Technical field

The invention relates to an electric pulse tool adapted to perform tightening operations where torque is delivered in pulses and a method for controlling an electric tool adapted for performing pulsed tightening operations, where torque is delivered in pulses to tighten a screw joint.

Background

During a tightening operation, in which an electric tool is used for tightening a joint, torque is applied to the joint in pulses by a motor housed inside the electric tool. Often it is desired to control the tightening such that a specific torque or clamp force is installed into the joint. The applied torque may be monitored by a torque sensor, but it may also be monitored by a sensor that monitors the retardation of the output shaft so as to indirectly monitor the applied torque.

It is often important to achieve high accuracy when using electric tools. For instance when the electric tool is used in production is important that critical joints are tightened correctly. Therefore electric tools are often adapted to tighten screw joints to a specific target value. However even though the joints are tightened to the correct target value the clamp force in the joints can vary. This since the friction in the joints can vary between different joints. Adapting the electric tool to tighten screw joints as accurate as possible is thus not a guarantee that the correct clamp force is installed in the joints.

Hence, there exists a need for an improved electric tool,

Summary

An object of the present disclosure is to provide an electric tool that can tighten joints so that a correct clamp force is installed in the joints.

This object is achieved in accordance with a first aspect of the disclosure by an electric tool for performing tightening operations, where torque is delivered in pulses to tighten a screw joint. The electric tool comprising an electric motor drivingly connected to an output shaft. The electric tool further comprising a processor and a memory storing software instructions that, when executed by the processor cause the electrical pulse tool to control the speed of motor, so that the electric tool provide torque pulses on the output shaft until a torque threshold value is reached. Start measuring an angular displacement of the output shaft when the torque threshold value is reached and provide torque pulses on the output shaft until a target angular displacement of the output shaft is reached.

According to the first aspect, the electric power tool provides an inventive solution to the concerns described above by providing torque pulses on the output shaft until a torque threshold value is reached and then start measuring an angular displacement of the output shaft when the torque threshold value is reached.

Thereafter provide torque pulses on the output shaft until a target angular displacement of the output shaft is reached. Thus it is possible to tighten a screw joint towards an angel target instead of a torque target. An advantage with this approach, is that the spread in clamping force becomes lower compared to if the screw joint is tightened to a target torque. Tightening towards a target torque is strongly affected by the friction. The friction however varies between different screws, which makes the spread in clamping force higher compared to if the screw joint is tightened to a target angle.

According to one embodiment, the pulses are provided by a hydraulic pulse unit coupled to the electric motor, the hydraulic pulse unit intermittently couples the electric motor via a hydraulic coupling mechanism to the output shaft. Thus the strategy according to the present disclosure can be used in an electric tool comprising a hydraulic pulse unit. Thereby providing the possibility to tighten to a target angle with an electric hydraulic pulse tool. An advantage, as mention above, is that the spread in clamp force can be reduce when tightening a screw joint to a target torque instead of tighten to target torque as in prior art solutions.

According to one embodiment, the motor is driven in a pulsed manner to provide pulses on the output shaft. In this embodiment the pulses are provided by acceleration the motor within the inherent play that exist in the gearbox between the motor and the output axel. In other embodiment the motor is accelerated within a certain play unit that is provided between the motor and the output axel. Hereby rotational energy is built up in the tool.

This rotational energy is then transferred to the screw as a torque pulse, when the play between the motor and the output axle is closed.

According to one embodiment, the torque threshold value is a preset value. Herby the torque threshold value can be set to be the same for same type of screw joints. Thus ensuring that screw joints of the same type are tightened to a target angle from the same torque threshold value.

According to one embodiment, the electric tool is further operative to determine the torque threshold value based on a target parameter for the tightening, such that the torque threshold value increases when the target parameter increase and the torque threshold value decreases when the target parameter decreases. Herby it is possible to automatically determine the torque threshold based on the target parameter. When the target parameter increase the torque threshold increases. This since increasing the target parameter indicates that the screw joint is tightened to a higher torque. This is true both a target parameter being torque and angle.

According to one embodiment, the torque threshold value is 31 per cent or less of the electric tools maximum torque value. Herby is possible to start measuring an angular displacement of the screw joint before a substantially angular displacement of the screw has occurred. Thus ensuring a lower spread in the clamp force between different screw joints. In a tightening where the angular displacement is started to be measured before the torque threshold value is 31 per cent of the electric tools maximum torque value the screw has not been significantly turned. Therefore the angular measurement will be performed over an angular interval long enough to have impact on the characteristics of the tightening. Thus it is possible to tighten a screw towards an angle target instead of a torque target, since the angle measurement is started at a torque value of 31 percent or less of the electric tools maximum torque value.

