EP2146822B1 - Power screwdriver - Google Patents

Abstract

The invention relates to a power screwdriver (10) comprising a motor (12) as the drive, a desired torque default element (52) and an actual torque determination element (46), a torque gradient determination element (48) and a motor control (40) which controls the motor (12) depending on the torque gradient (dmd_lst/dt). The invention is characterized by a torque threshold determination element (50) which provides a torque threshold value (Md_Lim, Md_Lim1, Md_Lim2) that depends on the torque gradient (dmd_lst/dt) and lies below the desired torque value (Md_Soll). If the actual torque value (md_Ist) exceeds the torque threshold value (Md_Lim, Md_Lim1, Md_Lim2), a motor control (40) presets a speed reduction for the motor (12) or already completely switches off the motor (12). The power screwdriver (10) according to the invention avoids torque overshoots and yet allows the desired torque value (Md_Soll) to be exactly reached in the shortest time possible.

Description

The invention relates to a power wrench according to the preamble of the independent claim.

State of the art

In the DE 23 26 027 A is a power-driven screwdriver is described, which provides a predetermined torque setpoint. The torque applied by the screwdriver is detected indirectly on the basis of the current flowing through the electric motor. Due to the mains connection, the starting point is an operating voltage of the electric motor, which is always the same and constant. If the torque setpoint has not yet been reached, the screwdriver turns at the maximum possible speed, which depends on the torque setpoint to be applied. Due to the inertia of the rotating parts of the screwdriver, such as electric motor and in particular gear, the screw is still rotated depending on the caster after reaching the torque setpoint.

The in the DE 23 26 027 A1 occurring problem due to the further rotation of the screwdriver when reaching the torque setpoint is of the DE 103 41 975 A1 addressed. Described is an electronic torque limiting device for an example used in a battery-powered screwdriver electric motor. The starting point is an electronic torque limitation, in which the current flowing through the electric motor is used as a measure of the torque. Such a procedure is referred to as inaccurate, because in particular at high speeds after switching off the electric motor by the kinetic energy of the rotating masses can occur with the result that a screw is tightened with a higher torque than the predetermined torque setpoint. To avoid based on the inertia or the dynamics of the transmission torque peak is proposed to set the maximum value of the permissible electric motor current in dependence on the speed of the electric motor. According to one embodiment, a torque setpoint can be set, which is converted into a maximum value of the electric motor current. The higher the maximum value of the electric motor current is set, the lower may be the maximum speed of the electric motor.

In the EP 0 187 353 A2 is described a screwdriver, the electric motor is powered by the AC mains. It is based on the knowledge that the electric motor provides a maximum and specific torque under load at standstill, this torque depends on the provided voltage or the load current according to the respective motor characteristic. The tightening torque setpoint is achieved at a low speed or even when the wrench is stopped, thus avoiding overshoot of the torque setpoint by an overrun.

There is also a compensation circuit which is able to compensate for fluctuations in the mains voltage in order to eliminate the influence on the torque actual value. When the supply voltage drops, the phase angle of a triac drive is increased, so that a higher average voltage is applied to the electric motor.

In the DE 196 26 731 A1 is a battery-powered small screwdriver described, which includes a switching element, which the electric motor by shorting off. The switching element is actuated by a depth stop. The abrupt deceleration of the electric motor reduces overshoot. It should be noted, however, that such short-circuiting of the electric motor is possible only at comparatively low torques to be delivered, for example, 100 Nm and low-power electric motors, even in low-power electric motors in the case of short-circuiting a high-speed rotating electric motor with a significant short-circuit current and the related electromagnetic interference must be expected. The short-circuit current loads both a collector of a DC motor realized as an electric motor and the switching element used for shorting the electric motor to a considerable extent.

The DE 43 10 936 A1 discloses a power wrench that is completely shut down when it has reached a torque setpoint. The value at which the power wrench is turned off is adjustable and is below the magnitude of the torque overshoot below a target value. The shutdown is used to advance the tool and fastener to the target torque due to overshoot.

In the DE 103 45 135 A1 is a small battery-powered screwdriver described that contains a lithium-ion battery for power.

In the DE 201 13 184 U1 and for example the DE 196 47 813 A1 are provided as power tools designed electric motor driven screwdriver, each having a support arm for providing a counter torque when tightening or loosening screw.

