[craigsmansion]

At the heart of every mechanical watch lies a coiled spring. Within that spring energy is stored that wants to be released.

This energy is released through the wheel train, which is a set of cogs with reductions (some of them attached to the hands).

Left to its own device this system would just spin as quickly as possible until the spring is unwound.

To prevent this, somewhere in the train there is something called a lever escapement. Simple put, this puts a stick in a cog at regular intervals to prevent it from spinning and unwinding freely. This stick is timed by a weight that spins from left to right. This stick literally is what makes watches tick, since that's where the ticking sound comes from.

Spring Drive doesn't poke a stick into a wheel to prevent the system from freely unwinding. What it does do is add magnets to the wheel and stationary coils around it. The wheel can't spin freely since it has to overcome the magnetic induction as the magnets pass the coils. That's the reason SpringDrive doesn't tick and the hands glide instead of stopping and starting.

If it's still unclear, try looking up escapement levers first. There should be a lot of clear animated explanations online.

[twic]

I understand (roughly!) the basic idea of a watch mechanism, and how mechanical escapements work.

What i don't understand is exactly how the spring drive keeps the wheel moving at a constant speed.

I appreciate that this involves magnets, and eddy currents induced by those magnets, but that's a description, not an explanation. What is the mechanism, physically and mathemetically, that keeps the speed constant?

[craigsmansion]

From a physics angle we're mostly dealing with electromagnetic induction. Faraday's law of electromagnetic induction states:

"that a voltage is induced in a circuit whenever relative motion exists between a conductor and a magnetic field and that the magnitude of this voltage is proportional to the rate of change of the flux".

The magnetic field here is attached to the glide wheel (rotor). The conductor is some stationary spools placed near the glide wheel. This means the voltage output depends on how fast the glide wheel spins (rate of change of the flux).

The other part of Springdrive comes from Lenz’s Law of Electromagnetic Induction:

"the direction of an induced electro magnetic field is such that it will always opposes the change that is causing it"

That means that the current creates a electromagnetic field of its own that opposes the changes in the magnetic field that caused it.

So that opposing field depends on the rate of flux according to Faraday. That rate of flux directly correlates to the passage of time (the spinning of the glide wheel).

By adjusting the placement and configuration of the circuit (the spools) we can influence the flux, thereby influence the induced voltage, thereby influence the induced magnetic field that opposes the change that is causing it, which is the spinning of the rotor that correlates to the passage of time, to a degree that hopefully is around 1 second per second (or 1 rotation of the glidewheel per 1/8 second).

Note that if the glide wheel accelerates for some reason, the change of flux changes, the voltage changes, and the opposing magnetic field increases, forcing the speed of the glide wheel down. On the other hand, if the glide wheel slows down, the opposing magnetic field decreases and allows the glide wheel to spin faster.

So technically the "smooth" second hand isn't smooth at all, but slows down or speeds up about 8 times a second, so it's not exactly "at a constant speed," but since it's not a start-stop motion, it sure looks smooth to an observer.

[SamReidHughes]

Inertia. The torque applied by electromagnetic force on the rotor plus that of friction and air resistance equals the torque applied (indirectly through a gear train) by the mainspring. The net torque is zero and the rotor keeps going at the same rate of rotation.

The integrated circuit counts how many times the rotor has spun compared to how many quartz crystal oscillations have happened and adjusts the electromagnetic force somehow. My guess is by increasing/decreasing the power passing through some resistor (creating heat).