Device and method for securing a drive control against supply voltage failures

Abstract

Device (10) for securing a drive control against supply voltage failures with: a drive circuit (12) for an electric drive (14), comprising a bridge circuit (16) electrically connectable or connected to the drive (14) and an energy storage unit (18) electrically connected to the bridge circuit (16), wherein the drive circuit (12) is configured to be operated at a first voltage level; a control circuit (24) for the electric drive (14), comprising a control unit (26) for controlling the bridge circuit (16), wherein the control circuit (24) is configured to operate at a second voltage level which differs from the first voltage level; and a voltage converter (36) which electrically connects the drive circuit (12) to the control circuit (24); wherein the drive circuit (12) and the control circuit (24) are configured to be operated with a supply voltage from a common supply voltage source; wherein the control unit (26) is configured to detect a drop in the supply voltage and, in response to the drop in the supply voltage, to activate the voltage converter (36); and wherein the voltage converter (36) is configured to supply the control circuit (24) with energy from the energy storage unit (18) in response to an activation signal from the control unit (26).

Description

Field of invention

[0001] The invention relates to securing a drive control for an electric drive, for example a brushless DC motor, against failures of the supply voltage in order to enable extended operation of the drive control. Background and state of the art

[0002] In electric drives, such as brushless DC motors, the drive circuit with its bridge rectifier and the control circuit for the electric drive are often powered from a common supply voltage source. Fluctuations or failures in the supply voltage then cause not only the drive circuit to fail, but also the motor control. This can lead to critical operating conditions that impair the function and safety of the system.

[0003] It is therefore desirable to transition the system to a safe state in a controlled manner in the event of a power failure, or at least to save the essential operating parameters, thus facilitating the restart of the electric drive after power is restored. To achieve this safe state, the control and actuating devices within the system, such as the microcontroller and the motor output stage, should continue to receive power for a certain period despite the voltage drop. One option is to provide a separate emergency power supply independent of the main supply voltage. However, this is complex and requires additional components within the system, which undesirably increase both the size and manufacturing costs.

[0004] From the patent application DE 10 2012 102 869 A1, a method for controlling a brushless electric motor by means of pulse width modulation is known, in which braking energy is fed back into the voltage supply during the braking phase.

[0005] From the patent application DE 101 59 639 A1, a method and a device for maintaining the supply voltage of a power converter electronics during an armature short-circuit braking of a power converter-fed rotating field machine are known, in which an armature short circuit is maintained by a power converter in the event of a network fault, even if the braking process takes more time than the buffer time of the DC link capacitor, thereby making a matrix converter usable as an industrial power converter.

[0006] Against the background of the prior art, the invention aims to provide a compact, efficient device for protecting a drive control against failures of the supply voltage. Overview of the invention

[0007] This problem is solved by the device and the method according to independent claims 1 and 9, respectively. The dependent claims relate to advantageous embodiments.

[0008] An inventive device for protecting a drive control system against supply voltage failures comprises a drive circuit for an electric drive with a bridge circuit that can be electrically connected to or is connected to the drive, and an energy storage unit that is electrically connected to the bridge circuit. The drive circuit is configured to operate at a first voltage level. The device further comprises a control circuit for the electric drive with a control unit for controlling the bridge circuit. The control circuit is configured to operate at a second voltage level that differs from the first voltage level. The device also includes a voltage converter that electrically connects the drive circuit to the control circuit.The drive circuit and control circuit are configured to be operated with a supply voltage from a common supply voltage source, wherein the control unit is configured to detect a drop in the supply voltage and, in response to the drop in the supply voltage, to activate the voltage converter, and wherein the voltage converter is configured to supply the control circuit with energy from the energy storage unit in response to an activation signal from the control unit.

[0009] The invention is based on the understanding that a system comprising an electric drive, with energy stored electrically in the drive circuit and mechanically in the drive, has an energy storage capacity independent of the supply voltage. This energy storage can be used to maintain the operation of the control circuit in the event of a supply voltage failure until the system has been shut down in a controlled manner. According to the invention, protecting the drive control against supply voltage failures can therefore be achieved using only a voltage converter and the components already required for motor operation. An additional emergency power supply is not necessary. A drive with the device according to the invention can therefore be manufactured in a particularly compact and cost-effective manner.