Such screwdrivers are referred to as power wrenches, because the torque provided can amount to, for example, 10,000 Nm, which could not be applied without the support arm of an operator of the power wrench. With increasing torque during the screwing deforms the Support arm elastic, whereby the support arm absorbs energy. During the screwing process, the support arm clamps the screwdriver on the screw connection. The support arm takes not only the energy occurring during the screwing, but also after the shutdown of the power wrench still in the rotating masses such as the electric motor and in particular the transmission existing rotational energy by deforming.

In the DE 196 20 782 A1 is given a method for producing a screw in which the temporal torque curve is detected as a gradient. A distinction is made between a first and a second torque increase, wherein the first torque increase is associated with a thread cutting operation and the second torque increase with the tightening of the screw connection. If the second torque gradient decreases, this is interpreted as a thread deformation and the screwdriver is switched off.

The invention has for its object to provide a power wrench, in particular a battery-powered power wrench, which allows the achievement of a predetermined torque setpoint for a screw without the risk of torque overshoot in an optimal manner.

The object is achieved by the features specified in the independent claim.

Disclosure of the invention

The power wrench according to the invention has the features of claim 1.

The power wrench according to the invention makes it possible, on the basis of the torque gradient determination, to fall short of hard and soft screwdriving cases. Based on the determined torque gradient and the adjusted torque setpoint, the torque threshold setting may selectively set the torque threshold below the torque setpoint such that torque overshoots due to the speed reduction and the complete shutdown of the electric motor when the Torque actual value has reached the torque setpoint.

Advantageous developments and refinements of the power wrench according to the invention will become apparent from the dependent claims.

An embodiment provides that the electric motor control the electric motor at a torque actual value, which is below the torque threshold, the maximum possible speed of the electric motor sets. Accordingly, the maximum possible power is made available to the electric motor, whereby the maximum possible rotational speed is set under the given load conditions. With this measure, the screw can be made in the shortest possible time, without the risk of torque overshoot exists.

An embodiment provides that the torque threshold setting determines the difference between the torque setpoint and the torque threshold as a function of the torque gradient. This measure covers the entire spectrum from soft to hard screwdriving cases. The torque threshold setting adds the difference a larger torque gradient to a higher value than a smaller torque gradient so that torque overshoot is avoided in both hard and soft tightening.

One embodiment provides that the torque threshold definition contains a table in which torque gradients and torque setpoints are stored for determining the torque threshold value. Alternatively, it can be provided that the torque threshold setting extrapolates the torque threshold based on the determined torque gradient, the torque actual value and the set torque setpoint.

Another embodiment provides a motor current detection, which detects the motor current as a measure of the actual torque value. The motor current detection can be realized, for example, as a low-impedance shunt, which is cheaper to implement compared to an electromagnetic motor current detection.

Another embodiment provides a data carrier in which characteristic values of the screw connection are stored and / or which is provided for the storage of recorded data of the screw connection to be produced. The data carrier contains at least the predetermined torque setpoint. At least the actually achieved torque actual value of the screw connection can be stored. The data carrier may further contain parameters such as calibration data of the power wrench or be provided for storing such characteristics.

The data carrier can be assigned to the power screwdriver. According to another embodiment, the power wrench on means for signal transmission to a arranged outside the power wrench disk.

A development provides a voltage limiter circuit which limits the motor voltage occurring at the electric motor to a predetermined limiting voltage. The limiting voltage is preferably set at least to the nominal operating voltage of the electric motor, so that the electric motor can contribute to reducing an optionally stored in a support arm of the power wrench towards the end of the screwing energy by operating the electric motor in the generator mode, without the electric motor applies a counter-torque.

The voltage limiter circuit preferably includes a bipolar limiter diode and / or a varistor.

Another development of the power wrench according to the invention provides as a power source for the electric motor before a lithium-based accumulator due to its comparatively high energy density. Can be used, for example, a lithium-ion battery (Li-ion battery) or, for example, a lithium-polymer battery (Li-polymer battery).

If the supply voltage is provided by an accumulator, a battery voltage drop compensation circuit is preferably provided, which compensates the influence of a sinking supply voltage on the achievement of the set torque setpoint, which occurs in particular when the torque actual value is obtained from the motor current. A simple implementation of the battery voltage drop compensation circuit provides that the battery voltage drop compensation circuit either increases the setpoint torque setpoint or reduces the determined actual torque value when the supply voltage drops. As a result, an intervention in the power section of the electric motor is avoided.

