1. Spring-drive (loaded)/clockwork via mechanical governor (thanks to centrifugal weights) – LOW accuracy

2. Electric drive (DC) motor via mechanical governor (thanks to centrifugal switch contacts) – LOW accuracy

3. Electronic DC motor control circuits via
a. Current feedback – MEDIUM accuracy
b. Tachometer feedback (thanks to AC tachogenerator) – GOOD accuracy

4. Crystal sync via closed loop servo feedback (thanks to optical tachometer/encoder or tachogenerator/encoder) – ULTIMATE accuracy

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