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Analyzing the internal mechanics of a watch stepper motor

Motors & Generators with MagNet

Presented here is the internal mechanics of a watch employing a stepper motor. The watch is simulated using Infolytica's Transient 3D with Motion solver.

The device consists of a steel frame (12.1 mm across) with a stranded coil wrapped around it. The rotor of the stepper motor is made entirely of a neodymium permanent magnet with a fixed direction of magnetization. Instead of modeling all of the underlying gears attached to the rotor, a moment of inertia is applied to correctly simulate the motion. Friction is also applied to the moving body to simulate the contact between the rotor and its gears.

Analyzing the internal mechanics of a watch stepper motor

METHODS and RESULTS

CURRENT REPORTED for the COIL

A 1-volt pulse is applied across the coil from 0 to 3.9 ms. The graph on the left shows the current reported for the coil by MagNet. Once the voltage pulse is complete, current is still flowing within the coil due to the induced currents from the spinning rotor and self-induced effects.

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POSITION of the ROTOR vs TIME

The voltage applied to the coil for 3.9 ms imparts a spin to the neodymium rotor causing it to rotate 180 degrees. By alternating the sign of the consecutive voltage pulses, the rotor will begin achieving complete revolutions. The graph on the left displays the position vs time of the rotor.

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FORCES APPLIED on ROTOR

Due to the permanent magnet rotor and the applied voltage, the flux density field creates a torque about the axis of rotation. This torque is responsible for the motion and it can be seen in the graph on the right. The net torque is the combination of the magnetic torque along with the friction that dampens the motion.

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REMESH REGION

For each motion time step, the remesh region is recreated and reconnected to the surrounding mesh. With only 1% of the mesh elements in the remesh region, remeshing is a minor step when compared to the actual solving time.

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