Further advantageous embodiments and further developments of the power wrench according to the invention will become apparent from the following description. Embodiments of the power wrench according to the invention are shown in the drawing and explained in more detail in the following description.

• Figur 1 eine Skizze eines erfindungsgemäßen Kraftschraubers,
• Figur 2 ein Blockschaltbild einer Ansteuerschaltung des erfindungsgemäßen Kraftschraubers,
• Figur 3 Drehmoment-Verläufe in Abhängigkeit von der Zeit und
• Figuren 4a und 4b unterschiedliche Ausgestaltungen einer Spannungsbegrenzer-Schaltung.

Show it:

• FIG. 1 a sketch of a power wrench according to the invention,
• FIG. 2 a block diagram of a drive circuit of the power wrench according to the invention,
• FIG. 3 Torque curves as a function of time and
• FIGS. 4a and 4b different embodiments of a voltage limiter circuit.

FIG. 1 shows a sketch of a power wrench 10, which includes an electric motor 12 as a drive, which drives a socket 16 via a gear 14. The power wrench 10 includes a support arm 18 which provides a counter-torque during the screwing operation. In the exemplary embodiment shown, it is assumed that a battery-operated power wrench 10 contains a battery part 20 in which an accumulator 22 is accommodated. Commissioning of the power screwdriver 10 takes place with a switch 24. For controlling the electric motor 12, a drive circuit 26 is provided, to which a data carrier 28 and a transmitting / receiving device 30 are assigned.

In the exemplary embodiment shown, a DC motor 12 is assumed, which is preferably driven by a pulse width modulated signal which determines the average operating voltage of the electric motor 12.

In FIG. 2 For example, an electric motor driver 40 is shown which provides a pulse width modulated signal s_PWM that either fully opens or completely closes a switching element 42, such as a MOS field effect transistor, where the period and / or pulse duration may be variable.

The duty cycle of the pulse width modulated signal s_PWM, which reflects the ratio of duty cycle to period, sets the mean motor voltage u_Mot and thereby allows influencing the power provided to the electric motor 12 or the rotational speed of the electric motor 12th

After closing the switch 42, a motor current i_Mot flows as a function of the pulse duty factor of the pulse width modulated signal s_PWM, as a function of the supply voltage u_Batt and as a function of the load of the electric motor 12.

The motor current i_Mot is used as a measure of the torque applied by the electric motor 12 and thus as a measure of the torque actual value provided at the socket 16. In the exemplary embodiment shown, the motor current i_Mot is detected with a motor current detection 44, which is implemented as a low-resistance resistor or shunt of, for example, 0.01 ohms. The voltage drop u_Sens which occurs as a measure of the motor current i_Mot at the shunt 44 is amplified in a torque actual value determination 46, which contains, for example, an OpAmp connected as a differential amplifier, and is provided as a measure of the actual torque value md_Ist. Preferably, a signal smoothing device not shown in detail is provided, which frees the torque actual value md_Ist at least from high-frequency interference signals.

The torque actual value md_Ist is provided to the electric motor drive 40, a torque gradient determination 48 and a torque threshold setting 50. The torque gradient determination 48 determines the gradient dmd_Ist / dt of the torque actual value md_Ist by determining at least one time differential quotient. Preferably, the differential quotient is approximated and by the difference quotient.

The torque gradient determination 48 provides the torque gradient dmd_Ist / dt of the torque threshold setting 50, which is based on the torque gradient dmd_Ist / dt, the actual torque value md_Ist, that of a torque setpoint specification 52nd provided torque setpoint Md_Soll and a torque minimum value Md_Min a torque threshold Md_Lim determines which of the electric motor driver 40 is available.

The determination of the torque threshold Md_Lim in the torque threshold setting 50 is made on the basis of in FIG. 3 shown temporal torque curves explained in more detail. FIG. 3 shows a first screw SF1, which corresponds to a hard screw case, in which a comparatively rapid change of the torque actual value md_Ist occurs. FIG. 3 shows a second screw SF2, which corresponds to a soft screw case, in which a comparatively slow change of the torque actual value md_Ist occurs.

