Load-cell measurement chain
CJ Ackley | Electrical | Project brief

STATUS
Measurement brief; sensor selection pending. Detailed design and analysis pending.

THE PROBLEM
A bench weighing station can show more digits than its measurements justify. Sensor sensitivity, excitation, electrical noise, drift, and calibration all affect the reported mass. This project develops the signal chain and a traceable uncertainty budget.

DESIGN QUESTION
Which errors dominate the useful measurement, and which changes are worth making?

SCOPE
- Specify bridge excitation, input range, gain, conversion, and sample rate.
- Compare filtering choices against settling time and noise requirements.
- Define zero, span, repeatability, and temperature checks.

CONSTRAINTS
Sensor: Specified strain-gauge bridge and rated capacity
Range: Useful mass range and overload margin to be agreed
Response: Settling time and reporting interval to be defined
Evidence: Datasheet limits distinguished from measured behavior

ENGINEERING DECISIONS (PROPOSED)
Filtering and measurement uncertainty
Compare filter settings using the same input noise and step. Propagate calibration, offset, sensitivity, reference, and temperature contributions before deciding whether more conversion resolution would help.

ALTERNATIVE
A longer averaging window improves the display of a steady load, but can conceal a changing load and delay the reported result.

TRADEOFF
The selected filter is a compromise between noise and settling time. Extra nominal bits do not remove drift or calibration uncertainty.

PROPOSED TOOLS AND METHODS
Signal-chain model, Uncertainty worksheet, Synthetic signal tests

VERIFICATION SCENARIO (NOT YET RUN)
A load step occurs while the zero drifts
Apply known synthetic steps with noise and slow offset drift, then compare the candidate filters and zero-correction policies.

PASS CONDITION
Report settling time and steady residual error using declared definitions. Do not let automatic zero correction absorb a real sustained load.

EVIDENCE TO RETAIN
Signal and headroom calculations, uncertainty contributions with correlation assumptions, reproducible test vectors, and a calibration worksheet.

MEASUREMENT AND LIMITS
Separate resolution, repeatability, bias, and combined uncertainty. Synthetic signals verify processing behavior; calibrated physical loads are needed to establish measurement performance.

COMPLETION CRITERIA (PLANNED)
- Every signal-chain stage has range and headroom checks.
- Filtering meets the agreed noise and response requirements on defined test signals.
- The physical calibration plan includes zero, span, repeatability, and temperature checks.

VERIFICATION PLAN
Signal budget
Track minimum and maximum signal through each stage, including headroom.

Uncertainty
Propagate sensitivity, reference, offset, and calibration contributions.

Test signals
Use synthetic steps and noise to check the processing; reserve accuracy claims for calibration.

PLANNED DELIVERABLES
- Signal-chain specification and uncertainty workbook
- Calibration and filtering code with reference test cases
- Wiring concept and a repeatability and drift test plan

SCOPE NOTE
No calibrated accuracy or trade-use certification is claimed. Mechanical installation and real sensor behavior need physical testing.

NEXT STEP
Choose the load cell and define the mass range, resolution, and settling requirement.
