Safety circuit for hand tools, and method for safe operation thereof
First Claim
1. Method of deenergizing an electric power tool connected to a two-wire power supply connection havingan electric motor supplied by network power at network frequency from said two-wire power supply connection and having a metallic bit holder (6) holding a metallic cutter bit therein, said deenergizing method being effective if the bit inadvertently contacts a metallic object (16, 17) regardless of whether said metallic object is positively grounded,comprising the steps ofenergizing said power tool with power at power network frequency;
- furnishing a test circuit supply voltage;
utilizing said test circuit supply voltage for generating an alternating electrical test energy of a frequency substantially elevated above power line frequency, and having a range of between 2 and 20 kHz;
connecting the alternating electrical test energy at a voltage level of substantially below power line voltage to said bit holder by a capacitor (9) having a coupling impedance which is high with respect to power line frequency and an impedance which is low with respect to said elevated frequency of the electric test energy;
coupling the electrical test energy to at least one of the wires of the two-wire power supply connection;
sensing the level of electrical test energy at said elevated frequency applied to said bit holder;
measuring change in said sensed electrical test energy applied to the bit holder, and then towards said metallic object; and
controlling energization of the power tool by disconnecting energization thereof if change in test energy applied to the bit, above a predetermined change level, is sensed.
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Accused Products
Abstract
To detect if a tool bit, such as a drill, a saw blade, or the like, upon being worked into a wall, meets a metallic obstruction, such as a conduit, reinforcement rod, hydraulic pipe or the like, an alternating voltage, preferably between 2 and 20 kHz, is coupled through a coupling capacitor (9) to the tool bit, and current flow to the tool bit is detected by a current measurement stage (10; 31, 32) to disconnect energy to the drive motor (5), for example by disabling firing of a thyristor (15), or closing a valve of a compressed-air tool. The coupling capacitor should have a capacity small enough to provide high impedance to network power frequency, and to insure a substantial change in current being supplied to the tool bit holder, typically a chuck, if the tool bit meets a metallic object in the wall. The a-c supply circuit can be formed as part of a trigger circuit for the thyristor, by pulse-energizing a trigger diode (28) upon repetitive charge of a capacitor (26) through a charging network (22, 23, 24, 25).
152 Citations
15 Claims
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1. Method of deenergizing an electric power tool connected to a two-wire power supply connection having
an electric motor supplied by network power at network frequency from said two-wire power supply connection and having a metallic bit holder (6) holding a metallic cutter bit therein, said deenergizing method being effective if the bit inadvertently contacts a metallic object (16, 17) regardless of whether said metallic object is positively grounded, comprising the steps of energizing said power tool with power at power network frequency; -
furnishing a test circuit supply voltage; utilizing said test circuit supply voltage for generating an alternating electrical test energy of a frequency substantially elevated above power line frequency, and having a range of between 2 and 20 kHz; connecting the alternating electrical test energy at a voltage level of substantially below power line voltage to said bit holder by a capacitor (9) having a coupling impedance which is high with respect to power line frequency and an impedance which is low with respect to said elevated frequency of the electric test energy; coupling the electrical test energy to at least one of the wires of the two-wire power supply connection; sensing the level of electrical test energy at said elevated frequency applied to said bit holder; measuring change in said sensed electrical test energy applied to the bit holder, and then towards said metallic object; and controlling energization of the power tool by disconnecting energization thereof if change in test energy applied to the bit, above a predetermined change level, is sensed. - View Dependent Claims (2, 3)
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4. Safety circuit used in power tools, particularly hand tools, used in deenergizing the power tool upon contact of a bit with a metallic object (16, 17), regardless of whether said metallic object is grounded, said power tool having a two-wire power supply connection and an electrically conductive bit holder (6), comprising
a source of electrical test energy having a frequency elevated substantially above power line frequency and a test voltage substantially below power line voltage, said source having two test energy output terminals; -
a capacitor (9) applying said test energy from one terminal of the electrical test energy source to the bit holder (6), said capacitor (9) having a coupling impedance which is high with respect to power line frequency and an impedance which is low with respect to the elevated frequency of electric test energy; coupling means (34;
22-25,
26) for high frequency coupling the other terminal of the test energy source to at least one wire of the two-wire power supply connection;a test current measuring circuit (10, 30,