According to one embodiment, the speed of the motor is controlled by continuously monitor the actual position of the motor. Herby an angle sensor can be arranged to determine the position of the motor. According to one embodiment, the torque pulses before the torque threshold value is reached is provided with a torque value of less than 15 per cent of the electric tool max torque value. Herby is possible to start measuring an angular displacement of the screw joint before a substantially angular displacement of the screw joint has occurred. Thus ensuring a lower spread in the clamp force between different screw joints.

In accordance with a second aspect the disclosure relates to a method for controlling an electric tool for performing tightening operations, where torque is delivered in pulses to tighten a screw joint. The method comprising the steps of: controlling the speed of a motor, so that the electric tool provides torque pulses on an output shaft until a torque threshold value is reached. Start measuring an angular displacement of an output shaft when the torque threshold value is reached. And controlling the speed of the motor, so that the electric tool provides torque pulses on the output shaft until a target angular displacement of the output shaft is reached.

Advantages and of embodiments according to the second aspect have been described above in relation to the embodiments of the first aspect.

Brief description of the drawings

The invention will now be described in more detail and with reference to the accompanying drawings, in which:

Fig. 1 shows a longitudinal section through the electric tool according to an exemplary embodiment of the present disclosure. Fig. 2 shows example diagram of torque pulses according to an exemplary embodiment of the present disclosure.

Fig. 3 illustrates a flow chart according to an exemplary embodiment of the present disclosure.

Detailed description

Aspects of the present disclosure will be described more fully hereinafter with reference to the accompanying drawings. The device, method and computer program disclosed herein can, however, be realized in many different forms and should not be considered as being limited to the aspects set forth herein. Like numbers in the drawings refer to like elements throughout.

The terminology used herein is for the purpose of describing particular aspects of the disclosure only, and is not intended to limit the disclosure. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise.

In fig. 1 an electric tool 10 in accordance with a specific embodiment of the disclosure is schematically shown. The electrical pulse tool 10 is configured to perform tightening operations where torque is delivered in pulses to tighten screw joints. For this purpose the electrical pulse tool 10 comprises a bidirectional electric motor 12 which is arranged to deliver torque in two opposite rotational directions, i.e. clockwise and counter clockwise.

The electric tool 10 further comprises a handle 22, which is of a pistol type in the shown embodiment. The disclosure is however intended to cover any type of pulse tools. A power supply 24, such as a battery, is arranged in the lower part of the handle and a trigger 23 is arranged for manipulation of the operator so as to power the electric motor 12. The power supply may also be a connection to an electric cable.

Further, the electric tool comprises an output shaft 14 and a position sensor 25 arranged to determine an angular displacement of the motor. The shown embodiment further comprises hydraulic pulse unit 13. The hydraulic pulse unit 13 comprising an inertia body 18 that houses a piston activated rotator 19. The inertia body 18 is rigidly connected to the rotor 20 of the motor 12. The rotor 20 is in the shown embodiment arranged coaxially inside a stator of the motor 12. A pulse is generated as cam surfaces (not shown) on the inside of the inertia body 18 interacts with the pistons so as to force the rotator 19 to rotate in a conventional manner well known in the art. The rotator is connected to the output shaft 14. Herby the pulses are transferred to the screw being tightened via an output shaft 14.

The disclosure is however not limited to electric tools with a pulse unit. Pulses may also be produced in electric tools with a direct connection between the motor and the output shaft by pulsing the motor of the electric tool. The disclosure also covers such electric tools.

The electric tool 10 further comprising a processor 16 arranged to control the electric motor 12. The electric tool 10 also comprises a memory 26 containing instructions executable by the processor 16.

The inventors have realised that less scatter in the clamp force of the screw joints can be achieved by tightening screw joints to a target angle instead of to a target torque. An advantage with this solution is that different screws are turned the same angle. As mention above, since the friction between screw joints vary, the clamp force between different screw joints becomes much less if screws are tightened to a target angle instead of to a target torque.

