Add quadrature encoder starter course

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.