Estimation of an angle of a tightening made by auto-cycling of a hydraulic torque wrench system

Abstract

Method of estimating an angle of a tightening made by auto-cycling a hydraulic torque wrench system (1). The method comprises, based on a pressure signal, estimating an angle (AstartCycle) that the tightening has progressed during a first cycle (TC2) as counted from a predetermined pressure (PstartAngle), the estimation being based on: a relation between a pressure and / or time range from a predetermined pressure (PstartAngle) to an exit pressure (Pex1) and a pressure and / or time range from an entry pressure (Pent1) to the exit pressure (Pex1) in the first cycle (TC2), and a predetermined angle (Afull) corresponding to a full stroke of the piston.

Description

[0001]ESTIMATION OF AN ANGLE OF A TIGHTENING MADE BY AUTO-CYCLING OF AHYDRAULIC TORQUE WRENCH SYSTEMField of the inventionThe present invention generally relates to the field of hydraulic torque wrench systems. In particular, the present invention relates to such hydraulic torquewrench systems that can be auto cycled.Background of the invention 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. 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.In tightening technique on an industrial level, it is generally desirable to recordcertain parameters of a tightening for the purpose of traceability and futureaudit. Such parameters may typically include the torque and the angle of thetightening. In hydraulic torque wrench systems, the pressure correlates to thetorque and, thus, the pressure is typically used as an indicator of the torque of the tightening. The angle of the tightening can be measured by an encoder coupled to the output shaft of the tool. However, such an encoder adds to the cost of the tool and requires additional cabling from the wrench to the controlunit, the latter typically being located at the pump unit.Summary of the inventionIt would be advantageous to achieve a method and control device overcoming, orat least alleviating, the above mentioned drawbacks. In particular, it would bedesirable to provide a method and a control device enabling estimation of anangle of a tightening performed by a hydraulic torque wrench system withoutthe need for additional sensors.To better address one or more of these concerns, a method and a control devicehaving the features defined in the independent claims are provided. Preferableembodiments are defined in the dependent claims.Hence, according to a first aspect, a method of estimating an angle of a tighteningmade by auto-cycling a hydraulic torque wrench system is provided. The auto-cycling comprises regulating a hydraulic pressure of the system to alternatinglyadvance and retract a piston of the system for tightening a joint. The method comprises:- receiving a signal indicative of said hydraulic pressure (over time);- based on the signal, detecting a first cycle of the tightening, in which apredetermined pressure is exceeded during advance of the piston, wherein said predetermined pressure is the pressure from which a total angle of the tightening is to be counted;- based on the signal, estimating an entry pressure of said first cycle,wherein the entry pressure is the pressure corresponding to when installation of torque to the joint is started in said first cycle (i.e. at which pressure the fastener starts to turn);- based on the signal, estimating an exit pressure of said first cycle, whereinthe exit pressure is the pressure corresponding to when installation of torque to the joint is ended in said first cycle (i.e. at which pressure the fastener stops to turn); and- estimating an angle that the tightening has progressed during said firstcycle as counted from said predetermined pressure. The estimation is based on:a relation (e.g. a ratio) between the pressure and / or time range from the predetermined pressure to the exit pressure and the pressure and / or time range from the entry pressure to the exit pressure in the first cycle, and a predetermined angle corresponding to a full stroke of the piston (i.e. the angle that the fastener / ratchet turns as the piston travels a full stroke). According to a second aspect, a control device is provided. The control device is configured to perform the method according to the first aspect. The inventor has realized that, since the pressure increase is (at least almost)linear between the entry pressure and the exit pressure of a cycle, i.e. as thefastener turns and torque is installed, both the relation between differentpressure ranges, as well as between different time ranges (somewhere betweenthe entry and exit pressure), are representative of the relation between thecorresponding angle ranges that the fastener has turned during these pressure / time ranges. Thus, since the tightening angle of a full stroke cycle isknown, an angle of a partial stroke can be estimated / calculated based on suchpressure / time ranges.So, the ratio of the pressure range between the entry and exit pressures to thepressure range between the predetermined pressure and the exit pressurecorresponds to the ratio of the angle range that the fastener has turned ascounted from the entry pressure to the exit pressure to the angle range that thefastener has turned as counted from the predetermined pressure to the exitpressure. In similarity, the ratio of the time range as counted between the entrypressure and the exit pressure to the time range as counted between thepredetermined pressure and the exit pressure corresponds to the ratio of theangle range that the fastener has turned as counted between the entry