Circuit for suppressing resonances

Abstract

A machine tool containing a drive, preferably a brushless DC motor, a control unit with an inverter for regulating the rotational speed of the drive, and an energy supply, wherein at least one first series resonant circuit is included which is parallel to the inverter and to a DC link capacitor.

Description

[0001] Circuit for suppressing resonances

[0002] The present invention relates to a machine tool comprising a drive, preferably a brushless DC motor, a control unit with an inverter for speed control of the drive and a power supply.

[0003] Electrically powered machine tools typically use speed-controlled electric motors as their drive system. These motors are usually controlled by a control unit (also known as power electronics) with an inverter. The inverter is switched using a PWM frequency (Pulse Width Modulation). This pulsed inverter current draw results in a pulsed output. Consequently, the inverter current contains frequency components at the PWM frequency and their multiple harmonics. These frequency components can cause oscillations with the power source (e.g., the battery pack in battery-powered machine tools or the mains power supply in mains-powered machine tools).The impedance of the power source, together with the DC link capacitance (also known as buffer capacitance or bulk capacitance) of the control device, forms an unwanted resonant circuit, which can generate unnecessarily high circulating currents. These currents place an unnecessary load on the power source and cause additional heating in the power source and the connecting cable to the control device.

[0004] The object of the present invention is therefore to solve the problem described above.

[0005] The problem is solved by the subject matter of independent patent claim 1.

[0006] Further advantageous embodiments of the subject matter according to the invention are contained in the corresponding dependent claims.

[0007] The problem is solved in particular by a machine tool containing a drive, preferably a brushless DC motor, a control unit with an inverter for speed control of the drive and a power supply.

[0008] According to the invention, at least one first series resonant circuit is included in parallel with the inverter and an intermediate circuit capacitor. The first series resonant circuit comprises a first coil with a first inductance value and a first capacitor with a first capacitance value.

[0009] The series resonant circuit can also be referred to as an LC series circuit or resonant circuit.

[0010] According to an alternative embodiment, it is possible that at least one second series resonant circuit is included, which is arranged in parallel to both the inverter and an intermediate circuit capacitor as well as to the first series resonant circuit.

[0011] It should be noted that the first series resonant circuit has a first coil with a first induction value and a first capacitor with a first capacitance value, and the second series resonant circuit has a second coil with a second induction value and a second capacitor with a second capacitance value.

[0012] The first induction value can be higher or lower than the second induction value. However, it is also possible that the first induction value of the first coil of the first series resonant circuit and the second induction value of the second coil of the second series resonant circuit are identical.

[0013] The first capacitance value is higher or lower than the second capacitance value. However, it is also possible that the first capacitance value of the first capacitor in the first series resonant circuit and the second capacitance value of the first capacitor in the second series resonant circuit are identical.

[0014] According to an alternative embodiment, it may be possible that the at least first and / or second series resonant circuit contains at least one resistive element.

[0015] The resistive element can be designed as a discrete resistor.

[0016] In the case that both the first and second series resonant circuits each contain a resistive element, it can be advantageous for the resistive element of the first series resonant circuit to have a higher or lower resistance value than the resistive element of the second series resonant circuit. However, it is also possible for the resistive element of the first series resonant circuit and the resistive element of the second series resonant circuit to have an identical resistance value.

[0017] According to a further alternative embodiment, it is possible that the at least first or second series resonant circuit contains at least one coil with an inductance value and a capacitor with a capacitance value, and wherein the switching frequency of a control device for the drive is essentially equal to a resonant frequency based on an inductance value and capacitance value.

[0018] Each series resonant circuit cancels out a frequency component which can then no longer excite an oscillation with the energy source.

[0019] The number of parallel resonant circuits indicates how many frequency components one wants to suppress (e.g. a fundamental oscillation, a first harmonic, a second harmonic, etc.).

[0020] However, it can also be useful to suppress only one fundamental frequency or only one overtone.

[0021] The bandwidth, or quality factor, of the series resonant circuit can be adjusted via the ratio of its inductance to its capacitance. Generally, a higher bandwidth for the inductance and capacitance values ​​is advantageous to improve robustness against deviations in the inverter's PWM frequency.

[0022] If the resonant frequency of the series resonant circuit is matched to the PWM frequency of the control device, the impedance for the fundamental frequency component of the inverter current is minimized, or, if the resistance value is negligibly small, it represents a short circuit for this current component.

