Method for braking a machine tool, and machine tool

Abstract

The present invention relates to a method for braking a machine tool, wherein the machine tool is a mains-operated machine tool, and electrical energy that is generated when braking the machine tool is fed back into an intermediate circuit of the machine tool and subsequently fully dissipated as loss power in components of the machine tool. The invention makes use of the basic concept that the braking power is reduced when the intermediate circuit or the intermediate circuit capacitance has reached its limit for absorbing electrical energy. This reduction in braking power can be achieved by shortening a current space vector with the help of correction factors or by rotating it within a space vector representation. The invention also relates to a machine tool for carrying out the braking method.

Description

[0001] Hilti Corporation in Schaan

[0002] Principality of Liechtenstein

[0003] METHOD FOR BRAKING A MACHINE TOOL AND MACHINE TOOL

[0004] The present invention relates to a method for braking a machine tool, wherein the machine tool is a mains-operated machine tool and electrical energy released during braking of the machine tool is fed back into an intermediate circuit of the machine tool and converted into power loss in components of the machine tool. The invention makes use of the basic idea that the braking power is reduced when the intermediate circuit and the power loss in various components of the machine tool have reached their limits for absorbing electrical energy. This reduction in braking power can be brought about by shortening a current space vector or by rotating it in a space vector representation with the aid of a correction factor. In a second aspect, the invention relates to a machine tool for carrying out the braking method.

[0005] Background of the invention:

[0006] In the field of machine tools, it is known to brake machine tools to complete a task or for safety reasons. The following braking methods are known in particular in the prior art: short-circuit braking, the use of a brake chopper as a braking resistor, and recuperation.

[0007] During short-circuit braking, the machine tool motor is short-circuited. This can be achieved, for example, by switching on all of the low-side semiconductors of the motor inverter at the same time. An initially high short-circuit current can develop, which then changes into a continuous short-circuit current. The short-circuit current generates a braking torque in the machine tool motor with which the machine tool or its tool can be braked. The disadvantage of short-circuit braking is that the user has little influence on the braking torque or the braking time. The level of the short-circuit current is determined solely by the motor parameters. In addition, the machine tool electronics must be able to carry the comparatively high initial short-circuit current at the start of short-circuit braking. If the machine tool is not designed to withstand such short-circuit currents, this can lead to damage to the machine tool or its tool.their electronics. When using a brake chopper as a braking resistor, the machine tool motor can be braked in a controlled manner so that it becomes a generator. The energy output can be fed back into an intermediate circuit of the machine tool’s electronics. The machine tool can have a brake chopper as a braking resistor, whereby the brake chopper is designed to connect a resistor to the intermediate circuit in order to convert the energy in the braking resistor into heat. Although the use of a brake chopper allows more freedom in achieving certain desired braking torques or braking times, one disadvantage of using a brake chopper is that at least one additional hardware component has to be installed in the machine tool. This increases the weight of the machine tool and its volume. A larger volume can, for example, have a negative impact on ergonomics orThe handling of the machine tool suffers. The installation of a brake chopper often also requires additional electronic components, such as MOSFETs, gate drivers, or connecting wires, which can complicate the manufacture and assembly of the machine tool. Especially if the brake chopper is designed for high pulse power, the costs for the machine tool and the space required inside the machine tool can increase significantly.

[0008] Regeneration is only possible if regenerative energy supply devices are used in the machine tool. The machine tool motor can be braked in a controlled manner during recuperation, thus acting as a generator. The energy released can be fed back into the intermediate circuit and from there transferred to the energy supply devices. Mains-powered machine tools typically use a rectifier, which only allows energy to flow from the mains to the intermediate circuit. Regeneration is not possible in these cases.

[0009] The object underlying the present invention is to overcome the deficiencies and disadvantages of the prior art described above and to provide a method for braking a machine tool, as well as a machine tool, so that on the one hand the most effective protection for a user of the machine tool can be enabled, and on the other hand a compact, handy machine tool can be provided. Furthermore, the released energy should be fed back into the intermediate circuit as best as possible and then converted essentially entirely as power loss in components of the machine tool in order to be able to provide a machine tool that is as resource-efficient as possible and does not require any additional components, such as a brake chopper. This object is achieved by the subject matter of the independent claims.Advantageous embodiments of the subject matter of the independent claims can be found in the dependent claims.

[0010] Description of the invention:

[0011] According to the invention, a method for braking a machine tool is provided, wherein the machine tool is a mains-operated machine tool and electrical energy released when the machine tool is braked is fed back into an intermediate circuit of the machine tool and converted into power loss in components of the machine tool. The braking method is characterized by the following method steps: a) operating the machine tool under specification of a first current value, b) feeding the electrical energy released when the machine tool is braked back into the intermediate circuit of the machine tool, c) reducing a braking power of the machine tool if a limit value of the voltage of the intermediate circuit is exceeded, d) continuing to operate the machine tool with target current values, wherein the target current values ​​are determined based on the reduction in the braking power of the machine tool and the first current value.

[0012] The feedback into the intermediate circuit always entails losses; power losses always arise in components of the machine tool, which are converted into heat there. These components can be, in particular, the motor and electronics. It is advantageous if the machine tool adopts an operating point with poorer efficiency during the braking process in order to generate higher losses in the components of the machine tool. In the sense of the invention, it is particularly preferred that the electrical energy released when the machine tool is braking is fed back into an intermediate circuit capacitance or into an intermediate circuit capacitor of the machine tool. The intermediate circuit can preferably comprise one or more intermediate circuit capacitors, which are characterized by the intermediate circuit capacitance.Preferably, the intermediate circuit capacitance can be determined by the intermediate circuit capacitor or the plurality of intermediate circuit capacitors of the intermediate circuit.