31) connected between the electrical test energy source and the capacitor (9) and sensing change of current to said bit and bit holder; anda disconnect circuit connected to and controlled by the test current measuring circuit (10) and disconnecting energization from said two-wire power supply connection to the power tool when the test current measuring circuit (10) detects change in test current above a predetermined level, upon contact of the bit with said metallic object (16,
17). - View Dependent Claims (5, 6, 7, 8, 9, 10, 11, 12)
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13. Safety circuit used in power tools, particularly hand tools, used in deenergizating the power tool upon contact of a bit with an electrically conductive object (16, 17), said power tool having an electrically conductive bit holder (6) and an electric motor (5), comprising
a high-frequency electrical energy source providing electrical energy having a frequency substantially above power line frequency and a voltage substantially below power line voltage; -
coupling means (9) applying the voltage from the electrical energy source to the bit holder (6); a current measuring circuit (10, 30,
31) connected between the electrical energy source (11) and the coupling means (9);a disconnect circuit (14, 15;
24, 25, 26, 28,
29) connected to and controlled by the current measuring circuit (10, 30,
31) and disconnecting energization to the motor (5) when the current measuring circuit detects current above a predetermined level, said disconnect circuit includinga semiconductor switch connected to the motor (5); a trigger circuit connected to the semiconductor switch (15) and thereby controlling the motor (5); the current measuring circuit (10, 30,
31) being connected to and in turn controlling the trigger circuit; anda direct current source providing power to the trigger circuit; said trigger circuit including a trigger diode (28) and a capacitor (26) charged by direct current from the direct current source, the trigger diode (28) periodically discharging the capacitor (26) to form therewith an oscillatory circuit; and connection means coupled to the oscillatory circuit and forming said high frequency energy source.
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14. Safety circuit used in power tools, particularly hand tools, for de-energizing the power tool upon contact of a bit with a metallic object (16, 17) regardless of whether said metallic object is grounded, said power tool having an electrically conductive bit holder (6) and a two-wire power supply connection, comprising
a source of electrical test energy having a frequency substantially above power line frequency and a test voltage substantially below power line voltage; -
a capacitor (9) applying said test energy from the electrical test energy source to the bit holder (6), said capacitor (9) being capable of effectively blocking power line frequency current; a test current measuring circuit connected between the electrical test energy source and a capacitor (9), and sensing change of current to said bit and bit holder, said test current measuring circuit comprising a resistor (32) serially connected between the source of electrical test energy and the capacitor (9); a transistor (31) having its base-emitter path connected in parallel to the resistor (32); and a response circuit (30) including the collector of the transistor (31), the transistor changing conduction state when the voltage across the resistor (32) due to current flow to said bit and bit holders exceeds the base-emitter voltage of the transistor; and a disconnect circuit (14, 15, 24, 25, 26,
29) connected to and controlled by the test current measuring circuit and disconnecting energization from said two wire power supply connection to the power tool, when the test current measuring circuit detects change in test current above a predetermined level, upon contact of the bit with said metallic object (16,
17).
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15. A power tool, particularly motor-driven hand tool, having
an electric motor (5); -
two-wire power supply lines (3,
4) supplying electrical power to said motor at network frequency;an electrically conductive bit holder (6) driven by the motor; a motor controlled triac (15) connected for energization of the electric motor from said power lines; and a triac trigger for rendering said motor controlled triac conductive, in combination with a safety circuit for deenergizing the motor upon contact of a bit in the bit holder with an electrically conductive object (16), said safety circuit comprising a source of electrical energy coupled to at least one of the power line conductors, and having a frequency substantially elevated above power line frequency and a voltage substantially below power line voltage; capacitor means (9) dimensioned to have a high impedance at power line frequency and a low impedance at said elevated frequency, and applying the electrical energy from the electrical energy source to the bit holder (6); a current measuring circuit connected between the electrical energy source and the capacitor means (9); and a disconnect circuit connected to and controlled by the current measuring circuit and disconnecting energization to the motor when the current measuring circuit detects current at said elevated frequency above a predetermined level, said disconnect circuit including an oscillator circuit which serves both as said triac trigger and as the source of said electrical energy having said elevated frequency.
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Specification