EP2527091B1 - Screwdriver and method for controlling a screwdriver - Google Patents

Description

The invention relates to a screwdriver with a drive comprising an electric motor which is coupled to a drive shaft for driving a tool for tightening a screw with a predetermined torque, with a current sensor for detecting a motor current, with a control device for controlling the drive, the is designed to decelerate the drive upon reaching a braking criterion.

Such a screwdriver is from the DE 10 2008 033 866 A1 known.

The known screwdriver comprises a limiting device for limiting a delivery torque provided on the output side of the drive train to a maximum torque value. The control device controls the drive motor, monitors the drive train, switches it off when a shutdown criterion is reached, and actuates a braking device for actively braking the drive motor by means of a rotating field opposite the respective direction of rotation of the drive motor. The limiting device for limiting the output side provided output torque to a torque maximum value controls the energizing means via the drive motor for braking operation under at least one braking condition, wherein a quotient of the output torque and an existing in the drive train rotational energy is less than or equal to a maximum value. In this way it should be prevented that damage occurs by excessive braking especially in small screws or delicate workpieces.

The disadvantage of this known arrangement is in particular that constantly very many, the tightening torque influencing individual parameters must be monitored and thus a very high computational effort is required, which is either detrimental to the accuracy or the screwing time.

From the WO 2009/107563 A2 is a screwdriver with a brushless EC motor known in which the motor current and the number of revolutions of the motor shaft is used to estimate the tightening torque of a screw. The drive is stopped when the estimated tightening torque exceeds a certain estimated value.

At another from the DE 103 41 975 A1 Known screwdriver torque limiting device is provided, which is limited by a control of the motor current as a function of the speed to a maximum value.

The disadvantage of this screwdriver is that the tightening torque is subject to considerable inaccuracy.

For many manual and industrial screwdriving applications, it is important to provide a screwdriver with a high level of repeat accuracy for a given tightening torque for a large number of screwed connections. For example, it is important for visible after installation wood fittings, both for the visual appearance, as well as for the mechanical support that each screw head is always the same deep sunk. The same applies to sheet metal fittings in a slightly modified form. For example, sheet metal fittings should be tightened on a façade with uniformly high torque. If the screw is tightened too weak, the screw head does not completely seal off the resulting screw hole and moisture can penetrate. On the other hand, if the screwdriver shuts down too late, there is the possibility that the screw head will be torn off and a costly rework will be required.

Against this background, the object of the invention is to provide a screwdriver and a method for controlling a screwdriver with a high repeat accuracy for a predetermined tightening torque for a large number of screw connections. This should be guaranteed to work as effectively as possible.

This object is achieved with a screwdriver according to the type mentioned above in that the control device is designed on the basis of the predetermined torque for calculating a limit value for the motor current at which the drive is put into a waiting state, and that the control device when exceeding the limit value for the motor current for the determination of the braking criterion on the basis of the predetermined torque for calculating an activation time or a rotation angle at which the braking process is initiated, at least as a function of the kinetic energy of the drive at the time of exceeding the limit for the motor current is formed, wherein the electric motor is energized during the waiting state until the initiation of the braking process, not or only partially energized.

• Erfassen eines vorbestimmten Anzugsmoments;
• auf der Basis des vorbestimmten Anzugsmoments Berechnen eines Grenzwertes für den Motorstrom, bei Überschreiten dessen der Antrieb in einen Wartezustand überführt wird;
• Anziehen der Verschraubung durch Antreiben des Werkzeugs;
• Überwachen des Motorstroms und Überführen des Antriebs in den Wartezustand bei Überschreiten des Grenzwertes für den Motorstrom;
• Bestimmen einer kinetischen Energie des Antriebs zum Zeitpunkt des Überschreitens des Grenzwertes für den Motorstrom;
• Bestimmen eines Bremskriteriums zur Einleitung einer Bremsung des Antriebs zumindest auf der Basis der kinetischen Energie des Antrieb zum Zeitpunkt des Überschreitens des Grenzwertes für den Motorstrom;
• Einleiten eines Bremsvorgangs bei Erreichen des Bremskriteriums;
• wobei der Antriebsmotor während des Wartezustands bis zum Einleiten des Bremsvorgangs weiter bestromt wird, gar nicht oder nur teilweise bestromt wird.

