Safety circuit for hand tools, and method for safe operation thereof

US4722021A · kind A · utility

Assignee

• Robert Bosch GmbH · DE

Inventors

• Friedrich Hornung · Stuttgart, DE
• Fritz Schadlich · Echterdingen, DE
• Martin Gerschner · Echterdingen, DE
• Klaus Gunther · Voerde, DE

Key dates

• Filing date: Oct 3, 1986
• Grant date: Jan 26, 1988
• Expiry date: Oct 3, 2006

Classification

• Technology area (CPC Y): Emerging Cross-Sectional Technologies
• CPC primary: Y10S388/937
• WIPO field: Electrical machinery, apparatus, energy
• WIPO sector: Electrical engineering

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).