MOTOR CONTROL UNIT FOR A POWER TOOL

Abstract

A motor control unit for a brushless electric motor of a power tool, in particular a hand-held power tool, having an electronic control device having a power limiting device designed to limit the power absorption of the electric motor in order to restrict the power absorption of the electric motor to a maximum permissible maximum power value and to adjust this maximum power value during the operation of the electric motor as a function of the at least one temperature value. A corresponding power tool and a method are also provided.

Claims

1. A motor control unit for a brushless electric motor of a power tool or a hand-held power tool, the motor control unit comprising: a temperature sensor via which at least one temperature value that is influenced by the operation of the electric motor of a power tool component is determined; a power measuring device that is set up to record a power absorption value that characterizes a power absorption of the electric motor; and an electronic control device that has a power limiting device, which, in order to limit the power absorption of the electric motor, limits the power absorption of the electric motor to a maximum permissible power value and adjusts a maximum power value during an operation of the electric motor as a function of the at least one temperature value.

2. The motor control unit according to claim 1, wherein the power measuring device includes a current measuring device via which a current absorbed by the electric motor is detected as a current value, and wherein the power absorption value is a current value.

3. The motor control unit according to claim 1, wherein the control device is programmable and programmed to limit the power absorption of the electric motor to the maximum permissible maximum power value and to adjust this maximum power value during the operation of the electric motor as a function of the at least one temperature value.

4. The motor control unit according to claim 3, wherein the control device is programmable and programmed to reduce the maximum permissible maximum power value with increasing temperature linearly or in accordance with a non-linear progression, and to increase it with increasing setpoint speed linearly or in accordance with a non-linear progression.

5. The motor control unit according to claim 1, wherein the electronic control device has a speed control which is set up or programmed to set the speed of the electric motor to be set as a function of a setpoint, independently of the current to regulate it as long as the maximum power value is not exceeded and/or provided that the at least one temperature value does not exceed a pre-determined temperature.

6. The motor control unit according to claim 1, wherein the control device has a speed control which is set up or programmed to regulate a speed n, wherein the speed n is dependent on a setpoint speed specified by the user, the maximum power value or the maximum motor current and the at least one temperature value, and wherein the speed is a function of the setpoint speed of the maximum power value or of the motor current and the at the least one temperature value.

7. The motor control unit according to claim 1, wherein the maximum power value is a function of the at least one temperature value and the setpoint speed, and, wherein the maximum motor current is a function of the at least one temperature value and the setpoint speed.

8. The motor control unit according to claim 1, wherein the control device is set up or programmed so that the limitation of the power absorption of the electric motor or a programmed current limitation is superimposed on the speed control.

9. The motor control unit according to claim 1, wherein the control device is set up or programmed so that the limitation of the power absorption of the electric motor or a programmed current limitation is subordinate to the speed control.

10. The motor control unit according to claim 1, wherein the control device is set up or programmed to reduce the maximum power value during the operation of the electric motor, which also influences the speed in particular, in the event of an increase in at least one temperature value under load.

11. The motor control unit according to claim 1, wherein the control device has an overtemperature cut-off that interrupts the power absorption of the electric motor when the at least one temperature value reaches or exceeds a specified overtemperature value.

12. A power tool comprising: a brushless electric motor; a drive shaft coupled to the electric motor and an output shaft coupled to the drive shaft via a gear unit, to which a tool is adapted to be connected; and a motor control unit according to claim 1.

13. The power tool according to claim 12, wherein the power tool is as an angle grinder.

14. The power tool according to claim 12, wherein the power tool is a hand-held power tool.

15. A method for controlling a brushless electric motor of a power tool via a motor control unit according to claim 1, the method comprising: detecting the at least one temperature value of a power tool component influenced by the operation of the electric motor; detecting the power absorption value characterizing the power absorption of the electric motor; setting the power absorption of the electric motor to a maximum permissible maximum power value; and adjusting the maximum power value as a function of the at least one temperature value.

15. A computer program or storage medium comprising a computer program code having instructions which, when the computer program is executed by a computer, cause it to perform the method according to claim 15.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0047] The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitive of the present invention, and wherein:

[0048] FIG. 1 shows a diagram showing speed characteristics of a variable speed electric motor, in particular the, in particular brushless, electric motor of a power tool, in particular an angle grinder, which contains the motor control unit according to the invention in an exemplary design;

[0049] FIG. 2 shows a diagram with a characteristic of the power limitation in the exemplary inventive motor control unit of the electric motor of the power tool according to the invention mentioned in FIG. 1;

[0050] FIG. 3 shows a diagram with a circuit diagram of the inventive electronic motor control unit according to the embodiment of the electric motor of the exemplary power tool of the invention mentioned in FIG. 1; and

[0051] FIG. 4 shows the exemplary power tool according to the invention mentioned in FIG. 1.

