Week 14 · Improve: Stopping needs space

Session 4 of 4 · Detect obstacles · Phase 4

Plan about 15 minutes for explanation, 30 minutes for practical work and 10–15 minutes for documentation. A longer build may continue into the next session: stop safely, commit the current state and record the next check. Desktop simulations count as software evidence; label them clearly and record physical validation separately.

Engineering challenge

Can your detector distinguish a hazard while leaving enough time to stop? This session focuses on stopping needs space.

Before you start

The previous week’s recorded baseline and Week 13, Improve. For later sessions this week, retain the preceding session’s files and predictions.

Equipment: Desktop Python, editor, paper and ruler; for physical work, the configured 3pi+ 2040, clear floor mat and hardware checklist. Week 3 additionally uses the separate low-voltage LED circuit described in its procedure.

For any motion, verify the stop button, short time limit and clear floor area first. Keep the wheels raised for a new device program until its commands and stop behaviour are checked. A hazard or uncertain input is a reason to stop and document, not to force the trial to finish.

Theory and mathematics

Stopping needs space

Detection delay includes sampling, filtering, computation and motor response. During reaction time τ, a robot at speed v travels vτ before braking effects are considered. Total stopping distance must be measured, because cutting drive does not mean immediate rest. Design a floor boundary so the hazard is detected before the danger zone, with margin for worst observed stopping and uncertainty. A bump sensor detects contact after contact happens; use soft lightweight test barriers at low speed and do not call it non-contact collision avoidance.

Worked example — illustrative values

With fictional TP=8, FP=2, FN=2, TN=8: precision=8/10=0.8, recall=8/10=0.8, accuracy=16/20=0.8. At 0.10 m/s and a 0.20 s processing/reaction delay, the robot travels 0.020 m before additional stopping distance. These examples are not measured safety margins.

Write the calculation in your notebook before running code. State which values you measured, which you assumed and which the program calculates. A correct numerical calculation cannot rescue an incorrect physical assumption.

Run and explain the model

The following is desktop Python, not a ready-to-run motor program. Download this week’s example, save it in your student repository and run python3 code/w14.py from the repository root. The same small model is reused across the week so you can learn it, build with it, test it and revise it.

# Desktop Python teaching example. Numerical inputs are illustrative.
truth = [True, True, False, False]
readings = [810, 440, 500, 125]  # fictional
predicted = [value > 470 for value in readings]
for actual, estimate in zip(truth, predicted):
    print("TP" if actual and estimate else "FN" if actual else "FP" if estimate else "TN")

Compare the unchanged baseline and your proposed change on the same inputs. Keep the original files so another reader can reproduce the comparison. If an exception appears, read its final line, identify the input or assumption that caused it and make the smallest explained correction. Do not delete validation merely to obtain output.

Understanding the model and its limits

The two Boolean lists represent independent ground truth and detector output. Their paired outcomes give one true positive, one false negative, one false positive and one true negative. In this four-case toy example, hazard recall is 1/(1+1)=0.5 and false-positive fraction among safe cases is 1/(1+1)=0.5. These are illustrative counts, not a sensor performance claim. Choose a threshold on development labels, then evaluate fresh labels. A missed hazard and an unnecessary stop have different consequences; record both rather than choosing a threshold solely by total accuracy.

Practical instructions

1. Choose a detector rule based on missed hazards and nuisance stops rather than accuracy alone.
2. Estimate reaction travel using your slow speed and measured loop interval.
3. If previous hardware checks pass, measure three bounded stops in a generous clear lane; never approach stairs.
4. Document a tested stop margin and restrict the operating envelope if it is inadequate.

Experiment

Test three stops at a verified low speed. Use the largest observed distance plus a justified margin, not the average alone.

Before testing, record your prediction, changed factor, measured response, fixed conditions and stopping rule. Save every attempted run, including failures, with a condition and source version. If hardware is unavailable, use an explicitly labelled synthetic/replay dataset and list the physical question it cannot answer. Do not invent completed trials.

Deliverable

A documented stopping envelope or a clearly failed margin test that prevents unsafe route use.

Save notebook/w14-s4.md, the relevant code revision, raw CSV or test-case records, and one labelled diagram/plot/table. Link the files relatively from your notebook. Use the entry template and report guide.

Completion criteria

• Explain stopping needs space in your own words using this session’s example and its units/assumptions.
• Produce the specific evidence above: A documented stopping envelope or a clearly failed margin test that prevents unsafe route use.
• Keep predictions and raw outcomes, distinguish observations from interpretation, and explain one limitation or unresolved failure.
• Review the Git diff, commit the session’s intended files and state the next experiment or safe continuation point.

A documented failed prediction can meet the learning criteria. A missing physical trial must remain marked untested; software success alone does not validate the robot.

Reading and video

• Focused reading: Robotics Lab: uncertainty and classification primer. Study task: Build a four-cell confusion table and identify the missed-hazard cell.
• Video/lecture option: MathWorks: What is PID control?. Study task: Draw the closed-loop signal path and label the measured quantity. Watch a relevant 5–10 minute excerpt or use the linked notes if video is inaccessible. This is supporting conceptual material; hardware in a demonstration may differ from yours.
• Practical reference: Engineering handbook and hardware setup. Manufacturer/API references and video metadata were checked on 2026-10-09; recheck the actual firmware before transferring code.

Reflection and next step

Which assumption most affected your result? Point to one observation that supports your explanation and one alternative explanation the evidence has not ruled out. Write a specific next test with a changed factor and measurable outcome, then proceed through the week’s Learn → Build → Experiment → Improve cycle.