[0023] This frequency component no longer affects the energy source, as the series resonant circuit short-circuits this frequency component, thus preventing the excitation of an oscillating construct consisting of the impedance of the energy source and the DC link capacitance.

[0024] However, since the inverter current contains harmonic currents in addition to the fundamental frequency, complete suppression of oscillations with only one series resonant circuit is not possible, as this can only be tuned to a specific resonant frequency. The harmonics can be suppressed by additional series resonant circuits arranged in parallel.

[0025] According to another alternative embodiment, the power supply may be designed in the form of at least one accumulator or a mains power source. The accumulator is configured as a power supply with a direct current or a direct voltage, and the mains power source is configured as a power supply with an alternating current or an alternating voltage. Further advantages will become apparent from the following description of the figures. The figures illustrate various embodiments of the present invention.

[0026] The figures, the description, and the claims contain numerous features in combination. A person skilled in the art will expediently consider these features individually and combine them into meaningful further combinations.

[0027] They show:

[0028] Figure 1 shows a side view of a machine tool with a power supply designed as an accumulator;

[0029] Figure 2 shows a circuit diagram of the object according to the invention in a first embodiment;

[0030] Figure 3 shows a circuit diagram of the object according to the invention in a second embodiment;

[0031] Figure 4 shows a circuit diagram of the object according to the invention in a third embodiment;

[0032] Figure 5 shows a circuit diagram of the invention according to a fourth embodiment; and

[0033] Figure 6 shows a circuit diagram of the invention according to a fifth embodiment.

[0034] Examples of implementation:

[0035] Figure 1 shows a machine tool 1.

[0036] The machine tool 1 shown in Figure 1 is designed in the form of a cordless screwdriver according to an exemplary embodiment.

[0037] According to an alternative embodiment, the machine tool 1 can also be designed in the form of a saw, a grinding device, a hammer drill or the like.

[0038] The power tool 1, designed as a cordless screwdriver, essentially comprises a housing 2, a handle 3 and a tool holder 4.

[0039] The housing 2 has a front end 2a, a rear end 2b, a top end 2c and a bottom end 2d.

[0040] The tool holder 4 is positioned at the front end 2a of the housing 2. The tool holder 4 serves to receive and hold a tool 7. In the exemplary embodiment, the tool is designed as a screwdriver bit.

[0041] A first end 3a of the handle 3 is positioned at the lower end 2d of the housing 2. An interface 6 is provided at the second end 3b of the handle 3.

[0042] As also shown in Figure 1, the handle 3 has an activation switch 8 with which the machine tool 1 can be set to an activation state or deactivation state.

[0043] A power supply 5 can be repositionably attached to interface 6.

[0044] The interface 6 of the machine tool 1 contains a positive contact, a negative contact, and a communication contact. Neither the positive contact, the negative contact, nor the communication contact of interface 6 of the machine tool 1 are shown in the figures.

[0045] In the present embodiment, the power supply 5 is designed in the form of a single accumulator. Alternatively, more than one accumulator can also be provided as the power supply for the machine tool 1.

[0046] Power supply 5 is used to supply the machine tool with electrical energy.

[0047] Inside the housing 2, there is essentially a drive 9 in the form of an electric motor, a gearbox 10, a drive shaft 11 and a control device 12 positioned.

[0048] The drive 9, designed as an electric motor, the gear unit 10, the drive shaft 11 and the tool holder 4 are arranged inside the housing 2 in such a way that a torque generated in the electric motor 9 can be transmitted to the gear unit 10, the drive shaft 11 and finally to the tool holder 4 or to the tool 7.

[0049] The drive 9, designed as an electric motor, is in the form of a brushless DC motor.

[0050] According to a first embodiment, the control unit 12 includes an inverter 16, a first series resonant circuit 17 and an intermediate circuit capacitor 18, see Figure 2.

[0051] The first series resonant circuit 17, the intermediate circuit capacitor 18 and the inverter 16 are arranged parallel to each other.

[0052] The inverter 16 can also be referred to as an inverter and in turn contains a control device 19 with switching elements 20. In this case, the control device 19 is designed as an FL bridge with a number of switching elements 20.

[0053] The switching elements 20 are designed as transistors. The transistors are designed as MOSFETs.

[0054] The inverter 16 serves to generate a switching frequency for the switching elements 20 of the control device 19. In this case, the switching elements 20 are controlled by pulse width modulation (PWM). The switching frequency is then based on a PWM frequency.

