Power tool

Abstract

The present application provides an electric power tool, in which the overloading state of a motor is determined without detecting the current flowing through the motor, and the power supply to the motor is limited. The electric power tool comprises a detection unit that detects the actual rotation speed of an output shaft that transmits driving force from the motor to the tool; a switching element provided in the current path from an AC power source to the motor; a control unit that controls the effective voltage applied to the motor by causing the switching element to perform switching operation in such a manner that the actual rotation speed detected by the detection unit reaches a target rotation speed; and a torque limiting unit that limits the effective voltage controlled by the control unit to reduce or stop the rotation of the motor when the difference between the actual rotation speed detected by the detection unit and the target rotation speed is equal to or greater than a threshold value.

Description

Technical Field

[0001] This invention relates to power tools configured to receive power from an AC power source to drive a motor. Background Technology

[0002] The grinding mechanism described in Patent Document 1 is as follows: the rotation of the motor is transmitted to the main shaft by means of the rotating shaft of the motor and two bevel gears provided on the main shaft. Furthermore, the bevel gears on the main shaft side are fixed to the main shaft in a state between the bearings clamped on the main shaft and the fixed member that restricts the axial movement of the main shaft.

[0003] Existing technical documents

[0004] Patent documents

[0005] Patent Document 1: Japanese Patent Application Publication No. 2019-51582 Summary of the Invention

[0006] In the aforementioned grinding machine, when a large load is applied to tools such as disc-shaped grinding stones mounted on the spindle, the bevel gear slides around the spindle, which can suppress the application of excessive torque from the spindle to the motor's rotation shaft. That is, in Patent Document 1, the bevel gear fixed to the spindle functions as a mechanical torque limiter. However, in a grinding machine, if a mechanical torque limiter is constructed, the head portion of the grinding machine becomes heavy due to the fixing mechanism that fixes the bevel gear by allowing it to slide.

[0007] On the other hand, as torque limiters, there are also known torque limiters based on current detection. These limiters determine that the motor is overloaded when the current flowing through it exceeds a specified value, and then limit the power supply to the motor. However, torque limiters based on current detection have the following problem: a current detection unit needs to be installed in the motor's drive system, which increases the cost of the power tool.

[0008] One objective of the present invention is to enable the determination of an overload condition of a motor in an electric tool configured to receive power from an AC power source to drive the motor, without detecting the current flowing through the motor, and to limit the power supply to the motor.

[0009] An electric tool according to one aspect of the present invention includes: a motor, an output shaft, a detection unit, a power supply path, a switching element, a control unit, and a torque limiting unit.

[0010] The motor is configured to generate driving force, and the output shaft is configured to receive this driving force and rotate to drive the tool. Additionally, the detection unit is configured to detect the actual rotational speed of the output shaft.

[0011] The power supply path is configured to transmit AC power from an AC power source to the motor, and a switching element is disposed on this power supply path. Furthermore, the control unit is configured to switch the element in such a way that the actual rotational speed detected by the detection unit reaches a preset target rotational speed, thereby controlling the effective voltage applied to the motor. In other words, the control unit provides feedback control of the voltage applied to the motor so that the output shaft rotational speed reaches the target rotational speed.

[0012] In addition, the torque limiting unit is configured such that when the difference between the actual rotational speed detected by the detection unit and the target rotational speed is above a threshold, the effective voltage controlled by the control unit is limited, thereby reducing or stopping the rotation of the motor.

[0013] Therefore, the power tool according to the present invention can protect the motor from overload conditions without the need for a mechanical torque limiter.

[0014] Furthermore, according to the power tool of the present invention, the overload condition of the motor can be determined without the need for a current detection unit to detect the current flowing through the motor, and the voltage applied toward the motor can be limited. Therefore, an electrical torque limiter can be implemented at low cost.

[0015] In other words, although the overload condition of the motor is determined by a detection unit in this invention, the detection unit is used to control the actual speed of the output shaft to the target speed in the control unit. Therefore, it is not necessary to separately configure it to function as a torque limiter. Thus, the power tool according to this invention can realize the function of a torque limiter at low cost.

[0016] However, the motor can be a brushed motor. Furthermore, the control unit can be configured to control the effective voltage by controlling the conduction angle of the AC voltage applied to the motor; the torque limiting unit can be configured to reduce or stop the motor's rotation by limiting the conduction angle.

[0017] Alternatively, the control unit can be configured to set a command speed such that the speed approaches the target speed over time after control begins, and control the effective voltage based on the command speed.

[0018] In addition, the torque limiting unit can be configured such that after the difference between the commanded speed and the target speed becomes below a preset start determination value, it begins to determine whether the difference between the actual speed and the target speed is above a threshold (in other words, the determination of overload state).

