Encoder Wheel

encoder_wheel.py

Overview

encoder_wheel.py is a synthetic wheel-odometry source that emits pseudo-random tick counts on each wheel revolution at a fixed rate. It enables movement stacks—dead-reckoning, PID controllers and loggers—to run in simulation or CI without real encoder hardware.

Interfaces (strongly-typed, stateless)

Direction	Topic	Semantics
Provided	/{robot}/encoder_wheel	std_msgs/Int32 – tick count per revolution (0–999)

Contract

Pre-conditions

• Node launched with valid robot_id CLI argument.

• Downstream expects wheel ticks at ~10 Hz.

Post-conditions

• Publishes exactly one Int32 message each loop iteration.

• data field is an integer uniformly drawn from [0, 999].

• No other side-effects or retained state.

Invariants

• Loop frequency = _RATE ± ROS scheduler jitter.

• Tick range fixed by _MAX_TICK.

Quality-of-Service KPIs

Metric	Target	Rationale
Message rate	10 Hz ± 0.5 Hz	Keeps odometry loops stable.
Latency	< 100 ms	Avoids stale encoder data.
CPU load	< 1 %	Safe on embedded CPU.

Implementation notes

• Uses random.randint(0, _MAX_TICK) for uniform tick generation.

• All logic resides in wheel_encoder(); the main loop only handles timing.

• Seed the RNG (random.seed(…)) before wheel_encoder() to reproduce traces.

encoder_wheel.wheel_encoder() → None

Initialise ROS publisher and broadcast random tick counts until shutdown.

Workflow

1. Read robot_id from CLI and initialise node <robot>_encoder_wheel>.

2. Advertise /robot/encoder_wheel (queue_size=10).

3. Loop at _RATE Hz: a. Generate ticks = randint(0, _MAX_TICK).

   2. Publish Int32(ticks).

   3. Sleep to maintain rate.