[0013] The method enables the shortest possible braking time, while also providing a compact, manageable machine tool—preferably without a brake chopper. The particularly short braking time allows the machine tool user to be particularly effectively protected from injury. The proposed method advantageously enables electrical energy to be fed back into the intermediate circuit of a machine tool by utilizing the power losses of various components of the machine tool.

[0014] This method advantageously allows the released braking energy to first charge the DC link to a maximum permissible limit and then reduce the braking power such that the voltage limit in the DC link is not exceeded. The machine tool's electronics and its motor can initially be operated at an optimal operating point with particularly low power loss in order to charge the DC link to the permissible voltage limit. If the braking power needs to be reduced, the machine tool's electronics and its motor can then be operated at an inefficient operating point with higher or highest possible power loss.

[0015] To achieve an optimal operating point, the machine tool can, for example, be operated along a power loss-optimized characteristic curve. The formulation that the machine tool is operated along a power loss-optimized characteristic curve preferably means, within the meaning of the invention, that the losses during operation of the machine tool can be minimized using the invention. These can be, for example, iron and / or copper losses. The formulation that the machine tool is operated along a power loss-optimized characteristic curve can, within the meaning of the invention, also mean that the machine tool is operated in an optimized manner with regard to current yield and / or efficiency.

[0016] It is preferred within the meaning of the invention that the machine tool is operated along an MTPA characteristic curve in a space vector representation, where the abbreviation MTPA stands for “maximum torque per ampere”, i.e. for a maximum torque per ampere. This advantageously allows the machine tool to be operated with an optimum current yield. It has been shown that the machine tool can be operated in this way with particularly low losses or power losses. Preferably, in the context of the invention, a maximum motor stator target current l_S, max is specified, which within the meaning of the invention is preferably also referred to as maximum motor current l_S, max. This maximum motor current l_S, max can thus represent a first current value within the meaning of the invention.However, if the braking power is too high, the DC link voltage limit, which is intended to ensure safe operation of the DC link capacitance, may be exceeded. In accordance with the invention, it is preferred that the DC link voltage limit be referred to as U_ZK, max.

[0017] Alternatively or additionally, the first current value can be the motor target current l_S, target. According to the invention, it is preferred that the exceeding of a limit value of the intermediate circuit voltage is prevented by active control of the intermediate circuit voltage. For this purpose, a reduction in the braking power of the machine tool is initiated, preferably after an exceeding of the intermediate circuit voltage limit value has been detected. According to the invention, target current values ​​are calculated and output, the determination of the target current values ​​being based on the reduction in the braking power of the machine tool and the first current value. According to the invention, it is preferred that the target current values ​​from method step d) of the proposed method are target current values ​​for the d and q axes of the space vector representation. The target current values ​​for the d and q axes can thus be changed orrepresent second current values ​​with which the machine tool can continue to operate. By continuing to operate the machine tool with the modified target current values, preferably for the d and q axes of the space vector representation, a reduction in the machine tool's braking power can be ensured so that the limit value of the intermediate circuit voltage is not exceeded, while still enabling safe and rapid braking of the machine tool.

[0018] In a particularly preferred embodiment of the invention, the machine tool can, for example, be operated with a target motor current l_S, soll. Furthermore, a maximum motor current l_S, max can be specified, whereby the braking power of the machine tool is reduced if the limit value of the intermediate circuit voltage is exceeded. The braking power of the machine tool can be reduced in particular by rotating a current space vector l_S, max in the space vector representation by applying a changed braking angle ß_brems such that a length of the current space vector l_S, max remains essentially unchanged. By rotating the current space vector l_S, max, the changed target current values, preferably for the d and q axes of the space vector representation, can be determined and the braking power of the machine tool can be reduced in this way.In this embodiment of the invention, it is preferred that the target current values, preferably for the d and q axes of the space vector representation, are determined with the aid of the maximum stator current l_S, max and the braking angle ß_brems, wherein the target current values, preferably for the d and q axes, are to be used as the output or manipulated variable for the further braking process.

[0019] In one embodiment, the invention relates to a method for braking a machine tool, wherein the machine tool is a mains-operated machine tool and electrical energy released when the machine tool is braked is fed back into an intermediate circuit of the machine tool and converted into power loss in components of the machine tool.The method is characterized by the following method steps: a) operation of the machine tool along an MTPA characteristic curve in a space vector representation under specification of a first current value, b) feedback of the electrical energy released when braking the machine tool into the intermediate circuit of the machine tool, c) reduction of a braking power of the machine tool if a limit value of the voltage of the intermediate circuit is exceeded, d) continued operation of the machine tool with target current values ​​for the d and q axes of the space vector representation, wherein the target current values ​​for the d and q axes are calculated based on the reduction in the braking power of the machine tool and the first current value.

[0020] According to the invention, it is preferred that a correction factor k_red can also be determined in the case of a rotation of the current space vector l_S,max. Preferably, the correction factor k_red can be applied to an angular position of the current space vector l_S,max, so that a rotation of the current space vector l_S,max with a modified braking angle ß_brems is obtained.

