EP2139645B1 - Power screwdriver - Google Patents

Abstract

The invention relates to a power screwdriver (10) comprising a motor (12) and a control circuit (22) which switches the motor (12) off by means of a switch-off signal (s_Stop) when it reaches a preset desired torque value (Md_Soll), and a supporting arm (18) which absorbs energy during screwing. The invention is characterized by a voltage limiting circuit (46) that limits the motor voltage (u_Mot) occurring on the motor (12) when the energy stored in the supporting arm (18) of the motor (12), which is driven as the generator and rotates counter to the direction of drive, is dissipated, to a predetermined limiting voltage (u_Lim). Said limiting voltage (u_Lim) is fixed in such a manner that the motor (12) is capable of rotating without counter-torque counter to the direction of drive when operated in the generator mode and that yet no inadmissible overvoltages occur.

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 one Torque limit, 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 setting of the average operating voltage of the electric motor is carried out with a switching element, which is realized for example as a triac. The average operating voltage of the electric motor or the load current can be adjusted with a potentiometer, whereby the maximum torque at engine standstill or at low engine speeds is changeable and adjustable. 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 in the triac drive increases, so that a higher average voltage is applied to the electric motor.

In the DE 196 26 731 A1 a battery-operated small screwdriver is described which includes a switching element which shuts off the electric motor by shorting. 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.

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

Generally known prior art is to provide a freewheeling circuit parallel to an electric motor, which enables the current reduction of the inductive energy stored in the inductive portion of the electric motor after switching off the electric motor. The freewheeling circuit can be realized, for example, as a switched freewheeling circuit in which, for example, a MOS field effect transistor connected in parallel with the electric motor is switched on simultaneously with the switching off of the power supply and thus bridges the electric motor, so that the motor current can be reduced. In the simplest case, the freewheeling circuit is realized with a parallel to the electric motor connected freewheeling diode. Such a freewheeling circuit only allows the further flow of the motor current after switching off the power supply, wherein the voltage at the motor is not defined in the active freewheeling circuit, but by the Forward voltage of the current-carrying freewheeling device used depends, which is highly temperature-dependent and in particular depends on the amount of freewheeling current.

In the DE 201 13 184 U1 and for example the DE 196 47 813 A1 are provided as hand tool machines 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 process, the support arm deforms elastically, 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.

The tension can be achieved for example by a slip clutch, which mechanically disengages when reaching the torque setpoint. Particularly at low torque setpoints, the drive unit can release the voltage by setting a defined power. In both methods, the greatly different mass ratio of the rotating drive unit with respect to the transmission mass has a negative effect on the transmission and the electric motor.

Another generic battery-powered screwdriver goes out US 2005/0279198 A1 out.

For screwdrivers, especially for power wrenches, which can provide a very high torque, it is essential that the in

Support arm can store stored energy controlled, so that the power wrench can be removed from the screw. Due to the generally high reduction ratio of the transmission can not be ruled out that the electric motor starts due to the energy stored in the support arm to rotate counter to the drive direction.

The invention has for its object to provide a power screwdriver, in particular a battery-powered power wrench, which allows a safe degradation of the energy stored in the support arm after switching off the power wrench.

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

Disclosure of the invention

The power wrench according to the invention has an electric motor and a drive circuit which electronically shuts off the electric motor upon reaching a set torque setpoint with a shutdown signal. Furthermore, a support arm is provided, which receives energy during the screwing process. The power wrench according to the invention is characterized by a voltage limiter circuit which limits the motor voltage occurring at the electric motor of the energy stored in the support arm as a generator operated, rotating counter to the drive direction of the electric motor to a predetermined limiting voltage.

The voltage limiter circuit provided according to the invention initially ensures that the energy stored in the support arm during the screwing operation can be consumed after the electric motor has been switched off when the torque setpoint is reached by driving the electric motor via the transmission in generator operation, the electric motor being below the predetermined value Limiting voltage in a wide speed range no significant counter-torque builds.

In particular, the voltage limiter circuit provided according to the invention protects the drive circuit against inadmissibly high voltages, which could occur at a high energy stored in the support arm after switching off the electric motor upon reaching the torque setpoint corresponding to a high speed of the electric motor in the generator mode.

