspond to, or be slightly lower than, the detected gradient. For example, the reference end-of-stroke gradient (or predetermined gradient threshold) may be comprised within the range of 70-90% of the detected gradient. If the end-of-stroke part of the curve (in the present example, from point 23 to Pmax) contains several gradients 25a, 25b, as a consequence of the pump shifting stage, each one of these gradients 25a, 25b may be detected and saved as a reference end-of-stroke gradient (or predetermined gradient threshold) associated with the respective pump stage.

[0065] According to embodiments, other reference values may (additionally or alternatively) be determined based on the one or more learning cycles L1-L3. For example, a predetermined retract time threshold may be set based on (such as equal to, or slightly lower than) a retract time for a complete stroke of the piston measured during the one or more learning cycles L1-L3. According to a further example, a predetermined advance time threshold may be set based on (such as equal to, or slightly lower than) an advance time for a complete stroke of the piston measured during the one or more learning cycles L1-L3.

[0066] It will be appreciated that any one of the reference values referred to above may be determined based on average values detected during a plurality of learning cycles L1-L3 in order to enhance the accuracy of the system. Further, it may be envisaged that any one of the reference values referred to above may be updated during subsequent cycling of a tightening operation.

[0067] Auto-cycling of the system for performing a tightening according to embodiments will now be described with reference to Figures 3 to 9.

[0068] Figure 3 is a pressure-time curve of a complete tightening according to an example. The tightening operation comprises a number of cycles TC0-TC4. In the present example, the target pressure is set to 300 bar and the pump unit will operate in two stages to get there.

[0069] According to an embodiment, what may be referred to as a trial cycle TCO may be performed as an initial cycle of the tightening. The purpose of the trial cycle may not just be to start the tightening operation, but also to estimate in which state the joint is in so as to be able to act accordingly, such as to estimate an exit pressure of the initial cycle or to terminate the tightening in case it is estimated as a rehit.

[0070] First, it may be awaited that any play in the mechanical connection between the piston and the output interface (such as in the pawl and ratchet mechanism) has started to close. As already explained with reference to the learning cycles in Figure 2, this event may be preceded by a small peak / bump in the pressure. This peak / bump may be detected by the control device and awaited to pass.

[0071] Then, the control device may monitor the pressure signal of the trial cycle in order to detect if any one of a number of conditions a)-c) occurs. These conditions a)-c) will in the following be described with reference to Figures 4 to 6.

[0072] Condition a) may occur when the joint is in a rundown phase, i.e. before the joint reaches snug, or at least first in a rundown phase and then, during the trial cycle, the joint may reach snug. An example of a pressure / time curve resulting from a rundown trial cycle is illustrated in Figure 4.

[0073] Once the initial peak / bump 61 has passed, a period 62 of steady pressure may follow as the play in the mechanical connection between the piston and the output interface closes and as the piston then advances and turns the fastener of the joint with very little resistance as the fastener runs freely. As the piston reaches the end of stroke, the gradient 67 of the pressure may get significantly steeper. In the example illustrated in Figure 4, at point 68, the pump shifts stage, whereupon the gradient 69 may get slightly reduced. Pressure may continue to be applied until the target pressure is reached.

[0074] Now, this condition may be detected by the control device by detecting that the gradient of the signal exceeds and stays over the predetermined gradient threshold (alternatively referred to as the reference end-of-stroke gradient) until the target pressure is reached. The time it takes to then retract the piston (in the trial cycle) back to its’ start position (or at least to a given point close to the start position) for the next cycle may then be measured and compared with the predetermined retract time threshold. If that retract time is longer than the predetermined retract time threshold, the requisites for condition a) are fulfilled. That retract time being longer than the predetermined retract time threshold is an indication of that the piston has travelled a full advance stroke during the trial cycle.

[0075] In the example in Figure 4, gradient 67 may be detected as exceeding the predetermined gradient threshold associated with the first pump stage and gradient 69 may be detected as exceeding the predetermined gradient threshold associated with the second pump stage.

[0076] As condition a) is detected, the control device may estimate an exit pressure of the trial cycle (i.e. the pressure installed in the joint at the end of the advance stroke of the trial cycle). This may be done in different ways.

