Single-turn absolute encoders provide unique position data for every angle within one revolution (0–360°).
When an axis can rotate through more than one revolution (a wrist joint in a robot arm, a rotary table axis, a valve actuator) a single-turn absolute encoder cannot distinguish between, for example, position 30° on the first revolution and position 30° on the fifth revolution.
Multi-turn absolute encoders extend the absolute measurement range across a defined number of revolutions while maintaining the fundamental advantage of absolute encoders: no homing cycle required at power-up.
Why Multi-Turn Capability Is Needed
Robot Joint Applications
Robotic wrist joints (particularly the 4th, 5th, and 6th axes of a 6-axis arm) may rotate through ±540° or more during programming.
If a single-turn absolute encoder is used, the controller cannot determine the joint’s absolute position after power-up without a homing cycle.
A multi-turn absolute encoder provides immediate, definitive position knowledge: “this joint is at 127.3° on its second clockwise revolution”, without any motion.
Valve Actuator Position
Industrial valves driven by multi-turn actuators may require 10–30 rotations of the handwheel shaft to fully open or close.
The actuator position sensor must resolve the valve position across all 10–30 turns. A multi-turn encoder with 16-bit single-turn resolution and 12-bit multi-turn count (4,096 turns) provides 28 bits of total absolute position across 4,096 revolutions.
Lead Screw and Ball Screw Axis with Direct Motor Encoder
When a linear axis is driven by a ball screw with a motor encoder (not a separate linear scale), the motor encoder counts through multiple revolutions as the carriage traverses the axis.
For a 25 mm pitch ball screw with a 1 m axis travel: 1,000 mm / 25 mm = 40 revolutions from end to end. A single-turn absolute encoder cannot resolve position across 40 revolutions; a multi-turn encoder with 6-bit multi-turn count (64 turns) is sufficient.
Implementation Method 1: Gear Reduction with Additional Encoder Tracks
The classical multi-turn implementation uses a gear reduction stage connected to the encoder shaft. The reduced gear output drives additional encoder tracks that count revolutions:
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Architecture:
- The main encoder disc provides high-resolution single-turn absolute position (e.g., 18-bit, 262,144 positions per revolution)
- A gear train (reduction ratio N:1) drives a secondary disc
- The secondary disc provides lower-resolution position data across N turns (e.g., 12-bit, 4,096 turns)
- Total absolute range: 18 + 12 = 30 bits across 4,096 turns = 1,073,741,824 unique positions
Advantages:
- Inherent absolute position at power-up, no battery required
- Works over unlimited revolution count (within the gear reduction ratio)
- Widely qualified technology (used in servo drives for decades)
Disadvantages:
- Gear backlash introduces error in the multi-turn count at revolution boundaries
- Requires additional mechanical elements (gears, additional encoder track), larger form factor
- Resolution of the multi-turn count is limited by the gear reduction ratio
Gear backlash management: At the boundary between revolution N and revolution N+1, the gear position must uniquely determine the correct turn count.
If backlash allows the gear to be in an ambiguous position at this boundary, the turn count can be wrong by ±1. High-quality encoder designs use anti-backlash gears or design the multi-turn track so that any gear position in the backlash zone still maps to a unique, unambiguous turn count (using overlapping margin zones).
Implementation Method 2: Battery-Backed Revolution Counter
An alternative architecture uses a non-volatile revolution counter rather than gear reduction:
- The main encoder disc provides single-turn absolute position as in a standard absolute encoder
- A direction-detecting circuit monitors the incremental signal from the encoder
- Each time the encoder crosses the 0/360° boundary, the revolution counter increments or decrements
- The revolution count is stored in non-volatile memory (battery-backed SRAM or EEPROM)
- Total position = (revolution count × 360°) + single-turn position
Advantages:
- No gear train, smaller, simpler form factor.
- No gear backlash error.
- Revolution count stored in non-volatile memory even through power cycles.
- Resolution of multi-turn count is not limited by gear ratio.
Disadvantages:
- Requires battery (or energy harvesting) to maintain the counter during power-off.
- Battery has finite life, typically 3–10 years depending on battery capacity and quiescent current.
- If battery fails or is disconnected, multi-turn count is lost and homing may be required
Battery life calculation example:
If the revolution counter circuit draws 10 µA quiescent current from a 50 mAh lithium cell: Battery life = 50 mAh / 0.010 mA = 5,000 hours = 208 days, too short.
Modern designs using micropower SRAM and energy-harvesting from the encoder signal during rotation achieve < 1 µA quiescent current: Battery life = 50 mAh / 0.001 mA = 50,000 hours = 5.7 years.
Energy-Harvesting Alternative
To eliminate the battery entirely: a small energy storage element (supercapacitor) accumulates charge during normal powered operation. If power is interrupted, the supercapacitor provides sufficient energy to maintain the counter for the expected power-off duration.
For applications that are unlikely to be without power for more than a few hours (most industrial machinery), a properly sized supercapacitor eliminates the battery life constraint entirely.
Capacitive Multi-Turn Encoder Design
Capacitive (Electric Encoder) multi-turn designs use the same holistic capacitive measurement principle as single-turn designs, combined with a revolution counter:
- Single-turn resolution: 14–26 bits
- Multi-turn count range: up to 4,096 turns (12-bit revolution counter)
- Total absolute range: 26 + 12 = 38 bits (theoretically, limited in practice by application range)
The revolution counter in the Electric Encoder uses energy harvesting, no battery required. The capacitive sensing circuit itself generates a small amount of charge during each revolution that is sufficient to maintain the revolution count in memory.
Application Checklist for Multi-Turn Encoder Selection
When selecting a multi-turn encoder:
- Maximum number of turns required: Determines multi-turn bit count.
- Resolution per turn required: Determines single-turn bit count.
- Battery vs. gear vs. energy-harvesting: Determined by power availability and maintenance constraints.
- Absolute position at power-up: Multi-turn encoders provide this; confirm the drive/controller supports multi-turn readout.
- Protocol compatibility: Multi-turn position words require wider serial frames (e.g., 38-bit BiSS-C vs. 26-bit).
To expand your understanding, take a look at our guide on
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