The torque gradient determination 48 determines after the start of the screwing process the torque gradient dmd_Ist / dt, which can be approximated for example by at least one difference quotient. In the illustrated embodiment according to FIG. 3 It is assumed that the torque gradient determination 48 determines at least one difference quotient after exceeding the torque minimum value Md_Min on the basis of a time interval dti. The time interval dti is to be set in such a way that the expected fastest possible torque increase and the lowest set torque setpoint Md_Soll ensure that the torque threshold setting 50 can determine and provide a torque threshold Md_Lim1, Md_Lim2.

The torque minimum value Md_Min is set, for example, to a torque actual value md_Ist which is slightly above the expected joining torque of the screw connection. With this measure can ensure that the actual torque gradient dmd_Ist / dt of the screw is determined.

Based on the set torque setpoint Md_Soll, the preferably predetermined torque minimum value Md_Min, the determined torque actual value md_Ist and on the basis of the torque gradient dmd_Ist / dt, the torque threshold setting 50 sets the first torque threshold Md_Lim1 and for the second screwdriver SF2 the second torque threshold Md_Lim2 fixed. The torque threshold values Md_Lim1, Md_Lim2 are each below the torque setpoint Md_Soll. The first torque threshold Md_Lim1 is a first difference d1 below the torque setpoint Md_Soll and the second torque threshold Md_Lim2 is a second difference d2 below the torque setpoint Md_Soll.

The torque threshold setting 50 may set the threshold Md_Lim1, Md_Lim2 based on stored tables. According to another embodiment, functional relationships between the mentioned input variables are stored in the torque threshold setting 50, so that the torque threshold values Md_Lim1. Md_Lim2 can be extrapolated from the current actual torque value md_Ist. In the simplest case, the functional relationship can be based on a straight line equation so that the expected torque curve can be completely specified by the slope and a point of the straight line. The torque threshold values Md_Lim1, Md_Lim2 or the functional relationships required for determining the threshold values Md_Lim1, Md_Lim2 are preferably determined experimentally and stored in the torque threshold setting 50.

In the first tightening case SF1, it is assumed that the first torque threshold value Md_Lim1 will be reached at a first time ti1. The first torque threshold Md_Lim1 or the first difference d1 are adapted to a hard screwdriving case, which was detected on the basis of the determined torque gradient dmd_Ist / dt. The first difference d1 is comparatively large.

In the second screwing SF2, it is assumed that the second torque threshold Md_Lim2 will be reached at a fourth time ti4. The second torque threshold Md_Lim2 or the second difference d2 are adapted to a soft screw case, which was detected on the basis of the determined torque gradient dmd_Ist / dt. The second difference d2 is comparatively small.

A first comparator 54 contained in the electric motor drive 40 compares the torque threshold value Md_Lim, Md_Lim1, Md_Lim2 with the actual torque value md_Ist and provides a control signal s_Mot depending on the result of the comparison. The control signal s_Mot ensures that the pulse width modulated signal s_PWM drives the electric motor 12 with a lower power than before, so that the electric motor 12 is given a speed reduction.

The speed reduction or the complete shutdown after reaching the torque threshold Md_Lim, Md_Lim1, Md_Lim2 substantially prevents the actual torque value md_Ist from overshooting, which would cause the screw connection to be screwed to a torque higher than the torque setpoint Md_Soll would.

The overshoot is caused by the existing in the electric motor 12 and in particular in the transmission 14 kinetic energy towards the end of the screwing. In this regard, in particular the hard screw SF1 critical, because in a relatively short time ti the torque setpoint Md_Soll is reached. At the in FIG. 3 the embodiment shown is to illustrate the Problem assumed that despite the speed reduction or the complete shutdown of the electric motor 12 after exceeding the first torque threshold Md_Lim1 the increase of the torque actual value md_Ist until a second time ti2 almost without reduction of the torque gradient dmd_Ist / dt , The speed reduction of the electric motor 12 initiated by the control signal s_Mot and predetermined by the pulse-width-modulated signal s_PWM therefore only has an effect from the second time ti2.

