Encoders: incremental, absolute, magnetic

An encoder turns mechanical motion into a digital signal. Without one, your robot has no idea where its joints actually are. Three types dominate hobby and industrial robotics; each has a different sweet spot, different failure modes, and different quirks. Picking the right one for each joint matters as much as picking the right motor.

Type 1: Incremental optical / quadrature

A disk with alternating dark and light slots, two photo-detectors offset 90°. Each transition produces a pulse; the phase between channels A and B tells you direction.

• Resolution: typically 100–10,000 PPR (pulses per revolution). With 4× quadrature decoding (count both edges of both channels), you get 4× that in counts.
• Output: digital pulse trains; needs a microcontroller pin with edge interrupts or hardware quadrature decoder.
• Strengths: cheap (~$5–30), high-resolution, widely supported.
• Weaknesses: relative only — needs a "home" reference at startup; can miss counts at high speeds; fragile (light path can be obscured by dust).

Common in: hobby DC motors with built-in encoders (Pololu metal-gearmotors), Dynamixel-class servos with add-on encoders, retrofitted lab arms.

Type 2: Absolute encoder

The disk has a coded pattern (binary, gray code, or analog) such that any single position is uniquely identified. No homing needed.

• Resolution: 12–22 bits per revolution (4096 to 4M positions).
• Output: SSI, BiSS-C, EnDat, SPI — serial protocols specific to the encoder.
• Strengths: knows position immediately on power-up; no missed counts; multi-turn variants track total rotations.
• Weaknesses: expensive ($50–500); proprietary protocols add complexity.

Common in: industrial servos, joint encoders on serious arms (Franka, UR), aerospace.

Type 3: Magnetic (Hall-effect or magnetoresistive)

A small magnet on the rotor; sensor chip(s) detect rotation. Most modern hobby BLDCs use these.

• Resolution: 12–14 bits typical (4096–16384 positions per revolution).
• Output: SPI (AS5048, AS5047), I²C, or analog. Often integrated into BLDC motor controllers.
• Strengths: cheap (~$3–20), no optics to fail, dust-immune, contactless (no wear).
• Weaknesses: lower resolution than optical; sensitive to magnetic interference.

The 2026 hobby default for BLDC motors is AS5048A or its successors. Used in ODrive, MJBots Moteus, SimpleFOC.

Other types worth knowing

• Resolvers: absolute, military-grade, sine/cosine analog output. Robust to vibration and temperature; common in aerospace and industrial servos.
• Inductive (Inductosyn): like resolvers but linear; precision machine-tool stages.
• Tachometer: velocity-only output (a small DC generator). Used in legacy systems; rare in 2026.

The decision matrix

Need	Pick
Hobby diff-drive wheels	Incremental optical (built into motor)
BLDC motor commutation	Magnetic (AS5048)
Industrial arm joints	Absolute (BiSS-C)
Aerospace, harsh environments	Resolver

Reading the encoder

Quadrature in firmware

Two GPIO pins, edge interrupts on both. Decoder logic:

volatile int32_t count = 0;
void on_a_change() {
    if (digitalRead(PIN_B) == digitalRead(PIN_A)) count++;
    else count--;
}
void on_b_change() {
    if (digitalRead(PIN_A) != digitalRead(PIN_B)) count++;
    else count--;
}

Software decoding works up to ~10 kHz before missed pulses. For higher rates use a hardware quadrature decoder peripheral (STM32 timers, ESP32-S3 PCNT, RP2040 PIO).

SPI absolute encoder

Read a register over SPI; convert to angle.

SPI.transferBytes(query, buf, 2);
uint16_t raw = (buf[0] << 8) | buf[1];
float angle_rad = (raw & 0x3FFF) * 2.0 * M_PI / 16384.0;

Keep the SPI clock fast (10+ MHz) so the read doesn't latency-limit the control loop. AS5048A datasheet has the magic bytes.

The gotchas

• Backlash: between motor encoder and joint output (through a gearbox), there's mechanical backlash. The encoder reports motor angle; joint angle is offset. Either: put the encoder on the joint output, or model + compensate the backlash.
• Magnet placement: magnetic encoders need the magnet within 0.5–1 mm of the chip and aligned to ~1°. Misalignment → wrong readings.
• Aliasing at high speed: at >100 kHz pulse rate, even hardware decoders can miss. Use higher-rate timers or upgrade to absolute.
• Index pulse for homing: incremental encoders often have a Z (index) pulse once per revolution. Use it for homing.
• Multi-turn vs single-turn absolute: a single-turn absolute knows position within 1 revolution; loses count on power cycle if the joint moves more than 360°. Multi-turn tracks total revolutions in non-volatile memory.

What goes wrong in production

• Encoder cable break: noise on the lines. Filter; check for sudden jumps; trigger a fault.
• Slipped magnet: AS5048 magnet glued to a shaft can slip after thermal cycling. Re-mount with stronger adhesive or a press-fit collar.
• EMI from motor: BLDC's switching noise can corrupt encoder signals. Twisted-pair cables, ferrite beads, shielded cables.
• Software counts overflow: 32-bit counter at 1000 RPM × 10000 PPR overflows in days. Use 64-bit, or reset / handle wraparound.

Multi-rate filtering

Raw encoder reads are positions. To get velocity, you take a finite difference. Common implementations:

• Period method: time between consecutive pulses. Accurate at low speed; noisy at high speed.
• Pulse-count method: count pulses per fixed time window. Accurate at high speed; quantized at low speed.
• M/T method: switch between the two based on speed. Standard in production servo drives.

Exercise

Wire a hobby DC motor with quadrature encoder to an ESP32. Implement quadrature decoding via interrupts; print position and velocity at 100 Hz. Spin the motor by hand; watch the count go up and down. Then implement a position PID loop. The first time the motor goes to a commanded angle and stops there is the moment encoder feedback becomes intuitive.

Next

IMUs, magnetometers, and sensor quality — the inertial sensors that complement encoders for body pose estimation.