Controlling a motor of a pulse tool using upper and lower limits

Abstract

A method of controlling a motor of a pulse tool is provided. The method comprises: determining a parameter value indicative of a torque and / or angle increase achieved by a just performed pulse (n); determining a motor power control parameter value (M) for the next pulse (n+1) based on said determined parameter value (d) indicative of the torque and / or angle increase and such that said motor power control parameter value is kept between an upper limit (Mmax) and a lower limit (Mmin); controlling the motor according to the determined motor power control parameter value to create the next pulse. Further, the upper and lower limits are set so as to converge towards a predetermined final target window (Mw) defining an allowable range within which the motor power control parameter value for the last pulse of the tightening operation is to be comprised. Thereby, less spread in installed torque is provided.

Description

[0001] CONTROLLING A MOTOR OF A PULSE TOOL USING UPPER AND LOWER LIMITS

[0002] Field of the invention

[0003] The present invention generally relates to the field of controlling motors of pulse tools. In particular, the present invention relates to pulse strategies for such pulse tools.

[0004] Background of the invention

[0005] In industrial assembly, the requirements on ergonomics for operators as well as precision in installed torque in a joint are of high importance. Therefore, so called pulse tools are often used. Pulse tools deliver the tightening torque in pulses, wherein the output torque is zero between each torque pulse. This may be achieved either by means of a hydraulic pulse unit that intermittently couples the motor to the output shaft, or by the motor of the tool itself being driven in a pulsed manner. Such pulse tools are known for achieving high precision in installed torque as well as low reaction forces for the operators due to the pulsing technique.

[0006] Another desire in industrial assembly is rapid tightening operations since this may save time (and therefore costs) in the assembly procedure. This desire may typically contravene the requirement on high accuracy in installed torque. To speed up a tightening operation, high motor power must be used. However, high motor power may result in lower precision in installed torque. In conventional pulse tools, the same motor power (and thereby the same energy) is used for the pulses throughout the tightening operations.

[0007] WO 2021151674 Al discloses a pulse tool allowing a user to set different power levels to be used during different stages of the tightening. Thus, the user can adjust the power level to be high in the beginning of a tightening up to a certain torque threshold, and then low as the torque exceeds that torque threshold, so that the tightening is performed with a lower power close to the target torque. Thus, the user is able to take the characteristics of the joint into consideration when setting the power for the pulses up to a certain torque threshold. It is possible to adapt the power so that the joint is tightened as fast as possible up to a certain torque threshold. It is also possible to achieve a more accurate tightening since the power can be set to a lower value close to the target torque.

[0008] A shortcoming with this solution is that it requires manual input and knowledge of the characteristics of the joints that the strategy is to be applied to. Further, the pre-set power levels may not be optimal for all the joints to be tightened, in terms of speed and accuracy.

[0009] 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 provide a method and a control device that enable a better tailored trade-off between speed and accuracy of tightenings of different joints.

[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 an aspect, a method of controlling a motor of a pulse tool is provided. The pulse tool is arranged to provide torque in pulses during a tightening operation to tighten a threaded joint. The method comprises:

[0013] - determining a parameter value indicative of a torque and / or angle increase achieved by a just performed pulse;

[0014] - determining a motor power control parameter value for the next pulse based on said determined parameter value indicative of the torque and / or angle increase and such that said motor power control parameter value is kept between an upper limit and a lower limit;

[0015] - controlling the motor according to the determined motor power control parameter value to create the next pulse; and repeating these steps until a target torque and / or angle is reached (and the tightening operation is ended).

[0016] Further, the upper and lower limits are set so as to converge towards a predetermined final target window defining an allowable range within which the motor power control parameter value for the last pulse of the tightening operation is to be comprised.

[0017] With the present aspect, the motor is controlled to output a power in the next (i.e. immediately succeeding) pulse dependent on the torque and / or angle increase achieved in the just performed (i.e. immediately preceding) pulse. Thereby, a control method is provided that will adjust the power (energy) of the motor during the tightening dependent on the characteristics of the joint. This enables a trade-off between speed and accuracy of the tightening that is tailored to the joint being tightened. For example, a soft joint will imply a lower increase in torque and higher increase in angle per pulse as compared to a hard joint. Due to the relatively small torque increase steps, softer joints may typically take longer time to tighten. The control method may detect this and determine a motor power control parameter value so as to provide an increase of the motor power in the next pulse. In this way, the tightening will proceed faster as compared to if no adjustment of the motor power was done. On the other hand, if the torque increase is higher (and the angle increase is lower), as a consequence of a harder joint, the control method may determine a motor power control parameter value so as to decrease the motor power in the next pulse, whereby the torque step per pulse will become smaller, which in turn will result in a better precision in the finally installed torque.

