Tricky Circuit

Stepper: commands with a direction

Turn counted steps into motion

A stepper motor advances its shaft in discrete steps. A full step is one complete advance; this example uses 1.8° per step. A motor driver is the power circuit that sequences current through two coils, wire windings that make magnetic fields.

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EXPERIMENT BENCH

Change the circuit settings, predict the readings, then check a challenge. These are learning models, with the assumptions explained in each lesson.

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Why it works

The selected count previews one command from zero, not a running position counter. Turning the lab off clears the preview, not a real shaft. Torque, missed steps, current regulation and mechanical motion are omitted.

n is the step count, θ signed angle and r signed turns. Direction 0 means forward; 1 means reverse. Bipolar operation sends current both ways through each winding, not transistor regions. A and B name the two independent windings; +/− are current-direction commands. Phase 0 is the chosen starting reference for the four-state sequence, not a measured current. A negative angle marks direction.

Difficulty

  1. Build · Build it

    Power on with n = 25 and direction 0, forward. Get θ = 45° and r = 0.125 turns.

    Need a hint?

    n = 25 → θ = +45°; r = +0.125

  2. Explore · Change the command

    Set n = 100 and direction 1, reverse. Get θ = −180° and r = −0.5 turns.

    Need a hint?

    n = 100 → θ = −180°; r = −0.5

  3. Solve it · Solve the challenge

    Set n = 200 and direction 0, forward. Get θ = 360° and r = 1 turn.

    Need a hint?

    n = 200 → θ = +360°; r = +1

Extra reading and circuit recipes

Turn counted steps into motion

A printer can request small moves by counting steps. The command has an amount and a direction, while the driver must turn that request into winding currents.

θ = ±n × 1.8°; r = θ / 360°

Multiply the count by the full-step angle, then apply the direction sign. Divide by 360° to express the result in turns. The driver sequences magnetic fields; the arithmetic alone does not simulate that motion.

A useful pattern

For a 1.8° full-step example, 200 steps make 360°. Other motor step angles exist; 200 is not a universal motor constant.

Beyond the model

A commanded count cannot prove actual position. If a physical motor cannot follow its load, a step can be missed even though the command was sent.

A useful trap to avoid

Changing the count from 25 to 100 selects a new 100-step preview; it does not add 100 to the old 25. Forward and reverse are conventions chosen for this diagram.

Keep n = 50 and switch from forward to reverse. What changes?

  1. only the sign: +90° becomes −90°
  2. the angle magnitude doubles
  3. the count becomes 100
Read more

The count and 1.8° step size stay fixed. The direction changes the sign of the same 90° command.

Build it

Set n = 25, direction 0. Twenty-five full steps command one eighth of a turn.

n = 25; θ = 45°; r = 0.125

Change the command

Set n = 100, direction 1. The preview commands half a turn in reverse.

n = 100; θ = −180°; r = −0.5

Try another path

Compare 25 forward with 25 reverse, then 200 forward with zero. One whole turn returns a shaft to the same orientation, but is a different motion command.

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