Reliability, Verification & Maintainability

Reliability is the probability of performing the required function for a stated time under stated conditions. It is not the absence of complaints during a short demonstration.

Design from failure backward

Create a failure-mode table with function, failure mode, local effect, system effect, cause, existing control, severity, occurrence evidence, detectability, and action. Use rankings to prioritize conversation, not to manufacture a magical risk number. A low-probability severe hazard still demands risk reduction.

Common automation failures include contamination, connector loosening, cable flex fatigue, sensor drift, lost calibration, seal leakage, overheated drives, software state mismatch, lubricant loss, and inaccessible wear parts. Include human recovery actions; a jam that is harmless in automatic operation may be dangerous during intervention.

Verification ladder

1. Inspection proves identity, material, dimensions, workmanship, and configuration.
2. Analysis predicts stress, motion, heat, flow, and timing.
3. Component tests estimate uncertain parameters and margins.
4. Subsystem tests exercise interfaces and faults.
5. System tests prove requirements across operating cases.
6. Validation observes the system doing real work with intended users.

NIST’s robotics program emphasizes performance requirements, metrics, test methods, and validation because adoption risk falls when capability can be measured ¹.

Availability and repair

In a simple repairable system, inherent availability is often approximated as

At 500 h mean time between failures and 5 h mean repair, \(A=99.0\%\). Cutting repair to 1 h raises it to 99.8% without changing failure rate. Real operational availability also includes planned maintenance, logistics, and organizational delay.

Make maintenance a design input

Provide safe isolation, access envelopes, lift points, drain and bleed points, replaceable wear surfaces, connector keying, labels, diagnostics, baseline measurements, and controlled backups. Estimate the tools, skill, time, and exposure required for each planned task.

Course review

Submit a design dossier for a bounded transmission or mechanism: requirements, load cases, calculations, material/process choice, tolerance stack, risk table, verification matrix, and maintenance task analysis. End with the three uncertainties that most threaten function and the experiments that retire them.