Regarding the method, the object of the invention is achieved by a method for controlling a screwdriver to a screw with a tightening predetermined torque by means of a drivable by a drive tool, comprising the following steps:

• Detecting a predetermined tightening torque;
• on the basis of the predetermined tightening torque calculating a limit value for the motor current, when the drive is exceeded in a waiting state is exceeded;
• Tightening the screw connection by driving the tool;
• Monitoring the motor current and transferring the drive to the standby state when the motor current limit is exceeded;
• Determining a kinetic energy of the drive at the time of exceeding the limit value for the motor current;
• Determining a braking criterion to initiate braking of the drive based at least on the kinetic energy of the drive at the time the motor current limit is exceeded;
• Initiating a braking process when the braking criterion is reached;
• wherein the drive motor is energized during the waiting state until the initiation of the braking process, not or only partially energized.

The object of the invention is achieved in this way.

According to the invention, in a first phase of the screwing operation, the drive is initially driven without limitation until the previously determined Criterion for the wait state is reached when a limit for the motor current is exceeded. If the limit of the motor current is exceeded, the kinetic energy of the drive is also detected at this time and used for the determination of the braking criterion for braking. During the waiting state until the initiation of the braking process, the motor is either further energized or not energized or only partially energized.

In this way, there is a very high repeatability for a predetermined torque to a variety of glands.

The torque with which a screw is tightened after the end of the screwing operation is effected by three components: A first component is the torque generated by energization of the electric motor until reaching the waiting state of the drive. This represents the largest component of the tightening torque. A second part of the tightening torque is based on the transmitted torque from the time the limit value is exceeded until the onset of braking. If the motor continues to be energized during this time, which is preferred according to the invention, this proportion is likewise generated by the motor current. The third part of the tightening torque is generated by the kinetic energy of the drive during the braking process. The more energy is removed from the drive by the braking process, the lower the torque component of the tightening torque during the braking process. This has the advantage that the time between the exceeding of the limit value for the motor current and the standstill of the screwdriver is low. Thus, the shutdown time of the drive can be set later, whereby the repeatability increases.

In a further embodiment of the invention, the control device is designed for calculating an average value, in particular a moving average, of the motor current which is used for the calculation of the limit value for the motor current.

In this way, fluctuations of the motor current can be compensated by the commutation of the electric motor and asymmetries and taken into account in an average value of the motor current. This improves the accuracy.

According to a further embodiment of the invention, the limit value for the motor current is derived from a directly proportional relationship between the torque and the motor current. This is particularly advantageous when the electric motor is an electronically commutated electric motor, in particular a permanent magnet synchronous motor (PMSM) or BLDC motor. For these motors, the output torque is directly proportional to the motor current. In this way, the predetermined torque can be derived in a particularly simple manner from a measurement of the motor current.

According to a further embodiment of the invention, at least one sensor for detecting at least one parameter is provided, which is selected from the group consisting of a temperature of a transmission to which the electric motor is coupled, a temperature of the electric motor, a rotational speed of the electric motor, a transmission ratio of the transmission and a supply voltage of the screwdriver consists.

Since the kinetic energy of the drive at the time of exceeding the limit for the motor current depends on a number of parameters, it makes sense to monitor these parameters by means of sensors and to take into account in the determination of the braking criterion.

Preferably, in this case, the kinetic energy of the drive at the time of exceeding the limit value for the motor current is estimated on the basis of parameters that include at least the rotational speed of the drive, and the moment of inertia of the drive, ie, the rotating parts of the drive.

As far as the drive comprises an electric motor and a gearbox coupled thereto, the rotational speed of the electric motor and the gear ratio of the gearbox or of the drive are taken into account as parameters.

According to a further embodiment of the invention, the parameters comprise at least one further parameter which is selected from the group consisting of a supply voltage of the screwdriver, a temperature of the electric motor and a temperature of a transmission coupled to the electric motor.