[0010] A failure of the supply voltage, as defined by the invention, can encompass any deviation of the supply voltage and / or supply current from a range in which regular motor operation is possible. This includes both fluctuations and a complete failure of the supply voltage.

[0011] The first voltage level, at which the drive circuit is operated, is preferably higher than the second voltage level, at which the control circuit is operated. Accordingly, the energy stored in the drive circuit is generally significantly higher than the energy stored in the control circuit. The device according to the invention makes the energy stored in the drive circuit and in the electric drive usable for maintaining the voltage supply of the control circuit.

[0012] The voltage converter can, in particular, include a digitally controllable device for the interval-wise transfer of charge between different voltage levels.

[0013] The energy storage unit may include a capacitor, in particular a smoothing capacitor in a bridge circuit. Such capacitors can be used, for example, to buffer the various supply voltages during the commutation of a brushless DC motor and are typically connected to the corresponding voltage sources via diodes to prevent discharge through the sources, which become low-impedance during a voltage failure or dip. These capacitors represent a substantial energy storage capacity that can be supplied to the control unit via the voltage converter in the event of a voltage failure.

[0014] In a preferred embodiment, the control unit comprises a microcontroller.

[0015] The device according to the invention can also include the electric drive, preferably a brushless DC motor.

[0016] In one embodiment, the device according to the invention also includes the supply voltage source. The supply voltage can, for example, be converted by means of a transformer into a first voltage level and a second voltage level, which are made available to the drive circuit and the control circuit, respectively.

[0017] In a preferred embodiment, the control unit is configured to activate the voltage converter in response to a drop in the supply voltage, in particular the voltage level in the drive circuit and / or in the control circuit, below a predetermined first threshold.

[0018] For example, many microcontrollers have internal overvoltage interrupts (undervoltage interrupts) that are triggered before the microcontroller's operating voltage falls below its normal operating range. These internal interrupts do not require any additional resources such as connection pins or analog-to-digital converters. They can be easily used to drive the voltage converter to transfer power from the drive circuit to the control circuit. An interrupt causes a temporary pause in a running program, such as a microcontroller's program, triggered by an interrupt request.

[0019] In a further development of the invention, the control unit is configured to deactivate the voltage converter in response to an increase in the supply voltage, in particular the voltage level in the drive circuit and / or in the control circuit, above a predetermined second threshold.

[0020] To deactivate the voltage converter, an overvoltage interrupt, which many microcontrollers are equipped with, can be used. For example, after a power supply failure, energy can be transferred from the drive circuit to the control circuit via the voltage converter until the second threshold is reached, such as when an internal overvoltage interrupt of the microcontroller is triggered. The voltage converter is then deactivated until the voltage at the control circuit drops below the first threshold and the voltage converter is reactivated. This sequence can be repeated for several cycles.

[0021] In a preferred embodiment, the drive is a brushless DC motor.

[0022] The bridge circuit can include several switching elements, especially semiconductor switching elements such as MOSFETs.

[0023] In a preferred embodiment, freewheeling diodes are connected in parallel to each of the switching elements.

[0024] Such a bridge circuit allows the control of a multi-phase brushless DC motor, for example in a star connection.

[0025] In another preferred embodiment, the bridge circuit comprises three half-bridges. Such a bridge circuit is also referred to as an H6 bridge and essentially comprises six switching elements, which can be implemented, in particular, by semiconductor switching elements such as MOSFETs. H6 bridges can also be used, in particular, to drive a three-phase brushless DC motor. Brushless DC motors are used, for example, in industrial applications, for actuators in automobiles, or to drive fans or storage disks in a hard disk drive. Such brushless DC motors can have various types of bearings for supporting the rotor. For example, such a motor can have one or more ball bearings, plain bearings, roller bearings, or fluid dynamic bearings.

[0026] In a preferred embodiment, the control unit is configured to control the bridge circuit in response to a drop in the supply voltage in such a way that the electric drive feeds energy into the energy storage unit and / or to the voltage converter.

[0027] In this embodiment, the energy stored in the inertia of the drive's mass is fed directly into the energy storage unit and / or to the voltage converter. From there, it can be transferred to the control circuit to maintain the operation of the control unit in the event of a power failure. In this embodiment, the drive thus acts as a mechanical energy storage device to extend the operation of the control unit during a power failure.

