Electricity: motive power systems – Braking
Reexamination Certificate
1999-06-04
2001-05-22
Martin, David (Department: 2837)
Electricity: motive power systems
Braking
C318S375000, C318S376000, C318S757000
Reexamination Certificate
active
06236177
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to motors for electric power tools. More particularly, the invention relates to a braking and control circuit for such a motor.
AC universal motors have commonly been used in electric power tools, such as drills, circular saws and other types of equipment. Generally, such motors provide a high-power, light-weight power source for these power tools. Typically, the universal motor includes a housing, a stator assembly having a run coil, and a rotatable shaft or arbor having an armature mounted thereon. Current flows through the run coil and creates a magnetic field which interacts with the magnetic field of the armature. This interaction rotatably drives the arbor. To drive a tool element, such as a drill bit or a saw blade, the tool element may be mounted directly on the arbor or be coupled to the arbor by a gear transmission or the like.
Conventional universal motors tend to coast, i.e., the arbor continues to rotate for some time after the motor is disconnected from the electrical power source. This coasting generally results from the rotational momentum of the arbor, the transmission, and the tool attachments. To prevent or limit coasting, the motor often includes a braking arrangement.
A typical braking arrangement includes a dynamic braking circuit which relies on passive generation of free wheeling current in the stator to produce a counter-electromagnetic force (counter-EMF) to stop the rotation of the arbor and to, thereby, brake the motor. One such dynamic braking circuit is shown and described in U.S. Pat. No. 5,294,874.
SUMMARY OF THE INVENTION
One problem with existing braking arrangements, such as the above-described dynamic braking circuit is that, if the motor is disconnected from the power source and reconnected in a closed loop at a point or phase in the power cycle of the alternating current at which there is little or no voltage, the braking circuit will not generate the necessary counter-EMF to brake the arbor.
Another problem with existing braking arrangements is that the conditions in which braking required are different for different power tools. Therefore, for different power tools, the braking arrangement must be configured to accommodate the different braking conditions. For example, in some power tools, such as portable drill presses, braking is required when the drill press accidentally disconnects from the workpiece (“breakaway”) during drilling operations, a safety-related braking condition. In other power tools, such as circular saws, braking is required when the tool element, such as the saw blade, binds on the workpiece and the power tool is jerked or kicks back, another safety-related braking condition. Further, in some power tools, braking may be desired each time the operator releases the trigger so that the blade stops quickly and the operator can move to the next drilling or cutting operation, a productivity-related braking condition.
Yet another problem with existing braking arrangements is that, if the motor is braked too quickly, the arcing occurs between the rotor and the commutator brushes, thereby reducing the life of the motor and the brushes. This arcing can be especially problematic if the motor is braked frequently, i.e., productivity-related braking. However, if the motor is not braked quickly enough, the braking can be ineffective, i.e., in a safety-related braking condition.
The present invention provides a power tool including a braking and control circuit that alleviates the problems with existing braking arrangements. The present invention provides a braking and control circuit including a microcontroller-based control circuit. The microcontroller assures control of switch means, such as triacs, switches and relays, and ensures that braking is effectuated regardless of the phase in the power cycle of the alternating current. Also, the microcontroller is programmable so that the braking and control circuit accommodates different braking conditions for different power tools and accommodates combinations of braking conditions for the same power tool. Further, the microcontroller is programmable to configure the braking and control circuit so that the braking force applied to the motor and the stopping time of the motor are regulated and adjustable. This may be accomplished by outputting a control signal so that the switch means skips cycles in the alternating current or by otherwise adjusting the operation of the switch means.
The present invention provides a braking and control circuit for an electric motor, the motor including a housing, a stator supported by the housing, and a shaft rotatably supported by the housing, wherein the stator is selectively connected with a power source to rotatably drive the shaft. The braking and control circuit comprises first switch means for selectively disconnecting the motor from the power source, second switch means electrically connected across the motor; and control means electrically connected with at least one of the first switch and the second switch means and operable to output a control signal to control the at least one of the first switch means and the second switch means to brake the motor.
The control means is preferably electrically connected with the first switch means and with the second switch means. Preferably, the control means outputs a first control signal to the first switch means so that the first switch means disconnects the motor from the power source. At approximately the same time or shortly thereafter, the control means also preferably outputs a second control signal to the second switch means so that the second switch means connects the motor in a closed loop and generates a counter-electromagnetic force to brake the motor. Also, the control means preferably selectively outputs the first control signal to the first switch means so that the first switch means selectively disconnects and reconnects the motor and the power source and selectively outputs the second control signal to the second switch means so that the second switch means selectively connects and disconnects the motor in a closed loop to regulate a braking force applied to brake the motor.
The control means preferably includes a microcontroller operable to output the control signal and programmable to optimize braking of the motor. Preferably, the microcontroller is programmable to change the stopping time of the motor and to change the braking force applied to the motor. Also, the microcontroller is preferably programmable to output the control signal on selected ones of the plurality of cycles of the alternating current to control the first switch means and the second switch means on the selected ones of the plurality of cycles to brake the motor and to output the control signal at a point in the alternating current so that a desired voltage is supplied to brake the motor.
The present invention also provides a power tool comprising a housing, an electric motor, and braking and control means for controlling and braking the motor. The braking and control means includes switch means electrically connected with the motor and control means electrically connected with the switch means and operable to output a control signal to control the switch means to brake the motor.
Preferably, the control means includes a microcontroller operable to output the control signal. The switch means are preferably operable to selectively disconnect the motor from the power source, and the braking and control means preferably further includes second switch means electrically connected with the motor and operable to selectively connect the motor in a closed loop. The microcontroller preferably outputs a first control signal to the first switch means so that the first switch means disconnects the motor from the power source and outputs a second control signal to the second switch means so that the second switch means connects the motor in a closed loop and generates a counter-electromagnetic force to brake the motor.
Preferably, the microcontroller is programmable to configur
Willhide Joseph W.
Zick Jonathan A.
Leykin Rita
Martin David
Michael & Best & Friedrich LLP
Milwaukee Electric Tool Corporation
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