[0018] In conclusion, the present invention enables an automated procedure that provides a trade-off between speed and accuracy of tightening operations better adapted to different joints.

[0019] It will be appreciated that either one, or both of, a torque increase and an angle increase may be determined for the previous pulse. They are both parameters that, in opposite ways, reflects the stiffness of the joint. A pulse created with a given motor power will result in a larger torque increase and a smaller angle increase for a hard joint as compared to a soft joint.

[0020] Further, the inventor has realized that with an adaptive pulsing strategy (that is, a pulsing strategy wherein the pulse energy for a pulse is based on the torque / angle increase of the previous pulse), the energy in the final one or few pulses will differ, sometime significantly, between different tightenings.

[0021] Different pulse energy means different pulse speed, and the friction in the joint varies dependent on the pulse speed. With different friction, the force exchange between the tool and the joint will differ, that is, how much actual clamping force (N) is installed in the joint per torque (Nmj delivered by the tool. This may lead to a spread in installed clamping force between different joints.

[0022] To reduce such spread in installed clamping force, the present aspect provides upper and lower limits for the motor power control parameter, which upper and lower limits converge towards a predetermined final target window for the motor control parameter value of the last pulse. This means that different tightenings (of different joints) will be finalized with more similar pulse energy. In other words, a corridor (or envelope) is provided, within which the motor control parameter (and thereby the corresponding pulse energy) is allowed to vary. The corridor may be broader in the beginning of the tightenings to provide more room for the adaptive algorithm to operate and vary the pulse energy, and more narrow towards the end of the tightening to reduce the spread in energy of the last pulse between different tightenings.

[0023] According to an embodiment, the final target window may be predetermined based on the target torque for the tightening operation.

[0024] For example, a lower final target window may be set for a lower target torque and a higher final target window may be set for a higher target torque, whereby an improved trade-off is provided between precision in final installed torque and speed of the tightening. In case the tightening is controlled based on angle (up to a target angle), the torque may be monitored as an auxiliary parameter and a target torque may be pre-set as well, based on which the final target window may be predetermined.

[0025] According to an embodiment, the final target window may be predetermined based on the characteristics of the pulse tool (such as on the maximum output power of the motor, characteristics of the hydraulic pulse unit, torsional behaviour of the tool etc.). For example, the minimum value of the final target window may be adapted such that the tool delivers a desired torque increase and a desired accuracy in the last pulse for the hardest plausible joint.

[0026] According to an embodiment, the width (or size) of the final target window may correspond to less than 40% of a maximum output specified for the motor, such as less than 30% of a maximum output specified for the motor, such as less than 20% of a maximum output specified for the motor. A more narrow width of the final target window will provide less spread in the final installed torque in different joints. However, a too narrow window may make tightenings of soft joints slower than desired (considering that the minimum value of the final target window preferably should be kept at a level low enough to provide a desired accuracy in the final installed torque for hard joints). For example, the width (or size) of the final target window may correspond to more than 5% of a maximum output specified for the motor.

[0027] According to an embodiment, determining the parameter value indicative of the torque and / or angle increase may comprise determining a difference between a received input indicative of a torque and / or angle achieved in the just performed pulse and a received input indicative of a torque and / or angle achieved in the pulse just before the just performed pulse.

[0028] For example, the parameter value indicative of the torque and / or angle increase may be based on input received from a torque transducer and / or from an angle encoder and / or any other means for measuring or estimating the torque and / or angle in a performed pulse. According to an embodiment, the method may further comprise, before the tightening operation starts, receiving input (e.g. from a user interface) indicative of a desired trade-off between speed and accuracy of the tightening operation.

[0029] Determining the motor power control parameter value for the next pulse may be further based on said received input indicative of the desired trade-off between speed and accuracy.

[0030] For example, if the input indicates a desire of higher speed at the cost of lower accuracy, the motor power control parameter value for the next pulse may be determined to be higher as compared to if the input indicates a desire of higher accuracy at the cost of lower speed.

[0031] For example, a user may be able to select between two or more modes of operation for the pulse tool, such as one accurate mode, one normal mode and one speedy mode.