[0028] In this configuration, the electric drive interacts with the bridge circuit as a DC / DC converter or synchronous converter.

[0029] In a preferred embodiment, the control unit is configured to control the bridge circuit for pulse width modulation of the electric drive.

[0030] By varying the duty cycle of the pulse width modulation, i.e., the ratio of on-time to off-time, the power or speed of the drive can be appropriately adjusted.

[0031] In a preferred embodiment, the control unit is configured to operate the bridge circuit in active freewheeling mode during the off-times of the pulse width modulation.

[0032] In this configuration, a switch in the bridge circuit is closed during the off-times to bypass the freewheeling diode connected in parallel to the switch.

[0033] On the one hand, this reduces the power loss at the switch in question. On the other hand, the active freewheeling mechanism allows bidirectional current flow through the switch.

[0034] In a further development of the invention, the control unit is configured to reduce the duty cycle of the pulse width modulation in response to a drop in the supply voltage.

[0035] This makes it possible to reverse the direction of the current flow during freewheeling, so that energy is stored in the motor inductance during this freewheeling phase and fed back into the energy storage unit and / or to the voltage converter at the end of the freewheeling phase.

[0036] The invention also relates to a method for protecting a drive control against supply voltage failures, comprising the step of providing a first voltage from a supply voltage to a drive circuit for an electric drive, wherein the drive circuit includes a bridge circuit electrically connectable to or connected with the drive and an energy storage unit electrically connected to the bridge circuit. The method according to the invention further comprises providing a second voltage from the supply voltage to a control circuit for the electric drive, wherein the second voltage differs from the first voltage and wherein the control circuit includes a control unit for controlling the bridge circuit.Furthermore, the method according to the invention comprises detecting a drop in the supply voltage, activating a voltage converter which electrically connects the drive circuit to the control circuit in response to the detected drop in the supply voltage, and supplying the control circuit with energy from the energy storage unit via the voltage converter.

[0037] In a preferred embodiment, the method comprises the step of activating the voltage converter in response to a drop in the supply voltage, in particular the voltage level in the drive circuit and / or in the control circuit, below a predetermined first threshold.

[0038] In a further development of the invention, the method comprises deactivating the voltage converter in response to an increase in the supply voltage, in particular the voltage level in the drive circuit and / or in the control circuit, above a predetermined second threshold.

[0039] In a preferred embodiment, the method comprises the step of supplying energy to the energy storage unit and / or to the voltage converter in response to a drop in the supply voltage.

[0040] In a preferred embodiment, the method can include controlling the bridge circuit for pulse width modulation, particularly in active freewheeling mode.

[0041] In a further development, the method includes reducing the duty cycle of the pulse width modulation in response to the drop in the supply voltage, so that during the freewheeling phase in the off-time, energy is stored in the motor inductance of the electric drive and fed back into the energy storage unit and / or to the voltage converter at the end of the freewheeling phase. Description of preferred embodiments

[0042] The features and numerous advantages of the solution according to the invention can best be explained by means of a detailed description of preferred embodiments with reference to the accompanying figures, in which: Fig. Figure 1 shows a schematic block diagram of a device for securing a drive control against supply voltage failures according to an embodiment of the invention; Fig. 2 a schematic circuit diagram of a voltage converter as used in the device according to Fig. 1. It can be used, shows; Fig. Figure 3 shows a simplified block diagram of a bridge circuit for a three-phase brushless DC motor according to an embodiment of the invention; Fig. 4 schematically shows the course of the current in the motor phases of the three-phase brushless DC motor according to Fig. 3 induced voltages over a full revolution; Fig. 5a the circuit configuration of the bridge circuit and the current flow in the circuit of the Fig. 3 shows during the start-up phase of PWM operation; Fig. 5b the circuit configuration of the bridge circuit and the current flow in the circuit of the Fig. 3 illustrated in a pulse width modulation operation during the switch-off phase of the pulse width modulation operation; Fig. 5c shows the time course of the current intensity in pulse width modulation operation during the switch-on and switch-off times; Fig. 5d the circuit configuration and the current flow in the circuit of the Fig. 3 illustrated when the current direction is reversed in pulse width modulation operation with active freewheeling; Fig. Figure 5e schematically represents the time course of the current strength in pulse width modulation operation with active freewheeling and feedback into the source Q; Fig. 5f the circuit configuration and the current waveform of the circuit according to Fig. 3 shows when energy is fed back into the source Q; and Fig. 6a a simplified representation of the circuit diagram of the Fig. 3 in a first commutation phase; Fig. 6b a summarized representation of the circuit diagram of the Fig. 6a shows; Fig. 7a / 7b the current flow in the circuit of Fig. 6b in step-down converter operation with switch S1 switched on ( Fig. 7a) or with switch S1 switched off ( Fig. 7b) show; and Fig. 8a / 8b the current flow in the circuit of Fig. 6b in boost converter operation with switch S4 switched on ( Fig. 8a) or with switch S4 switched off ( Fig. 8b) show.