The torque setpoint Md_Soll is reached at a third time ti3 with a reduced torque gradient dmd_Ist / dt. At the third time ti3 a shutdown of the electric motor 12 is provided. This switch-off is initiated by a stop signal s_Stop, which provides a second comparator 56 arranged in the electric motor drive 40 as a function of the comparison result between the torque setpoint Md_Soll and the actual torque value md-Ist.

In the case of soft screwdriver SF2, in contrast to hard screwdriving SF1, after reaching the second torque threshold value Md_Lim2, a comparatively longer period of time is available until the torque setpoint Md_Soll is reached. Therefore, the second torque threshold Md_Lim2 may be much closer to the torque setpoint Md_Soll, corresponding to a smaller difference d2. In this case as well, after the second torque threshold value Md_Lim2 has been reached, the speed reduction of the electric motor 12 is initiated. Due to the resulting reduction in the torque gradient dmd_Ist / dt after the second torque threshold Md_Lim2 has been exceeded, SF2 also occurs in the case of the soft torque limit Overshoot prevents the gland from exactly matching the torque setpoint Md_Soll is reached, which is reached at a fifth time ti5.

In the embodiment shown, it is assumed that for the power supply of the electric motor 12 of the battery 22 is provided, which is preferably realized as a lithium-based accumulator, which is characterized by a high energy density. Can be used, for example, a lithium-ion battery (Li-ion battery) or, for example, a lithium-polymer battery (Li-polymer battery). The battery 22 provides the supply voltage u_Batt. Although the discharge characteristic of a battery, in particular of a lithium-based accumulator, is relatively flat, even a small voltage drop has an immediate effect on the achievement of the torque setpoint Md_Soll, if the motor current i_Mot is used as a measure for the torque actual value md_Ist a lesser motor current i_Mot sets with decreasing supply voltage u_Batt.

Therefore, a battery voltage drop compensation circuit 60 is provided, which compensates the influence of a sinking supply voltage u_Batt on reaching the set torque setpoint Md_Soll.

In principle, the supply voltage u_Batt could be directly stabilized and kept constant, although power semiconductor devices would be required, which are relatively expensive on the one hand and on the other hand because of the high expected currents to 100 A, for example, are too voluminous to be accommodated in the power wrench 10 can ,

The battery voltage drop compensation circuit 60 therefore preferably intervenes with a compensation signal s_Batt_Komp in the torque setpoint input 52 or in the actual torque value determination 46, the torque setpoint Md_Soll being increased when the supply voltage u_Batt decreases.

The battery voltage drop compensation circuit 60 may include, for example, a reference voltage source to which the supply voltage u_Batt is compared. With decreasing difference between the reference voltage and the supply voltage u_Batt during the discharge process of the battery 22, the compensation signal s_Batt_Komp is constantly increased, the increase corresponds to a virtual reduction of the motor current i_Mot to compensate for the actually lower motor current i_Mot with decreasing supply voltage u_Batt in the signal evaluation.

During operation of the power wrench 10, the support arm 18 provides the required counter-torque to the torque transmitted by the socket 16 to the screw connection. The support arm 18 is to fix the preparation of the screwing on a suitable support. During the screwing occurs depending on the increasing torque correspondingly increasing deformation of the support arm 18, which corresponds to a storage of energy. The stored energy in the support arm 18 has after switching off the power wrench 10 when reaching the set torque setpoint Md_Soll the maximum value.

Due to the deformation of the support arm 18, the socket 16 and thus the entire power wrench 10 is clamped on the screw. After switching off the electric motor 12, the stored energy in the support arm 18 causes the electric motor 12, starting from the socket 16, is driven backwards via the gear 14, wherein the electric motor 12 begins to rotate in the opposite direction to the drive direction.

The electric motor 12 is therefore operated as a generator in the degradation of stored energy in the support arm 18. For quick and easy removal of stored energy in the support arm 18, the electric motor 12 should be able to rotate freely without applying a counter-torque, which would complicate and extend the discharge process. The electric motor 12 should therefore not be short-circuited or low-resistance bridged in this operating state, wherein even at a low generator voltage, a high motor current i_Mot, corresponding to a high counter-torque would occur. It should be noted here that in generator mode the motor voltage u_Mot reverses due to the other direction of rotation and the motor current i_Mot therefore flows in the opposite direction, provided that the current path is available.

