Add quadrature encoder starter course

lessons/01-reading-encoder-signals/01-meet-the-quadrature-encoder/meet-the-quadrature-encoder.md

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+---
+title: Meet the Quadrature Encoder
+summary: Learn what the A and B channels are and why their phase offset lets you count motion.
+chapter: Reading Encoder Signals
+order: 1
+tags:
+  - encoders
+  - motors
+  - feedback
+estimated_minutes: 8
+---
+
+# Meet the Quadrature Encoder
+
+A quadrature encoder gives you two square-wave signals, usually called `A` and `B`.
+
+Those signals toggle as the shaft turns. The important detail is that they do not toggle at the same time. One channel leads the other by a quarter of a cycle, which is where the name quadrature comes from.
+
+That phase offset gives you two useful facts:
+
+1. You can count edges to measure motion.
+2. You can compare which signal leads to determine direction.
+
+## What you should observe
+
+- Slow rotation produces clean transitions you can inspect with a logic analyzer.
+- Reversing the shaft swaps which channel leads.
+- More pulses per revolution means finer position measurement.
+
+## Checkpoint
+
+Capture both channels while rotating the shaft by hand and confirm that one channel leads the other.

lessons/01-reading-encoder-signals/02-counting-direction-and-steps/counting-direction-and-steps.md

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+---
+title: Counting Direction and Steps
+summary: Decode edges from the A and B channels to recover step count and direction.
+chapter: Reading Encoder Signals
+order: 2
+tags:
+  - decoding
+  - interrupts
+  - firmware
+estimated_minutes: 10
+---
+
+# Counting Direction and Steps
+
+Once you can read the A and B channels, the next job is decoding.
+
+The most common approach is:
+
+- trigger on an edge
+- read both channels
+- use a lookup table or state machine to update the count
+
+If your decoder sees illegal state jumps, you are probably dropping edges or reading a noisy signal.
+
+## Practical notes
+
+- Start with low shaft speed.
+- Add debounce or filtering only if the hardware needs it.
+- Test forward and reverse explicitly.
+
+## Checkpoint
+
+Write firmware that increments a counter when the shaft turns forward and decrements when it turns backward.

lessons/02-using-encoder-feedback/01-measuring-wheel-speed/measuring-wheel-speed.md

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+---
+title: Measuring Wheel Speed
+summary: Convert encoder counts over time into rotational speed for a drive wheel.
+chapter: Using Encoder Feedback
+order: 1
+tags:
+  - speed
+  - odometry
+  - robotics
+estimated_minutes: 9
+---
+
+# Measuring Wheel Speed
+
+Position counts become useful control signals once you add time.
+
+To estimate wheel speed:
+
+1. sample the encoder count at a fixed interval
+2. subtract the previous count from the current count
+3. divide by the sample period
+4. convert counts per second into revolutions per second or wheel surface speed
+
+## Why this matters
+
+Speed feedback is the bridge between open-loop motor commands and predictable robot motion.
+
+If two drive motors get the same PWM value but spin at different speeds, your robot will drift. Encoder-based speed estimation lets you detect and correct that mismatch.
+
+## Checkpoint
+
+Plot wheel speed over time while ramping the motor command up and down.

lessons/02-using-encoder-feedback/02-closing-the-loop-with-a-pid/closing-the-loop-with-a-pid.md

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+---
+title: Closing the Loop with a PID
+summary: Use encoder speed feedback to hold a target wheel speed with a simple controller.
+chapter: Using Encoder Feedback
+order: 2
+tags:
+  - control
+  - pid
+  - robotics
+estimated_minutes: 12
+---
+
+# Closing the Loop with a PID
+
+With encoder speed feedback in place, you can command a target speed and adjust motor output based on error.
+
+The basic control loop is:
+
+- measure current speed
+- compute `error = target - measured`
+- update the controller
+- send the new motor command
+
+Start simple. A proportional controller is often enough to prove the loop works before adding integral or derivative terms.
+
+## What good behavior looks like
+
+- the wheel reaches the target speed quickly
+- overshoot stays limited
+- the response remains stable when load changes
+
+## Checkpoint
+
+Tune a controller so the wheel returns to the target speed after you briefly drag on the tire by hand.