[0043] Fig. Figure 1 is a block diagram of a device 10 for protecting a drive control against supply voltage failures. The device 10 comprises a drive circuit 12 for an electric drive 14, for example a brushless DC motor M with load L.

[0044] The drive circuit 12 comprises a bridge circuit 16 electrically connected to the drive 14 and a buffer capacitor 18 electrically connected to the bridge circuit 16. The buffer capacitor 18 buffers the first supply voltage applied to the drive circuit 12, which is provided by a first voltage source 20.

[0045] In the schematic block diagram of the Fig. Figure 1 shows only a single buffer capacitor 18. However, a drive circuit 12 can also include several such capacitors, depending on the circuit configuration. The capacitors can, for example, be connected to the first voltage source 20 via diodes to prevent discharge through the sources, which become low-impedance during a voltage failure or dip. This function is performed by the diode 22 between the buffer capacitor 18 and the voltage source 22.

[0046] Circuit 10 further comprises a control circuit 24 for controlling the electric drive 14 via the bridge circuit 16. The control circuit 24 comprises a microcontroller 26, which is connected via measuring and control lines 28 (in Fig. (1 shown as a single line for clarity) is connected to the bridge circuit 16. The microcontroller 26 controls the bridge circuit 16 for commutation of the drive 14 via the control lines 28. The microcontroller 26 can monitor the voltage level of the supply voltage at the bridge circuit 16 via the measuring lines 28.

[0047] The control circuit 24 includes a buffer capacitor 30 electrically connected to the microcontroller 26. The microcontroller 26 and the buffer capacitor 30 are supplied via a second voltage source 32 for the control circuit 24. A diode 34 between the buffer capacitor 30 and the second voltage source 32 serves as protection against the discharge of the buffer capacitor 30 via the voltage source 32 for the control circuit 24, which has a low resistance in the event of a fault.

[0048] In many applications, the drive circuit 12 and the control circuit 24 are operated at different voltage levels. For example, in automotive applications, the drive circuit 12 may be operated at a higher voltage level (e.g., 36 V) than the control circuit 24 (e.g., 12 V or 5 V). However, in many applications, it is advantageous to supply the drive circuit 12 and the control circuit 24 from a common voltage source. For example, both the first voltage source 20 for the drive circuit 12 and the second voltage source 32 for the control circuit 24 can be supplied from a common supply voltage source (in Fig. (1 not shown) are supplied. The supply voltage for the control circuit 24 is then, for example, the stepped-down supply voltage for the drive circuit 12.

[0049] Such a configuration with a common supply voltage source for the drive circuit 12 and the control circuit 24 is advantageous in practice because the control unit can be designed more compactly and manufactured more cost-effectively. However, it carries risks because a failure of the supply voltage to the drive 14 also always affects the microcontroller 26. A failure of the microcontroller 26 together with the bridge circuit 16 can result in insufficient time to bring the drive 14 to a safe state in a controlled manner or to save its operating parameters. The buffer capacitor 30 of the control circuit 24 often does not have sufficient capacitance to maintain the functionality of the microcontroller 26 for a sufficiently long time. In contrast, the buffer capacitor 18 of the drive circuit 12, which operates at a higher voltage, typically has a significantly higher capacitance.

