Control system for synchronous motor employed as servomotor

Claim 1

A control system for a synchronous motor employed as a servomotor comprising: a pulse coder having a first pulse coder element which has at least one train of angular portions of which the cumulative angle corresponds to one pair of the field poles of said synchronous motor and each of which is produced by splitting the cumulative angle of said angular portions to predetermined portions utilizing a code system to enable each of said angular portions to represent the absolute angular positions thereof, said first pulse coder element generating signals representing the absolute angular position of one of said field poles with respect to a reference angular position determined following the windings of the stator of said synchronous motor, and having a second pulse coder element which has a train of smaller angular portions of which the cumulative angle covers each of said angular portions of said train of said first pulse coder element and each of which is produced by splitting the cumulative angle of said smaller angular portions to predetermined portions, said second pulse coder element generating incremental pulses corresponding to each of said smaller angular portions, an arithmetic unit comprising a means to recognize said signals generated by said first pulse coder element, a counter to count said incremental pulses generated by said second pulse coder element, said counter being triggered by a change of said signals generated by said first pulse coder element, a means to detect the absolute angular position of one of said field poles with respect to said reference angular position determined following the windings of the stator of said synchronous motor at a scale corresponding to the interval of said incremental pulses, and a means to generate a first trigonometrical function signal following said absolute angular position of one of said field poles with respect to said reference angular position determined following the windings of the stator of said synchronous motor and at least one other trigonometrical function signal having a predetermined amount of phase difference with respect to said first trigonometrical function signal, and an alternating current voltage generator which is triggered by said first trigonometrical function signal and said at least one other trigonometrical function signal generated in said arithmetic unit and which generates a poly-phase alternating current voltage of which each component varies the frequency from zero to a predetermined amount but which maintains a predetermined amount of phase difference with respect to said absolute angular position of one of said field poles of the stator of said synchronous motor, whereby said synchronous motor is allowed to run maintaining a predetermined amount of internal phase angle regardless of the rotation speed thereof.