DETAILED DESCRIPTION

[0052] FIG. 1 shows a diagram of motor characteristics, in particular speed characteristics, in which the speed n (10; ordinate) is plotted against the torque M (20; abscissa). These are characteristic curves of a variable speed electric motor, in particular the, in particular brushless, electric motor of an electric motor according to the invention and exemplary power tool, especially an angle grinder. In particular, it is a shunt motor.

[0053] The speed characteristic 30 is the natural drive characteristic at nominal voltage:

[00001]$n=n_0-m*M,$

wherein n is the speed, n.sub.0 is the unregulated idle speed 11, m is the characteristic gradient, M is the torque. The intersection of the characteristic 30 with the abscissa denotes the locking torque 24 of the drive characteristic.

[0054] The nominal voltage of a DC motor is the voltage at which the motor is to operate under nominal conditions. Nominal conditions refer to the operating parameters specified for the motor, including the nominal speed, nominal power, nominal amperage and nominal voltage.

[0055] The nominal speed is set according to the speed characteristic, and the choice of nominal voltage depends on where the maximum idle speed should be. For example, at increased voltage, the natural drive characteristic 32 is above the drive characteristic 30 of the nominal voltage and has the same gradient and a higher idle speed. The nominal current of the motor is the result of the maximum permissible continuous load on the motor under thermal conditions. The continuous operating range is limited by the two criteria of the permissible continuous torque and the limit speed. For thermal reasons, the motor can only be loaded with the maximum permissible continuous current, but higher currents (torques) are permitted for short periods of time as long as the winding temperature remains below the critical value. The maximum permissible continuous current also depends on the specified setpoint speed and the associated cooling capacity of the fan wheel, which is usually connected to the motor shaft in power tools. Phases of increased currents are temporary, wherein the thermal time constant of the winding or other thermally determining power tool components, e.g., power semiconductors, is a measure of how long such short-term overloads may last. The order of magnitude of the times with overload depends on the motor current and starting temperature of the thermally determining power tool component and ranges from a few seconds to a few minutes.

[0056] The winding is heated by power heat losses, and it is important that the resulting heat is effectively dissipated to avoid overheating. This is especially important for fully enclosed motors. According to the invention, overheating and overloading are avoided by limiting the power absorption of the electric motor as a function of the (increased) temperature. The user receives feedback for their work with the power tool through a reduced speed due to the power absorption limitation.

[0057] The maximum permissible continuous current is preferably chosen such that it does not exceed the maximum winding temperature under standard conditions (25? C. ambient temperature, no heat dissipation via the flange, free air circulation). This nominal current is strongly dependent on the type of winding, wherein thin-wire windings may have smaller nominal currents than thick-wire windings. Graphite brushed motors have an increased friction loss at higher speeds, which is eliminated with EC motors. Iron losses in the form of magnetic reversal and eddy current losses also increase with increasing speed in EC motors and generate additional heating. Simultaneously, the cooling air flow increases with higher speeds and, depending on the thermal design, also the cooling capacity, since the fan wheel in power tools is usually firmly connected to the motor shaft. The nominal torque that is assigned to the nominal current depends on the electromagnetic design of the respective motor. The maximum speed of the DC motor is mainly limited by the commutation system. But also other drive elements such as gears, bearings, imbalances or maximum spin speeds are relevant.

[0058] In the diagram in FIG. 1, some straight lines are plotted with a gradient that differs from the gradient m of the natural drive characteristic, namely they have a steeper slope m.sub.2, i.e., with a higher value of m.sub.2>m:

[0059] The drive characteristic 40 with power limitation at the highest speed setting and low temperature,

[0060] The drive characteristic 41 with power limitation at the highest speed setting and high temperature,

[0061] The drive characteristic 42 with power limitation at the lowest speed setting and low temperature, and

[0062] The drive characteristic 43 with power limitation at the lowest speed setting and high temperature.

[0063] These are characteristic curves that occur in an inventive motor having a power limiting device according to the embodiment.