[0050] With the solution according to the invention, the energy stored in the buffer capacitor 18 can be used for the continued operation of the control circuit 24 in the event of a supply voltage failure. For this purpose, the control circuit 24 is electrically connected to the drive circuit 12 via a voltage converter 36, which converts the higher supply voltage of the drive circuit 12 into the required lower voltage of the control circuit 24. The voltage converter 36 can be controlled by the microcontroller 26 via a control line 38. The microcontroller 26 is connected via a measuring line 40 to a voltage measuring device, which detects a drop in the supply voltage in the drive circuit 12 and / or in the control circuit 24. The voltage measurement can be carried out, for example, using two resistor elements 42a, 42b.If the voltage detected in the drive circuit 12 or in the control circuit 24 falls below a predetermined threshold level, the microcontroller 26 activates the voltage converter 36 via the control line 38, so that energy to maintain the operation of the control circuit 24, in particular the microcontroller 26, is supplied from the buffer capacitor 18 to the control circuit 24.

[0051] The energy stored in the drive circuit 12 is therefore made available for the operation of the control circuit 24 in the event of a failure or fluctuation of the supply voltage, so that the microcontroller 26 can continue to control the bridge circuit 16 as intended and bring the drive 14 into a safe operating state. At the same time, due to the solution according to the invention, the microcontroller 26 has sufficient energy available to store the operating state of the drive 14, so that a safe restart of the drive 14 is possible after the supply voltage is restored.

[0052] Within the framework of the solution according to the invention, any converter device or voltage converter suitable for converting the higher supply voltage of the drive circuit 12 into the lower supply voltage of the control circuit 24 can be used in the circuit 10. An exemplary circuit configuration for the voltage converter 36 is shown in Fig. Figure 2 shows this circuit. It comprises a transistor 44, whose source is connected to the terminal for the buffer capacitor 18 of the drive circuit 12 and whose sink is connected to the terminal for the buffer capacitor 30 of the control circuit 24. A resistor 46 is connected in series with the transistor 44 of the voltage converter 36 to limit the current. The gate of the transistor 44 is driven by a control transistor 48, whose gate is connected to the control line 38 and, via that, to the microcontroller 26. Furthermore, the emitter terminal of the control transistor 48 is connected to ground (GND) via a resistor 56. The collector terminal of the control transistor 48 is connected to the transistor 48 via a resistor 52 and to the buffer capacitor 18 via a resistor 54.

[0053] The device and method according to the invention for securing a drive control against supply voltage failures can be used for the drive control of a large number of different electric drives. Fig. Figure 3 shows an example of a drive circuit 12 for driving a three-phase brushless DC motor in a star connection. The drive 14 is shown in the schematic block diagram of the Fig. 3 represented by three phases U, V, W.

[0054] The bridge circuit 16 connected to the motor 14 comprises six semiconductor switching elements S1, S2, S3, S4, S5, S6, each of which has freewheeling diodes 50-1 to 50-6 connected in parallel. Such a bridge circuit 16 is well known for controlling a three-phase brushless DC motor 14. Of the other components of the in Fig. The drive circuit 12 shown in 1 is in the simplified block diagram of the Fig. For clarity, only the buffer capacitor 18 is shown in Figure 3. The semiconductor switching elements S1 to S6 are symbolically connected to resistors 58, 60, 62 of the motor phases U, V, W.

[0055] Fig. Figure 4 illustrates the course of the operation of the circuit according to Fig. 1 and Fig. 3. Voltages induced in the motor phases U, V, W (back electromotive force, BEMF) over a full motor revolution from 0° to 360° electrically, where the voltages are normalized over a maximum phase voltage U0. Fig. Figure 4 also shows, for illustration purposes, the voltage differences UV (UV), UW (UW), VW (VW), VU (VU), WU (WU) and WV (WV) derived from the phase voltages.

[0056] To achieve optimal torque of the drive 14, the bridge circuit 16 used for commutation is generally controlled such that the current flows through the two motor phases whose BEMF has the greatest possible voltage difference. From the in Fig. The voltage curve shown in section 4 shows that in the range between 0° and 60°, the differential voltage UV spanned by the motor phases U and V is the largest voltage present. Therefore, in the range between 0° and 60°, commutation occurs between phases U and V. For the drive 14 to draw energy from the source Q, the current I must flow through the two motor phases in such a way that it counteracts the BEMF. This can be achieved, for example, by closing switches S1 and S5 of the bridge circuit 16. Fig. Figure 5a shows the corresponding circuit configuration. Current flow and direction are shown in the circuit diagram. Fig. 5a is indicated by dashed lines or arrows.