[0021] In a further particularly preferred embodiment of the invention, the braking power of the machine tool can be reduced by determining at least one correction factor k_red and applying it to the maximum motor current l_S, max. This makes it possible to obtain a reduced setpoint value l_S, red for the motor current, based on which the changed setpoint current values, preferably for the d and q axes of the space vector representation, are then determined and the braking power of the machine tool is reduced in this way. The reduced setpoint value l_S, red for the motor current can be passed on to a motor current controller so that the motor current can be regulated or adjusted based on the reduced setpoint value l_S, red. In the sense of the invention, it is preferred that in this embodiment the braking power of the machine tool is reduced by shortening the current space vector.In other words, the application of the correction factor k_red to the maximum motor current l_S, max leads to a shortening of the current space vector, which advantageously allows the braking power of the machine tool to be reduced.

[0022] In a further particularly preferred embodiment of the invention, the machine tool can be operated subject to a specification of the maximum motor current l_S, max. The braking power of the machine tool can be reduced either by determining at least one correction factor k_red and applying it to the maximum motor current l_S, max, so that a reduced setpoint l_S, red is obtained for the motor current, or by rotating the current space vector l_S, max in the space vector representation, so that a length of the current space vector l_S, max remains essentially unchanged. The rotation of the current space vector l_S, max can preferably be effected by applying a modified braking angle ß_brems to the current space vector l_S, max.

[0023] By rotating the current space vector l_S, max, an operating point of the machine tool motor can be set to a lower torque and thus also a lower braking power without reducing the stator current amplitude. This high stator current can then cause further high losses within an inverter, the electronics and / or the motor, so that the braking energy can be absorbed and converted into heat. This can advantageously create a larger power sink and make the braking process even faster. In particular, by applying the changed braking angle ß_brems and the resulting rotation of the current space vector l_S, max in the space vector representation, a regenerative braking operation entailing high losses can be provided for a machine tool to decelerate its tool. The proposed method initially does not require a braking chopper.Preferably, the braking angle ß_brems can also be referred to as “stator current space vector angle ß_brems” in the sense of the invention.

[0024] According to the invention, it is preferred that when the current space vector l_S, max is rotated to reduce the braking power, the DC link voltage regulator is used to determine or output a correction factor as a manipulated variable. Such a reduction in braking power is particularly preferred when the DC link capacitance and an optional braking resistor, such as a braking chopper, have reached their maximum power consumption. This advantageously ensures that the DC link voltage limit is maintained and the DC link capacitance is not damaged. In particular, by rotating the current space vector, an operating point in the space vector representation with a reduced braking torque can be obtained. The reduced braking torque can be obtained in particular by a shortened current space vector and / or by a rotated current space vector.

[0025] The correction factor k_red can be generated as an output variable by various sources and controllers. For example, a correction factor k_red can be generated by a DC link voltage regulator. For the correction factor k_red, the duty cycle of an optional brake chopper can also be taken into account when calculating the braking angle using a duty cycle controller. This is particularly true if the machine tool has a brake chopper. Within the meaning of the invention, it is preferred that the correction factor k_red also includes a dependency related to the duty cycle of the brake chopper. This is particularly true if the machine tool has a brake chopper. The correction factor k_red can thus represent the DC link voltage regulator correction factor or a correction factor that maps the duty cycle of the brake chopper via a duty cycle controller.If a brake chopper is used within the scope of the present invention, the braking time can advantageously be further shortened because the provision of the brake chopper creates an additional absorption possibility for excess energy that is released when the machine tool is braked.

[0026] The ability of a brake chopper to absorb electrical energy may be limited if a duty cycle of 100% or substantially 100% is reached. To circumvent such a limitation due to the duty cycle, it may be preferable within the meaning of the invention to use a comparatively low-resistance brake chopper. The brake chopper may preferably have an electrical resistance in a range of 10 to 200 ohms. Such brake choppers or braking resistors are preferably referred to as "low-resistance brake choppers" within the meaning of the invention.

[0027] If the machine tool includes a brake chopper, energy distribution in the proposed braking method can be described as follows: First, the released braking energy is fed back into the intermediate circuit or into the intermediate circuit capacitance. If the intermediate circuit capacitance can no longer absorb the braking energy or braking power, i.e., if the intermediate circuit capacitance has reached its voltage limit, the remaining excess energy can be fed into the brake chopper. If the brake chopper can no longer absorb the braking energy or braking power, the braking power of the motor can be reduced by a smaller braking torque. Preferably, the braking angle ß_brems can also be referred to as the "stator current space vector angle ß_brems" within the meaning of the invention. This braking angle ß_brems can be calculated in the context of the present invention as follows: ßbrems =cred ' ( axle, MTPA ~ axle, 0 Nm ) + axle, 0 Nm-

[0028] The parameter ßbrems, TPA preferably corresponds to the angle on the MTPA characteristic curve, while the parameter ßbrems, o Nm corresponds to the current space vector angle with which an operating point with a torque generation of 0 Nm or essentially 0 Nm is reached. This operating point can, for example, lie in the third quadrant or the fourth quadrant of the space vector representation. The third quadrant should explicitly include the boundary to the second quadrant (upper limit of the third quadrant), and the fourth quadrant should explicitly include the boundary to the first quadrant (upper limit of the fourth quadrant). As can be seen in Figure 2a, the intersection point of the current limit circle K with the 0 Nm characteristic curve of the space vector representation lies at the boundary between the third and second quadrant, and preferably no torque is generated. As can be seen in Figure 2b, the intersection point of the current limit circle K with the 0 Nm characteristic curve can also lie in the fourth quadrant.From the maximum stator current l_S, max and the current space vector angle ßbrems, the target current values, preferably for the d-axis and the q-axis of the space vector representation, can advantageously be calculated.