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

An embodiment provides that the limiting voltage corresponds in magnitude to at least the nominal operating voltage of the electric motor. On the one hand, this results in a sufficient speed range during generator operation without a current flow occurring which can only occur when the limiting voltage is reached. On the other hand, components with comparatively low permissible maximum operating voltages can be used, since the total occurring during operation of the electric motor motor voltages is limited in terms of the nominal operating voltage of the electric motor.

Another embodiment provides that the limiting voltage corresponds at most to the permissible DC protective voltage for electrical appliances. The protective extra-low voltage in the sense of the present application corresponds to the voltage which is legally permitted without special provisions for electrical insulation must be met. The protective low voltage is for example 42 volts.

Further embodiments relate to the realization of the voltage limiter circuit. A first realization possibility sees two in series, against each other poled Zener diodes before. Another possibility of realization provides for a bipolar limiter diode.

Another possibility of realization provides for a varistor.

Another possibility for realization provides for the use of a voltage limiter circuit which contains an electronic load.

While the voltage limiter circuits realized with diodes and transistors have a high response speed, the varistor can briefly absorb and thermally dissipate a comparatively high power.

A combination of different components makes it possible to optimize for different requirements.

An advantageous development of the power wrench according to the invention provides that the drive circuit provides the shutdown signal upon reaching the set torque setpoint based on a comparison of the torque setpoint with a torque actual value obtained from the electric motor current. The detection of the electric motor current, which is used as the basis for a measure of the torque provided by the screwdriver, can be done with simple circuit technology and is therefore considerably cheaper than a mechanical solution such as a slip clutch.

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).

The falling during operation of the power wrench supply voltage due to the decreasing battery voltage during the discharge process is Advantageously compensated by a Akkuspannungsabfall compensation circuit, so that the falling operating voltage has no influence on the achievement of the set torque setpoint.

Instead of an intervention in the power section of the control electronics is provided according to an embodiment that the Akkuspannungsabfall compensation circuit either increases the preset torque setpoint with decreasing battery voltage or reduces the indirectly detected on the basis of the electric motor current torque actual value. This virtually shifts the characteristic curve of the electric motor.

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 und

die Figuren 3a - 3d 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 and

the FIGS. 3a-3d different embodiments of a voltage limiter circuit.

FIG. 1 shows a sketch of a power wrench 10 which includes an electric motor 12 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 embodiment shown, it is assumed that a battery-powered power wrench 10, which includes an accumulator 20 which is housed in a battery part 22. The commissioning of the power wrench 10 is carried out with a switch 24. For controlling the electric motor 12, a drive circuit 26 is provided.

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, a pulse width modulator 30 is shown that provides a pulse width modulated signal s_PWM that either fully opens or completely closes a switching element 32, such as a MOS field effect transistor. The period duration and / or the pulse duration of the pulse width modulated signal s_PWM can be variable.

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

After closing the switch 24, 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 provided by the power wrench 10 on the socket 16. In the exemplary embodiment shown, the motor current i_Mot is detected with a shunt 34, which is realized as a low-resistance 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 34 is amplified in a sensor signal conditioning 36 and fed to a signal smoothing 38 as a measure for the torque actual value md_Ist, which provides a smoothed actual torque value mdm_Ist of a screwdriver shutdown 40 ,

The sensor signal conditioning 36 contains, for example, a Op Amp connected as a differential amplifier. The signal smoothing 38 is realized, for example, as a resistor-capacitor combination which has a low-pass filter function or an integrating property, which leads to a sliding averaging.

The optionally provided signal smoothing 38 largely suppresses noise and current spikes, which can lead to an erroneous shutdown of the power wrench 10.

The screwdriver shutdown 40 is realized, for example, with a comparator-connected OpAmp, to which the smoothed torque actual value mdm_Ist or the actual torque value md_Ist and a torque setpoint input 42 provided torque setpoint Md_Soll are made available. The torque setpoint input 42 is preferably a potentiometer whose setting wheel, which is accessible to an operator of the power wrench 10, is labeled with the different torque setpoints to be specified.

