Biaxial stepper

Two bipolar stepper motors on one axis, one shaft inside the other, so one body turns two hands like a clock.

Aliases: dual shaft stepper, coaxial stepper, clock stepper.

Pins

Pin	What it does
A1+, A1-	Motor 1, coil A: the outer hand.
B1+, B1-	Motor 1, coil B.
A2+, A2-	Motor 2, coil A: the inner hand.
B2+, B2-	Motor 2, coil B.

Properties

The same as the NEMA 17 for each motor: resistance (1.5 ohms a coil), current (1.7 A rated), steps (200 a turn), max_steps (1000 full steps a second), and the starting angle of the outer hand and inner_angle of the inner.

Driving it

Each motor wants its own pair of H-bridges: two A4988 or DRV8825 boards, or an L298N each in full step. Then it is STEP and DIR per hand.

What the model gets right

Each motor is the NEMA 17 model on its own four wires: the two coil currents point a field, the rotor follows it the short way around at up to max_steps, slips a whole electrical cycle when driven faster, and holds nothing below 5% of its rated current.

What it does not model

What the NEMA 17 leaves out: inductance, back EMF, torque and load, and resonance. The small dual-shaft gauge motors this stands for have gearing of their own; set steps to match.

Common mistakes

Swapping the two motors' wires: the hour hand races round like a minute hand.

See it in action

Two hands runs it as a clock, a minute every second. Open it at /templates.