We adopt the Motorola MC68B09 MPU (microprocessing unit) as the main microprocessor and the MC68B09E MPU as the submicroprocessor. They operate in parallel at the E (enable) clock frequency, 2 MHz. The schematic diagram of this measuring instrument is shown in Fig. 3.
Fig. 3 Schematic diagram of instrument.
It is first necessary to measure the period T in order to synchronize the operation of this instrument with the rotation of a synchronous generator which may fluctuate with time owing to various causes. In order to achieve this, the d-axis pulse generated by a d-axis pickup is shaped and given to the main MPU. The d-axis pulse also serves as the time origin for Fourier expansion. The main MPU obtains a period T by measuring a time space between two neighbouring d-axis pulses and generates the pulse train whose period is T/36. Three 12-bit ADCs simultaneously start to convert the induced voltages of three search coils into digital values using this pulse train.
The main MPU samples induced voltages of three search coils 36 times in one interval between two d-axis pulses and sends the set of voltages which have been sampled in that interval to the subMPU. The subMPU calculates airgap flux distribution from this set based on eqns. 6, 14 and 10. The main MPU takes in the values of airgap flux distribution in that interval from the subMPU and sends them to 12-bit DACs.By repeating these processes, synchronization with the d-axis pulse can be realized and airgap flux distribution can be obtained at every cycle. Note that samplings in an interval are executed according to the preceding period and the airgap flux distribution is calculated in the following interval.