[0029] It is preferred in the sense of the invention that the braking torque is reduced by the rotated braking angle or stator current space vector angle ß_brems, while the losses in the inverter and / or in the motor of the machine tool continue to absorb the maximum braking power due to the essentially constant motor current amplitude, so that in summary, a particularly rapid braking of the tool of the machine tool can be achieved with the invention.

[0030] According to the invention, it is preferred that an operating range of a current space vector l_S is limited in its length and / or in its current vector angle by a correction factor k_red, so that a limit value U_ZK, max of the intermediate circuit capacitance is not exceeded. Preferably, an operating range of a current space vector l_S can be limited by a current limit of an inverter of the machine tool, wherein in particular the provision of a correction factor k_red, which can be applied to a length and / or an angle of the current space vector l_S, advantageously results in the limit value for the voltage of the intermediate circuit capacitance not being exceeded. According to the invention, it is preferred that the longest current space vector l_S lies on the MTPA characteristic curve, which is preferably limited by the current limit of the inverter of the machine tool when unlimited braking of the machine tool occurs with the greatest possible braking torque.

[0031] According to one embodiment of the present invention, exceeding the voltage limit value of the intermediate circuit capacitance can be prevented by determining a correction factor k_red and applying it to the maximum motor current l_S, max, so that a reduced setpoint l_S, red for the motor current is obtained. Advantageously, this enables active control of the intermediate circuit voltage and provides a balanced compromise between maximum braking deceleration and protection of the intermediate circuit. The determination of the one correction factor k_red preferably takes place in a controller provided for this purpose, wherein a voltage correction factor k_red can be provided by a voltage regulator within the electronics of the machine tool. The voltage regulator is preferably a voltage regulator of the intermediate circuit.Preferably, the intermediate circuit voltage regulator is thus configured to determine an intermediate circuit voltage correction factor k_red. The corresponding correction factor k_red can preferably assume values ​​in a range between 0 and 1. A reduced motor stator target current l_S, red is preferably output as the output variable of this control or correction process. Within the meaning of the invention, it is preferred that this reduced motor stator target current l_S, red is also referred to as a reduced motor current l_S, red. The fixed maximum motor current l_S, max can thus be multiplied by the correction or reduction factor k_red, so that a reduced target current value l_S, red can be passed on to the motor current regulator.Preferably, a first current value, for example the maximum motor current l_S, max, can be multiplied by the intermediate circuit voltage correction factor k_red in order to obtain target current values, preferably for the d- and q-axes, as second current values ​​within the meaning of the invention, with which the machine tool can continue to be operated.

[0032] The corresponding space vector representation (cf. Figure 1) shows the operating range AB of the so-called current space vector l_S. Without a limitation of the braking power, the longest current space vector l_S results on the MTPA characteristic curve. The current space vector l_S represents in particular a stator current space vector, i.e. in particular the space vector for the stator current of the electric motor of the machine tool is shown. In the sense of the invention, it is preferred that the current space vector l_S is limited by a current limit of the inverter of the electronics of the machine tool. This current limit of the inverter is also shown in the space vector representation shown in Figure 1, specifically as circle K. Preferably, when the intermediate circuit voltage limit (U_ZK, max) is reached, the current space vector l_S can be shortened to such an extent that the intermediate circuit voltage limit is not exceeded.The shortening of the current space vector l_S is preferably carried out by applying the correction factor k_red.

[0033] The term “correction factor k_red” is a generic term for correction factors of various origins. This can, for example, be the correction factor k_red for limiting the maximum motor current l_S, max to a reduced motor current l_S, red, whereby in the context of the invention at least one correction factor k_red is applied to the setpoint for the maximum motor current. A further correction factor k_red can, for example, ensure that the duty cycle is limited when a braking chopper is used, i.e. when electrical energy is not only fed back into the intermediate circuit capacitance or converted into motor and electronics losses, but also into a braking resistor, which in the sense of the invention is preferably referred to as a “braking chopper”. In the sense of the invention, it is preferred that the correction factor k_red lies in a value range between 0 and 1.

[0034] It is preferred in the sense of the invention that the correction factor k_red is determined by a voltage regulator of the intermediate circuit voltage or that the correction factor k_red is used to limit a duty cycle by means of a duty cycle regulator.

[0035] Voltage regulators of the intermediate circuit capacitance are provided, in particular, for determining the correction factor k_red when no brake chopper is used. The machine tool or its electronics may furthermore or additionally comprise further regulators. In particular, when a brake chopper is used in the context of the invention, the machine tool or its electronics may comprise two regulators. For example, a first regulator may be provided, which is preferably designed as a two-point regulator with hysteresis and is configured to control the brake chopper. It is preferred within the meaning of the invention that this first regulator is designed as a comparator with switching hysteresis, wherein the first regulator preferably comprises an upper switch-on threshold and a lower switch-off threshold or is characterized by such thresholds.The first controller is designed in particular to compare an intermediate circuit voltage U_ZK of the intermediate circuit capacitance with a reference voltage U_Chopper of the brake chopper. The output signal of the first controller can preferably represent or form a signal for pulse width modulation (PWM), wherein the first controller outputs a high-level signal in particular when the voltage U_ZK of the energy supply device is greater than the reference voltage U_Chopper of the brake chopper. In other words, the first controller is designed to output a PWM signal which is a high level in particular when U_ZK > U_Chopper. The PWM signal can have a duty cycle, wherein the duty cycle can be formed, for example, by a low-pass filter. The switching threshold U_Chopper preferably corresponds to the limit value U_ZK, max of the maximum permissible intermediate circuit voltage.