In this way, the repeat accuracy of the tightening torque can be further increased since the actual engine speed is dependent on the supply voltage, i. the battery voltage, if an accumulator is used for power supply, as well as the temperature of the engine and transmission is dependent. Likewise, the speed is influenced by the position of the accelerator switch.

By taking into account these influences, the repeat accuracy in maintaining the predetermined tightening torque is improved.

According to a further embodiment of the invention, the control device is designed to store a functional relationship between the braking criterion and the parameters that are used to determine the braking criterion, and to determine the braking criterion, taking into account the functional relationship.

The parameters which are included in the determination of the braking criterion, in particular on the basis of the kinetic energy of the drive at the time of exceeding the limit value for the motor current, are preferably determined empirically and stored the functional relationship in a memory. The control device can then take these parameters into account when determining the braking criterion. For this purpose, the parameter values, which are generally determined analogously, are preferably converted into digital values via A / D converters and can then be taken into account simply in an additive or subtractive manner in the determination of the braking criterion.

According to a further embodiment of the invention, the predetermined torque is adjustable, preferably by means of a suitable actuator, such as with the aid of a potentiometer.

More preferably, in this case, the potentiometer at lower torques on a higher resolution than at higher torques.

This can be realized either by a potentiometer with a corresponding characteristic curve, for example in a logarithmic or expotential characteristic curve or else in software via the control device. Similarly, a read-in value may be multiplied by a proportionality constant to obtain a linear progression of torque provided to the tool through the potentiometer's adjustment path.

A more sensitive adjustment of the tightening torque at low tightening torques is advantageous, since the tightening torques for tightening, in particular at lower values, generally have to be observed more accurately, while at high tightening torques a certain deviation is less significant.

For braking the electric motor can be carried out according to a further embodiment of the invention, a short-circuiting of the electric motor for a first period of time. This is the conventional short-circuit braking in electric motors.

According to a further embodiment of the invention, however, the control device is additionally designed for the braking of the electric motor for energizing the electric motor for a second period of time counter to its original direction of rotation.

By combining a braking of the electric motor by shorting for a first period of time and by energizing the electric motor for a second period contrary to its original direction of rotation can be a much faster braking (so-called hybrid braking) can be achieved.

This method is described in detail in the German patent application with the file number 10 2010 032 335.7 which is not prepublished, and which is hereby incorporated by reference in its entirety.

With a faster deceleration of the drive, a smaller predetermined torque can be achieved. Furthermore, the glands become more reproducible and more accurate as the amount of energy transferred during the braking phase becomes smaller. A certain disadvantage is that, in the case of the use of a rechargeable battery with active counter-energization, energy is required from the rechargeable battery and the number of screwings per rechargeable battery charge thus drops.

In accordance with a further embodiment of the invention, the control device may have a pulse width modulation controller for controlling the electric motor. In this case, the pulse width modulation control can be used, in particular, to effect a pulse-width-modulated current supply in the direction of rotation, a pulse width-modulated short circuit or a pulse-width-modulated current supply counter to the original direction of rotation of the electric motor.

It is possible to achieve both a simple control of the average motor current during the drive phase and a control of the short-circuit current during the braking phase or the counter-energization during the braking phase.

According to a further embodiment of the invention, the screwdriver on an accumulator for power supply. Preferably in the form of a nickel-cadmium rechargeable battery, a lithium-ion rechargeable battery, a lithium-polymer rechargeable battery or a nickel-metal hydride rechargeable battery.

By using a rechargeable battery, the practicality of the screwdriver can be significantly improved, as leads, which can affect handling during operation, are eliminated. The state of charge of the accumulator is preferably taken into account in the determination of the braking criterion.

In a further embodiment of the invention, the electric motor has a permanently excited rotor, which is associated with at least one position sensor, preferably a Hall sensor, for detecting the position of the rotor.

Alternatively, the control device for indirect detection of the position of the rotor via the mutual induction of a field winding of the electric motor may be formed.