[0064] The setpoint speed range 14 permitted in operation lies between the lowest setting 13 and the highest setting 12 of the setpoint speed n.sub.soll. At the highest speed setting 12, there is a variable speed characteristic range 33 in which the speed remains constant even as the torques increase. It is the working area of the electric motor in which continuous operation is possible, namely up to torque 22. The further course of the characteristic curve to even higher torques now depends on the temperature measured on the electric motor, which varies depending on the state:

[0065] A cold machine starts with a power limitation that allows part of the natural motor characteristic 31 and only limits the power of the drive at torque 23 by generating the steeper speed drop in the characteristic load 40 that starts there. This behavior applies to temperatures below 71 (FIG. 2). From then on, with increasing temperature, this characteristic load 40 (as a result of a reduced power limitation) then wanders to the characteristic load 41. Therefore, first the drive characteristic 40 and then the drive characteristic 41 are explained:

Drive Characteristic 40

[0066] The drive characteristic 40 is created by the power limitation at the highest permissible power for the speed setting 12. This is effective from the cold state up to a temperature of 71, i.e., even at temperatures below 71. At these temperatures, the power limitation is only reached above torque 23 and can be used, for example, to prevent a shutdown by other protective mechanisms, which are only intended to intervene in the event of a malfunction of the drive. Therefore, the user can use a section of the natural characteristic 31, which allows for the speed to decrease up to torque 23 with the gradient m and above this torque, with the gradient m.sub.2. The characteristic load 40 forms the part of the overload range generated by the power limitation at the highest setting 12 and at low temperature 71 and continues until standstill.

[0067] From 71 onwards, the power limitation is gradually restricted up to 73 until finally the characteristic load 41 acts at 73.

Drive Characteristic 41

[0068] From the high temperature 73 (see FIG. 2) and up to the cut-off temperature 74, the minimum current permitted by the power limitation for the highest speed setting 12 applies, which generates the drive characteristic 41this is the drive characteristic at power limitation at the highest setting 12 and at the highest temperature 73, which already acts at torque 22. In the event of an overload above the characteristic curve 33, which can be used in continuous operation, the user immediately experiences a drop in speed with the gradient m.sub.2 and can adjust their handling of the power tool accordingly. In particular, it allows the user to work more continuously with the power tool. The drive characteristic 41 generated by the power limitation continues until it comes to a standstill.

[0069] At the lowest speed setting 13 there is a variable speed characteristic range 34, in which the low speed remains constant even with increasing torques at least up to torque 21. Again, this is the working area for the continuous operation of the electric motor. The further course of the characteristic curve to even higher torques now again depends on temperature measured at the electric motor or another thermally determining power tool component, which varies according to the state:

Drive Characteristic 42

[0070] If the engine runs at the lowest speed setting 13 and at temperatures lower than or equal to 71 (see FIG. 2), the highest current provided by the power limiting device for the lowest speed setting applies and results in the characteristic load 42 at the appropriate loadthis is the drive characteristic at the power limitation at the lowest setting 12 and low temperature 71.

Drive Characteristic 43

[0071] From the high temperature 73 (see FIG. 2) and up to the cut-off temperature 74, the minimum permissible current by the power limitation for the lowest speed setting 13 applies. As a result, the working characteristic runs above torque 21 on the characteristic load 43, which continues until standstill.

[0072] At a high temperature 72 just below the critical temperature 73, a warning device of the power tool is activated, which warns the user when the critical temperature 73 is reached. If the maximum permissible temperature of 74 (cut-off temperature) is exceeded, the overtemperature circuit will switch off the engine.

[0073] FIG. 2 shows a P.sub.max(n.sub.soll; T) characteristic of the power limitation according to the invention. The maximum available power in the form of a maximum available current I.sub.max is determined here as a function of a pair of values (n.sub.soll; T) of the applied setpoint speed n.sub.soll and the measured motor temperature T. This characteristic is determined for a power tool in a test bench and then stored in a data memory of the control device.

[0074] There is now a specific setpoint speed 60 and a specific temperature 70. The setpoint speed 60 is relative and preferably corresponds to the position of a control element (potentiometer, rotary wheel, lever or other operating device) of the machine. The maximum power absorption for a specific position (power limitation 50 ) is then determined by the unambiguous point P.sub.max(n.sub.soll; T) in FIG. 2.

[0075] The maximum power absorption P.sub.max determines the actual speed n as soon as this power is reached: if the temperature rises, then the actual speed is always reduced at this power, because the dependence n(n.sub.soll; P.sub.max(T)) applies. If the user selects the maximum setpoint speed 62 (n.sub.soll_max at 12 ), this results, at a low temperature of 71, in a maximum achievable power absorption value P.sub.max?I.sub.max, 52 on the very outside in FIG. 2.