pressureand the exit pressure to the angle range that the fastener has turned as countedbetween the predetermined pressure and the exit pressure. In other words, apressure range ratio, within the limits of the entry and exit pressures, is (at least substantially) the same as the corresponding angle range ratio. Also, a time range ratio (of time ranges between specific pressure readings), within the limits of the entry and exit pressures, is (at least substantially) the same as thecorresponding angle range ratio.With the present aspects, it is possible, without using additional sensors (such asan encoder), to estimate a tightening angle of the first cycle (as counted from thepassing of the predetermined pressure). With the present aspects, only thepressure signal is required to make an estimation of the angle. It is particularly advantageous to base the angle estimation on the above described pressure ranges (instead of the time ranges), since that estimation will be more robust to potential changes in the pressure build up rate, such as if the pump shifts stage during the first cycle, whereby the pressure build up rate slows down. It is in particular the angle of the first cycle of the tightening (i.e. the cycle in which the predetermined pressure is passed) that is tricky to estimate since it typically only constitutes a part of a full stroke while the piston may indeed havetravelled a full stroke during the cycle. As a consequence, the retract / advancetimes of the piston cannot be utilized for the purpose of estimating the angle.The angle of the remainder of the tightening can be estimated in different ways, as will be described further on in the present specification. In industrial tightening, the total angle of a tightening is typically counted asfrom passing a particular (predetermined) torque (which for a hydraulic torquewrench system corresponds to a particular pressure), and thus not from the very beginning of the rundown, or even from the very beginning of the torque build up phase of the tightening. This particular torque / pressure may e.g. be apredetermined percentage (e.g. 50%) of the target torque / pressure.In the present specification, when referring to an entry pressure of a specificcycle, it is meant the pressure corresponding to when installation of torque tothe joint is started in that cycle. Further, when referring to an exit pressure of aspecific cycle, it is meant the pressure corresponding to when installation oftorque to the joint is ended in that cycle.It will be appreciated that, even though the cycle in which the predeterminedpressure is passed is referred to as the “first cycle” in the present specification, itmay not necessarily be the very first (initial) cycle of the tightening. For example,it may be envisaged that the predetermined pressure is passed as the tightening has already progressed on or a few cycles. 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.According to an embodiment, estimating the angle of the first cycle may be basedon the equation: or any equivalent equation, wherein: AstartCycleis the angle of the first cycle, Afull is the predetermined angle corresponding to a full stroke of thepiston, Pex1 is the exit pressure of the first cycle, Pent1 is the entry pressure of the first cycle, and PstartAngle is the predetermined pressure from which the total angle of thetightening is to be counted.An example of an equivalent equation is: Other equivalent equations may also be envisaged. Further, a corresponding equation for the time range ratio may alternatively (oradditionally) be used as a basis for estimating the angel of the first cycle: wherein: Tex1is the time when the exit pressure of the first cycle is passed, Tent1 is the time when the entry pressure of the first cycle is passed, andTstartAngle is the time of the passing of the predetermined pressure fromwhich the total angle of the tightening is to be counted.According to an embodiment, the method may further comprise estimating thetotal angle of the tightening. This may include adding to the estimated angle ofthe first cycle, for each subsequent cycle detected as a full stroke cycle (i.e. acycle in which the piston has travelled a full stroke), said predetermined angle.A cycle may e.g. be detected as a full stroke cycle in case an end of stroke is detected during the cycle. The end of stroke is when the piston hits the end of its’ stroke, whereby the pressure gradient increases drastically as the pump then works against a standing still piston. According to an embodiment, the method may further comprise estimating an angle that the tightening progressed in a last cycle of the tightening. The last cycle of the tightening is the cycle in which the pressure (and corresponding torque) reaches its’ pre-set target during advance of the piston(i.e. before the piston has reached the end of its’ stroke). Hence, the last cycle ismost commonly not a full, but a partial stroke. The estimation of the angle of the last cycle may be made in different ways. For example, it may be based on the same principle as that of the first cycle, i.e. by utilizing the fact that a pressure / time range ratio, during advance of the piston and application of torque to the joint, (at least substantially) equals the corresponding angle range ratio. So, according to an embodiment, the method may further comprise:- based on the signal, estimating an entry pressure and an exit pressure ofthe last cycle; and- based on the signal, estimating an entry pressure and an exit pressure ofat least one full stroke cycle preceding the last cycle;wherein estimating the angle of the