In both cases, such a position detection, a sense of rotation or a speed detection is possible. In this way, can be dispensed with a separate speed sensor.

Also in this way the object of the invention is achieved.

Fig. 1

eine schematische Darstellung eines erfindungsgemäßen Schraubers;

Fig. 2

eine vereinfachte schematische Darstellung einer mit dem Elektromotor gekoppelten Steuereinrichtung;

Fig. 3

ein Flussdiagramm für die Steuerung des Schraubers;

Fig. 4

den Zusammenhang zwischen der Versorgungsspannung und der Drehzahl des verwendeten BDLC-Motors;

Fig. 5

die Dauer eines Bremsvorgangs in Abhängigkeit von der Drehzahl des Elektromotors bei Einleitung der Bremsung;

Fig.6

den Stromverlauf bei einem beispielhaften Schraubvorgang mit anschließendem Wartezustand bis zu einer anschließenden Bremsung über der Zeit mit einer langsamen Drehzahl;

Fig. 7

einen vergrößerten Ausschnitt aus der Darstellung gemäß Fig. 6 im Bereich des Wartezustands bis zum Bremsbeginn und anschließender Bremsung;

Fig. 8

den Verlauf des Motorstroms in Abhängigkeit von der Zeit bei einer weiteren Verschraubung mit einer hohen Drehzahl und

Fig. 9

einen Ausschnitt aus der Darstellung gemäß Fig. 8 im Bereich des Wartezustands bis zum Bremsbeginn und anschließender Bremsung;.

Further features and advantages of the invention will become apparent from the following description of a preferred embodiment of the invention with reference to the drawing. Show it:

Fig. 1

a schematic representation of a screwdriver according to the invention;

Fig. 2

a simplified schematic representation of a coupled to the electric motor controller;

Fig. 3

a flow chart for the control of the screwdriver;

Fig. 4

the relationship between the supply voltage and the speed of the BDLC motor used;

Fig. 5

the duration of a braking operation as a function of the rotational speed of the electric motor when braking is initiated;

Figure 6

the current flow in an exemplary screwing followed by wait state until a subsequent braking over time at a slow speed;

Fig. 7

an enlarged section of the illustration according to Fig. 6 in the area of the waiting state until the start of braking and subsequent braking;

Fig. 8

the course of the motor current as a function of time in another screw with a high speed and

Fig. 9

a section of the illustration according to Fig. 8 in the area of the waiting state until the start of braking and subsequent braking;

In Fig. 1 an inventive screwdriver is shown in simplified form and designated overall by the numeral 10.

The screwdriver 10 has a housing 11 with a pistol-shaped handle. In the housing 11, a drive 12 is accommodated, which consists of an electric motor 14 and a gear 16 which is driven by the electric motor 14 via the motor shaft 18. The gear 16 drives a drive shaft 20, at the outer end thereof a receptacle for a tool 22, e.g. a bit, is provided.

Although the screwdriver 10 could be operated in principle by means of a mains voltage, this has in the present case an accumulator 40 as an energy source, which is interchangeable received at the bottom of a pistol-shaped handle. The accumulator 40 can be, for example, a lithium-ion accumulator. The screwdriver 10 is controlled by a central control device 36, which is coupled to the electric motor 14, the transmission 16 and a throttle switch 32 via suitable connection lines. Furthermore, the control device 36 is coupled to the accumulator 40 with the interposition of a current sensor 38 and a voltage sensor 39.

In order to set a predetermined tightening torque for a screw connection, an actuating element 34 is provided in the region of the housing 11 facing away from the tool 22, which may be, for example, a potentiometer. Again, this is suitably connected in line with the control device 36. Furthermore, 16 sensors 24, 26, 28, 30 are provided on the electric motor 14 and the transmission, which are designed to detect the transmission temperature, the engine temperature, the rotational speed of the electric motor 14 and a gear recognition and connected to the control device 36 in a suitable manner are.

On the adjusting element 34, a tightening torque for tightening a screw can be adjusted. For this purpose, the corresponding potentiometer preferably has a higher resolution at lower torques and a lower resolution at higher torques, so that especially at low torques a sensitive adjustment is possible.

Fig. 2 schematically shows a simplified representation of the control of the screwdriver.

The electric motor 14 is formed in the present case as a permanent-magnet EC motor in the form of a BLDC motor. It has a rotor 42 with two permanent-magnetic poles and three field windings 60, 62, 64, which are arranged in the described case in the form of a star connection. It is understood that alternatively a triangular circuit could be used and that both the number of field windings and the number of poles could be varied. The excitation windings 60, 62, 64 are controlled by associated driver circuits 48, 50, 52 from the central control device 36.

In the electronically commutatable electric motor 14, it is necessary to effect the rotating field via the motor control. Such a control of the excitation windings 60, 62, 64, among other things, makes it possible to dispense with brushes for contacting, so that the electric motor 14 can be designed to be low-maintenance or virtually maintenance-free overall. For detecting the position of the rotor 42, a position sensor 58 may be provided be coupled to the controller 36. The position sensor 58 may be designed as a Hall sensor, for example. The position sensor 58 is designed to detect the magnetic field of the pole pairs of the rotor 42 or a separately provided on the motor shaft 18 sensor disc (not shown) for determining the position.

The drivers 48, 50, 52 serve to selectively drive individual excitation windings 60, 62, 64, for example with a high signal, a low signal or else a zero signal in order to form the alternating field. The signal can basically have a block-shaped, a sinusoidal or a pulse-width-modulated curve. The drivers 48, 50, 52 may in particular be integrated or discrete power transistors. The outputs of the drivers 48, 50, 52 are connected to the field windings 60, 62, 64.

In Fig. 2 are also simplified short-circuit switch 54, 56 indicated, which are adapted to short-circuit the field windings 60, 62, 64 in the event of braking. The short-circuit switches 54, 56 can be coupled to the excitation windings 60, 62 in order to short-circuit them. The short-circuiting switches 54, 56 illustrated here are designed, for example, to short-circuit the exciter winding 60 with the exciter winding 62. In a similar manner, further short-circuiting switches can be provided, for example, to short-circuit the excitation winding 60 with the field winding 64 or the field winding 62 with the field winding 64.

It is understood that the short-circuiting switches 54, 56 are shown as illustrative only as separate elements. The functionality of the shorting switches 54, 56 can also be accomplished via the controller 36 using the drivers 48, 50, 52.

In Fig. 2 schematically a microprocessor 37 is indicated, and a memory 43 which serves as a program memory and in which also parameterized characteristics can be stored. Furthermore, the control device 36 comprises a clock generator 44 and a pulse width modulation controller (PWM) 46. It goes without saying in that the said components can readily be fully integrated or designed as part of the control device 36 or discretely.

According to the invention, the screwdriver 10 is designed to tighten screws with a high repeat accuracy with a predetermined torque that is preset.

This will be explained below on the basis of FIGS. 3 to 9 explained in more detail.

The EC motor in the form of a BLDC motor used according to the invention has a direct proportionality between the motor current I _M and the generated torque M: M [Nm] = k _t * I _M [A]. In this case, the motor current I _{M is} measured in amperes at the base of the circuit (bus current), cf. Current sensor 38 in Fig. 1 , k _t is the proportionality constant.

In the BLDC motor 14 used, the speed is also proportional to the applied voltage. This connection is in Fig. 4 in which the speed n [1 / min] is plotted against the voltage U [V]. This relationship can be represented as n [1 / min] = k _e * U [V]. Here k _e is a proportionality constant. This describes how high the induced voltage U in the motor 14 is at a certain speed n. In order to allow a current plus and thus to generate a torque, a difference between the external voltage (battery voltage) and the internal, induced voltage EMK is necessary. The larger the available voltage U, the faster the motor rotates.

In Fig. 5 Furthermore, the relationship between the braking time t [ms] and the speed n [1 / min] is shown. It can be seen that the braking time at the maximum speed of about 24000 rpm is about 25 ms, while at a speed of only about 7000 rpm it is 5 ms. Due to the hybrid brake used according to the DE 10 2010 032 335.7 , which is hereby incorporated by reference in its entirety, results as shown in FIG Fig. 5 a nearly linear relationship between the braking time and the initial number of revolutions of the electric motor 14. It is understood that both speed and braking time are engine-specific and can thus change.

