Incremental encoders (covered last time) count edges from wherever they happened to be at power-up. That's fine for a mouse wheel, but a disaster for a robot arm that needs to know "am I about to swing into the wall?" before it moves. Absolute encoders report a unique code for every shaft position — no homing routine, no lost-count failures after a power glitch.

The two main flavors:

• Optical Gray-code disc: A glass or mylar disc with concentric tracks of opaque/transparent patterns. A 13-bit encoder has 13 tracks and 13 photodetectors, giving 8192 unique positions (0.044° resolution). The bits are arranged in Gray code, not binary — adjacent positions differ by exactly one bit, so a misaligned read can never produce a wildly wrong answer (e.g. 0111 → 1000 in binary changes 4 bits at once; in Gray code, 0100 → 1100 changes only one).
• Magnetic on-axis: A diametrically magnetized disc magnet glued to the shaft end, with a Hall-array IC (AS5048, AMS AS5600, MA730) sensing the field vector. The chip does CORDIC math internally and spits out 12–14 bits over SPI, I²C, or as a PWM duty cycle. Cheap (~$3), rugged, no glass to crack — the modern default for hobby and industrial servos alike.

Single-turn vs multi-turn: A bare absolute encoder only knows position within one revolution. For multi-turn knowledge (e.g. lead-screw position over 50 turns), you either add a mechanical gear-reduction stage with a second encoder, or use a battery-backed turn counter — a tiny ultra-low-power counter that keeps ticking from a coin cell when main power is off. Lose the battery, lose the count.

Resolution math, rule of thumb: Resolution in degrees = 360° / 2ⁿ. So 10-bit = 0.35°, 12-bit = 0.088°, 14-bit = 0.022°. But accuracy is usually 5–20× worse than resolution due to magnet eccentricity, nonlinearity, and noise — an AS5048 datasheet brags 14-bit resolution but only ±0.05° accuracy after calibration. Don't confuse the two when sizing a system.

Real-world example: A 6-axis collaborative robot arm (think Universal Robots) uses absolute magnetic encoders on every joint, both pre- and post-gearbox. On power-up the controller instantly knows all six joint angles to ~0.01°, so the arm can resume a paused task without re-homing. Combine that with the post-gearbox encoder catching backlash, and you get the smooth, predictable motion that makes cobots safe to work alongside.

SPI gotcha: Most magnetic encoder ICs use SPI mode 1 (CPOL=0, CPHA=1), not the more common mode 0. Wire it up assuming mode 0 and you'll get garbage data with no error — check the datasheet timing diagram before debugging your code for three hours.