robotics//sensor//measurement chain

A measurement chain is the sequence of stages that carries a physical quantity from the sensing element to a number in software, and every stage adds its own error, so it is the map an engineer follows to find out where a reading went wrong. Between the physics and the code there are always the same links:

A measurement chain is the sequence of stages that carries a physical quantity from the sensing element to a number in software, and every stage adds its own error, so it is the map an engineer follows to find out where a reading went wrong. Between the physics and the code there are always the same links:

1Sensortransduces the quantity2Conditioningpower, amplify, filter, isolate3Convertersamples and quantizes4Buscarries the number5Softwaretimestamp, units, calibration, filtering

Signal conditioning powers the sensor, amplifies signals of millivolts, filters before sampling so that nothing above half the sampling rate folds back (anti-aliasing filter), isolates galvanically, and compensates what can be compensated, such as the cold junction of a thermocouple. The analog-to-digital converter samples and quantizes, the serial bus carries the number to the processor, and the software gives it a time, converts it to units, applies the calibration and filters it.

Each link has its typical faults. In the sensor: bias, scale, noise, temperature. In the conditioning: amplifier offset, interference picked up by the wiring (the 50 Hz of the mains is a classic) and ground loops, currents that flow through the signal ground because two earth points sit at different potentials. In the converter: quantization and aliasing. In the bus: lost frames, or a sensor that stops answering while the driver repeats its last value. In the software: the wrong time, swapped units, a calibration applied twice.

The worst errors look like physics. A frozen value reads as a very stable process, and a bias leads the estimator and the controller to the wrong place with total confidence; neither raises an alarm, which is why the chain is checked link by link before the data are blamed on the plant (sensor error model).

In a modern sensor half the chain lives inside the chip: an IMU integrates the sensing element, amplifier, converter and a digital filter, and hands over numbers already filtered by a filter that perhaps nobody configured (smart sensor). Read the whole datasheet, because half of the measurement model is written there.