The basic principle of the screwdriver according to the invention is first to drive the drive 12 or the electric motor 14 in a first phase of the screwing until the motor current I _M exceeds the limit value I ₁ . Thereafter, a wait state begins until the start of braking. At the time of exceeding the limit value I ₁ by the motor current I _M , the kinetic energy E _kin contained in the rotating parts of the drive is detected and used to calculate a time at which the braking of the motor begins. This will be explained below with reference to Fig. 3 explained in more detail. During the wait state, ie from the time of exceeding the limit I ₁ by the motor current I _M until the initiation of braking, the motor is preferably energized further.

The tightening torque of the fitting consists of three parts: The first (and largest) part of the tightening torque is generated in the phase until the start of the waiting state when the limit value I _{1 is} exceeded by the motor current I _M. The torque _M generated by the motor current I _M is in this case directly proportional to the motor current I _M , as explained above. A second portion of the tightening torque is also generated by the motor current I _M during the waiting state until the time of application of the braking, since the electric motor 14 is preferably energized further during this phase. The third portion of the tightening torque is due to the kinetic energy of the drive 12 during the braking phase until the speed 0 is reached. The more energy is extracted from the drive with the help of the hybrid brake or short-circuit brake, the lower the torque deviation after exceeding the threshold I ₁ .

The tightening torque additionally applied after exceeding the limit value I ₁ by the motor current I _M depends on various parameters. These include the battery voltage, the engine temperature, the gearbox temperature, the position of the accelerator switch and the selected gear.

The voltage of the accumulator 40 decreases, the less residual charge in the accumulator 40 is present. In a fully charged accumulator, the speed n of the electric motor 14 is greater than in an almost empty accumulator 40th

By changing the winding temperature of the electric motor 14, its current consumption and thus also the torque output at a certain current I _{M changes} . This in turn affects the speed n.

As the transmission temperature increases, the viscosity decreases and the efficiency of the transmission 16 increases as friction decreases.

The user of the screwdriver 10 may also influence the speed n of the screwdriver 10 by means of the accelerator switch 32.

Since the screwdriver 10 may include both a single and a multi-speed transmission, also the set gear should be detected. Since both the output torque M and the output speed depend thereon, parameters within the controller 36 and microprocessor 37, respectively, may be adjusted to obtain the best possible tightening result.

To take into account and compensate for these parameters, the signals of the sensors 24, 26, 28, 30 for the transmission temperature, the engine temperature, the rotational speed and the gear recognition and the voltage sensor 39 for monitoring the battery voltage of the control device 36 are supplied. The deviations caused by these parameters are determined empirically and stored in the memory 43, so that the control device 36 can make a corresponding compensation.

The sequence of a screw connection is in the flowchart 70 according to Fig. 3 and is explained below:

After the start 72, a shut-off torque set with the aid of the adjusting element 34 is first read in, cf. Step 74. In the next step 76 becomes calculated from the read-off switching torque from the known direct proportional relationship between torque M and motor current I _M, a limit I ₁ for the motor current at which a shutdown of the drive 12 takes place. In a first approximation, the set shutdown torque M is divided by the proportionality constant k _{t in} order to determine the motor current. A certain value Δ is subtracted therefrom, to permit compensation for the after exceeding the limit value I ₁ for the motor current I _M are still generated torque components _IM. For example, 10% of the calculated motor current is subtracted to determine the limit I ₁ . Subsequently, the screwing process is started according to item 78. During the screwing process, the motor current I _M is constantly measured by means of the current sensor 38, wherein averaging takes place.