last cycle is based on: arelation (e.g. ratio) between the pressure and / or time range from theentry pressure of the last cycle to the exit pressure of the last cycle and thepressure and / or time range from the entry pressure of said at least one fullstroke cycle to the exit pressure of said at least one full stroke cycle, andthe predetermined angle corresponding to a full stroke of the piston.Hence, the ratio / relation between pressure / time ranges in different cycles are utilized in order to estimate the angle of the last cycle. The estimation of the angle may be based on the pressure / time range between the entry and exit pressure of a single preceding cycle, such as of the one cycle just before the last cycle, or on an average pressure and / or time range between the entry and exit pressures of several preceding full stroke cycles. The present embodiment is advantageous in that the angle of the last cycle can be estimated without the need of additional sensors and without relying on advance / retract times of the piston which makes the estimation more accurate.The exit pressure of the last cycle may e.g. simply be estimated as equalling thetarget pressure. According to another embodiment, the method may further comprise:- determining a retract time of the last cycle; and- determining a retract time of at least one full stroke cycle preceding thelast cycle, wherein estimating the angle of the last cycle is based on: a relation of the retract time of the last cycle and the retract time of the atleast one full stroke cycle preceding the last cycle, andthe predetermined angle corresponding to a full stroke of the piston.Basing the estimation of the angle of the last cycle on the retract times may however be slightly less accurate than basing it on the pressure / time ranges as described in the previous embodiment since the first part of the advance strokeof a cycle closes the play in the ratchet, wherein the fastener does not move. Thisfirst part will add to the retract time but not to the tightening angle, whereby theratio of the retract times till have a slightly lower correspondence to the anglerange ratio. For example, this embodiment may be used as a fallback in case estimation of the angle of the last cycle according to the previous embodiment for somereason fails and / or is expected / determined to be unacceptably inaccurate.For example, the method may further comprise detecting an indication that the estimation of the angle of the first cycle and / or the estimation of the angle of the last cycle is / will be unacceptably inaccurate. For example, in response to detecting such an indication, the estimation of the first cycle and / or the estimation of the angle of the last cycle, respectively, may be further based on a compensation value / compensation function. Such an indication may be detected in different ways. For example, it may be detected by detecting that the pump has shifted stage during any one of thepressure / time ratios determined. Such stage shift may e.g. simply be signalled bythe pump. A stage shift of the pump lowers the pressure build-up rate.Alternatively (or as a complement), such an indication may be detected byanalysing (comparing) pressure gradients of the determined pressure / timeranges.In case the pressure gradients have a poor match (such as less than apredetermined percentage), that may be an indication that, in particular, theratio of the determined time ranges may be more or less off from the ratio of the corresponding angle ranges, which may make the estimation of the angle less accurate. This way of detecting the indication that the estimation of the angle of the first cycle and / or the estimation of the angle of the last cycle is / will be unacceptablyinaccurate may catch up both situations where the pump has shifted stageduring any one of the determined pressure / time ranges and also other situations causing disruptions / discontinuities in the pressure build up rate during the determined pressure / time ranges.For example, the method may further comprise:- determining if a pressure gradient between the entry pressure of said atleast one full stroke cycle and the exit pressure of said at least one full stroke cycle matches a pressure gradient between the entry pressure of the last cycleand the exit pressure of the last cycle.For example, the method may further comprise:- determining if a pressure gradient between the entry pressure of the firstcycle and the exit pressure of the first cycle matches a pressure gradientbetween the predetermined pressure and the exit pressure of first cycle.The gradients may e.g. be average gradients between said pressure readings.The gradients may e.g. be considered to match if they differ less than to apredetermined extent, such as less than 15%.If the gradients are found not to match, further measures may be taken so as tocompensate for the differing gradients, such as adding the above mentioned compensation value and / or compensation function to the basis for the estimation of the angle of the cycle in question. For example, an addition of a predetermined percentage (i.e. a compensation value) may be added to one of the time ranges. Alternatively, a compensation function may be applied to the calculation of the angle of the cycle in question. According to an embodiment, estimating the entry and exit pressures of a particular cycle (such as the first or last cycle) may be made by detecting a change in the pressure gradient of the signal (which may be referred to as a knee). This may be done in different ways. 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. Further, the estimation of the exit pressure may be based on the estimated first and second average gradients. 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. For example, the first and second intervals may each range a predetermined number of data points (forwards and backwards, respectively) from the data point. The average gradients may not necessarily be an arithmetic mean gradient of the interval, but rather an approximation / representation thereof. 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. It will be appreciated that the entry pressure may be estimated in a similarmanner as described in the above embodiments.According to an embodiment, the steps of detecting the first cycle and estimating the entry and exit pressures of the first cycle are made in retrospect, such as after the first cycle is completed, such as after the tightening is completed. Optionally, also any one of the other steps as described in the previous embodiments may be performed in retrospect, such as after the tightening is completed. Making the angle estimation in retrospect may facilitate the required data processing. Estimating an angle of the tightening may typically be made for the purpose of saving it as a result value of the tightening, e.g. together with other parameterssuch as final torque etc. Hence, it may not be needed during the tighteningprocess itself, but merely afterwards. According to an embodiment, the estimation of the entry and exit pressures of the first cycle (or of any of the other cycles) may be made based on a derivative of the signal. 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. 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. According to an embodiment, the hydraulic torque wrench system may comprise a control device according to the second aspect. According to an embodiment, a 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 the preceding embodiments. According to an embodiment, a 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 the preceding embodiments. It is noted that embodiments of the invention relate to all possible combinations of features recited in the claims. Further, it will be appreciated that the variousembodiments described for the method are all combinable with the controldevice as defined in accordance with the second aspect of the present invention.Brief description of the drawingsThese and other aspects will now be described in more detail in the followingillustrative and non-limiting detailed description of embodiments, withreference to the appended drawings. Figure 1 shows a hydraulic torque wrench system according to an embodiment.Figure 2 shows a time-pressure curve of three learning cycles according to anembodiment.Figure 3 shows a time-pressure curve of a tightening operation according to anembodiment. Figure 4 shows an example of a rundown trial cycle according to an example. Figure 5 shows an example of a rehit trial cycle according to an example. Figure 6 shows an example of a partial rehit trial cycle according to an example. Figure 7 shows an enlargement of the curve of two of the cycles of the tightening operation of Figure 3. Figure 8 shows a further enlargement of the curve of Figure 7. Figure 9 shows an enlargement of the curve of three of the cycles of the tightening operation of Figure 3. Like reference numerals refer to like elements throughout the description. Detailed description of embodimentsA hydraulic torque wrench system 1 according to an embodiment will bedescribed with reference to Figure 1. The system 1 comprises a wrench 2hydraulically 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.The pump unit 4 may comprise a single- or multistage pump. For example, thepump 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 typesof single- or multistage pumps may also be envisaged.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. 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. 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. 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. 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 disengagementof one of the pistons of the pump). The pressure continues to rise until the targetpressure 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. 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 gradientmay e.g. correspond to, or be slightly lower than, the detected gradient. Forexample, 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. 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 (suchas equal to, or slightly lower than) a retract time for a complete stroke of thepiston measured during the one or more learning cycles L1-L3. According to afurther 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 strokeof the piston measured during the one or more learning cycles L1-L3.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. Auto-cycling of the system for performing a tightening according to embodiments will now be described with reference to Figures 3 to 9. 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. According to an embodiment, what may be referred to as a trial cycle TC0 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. 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. Thispeak / bump may be detected by the control device and awaited to pass.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. 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. 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 pistonreaches the end of stroke, the gradient 67 of the pressure may get significantlysteeper. 