In particular, it has been found on the basis of experiments that in generator operation significant motor voltages u_Mot can occur, which are significantly above the nominal operating voltage of the electric motor 12. In an electric motor 12 with a rated operating voltage of, for example, 28 volts, voltage spikes up to over 200 volts were detected with a pulse duration of several 100 ns. Such high-energy pulses can lead to the destruction of components of the drive circuit 26, in particular to the destruction of the switching element 42.

Therefore, the voltage limiter circuit 70 is provided, which limits the motor voltage u_Mot occurring at the electric motor 12 of the electric motor 12 operated as a generator when the energy stored in the support arm 18 is reduced and counter to the drive direction to a predetermined limiting voltage u_Lim.

The voltage limiter circuit 70 is not comparable to a freewheel which essentially short circuits only the electric motor 12. The voltage limiter circuit 70 allows the targeted specification of the limiting voltage u_Lim, so that the electric motor 12 during generator operation in the destruction of the energy stored in the support arm 18 at least until reaching the limiting voltage u_Lim generates no counter-torque. In this operating state, a motor current i_Mot occurs in the reverse direction compared to the normal operation only when the motor voltage u_Mot in the generator operation, the limiting voltage u_Lim tries to exceed.

However, the voltage limiter circuit 70 can take over the function of a freewheel, wherein during the freewheel, in which the direction of the motor current i_Mot does not turn around, the limiting voltage u_Lim occurs as a motor voltage u_Mot. Optionally, a not shown in detail switched freewheel can be provided, which is driven by the pulse width modulated signal s_PWM.

The voltage limiter circuit 70 can be realized in different ways. At the in FIG. 4a In the embodiment shown, the voltage limiter circuit 70 includes a bipolar voltage limiter diode 72, which is also referred to as TVS (Transient Voltage Suppressor). The voltage limiter diode 72 includes two zener diodes integrated in a single device. At the in FIG. 4b In the embodiment shown, the voltage limiter circuit 70 includes a varistor 74.

While diodes 72 enable a very fast response to voltage pulses, a varistor 74 can receive and derive a higher energy, at least in the short term. Depending on the requirements, therefore, a combination of diodes 72 and a varistor 74 may be provided.

The limiting voltage u_Lim is initially set to a value at which no limitation of the motor voltage u_Mot can occur in the normal drive mode of the electric motor 12. The limiting voltage u_Lim is thus set to a value of at least 28 volts in a 28 volt electric motor 12. Since the motor voltage u_Mot reverses in generator operation of the electric motor 12, the voltage limiter circuit 70 must provide the limiting voltage u_Lim, in particular for the motor voltage u_Mot, with reversed polarity, since the risk of overvoltage exists in generator operation in particular. In the embodiment shown with the in FIG. 2 registered polarity of the supply voltage u_Batt occurs in generator operation of the electric motor 12, the positive potential of the motor voltage u_Mot on the switching element 42, while the negative potential is applied to the battery 22.

Appropriately, a limiting voltage u_Lim is given, which corresponds at least to the amount of the nominal operating voltage of the electric motor 12. According to another embodiment, at least the effective in the generator mode of the electric motor 12 limiting voltage u_Lim is set to the value of a so-called protection low voltage, which may be set by law. A protective low voltage in this sense should be defined by the fact that on an electrical device, in the present case the power wrench 10, live parts that can be touched must not exceed the protective extra-low voltage. If this could be the case, special measures must be taken to protect against contact. The protective low voltage is for example 42 volts.

Another development of the power wrench 10 according to the invention provides a data carrier 80, which contains data for the screw connection, such as at least the torque setpoint Md_Soll, and / or for receiving data, such as the torque actual value md_Ist actually achieved, is prepared, which are stored at least at the end of the screwing process. The data carrier 80 may further include calibration data of the power wrench 10 and / or be prepared for storing characteristics of the power wrench 10. Preferably, the data carrier 80 is realized as a mobile data carrier, for example as a low-cost RFID.

Another development of the power wrench 10 according to the invention provides means 82 for signal transmission, for example, a transmitting / receiving device 82, which is designed for receiving and / or transmitting data relating to screwing and / or characteristics of the power wrench 10. The transmitting / receiving device 82 is preferably designed to cooperate with a data carrier, not shown in detail, for example, a mobile data carrier, which may correspond to the data carrier 80. Unless it is at this Disk is an already mentioned RFID, the transmitting / receiving device 82 to a high-frequency transmitter and / or high-frequency receiver, wherein the transmission / reception frequency is tuned to the transmission / reception frequency of the data carrier.

