EMBEDDED RUST · no_std → WASM

Timers, PWM & Proportional Drive

Use PWM motor channels and the millisecond clock to replace bang-bang control with a smooth proportional controller that settles in the pad without overshoot.

01Challenge

The starter slams the motors to full, then to zero. Watch it overshoot and wobble and miss the tighter tolerance. Rebuild it so the rover eases in and stops clean.

02Model

A motor pin is digital: on or off. Getting HALF speed takes more than one command. You blink it fast: on 60% of each tiny window. That's PWM: the duty cycle IS the average power, and p.motors.set(0.6, ..) is that duty. Bang-bang uses only 0% or 100%, so it overshoots. Proportional control scales the duty by how far you still are: lots of error → more power, near the goal → gentle. The timer gives the steady tick that makes 'per second' mean something.

▶ Live · scrub & hover — a pin that is only ever ON or OFF delivers any average power — duty × supply

03Guided practice

1. PWM is duty. p.motors.set(d, d) drives at duty d ∈ [-1,1], the average power. 2. Proportional forward. Replace bang-bang with let forward = (0.8 * dist).min(0.6).max(0.0); 3. Keep steering. let turn = 1.2 * bearing; then p.motors.set(forward - turn, forward + turn); 4. Timing. millis() returns the loop clock in ms if you want to rate-limit.

04Feedback

PASS WHEN The rover settles within 0.12 m of the pad: proportional drive, no bang-bang overshoot.

If your run fails, check:

Still overshooting past 0.12 m. You're using a constant forward. Make it proportional: forward = (0.8 * dist).min(0.6).
Rover crawls and times out. Gain too low / clamp too small. Raise toward 0.8, cap at 0.6.
Wobbles at the pad. Reduce forward near zero so it can settle.

05Retrieve & space

PWM: a duty of 0.6 means the pin is on ____ % of each window (60).
From the Python PID lesson: a proportional term scales effort by the ____ (error/distance).
Why .min(0.6).max(0.0) on forward? (keep duty in a safe, non-reversing range).

06Mastery & project

Firmware that drives into a TIGHT tolerance using proportional PWM: the bare-metal version of the PID intuition.

The smooth-drive primitive the capstone reuses; sets up timing for the async tier.

← GPIO, Peripherals & the Control Loop Async Tasks & the Real-Time Loop →

EMBEDDED RUST · no_std → WASM

Timers, PWM & Proportional Drive

PWM duty + a proportional law in real Rust: smooth the rover into the pad.

You read

You change

the firmware you write in the control loop

Fixed

the emulated chip, its peripherals, and the wiring