[0032] The present embodiment is advantageous in that it allows a user to tailor the trade-off between speed and accuracy to a specific application.

[0033] According to an embodiment, the upper and lower limits and / or the width of the final target window may be based on the received input indicative of the desired trade-off between speed and accuracy. If a speedier tightening is desired, a wider corridor and / or final target window may be set to allow for a greater variation in pulse energy towards the end of the tightening, which for tightening of relatively soft joints means speedier tightenings. If a more accurate tightening is desired, a narrower corridor and / or final target window may be set, which may imply a more strict limitation towards the end of the tightening which means less spread in the finally installed clamp force (that is, a higher accuracy).

[0034] According to an embodiment, the method may further comprise receiving input indicative of a (total) torque and / or total angle achieved in the just performed pulse (i.e. the total torque and / or angle achieved by the tightening so far), wherein determining the motor power control parameter value for the next pulse may be further based on the received input indicative of a (total) torque and / or total angle achieved in the just performed pulse. In this way, the motor power control parameter value for the next pulse may also be based on when in the tightening operation the next pulse is, that is, how close to the target torque / angle the next pulse is. In the beginning of the tightening operation, high pulse energy (motor power) is beneficial as it provides a speedy first part of the tightening, while low pulse energy (motor power) is better towards the end of the tightening operation as it will improve the accuracy of the finally installed torque.

[0035] According to an embodiment, the motor power control parameter value for the next pulse may be determined to be lower with a larger torque increase and / or smaller angle increase as indicated by said determined parameter value, and higher with a smaller torque increase and / or larger angle increase as indicated by said determined parameter value.

[0036] A smaller torque increase, as well as a larger angle increase, may indicate that the joint being tightened is relatively soft. This means that there is room for increasing the motor power output while still achieving a good accuracy in the finally installed torque. Analogously, a larger torque increase, as well as a smaller angle increase, may indicate that the joint being tightened is relatively hard. This means that it is better to decrease the motor power output to be able to achieve a good accuracy in the finally installed torque even though it comes at a cost of a slightly less rapid tightening as compared to if no adaptation of the motor power output had been made.

[0037] The present embodiment may be realised in different ways. For example, one or more thresholds for the torque and / or angle increase may be pre-set, such that if the torque increase in the just performed pulse is larger than a predetermined first threshold value, the motor power control parameter value is determined to be lower (e.g. by a predetermined amount) for the next pulse than that of the just performed pulse. Further, if the torque increase in the just performed pulse is smaller than a second predetermined threshold value (being lower than the first predetermined threshold value), the motor power control parameter value is determined to be higher (e.g. by a predetermined amount) for the next pulse than that of the just performed pulse. Similar thresholds may be set for the angle increase but with opposite logic.

[0038] According to another example, values of the torque / angle increase in the forgoing pulse may be correlated with specific motor power control parameter values for the next pulse in a look-up table or with a predetermined function.

[0039] In the present specification, the term “motor power control parameter” means a parameter that, when adjusted will make the output power (energy) of the motor to be adjusted accordingly. It may itself be set as a control target for a motor controller, or it may be converted to a control target for the motor controller. The motor power control parameter may e.g. be any one of: motor power, motor current, energy output from the motor, motor torque and motor speed (that, is the peak motor speed in the next pulse cycle). For example, if the motor power control parameter value is determined in terms of motor power, the determined motor power value may itself be used as input to the motor controller, or it may be converted to e.g. a motor current value, which in turn may be used as input to the motor controller. In this context, a motor controller means a device (e.g. being a part of a control device of the pulse tool) that, based on a received control target, regulates the motor to achieve that control target.

[0040] According to an embodiment, the pulses may be provided by a hydraulic pulse unit of the pulse tool, wherein the hydraulic pulse unit intermittently couples the motor via a hydraulic coupling mechanism to an output shaft of the pulse tool.

[0041] In this kind of pulse tool, the motor is driven to output a continuous (in contrast to intermittent) torque / power / energy during the tightening, and it is the hydraulic pulse unit that creates the pulses. The continuous torque / power / energy of the motor may be adjusted according to the repeatedly determined motor power control parameter value during the tightening.

[0042] Alternatively, the motor may itself be driven in a pulsed (intermittent) manner to provide the pulses on the output shaft of the pulse tool, wherein the output power / torque / energy of the motor is zero between the pulses. Hence, the motor delivers no torque / power / energy between the pulses. The intermittent torque / power / energy of the motor may be adjusted according to the repeatedly determined motor power control parameter value during the tightening.