Claims (16)

1. A power screwdriver, comprising an electric motor (12) as a drive, a torque setpoint specification element (52) and an actual torque value determination element (46), a torque gradient determination element (48) and an electric motor triggering unit (40) which triggers the electric motor (12) depending on the torque gradient (dmd_Ist/dt), characterized in that a torque threshold determination element (50) is provided, that the torque threshold determination element (50) provides a torque threshold value (Md_Lim, Md_Lim1, Md_Lim2) which depends on the torque gradient (dmd_Ist/dt) and lies beneath the torque setpoint (Md_Soll), and that the electric motor triggering unit (40) provides the electric motor (12) with a speed reduction when the actual torque value (md_Ist) exceeds the torque threshold value (Md_Lim, Md_Lim2) and switches off the electric motor completely when the actual torque value (Md_Ist) reaches the torque setpoint (Md_Soll).
2. A power screwdriver according to claim 1, characterized in that the electric motor triggering unit (40) provides the electric motor (12) with the maximum possible speed of the electric motor (12) at an actual torque value (md_Ist) which lies beneath the torque threshold value (Md_Lim, Md_Lim1, Md_Lim2).
3. A power screwdriver according to claim 1 or 2, characterized in that the torque threshold determination element (50) determines the difference (d1, d2) between the torque setpoint (Md_Soll) and the torque threshold value (Md_Lim, Md_Lim1, Md_Lim2) depending on the torque gradient (dmd_Ist/dt).
4. A power screwdriver according to claim 3, characterized in that the torque threshold determination element (50) determines the difference (d1, d2) to be higher at a higher torque gradient (dmd_Ist/dt) than at a lower torque gradient (dmd_Ist/dt).
5. A power screwdriver according to claim 1, characterized in that the torque threshold determination element (50) contains a table in which the torque gradients (dmd_Ist/dt) and torque setpoints (Md_Soll) are stored for determining the torque threshold value (Md_Lim, Md_Lim1, Md_Lim2).
6. A power screwdriver according to claim 1, characterized in that the torque threshold determination element (50) extrapolates the torque threshold value (Md_Lim, Md_Lim1, Md_Lim2) on the basis of the determined torque gradient (dmd_Ist/dt), the actual torque value (md_Ist) and the torque setpoint (Md_Soll).
7. A power screwdriver according to claim 1, characterized in that a motor current detection element (44) is provided which detects the motor current (i_Mot) as a measure for the actual torque value (md_Ist).
8. A power screwdriver according to claim 1, characterized in that a data medium (80) is provided in which the characteristic values of the screwed joint and/or the power screwdriver (10) are stored, and/or which is provided for storing the detected data of a screwed joint or characteristic values of the power screwdriver (10).
9. A power screwdriver according to claim 8, characterized in that the power screwdriver (10) comprises means (82) for signal transmission to a data medium arranged outside of the power screwdriver (10).
10. A power screwdriver according to claim 1, characterized in that a voltage limiter circuit (70) is provided which limits the motor voltage (u_Mot) occurring in the electric motor (12) to a predetermined limit voltage (u_Lim) which is fixed at least to the nominal operating voltage of the electric motor (12).
11. A power screwdriver according to claim 10, characterized in that the voltage limiter circuit (70) contains a bipolar limiter diode (72).
12. A power screwdriver according to claim 10, characterized in that the voltage limiter circuit (70) contains a varistor (74).
13. A power screwdriver according to claim 1, characterized in that a storage battery (22) is provided for providing the supply voltage (u_Batt).
14. A power screwdriver according to claim 13, characterized in that the storage battery (22) is a lithium-based storage battery (Li-ion battery, Li-polymer battery).
15. A power screwdriver according to claim 13 or 14, characterized in that a battery voltage drop compensation circuit (60) is provided which compensates the influence of a decreasing supply voltage (u_Batt) on reaching the set torque setpoint (Md_Soll).
16. A power screwdriver according to claim 15, characterized in that the battery voltage drop compensation circuit (60) increases the set torque setpoint (Md_Soll) when the supply voltage (u_Batt) decreases.

Images