[0077] For example, the exit pressure may by default be estimated and set to zero, assuming that the joint is still in rundown phase at the end of the trial cycle. This may result in that, in embodiments wherein the exit pressure of the previous cycle is used as a bar for enabling end-of-stroke detection, end-of-stroke detection is enabled from start (i.e. from zero pressure), or at least from when the initial bump / peak has just passed, in the following cycle. However, if the fastener reached snug during the trial cycle, the actual exit pressure will not be zero, but some pressure will have been installed in the joint. The following cycle will then start with (after any play in the mechanical connection is closed) a steep pressure gradient, exceeding the reference end-of-stroke gradient, as the pressure builds to the level of the actual exit pressure of the trial cycle, at which the piston will start to move. As the gradient exceeds the reference end-of-stroke gradient, the control device may erroneously detect end-of-stroke and command retraction of the piston before it has even started to move. In order to detect this situation, the control device may measure the time it takes to reach end-of- stroke in that following cycle and compare that with the predetermined advance time threshold. If it turns out shorter than the predetermined advance time threshold, that may be an indication of that the piston has not moved (or just moved a little bit) before end-of-stroke was detected and, consequently, that the exit pressure of the trial cycle was in fact not zero. In order to remedy that, the control device may command the system to perform another (second) trial cycle to again detect if any one of conditions a)-c) occurs.

[0078] Alternatively, the exit pressure of the trial cycle may be estimated by actually analysing the pressure signal to find a point (or small range) where the pressure starts to rise with an end-of-stroke gradient. This may be done in the same manner as the exit pressure is estimated for the intermediate cycles of the tightening operation as will be described in more detail with reference to Figure 8 further below.

[0079] The tightening operation may then be continued by further auto-cycling.

[0080] Condition b) may occur when the joint is a rehit. That means that the target torque (or more) is already installed in the joint. An example of a pressure / time curve resulting from a rehit trial cycle is illustrated in Figure 5.

[0081] After the initial bump / peak 71 and closing of the play in the mechanical connection between the piston and the output interface (see flat section 72), the pressure may directly rise with a steep gradient 77 as the piston will not move any further. In the example illustrated in figure 5, the pump shifts stage at point 78, whereby the gradient 79 may become slightly lower. Pressure may be applied up to the target pressure, whereupon retraction may be commanded.

[0082] Now, this condition may be detected by the control device by detecting that the gradient of the signal exceeds and stays over the predetermined gradient threshold (alternatively referred to as the reference end-of-stroke gradient) until the target pressure is reached. The time it takes to then retract the piston (in the trial cycle) back to its’ start position for the next cycle may then be measured and compared with the predetermined retract time threshold. If that time is shorter than the predetermined retract time threshold the requisites for condition b) are fulfilled. That time being shorter than the predetermined retract time threshold is an indication of that the piston has not travelled a full stroke, such as maybe barely moved at all.

[0083] In the example in Figure 5, gradient 77 may be detected as exceeding the predetermined gradient threshold associated with the first pump stage and gradient 79 may be detected as exceeding the predetermined gradient threshold associated with the second pump stage.

[0084] As condition b) is detected, the control device may command termination of the tightening as (at least) the target torque is already installed in the joint.

[0085] Condition c) may occur when the joint is a partial rehit. That means that some torque (but less than the target torque) is already installed in the joint. An example of a pressure / time curve resulting from a partial rehit trial cycle is illustrated in Figure 6.

[0086] After the initial bump / peak 81 and closing of the play in the mechanical connection between the piston and the output interface (see flat section 82), the pressure may first rise with a steep gradient until it reaches a level (see point 88) able to start to move the piston (and turn the fastener). As the piston (and fastener) starts to move, the pressure gradient 89 will be reduced to some extent.

[0087] Now, this condition may be detected by the control device by detecting that the gradient of the signal first exceeds the predetermined gradient threshold (see gradient 87) (alternatively referred to as the reference end-of-stroke gradient), and then falls below the predetermined gradient threshold (see gradient 89) for a predetermined amount of time (for the purpose of hysteresis in order to avoid small irregularities in the pressure signal to trigger condition c) detection).

[0088] As soon as condition c) is detected (e.g. at point 85), the control device may command termination of the trial cycle. Hence, the trial cycle may be terminated before the piston hits the end-of-stroke and the target pressure is reached. The control device may then estimate the exit pressure of the trial cycle. That is, the pressure that occurred just before the termination of the trial cycle may be estimated. This may e.g. be made by analysing the signal so as to identify the peak pressure of the trial cycle. Alternatively, level of the pressure read at time of detection of condition c) or read at the time of commanding retraction may be estimated and set as the exit pressure of the trial cycle.

[0089] The tightening operation may then be continued by further auto-cycling.

