Safety device for an electric power tool

Claim 1

A safety device for an electric power tool, comprising: an electric power tool; an on-off switch in a power circuit line connected to said tool for switching power to said tool on and off; a motor means for driving a tool; electronic control means connected to said motor means for controlling operation of said motor means; a tachometer-generator means for controlling speed of said motor means, said tachometer-generator means emitting a signal with frequency proportional to angular speed of said motor means; an adjustable timer means; a synchronizing stage connected to said adjustable timer means and having an input connected to said tachometer-generator means, said signal from said tachometer-generator means being applied to said synchronizing stage, said synchronizing stage triggering said adjustable timer means, said timer means generating a pulse signal corresponding to a reference value and having a length corresponding to maximum speed; a comparator means connected to said timer means for comparing said pulse signal with said signal from said tachometer-generator means, said comparator means generating a signal when actual speed is less than said reference value; an ignition amplifier connected to an electronic switching means, said signal from said comparator means being applied to said electronic switch means through said ignition amplifier; a relay means connected in parallel with said motor means, said relay means having a switching contact for switching current supplied to said motor means; an auxiliary contact on said relay means, said relay means being able to maintain itself in an actuated state through said auxiliary contact until said on-off switch in said power circuit line is switched to off position, so that said motor means is prevented from operating when the speed drops below a predetermined magnitude, said auxiliary contact bridging said electronic switch.