[0036] In an alternative embodiment of the invention, the first controller can also be designed as a PI controller. It is preferred within the meaning of the invention that the output variable of the first controller, which is preferably designed as a PI controller, is a duty cycle for the brake chopper. Control pulses for the brake chopper can preferably be generated from this duty cycle. A PWM module, for example, can be used for this purpose. In other words, the PWM module can be designed to generate control pulses for a brake chopper from a duty cycle output by a PI controller.

[0037] The machine tool or its electronics can have a second controller used to limit duty cycles. The second controller can be designed, in particular, to regulate the duty cycle to a setpoint value D_Setpoint of a maximum of approximately 95% of the switch-on duty cycle of the brake chopper. The second controller can output a correction or reduction factor k_red as an output variable, which lies in a value range between 0 and 1. According to the invention, it is preferred that the correction factor k_red be multiplied by the maximum motor stator setpoint current l_S, max. In this way, the reduced motor stator setpoint current l_S, red can advantageously be obtained or determined.

[0038] The machine tool or its electronics can have a limiting device for a rate of change of the motor current. The limiting device is preferably designed to provide a smooth transition from driving to braking of the machine tool or its tool. The limiting device can preferably also be referred to as a "rate of change limiter of the motor current" within the meaning of the invention. It is preferred within the meaning of the invention that the limiting device avoids a jump in the motor stator target current to the maximum motor stator current l_S,max by only permitting a maximum rate of change of the motor stator target current. The rate of change is preferably specified in the physical unit "amperes per second", i.e., amperes / second or A / s. A limiting device can be provided as an optional component on the machine tool.The limiting device can then be represented as an additional block in the block diagrams of Figures 3-6, which enables a particularly smooth transition from driving to braking. The proposed braking method is described using field-oriented control (FOC). However, within the meaning of the invention, it is also preferable to implement the method with block commutation. In the language of space vector representation, the operating range of the current or stator current space vector is shown in Figures 2a and 2b.

[0039] The machine tool may include a brake chopper, wherein electrical energy released when the machine tool's tool is braked is absorbed by the brake chopper. The braking method that utilizes the brake chopper represents a braking method with efficient regenerative braking operation with a brake chopper. It is preferred within the meaning of the invention that the stator current l_S is only reduced when the limit value of the intermediate circuit voltage, U_Zk, max, has reached the corresponding limits. It may also be preferred within the meaning of the invention that the stator current l_S is reduced when the duty cycle of the brake chopper has reached its limit.

[0040] The use of a brake chopper is particularly advantageous when the power consumption is limited by the intermediate circuit capacitance and the losses in the components, for example because critical limit values ​​of the intermediate circuit and / or the power losses of the components are exceeded. According to the invention, it is preferred that the brake chopper can absorb additional electrical energy that is released when the machine tool is braked. Preferably, the intermediate circuit and the component losses can absorb a basic amount of electrical energy, and the brake chopper can be provided as an optional component on the machine tool to absorb additional electrical energy. This advantageously makes it possible to provide a particularly high braking power. According to the invention, it is preferred that the maximum power loss within the brake chopper is limited depending on the design.The limitation is achieved in particular when a duty cycle, in particular a switch-on duty cycle, of the brake chopper is 100% or substantially 100%. The duty cycle or the switch-on duty cycle can preferably also be referred to as duty cycle D within the meaning of the invention. If such a duty cycle or switch-on duty cycle of 100% or substantially 100% is achieved, it is preferred within the meaning of the invention that the braking power is reduced by a reduced motor current, in particular a reduced motor stator target current l_S, red. This is particularly because in this case the energy supply device and the brake chopper have reached their maximum power consumption. In one embodiment of the invention, the machine tool can be operated subject to a specification of a maximum motor current l_S, max, in particular maximum motor stator target current l_S, max.A correction factor k_red can be determined by a voltage regulator of the intermediate circuit voltage, wherein the correction factor can be applied to the maximum motor current l_S, max. In this way, a reduced setpoint l_S, red for the motor current can be obtained, which can then be converted into the second current values ​​for continued operation of the machine tool, in particular the setpoint current values ​​for the d and q axes in the space vector representation. In the sense of the invention, it is preferred that the reduced setpoint l_S, red for the motor current can be passed on to a motor current controller so that the motor current can be regulated or adjusted based on the reduced setpoint l_S, red. This method results in a reduction in braking power by shortening the stator current space vector on the MTPA characteristic curve in the space vector representation.In particular, correction factors can be applied to reduce the value of a maximum motor current l_S, max to a reduced setpoint l_S, red for the motor current.

[0041] In a further embodiment of the invention, the machine tool can determine a correction factor k_red, which is applied to the maximum motor current l_S, max in order to obtain the reduced setpoint l_S, red for the motor current. The reduced setpoint l_S, red for the motor current can be passed on to a motor current controller so that the motor current can be regulated or adjusted based on the reduced setpoint l_S, red. This method also results in a reduction in braking power by shortening the stator current space vector on the MTPA characteristic curve in the space vector representation. In particular, the at least one correction factor can be applied to reduce the value of a maximum motor current l_S, max to a reduced setpoint l_S, red for the motor current.