In the following query 82 it is checked whether the averaged motor current I _M exceeds the previously determined limit value I ₁ . If this is not the case, then the electric motor 14 is driven further. As soon as the limit I _{1 is} exceeded, the current speed n is detected in the following process 84 and the kinetic energy is calculated according to number 86. The kinetic energy E _{kin of} the drive 12 results in a first approximation from the formula E _kin = 1/2 Jn ² , where J is the moment of inertia of the drive 12 and n is the speed of the drive 12. The other influencing factors, such as battery voltage, engine temperature, gearbox temperature, position of the accelerator switch and set gear ratio are taken into account parametrically.

Subsequently, according to number 88, the application time t _{1 of} the brake is calculated and the screwing operation first continues according to number 90. Achieved according to the following query 92, the time t the braking time t> t ₁ so the engine is decelerated according to 94 until it finally comes to a standstill in accordance with 96.

It is understood that instead of the determination of a braking time, alternatively, the cumulative angle of rotation could be determined in case of exceeding the braking is initiated. For determining the angle of rotation, for example, the position sensor 58 could be used.

In the FIGS. 6 and 7 For example, a screwing process is shown with a slow speed. Fig. 6 shows by way of example the course of the motor current I _M as a function of the time t. The screwing process starts first in the diagram with the idle. The start is not shown. The first, very long increase marks, for example, the thread furrows of a self-tapping sheet metal screw. Then there is a small plateau, where the current rises only very slightly. This area extends from the end of the thread furrow to the head rest. The headrest is characterized by a very strong rising current. If the current I _M exceeds the threshold I ₁ , then the time (or alternatively the angle of rotation) is calculated until the beginning of the deceleration. Since in this screw the voltage and thus the speed n is very low, the time between the exceeding of the threshold I ₁ and the beginning of the braking process t _{1 is} very long, the actual braking time, however, short. The subsequent rapid increase in current at 95 during the braking process can be explained by the fact that it is braked actively, ie with the help of the hybrid brake. If, on the other hand, only one short-circuit brake is used during the braking process, then the current would be 0 from the beginning of the braking process.

In Fig. 7 is a section of Fig. 6 shown enlarged.

The time between the beginning of the waiting state at I _M > I ₁ until the start of braking at t> t ₁ is better evident from this.

In the FIGS. 8 and 9 the course of a screw connection is shown at a high speed. The course essentially corresponds to that in Fig. 6 shown course. Due to the increased speed, the period between the exceeding of the limit value I _{1 of} the motor current I _M and at the beginning of the braking process t _1, however, is less than in the illustration according to FIG Fig. 6 and / or 7. The time of the active deceleration is extended. However, the time between the exceeding of the limit value for the current I ₁ and the end of the deceleration process is the same length as at low speed. 95 denotes the braking process. According to the hybrid braking used, the braking current is limited to a maximum value, as can be seen by the horizontal course to the end of the braking process.

Claims (15)