[0019] If the torque limiting unit is configured in this way, it is possible to suppress the overload condition of the motor from the start of motor drive by the control unit until the motor speed increases, thereby limiting the effective voltage applied to the motor.

[0020] Next, the power tool of the present invention may include a target setting unit configured to accept a target rotational speed setting. Furthermore, the torque limiting unit may be configured to set a start determination value based on the target rotational speed set by the target setting unit.

[0021] In addition, the torque limiting unit can be configured to limit the effective voltage controlled by the control unit even if the actual speed does not reach the preset minimum speed after a predetermined time has elapsed after the control unit starts controlling the motor.

[0022] If the torque limiting unit is configured in this way, when a large load is applied to the output shaft at the start of motor drive and the output shaft speed does not increase, the overload condition of the motor can be quickly detected, and the rotation of the motor can be reduced or stopped.

[0023] In addition, the torque limiting unit can be configured to set a threshold based on the target speed set by the target setting unit.

[0024] Furthermore, the torque limiting unit can be configured such that when the target speed set by the target setting unit is lower than the preset limit implementation speed, the effective voltage controlled by the control unit is not limited.

[0025] In other words, when the target speed is set low, even if the output shaft rotation decreases due to the load applied to the tool from the outside, there is no large current flowing through the motor because the difference between the actual speed and the target speed is small. Therefore, in this case, the torque limiting unit stops functioning.

[0026] Next, the torque limiting unit can be configured such that when limiting the effective voltage controlled by the control unit, if the actual rotational speed exceeds a preset return speed, the effective voltage limitation is released. Accordingly, when the motor returns from an overload state, the motor control by the control unit can quickly return to normal control.

[0027] Furthermore, the power tool of the present invention may include a reporting unit configured to report a torque limiting unit that limits the effective voltage. Accordingly, the user can monitor the motor for overload based on reports from the reporting unit and take appropriate measures such as stopping the motor drive.

[0028] Additionally, the reporting unit can be configured to stop reporting when the limiter releases the effective voltage limit or when the power switch of the power tool is turned off.

[0029] Next, the power tool of the present invention can be equipped with a microcomputer. Furthermore, the microcomputer can be configured to include a control unit and a torque limiting unit.

[0030] In this case, the control unit and torque limiting unit can be implemented as one of the programs executed by the microcomputer, which simplifies the device configuration compared to the case where the control unit and torque limiting unit are constructed from dedicated circuits. Attached Figure Description

[0031] Figure 1 This is a perspective view showing the configuration of the grinding machine according to the embodiment.

[0032] Figure 2 This is a cross-sectional view showing the configuration status of the controller inside the grinding mill.

[0033] Figure 3 It is a block diagram representing the circuit configuration of the controller.

[0034] Figure 4 It means Figure 3 The flowchart shown illustrates the control processing performed in the microcomputer.

[0035] Figure 5 It means Figure 4 The flowchart shown details the torque limit determination process.

[0036] Figure 6 It means Figure 4 , 5 The diagram illustrates specific examples of the various parameters used in the processing.

[0037] Symbol Explanation

[0038] 2…grinding machine; 12…motor; 12A…speed detection unit; 14…rotating shaft; 18…spindle; 26…AC power supply; 28…power switch; 33…power path; 34…bidirectional thyristor; 50…dial; 56…display unit; 60…motor output control unit; 70…torque limiting unit. Detailed Implementation

[0039] Hereinafter, exemplary embodiments of the present invention will be described with reference to the accompanying drawings. Figure 1 Please provide an explanation.

[0040] like Figure 1 As shown, the grinding machine 2 of this embodiment is mainly composed of a motor housing 4, a gearbox 6, a rear housing 8, and a wheel cover 10.

[0041] The motor housing 4 is a cylindrical shell, inside which the motor 12 is housed. For example... Figure 2 As shown, the motor 12 is arranged inside the motor housing 4 such that the rotation shaft 14 of the motor 12 is parallel to and substantially aligned with the central axis of the motor housing 4. Furthermore, the rotation shaft 14 of the motor 12 protrudes toward the gearbox 6.

[0042] The rear housing 8 is located at one end of the central shaft of the motor housing 4 (specifically, the side opposite to the gearbox 6). The rear housing 8 is configured such that the user can hold it with one hand at the rear end of the grinder 2. Furthermore, a power cord 16 for introducing AC voltage from an AC power source such as a commercial power supply is led out from the rear end of the rear housing 8.

[0043] The gearbox 6 is located on the side of the motor housing 4 opposite to the rear housing 8, and the main shaft 18 protrudes in a direction orthogonal to the rotation axis 14 of the motor 12. The main shaft 18 is provided with an inner flange 20 for positioning and fixing specified tools such as disc-shaped grinding stones.