[0043] According to a second aspect, a control device for controlling a motor of a pulse tool is provided. The pulse tool is arranged to provide torque in pulses during a tightening operation to tighten a threaded joint. The control device is configured to perform the method according to the first aspect.

[0044] According to an embodiment, a system is provided that comprises a pulse tool arranged to provide torque in pulses during a tightening operation to tighten a threaded joint, the pulse tool comprising a motor; and a control device according to the second aspect for controlling the motor of the pulse tool.

[0045] 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 according to the first aspect.

[0046] According to an embodiment, a computer-readable storage medium is provided comprising instructions which, when executed by a computer (such as the control device), cause the computer to carry out the method according to the first aspect.

[0047] It is noted that embodiments of the invention relates to all possible combinations of features recited in the claims. Further, it will be appreciated that the various embodiments described for the method are all combinable with the device as defined in accordance with the second aspect of the present invention.

[0048] Brief description of the drawings

[0049] 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.

[0050] Figure 1 shows a pulse tool comprising a hydraulic pulse unit according to an embodiment. Figure 2 shows a pulse tool with an intermittently driven electric motor according to an embodiment.

[0051] Figure 3 shows graphs of how different parameters varies during a tightening according to an embodiment.

[0052] Figure 4 shows a method according to an embodiment.

[0053] Figure 5 shows a graph illustrating how the motor power output varies during five tightenings of five different joints according to an embodiment.

[0054] Figure 6 shows a graph illustrating how the motor power output varies during five tightenings of five different joints according to another embodiment.

[0055] Figure 7 shows a graph illustrating how the motor power output varies during five tightenings of five different joints according to yet another embodiment.

[0056] All the figures are schematic, not necessarily to scale, and generally only show parts which are necessary in order to elucidate the embodiments, wherein other parts may be omitted. Like reference numerals refer to like elements throughout the description.

[0057] Detailed description of embodiments

[0058] A pulse tool 1 according to an embodiment will be described with reference to Figure 1. The pulse tool 1 may be arranged to tighten threaded fastener during industrial assembly. The pulse tool 1 comprises a motor 12, which preferably may be an electric motor 12. The motor 12 may comprise a rotor 14 and a stator 13. The tool 1 may further comprise an output shaft 16. The output shaft 16 may be arranged at a front end 10a of a housing 10 of the tool 1. The tool 1 may further comprise a hydraulic pulse unit 15 arranged to intermittently couple the motor 12 via a hydraulic coupling mechanism to the output shaft 16. For example, the pulse unit 15 may comprise an inertia drive member 18 coupled to the rotor 14 in a rotatable fixed manner. Hence, the inertia drive member 18 will rotate together with the motor 12. The inertia drive member 18 may comprise a cylindrical fluid chamber 19, into which an impulse receiving portion 11 of the output shaft 16 extends. An arrangement including cam profiles and pistons is arranged to intermittently transfer rotational energy from the inertia drive member 18 to the impulse receiving portion 11. Such an arrangement is known as such to the skilled person and will not be described further herein. An example of a hydraulic pulse unit is described in WO 9114541 Al.

[0059] In this type of pulse tool 1, the motor 12 will deliver a continuous, relatively low, torque, such as for example a torque of around 1 Nm. The hydraulic pulse unit 15 will then convert this continuous torque from the motor 12 to intermittent torque pulses on the output shaft 16, wherein each torque pulse e.g. may be around 15-55 Nm. Such a pulse tool 1 may in the following be referred to as a hydraulic pulse tool.

[0060] The tool 1 may further comprise a control device 20. The control device 20 may comprise processing circuitry 201 and a memory 202. The control device 20 may be configured to control the motor 12.

[0061] Alternatively, the control device 20 may be arranged outside, and in communication with, the tool 1.

[0062] The tool 1 may further comprise a sensor 3 arranged to sense a parameter indicative of a torque and / or angle achieved by a torque pulse delivered by the output shaft 16. For example, the sensor may comprise an angle encoder 3 arranged to measure an angular position and velocity of the output shaft 16 (or of a component in fixed rotational relation to the output shaft 16). The angular encoder 3 may sense an angle increase achieved by a just performed pulse. It may also sense a retardation of the angular velocity of the output shaft 16 resulting from the transfer of torque from the tool to the fastener. This retardation of the angular velocity is indicative of the torque in the performed pulse and may thus be used for estimating / calculating the torque achieved by the performed pulse.