[0090] Optionally, instead of analysing the pressure signal directly, a derivative thereof may be analysed in order to detect any one of conditions a)-c).

[0091] The exit pressure estimated for the trial cycle may be used for further control of the tightening and / or for estimating one or more parameters of the tightening as will be described in more detail below.

[0092] Further auto-cycling according to an embodiment will now be describe with reference to Figure 7 showing an enlargement of two of the cycles TC2, TC3 of the curve of Figure 3.

[0093] Firstly, with reference to Figure 7, it will be explained how the pressure typically may change during one or more intermediate cycles of the tightening.

[0094] In similarity with the learning cycles L1-L3 and the trial cycle TCO, the start of a first one TC2 of the cycles, the pressure may first build while the piston stands still, up to a point 31, where a bump / small peak in the pressure may occur as the static friction between the piston and cylinder as well as any retract pressure are overcome, whereupon the piston starts to advance. The pressure may then level out for a short period 32 after the initial pressure peak 31 as the piston moves (but not the ratchet and output interface / fastener) to close the play between the ratchet and the pawl.

[0095] When the pressure is high enough to turn the output interface / fastener, the piston advances and applies torque to the joint (with start from point 33 in the illustrated example). This may be reflected in the pressure signal as a steady increase in the pressure. Hence, the pressure during this part of the curve (that is, after any gap in the pawl ratchet mechanism is closed and then as long as the piston advances) corresponds to the torque installed in the joint. As a full advance stroke is completed, the piston reaches it’s dead end and stops moving, whereupon the pressure starts to build more rapidly. In the illustrated example, this occurs at point 34a. This may eventually be detected by the control device, (e.g. at point 35a), which in response may initiate retraction of the piston whereupon the pressure decreases down to the base line again.

[0096] The pressure at the end of the advancing stroke (that is, at point 34a) may be referred to as an exit pressure Pexiof that cycle TC2. In other words, the exit pressure Peximay be the pressure occurring just when the piston reaches the end of its stroke. The exit pressure Pexiis the pressure corresponding to the torque installed in the joint during the just performed cycle TC2.

[0097] Once the piston has been fully retracted, another cycle TC3 begins. In similarity with the previous cycle of the present example, the pressure may first build rapidly until the piston starts to move, then level out for a short period of time as the piston moves to close the gap between the pawl and the ratchet. Once the pawl and ratchet are engaged, the piston will again stop (at point 36) as the pressure is still too low to turn the fastener. The pressure now builds rapidly, see slope 37 of cycle TC3, until (approximately) the exit pressure Pexiof the previous cycle is overcome (at point 38), whereupon installation of torque in the joint starts again and the gradient of the curve decreases to some extent. In the illustrated example, a small notch 40 occurs in the pressure during the advancing of the piston as a consequence of the pump unit shifting stage. As the piston again reaches the end of its stroke (at point 34b), the pressure will drastically increase until retraction of the piston is again initiated, whereupon the pressure drops down to the base line again.

[0098] The system may continue to cycle until the pressure target Pmax is reached during advancing of the piston (with no end-of-stroke detected).

[0099] Embodiments of the present disclosure aims at making an accurate detection of when the piston reaches the end of its stroke, so the control device know when to command retraction of the piston. As can be seen in Figures 3 and 7, the pressure gradient may, at several occasions, be similar to the end-of-stroke gradient. For example, the very first steep pressure increase up to point 31 as well as the pressure increase 37 just before the fastener starts to turn (between points 36 and 38) are very similar (or even equal) to the pressure increase caused by the piston having reached the end of its stroke. To avoid detection of any of these slopes as an indication of the end of the stroke, embodiments of the present disclosure have been developed.

[0100] The control device of the system may be configured to analyse the pressure signal in order to estimate / detect the exit pressure Pexiof a just performed cycle (which may be referred to as a first cycle, such as e.g. cycle TC2 in Figure 7). This analysis may e.g. be performed after the completion of the advance stroke of the piston, or even after completion of the whole cycle.

[0101] In order to identify the exit pressure, data points of the pressure signal may e.g. be analysed as will be described in the following with reference to Figure 8. Figure 8 shows an enlarged portion of the pressure signal at the transition from advancement of the piston to the end of the stroke (such as at point 34a of the first cycle TC2). The pressure signal may be analysed with respect to a plurality of its’ data points 50, 51 (for the sake of simplicity and clarity, only three data points are marked with reference numbers 50 and 51 in Figure 8).