[0042] It is preferred within the meaning of the invention that the motor of the machine tool is controllable using field-oriented control or block commutation. In a preferred embodiment of the invention, it is preferred that the braking angle ß_brems can be changed in order to reduce the braking power of the machine tool and to rotate the current space vector l_S, max in the space vector representation. If the motor of the machine tool is controlled using block commutation, it can also be preferred within the meaning of the invention that the commutation angles are changed in order to reduce the braking power of the machine tool. In a preferred embodiment, the commutation angles are changed in such a way that the resulting commutation blocks are lagging.The machine tool may include a brake chopper, wherein the brake chopper is configured to absorb excess electrical energy released when the tool of the machine tool is braked. In particular, the brake chopper is configured to absorb, in addition to the intermediate circuit capacitance and the power losses in the components, additional electrical energy that the aforementioned components can no longer absorb due to their limited power consumption. This electrical energy is preferably referred to as "excess electrical energy" in the context of the present invention.

[0043] An exemplary embodiment of the invention will be described below, namely a braking method with efficient regenerative braking operation using a brake chopper, as shown by way of example in Figure 4. In this embodiment of the invention, the PWM signal of the brake chopper can be generated as follows, wherein the PWM signal is provided in particular by the first controller, for example a two-point controller. The first controller can comprise a comparator or be formed by a comparator. The comparator is designed to compare the intermediate circuit voltage u_ZK with the reference voltage U_Chopper. The level of the reference voltage U_Chopper preferably corresponds to the voltage of the intermediate circuit voltage limit value U_ZK, max.

[0044] If the intermediate circuit voltage u_ZK—possibly plus a hysteresis contribution—is greater than the reference voltage U_Chopper, a high level can be output at the PWM signal U_GatedriverinputPWM. Otherwise, a low level is output. According to the invention, it is preferred that the intermediate circuit voltage ripple and / or the resulting switching frequency be adjusted via the hysteresis width.

[0045] According to the invention, it is preferred to determine the duty cycle of the PWM signal using a low-pass filter, preferably an RC low-pass filter. PID controllers, for example, can also be used as controllers for the duty cycle.

[0046] Further advantages emerge from the following description of the figures. The figures, the description, and the claims contain numerous features in combination. The skilled person will also expediently consider the features individually and combine them into further meaningful combinations.

[0047] In the figures, identical and similar components are numbered with the same reference numerals.

[0048] They show:

[0049] Fig. 1 Working range of a possible stator current space vector in a space vector representation for efficient generator braking operation

[0050] Fig. 2a, b Working range of a possible stator current space vector in a space vector representation with maximum lossy generator braking operation

[0051] Fig. 3 Possible block diagram «braking» with efficient regenerative braking operation without brake chopper

[0052] Fig. 4 possible block diagram «braking» with efficient regenerative braking operation with brake chopper

[0053] Fig. 5 Possible block diagram «braking» with maximum lossy regenerative braking operation without brake chopper

[0054] Fig. 6 Possible block diagram «braking» with maximum lossy regenerative braking operation with brake chopper

[0055] Fig. 7 schematic representation of a preferred embodiment of the proposed machine tool

[0056] Implementation examples and figure descriptions:

[0057] Figure 1 shows a possible space vector representation depicting the operation of a machine tool 10. In particular, Figure 1 shows the operating range AB of a possible stator current space vector l_S in such a space vector representation during efficient regenerative braking operation of the machine tool 10. The I_d value of the current flowing through the motor 12 of the machine tool 10 is shown on the x-axis of the space vector representation. A machine tool 10 is shown schematically in Fig. 8.

[0058] The y-axis of the space vector representation shows the I_q value of the current flowing through the motor 12 of the machine tool 10. The value l_q represents the torque-generating component of the current, while the value l_d represents the field-generating current component.

[0059] Figure 1 shows the four quadrants 1, 2, 3 and 4 of a space vector representation. The first quadrant 1 is characterized by a negatively increasing torque \ M and generator operation G. The second quadrant 2 is characterized by a positively increasing torque ^M and motor operation M. The third quadrant 3 is characterized by a negatively increasing torque \ M and motor operation M. The fourth quadrant 4 is characterized by a positively increasing torque ^M and generator operation G. The increasing and decreasing torques of the torque hyperbolas are symbolized in Figures 1 and 2 by dashed arrows. The torque hyperbolas are preferably formed by operating points with the same torque. A circle K is drawn in Figure 1, where circle K represents the current limit of an inverter of machine tool 10. Circle K orThe current limit of the inverter lies equally in the four quadrants 1, 2, 3, and 4 of the space vector representation, which means that a center point of the circle K coincides with the intersection point of the y- and x-axes of the space vector representation. The operating range AB of the stator current space vector l_S is shown in the third quadrant 3 of the space vector representation. In the space vector representation shown in Figure 1, the operating range AB of the stator current space vector l_S coincides with the MTPA characteristic curve of the machine tool 10. The motor 12 of the machine tool 10 is therefore advantageously operated at an efficiency-optimized operating point in which the current heat losses are minimal.

[0060] The 0 Nm characteristic curve N runs through the first quadrant 1 and the fourth quadrant 4, essentially parallel to the y-axis of the space vector representation. Furthermore, a second 0 Nm characteristic curve N2 results, which runs on the x-axis or coincides with the x-axis (therefore not shown). The space vector representation depicted in Figure 1 shows a machine tool 10 and its operation, in which the braking power of the machine tool 10 is reduced by determining at least one correction factor k and applying it to a maximum motor current l_S, max of the machine tool 10, so that a reduced setpoint l_S, red for the motor current is obtained. This corresponds to efficient regenerative braking operation of the machine tool 10.