[0063] As an alternative, or as a complement, the sensor may comprise a strain gauge (not shown) arranged to sense the strain in the output shaft 16 (or of a component in fixed rotational relation to the output shaft 16), which is indicative of the torque in the performed pulse. For example, the sensor may comprise a torque transducer.

[0064] Figure 2 depicts another exemplary embodiment of a pulse tool 101 of the present disclosure. In this embodiment, the pulses are created by driving an electric motor 112 in a pulsed (intermittent) manner. The pulses may be created as the electric motor 112 accelerates within a play in a gear arrangement 111 between the electric motor 112 and the output shaft 116. The energy from the motor 112 is then transferred to the output shaft 116 and a pulse is created on the output shaft 116. The electric motor 112 may comprise a rotor and a stator. Such a pulse tool 101 may in the following be referred to as a direct driven pulse tool.

[0065] For example, the gear arrangement 111 may comprise a play unit (not shown). Alternatively, the play unit may be arranged separately from the gear arrangement 111. The purpose of the play unit is to add play to the play that exists in the gear arrangement 11. An advantage with a separate play unit is that the amount of play can be chosen. In case a larger play is chosen in the play unit, there will be more time to control the speed of the electric motor 112 before the play in the play unit is closed and the electric motor 112 couples to the output shaft 116 to provide a torque pulse on the output shaft 116.

[0066] The tool 101 may further comprise similar components (such as a sensor 3, and a control device 20) as the tool 1 described with reference to Figure 1.

[0067] The principles of an embodiment of the invention will now be described with reference to Figures 3 and 4. Figure 3 illustrates how a determined motor power control parameter value M (a), actual motor (and inertia drive member) speed (b), pulse torque (c) and installed torque in the joint (d) will vary throughout a torque build-up phase (following a rundown phase) of a tightening operation performed by a pulse tool, such as by any one of the pulse tools described above. Figure 4 illustrates a method according to an embodiment.

[0068] The control device 20 may optionally be configured to, before the tightening operation starts, receive 401 input (e.g. via a user interface) indicative of a desired trade-off between speed and accuracy of the tightening operation. For example, the user may select between an accurate, normal and speedy mode.

[0069] First, the rundown of the tightening operation is completed. A start motor power control parameter value that the control device may control the motor according to may be pre-set for the first one or few pulses. Optionally, this start motor power control parameter value may be the same as the rundown motor power control parameter value.

[0070] Further, the control device 20 is configured to determine 402 a parameter value indicative of a torque and / or angle increase achieved by a just performed pulse n. This may e.g. be made by calculating the difference d between the torque achieved by the just performed pulse n with the torque achieved by the pulse n-1 performed just before the just performed pulse n (see graph c) in Figure 3). An angle increase may e.g. be derived directly from the angle encoder.

[0071] The control device 20 may be further configured to receive input indicative of a torque and / or total angle achieved in the just performed pulse (i.e. the total torque and / or angle achieved by the tightening so far).

[0072] The control device 20 is further configured to determine 403 a motor power control parameter value M (see graph a) in Figure 3) for the next pulse n+1 based on the determined parameter value indicative of the torque and / or angle increase. The motor power control parameter value M is determined so as to be kept between an upper limit Mmax and a lower limit Mmin. The upper and lower limits Mmax, Mmin may be predetermined so as to converge towards a final target window Mw. The final target window Mwis a predefined range / window, in which the motor power control parameter value M of the final pulse of the tightening is to be comprised. Hence, the algorithm governed by the control device 20 may adjust the motor power control parameter value M within a corridor defined by the upper and lower limits Mmax, Mmin such that it ends up within the final target window Mw.

[0073] The upper and lower limits Mmax, Mmin may e.g. be set for a majority of, such as the entire, torque build up phase of the tightening operation. For example, the upper and lower limits Mmax, Mmin may be set so as to lead (all the way) to the final target window Mw.

