Signals, Noise, Sampling & Conditioning

A sensor produces a signal plus bias, noise, interference, quantization, drift, and delay. Signal conditioning makes that mixture compatible with the controller and the decision.

Scale and resolution

An ideal 12-bit converter over 0–10 V has \(10/4096=2.44\) mV per count. If a pressure transducer maps 0–10 V to 0–1 MPa, one count represents about 244 Pa. That is resolution, not accuracy. Offset, gain error, nonlinearity, noise, and sensor uncertainty remain.

Sampling and aliasing

Sampling at \(f_s\) cannot uniquely represent arbitrary content above \(f_s/2\). An anti-alias filter limits analog bandwidth before conversion. Choose sample rate from relevant plant and fault dynamics, not a slogan. A temperature loop may need seconds; motor current control may need microseconds.

Noise paths

Interference couples capacitively, inductively, conductively through shared impedance, or radiatively. Manage it with physical separation, twisted pairs, appropriate shielding and termination, differential measurement, isolation, short return loops, filtered power, and intentional grounding. “Ground” is not an abstract zero-voltage bucket; currents create voltage differences.

Filtering with consequence

A first-order low-pass filter reduces high-frequency noise but delays response. If a filter time constant is 100 ms, a real 50 ms impact may be strongly attenuated. Separate control filtering, display smoothing, and safety detection. Preserve raw data when possible for diagnosis.

Calibration chain

Trace physical input to sensor output, conditioning, conversion, scaling, displayed engineering unit, alarm, and control decision. Check zero, span, repeatability, hysteresis, and relevant intermediate points. Record as-found and as-left values.

Lab

With a safe low-voltage sensor, log raw and filtered signals at several sample rates. Inject a known slow change and a brief transient. Plot amplitude and delay. Your report must state which filter is acceptable for control, display, and fault detection—and why one setting should not automatically serve all three.