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. Now, this condition may be detected by the control device by detecting that thegradient of the signal exceeds and stays over the predetermined gradientthreshold (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 tothe start position) for the next cycle may then be measured and compared withthe predetermined retract time threshold. If that retract time is longer than thepredetermined 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. In the example in Figure 4, gradient 67 may be detected as exceeding thepredetermined gradient threshold associated with the first pump stage andgradient 69 may be detected as exceeding the predetermined gradient threshold associated with the second pump stage. 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. 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 whenthe initial bump / peak has just passed, in the following cycle. However, if thefastener 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 whichthe piston will start to move. As the gradient exceeds the reference end-of-strokegradient, 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. 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. The tightening operation may then be continued by further auto-cycling. 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. 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. Now, this condition may be detected by the control device by detecting that thegradient of the signal exceeds and stays over the predetermined gradientthreshold (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 bemeasured and compared with the predetermined retract time threshold. If thattime is shorter than the predetermined retract time threshold the requisites forcondition 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. In the example in Figure 5, gradient 77 may be detected as exceeding thepredetermined gradient threshold associated with the first pump stage andgradient 79 may be detected as exceeding the predetermined gradient threshold associated with the second pump stage. 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. 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. 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. Now, this condition may be detected by the control device by detecting that thegradient of the signal first exceeds the predetermined gradient threshold (seegradient 87) (alternatively referred to as the reference end-of-stroke gradient),and then falls below the predetermined gradient threshold (see gradient 89) fora predetermined amount of time (for the purpose of hysteresis in order to avoid small irregularities in the pressure signal to trigger condition c) detection). 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 thepeak pressure of the trial cycle. Alternatively, level of the pressure read at timeof 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. The tightening operation may then be continued by further auto-cycling.Optionally, instead of analysing the pressure signal directly, a derivative thereofmay be analysed in order to detect any one of conditions a)-c). 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. 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. 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. In similarity with the learning cycles L1-L3 and the trial cycle TC0, 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. 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 thepiston advances) corresponds to the torque installed in the joint. As a fulladvance 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. The pressure at the end of the advancing stroke (that is, at point 34a) may be referred to as an exit pressure Pex1of that cycle TC2. In other words, the exit pressure Pex1may be the pressure occurring just when the piston reaches the end of its stroke. The exit pressure Pex1is the pressure corresponding to the torque installed in the joint during the just performed cycle TC2. 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 Pex1of 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. The system may continue to cycle until the pressure target Pmax is reachedduring advancing of the piston (with no end-of-stroke detected).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 (betweenpoints 36 and 38) are very similar (or even equal) to the pressure increasecaused 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. The control device of the system may be configured to analyse the pressure signal in order to estimate / detect the exit pressure Pex1of 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. 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 datapoints are marked with reference numbers 50 and 51 in Figure 8).For each data point 50, a first interval I1 of data points 51 reaching forward fromthe data point 50 and a second interval I2 of data points 51 reaching backwardfrom the data point 50 may be determined / selected. Then, an average gradient dP1, dP2 of each interval I1, I2 may be estimated / calculated. For example, linear regression may be performed on the data points in each interval I1, I2, wherein the gradient (e.g. k-value) of the resulting linear function may be determined as the average gradient dP1, dP2. Other methods of estimating average gradients dP1, dP2of the intervals I1,I2, may also be envisaged.Further, the estimation of the value of the exit pressure may be based on thedetermined first and second average gradients dP1, dP2. For example, a score may be set for each data point 50 based on the difference between the first and second average gradients dP1, dP2. For example, the score may be e.g. the actual calculated difference, or a ratio between the average gradients dP1, dP2. Then, the data point 50 having the score indicating the largest difference between the first and second average gradients dP1, dP2 may be selected as the data point representing the exit pressure. 