[0044] Additionally, a locking nut 22 is screwed onto the portion of the spindle 18 further from the inner flange 20. The locking nut 22 is used to clamp the tool between itself and the inner flange 20.

[0045] The main shaft 18 is rotatably fixed inside the gearbox 6 by means of bearings. In addition, the gearbox 6 houses a gear mechanism, which includes a bevel gear fixed to the main shaft 18 and a bevel gear fixed to the rotating shaft of the motor 12.

[0046] Therefore, the rotation of the motor 12 is transmitted to the spindle 18 via the gear mechanism, and the spindle 18 rotates accordingly to the rotation of the motor 12. Thus, if the tool is fixed to the spindle 18 in advance by means of the locking nut 22, the motor 12 can be used to rotate the tool to perform operations such as grinding, polishing, and cutting.

[0047] The wheel cover 10 is used to protect the user from the scattering of fragments of the workpiece or tool generated during the operation. Therefore, the wheel cover 10 is formed in a generally semi-circular shape to cover a portion (approximately half) of the tool fixed to the spindle 18 from the gearbox 6 side.

[0048] In addition, a handle 24 is provided on the gearbox 6 for easy attachment and removal. The handle 24 is held by the user when the grinder 2 is in use, allowing the tool to be positioned by hand. Therefore, the handle 24 protrudes from the gearbox 6 in a direction orthogonal to the rotation axis 14 of the motor 12 and the central axis of the spindle 18.

[0049] Additionally, a controller for driving and controlling the motor 12 is housed within the gearbox 6. For example... Figure 2 As shown, the controller includes a control controller 30 for controlling the rotation of the motor 12, and a drive controller 32 for driving the motor 12 according to instructions from the control controller 30. The two controllers 30 and 32 are configured to clamp the motor 12 within the gearbox 6.

[0050] like Figure 3As shown, the drive controller 32 includes a bidirectional thyristor (so-called bidirectional silicon controlled rectifier) ​​34 and a motor drive circuit 36. It should be noted that the bidirectional thyristor 34 is an example of the switching element of the present invention.

[0051] A bidirectional thyristor 34 is disposed on the power supply path 33, which is configured to transmit AC power input from AC power source 26 via power supply line 16 to motor 12. Motor 12 is a brushed AC motor.

[0052] Therefore, for motor 12, when the bidirectional thyristor 34 is turned on, an AC voltage supplied from AC power supply 26 is applied, thereby generating a driving force to rotate the rotating shaft 14 and even the main shaft 18. It should be noted that motor 12 is provided with a speed detection unit 12A for detecting the rotational speed of the rotating shaft 14. This speed detection unit 12A corresponds to an example of the detection unit of the present invention.

[0053] The motor drive circuit 36 ​​is configured to turn the bidirectional thyristor 34 on or off according to instructions from the controller 30, thereby controlling the conduction angle of the AC voltage applied to the motor 12. Therefore, the effective value (i.e., effective voltage) of the AC voltage applied to the motor 12 changes accordingly with the conduction angle controlled by the motor drive circuit 36.

[0054] A large current flows through the bidirectional thyristor 34 to drive the motor 12, and a current also flows through the motor drive circuit 36 ​​to drive the bidirectional thyristor 34. Therefore, the drive controller 32 is prone to overheating. Thus, as... Figure 2 As shown, the drive controller 32 is housed in a metal housing with heat dissipation fins 38 on the outer wall.

[0055] Next, a power switch 28 is provided on the power path 33 to switch the on / off state by user operation. Therefore, AC voltage can be applied to make the motor 12 rotate only when the power switch 28 is in the on state.

[0056] It should be noted that the power switch 28 is located on the motor housing 4. In addition, the motor housing 4 is provided with a dial 50 that switches the target speed of the motor 12 in stages by the user's operation and an LED display 56.

[0057] The dial 50 is an example of the target setting unit of the present invention, such as... Figure 6 As illustrated, the configuration is such that, corresponding to the dial positions “1” to “5”, the target speed of motor 12 can be set to 5 stages.

[0058] Furthermore, the display unit 56 is an example of the reporting unit of the present invention, configured to detect the overload state of the motor 12 by means of the operation of the torque limiting unit 70 (described later), and to report the event by illuminating an LED when the rotation of the motor 12 is suppressed or stopped. It should be noted that the reporting unit of the present invention can be configured to report the overload state using an alarm sound or voice.

[0059] Next, the controller 30 includes: a microcomputer 40, a power supply circuit 42, a zero-crossing detection circuit 44, a switch input detection circuit 46, a dial position detection circuit 48, a rotation signal input circuit 52, and a display circuit 54. It should be noted that in the following description, the microcomputer 40 will be referred to as MCU 40.