[0061] Figures 2a and 2b show a space vector representation of a maximum-loss generator braking operation of a machine tool 10. In contrast to Figure 1, in the braking method shown in Figure 2, a current space vector l_S, max is rotated in the space vector representation by applying a changed braking angle ß_brems, so that a length of the current space vector l_S, max remains essentially unchanged and the braking power of the machine tool 10 is reduced. The current space vector l_S, max is initially located in the third quadrant 3 of the space vector representation, but as a result of the rotation in Figure 2a, it reaches the boundary between the second quadrant 2 and the third quadrant 3. The current space vector l_S, max is initially located in the third quadrant 3 of the space vector representation, but as a result of the rotation in Figure 2b, it reaches the fourth quadrant 4.The braking angle ß_brems is also shown in the fourth quadrant 4 of the space vector representation. The rotated current space vector l_S, max intersects the circular area K in the fourth quadrant 4, which represents the current limit of the inverter of the electronics of the machine tool 10. This intersection point of the rotated current space vector l_S, max and the circular area K coincides with the intersection point of the circular area K and the 0 Nm characteristic curve N in the exemplary embodiment of the invention shown in Figure 2. The operating range AB is therefore limited to operating points between the states "maximum braking torque" and "no braking torque, i.e., 0 Nm".

[0062] It should be noted that the block diagrams shown in Figures 3 to 6 show exemplary braking methods for a machine tool 10 in which the motor 12 is operated with a field-oriented control. The proposed method can, of course, also be implemented with a machine tool 10 in which the motor 12 is controlled with block commutation. The motor 12 of the machine tool 10 is a brushless motor, which can preferably deliver a power of more than 1,800 watts (W).

[0063] Fig. 3 shows a possible block diagram of "braking" with efficient regenerative braking of a machine tool 10 without a brake chopper 18. In the braking process shown in Figure 3, the intermediate circuit voltage regulator 20 is configured to output a correction factor k_red for the voltage of the intermediate circuit capacitance 14. Furthermore, a speed controller 30 is provided, which controls the speed n of the machine tool 10. Target current values ​​are obtained as output or manipulated variables of the braking process and can be passed on to the motor current controllers 34, 36. The target current values ​​can preferably be target current values ​​for the d and q axes of the space vector representation. The motor current controllers 34, 36 represent, in particular, a d-current controller 34 and a q-current controller 36, with the motor current controllers 34, 36 passing their control commands to the pulse width modulation PWM.

[0064] Fig. 4 shows a possible block diagram for "braking" with efficient regenerative braking operation of a machine tool 10 with a brake chopper 18. The block diagram shown in Figure 4 shows the speed controller 30 and the motor current controllers 34, 36, which are already known from Figure 3 and the block diagram shown there. Furthermore, Figure 4 shows a duty cycle controller 28, which, within the meaning of the invention, can preferably also be referred to as a "second controller" and is used to limit the duty cycle to a setpoint value D_setpoint of, for example, 95%. The output value of this second controller is a correction factor k_red, which lies between the values ​​0 and 1. This correction factor k_red can be multiplied by the current l_S, max to obtain the reduced motor stator setpoint current l_S, red.

[0065] Furthermore, the block diagram shown in Figure 4 shows a two-point controller 24 with hysteresis, which is preferably referred to as the "first controller" in the sense of the invention. This first controller 24 is preferably used to control the braking resistor 18 referred to as the "brake chopper". The first controller 24 can, for example, be designed as a comparator with switching hysteresis and be set up to compare an intermediate circuit voltage with a reference voltage. The intermediate circuit voltage can preferably also be referred to as voltage u_ZK, while the reference voltage is referred to as "U_Chopper" in Figure 4. The first controller 24 outputs a PWM signal as an output signal, which outputs a high level when the battery voltage is greater than the reference voltage, i.e. u_ZK > u_Chopper. A duty cycle of the PWM signal can, for example, be formed by the low-pass filter 32 shown in Figure 4.It is preferred in the sense of the invention that values ​​or quantities which vary over time are designated with lower case letters, while constant values ​​or quantities, such as limit values ​​or other predetermined values, are designated with upper case letters.

[0066] Fig. 5 shows a possible block diagram of “braking” with maximum lossy regenerative braking operation of a machine tool 10 without brake chopper 18. Similar to the block diagram of Figure 3, the correction factor k_red is output in the braking method described in Figure 5.

[0067] In particular, the correction factor k_red can be determined by the DC link voltage regulator 20. The stator current space vector angle or braking angle ß_brems can be calculated from the correction factor k_red. To do this, the angles ß_brems, MTPA and ß_brems, 0 Nm are subtracted from each other and multiplied by the correction factor k_U, red. The ß_brems, 0 Nm can be added again to the product to obtain the braking angle ß_brems. The parameter ß_brems, MTPA preferably represents the angle on the MTPA characteristic curve, while the parameter ß_brems, 0 Nm corresponds to the stator current space vector angle in the fourth quadrant 4 of the space vector representation or the stator current space vector angle on the x-axis between the second quadrant 2 and the third quadrant 3 at which no torque is generated. This parameter ß_brems, 0 Nm in the fourth quadrant 4 preferably results from the intersection point of the current limit circuit K with the 0 Nm characteristic curve N.Using the braking angle ß_brems and the current l_S, max, the d and q setpoints can be calculated, which can be passed on to the d current controller 34 and the q current controller 36.

