Sensors, actuators and signal acquisition
Follow the complete physical signal chain—from the quantity being measured to the action taken—and design each conversion, assumption and failure response deliberately.
After this module, you should be able to:
- Describe a complete measurement and actuation chain
- Select sensors using meaningful performance characteristics
- Reason about conditioning, sampling and calibration
- Design diagnostic and verification coverage for physical interfaces
The system does not receive reality. It receives a representation.
A sensor converts a physical quantity—the measurand—into an electrical or digital representation. Conditioning, conversion, timing and software then transform that representation again. Each stage adds limits, uncertainty, delay and possible failure modes.
Choose the sensor for the behaviour you must distinguish.
| Characteristic | What it means in practice | Design question |
|---|---|---|
| Range | Minimum and maximum input without unacceptable damage or loss of performance. | What normal, transient and fault values are credible? |
| Accuracy | Closeness to the reference value under stated conditions. | What total error is allowed across temperature and life? |
| Precision / repeatability | Agreement between repeated measurements. | Can the system reliably distinguish the required change? |
| Resolution | Smallest observable increment in the output chain. | Is useful resolution limited by noise rather than converter bits? |
| Bandwidth and response | How quickly changing input is represented. | Which dynamics must be captured and which should be rejected? |
| Drift and hysteresis | Change with time and dependence on previous input direction. | What calibration or compensation is needed? |
| Cross-sensitivity | Response to temperature, vibration, supply or another unwanted influence. | Can environment mimic the quantity being measured? |
Sensor selection includes installation. Mechanical stress, cable routing, heat sources, grounding, connector resistance and ingress protection can dominate laboratory data-sheet performance.
Sampling must preserve the information the decision needs.
Condition before conversion
The analogue front end may provide excitation, protection, impedance matching, amplification, level shifting and filtering. Its output must remain within the converter’s input and common-mode limits for normal and credible abnormal conditions.
Control aliasing
The theoretical Nyquist threshold requires a sampling frequency greater than twice the highest frequency to be represented. A practical design also needs an analogue anti-alias filter, realistic transition band and margin for interference and timing uncertainty. Sampling faster does not recover information already distorted or clipped upstream.
Use an honest error budget
ADC bit depth is not system accuracy. Include sensor error, reference tolerance, offset, gain error, noise, non-linearity, temperature effects, quantisation, timing and calibration residuals. Effective number of bits can be far lower than nominal resolution.
Make time part of the data
For multiple channels, define whether samples are simultaneous or sequential, when timestamps are assigned and how acquisition aligns with control outputs. Stale but plausible data can be more dangerous than clearly invalid data.
An actuator converts information into energy.
The command signal is only one part of the actuation chain. The driver, supply, load, mechanical transmission, thermal behaviour and feedback path determine what actually happens.
- Define voltage, current, duty cycle, force, torque, speed, travel and thermal limits.
- Consider saturation, stiction, backlash, wear, stall, blocked motion and loss of supply.
- Specify the de-energised condition and whether it is genuinely safe for every operating state.
- Separate command monitoring from outcome monitoring: reading back an output register does not prove physical action.
- Use interlocks or independent cut-offs where software response is too slow or cannot be trusted for the hazard.
Worked example: closed-loop temperature control
Useful diagnostics include open- and short-circuit thresholds, rate-of-change limits, disagreement with an independent sensor and a check that temperature responds after heater energy is applied. Each diagnostic needs a detection time and a defined system response.
Verify the chain under realistic conditions.
Range, units, transfer functions, filtering, sampling, latency and diagnostic thresholds.
Allocated contributors, assumptions, worst-case combination and acceptance limits.
Reference standards, points, coefficients, storage, validity and recalibration triggers.
Known physical or electrical stimuli at nominal, boundary and abnormal conditions.
Open, short, stuck, drift, noise, stale data, saturation, jam and feedback loss.
Performance across temperature, supply, EMC, vibration, ageing and relevant loads.
Common failure patterns
A component figure is treated as end-to-end accuracy without installation, conditioning or software errors.
A sensor returns a well-formed number that is stale, out of context or physically implausible.
The controller confirms its requested output but does not detect a disconnected or jammed actuator.
Calibration compensates offset while noise, saturation, drift or non-linearity remain uncontrolled.
Further learning
- Texas Instruments — High-Speed Analog-to-Digital Converter BasicsSampling theory, ADC performance, selection, aliasing, clock jitter and evaluation.
- NASA Systems Engineering HandbookSystem design, interfaces, verification and technical management across the lifecycle.
Engineer the physical loop end to end.
A dependable embedded system controls the complete chain from measurand to data, decision, energy and verified physical outcome—including uncertainty, timing and failure behaviour at every conversion.