| 摘要 |
PURPOSE:To make a fault of a speed detector fail-safe by selecting a two phase signal from in a four phase signal, based on an open-phase detecting signal outputted from a one phase open-phase detecting means for detecting a fact that a signal of one phase has become an open-phase, and determining a two phase signal for measuring a revolving speed of a motor. CONSTITUTION:When an open-phase is generated at 'O' point, an open-phase detecting signal (e) is outputted from a one phase open-phase detector 55. The signal (e) is converted to a signal e' synchronizing with a signal -a by a circuit 70, and varied to the same signal as the signal e' by a circuit 71. A signal (a) is not outputted from a circuit 72 after the time point when a level of the signal e' has been varied to high from low. When the level of the signal e' is varied to high from low, a signal which has inverted a level of the signal -a is outputted from a circuit 73 instead of the signal -a outputted from a rotary encoder, and a signal (k) is obtained. As a result, even if a phase of the signal (a) becomes an open-phase at 'O' point, a signal of the same phase as the signal (a) is inputted to a speed controlling circuit 54, therefore, one phase open-phase can be made fail-safe. |