[0057] The motor operation and commutation state in the subsequent five phases 60° to 120°, 120° to 180°, 180° to 240°, 240° to 300° and 300° to 360° is determined accordingly by selecting the largest possible voltage difference from the curve diagram of the Fig. 4 and corresponding control of the bridge circuit 16.

[0058] When commutating to phases U and V at 0°, the inductances of these two phases initially limit the current rise. The current increases approximately linearly in this region and then remains at a constant level. In this constant region, the current is determined by the winding resistance of the two phases. At 60°, commutating to phases W and V occurs. The current drop before the phase change is again determined by the inductances of the two phases U and V.

[0059] The one in Fig. The switching state shown, in which switches S1 and S5 are closed, is maintained over the entire range from 0° to 60°, and is referred to as "full-block operation". In full-block operation, the maximum motor speed achievable at no load depends primarily on the level of the applied supply voltage Vcc, with the current flow through the motor windings being at its maximum for the given supply voltage Vcc.

[0060] In practice, varying the motor speed between standstill and the maximum speed achievable at the supply voltage Vcc is often achieved using pulse width modulation (PWM). In pulse width operation, one of the two switches, which are permanently active during full-block operation, is repeatedly switched on and off during a commutation phase. The duty cycle, i.e., the ratio between the on-time and off-time, determines the average voltage applied to the drive 14 and therefore its speed.

[0061] Starting from the in Fig. In the circuit configuration shown in 5a in full-block operation, for example, switch S1 can be opened for pulse width modulation while switch S5 remains closed. Fig. Figure 5b shows this circuit configuration. Current flow and direction are again represented by dashed lines and arrows, respectively. As shown in the block diagram of the Fig. As can be seen in Figure 5b, the current now flows through the freewheeling diode 50-4 belonging to switch S4. In pulse-width modulation (PWM) operation, switch S1 is periodically opened (off-time) and closed (on-time). In steady state, the current in Fig. 5c shows a periodic sawtooth-shaped current waveform.

[0062] If the resistance of the windings of phases U and V is assumed to be negligibly small, the current increases during the switching-on times according to the following equation: I=I0+(VCC−BEMF UV)×tLU+LV, where I0 is the current level at the beginning of the switch-on time, Vcc is the supply voltage, BEMF UV is the voltage induced in the motor phases U and V, LU and LV are the inductances of the phases U and V respectively, and t is the time.

[0063] During the off-time, however, the current decreases according to I=I1−BEMF UV×tLU+LV ab, where I1 denotes the current level reached at the beginning of the switch-off time.

[0064] Since the current flows through the freewheeling diode 50-4 belonging to switch S4 when switch S1 is open, losses P occur there, which result from the forward voltage UF of the diode 50-4 and the average value of the current I AVG during the switch-on time according to P=UF×I AVG calculate.

[0065] To reduce power losses at switch S4, the bridge circuit 16 can be operated in so-called "active freewheeling" mode. For this purpose, after switch S1 is switched off, switch S4 is switched on for approximately the duration of the switch-off time. The current then no longer flows through the freewheeling diode 50-4, but through switch S4. The losses across the on-resistance of switch S4 are significantly lower with modern semiconductor switches, such as MOSFETs, than the losses of a freewheeling diode 50-4.

[0066] To prevent unwanted current surges through the bridge branch consisting of switches S1 and S4, a short dead time is typically inserted between switching off switch S1 and switching on switch S4. A corresponding dead time can also be inserted between switching off switch S4 and switching on switch S1.

[0067] Pulse width modulation with active freewheeling makes it possible to feed the energy stored as kinetic energy in the drive 14 back into the drive circuit 12 or its buffer capacitor 18. In this way, the energy stored in the drive 14 can be supplied to the control circuit 24 via the voltage converter 36 to maintain the operation of the microcontroller 26 in the event of a supply voltage failure. To this end, the microcontroller 26, in response to a detected drop in the supply voltage across the voltage-sensing resistors 42a, 42b, controls the motor bridge 16 via the control line 28 to reduce the duty cycle of the motor commutation. If the duty cycle is small enough, i.e., the off-time is long enough, the off-current described by equation (2) becomes negative, i.e., its direction of flow reverses. The reversal of the current direction compared to that described in Fig. The current flow shown in 5b with switch S4 closed is in Fig. 5D representation.

