Speed control circuit for an electric power tool

• US 4,412,158 A
• Filed: 02/19/1981
• Issued: 10/25/1983
• Est. Priority Date: 02/21/1980
• Status: Expired due to Term

• Current Assignee: Black & Decker Incorporated (Stanley Black & Decker, Inc.)
• Original Assignee: Black & Decker Incorporated (Stanley Black & Decker, Inc.)
• Inventors: Lansdell, Derek A., Jefferson, Keith A.
• Primary Examiner(s): Weldon, Ulysses
• Application Number:
  US06/236,040
• Time in Patent Office: 978 Days
• Field of Search: 318/284, 318/286, 318/287, 318/293, 318/257, 318/268, 318/345 C, 318/345 G, 318/326, 318/327, 318/328, 318/461, 318/463, 318/464, 310/50, 73/488, 73/494, 324/160, 324/161, 324/173, 324/174
• US Class Current: 318/257
• CPC Class Codes: H02P 23/24 Controlling the direction, e.g. clockwise or counterclockwise
• CPC Class Codes: H02P 25/145 whereby the speed is regulated by measuring the motor speed and comparing it with a given physical value, speed feedback
• CPC Class Codes: Y10S 388/903 Protective, e.g. voltage or current limit
• CPC Class Codes: Y10S 388/907 Specific control circuit element or device
• CPC Class Codes: Y10S 388/908 Frequency to voltage converter
• CPC Class Codes: Y10S 388/915 Sawtooth or ramp waveform generator
• CPC Class Codes: Y10S 388/917 Thyristor or scr

Abstract

A method and apparatus for controlling the rotational speed of an electronically controlled power tool in both the forward and reverse direction. When the reversing switch is placed in the reverse direction, the electronic speed control circuit automatically limits the speed of the motor to a predetermined maximum value. The speed control circuit also is adapted to sense a change in the state of the reversing switch and remove power to the motor until it coasts down to a relatively slow speed or to zero before reapplying power in the opposite direction. The speed control circuit monitors the rotational speed of the motor by means of a tachogenerator comprising a ring magnet mounted on the rearward end of the rotor shaft of the motor outboard of the commutator-end bearings and a Hall effect device mounted to the peripheral flange of a carrier which encircles the ring magnet so that the ring magnet and Hall effect device are separated by a small radially extending gap.

Claims

1. An electric power tool, comprising:

an electric motor for driving an output spindle having connected thereto means for receiving a tool implement;

said motor having its commutator brushes slightly shifted from the neutral position to provide for better commutation in the forward direction of rotation;

switch means operable between forward and reverse positions for changing the direction of rotation of said motor from a forward direction to a reverse direction and vice versa by controlling the polarity of the power applied to said motor;

control circuit means for controlling the speed of rotation of said motor in both rotational directions by controlling the magnitude of the power applied to said motor, said control circuit means comprising a potentiometer, a ramp generator, reverse speed limiting means, a zero speed detector, a switch change detector, and a flip-flop;

said potentiometer being adjusted to provide a set speed signal to the ramp generator to determine the power applied to the motor;

said reverse speed limiting means being rendered operative when said switch means is in the reverse position to limit the ramp height attainable by said ramp generator to limit the maximum speed of the motor in said reverse direction to below that attainable in said forward direction to keep problems in the reverse direction due to poor commutation to an acceptable level;

said switch change detector actuating said flip-flop each time said switch means is actuated to disable the ramp generator and cut off supply of power to the motor; and

said zero speed detector actuating said flip-flop to remove its disabling effect and re-establish the supply of power to the motor when the motor speed has dropped to zero.

2. An electric power tool, comprising:

an electric motor having commutator brushes slightly shifted from the neutral position to provide better commutation in the forward direction of rotation;

switch means for changing the direction of rotation of said motor from a forward direction to a reverse direction and vice versa by controlling the polarity of the power applied to said motor;

speed sensing means for sensing the rotational speed of said motor and producing an actual speed signal in accordance therewith;

select means for selecting a desired motor speed and producing a set speed signal in accordance therewith;

speed control circuit means for controlling the rotational speed of said motor in both rotational directions by comparing said actual speed signal with said set speed signal and controlling the magnitude of the power applied to said motor in accordance with the difference therebetween;

reverse speed limit means for limiting the magnitude of the power available to said motor when said switch means is set for the motor to rotate in the reverse direction to limit the speed of rotation in the reverse direction to a maximum which is substantially below the maximum speed of rotation in the forward direction;

said reverse speed limit means causing power to be removed from said motor when said switch means is set for the motor to rotate in the reverse direction while the speed of rotation of said motor in the forward direction is greater than the value of said reverse speed maximum, power being re-applied to the motor when the motor speed drops to the level of the value of said reverse speed maximum; and

said select means comprising a manually adjustable potentiometer, and said reverse speed limiting means comprising a diode connected to a transistor, said transistor having its base-emitter junction short-circuited when said switch means is set for the motor to rotate in said forward direction and placing said diode in parallel with said potentiometer to limit the potentiometer output voltage when said switch means is set for the motor to rotate in said reverse direction.