[0060] The power supply circuit 42 is configured to convert the AC voltage input from the AC power supply 26 when the power switch 28 is in the ON state into a DC voltage, thereby generating a power supply voltage (DC constant voltage) Vcc for the MCU40 and its peripheral circuits to operate.

[0061] The zero-crossing detection circuit 44 is configured to detect the zero-crossing point of the AC voltage input from the AC power supply 26, and the detection signal of the zero-crossing point is input to the MCU 40.

[0062] In addition, the switch input detection circuit 46 is configured to detect that the power switch 28 is turned on when an AC voltage is input to the power path 33, and input a detection signal indicating this to the MCU 40.

[0063] In addition, the dial position detection circuit 48 is configured to detect the dial position of the dial 50 and input it to the MCU 40.

[0064] In addition, the rotation signal input circuit 52 is configured to shape the rotation signal waveform output from the speed detection unit 12A when the motor 12 rotates by a predetermined angle into a pulse signal and input it to the MCU 40.

[0065] In addition, the display circuit 54 is configured to illuminate the LEDs of the display unit 56 according to the report instructions output from the MCU 40.

[0066] Next, the MCU40 includes a CPU, ROM, RAM, and a bus connecting these components. Furthermore, the MCU40 functions as a motor output control unit 60, a switch determination unit 62, a command speed determination unit 64, a motor speed calculation unit 66, a display control unit 68, and a torque limiting unit 70 by executing a program through the CPU.

[0067] Here, the switch determination unit 62 detects whether the power switch 28 is in the on state based on the detection signal from the switch input detection circuit 46, and outputs the detection result to the motor output control unit 60 and the torque limiting unit 70.

[0068] In addition, the command speed determination unit 64 sets the target speed and command speed of the motor 12 based on the dial position input from the dial position detection circuit 48, and outputs the above speeds to the motor output control unit 60 and the torque limiting unit 70.

[0069] It should be noted that the commanded speed is a speed that is updated so that the motor 12 approaches the target speed over time after the start of driving. The motor output control unit 60 controls the conduction angle of the AC voltage applied to the motor based on this commanded speed.

[0070] In addition, the motor speed calculation unit 66 calculates the actual speed of the motor 12 based on the input interval of the pulse signal input from the rotation signal input circuit 52, and outputs the calculated actual speed to the motor output control unit 60 and the torque limiting unit 70.

[0071] Next, the motor output control unit 60 controls the conduction angle of the AC voltage applied to the motor 12 so that the actual speed of the motor 12 reaches the commanded speed when the power switch 28 is in the on state.

[0072] That is, the motor output control unit 60 sets the conduction angle so that the actual speed of the motor 12 reaches the commanded speed that is updated successively. And, according to the conduction angle, the timing for turning on the bidirectional thyristor 34 is set, and a control signal is output to the motor drive circuit 36 ​​at the timing.

[0073] As a result, the motor drive circuit 36 ​​performs so-called phase control by turning on the bidirectional thyristor 34 when the control signal is input, and uses this phase control to control the execution voltage applied to the motor 12.

[0074] That is, the bidirectional thyristor 34 is turned on by the application of a gate voltage and turned off at the next zero-crossing point of the AC voltage. Therefore, the motor drive circuit 36 ​​performs so-called phase control, that is, by applying a gate voltage to the bidirectional thyristor 31 at the input of a control signal, the bidirectional thyristor 34 is switched.

[0075] It should be noted that the motor output control unit 60 is an example of the control unit of the present invention. In addition, as described above, the bidirectional thyristor 34 is an example of the switching element of the present invention. However, the switching element is not limited to a bidirectional thyristor, and any semiconductor element such as a FET that can control the conduction angle through switching operation can be used.

[0076] Next, the torque limiting unit 70 determines whether the motor 12 is in an overload state based on the target speed and command speed set by the command speed determination unit 64 and the actual speed calculated by the motor speed calculation unit 66. Then, if the motor 12 is in an overload state, the rotation of the motor 12 should be reduced or stopped, and the conduction angle controlled by the motor output control unit 60 should be limited.

[0077] Furthermore, when the torque limiting unit 70 determines that the motor 12 is in an overload state and limits the conduction angle, the display control unit 68 outputs a display command to the display circuit 54, causing the LED of the display unit 56 to light up. As a result, the display unit 56 reports that the motor 12 is in an overload state and limits the rotation of the motor 12.

[0078] It should be noted that when the torque limiting unit 70 releases the conduction angle limitation, or when the switch determination unit 62 determines that the power switch 28 is turned off, the display control unit 68 stops outputting the display command and cancels the report from the display unit 56.

[0079] Next, according to Figure 4 and Figure 5 The flowchart below illustrates the control processing performed in the MCU40 to achieve the functions of the aforementioned parts. It should be noted that this processing is performed by the CPU executing programs stored in non-transient physical recording media such as ROM or non-volatile RAM.

[0080] like Figure 4 As shown, when the control process begins, the MCU 40 first initializes various variables used in subsequent processing in S100. Then, in S110, based on the input signal from the zero-crossing detection circuit 44, it determines the power supply frequency (AC frequency) of the AC power supply 26. It should be noted that this determination process involves determining whether the AC frequency is 50Hz or 60Hz, enabling control of the conduction angle of the AC voltage applied to the motor drive circuit 36.

[0081] Next, in S120, the process of the switch determination unit 62 is performed, that is, based on the input signal from the switch input detection circuit 46, it is determined that the power switch 28 has been turned on, and the process proceeds to S125.

[0082] In S125, the process of the display control unit 68 is executed. That is, when it is determined that the motor 12 is in an overload state and a limit phase angle is set in the process described later, a display command is output to the display circuit 54 to make the LED of the display unit 56 light up.

[0083] It should be noted that in S125, when the motor 12 returns from the overload state or when the power switch 28 is turned off, the output of display commands to the display circuit 54 is stopped, and the LEDs of the display unit 56 are turned off.

[0084] Next, in S130, it is determined whether the AC frequency was determined in S110 and whether the power switch 28 is in the on state in S120.

[0085] If it is determined in S130 that both the AC frequency and the on / off state of the power switch 28 have been determined, proceed to S140; otherwise, proceed to S100 and repeat the processing of S100 to S120.

[0086] In S140, the dial position determination process is performed. In the following S150, the target rotational speed corresponding to the dial position determined in S140 is determined. The dial position determination process is as follows: the operating position (i.e., dial position) of the dial 50 is determined based on the input signal from the dial position detection circuit 48.

[0087] It should be noted that when determining the target speed based on the dial position, the following should be used: Figure 6 An example mapping. This mapping, along with the program for this control processing, is stored in a non-transitional physical recording medium.

[0088] Furthermore, in this mapping, in addition to pre-setting the target speed corresponding to the dial position, the minimum speed and upper limit time for motor drive determination, the torque limit determination start ratio, threshold, limit phase angle, and return speed used in the processing described later are also pre-set. Additionally, the torque limit determination start ratio, threshold, limit phase angle, and return speed are each set according to the target speed.

[0089] After determining the target speed in S150, MCU40 enters S160 to determine whether the commanded speed matches the target speed. Then, if the commanded speed matches the target speed, it enters S200; if the commanded speed does not match the target speed, it enters S170.

[0090] In S170, it is determined whether the target speed is greater than the commanded speed. Then, if the target speed is greater than the commanded speed, proceed to S180, increase the commanded speed by a preset predetermined speed, and proceed to S200. Alternatively, if the target speed is less than the commanded speed, proceed to S190, decrease the commanded speed by a preset predetermined speed, and proceed to S200.

[0091] That is, in S170 to S190, when the target speed and the commanded speed are inconsistent, the commanded speed is updated by increasing or decreasing the commanded speed by a specified amount to bring it closer to the target speed. As a result, the commanded speed is updated sequentially after the start of this control process, gradually approaching the target speed over time.

[0092] It should be noted that the initial value of the commanded speed is the stop speed "0" before the motor 12 starts driving, which is set through the initialization process of S100. Then, through the processes of S150 to S190 described above, the function of the commanded speed determination unit 64 is realized.

[0093] Next, in S200, the processing of the motor speed calculation unit 66 is performed, that is, the speed (actual speed) of the motor 12 is calculated based on the input signal from the rotation signal input circuit 52.

[0094] Then, in the next step S210, an overload determination process is performed, that is, based on the actual speed of motor 12 calculated in S200, and the target speed and command speed of motor 12 set in the processes of S150 to S190, it is determined whether motor 12 is in an overload state. It should be noted that, in the following text, according to... Figure 5 The flowchart below provides a detailed explanation of the overload detection and handling process.

[0095] Next, in S220, it is determined whether the motor 12 is currently in an overload state based on the overload determination process in S210. Then, if the motor 12 is determined to be in an overload state, proceed to S240; if it is not determined to be in an overload state, proceed to S230.

[0096] In S230, motor control processing is performed and then proceeds to S250. This motor control processing updates the conduction angle of the AC voltage applied to the motor 12 in such a way that the actual rotational speed of the motor 12 reaches the commanded rotational speed (or even the target rotational speed).

[0097] On the other hand, in S240, a limiting phase angle is set to reduce the effective voltage of the AC voltage applied to the motor 12, and then proceeds to S250. The limiting phase angle is a limiting value of the conduction angle of the AC voltage applied to the motor 12.

[0098] It should be noted that regarding the limiting phase angle, such as Figure 6As illustrated, the conduction angle is set according to the target speed of motor 12: the higher the target speed, the larger the conduction angle. Furthermore, when the dial position is "5" and the target speed of motor 12 is at its highest, the limiting phase angle is set to 0°. This is because if motor 12 becomes overloaded during high rotation, a large current may flow through motor 12, potentially causing damage; therefore, the power supply to motor 12 is stopped.

[0099] Next, in S250, motor output processing is performed, that is, based on the conduction angle updated in S230, or the limiting phase angle set in S240, and the AC frequency determined in S110, the output timing is set, and a control signal is output to the motor drive circuit 36.

[0100] That is, in S250, phase control is performed to control the effective voltage applied to the motor 12. This phase control is achieved by controlling the conduction angle of the AC voltage applied to the motor 12 using the motor drive circuit 36. It should be noted that the processes in S220 to S250 realize the function of the motor output control unit 60. Then, after the process in S250 is executed, the process proceeds to S110, where the above series of processes are executed again.

[0101] Next, the overload determination process performed in S210 will be explained. This overload determination process is a process that realizes the function of the torque limiting unit 70, and is equivalent to an example of the torque limiting unit of the present invention.

[0102] like Figure 5 As shown, when the overload determination process begins, the MCU40 first measures the motor drive time from the start of the motor 12 drive to the present in S310, and then determines in S320 whether the motor 12 is currently in an overload state determination process.

[0103] In S320, if the overload condition of motor 12 is not determined, the process proceeds to S325 to determine whether the target speed of motor 12 exceeds the preset torque limit implementation speed. Then, if the target speed is below the implementation speed, the overload determination process ends.

[0104] This is because: the target speed is low, and even if the motor 12 is continuously driven, it is impossible for an overcurrent to flow in the power path 33, which would cause the motor 12 or the bidirectional thyristor 34 to malfunction. At this time, there is no need to detect the overload state of the motor 12 to suppress the rotation of the motor 12.

[0105] It should be noted that the implemented speed is the lower limit of the speed of motor 12 when the effective voltage is limited by torque limitation, which is equivalent to an example of the limitation of the implemented speed of the present invention.

[0106] Furthermore, regarding the implementation speed, similar to the minimum speed and upper limit time mentioned above, if it is set to a constant speed (e.g., 12000 rpm), then the torque limit can be disabled when the target speed is lower than the implementation speed (e.g., in the case of dial 1).

[0107] Next, if it is determined in S325 that the target speed is greater than the implemented speed, proceed to S330 to compare the actual speed of motor 12 calculated in S200 with the preset minimum speed.

[0108] In S330, if it is determined that the actual speed of motor 12 is lower than the minimum speed, then proceed to S340 to determine whether the motor drive time measured in S310 has exceeded the preset upper limit time.

[0109] It should be noted that the minimum speed and maximum time are parameters used to determine whether motor 12 becomes overloaded shortly after the start of the drive. Figure 6 As illustrated in the example mapping, a constant speed (2000 rpm) and a constant time (1 sec) are set.

[0110] In S340, if it is determined that the motor drive time has not exceeded the upper limit time, then proceed to S380. Alternatively, in S340, if it is determined that the motor drive time has exceeded the upper limit time, then in S350, it is determined that the motor 12 is in an overload state. In S360, a limiting phase angle is set to restrict the conduction angle of the motor 12, and then proceed to S380.

[0111] That is, if the speed of motor 12 does not reach the minimum speed during the period from the start of motor 12's operation until the upper limit time has elapsed, it is determined that motor 12 is in an overload state, and a limiting phase angle is set. It should be noted that, as explained above, the limiting phase angle is determined according to... Figure 6 The mapping shown in the example is set according to the target speed of motor 12: the higher the target speed, the larger the conduction angle.

[0112] Next, in S380, the starting speed for determining the torque limit is calculated, and the process proceeds to S390. This starting speed is the speed at which the difference between the target speed and the commanded speed falls below a predetermined starting determination value. Then, in S380, the target speed is multiplied by a value determined according to… Figure 6 The determination start ratio obtained from the mapping example is used to calculate the determination start speed (= target speed × determination start ratio).

[0113] Next, in S390, it is determined whether the difference between the commanded speed and the target speed falls below a predetermined starting judgment value by judging whether the commanded speed is greater than the judgment start speed calculated in S380. It should be noted that the starting judgment value is the target speed minus the judgment start speed.

[0114] Then, in S390, if it is determined that the commanded speed is below the starting speed of the determination, in other words, the difference between the commanded speed and the target speed is greater than the starting determination value, the overload state determination process ends.

[0115] This is because when the difference between the commanded speed and the target speed is greater than the initial judgment value, the motor 12 is on its way to accelerating toward the target speed, and the overload state of the motor 12 cannot be accurately determined based on the difference between the actual speed and the target speed.

[0116] On the other hand, in S390, if it is determined that the commanded speed is greater than the starting speed of the determination, in other words, the difference between the commanded speed and the target speed is less than the starting determination value, then proceed to S400 to calculate the speed difference between the actual speed and the target speed.

[0117] That is, in S400, since it is determined in S390 that the command speed has risen to approximately the target speed, the load status of motor 12 is detected by calculating the speed difference between the actual speed and the target speed.

[0118] Next, in S410, it is determined whether the motor 12 is in an overload state by judging whether the speed difference calculated in S400 is above a preset threshold. It should be noted that the threshold is based on... Figure 6 The mapping shown in the example is a value set according to the target rotational speed.

[0119] Then, in S410, if the speed difference is less than the threshold, it is determined that the motor 12 is not in an overload state, and the overload state determination process ends.

[0120] Furthermore, if in S410 it is determined that the speed difference exceeds a threshold, in S420 it is determined that motor 12 is in an overload state. Then, in the following S430, based on... Figure 6 The mapping shown in the example sets a limiting phase angle to restrict the conduction angle of motor 12, and ends the overload state determination process.

[0121] Next, if it is determined in S320 that the motor 12 is currently in an overload state, proceed to S440 to compare the actual speed of the motor 12 with the preset return speed.

[0122] It should be noted that, regarding the return speed, according to Figure 6The mapping shown in the example is set according to the target speed of motor 12. Figure 6 In the example mapping, the return speed is also set when the return speed is in the dial position "5". However, since the phase angle is limited to 0° when the dial position is "5", the actual speed will not be greater than the return speed.

[0123] In S440, if it is determined that the actual speed of motor 12 is greater than the return speed, it is determined that motor 12 has returned from the overload state to the normal load state, and thus enters S450, where the restriction on the conduction angle by the limiting phase angle is released, and the overload state determination process ends.

[0124] In addition, if S440 determines that the actual speed of motor 12 is below the return speed, it is determined that motor 12 has not returned from the overload state, and the overload state determination process ends.

[0125] As explained above, the grinding machine 2 of this embodiment is configured to control the drive of the motor 12 by setting a command speed based on the target speed so that the speed of the motor 12 reaches the target speed.

[0126] Then, after the drive of motor 12 is started, when the commanded speed rises to approximately the target speed, it is determined whether the speed difference between the actual speed of motor 12 and the target speed is above a threshold. If the speed difference is above the threshold, it is determined that motor 12 is in an overload state.

[0127] Therefore, in the grinding machine 2 of this embodiment, the motor 12 can be protected from overload conditions without the need for a mechanical torque limiter or an electrical torque limiter that determines the overload state based on the current flowing through the motor 12.

[0128] Furthermore, unlike mechanical torque limiters, it does not require a gear mechanism that allows the bevel gear connecting the spindle 18 and the rotating shaft 14 of the motor 12 to slide when the torque applied from the workpiece to the spindle 18 increases.

[0129] Therefore, according to this embodiment, the head part (i.e., gearbox part) of the grinding machine 2 can be made lighter than that of a grinding machine equipped with a mechanical torque limiter, thereby improving ease of use.

[0130] Furthermore, the torque limiting unit 70 in this embodiment determines the overload state of the motor 12 based on the actual speed, target speed, and commanded speed of the motor 12. Therefore, it does not need to detect the current flowing through the motor 12 as in an electrical torque limiter.

[0131] Therefore, the grinding machine 2 according to this embodiment has a simpler device configuration and can be implemented at low cost compared to a grinding machine equipped with an electric torque limiter.

[0132] In addition, in this embodiment, based on the target speed set by means of the dial 50, the overload state determination threshold, determination start ratio, limit phase angle, and return speed of the motor 12 are set.

[0133] Therefore, according to this embodiment, even if the target speed changes, the overload state of the motor 12 and the return from the overload state can be accurately determined, thereby providing good protection for the motor 12.

[0134] The above describes the methods for implementing the present invention. However, the power tools of the present invention are not limited to the above-described embodiments and can be implemented in various ways.

[0135] For example, in the above embodiment, the following scheme is described: the rotational speed of the rotating shaft 14 of the motor 12 is detected as the actual rotational speed, and the energization toward the motor 12 is controlled in such a way that the detected actual rotational speed reaches the target rotational speed.