The electric tool according to the present disclosure can accurately measure the angular displacement of an element of the screw joint during a tightening operation. This since the angular displacement of an element is measured when the torque threshold value is reached. Thus the electrical pulse tool 10 starts to measure an angular displacement of the output shaft when the torque threshold value is reached. By waiting until the torque threshold value is reached it will be possible to accurately measure the angular displacement of the screw or bolt. This since the measurement of the angular displacement is started from the same torque value. Since the torque threshold value is chosen to be a low torque value there will not be a significant angular displacement of the e.g. the screw, bolt or nut.

The software instructions are executed by the processor 16 the electrical pulse tool is operative to control the speed of the motor, so that the electric tool provide torque pulses on the output shaft until a torque threshold value is reached, Then start measuring an angular displacement of the output shaft when the torque threshold value is reached. And provide torque pulses on the output shaft until a target angular displacement of the output shaft is reached.

The processor 16 is a Central Processing Unit, CPU, microcontroller, Digital Signal Processor, DSP, or any other suitable type of processor capable of executing computer program code. The memory 26 is a Random Access Memory, RAM, a Read Only Memory, ROM, or a persistent storage, e.g. a single or combination of magnetic memory, optical memory, or solid state memory or even remotely mounted memory.

According to one aspect, the disclosure further relates to the above mentioned computer program, comprising computer readable code which, when run on the electric tool causes the electric tool to perform any of the aspects of the disclosure described herein.

According to one aspect of the disclosure the processor 16 comprises one or several of: a pulse module 161 adapted to provide torque pulses on the output shaft until a torque threshold value is reached; an angle module 162 adapted to start measuring an angular displacement of the output shaft when the torque threshold value is reached; a pulse module 163 adapted to provide torque pulses on the output shaft until a target angular displacement of the output shaft (14) is reached.

The pulse module 161, the angle module 162, and the pulse module 163 are implemented in hardware or in software or in a combination thereof. The pulse module 161, the angle module 162, and the pulse module 163 are according to one aspect implemented as a computer program stored in the memory 26 which run on the processor 16. The electric power tool is further configured to implement all the aspects of the disclosure as described herein.

Now turning to figure 2, which shows one example of a number of pulses in a tightening performed by the electric tool. In figure 2 the torque value of the torque pulses illustrated as a function of time t. In this example the tightening operation is illustrated as comprising 6 torque pulses 1-6. The tightening operation can however require fewer or more torque pulses. Each torque pulse will turn the output shaft an angular interval and thus increase the angular displacement of the output shaft.

As can be seen in figure 2 the electric tool 10 provides torque pulses 1 to 3 until the torque threshold value is reached. Next, the electric tool 10 starts to measure the angular displacement of the output shaft and provides torque pulses 4 to 6 on the output shaft. In the tightening illustrated in figure 2, the electric tool then provides tightening torque pulses until a target angular displacement of the output shaft is reached.

Figure 3 illustrates the steps in a method, performed in the electric tool 10 according to the above described exemplary embodiments. In a first step S10 the electric tool 10 controls the speed of the motor, so that the electric tool provides torque pulses on the output shaft until a torque threshold value is reached. In a next step S20 the electric tool 10 start measuring an angular displacement of the output shaft when the torque threshold value is reached. Thereafter in a next step S30 the electric tool 10 controls the speed of the motor, so that the electric tool provides torque pulses on the output shaft until a target angular displacement of the output shaft is reached.

According to one exemplary embodiment of the method, the torque threshold value is a preset value.

In another exemplary embodiment of the method, the torque threshold value is 31 per cent or less of the electric tools maximum torque value.

According to another exemplary embodiment of the method, the speed of the motor is controlled by continuously monitor the actual position of the inertia drive member. In a yet another exemplary embodiment of the method, the torque is determined based on the retardation magnitude of the motor. According to another exemplary embodiment the method comprising a further step of determining the torque threshold value based on a target parameter for the tightening, such that the torque threshold value increases when the target parameter increases and the torque threshold value decreases when the target parameter decreases. According to one embodiment the target parameter is torque. According to another embodiment the target parameter is angle.

Aspects of the disclosure are described with reference to the drawings, e.g., block diagrams and/or flowcharts. It is understood that several entities in the drawings, e.g., blocks of the block diagrams, and also combinations of entities in the drawings, can be implemented by computer program instructions, which instructions can be stored in a computer-readable memory.

In the drawings and specification, there have been disclosed exemplary aspects of the disclosure. However, many variations and modifications can be made to these aspects without substantially departing from the principles of the present disclosure. Thus, the disclosure should be regarded as illustrative rather than restrictive, and not as being limited to the particular aspects discussed above. Accordingly, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation.