Optoelectronic Position & Displacement Sensors

Optoelectronic position sensing deliberately modulates light with motion. An interrupter measures beam blockage; an encoder maps lines or code tracks to position; a reflective sensor measures returned light; a position-sensitive detector (PSD) divides photocurrent according to spot location.

Incremental encoder logic #

Quadrature channels A and B are spatially offset by one quarter cycle. Their phase order gives direction; edge count gives relative motion. An index pulse provides one reference per revolution. Count state is lost across power interruption unless the mechanism returns to a known reference.

With $N$ line periods/revolution and four-edge decoding, angular count size is $360^\circ/(4N)$. This is digital resolution, not absolute accuracy: disk eccentricity, line placement, bearing runout, interpolation, and missed edges add position error.

Analogue optical geometry #

For a one-dimensional PSD with end currents $I_1$ and $I_2$, a normalized estimate is proportional to

$$x\propto\frac{I_2-I_1}{I_1+I_2}.$$

The sum rejects first-order source intensity change and provides a useful “light present” diagnostic. Near low total current, division magnifies noise and the position result must be invalidated.

Receiver tradeoffs #

A transimpedance amplifier converts photocurrent to voltage. Higher feedback resistance raises gain but interacts with detector/input capacitance to reduce stable bandwidth and increase noise gain. Reverse bias reduces junction capacitance and speeds a photodiode but introduces dark current; zero-bias operation favors linear low-dark-current measurement.

Environmental failures #

Dust, oil, condensation, scratches, source aging, ambient light, misalignment, vibration, and cable crosstalk can all mimic motion or reduce margin. Differential channels and modulation help but do not remove obscuration.

Commission by viewing raw channel amplitudes and phase through an entire revolution, at minimum and maximum speed, with realistic contamination and supply variation. A clean digital count at rest does not prove adequate edge margin in motion.