[0068] Fig. Figure 5e shows the current's behavior over time between 0° and 60°. The regions of negative current, i.e., a reversal of the current flow direction, are shown in Fig. 5e is shown hatched. Since the BEMF UV is the driving force, energy is drawn from the drive 14 during the current flow reversal. The motor moves from the sink to the source, and is therefore slowed down.

[0069] When switch S4 is opened after the current direction is reversed, the two phase inductors LU and LV continue to drive the current in the same direction. As long as switch S1 is still open, the current flows through the freewheeling diode 50-1 associated with S1. When switch S1 is then closed, the current flows through switch S1, as shown in the circuit diagram. Fig. Figure 5f shows that energy is initially fed back from the drive 14 into the buffer capacitor 18 according to equation (1), where I0 is negative when switch S1 is turned on. After a certain time, however, the current becomes positive, i.e., it reverses its direction again, so that the Fig. The current profile shown in section 5a is established. From this point on, energy is again drawn from the motor capacitor 18.

[0070] The ratio between the on-time and off-time, i.e., the duty cycle, controls how much energy is fed into or taken out of the motor.

[0071] During regenerative braking in active freewheeling mode, the drive 14, in conjunction with the bridge circuit 16, acts as a synchronous converter or DC voltage transformer. For a simplified representation of this mode of operation, the circuit configuration of the Fig. 3. First, all components not involved in the commutation phase UV under consideration are removed, i.e., switches S3, S6 and phase W. The permanently open switch S2 can also be removed, and the permanently closed switch S5 can be replaced by a fixed connection. Fig. Figure 6a shows the resulting circuit configuration.

[0072] Combining the inductances and induced voltages of phases U and V corresponding to a phase U+V yields the representation of Fig. 6b, the circuit diagram of a synchronous converter. Resistors 58 and 60 were combined to form a single resistor 64.

[0073] In normal pulse width modulation operation, switch S1 is controlled with a predetermined duty cycle. Fig. Figure 7a shows the current flow with switch S1 closed (dashed lines), with the current direction indicated by arrows. Fig. Figure 7b shows the current flow with switch S1 open. Fig. 7a and Fig. 7b corresponds to a synchronous converter in step-down converter mode.

[0074] The Fig. 8a and Fig. Figure 8b shows the synchronous converter in boost converter mode. Fig. 8a, switch S4 is closed. The current flow is again represented by dashed lines, with the arrows indicating the current direction. Accordingly, Fig. 8b the current flow with switch S4 open.

[0075] Many microcontrollers 26 have internal overvoltage and undervoltage interrupts that are triggered before the operating voltage of the microcontroller 26 reaches values ​​at which reliable operation is no longer possible. These internal voltage interrupts do not require additional resources such as connection pins or analog-to-digital converters and can be used, according to the invention, in a simple manner to control the voltage converter 36. As described above with reference to the exemplary embodiments, the control can be effected in response to a detected voltage drop either by a fixed signal or by a pulse-width modulated signal. In both configurations, energy is drawn from the buffer capacitor 18 of the drive circuit 12 via the voltage converter 36 and fed into the buffer capacitor 30 of the control circuit.Energy can be drawn until the operating voltage of microcontroller 26 has risen sufficiently to trigger the internal overvoltage interrupt. Then, voltage converter 36 is switched off again until the voltage at microcontroller 26 has dropped sufficiently to trigger the internal undervoltage interrupt. The cycle then begins anew.

[0076] At the same time, the microcontroller 26 can monitor the voltage at the bridge circuit 16 via the measuring line 28 or via the measuring line 40 and the voltage measuring resistors 42a, 42b and control the bridge circuit 16 in such a way that the voltage at the bridge circuit 16 remains within a predefined range.

[0077] The scope of protection is defined by the following claims.