As soon as the smoothed torque actual value mdm_act or the torque actual value md_actual coincides with the torque command value Md_setpoint, the screwdriver shutdown 40 provides a stop signal s_stop, which is made available to the pulse width modulator 30. With the occurrence of the stop signal s_Stop upon reaching the predetermined torque setpoint Md_Soll the pulse width modulator 30 terminates the provision of the pulse width modulated signal s_PWM, whereby the switching element 32 is permanently closed and the electric motor 12 and the power wrench 10 is turned off.

In the exemplary embodiment shown, it is assumed that for the power supply of the electric motor 12, the battery 20 is provided, which is preferably realized as a lithium-based battery 20, which is characterized by a high energy density distinguishes. Can be used, for example, a lithium-ion battery or, for example, a lithium-polymer battery. The battery 20 provides the supply voltage u_Batt. Although the discharge characteristic of a battery, in particular of a lithium-based battery, runs relatively flat, even a slight voltage drop has an immediate effect on the achievement of the predetermined torque setpoint Md_Soll if the motor current i_Mot is used as a measure for the torque actual value md_actual, mdm_actual is, since with decreasing supply voltage u_Batt a lesser motor current i_Mot sets.

Therefore, a battery voltage drop compensation circuit 44 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, but 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 housed in power wrench 10 can ,

The battery voltage drop compensation circuit 44 therefore preferably intervenes in the screwdriver shut-off 40 with a compensation signal s_Batt_Komp, wherein when the supply voltage u_Batt drops, either the torque setpoint Md_Soll is increased or the actual torque value md_Ist is reduced.

The battery voltage drop compensation circuit 44 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 20, the compensation signal s_Batt_Komp is constantly increased, wherein the increase of a virtual reduction of the motor current i_Mot corresponds to the actual lower motor current i_Mot with decreasing supply voltage u_Batt to compensate for 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 screwdriver 10 on reaching the predetermined 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 power screwdriver 10 by the shutdown signal s_Stop provided by the screwdriver shutoff 40, the energy stored in the support arm 18 causes the electric motor 12, starting from the socket 16, to be driven backwards via the gear 14, wherein the electric motor 12 in the The direction opposite to the drive direction begins to rotate.

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 condition, which would occur even at a low generator voltage, a high motor current i_Mot, corresponding to a high counter-torque. 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 surges can lead to the destruction of components of the drive circuit 22, in particular to the destruction of the switching element 32.

Therefore, according to the invention, the voltage limiter circuit 46 is provided, which limits the motor voltage u_Mot occurring at the electric motor 12 of the electric motor 12 operating as a generator when the energy stored in the support arm 18 is reduced to a predetermined limiting voltage u_Lim.

The voltage limiter circuit 46 is not comparable to a freewheel which essentially short circuits only the electric motor 12. The voltage limiter circuit 46 allows the targeted specification of the limiting voltage u_Lim, so that the electric motor 12 during generator operation in the annihilation 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 46 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 be provided, which is driven by the pulse width modulated signal s_PWM.

The voltage limiter circuit 46 can be realized in different ways. At the in FIG. 3a shown embodiment, the voltage limiter circuit 46 includes two series-connected, mutually poled Zener diodes 50, 52. The breakdown voltages are preferably set equal. Apart from the forward bias voltages of the diodes 50, 52, the breakdown voltages are at least approximately equal to the clipping voltage u_Lim in both the positive and negative directions. In principle, different limiting voltages can be predetermined by a corresponding selection of the breakdown voltages of the zener diodes 50, 52 as a function of the polarity.

At the in FIG. 3b In the embodiment shown, the voltage limiter circuit 46 includes a bipolar voltage limiter diode 54, which is also referred to as TVS (Transient Voltage Suppressor). The Spannungsbegrenzerdiode 54 includes the two Zener diodes 50, 52 integrated into a single component, which is thus less expensive than individual Zener diodes and in particular can be equipped with less effort on a circuit board, resulting in further cost advantages in mass production.

At the in Figure 3c In the embodiment shown, the voltage limiter circuit 46 includes a varistor 56.