[0068] Fig. 6 shows a possible block diagram of “braking” with maximum loss-prone regenerative braking operation of a machine tool 10 with brake chopper 18, whereby the block diagram shown in Fig. 6 is largely a combination of elements of the block diagrams from Figs. 4 and 5.

[0069] Figure 7 shows a schematic representation of a preferred embodiment of the proposed machine tool 10. The machine tool 10 has a motor 12, which is preferably designed as a brushless motor. The machine tool 10 can have a tool 16, which can be designed, for example, as a disk-shaped cutting tool. The machine tool 10 shown in Figure 4 is preferably a cut-off machine with which cuts can be made in a substrate, such as concrete. The tool 16 of the machine tool 10 can be surrounded by a blade guard (without reference symbol) to protect the user of the machine tool 10 from flying chips and sparks. The machine tool 10 can be connected to a public or construction site power grid via a power cable 22 in order to be supplied with electrical energy.The machine tool 10 has an intermediate circuit 14 into which excess electrical energy released when the machine tool 10 is braked can be fed. The intermediate circuit 14 can have one intermediate circuit capacitor or several intermediate circuit capacitors with an intermediate circuit capacitance. The machine tool 10 can have a brake chopper 18 as an optional component. In the context of the present invention, electrical energy can be fed back into the intermediate circuit 14, in particular when the machine tool 10 is braked. Preferably, electrical energy can also be fed back into the brake chopper 18 of the machine tool 10, in particular when the absorption capacity of the other components of the machine tool 10, such as the absorption capacity of the intermediate circuit 14, has been exhausted.The machine tool 10 may further comprise one or more handles (without reference numerals) with which the user can transport the machine tool 10 or guide it during work.

[0070] List of reference symbols

[0071] 1 1st quadrant of the space vector representation

[0072] 2 2nd quadrant of the space vector representation

[0073] 3 3rd quadrant of the space vector representation

[0074] 4 4th quadrant of the space vector representation

[0075] 10 Machine tool

[0076] 12 Engine

[0077] 14 intermediate circuit

[0078] 16 tools

[0079] 18 brake choppers

[0080] 20 DC link voltage regulator

[0081] 22 Mains connection or power cable of the machine tool

[0082] 24 first regulator, especially DC link voltage regulator

[0083] 28 second controller, in particular duty cycle controller

[0084] 30 speed controllers

[0085] 32 low-pass filters

[0086] 34 d-current regulator

[0087] 36 q-current controllers

[0088] M engine operation

[0089] G Generator operation l_d torque l_q torque

[0090] ^M positively increasing torque

[0091] \ M negatively increasing torque K circuit range as current limit of the inverter

[0092] AB Working range of the stator current space vector N 0 Nm characteristic curve

[0093] MTPA MTPA characteristic curve (Maximum Torque per Ampere or maximum torque per Ampere ß_brake Braking angle

[0094] PWM pulse width modulation n speed t time

Claims

Patent claims 1. A method for braking a machine tool (10), wherein the machine tool (10) is a mains-operated machine tool and electrical energy released during braking of the machine tool (10) is fed back into an intermediate circuit (14) of the machine tool (10), characterized by the following method steps: a) operating the machine tool (10) under specification of a first current value, b) feeding back the electrical energy released during braking of the machine tool (10) into the intermediate circuit (14) of the machine tool (10), c) reducing a braking power of the machine tool (10) if a limit value of the voltage of the intermediate circuit (14) is exceeded, d) continuing to operate the machine tool (10) with target current values, wherein the target current values ​​are calculated based on the reduction in the braking power of the machine tool (10) and the first current value.

2. Method according to claim 1, characterized in that the machine tool (10) is operated along a power loss-optimized characteristic curve.

3. Method according to claim 1, characterized in that the machine tool (10) is operated along an MTPA characteristic curve in a space vector representation.

4. Method according to claim 3, characterized in that the target current values ​​are target current values ​​for the d- and q-axes of the space vector representation.

5. Method according to one of the preceding claims, characterized in that the braking power of the machine tool (10) is determined by the following method step is reduced: e) Rotation of a current space vector l_S, max in the space vector representation by applying a changed braking angle ß_brems, so that a length of the current space vector l_S, max remains essentially unchanged.

6. Method according to one of claims 1 to 4, characterized in that the braking power of the machine tool (10) is reduced by the following method steps: f) determining at least one correction factor k_red and g) applying the at least one correction factor k to a maximum motor current l_S, max, whereby a reduced setpoint value l_S, red for the motor current is obtained.

7. Method according to claim 6, characterized in that the at least one correction factor k_red lies in a value range between 0 and 1.

8. Method according to claim 6 or 7, characterized in that the at least one correction factor k_red is formed from the output variable of an intermediate circuit voltage regulator and / or is formed to limit a duty cycle of an optional brake chopper by means of a duty cycle regulator.

9. Method according to one of the preceding claims, characterized in that the machine tool (10) comprises a brake chopper (18), wherein electrical energy released during braking of the tool (16) of the machine tool (10) is absorbed by the brake chopper (18).

10. Method according to one of the preceding claims, characterized in that a control of a motor (12) of the machine tool (10) comprises a field-oriented control or a block commutation.

11. Machine tool (10) for carrying out the method according to one of the preceding claims, characterized in that a motor (12) of the machine tool (10) is a brushless motor.

12. Machine tool (10) according to claim 14, characterized in that the machine tool (10) comprises a brake chopper (18), wherein the brake chopper (18) is designed to absorb electrical energy which is released when the tool (16) of the machine tool (10) is braked.

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