Claims

[1] Device (10) for securing a drive control against supply voltage failures with: a drive circuit (12) for an electric drive (14), comprising a bridge circuit (16) electrically connectable or connected to the drive (14) and an energy storage unit (18) electrically connected to the bridge circuit (16), wherein the drive circuit (12) is configured to be operated at a first voltage level; a control circuit (24) for the electric drive (14), comprising a control unit (26) for controlling the bridge circuit (16), wherein the control circuit (24) is configured to operate at a second voltage level which differs from the first voltage level; and a voltage converter (36) which electrically connects the drive circuit (12) to the control circuit (24); wherein the drive circuit (12) and the control circuit (24) are configured to be operated with a supply voltage from a common supply voltage source; wherein the control unit (26) is configured to detect a drop in the supply voltage and, in response to the drop in the supply voltage, to activate the voltage converter (36); and wherein the voltage converter (36) is configured to supply the control circuit (24) with energy from the energy storage unit (18) in response to an activation signal from the control unit (26). [2] Device (10) according to claim 1, in which the control unit (26) is configured to activate the voltage converter (36) in response to a drop in the supply voltage below a predetermined first threshold. [3] Device (10) according to claim 2, in which the control unit (26) is configured to activate the voltage converter (36) in response to a drop in the supply voltage of the voltage level in the drive circuit (12) or in the control circuit (24) below a predetermined first threshold. [4] Device (10) according to one of claims 1 to 3, in which the control unit (26) is configured to deactivate the voltage converter (36) in response to an increase in the supply voltage above a predetermined second threshold. [5] Device (10) according to claim 4, in which the control unit (26) is configured to deactivate the voltage converter (36) in response to an increase in the supply voltage of the voltage level in the drive circuit (12) or in the control circuit (24) above a predetermined second threshold. [6] Device (10) according to one of the preceding claims, wherein the drive is a brushless DC motor (14). [7] Device (10) according to one of the preceding claims, in which the control unit (26) is configured to control the bridge circuit (16) in response to the drop in supply voltage such that the electric drive (14) feeds energy into the energy storage unit (18) and / or to the voltage converter (36). [8] Device (10) according to one of the preceding claims, wherein the control unit (26) is configured to control the bridge circuit (16) for pulse width modulation. [9] Device (10) according to claim 8, wherein the control unit (26) is configured to operate the bridge circuit (16) in active freewheeling during off-times of the pulse width modulation. [10] Device (10) according to claim 8 or 9, in which the control unit (26) is configured to reduce a duty cycle of the pulse width modulation in response to the drop in the supply voltage, so that the electric drive (14) feeds energy into the energy storage unit (18) and / or to the voltage converter (36) during the off times of the pulse width modulation. [11] Device (10) according to any one of the preceding claims, characterized by that the bridge circuit comprises three half-bridges. [12] Method for securing a drive control against supply voltage failures comprising the following steps: Applying a first voltage from a supply voltage to a drive circuit (12) for an electric drive (14), wherein the drive circuit (12) comprises a bridge circuit (16) that can be electrically connected to or is connected to the drive (14) and an energy storage unit (18) that is electrically connected to the bridge circuit (16); Applying a second voltage from the supply voltage to a control circuit (24) for the electric drive (14), wherein the second voltage differs from the first voltage and wherein the control circuit (24) includes a control unit (26) for controlling the bridge circuit; Detecting a drop in supply voltage; Activating a voltage converter (36), which electrically connects the drive circuit (12) to the control circuit (24), in response to the detected drop in the supply voltage; and Supplying the control circuit (24) with energy from the energy storage unit (18) via the voltage converter (36). [13] Method according to claim 12 comprising the step of activating the voltage converter (36) in response to a drop in the supply voltage below a predetermined first threshold. [14] Method according to claim 13 comprising the step of activating the voltage converter (36) in response to a drop in the supply voltage of the voltage level in the drive circuit (12) or in the control circuit (24) below a predetermined first threshold. [15] Method according to any one of claims 12 to 14 comprising the step of deactivating the voltage converter (36) in response to an increase in the supply voltage above a predetermined second threshold. [16] Method according to claim 15 comprising the step of deactivating the voltage converter (36) in response to an increase in the supply voltage of the voltage level in the drive circuit (12) or in the control circuit (24) above a predetermined second threshold. [17] Method according to any one of claims 12 to 16 comprising the step of feeding energy back into the energy storage unit (18) and / or to the voltage converter (36) in response to the drop in supply voltage. [18] Method according to any one of claims 12 to 17 comprising the step of controlling the bridge circuit (16) for pulse width modulation. [19] Method according to claim 18, characterized by that the bridge circuit is operated in active freewheeling mode at least temporarily. [20] Method according to claim 18 or 19 comprising the step of reducing a duty cycle of the pulse width modulation in response to the drop in the supply voltage, so that the electric drive (14) feeds energy into the energy storage unit (18) and / or to the voltage converter (36) during the off times of the pulse width modulation.

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