This in 3d figure The embodiment shown is based on a voltage limitation with an analog electronic load 58. The electronic load 58 can be realized with a transistor 60 which is connected in series with a loss resistor 62. For driving the transistor 60, a comparator 64 is provided, the motor voltage u_Mot as a measuring voltage u_Mess with the predetermined limiting voltage u_Lim compares and, depending on the comparison, opens the transistor 60 more or less. As a result, the voltage at the voltage limiter circuit 46 is regulated to the limiting voltage u_Lim and thus limited.

While the devices used in the voltage limiter circuits 46 - the zener diodes 50, 52, the voltage limiter diode 54 and the transistor 60 - allow a very fast response to voltage pulses, the varistor 56 can absorb and derive a higher energy at least in the short term. Depending on the requirements, therefore, a combination of diodes or transistors 50, 52, 54, 60 and a varistor 60 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 46 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 the generator mode of the electric motor 12, the positive potential of the motor voltage u_Mot on the switching element 32, while the negative potential is applied to the battery 20.

Expediently, the same amount of the limiting voltage u_Lim, which corresponds to at least the magnitude of the rated operating voltage of the electric motor 12, is predetermined for both polarities of the motor voltage u_Mot.

According to another embodiment, at least the effective in generator mode of the electric motor 12 limiting voltage u_Lim to the value of so-called protective extra-low voltage, which may be established 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. Preferably, the limiting voltage u_Lim set for the protective low voltage is also set to the same amount for both polarities of the motor voltage u_Mot.

Claims (13)

1. A power screwdriver, comprising an electric motor (12) and a control circuit (22) which switches the electric motor (12) off by means of a switch-off signal (s_Stop) upon reaching a preset reference torque value (Md_Soll), and a supporting arm (18) which absorbs energy during the screwing process, characterized in that a voltage limiting circuit (46) is provided which during the dissipation of the energy stored in the supporting arm (18) and while the electric motor (12) is operated as a generator and rotates against the drive direction limits the motor voltage (u_Mot) of the electric motor (12) occurring on the electric motor (12) to a predetermined limiting voltage (u_Lim).
2. A power screwdriver according to claim 1, characterized in that the limiting voltage (u_Lim) corresponds at least to the nominal operating voltage of the electric motor (12).
3. A power screwdriver according to claim 1, characterized in that the limiting voltage (u_Lim) corresponds at most to a protective low voltage.
4. A power screwdriver according to claim 1, characterized in that the voltage limiting circuit (46) contains two Zener diodes (50, 52) which are connected in series and are poled against one another.
5. A power screwdriver according to claim 1, characterized in that the voltage limiting circuit (46) contains a bipolar threshold diode (54).
6. A power screwdriver according to claim 1, characterized in that the voltage limiting circuit (46) contains a varistor (56).
7. A power screwdriver according to claim 1, characterized in that the voltage limiting circuit (46) contains an electronic load (58).
8. A power screwdriver according to claim 1, characterized in that the control circuit (22) provides the switch-off signal (s_Stop) upon reaching the set reference torque value (Md_Soll) by comparison of an actual torque value (md_Ist, mdm_Ist) obtained from the electric motor current (i_Mot) with the reference torque value (Md_Soll).
9. A power screwdriver according to claim 1, characterized in that a storage battery (20) is provided as the power source for the electric motor (12).
10. A power screwdriver according to claim 9, characterized in that a lithium-based storage battery (Li-ion storage battery, Li-polymer storage battery) is provided as the storage battery (20).
11. A power screwdriver according to claim 9 or 10, characterized in that a compensation circuit (44) for battery voltage drop is provided, which circuit compensates the influence of a decreasing operating voltage (u_Batt) on reaching the set reference torque value (Md_Soll).
12. A power screwdriver according to claim 9 and 11, characterized in that in the case of decreasing supply voltage (u_Batt) the compensation circuit (44) for battery voltage drop increases the reference torque value (Md_Soll) predetermined for reaching the set reference torque value (Md_Soll).
13. A power screwdriver according to claim 9 and 11, characterized in that in the case of decreasing supply voltage (u_Batt) the compensation circuit (44) for battery voltage drop reduces the actual torque value (md_Ist, mdm_Ist) detected for reaching the set reference torque value (Md_Soll).

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