System integration by interfaces

• Plan a staged integration sequence, compute timing and power margins, and isolate a failure at an electrical, data, timing, mechanical, or behavioral interface.

• engineering-workflow
• voltage-current-resistance
• programming-state
• power-regulation

Integration is where individually correct parts meet assumptions they did not know they had. A camera works on USB, motors work on a bench supply, and a screen works alone—then the combined robot resets when the wheels start. The useful response is not “integration is mysterious,” but an inventory of interfaces and budgets.

Each interface has a contract: connector and voltage, units and frame, update rate and latency, mechanical datum, ownership, startup order, and fault response. Budgets add system-wide demand. If electronics draw 0.7 A, two motors may each draw 1.2 A at the tested peak, and a margin factor is 1.25, the source path should be assessed for at least (0.7 + 2.4) × 1.25 = 3.875 A over the relevant duration. This calculation is not permission to ignore motor stall current, battery protection, connector rating, or transient voltage drop.

Integrate in thin vertical slices. First prove protected power and idle current. Then establish communication with actuators disabled. Exercise one motor with wheels raised, then sensing, then closed-loop low-speed motion, then planning. At each stage save a known-good checkpoint and acceptance evidence. A practical build decision is to add test points and log supply minimum, command age, reset cause, and subsystem heartbeat before packaging hides access.

A common misconception is that passing every subsystem test implies the assembled system will pass. Shared power impedance, CPU load, electromagnetic coupling, cable strain, coordinate assumptions, and scheduler contention appear only in combination. Another failure is connecting everything and debugging from the final symptom. Use a dependency-ordered matrix and change one connection or load at a time. Expected budgets are predictions; actual current, latency, and temperature require measurements with stated conditions. Never bypass fuses, guards, or current limits merely to make an integration test proceed.

Problem

A 5 V rail supplies a processor at 450 mA, camera at 320 mA, screen at 180 mA, and sensor peak at 70 mA. Add 30% margin and assess a 1.5 A regulator from current alone.

Steps

1. Peak listed load = 0.450 + 0.320 + 0.180 + 0.070 = 1.020 A.
2. With 30% margin, design current = 1.020 × 1.30 = 1.326 A.
3. A 1.5 A rating exceeds 1.326 A by 0.174 A, but thermal derating, input voltage, transient response, wiring drop, and startup peaks remain unverified.

Result

The regulator passes this current-only paper check with 174 mA headroom; it is not yet validated for the integrated build.

Sanity check

The margin adds 306 mA to 1.020 A, yielding 1.326 A, which is below but close to 1.5 A.

A robot resets only when camera streaming and both motors run together. All three pass alone. What is the strongest next test?

A. Randomly replace software modules until resets stop B. Capture supply voltage minimum, current, reset cause, and command timing while adding loads one at a time from a known-good state C. Bypass current protection to prevent shutdown D. Assume the camera library is wrong because streaming is visible

Your answer: ______________________________

Reasoning:

Goal

Integrate sensor, display/logging, and output mock-up through explicit acceptance gates.

Equipment

• Protected low-voltage supply
• microcontroller
• sensor or potentiometer
• LED output or safely driven motor
• multimeter
• serial console

Procedure

1. Write interface contracts and acceptance checks for power, sample message, display/log update, output command, and watchdog.
2. Measure idle supply voltage/current, then add sensor sampling and logging before enabling the output stage.
3. Enable a current-limited LED or raised-wheel motor test and record supply minimum, loop cadence, and command age.
4. Introduce a deliberate fault by changing the sensor unit label while preserving its numeric value; use the contract check to reject it, then restore the unit.
5. Stop upstream commands and verify the local output watchdog independently of the screen/logger.

What to observe

• Expected: every stage meets its own acceptance check before the next load or interface is enabled.
• Record actual voltage, current, cadence, and watchdog latency at each stage rather than substituting calculated budgets.
• Expected fault signature: a unit-contract failure is detected before a plausible number reaches actuation.

Transfer challenge

Create an integration order for a small printer analogue with a carriage, limit switch, and status screen, including the checkpoint before powered motion.

Cautions

• Power off before rewiring, use current limiting and protected low voltage, raise or mechanically restrain moving parts, and retain all protection during faults.

Learner: ____________________ Date: ____________________

Evidence artifact or file: ____________________________________________

Notes: