WO2010034162A1

WO2010034162A1 - Control device for controlling a motor in a low voltage dc power tool and control method thereof

Info

Publication number: WO2010034162A1
Application number: PCT/CN2008/072587
Authority: WO
Inventors: Bizhi Dai; Lei PENG
Original assignee: Bosch Power Tools (China) Co., Ltd.
Priority date: 2008-09-28
Filing date: 2008-09-28
Publication date: 2010-04-01

Abstract

A control device for controlling a motor (M) in a low voltage DC power tool and a control method thereof. The control device comprises: a switch unit (V1), coupling to the motor (M), and operative to adjust an input voltage of the motor (M) so as to implement a variable-speed function of the low voltage DC power tool; a driving voltage control unit (E1), coupling to the switch unit (V1), and operative to control the provision of a driving voltage (U2) to the switch unit (V1); and a driving voltage boost up unit (E2), coupling to the driving voltage control unit (E1), and operative to boost up the driving voltage (U2) of the switch unit (V1) when the driving voltage boost up unit (E2) detects that the driving voltage (U2) decreases below a desired level, wherein the driving voltage (U2) is provided from the driving voltage boost up unit (E2) to the switch unit (V1) via the driving voltage control unit (E1).

Description

CONTROL DEVICE FOR CONTROLLING A MOTER IN A LOW VOLTAGE DC POWER TOOL AND CONTROL METHOD THEREOF

FIELD OF THE INVENTION

[0001] The present invention relates to low voltage DC power tool, more particularly to a control device for controlling a motor in a low voltage DC power tool and control method thereof.

BACKGROUND

[0002] Low voltage (such as 7.2V or 3.6V) DC power tool has been widely used in many fields. As illustrated in Figure 1, a drill is a kind of conventional low voltage DC power tool. A low voltage DC power tool conventionally uses a Lithium-ion/polymer battery, a NiCd (Nickel Cadmium) battery, a NiMH (Nickel Metal Hydride) battery or other suitable batteries as a DC power source, which is operative to supply power to a motor, a switch unit and a driving voltage control unit in control circuit of the tool. Usually, MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is employed as the switch unit, which is operative to adjust input voltage of the motor so as to implement a variable- speed function of the tool. [0003] Output voltage of the power source may be expressed as:

R_m, where Vout is the output voltage of the power source, V_B is internal voltage of the power source, I is the current inside a circuit loop of the control circuit and R_1n is inherent internal resistance of the power source. For example, for a 3.6V Lithium-ion battery, its inherent internal resistance R_1n is about 40mohm. For a low torque output 3.6V DC power tool, its operation current can very easily reaches 2OA once it carries a heavy load. Therefore, considering the existence of the inherent internal resistance R_1n of the battery and large-current operation of the 3.6V DC power tool, the output voltage V_out of the battery would decrease even below 3.0V. [0004] Normally, for a 3.6V Lithium-ion battery power tool, it directly uses the output voltage V_out of the battery to drive the switch unit, such as a MOSFET. Therefore, the decrease of the output voltage V_out causes the driving voltage of the MOSFET to decrease even below 3.0V. It is well known that on-resistance of a MOSFET would increase dramatically when its driving voltage decreases. The increase of on-resistance of the MOSFET is harmful for the control circuit.

[0005] Firstly, the increase of on-resistance of the MOSFET results in the increase of voltage drop of the MOSFET, thus making the actual voltage supplied to the motor smaller. Especially for a 3.6V DC power tool, this phenomenon will become more obvious due to low rated voltage of the battery and inherent internal resistance thereof.

[0006] Secondly, the increase of on-resistance of the MOSFET results in that power dissipation (or referred to as heat dissipation) of the MOSFET becomes much more than a normal value. The increase of power dissipation of the MOSFET would easily cause failure of the MOSFET, thus resulting in failure of adjusting input voltage of the motor. [0007] It has been disclosed in the prior art that heat sink might be used in a low voltage DC power tool to solve the problem of the increase of power dissipation of the switch unit, such as a MOSFET. The heat sink usually has a large size, thus making the size of the tool larger. However, with the increasing demand of more potable, smaller and lighter low voltage DC power tool, it is needed to reduce the size of the tool. Therefore the use of heat sink could not meet such demand.

[0008] Figure 2 is a schematic diagram of a control circuit for controlling a motor in a conventional low voltage DC power tool. In Figure 2, M represents a DC motor which may be a DC brush motor, Ul represents a DC power source which may be a Lithium-ion /polymer battery, a NiCd battery, a NiMH battery or other suitable batteries, Vl represents a switch unit which may be a MOSFET or other suitable electronic witches, and El represents a driving voltage control unit.

[0009] El may comprise: a MCU (Micro Control Unit), for performing speed variation, safety protection and other related control functions; and a voltage driving circuit, for providing driving voltage to Vl. When the low voltage DC power tool operates, El determines whether to supply driving voltage to Vl, and thus controls the turn on or turn off of Vl. Accordingly, Vl could adjust the input voltage of M so as to implement a variable- speed function of the low voltage DC power tool.

[0010] As stated above, failure of Vl is very likely resulted by the decrease of its driving voltage due to the impact of a heavy load carried on the tool and inherent internal resistance of Ul.

[0011] Therefore, there is a need for a technique which could efficiently suppress the decrease of driving voltage of a switch unit in control circuit of a low voltage DC power tool, and thus avoid failure of the switch unit. SUMMARY

[0012] An object of the present invention is to provide a control device and method for boosting up driving voltage of a switch unit in control circuit of a low voltage DC power tool, so as to suppress the decrease of driving voltage of the switch unit.

[0013] In one aspect, the present invention provides a control device for controlling a motor in a low voltage DC power tool, comprising: a switch unit, coupling to the motor, and operative to adjust input voltage of the motor so as to implement a variable- speed function of the low voltage DC power tool; a driving voltage control unit, coupling to the switch unit, and operative to control provision of driving voltage to the switch unit; and a driving voltage boost up unit, coupling to the driving voltage control unit, and operative to boost up the driving voltage of the switch unit when the driving voltage boost up unit detects that the driving voltage decreases below a desired level, wherein the driving voltage is provided from the driving voltage boost up unit to the switch unit via the driving voltage control unit. [0014] In another aspect, the present invention provides a driving voltage boost up method, comprising: detecting a driving voltage; determining whether the level of the driving voltage decreases below a desired level; if the driving voltage is determined below the desired level, turning on an electric switch for a certain time period, such that the width of a control signal generated by the electric switch increases by a predetermined value and electric energy is accumulated during a duration time corresponding to the increased width of the control signal, or if the driving voltage is determined not below the desired level, turning on the electric switch for a time period, such that the current width of the control signal is maintained and electric energy is accumulated during a duration time corresponding to the current width of the control signal; turning off the electric switch and supplying the accumulated electric energy; and returning to the step of detection.

BRIEF DESCRIPTION OF THE DRAWING(S)

[0015] Figure 1 is a schematic diagram of a conventional low voltage DC power tool.

[0016] Figure 2 is a schematic diagram of a control circuit for controlling a motor in a conventional low voltage DC power tool.

[0017] Figure 3 is a schematic diagram of a control circuit for controlling a motor in a low voltage DC power tool according to an embodiment of the present invention.

[0018] Figure 4 is a block diagram of a driving voltage boost up control unit according to an embodiment of the present invention. [0019] Figure 5 is a flow chart of the process of an exemplary driving voltage boost up method according to an embodiment of the present invention.

[0020] Figure 6 is an exemplary circuit diagram implementing the driving voltage boost up control unit according to an embodiment of the present invention.

DETAILED DESCRIPTION

[0021] Figure 3 is a schematic diagram of a control circuit in a low voltage DC power tool, which includes a driving voltage boost up unit E2 according to an embodiment of the present invention. Except for E2, other components in Figure 3 are similar with those in Figure 2.

[0022] E2 is operative to boost up driving voltage of Vl, wherein the driving voltage is supplied from E2 to Vl via El. Specifically, E2 could boost up the driving voltage when it detects that the driving voltage decreases below a desired level, and could maintain the driving voltage when it detects that the driving voltage is not below the desired level. E2 may be implemented by a discrete analog circuit or a specific IC (such as a chip), which could be designed or programmed to perform the functions of E2.

[0023] Figure 4 is a block diagram showing the structure of E2. In Figure 4, it shows that E2 includes: a driving voltage output unit 41, a driving voltage detection unit 42, an electric energy accumulation and supply unit 43, an electric switch 44 and a driving voltage boost up control unit 45.

[0024] The driving voltage output unit 41 couples to El, and is operative to provide a driving voltage U2 to El. The driving voltage U2 is used to drive Vl under the control of El. The driving voltage output unit 41 may be an output filter, such as a capacitor. [0025] The driving voltage detection unit 42 couples to the driving voltage output unit 41, and is operative to detect the level of the driving voltage U2. The driving voltage detection unit 42 may be a voltage divider, by which a voltage feedback which reflects the driving voltage U2 outputted by the driving voltage output unit 41 could be obtained. For example, the driving voltage detection unit 42 may include a voltage divider circuit consisted of two resistances in series.

[0026] The electric energy accumulation and supply unit 43 couples to the driving voltage output unit 41 and a power source Ul, and is operative to accumulate electric energy from the power source Ul, which may be a Lithium-ion /polymer battery, a NiCd battery, a NiMH battery or other suitable batteries, and supply electric energy to the driving voltage output unit 41. For example, the electric energy accumulation and supply unit 43 may include an inductor for accumulating electric energy from the power source Ul. Meanwhile, when needed, the accumulated electric energy may be outputted from the inductor to the driving voltage output unit 41. Alternatively, a diode, such as a freewheel diode, may be arranged between the inductor and the driving voltage output unit 41, which turns on only when the voltage of the inductor is larger than that of the driving voltage output unit 41. Alternatively, a voltage filter (not shown) may be added between the electric energy accumulation and supply unit 43 and the power source Ul, which filters clusters in input voltage from the power source Ul.

[0027] The electric switch 44 couples to the electric energy accumulation and supply unit 43, and is operative to generate a control signal for controlling the accumulation and supply of electric energy on the electric energy accumulation and supply unit 43. The control signal may be any signal which could provide two or more signal values. For example, one signal value may be used to instruct the electric energy accumulation and supply unit 43 to accumulate electric energy, and another signal value(s) may be used to instruct the electric energy accumulation and supply unit 43 to supply electric energy. The width of the control signal may be defined as duration time of one signal value instructing the electric energy accumulation and supply unit 43 to accumulate electric energy. Preferably, the control signal is a kind of logic signal, of which a higher value may instruct to accumulate electric energy and a lower value may instruct to supply electric energy, or vice versa. The electric switch 44 may be a MOSFET which could generate a logic signal. For example, the turn on of the MOSFET may generate one value of the logic signal and instruct to accumulate electric energy, and the turn off of the MOSFET may generate another value of the logic signal and instruct to supply electric energy, or vice versa. The occurrence of a new value of the control signal triggers the change of the operation of the electric energy accumulation and supply unit 43, from accumulating to supplying, or from supplying to accumulating. [0028] During a time period that the electric switch 44 is turned on, i.e. when maintaining at one value of the control signal, the electric energy accumulation and supply unit 43 continuously accumulates electric energy. Inversely, during a time period that the electric switch 44 is turned off, i.e. when maintaining at another value of the control signal, the electric energy accumulation and supply unit 43 continuously supplies electric energy. It can be seen that the duration time for the electric energy accumulation and supply unit 43 to accumulate electric energy is determined by the width of the control signal. [0029] The driving voltage boost up control unit 45 couples to the driving voltage detection unit 42 and the electric switch 44, and is operative to perform a boost up function by which the driving voltage U2 could be boosted up.

[0030] Specifically, the driving voltage boost up control unit 45 receives a detected level of the driving voltage U2 from the driving voltage detection unit 42. When the driving voltage boost up control unit 45 determines that the detected level of the driving voltage U2 decreases below a desired level which is preset in the driving voltage boost up control unit 45, it turns on the electric switch 44 for a certain time period such that the width of the control signal generated by the electric switch 44 increases by a predetermined value. Then the driving voltage boost up control unit 45 turns off the electric switch 44 and continues to monitoring the detection result from the driving voltage detection unit 42. The above operations is performed repeatedly until the detection result from the driving voltage detection unit 42 shows that the level of the driving voltage U2 is not below the desired level. When determining the driving voltage U2 is not below the desired level, the driving voltage boost up control unit 45 turns on the electric switch 44 for a time period such that the current width of the control signal is maintained.

[0031] The driving voltage boost up control unit 45 may be implemented by discrete analog circuit totally. Also, the driving voltage boost up control unit 45 may be totally or partly implemented by a specific IC, such as a chip, which could be designed or programmed to perform all or some functions of the driving voltage boost up control unit 45. Moreover, the driving voltage boost up control unit 45 and the electric switch 44 may be integrated into one specific IC, such as a chip, which is designed to perform the functions of both of them. [0032] Figure 5 is a flow chart of the process of an exemplary driving voltage boost up method according to an embodiment of the present invention.

[0033] At step SlO, driving voltage U2 of Vl outputted by the driving voltage output unit 41 is detected by the driving voltage detection unit 42. The detection may be performed periodically at a certain time interval.

[0034] At step S20, it is determined by the driving voltage boost up control unit 45 whether the detected level of the driving voltage U2 decreases below a desired level which is preset in the driving voltage boost up control unit 45.

[0035] If step S20 determines the driving voltage U2 decreases below the desired level, the electric switch 44 is turned on, at step S30, by the driving voltage boost up control unit 45 for a certain time period, such that the width of a control signal generated by the electric switch 44 increases by a predetermined value and electric energy is accumulated on the electric energy accumulation and supply unit 43 during the duration of the increased width of the control signal. Then the process proceeds to step S50.

[0036] If step S20 determines the driving voltage U2 is not below the desired level, the electric switch 44 is turned on, at step S40, by the driving voltage boost up control unit 45 for a time period, such that the current width of the control signal is maintained and electric energy is accumulated on the electric energy accumulation and supply unit 43 during the duration of the current width of the control signal. Then the process proceeds to step S50. [0037] At step S50, the electric switch 44 is turned off by the driving voltage boost up control unit 45, and electric energy is supplied from the electric energy accumulation and supply unit 43 to the driving voltage output unit 41. Then the process returns to step SlO to perform the step of detecting again.

[0038] Figure 6 is an exemplary circuit diagram implementing the driving voltage boost up control unit according to an embodiment of the present invention.

[0039] In Figure 6, a capacitor C2 is used as the driving voltage output unit 41. Two resistances Rl and R2 in series are used as the driving voltage detection unit 42. An inductor Ll and a diode Dl in series are used as the electric energy accumulation and supply unit 43. A MOSFET V2 is used as the electric switch 44. A capacitor Cl is used as a voltage filter. E3 performs functions equivalent to those of the driving voltage boost up control unit 45, which could be implemented by a discrete analog circuit or a specific IC, such as a chip. The circuit in Figure 6 is only an example of the implementation of the voltage boost up unit E2 in Figure 3, wherein specific elements may be replaced by others that could perform corresponding functions.

[0040] The above structure and method are only embodiments of the present invention, which are not intended to limit the invention. Any changes, equivalent replacements and improvements made within the spirit and scope of the present invention should be included in the protecting scope of the present invention.

Claims

What is claimed is:

1. A control device for controlling a motor in a low voltage DC power tool, comprising: a switch unit, coupling to the motor, and operative to adjust input voltage of the motor so as to implement a variable-speed function of the low voltage DC power tool; a driving voltage control unit, coupling to the switch unit, and operative to control provision of driving voltage to the switch unit; and a driving voltage boost up unit, coupling to the driving voltage control unit, and operative to boost up the driving voltage of the switch unit when the driving voltage boost up unit detects that the driving voltage decreases below a desired level, wherein the driving voltage is provided from the driving voltage boost up unit to the switch unit via the driving voltage control unit.

2. The control device according to claim 1, wherein the driving voltage boost up unit comprises: a driving voltage output unit, coupling to the driving voltage control unit, and operative to provide the driving voltage to the driving voltage control unit; a driving voltage detection unit, coupling to the driving voltage output unit, and operative to detect the level of the driving voltage; an electric energy accumulation and supply unit, coupling to the driving voltage output unit and a power source for supplying electric energy for the low voltage DC power tool, and operative to accumulate electric energy from the power source and supply electric energy to the driving voltage output unit; an electric switch, coupling to the electric energy accumulation and supply unit, and operative to generate a control signal for controlling the accumulation and supply of electric energy on the electric energy accumulation and supply unit; and a driving voltage boost up control unit, coupling to the driving voltage detection unit and the electric switch, and operative to boost up the driving voltage via the electric switch when the detected level of the driving voltage decreases below the desired level.

3. The control device according to claim 2, wherein the driving voltage boost up control unit is operative to turn on the electric switch to increase the width of the control signal by a predetermined value when it detects that the driving voltage decreases below the desired level, and then turn off the electric switch, and the electric energy accumulation and supply unit is operative to accumulate electric energy for a duration time corresponding to the increased width of the control signal when the electric switch is turned on, and supply electric energy to the driving voltage output unit when the electric switch is turned off.

4. The control device according to claim 2 or 3, wherein the driving voltage boost up control unit is operative to turn on the electric switch to maintain the current width of the control signal when detecting the driving voltage is not below the desired level, and the electric energy accumulation and supply unit is operative to accumulate electric energy for a duration time corresponding to the current width of the control signal when the electric switch is turned on.

5. The control device according to claim 1, wherein the low voltage DC power tool uses a Lithium-ion/polymer battery, a NiCd (Nickel Cadmium) battery or a NiMH (Nickel Metal Hydride) battery as a power source.

6. The control device according to claim 1, wherein the driving voltage boost up unit is implemented by a discrete analog circuit or a specific IC.

7. The control device according to claim 2, wherein the driving voltage boost up control unit is implemented by a discrete analog circuit or a specific IC.

8. The control device according to claim 2, wherein the driving voltage output unit is a output filter; the driving voltage detection unit is a voltage divider for voltage feedback; the electric energy accumulation and supply unit is an inductor; and the electric switch is a MOSFET.

9. The control device according to claim 8, wherein the output filter is a capacitor; the voltage divider is a voltage divider circuit consisted of two resistances in series; and the electric energy accumulation and supply unit further includes a diode connected between the inductor and the capacitor, which turns on only when the voltage of the inductor is larger than that of the capacitor.

10. A driving voltage boost up method, comprising: detecting a driving voltage; determining whether the level of the driving voltage decreases below a desired level; if the driving voltage is determined below the desired level, turning on an electric switch for a certain time period, such that the width of a control signal generated by the electric switch increases by a predetermined value and electric energy is accumulated during a duration time corresponding to the increased width of the control signal; or if the driving voltage is determined not below the desired level, turning on the electric switch for a time period, such that the current width of the control signal is maintained and electric energy is accumulated during a duration time corresponding to the current width of the control signal; turning off the electric switch and supplying the accumulated electric energy; and returning to the step of detection.