[0055] According to the first embodiment, the first series resonant circuit 17 comprises a first coil 21 and a first capacitor 22. The first coil 21 has a first inductance value Li and the first capacitor 22 has a first capacitance value Ci.

[0056] The energy supply 5, designed as a battery, can be detachably connected to the machine tool 1 to supply the machine tool 1 with electrical energy. The battery 5 essentially comprises a battery housing 23, a number of energy storage cells 13, a battery interface 14, and a control unit 15.

[0057] The energy storage cells 13 can also be called battery cells and are arranged inside the battery housing 23.

[0058] The battery housing 23 essentially contains a lid element 23a, four side walls 23b and a bottom element 23c.

[0059] The battery interface 14 is located on the outside of the cover element 23a and serves for the electrical, electronic, and mechanical connection of the battery 5 to the machine tool 1 or a charging device. The charging device is used to charge the battery 5 with electrical energy and is not shown in the figures.

[0060] For electrical or electronic connection, the battery interface 14 has a positive contact, a negative contact and a communication contact.

[0061] The battery interface 14 can be connected to the interface 6 of the machine tool 1 such that the positive and negative contacts of the battery 5 and the machine tool 1 can be connected to each other. Furthermore, the communication contacts of the battery 5 and the machine tool 1 can be connected to each other.

[0062] The positive and negative contacts are used to create an electrical circuit when the accumulator 5 is connected to a machine tool 1 or a charging device. The communication contact is used to send and receive data and information in the form of electrical signals.

[0063] Alternatively or additionally, the accumulator 5 can also include radio communication (e.g. Bluetooth) or wireless communication.

[0064] The energy storage cells 13 serve to absorb, store and release electrical energy.

[0065] The energy storage cells 13 are designed in a cylindrical shape and based on lithium-ion technology.

[0066] Each energy storage cell 13 contains a contact device at one end, which serves to transmit electrical energy. The individual contact devices are connected to the control unit 15 of the accumulator 5 via corresponding lines.

[0067] Alternatively, the energy storage cells 13 can also be based on another suitable technology.

[0068] The cylindrical shape of the energy storage cells 13 is also optional, so any other suitable shape or geometry can be chosen. In particular, it is also possible for the energy storage cells 13 to be designed as pouch cells.

[0069] It is also possible that the accumulator 5 contains both cylindrical energy storage cells 13 and pouch cells. In particular, it is possible that the accumulator 5 contains only a single cylindrical energy storage cell 13 and a single pouch cell.

[0070] The control unit 15 regulates and controls various functions of the accumulator 5. These functions include, among others, controlling the input and output of electrical energy to and from the energy storage cells 13. Furthermore, the control unit 15 controls the amount of electrical energy to be received or released by the energy storage cells 13.

[0071] Figure 3 shows the control device 12 according to a second embodiment.

[0072] The control device 12 according to the second embodiment is essentially identical to the control device 12 according to the first embodiment. In contrast to the first embodiment, the control device 12 according to the second embodiment includes a first series resonant circuit 17 with a first coil 21, a first capacitor 22, and a first resistive element 24. The first resistive element 24 has a first resistance value Ri.

[0073] The first coil 21, the first capacitor 22 and the first resistor 24 are connected in series.

[0074] Figure 4 shows the control device 12 according to a third embodiment. The control device 12 according to the third embodiment differs from the control device 12 according to the first embodiment in that a second series resonant circuit 25 is connected in parallel to the first series resonant circuit 17. The first series resonant circuit 17 contains a first coil 21 with a first inductance value Li and a first capacitor 22 with a first capacitance value Ci. The second series resonant circuit 25 contains a second coil 26 with a second inductance value L2 and a second capacitor 27 with a second capacitance value C2. The first series resonant circuit 17 and the second series resonant circuit 25 are connected in parallel to each other.

[0075] Figure 5 shows the control device 12 according to a fourth embodiment. The control device 12 comprises a first series resonant circuit 17 and a second series resonant circuit 25, wherein the first series resonant circuit 17 includes a first coil 21 with a first inductance value Li, a first capacitor 22 with a first capacitance value Ci, and a first resistor 24 with a first resistance value Ri. The second series resonant circuit 25 includes a second coil 26 with a second inductance value L2, a second capacitor 27 with a second capacitance value C2, and a second resistor 28 with a second resistance value Ri. The first series resonant circuit 17 and the second series resonant circuit 25 are connected in parallel to each other.