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. 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. 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.Optionally, instead of analysing the pressure signal directly, a derivative thereofmay 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. The value of the estimated exit pressure Pex1of the just performed cycle TC2 may be saved by the control device. 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 Pex1 of 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. Preferably, the end-of-stroke detection may be performed, as from the exceeding of the exit pressure Pex1of 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. 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 Pex1 for 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. According to embodiments of the present disclosure, an angle of the tightening may be estimated. These embodiments will be described with reference to figure9 showing an enlargement of three of the cycles TC2-TC4 of the curve of Figure3. 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 PstartAngel. In the present example, this predetermined pressure PstartAngel is 50 bars. The control device may be configured to detect, by analysing the pressure signal, the cycle in which this predetermined pressure PstartAngel is passed. In the present example, that is the first cycle TC2. The control device of the system may be further configured to analyse the pressure signal in order to estimate an entry pressure Pent1and an exit pressure Pex1of the first cycle TC2. The entry pressure Pent1is the pressure at which torque is started to be installed in the joint, while the exit pressure Pex1 is the pressure at which installation of torque stops. The exit pressure Pex1 may 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. The entry pressure Pent1may 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. 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. Further, an angle AstartCycle of the first cycle TC2, as counted from the passing of the predetermined pressure PstartAngle, may be estimated. This estimation may be made based on a (determined) relation / ratio between the pressure range: Pent1 to Pex1 and the pressure range: PstartAngle to Pex1 and a predetermined full stroke angle Afull. For example, the following equation (or any of its’ equivalents) may be used: Alternatively (or as a complement), the estimation of the angle AstartCycle of the first cycle TC2 may be based on the relation / ratio between the time range: Tent1 to Tex1and the time range: TstartAngleto Tex1and a predetermined full stroke angle Afull. Here, the times Tent1, TstartAngle, and Tex1are the points in time when thepressures Pent1, PstartAngle , and Pex1 are passed.For example, the following equation (or any of its’ equivalents) may be used:Further, a total angle of the tightening may be estimated by adding to the angleAstartCycleof the first cycle TC2, one predetermined angle Afullfor each subsequent full stroke cycle performed. In the present example, one full stroke cycle TC3 is performed after the first cycle TC1, so one predetermined angle Afullis added to the angle AstartCycleof the first cycle TC2. Any cycle occurring between the first and the last cycles may be detected as a full stroke cycle. Further, an angle AendCycle 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). The angle AendCycle of the last cycle TC4 may be estimated in different ways. Preferably, it may be estimated by first estimating the entry and exit pressuresPentEnd, PexEnd of the last cycle TC4 as well as the entry and exit pressures PentFull,PexFullof at least one preceding full stroke cycle TC3. Then, the angle AendCycleof the last cycle TC4 may be estimated based on a (determined) relation / ratio between the pressure range: PentEnd to PexEnd and the pressure range: PentFull to PexFulland on the predetermined angle Afull. For example, the following equation (or any of its’ equivalents) may be used: Alternatively (or as a complement), the angle AendCycle 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: TentFull to TexFull and on the predetermined angle Afull. Here, the times TentEnd, TexEnd, TexFulland TentFullare the points in timewhen the pressures PentEnd, PexEnd, PexFull and PentFull are passed.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. 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 dP1 between the entry and exit pressures PentFull, PexFull 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, PexEndof 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. 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. 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 AendCycleof the last cycle TC4. The calculated / estimated total angle Atot(i.e. AstartCycle+ (Afullx number of intermediate full stroke cycles) + AendCycle) may then be saved, e.g. together with other parameters of the tightening. It will be appreciated that any of the above described embodiments may be performed based on a derivative of the pressure signal. 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. 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 arerecited in mutually different dependent claims does not indicate that acombination of these measures cannot be used to advantage.