[0074] The method further comprises to control 404 the motor according to the determined motor power control parameter value M to create the next pulse n+1. Hence, the motor power control parameter value M may constitute a control target when controlling the motor. The motor power control parameter value M may indicate a target energy of the next pulse n+1. The motor power control parameter may e.g. be expressed in terms of a motor power / energy / torque to be output by the motor in the next pulse cycle, or in terms of motor current to be input to the motor in the next pulse cycle, or in terms of motor speed, and in particular, a peak motor speed to be output by the motor in the next pulse cycle (see the peaks in graph b) in Figure 3). The speed of the motor will build up after a performed pulse (both for hydraulic and direct driven pulse tools) up to a peak speed. Then, the motor speed will rapidly drop to (at least essentially) zero as the torque is transferred to the joint (via the hydraulic coupling in case of a hydraulic pulse tool). The value of the peak speed will determine the energy of the pulse.

[0075] The motor power control parameter value M for the next pulse n+1 may be determined to be lower with a larger torque increase and / or smaller angle increase, and higher with a smaller torque increase and / or larger angle increase. Thus, the algorithm operated by the control device 20 may adapt the motor power to the stiffness of the joint. As illustrated in Figure 3, the determined motor power control parameter value M may e.g. first increase in a few steps and then decrease in the following steps until the end of the tightening operation. The smaller steps towards the end will improve the accuracy of the tightening while the larger steps in the beginning will speed up the tightening.

[0076] Optionally, the motor power control parameter value M for the next pulse n+1 may be determined also based on the received input indicative of the desired trade-off between speed and accuracy. If a speedier tightening is desired, the control device 20 may decide on generally higher motor power control parameter values as compared to if a more accurate tightening is desired. Further, if a speedier tightening is desired, the upper and lower limits Mmax, Mmin. and / or the final target window Mw, may be set wider as compared to if a more accurate tightening is desired.

[0077] The final target window Mwmay be predetermined based on the target torque for the tightening operation. For example, the width (size) of the final target window Mwand / or how high the final target window Mwis set may depend on the target torque for the tightening operation. A wider and higher final target window Mwmay be set for a higher target torque and a narrower and lower final target window Mwmay be set for a lower target torque.

[0078] Further, the width (or size) of the final target window may correspond to less than 40% of a maximum output specified for the motor, such as less than 30% of a maximum output specified for the motor, such as less than 20% of a maximum output specified for the motor and preferably more than 5% of a maximum output specified for the motor.

[0079] Optionally, the motor power control parameter value M for the next pulse n+1 may be determined further based on the received input indicative of a (total) torque and / or total angle achieved in the just performed pulse. The closer the target torque or target angle the received torque / total angle of the previous pulse is, the lower motor power control parameter value M may be determined for the next pulse n+1.

[0080] The steps of determining 402 the parameter value indicative of the torque and / or angle increase, determining 403 the motor power control parameter value M and controlling 404 the motor accordingly are then repeated until a target torque and / or angle is reached, whereby the tightening operation is ended.

[0081] Figure 5 illustrates how the control device 20 adapts the motor power output (y- axis) as five exemplary tightening operations P1-P5 of five different joints proceeds (the torque installed in the joint is represented on the x-axis). Each circle / diamond / square / triangle represents one pulse. All five tightening operations P1-P5 may start at a pre-set motor power control parameter value that in the present example corresponds to around 64% of a maximum power output of the motor.

[0082] In the tightening operation Pl, the control device 20 soon detects that the torque increase is relatively low (such as below a predetermined threshold) and therefore determines to increase the motor power control parameter value. As can be seen, the motor power control parameter value is soon on a maximum level, whereby the motor power output is 100% of the maximum output of the motor. However, the upper limit for the motor power control parameter value will soon force the output power of the motor to gradually decrease (on the lower side of the corresponding upper limit Pmaxfor the motor power output) as the tightening proceeds such that the motor output power ends up within a final power window Pwcorresponding to the final target window Mwfor the motor power control parameter (essentially) at the target torque of 30Nm and the tightening is ended. This may typically happen when tightening a very soft joint. The generally high motor power output during the tightening (and in particular in the beginning of the tightening) will make sure the tightening goes faster as compared to if the motor power output had stayed on the initial value of 64% until the end of the tightening.

[0083] In the tightening operation P5 on the other hand, the control device 20 soon detects that the torque increase is relatively high (such as above a predetermined threshold) and therefore determines to decrease the motor power control parameter value. As can be seen, the motor power control parameter value will gradually drop (and as a consequence, also the motor power output) until the target torque of 30Nm is reached and the tightening is ended. However, the lower limit for the motor power control parameter will maintain the output power of the motor on an upper side of the corresponding lower limit Pmaxfor the motor power output as the tightening proceeds such that the motor output power ends up within the final power window Pw(corresponding to the final target window Mwfor the motor power control parameter value). This may typically happen when tightening a very hard joint. The generally low motor power output during the tightening will make sure the tightening goes slower, whereby a higher accuracy of the final installed torque is provided, as compared to if the motor power output had stayed on the initial value of 64% until the end of the tightening.