[0136] However, the present invention can be applied to any power tool configured as follows: the rotational speed of the output shaft that rotates under the driving force of the motor 12, such as the rotational speed of the spindle 18, is detected as the actual rotational speed, and the energization toward the motor 12 is controlled in such a way that the detected actual rotational speed reaches the target rotational speed.

[0137] Furthermore, in the above embodiments, the grinding machine 2 is used as an example for explanation. However, the present invention is not limited to the grinding machine 2. Any electric tool configured to receive power from an AC power source to drive the motor 12 can be applied in the same way as the above embodiments.

[0138] In addition, in the above embodiments, the following scheme is described: the controller 30 is provided with an MCU 40, and the CPU in the MCU 40 executes the program stored in the non-transitional physical recording medium, thereby functioning as the control unit and torque limiting unit of the present invention.

[0139] However, the controller 30 may include, for example, multiple computers that function as a control unit or a torque limiting unit.

[0140] In addition, all or part of the functions of the above-mentioned parts, which are achieved by the CPU in the MCU40 executing the program stored in the non-transitional physical recording medium, can be implemented by hardware that combines logic circuits, analog circuits, etc.

[0141] Furthermore, the control controller 30 and the drive controller 32 can be configured as a single controller.

[0142] Furthermore, the multiple functions of one component in the above embodiments can be implemented by multiple components, or the single function of one component can be implemented by multiple components. Alternatively, the multiple functions of multiple components can be implemented by one component, or the single function implemented by multiple components can be implemented by one component. Additionally, a portion of the structure described in the above embodiments can be omitted.

[0143] In addition to being implemented with power tools, the present invention can also be implemented in various other ways, such as a system incorporating power tools, a program for enabling a computer to function as a power tool, a recording medium in a non-transitional manner such as a semiconductor memory containing the program, and a control method for power tools.

Claims

1. A power tool, characterized in that, have: A motor configured to generate driving force; An output shaft configured to receive the driving force, rotate, and drive the tool. The detection unit is configured to detect the actual rotational speed of the output shaft; A power supply path configured to transfer alternating current from an alternating power source to the motor; A switching element disposed on the energized path; The control unit is configured to cause the switching element to switch on and off in such a way that the actual rotational speed detected by the detection unit reaches a preset target rotational speed, thereby controlling the effective voltage applied to the motor. The torque limiting unit is configured to limit the effective voltage controlled by the control unit when the difference between the actual rotational speed detected by the detection unit and the target rotational speed is above a threshold, thereby reducing or stopping the rotation of the motor. as well as The target setting unit is configured to accept the target rotational speed setting. The control unit is configured to set a command speed such that it approaches the target speed over time after control begins, and to control the effective voltage based on this command speed. The torque limiting unit is configured such that, after the difference between the commanded speed and the target speed becomes below a preset start determination value, it begins to determine whether the difference between the actual speed and the target speed is above or below a threshold value, and is configured to: set the determination start speed by multiplying the target speed by a coefficient that is less than or equal to 1 based on the target speed, and set the start determination value by subtracting the determination start speed from the target speed. The coefficient takes a first value when the target rotational speed is within a first range, a second value when the target rotational speed is within a second range greater than the first range, and a third value that is less than the first value and greater than the second value when the target rotational speed is within a third range between the first and second ranges.

2. The power tool according to claim 1, characterized in that, The motor is a brushed motor.

3. The power tool according to claim 2, characterized in that, The control unit is configured to control the effective voltage by controlling the conduction angle of the AC voltage applied to the motor. The torque limiting unit is configured to reduce or stop the rotation of the motor by limiting the conduction angle.

4. The power tool according to claim 1, characterized in that, The torque limiting unit is configured to limit the effective voltage even if the actual rotational speed does not reach the preset minimum rotational speed during a predetermined period after the control unit starts controlling the motor.

5. The power tool according to claim 1, characterized in that, The torque limiting unit is configured to set the threshold based on the target rotational speed set by the target setting unit.

6. The power tool according to claim 5, characterized in that, The torque limiting unit is configured such that when the target rotational speed set by the target setting unit is lower than the preset limit rotational speed, the effective voltage is not limited.

7. The power tool according to claim 1, characterized in that, The torque limiting unit is configured such that, when limiting the effective voltage, if the actual rotational speed exceeds a preset return rotational speed, the limitation on the effective voltage is released.

8. The power tool according to claim 1, characterized in that, The power tool includes a reporting unit configured to report that the torque limiting unit limits the effective voltage.

9. The power tool according to claim 8, characterized in that, The reporting unit is configured to stop reporting when the torque limiting unit releases the effective voltage limitation or when the power switch of the power tool is turned off.

10. The power tool according to any one of claims 1 to 9, characterized in that, The power tool is equipped with a microcomputer. The microcomputer includes the control unit and the torque limiting unit.

Images