[0076] Furthermore, the first and second series resonant circuits 17, 25 are connected in parallel to the intermediate circuit capacitor 18 and the inverter 16.

[0077] Figure 6 shows the control device 12 according to a fifth embodiment. The control device 12 comprises a first and a second series resonant circuit 17, 25, wherein the first series resonant circuit 17 contains a first coil 21 with a first inductance L1, a first capacitor 22 with a first capacitance Ci, and a first resistor 24 with a first resistance Ri. The second series resonant circuit 25 contains only a second coil 26 with a second inductance L2 and a second capacitor 27 with a second capacitance C2. The first and second series resonant circuits 17, 25 are connected in parallel to each other. Furthermore, the first and second series resonant circuits 17, 25 are connected in parallel to the intermediate circuit capacitor 18 and the inverter 16.

[0078] It should be noted that both the inductance value L for the coils used and the capacitance value C for the capacitors used are chosen such that the switching frequency of a control device 19 for the drive 9 essentially corresponds to a resonant frequency f determined by formula A.

[0079] Formula A

[0080] The resonant frequency f of the series resonant circuit must ideally match the switching frequency or its harmonics precisely so that it short-circuits these frequency components. The resonant frequency f of the resistor and inductor can also be very close to the switching frequency or its harmonics, but the greater the difference, the less effective the series resonant circuit becomes.

[0081] Reference mark

[0082] 1 machine tool

[0083] 2 Machine tool housing

[0084] 2a front end of the housing

[0085] 2b rear end of the case

[0086] 2c upper end of the case

[0087] 2d lower end of the housing

[0088] 3 handle

[0089] 3a first end of the handle

[0090] 3b second end of the handle

[0091] 4 Tool holder

[0092] 5 Energy supply

[0093] 6. Machine tool interface

[0094] 7 tools

[0095] 8 activation switches

[0096] 9 Drive

[0097] 10 Gearbox device

[0098] 11 Drive shaft

[0099] 12 Control unit

[0100] 13 Energy storage cell

[0101] 14 Battery interface

[0102] 15. Accumulator control unit

[0103] 16 inverters

[0104] 17 first series resonant circuit

[0105] 18 Intermediate circuit capacitor

[0106] 19 Control device

[0107] 20 switching elements 21 first coil

[0108] 22 first capacitor

[0109] 23 battery housings

[0110] 23a Cover element of the battery housing

[0111] 23b Side walls of the battery housing

[0112] 23c Base element of the battery housing

[0113] 24 first resistance element

[0114] 25 second series resonant circuit

[0115] 26 second coil

[0116] 27 second capacitor

[0117] 28 second resistance element

[0118] Li first induction value

[0119] Ci first capacitance value l_2 second induction value

[0120] C2second capacitance value f resonance frequency

Claims

Patent claims 1. Machine tool (1) comprising a drive (9), preferably a brushless DC motor, a control unit (12) with an inverter (16) for speed control of the drive (9) and a power supply (5), characterized in that at least one first series resonant circuit (17) is included in parallel to the inverter (16) and an intermediate circuit capacitor (18).

2. Machine tool (1) according to claim 1 , characterized in that at least one second series resonant circuit (25) is included, which is arranged in parallel to both the inverter (16) and an intermediate circuit capacitor (18) as well as to the first series resonant circuit (17).

3. Machine tool (1) according to claim 1 or 2, characterized in that the at least first series resonant circuit (17) has a first inductance value (Li) and a first capacitance value (Ci) and the at least second series resonant circuit (25) has a second inductance value (L2) and a second capacitance value (C2).

4. Machine tool (1) according to at least one of claims 1 to 3, characterized in that the at least first and / or second series resonant circuit (17, 25) contains at least one resistance element (24, 28).

5. Machine tool (1) according to at least one of claims 1 to 4, characterized in that the at least first or second series resonant circuit (17, 25) contains at least one coil (21, 26) with an inductance value and a capacitor (22, 27) with a capacitance value, and wherein the switching frequency of a control device (19) for the drive (9) is essentially the same as a resonant frequency (f) based on an inductance value and capacitance value.

6. Machine tool (1) according to at least one of claims 1 to 5, characterized in that the energy supply (5) is designed in the form of at least one accumulator or a mains power source.

Images