Patent ID: 9240747
Filing Date: 2016-01-19
CPC Classification: B25F,H02H,H02K,H02P

Claim Text:
1. A method for determining rotational speed of an electric motor in a power tool powered by an alternating current (AC) input signal, comprising controlling voltage applied to the electric motor using a phase-controlled switch in accordance with a given firing angle of the phase-controlled switch corresponding to a conduction phase angle of the AC input signal, such that voltage of the AC input signal is applied through the phase-controlled switch to the electric motor, during each half cycle of the AC input signal, after the given firing angle but not before the given firing angle; measuring current through the electric motor; and determining, by a computer processor, rotational speed of the electric motor as a function of the given firing angle and the measured current, wherein the function is a linear combination of a first higher order polynomial related to the given firing angle and a second higher order polynomial related to the measured current, and wherein the computer processor resides in the power tool.