Testing Small Wind Turbine Generators - Renewable and ...

each of which has a precisely-timed beginning and end. Figure 2.19 illustrates timing withthe slice technique. One process, called moveSlice, is responsible for running the motor: itchecks the encoder and switches the motor phases when appropriate. It runs once every 2ms, and has 1 ms to complete. It runs very quickly, so it is always done before its allotted1 ms time interval has expired. The second process, called speedSlice, is responsible forcalculating and displaying the motor speed. It is initiated only once per second. It too isgiven only 1 ms to complete, because if it took longer, it would delay the more essentialmoveSlice process. Of course, speedSlice cannot finish its job in just 1 ms, so it continuesto operate in the vacant 1 ms time periods after each subsequent moveSlice operation.Figure 2.19. Control system timing, showing processes as scheduled and as performed.The full Dynamic C code, covering initialization and steady-state motor control, isprovided in Appendix B, Figures B.1 through B.6. It is heavily commented, describing indetail how the program runs the scheme oulined here.2.5.7 Encoder PropertiesAs calculated in Section 2.5.3, the motor must effect five sets of six phase transitions–or30 total phase transitions–for each revolution of the motor shaft. The resolution of theoptical encoder (1024 pulses per revolution) is about 34 times as great, so it has more thanadequate resolution for the application.The encoder outputs are sent through voltage followers before reaching the DSP, againfor the purpose of ensuring isolation and protection of the DSP. The encoder is poweredwith 5 volts DC, provided by a small DC power supply. This supplemental power supply28