[0076] At the highest permissible temperature 73 (FIG. 2), as compared to a lower temperature 72 or 71, an increase in the setpoint speed 60 by the user results in a smaller increase in the current I.sub.max provided by the power limiting device. Accordingly, at the low temperature 71, as compared to a higher temperature 72 or 73, an increase in the setpoint speed 60 by the user causes a greater increase in the current I.sub.max provided by the power limiting device.

[0077] If the user keeps the setpoint speed preselection of the power tool unchanged, e.g., the setpoint speed 61 constant, then, based on the maximum available current 51, the current I.sub.max provided by the power limiting device decreases in the event that the motor temperature rises from 71 to 72.

[0078] The user experiences the temperature-dependent power reduction in the upper load range due to the speed reduction, which produces the characteristic curve 41 at the highest temperature 73 and the highest speed 12, and the characteristic curve 40 at the low temperature 71. This speed reduction warns the user of the overload and allows them to adapt their way of working, in particular to reduce torque to ensure uninterrupted, more continuous work with the tool, in particular to avoid a motor shutdown due to overtemperature.

[0079] FIG. 2 does not show a specific speed n=n.sub.mot, because this is set depending on the load and the permissible current I.sub.max. The more current I.sub.max is allowed, the longer the speed control can keep the speed n.sub.mot high. But the speed control 110 is limited by the power limiting device 120 (in this case current limitation) when the current l.sub.aktuell for the pairing of setpoint speed n.sub.soll and current temperature T=T.sub.aktuell becomes too high. This characteristic from FIG. 2 is stored in a data memory 121 of the control device and, by programming the control device accordingly, sets the available power range for the electric motor and thus for the user.

[0080] In particular, the input device 101 or actuator may include: a push button, a throttle switch, an adjusting wheel/potentiometer or a digital setpoint setting with buttons and display or a remote-controlled setpoint setting via an electrical signal.

[0081] FIG. 3 shows a diagram with a circuit diagram of a motor control unit 100 of an exemplary power tool 1 according to the invention, which shows in particular the wiring of the electronic control device 150.

[0082] The user specifies-via input device 101, e.g., via actuator deflectiona setpoint n.sub.soll for the speed, and n.sub.soll is input to a speed control 110 and a power limiting device 120.

[0083] The speed control 110 acts by inputting a pulse width modulation to the driver 131 of a power setting 130, which provides the current for the electric motor 140. The electric motor uses a current depending on the mechanical load it experiences during operation, i.e., depending on the torque applied to the electric motor.

[0084] The electric motor delivers its current speed n.sub.aktuell back to the speed control 110, the temperature T measured on the electric motor by means of at least one temperature sensor to the power limiting device 120. The latter also processes the values supplied by the power measuring device 160 of the power level 130 of the current l.sub.aktuell currently used by the electric motor, which is proportional to the torque applied to the electric motor and the voltage U.sub.aktuell.

[0085] The power limiting device 120 calculates a power limitation from the currently entered values (n.sub.soll, n.sub.aktuell, T, l.sub.aktuell, U.sub.aktuell), in the form of a maximum power value P.sub.max, in this case a maximum permissible current value I.sub.max. This maximum power value limits the maximum duty cycle that can be output by the speed control 110 to the power level 130 for the control of the motor 140.

[0086] FIG. 4 shows a power tool designed as a hand-held machine tool 1, which has a motor control unit according to the invention. The machine tool is shown as an angle grinding machine. However, there is also the possibility that the portable machine tool 1 may have a different design, such as a circular sawing machine, core drilling machine or grinding machine. The gearbox housing 2a of the portable machine tool 1 is used to accommodate and/or support a gear unit 3. The gear unit 3 is designed as a right-angle gearbox and contains a rotationally driven output shaft 4 to which an insert tool unit 9 can be fixed by means of the quick-release device 7. A protective cover unit can be attached to the gearbox housing 2a or the mounting flange of a bearing plate in a known manner, while an additional handle can also be attached to the gearbox housing 2a in a known manner. The drive unit 5 of the portable machine tool 1 is mounted and/or stored in the fully enclosed motor housing. Preferably, the drive unit 5 drives the output shaft 4 in rotation about the axis of rotation R by means of interaction with the gear unit 3. The axis of rotation R of the output shaft 4 runs, at least essentially, perpendicular to a drive rotation axis 5a of the drive unit 5. The drive unit 5 is designed as an electric motor unit. Also arranged in the housing 2 is a circuit board 6, which contains the components for the electronic control and power supply of the electric motor, in particular the motor control unit 100 and the electronic control device 150.

[0087] The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are to be included within the scope of the following claims.