[0084] The control device 20 may operate analogously in the tightening operations P2- P4.

[0085] Each one of Figures 6 and 7 illustrates how the control device 20 may adapt the motor power output (y-axis) as five exemplary tightening operations P1-P5 of five different joints proceeds (the torque installed in the joint is represented on the x-axis] according to other embodiments. Figure 6 shows an embodiment, wherein the target torque is 20 Nm and the final target window is therefore set relatively low resulting in an allowable final motor power output Pwbetween 33% and 45% of the maximum output power. Figure 7, on the other hand, shows an embodiment, wherein the target torque is 50 Nm and the final target window is therefore set relatively high resulting in an allowable final motor power output Pwbetween 73% and 86% of the maximum output power.

[0086] 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.

[0087] 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

CLAIMS1. Method (400) of controlling a motor (12, 112) of a pulse tool (1, 101), the pulse tool being arranged to provide torque in pulses during a tightening operation to tighten a threaded joint, the method comprising:- determining (402) a parameter value (d) indicative of a torque increase and / or angle increase achieved by a just performed pulse (n);- determining (403) a motor power control parameter value (M) for the next pulse (n+1) based on said determined parameter value indicative of the torque increase and / or angle increase and such that said motor power control parameter value is kept between an upper limit(M max ) and a lower limit (Mmm);- controlling (404) the motor according to the determined motor power control parameter value to create the next pulse; and repeating these steps until a target torque and / or angle is reached, wherein the upper and lower limits are set so as to converge towards a predetermined final target window (Mw) defining an allowable range within which the motor power control parameter value for the last pulse of the tightening operation is to be comprised.

2. Method according to claim 1, wherein the final target window is predetermined based on the target torque for the tightening operation.

3. Method according to claim 1 or 2, wherein the final target window is predetermined based on the characteristics of the pulse tool.

4. Method according to any one of the preceding claims, wherein the width of the final target window corresponds to less than 40% of a maximum output specified for the motor, such as less than 30% of a maximum output specified for the motor, such as less than 20% of a maximum output specified for the motor.

5. Method according to any one of the preceding claims, wherein determining said parameter value indicative of the torque increase and / or angle increase comprises determining a difference (d) between a received input indicative of a torque and / or angle achieved in the just performed pulse and a received input indicative of a torque and / or angle achieved in the pulse (n-1) just before the just performed pulse.

6. Method according to any one of the preceding claims, further comprising, before the tightening operation starts, receiving (401) input indicative of a desired trade-off between speed and accuracy of the tightening operation.

7. Method according to claim 6, wherein the upper and lower limits and / or the width of the final target window are based on said received input indicative of the desired trade-off between speed and accuracy.

8. Method according to any one of the preceding claims, wherein the motor power control parameter value for the next pulse is determined to be lower with a larger torque increase and / or smaller angle increase as indicated by said determined parameter value, and higher with a smaller torque increase and / or larger angle increase as indicated by said determined parameter value.

9. Method according to any one of the preceding claims, wherein the motor power control parameter is any one of: motor power, motor current, motor torque, energy output from the motor, and motor speed.

10. Method according to any one of the preceding claims, wherein the pulses are provided by a hydraulic pulse unit (15) of the pulse tool (1), wherein the hydraulic pulse unit intermittently couples the motor (12) via a hydraulic coupling mechanism to an output shaft (16) of the pulse tool.

11. Method according to any one of claims 1 to 9, wherein the motor (112) is driven in a pulsed manner to provide the pulses on an output shaft (116) of the pulse tool (101), wherein the output of the motor zero between the pulses.

12. Control device (20) for controlling a motor of a pulse tool, the pulse tool being arranged to provide torque in pulses during a tightening operation to tighten a threaded joint, the control device being configured to perform the method according to any one of the preceding claims.

13. System comprising: a pulse tool arranged to provide torque in pulses during a tightening operation to tighten a threaded joint, the pulse tool comprising a motor; and a control device according to claim 12 for controlling the motor of the pulse tool.

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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