Spring Drive

Spring Drive is Seiko's name for a movement whose only power source is a wound mainspring, but whose rate is set electronically rather than by an escapement. Seiko Epson's patent application "Electronically controlled mechanical timepiece" (EP 0 905 587 A1, priority 26 September 1997) describes it: a generator is driven by rotation received from the train wheel, and "the rotation cycle of the generator is controlled in response to a control signal". The circuit contains "a quartz oscillator" producing "a reference frequency signal", against which the detected rotation is compared before the generator is braked electromagnetically.

Seiko's own product pages call the arrangement the tri-synchro regulator, which "controls the mainspring's mechanical energy", "converts a small part of this energy into electricity" and "generates a magnetic force to regulate the speed of the glide wheel". There is no escapement and no balance wheel: the glide wheel turns continuously against an electromagnetic brake, which is why the seconds hand sweeps rather than steps. The mainspring supplies all the energy; the electronics only decide how fast it may be spent.

The accuracy figures are Seiko's own claims. The Seiko Museum dates the first model to 1999 ("the world's first watch driven by the mainspring power and controlled by quartz at the same time") and describes "high precision comparable to that of a quartz watch (e.g., a monthly difference rate of less than ±15 seconds)". Seiko's product-knowledge page instead states "one-second-a-day accuracy", with a 72-hour power reserve. Both are Seiko statements about Seiko products; they are not reconciled on either page, and no independent body publishes a rating for Spring Drive.