1. A screwdriver comprising a drive (12) having an electric motor (14) which is coupled to a drive shaft (20) for driving a tool (22) for completing a screwdriving operation with a predetermined tightening torque, further comprising a current sensor (38) for detecting a motor current (I_M), further comprising a control device (36) for controlling the drive (12), the control device (36) being configured for braking the drive (12) when a braking criterion is reached, characterized in that the control device (36) is configured for calculating a treshold value (I₁) for the motor current (I_M) on the basis of the predetermined tightening torque, the drive (12) being moved to a standby state when the treshold value is exceeded, in that upon exceeding the treshold value (I₁) for the motor current (I_M) the control device (36) is configured for determining the braking criterion on the basis of the predetermined tightening torque to calculate an activation time (t₁) or a rotation angle (n₁), upon exceeding of which the braking process is initiated, at least as a function of the kinetic energy (E_kin) of the drive (12) at the time at which the treshold value (I₁) for the motor current (I_M) is exceeded, and in that during the standby state until the braking process is initiated, either current continues to be fed to the electric motor (14), or no current at all is fed, or current is fed to the electric motor (14) only to a certain extent.
2. The screwdriver of claim 1, characterized in that the control device (36) is configured for calculating a mean value for the motor current (I_M), which mean value is used for calculating the treshold value (I₁) for the motor current (I_M).
3. The screwdriver of claim 1 or 2, characterized in that the treshold value (I₁) for the motor current (I_M) is derived from a directly proportional relationship (M = k_t • I_M) between the torque (M) and the motor current (I_M).
4. The screwdriver of any of the preceding claims, characterized in that the electric motor (14) is an electronically commutated electric motor, preferably a permanent-magnet synchronous motor, further preferably a BLDC motor.
5. The screwdriver of any of the preceding claims, characterized in that at least one sensor (24, 26, 28, 30, 38, 39) is provided for detecting at least one parameter which is selected from the group consisting of a temperature of a gear mechanism (16) to which the electric motor (14) is coupled, a temperature of the electric motor (14), a rotation speed of the electric motor (14), a transmission ratio of the gear mechanism (16) and a supply voltage of the screwdriver (10).
6. The screwdriver of any of the preceding claims, characterized in that the control device is configured for determining the kinetic energy (E_kin) of the drive (12) at the time at which the treshold value (I₁) for the motor current is exceeded on the basis of parameters which include at least the rotation speed (n) of the drive (12) and a moment (J) of inertia of the drive (12).
7. The screwdriver of claim 6, characterized in that the parameters include the transmission ratio of the drive (12) and the rotation speed of the electric motor (14), and in that preferably the parameters include at least one further parameter which is selected from the group consisting of a supply voltage to the screwdriver (10), a temperature of the electric motor (14) and a temperature of a gear mechanism (16) which is coupled to the electric motor.
8. The screwdriver of any of the preceding claims, characterized in that the control device (36) is configured for storing a functional relationship between the braking criterion and the parameters which are used to determine the braking criterion, and for determining the braking criterion while taking into account the functional relationship.
9. The screwdriver of any of the preceding claims, characterized in that the predetermined tightening torque can be adjusted, preferably with the aid of a potentiometer (34), wherein preferably the potentiometer (34) has a higher resolution at lower tightening torques than at higher tightening torques.
10. The screwdriver of any of the preceding claims, characterized in that the control device (36) is configured for short-circuiting the electric motor (14) during a first time period (Δt₁) for braking the electric motor (14).
11. The screwdriver of any of the preceding claims, characterized in that the control device (36) is configured for feeding current to the electric motor (14) against its original direction of rotation during a second time period (Δt₂) for braking the electric motor (14).
12. The screwdriver of any of the preceding claims, characterized in that the control device (36) comprises a pulse-width-modulated control system (46) for actuating the electric motor (14).
13. The screwdriver of any of the preceding claims, characterized in that the electric motor (14) has a permanent-magnet rotor (42) which has at least one associated position sensor (58), preferably a Hall sensor, for detecting the position of the rotor (42).
14. Screwdriver according to one of claims 1 to 13, characterized in that the control device is configured for indirectly detecting the position of the rotor (42) by means of the mutual induction of a field winding (60, 62, 64) of the electric motor (14).
15. A method for controlling a screwdriver (10) for completing a screwdriving operation with a predetermined tightening torque by means of a tool (22) which can be driven by a drive (12), comprising the following steps:

    - detecting a predetermined tightening torque;

    - calculating a treshold value (I₁) for the motor current (I_M) on the basis of the predetermined tightening torque, the drive (12) being moved to a standby state when the said treshold value is exceeded;

    - completing the screwdriving operation by driving the tool (22);

    - monitoring the motor current (I_M) and moving the drive (12) to the standby state when the treshold value (I₁) for the motor current (I_M) is exceeded;

    - determining a kinetic energy (E_kin) of the drive (12) at the time at which the treshold value (I₁) for the motor current (I_M) is exceeded;

    - determining a braking criterion for initiating braking of the drive (12) at least on the basis of the kinetic energy (E_kin) of the drive (12) at the time at which the treshold value (I₁) for the motor current (I_M) is exceeded; and

    - initiating a braking process when the braking criterion is reached;

    - with current continuing to be fed to the drive motor (14), no current at all being fed to the drive motor (14) or current being fed to the drive motor (14) only to a certain extent during the standby state until the braking process is initiated.

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