[0102] For each data point 50, a first interval h of data points 51 reaching forward from the data point 50 and a second interval h of data points 51 reaching backward from the data point 50 may be determined / selected. Then, an average gradient dPi, d?2 of each interval h, h may be estimated / calculated. For example, linear regression may be performed on the data points in each interval h, h, wherein the gradient (e.g. k-value) of the resulting linear function may be determined as the average gradient dPi, d?2. Other methods of estimating average gradients dPi, d?2 of the intervals h, h, may also be envisaged.

[0103] Further, the estimation of the value of the exit pressure may be based on the determined first and second average gradients dPi, d?2. For example, a score may be set for each data point 50 based on the difference between the first and second average gradients dPi, d?2. For example, the score may be e.g. the actual calculated difference, or a ratio between the average gradients dPi, d?2. Then, the data point 50 having the score indicating the largest difference between the first and second average gradients dPi, d?2 may be selected as the data point representing the exit pressure.

[0104] Preferably, data points 50 having one of the two determined average gradients being negative may be discarded and not used as basis for estimating the exit pressure. Thereby, the risk of small notches / irregularities, which are not uncommon in pressure signals, being erroneously detected as exit pressure is reduced.

[0105] Preferably, the analysis of the data points may start from behind, such as from a data point after advancement of the piston is completed in the first cycle TC2, such as after the pressure peak of the first cycle TC2 or even after the complete first cycle TC2. The analysis may then be made, for data point by data point, backwards in the signal. In other words, the estimation of the exit pressure may be made retrospectively, in contrast to during the cycle.

[0106] According to an embodiment, the exit pressure of the trial cycle TC0 may be estimated in a similar manner as described above, in particular in case condition c] is detected.

[0107] Optionally, instead of analysing the pressure signal directly, a derivative thereof may be analysed in order to estimate / detect the exit pressure. Then, for example, the average gradients will correspond to the y-intercept values of the linear functions derived by the linear regression instead of its k-value. For example, the derivative of the signal may be derived by means of appropriate filtering of the signal. Other types of filtering of the signal may also be envisaged.

[0108] The value of the estimated exit pressure Pexiof the just performed cycle TC2 may be saved by the control device.

[0109] In the next cycle TC3, which may be referred to as a second cycle, monitoring of the pressure signal in order to detect the end of the advance stroke may not be started until the estimated exit pressure Pexiof the previous cycle is exceeded. That is, not until point 38 in the graph in Figure 7 is reached, the monitoring may start. Before that, end-of-stroke detection is disabled. This means that the risk of the first steep slopes 37, 39 of the curve of the cycle TC3, where the pressure builds against a standing still piston, being mistakenly detected as the end-of- stroke gradient is reduced.

[0110] Preferably, the end-of-stroke detection may be performed, as from the exceeding of the exit pressure Pexiof the previous cycle, continuously (or intermittently) during the cycle TC3. In this way, the end of stroke can be detected as soon as it occurs. For example, the gradient (derivative) of the pressure signal may be continuously (or intermittently) compared to the reference end-of-stroke gradient. When these matches, the end of the stroke may be detected. In response to the end-of-stroke detection, the control device may command retraction of the piston. In the example illustrated in Figure 7, the control device detects the end-of-stroke at point 35b. It may take a few milliseconds for the control device to identify the end-of-stroke and then to realize initiation of retraction of the stroke, as can be seen in Figure 7.

[0111] As long as the pressure has not reached the set pressure Pmaxduring advancement of the piston of the second cycle TC3 (indicating that the tightening operation is completed), the exit pressure PeX2 for the second cycle TC3 may be estimated / detected in the same manner as the exit pressure Pexifor the first cycle TC2 and used as a bar for enabling end-of-stoke detection in the next (third) cycle (such as in cycle TC4 in Figure 3). In other words, the control device may continue to estimate the exit pressure of each cycle and use it as a threshold for enabling end-of-stroke detection in the next cycle, until the tightening operation is terminated.

[0112] According to embodiments of the present disclosure, an angle of the tightening may be estimated. These embodiments will be described with reference to figure 9 showing an enlargement of three of the cycles TC2-TC4 of the curve of Figure 3.

[0113] It may be set in the control device that a total angle Atot of the tightening should be counted as from when the pressure installed in the joint passes / exceeds a predetermined value PstartAngei. In the present example, this predetermined pressure PstartAngei is 50 bars.

[0114] The control device may be configured to detect, by analysing the pressure signal, the cycle in which this predetermined pressure PstartAngei is passed. In the present example, that is the first cycle TC2.

[0115] The control device of the system may be further configured to analyse the pressure signal in order to estimate an entry pressure Penti and an exit pressure Pexi of the first cycle TC2. The entry pressure Penti is the pressure at which torque is started to be installed in the joint, while the exit pressure Pexiis the pressure at which installation of torque stops.

[0116] The exit pressure Peximay be estimated / determined by finding the bending of the curve resulting from the piston hitting the end of the stroke, e.g. in the same manner as described above with reference to Figure 8.

[0117] The entry pressure Penti may be estimated / determined in a similar way by finding the bending of the curve resulting from the piston starting to move, such as based on the same principles as those described with reference to Figure 8.

[0118] In some cases, the entry pressure of a particular cycle may be slightly lower than the exit pressure of the previous cycle due to losses in the system. Hence, estimating the actual entry pressure of a cycle (such as in any of the ways described above) may result in a more accurate estimation of the angle than if the entry pressure of a cycle is just automatically set to equal the exit pressure of the previous cycle. Indeed, the exit pressure of the previous cycle may be utilized in the determination of the entry pressure, e.g. by using it (or any predetermined percentage thereof) as a starting point for analysis of the signal. For example, stepping through the signal, data point by data point, for determining the above describe (with reference to Figure 8) score, may be started at a predetermined pressure above the exit pressure of the previous cycle.

[0119] Further, an angle Astartcycie of the first cycle TC2, as counted from the passing of the predetermined pressure PstartAngie, may be estimated. This estimation may be made based on a (determined) relation / ratio between the pressure range: Penti to Pexi and the pressure range: PstartAngie to Pexi and a predetermined full stroke angle Afuii. For example, the following equation (or any of its’ equivalents) may be used:

[0120] (Texi startAngle)

[0121] Astartcycie Afuu X

[0122] (P exl ~ Penti)

[0123] Alternatively (or as a complement), the estimation of the angle Astartcycie of the first cycle TC2 may be based on the relation / ratio between the time range: Tenti to Texi and the time range: TstartAngie to Texiand a predetermined full stroke angle AMI. Here, the times Tenti, TstartAngie , and Texiare the points in time when the pressures Penti, PstartAngie , and Pexiare passed.

[0124] For example, the following equation (or any of its’ equivalents) may be used:

[0125] Further, a total angle of the tightening may be estimated by adding to the angle

[0126] Astartcycie of the first cycle TC2, one predetermined angle Afuii for each subsequent full stroke cycle performed. In the present example, one full stroke cycle TC3 is performed after the first cycle TCI, so one predetermined angle Afuiiis added to the angle Astartcycie of the first cycle TC2.

[0127] Any cycle occurring between the first and the last cycles may be detected as a full stroke cycle.

[0128] Further, an angle Aendcycie of the last cycle TC4 of the tightening may be estimated. The last cycle TC4 is the cycle in which the maximum pressure Pmax is reached during advance of the piston (i.e. before end of stroke is detected).

[0129] The angle Aendcycie of the last cycle TC4 may be estimated in different ways. Preferably, it may be estimated by first estimating the entry and exit pressures PentEnd, PexEnd of the last cycle TC4 as well as the entry and exit pressures PentFuii, PexFuii of at least one preceding full stroke cycle TC3. Then, the angle Aendcycie of the last cycle TC4 may be estimated based on a (determined) relation / ratio between the pressure range: PentEnd to PexEnd and the pressure range: PentFuii to PexFuii and on the predetermined angle Afuii.

[0130] For example, the following equation (or any of its’ equivalents) may be used:

[0131] Alternatively (or as a complement), the angle Aendcycie of the last cycle TC4 may be estimated based on a (determined) relation / ratio between the time range: TentEnd to TeXEnd and the time range: TentFuii to TeXFuii and on the predetermined angle Afuii. Here, the times TentEnd, TeXEnd, TeXFuii and TentFuii are the points in time when the pressures PentEnd, PexEnd, PexFuii and PentFuii are passed.

[0132] For example, the following equation (or any of its’ equivalents) may be used: In case the last cycle is preceded by several full stroke cycles, average pressure / time ranges between the entry and exit pressures of two or more of these full cycles may be used as a basis for the angel estimation. Alternatively, only the time / pressure range between the entry and exit pressures of the just preceding full stroke cycle may be used.

[0133] In case the time range ratio approach is to be used, it may be beneficial to check whether there is an indication that the average pressure rate dPl between the entry and exit pressures PentFuii, PexFuii of the at least one preceding full stroke cycle TC3 deviates (more than to a predetermined extent) from the average pressure rate dP2 between the entry and exit pressures PentEnd, PexEnd of the last cycle TC4. If that is the case, the pump may have shifted stage during any one of these two cycles. In the present example, the pump has shifted stage during the full stroke cycle TC3 at point 40.

[0134] In case such a deviation is detected, a compensation (or correction) factor / function may be added to the estimation of the angle of the last cycle. The compensation factor / function may be adapted so as to compensate for the mismatch of the gradients in the calculation of the angle.

[0135] Alternatively, in case such deviation is detected, the ratio of the retract times of the last cycle TC4 and of at least one of the preceding full stroke cycles TC3 may be used as a basis for determining the angle Aendcycie of the last cycle TC4.

[0136] The calculated / estimated total angle Atot (i.e. Astartcycie + (AfUn x number of intermediate full stroke cycles) + Aendcycie) may then be saved, e.g. together with other parameters of the tightening.

[0137] It will be appreciated that any of the above described embodiments may be performed based on a derivative of the pressure signal.

[0138] The person skilled in the art realizes that the present invention by no means is limited to the embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims.

[0139] Additionally, variations to the disclosed embodiments can be understood and effected by the skilled person in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

27CLAIMS1. Method for controlling auto-cycling of a hydraulic torque wrench system (1), the auto-cycling comprising regulating a hydraulic pressure of the system to alternatingly advance and retract a piston (5) of the system for tightening a joint, the method comprising: receiving a signal indicative of said hydraulic pressure; based on the signal, estimating an exit pressure (Pexi) of a first cycle (TC2) of the tightening, wherein the exit pressure is the pressure corresponding to the torque installed in the joint at the end of the advance stroke of the cycle; and based on the signal, detecting an end of the stroke of the piston in a second cycle (TC3) of the tightening directly following said first cycle, wherein the detection of the end of the stroke in the second cycle is enabled only after the pressure in the second cycle has exceeded the estimated exit pressure of the first cycle.

2. Method as defined in claim 1, further comprising initiating retraction of the piston in response to the end of the stroke being detected.

3. Method as defined in claim 1 or 2, further comprising, for each one of a plurality of data points (50, 51) of the signal: for a first interval (h) ranging forward from the data point (50), estimating a first average gradient (dPi); and for a second interval (h) ranging backwards from the data point, estimating a second average gradient (d?2); wherein estimation of the exit pressure is based on the estimated first and second average gradients.

4. Method as defined in claim 3, further comprising, for each one of said plurality of data points, determining a score indicative of a differencebetween the first average gradient and the second average gradient, wherein estimation of the exit pressure is based on said score.

5. Method as defined in claim 4, wherein the exit pressure is estimated as the pressure of the one data point in a set of at least some of said plurality of data points having a determined score indicative of the largest difference between the first average gradient and the second average gradient, and wherein said set of at least some of said plurality of data points comprises only data points having positive first and second average gradients.

6. Method as defined in any one of claims 3 to 5, wherein the estimating of the first and second average gradients for the data points is made with start from a data point subsequent to the termination of advancement of the piston of the first cycle and then for data point by data point backwards.

7. Method as defined in any one of the preceding claims, wherein an evaluation of the signal in order to estimate the exit pressure is made retrospectively, after the pressure has peaked in the first cycle.

8. Method as defined in any one of the preceding claims, wherein an evaluation of the signal in order to detect the end of the stroke is made continuously or intermittently during the second cycle.

9. Method as defined in any one of the preceding claims, wherein the detecting of the end of the stroke of the second cycle comprises comparing a gradient of the signal with a reference end-of-stroke gradient.

10. Method as defined in claim 9, further comprising: before starting the tightening, and in response to an actuation input by a user, controlling the system to perform an idle stroke (LC1- LC3); during the idle stroke, detecting a gradient (25a, 25b) of the signal when the piston has reached the end of its stroke; and determining the reference end-of-stroke gradient based on the gradient detected during the idle stroke.

11. Method as defined in any one of the preceding claims, wherein the estimation of the exit pressure and / or the detecting of the end of the stroke is made based on a derivative of the signal.

12. Control device configured to perform the method as defined in any one of the preceding claims.

13. Hydraulic torque wrench system comprising a control device according to claim 12.

14. Computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the method as defined in any one of claims 1-11.

15. Computer-readable storage medium comprising instructions which, when executed by a computer, cause the computer to carry out the method as defined in any one of claims 1-11.

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