Claims

CLAIMS 1. Method of estimating an angle of a tightening made by auto-cycling ahydraulic 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, detecting a first cycle (TC2) of the tightening,in which a predetermined pressure (PstartAngle) is exceeded during advanceof the piston, wherein said predetermined pressure (PstartAngle) is thepressure from which a total angle (Atot) of the tightening is to be counted;- based on the signal, estimating an entry pressure (Pent1) of saidfirst cycle (TC2), wherein the entry pressure (Pent1) is the pressurecorresponding to when installation of torque to the joint is started in saidfirst cycle (TC2);- based on the signal, estimating an exit pressure (Pex1) of said firstcycle (TC2), wherein the exit pressure (Pex1) is the pressurecorresponding to when installation of torque to the joint is ended in saidfirst cycle (TC2); and- estimating an angle (AstartCycle) that the tightening has progressedduring said first cycle (TC2) as counted from said predeterminedpressure (PstartAngle), the estimation being based on:a relation between the pressure and / or time range from thepredetermined pressure (PstartAngle) to the exit pressure (Pex1) and thepressure and / or time range from the entry pressure (Pent1) to the exitpressure (Pex1) in the first cycle (TC2), anda predetermined angle (Afull) corresponding to a full stroke of thepiston.

2. Method according to claim 1, wherein estimating the angle (AstartCycle) ofthe first cycle (TC2) is based on the equation:or any equivalent equation, wherein: AstartCycleis the angle of the first cycle (TC2), Afull is the predetermined angle corresponding to a full stroke ofthe piston, Pex1is the exit pressure of the first cycle (TC2), Pent1 is the entry pressure of the first cycle (TC2), and PstartAngle is the predetermined pressure from which the total angle(Atot) of the tightening is to be counted.

3. Method according to claim 1 or 2, further comprising:- estimating the total angle (Atot) of the tightening, including addingto the estimated angle (AstartCycle) of the first cycle (TC2), for eachsubsequent cycle (TC3) detected as a full stroke cycle, saidpredetermined angle (Afull).

4. Method according to any one of the preceding claims, further comprising:- estimating an angle (AendCycle) that the tightening progressed in alast cycle (TC4) of the tightening.

5. Method according to claim 4, further comprising:- based on the signal, estimating an entry pressure (PentEnd) and anexit pressure (PexEnd) of the last cycle (TC4); and- based on the signal, estimating an entry pressure (PentFull) and anexit pressure (PexFull) of at least one full stroke cycle (TC3) preceding thelast cycle (TC4);wherein estimating the angle (AendCycle) of the last cycle (TC4) isbased on:a relation between the pressure and / or time range from the entrypressure (PentEnd) of the last cycle (TC4) to the exit pressure (PexEnd) of thelast cycle (TC4) and the pressure and / or time range from the entrypressure (PentFull) of said at least one full stroke cycle (TC3) to the exitpressure (PexFull) of said at least one full stroke cycle (TC3), andthe predetermined angle (Afull) corresponding to a full stroke ofthe piston.

6. Method according to claim 4 or 5, further comprising:- determining a retract time (TendCycle) of the last cycle (TC4); and- determining a retract time (TfullCycle) of at least one full stroke cycle(TC3) preceding the last cycle (TC4),wherein estimating the angle (AendCycle) of the last cycle (TC4) is based on: a relation (ratio) of the retract time (TendCycle) of the last cycle (TC4) and the retract time (TfullCycle) of the at least one full stroke cycle (TC3) preceding the last cycle (TC4), andthe predetermined angle (Afull) corresponding to a full stroke of thepiston.

7. Method according to claim 5 or 6, further comprising:- determining if a pressure gradient (dP1) between the entrypressure (PentFull) of said at least one full stroke cycle (TC3) and the exitpressure (PexFull) of said at least one full stroke cycle (TC3) matches a pressure gradient (dP2) between the entry pressure (PentEnd) of the lastcycle (TC4) and the exit pressure (PexEnd) of the last cycle (TC4).

8. Method according to any one of the preceding claims, wherein the stepsof detecting the first cycle (TC2) and estimating the entry and exitpressures (Pent1, Pex1) of the first cycle (TC2) are made in retrospect, suchas after the first cycle (TC2) is completed, such as after the tightening iscompleted.

9. Method as defined in any one of the preceding claims, wherein theestimation of the entry and exit pressures of the first cycle is made based on a derivative of the signal.

10. Control device configured to perform the method as defined in any one ofthe preceding claims.

11. Hydraulic torque wrench system comprising a control device according toclaim 10.

12. Computer program comprising instructions which, when the program isexecuted by a computer, cause the computer to carry out the method as defined in any one of claims 1-9.

13. 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 1-9.

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