Electric Encoder Technology: Capacitive Absolute Position Sensing Principles

Conventional absolute position sensors for rotary applications use either optical gratings or magnetic poles. Both approaches have well-documented limitations: optical sensors are sensitive to contamination, vibration, and require precise alignment; magnetic sensors are susceptible to external magnetic fields and have reduced accuracy-to-size ratios in EMI-heavy environments. 

The Electric Encoder uses a capacitive sensing principle that eliminates both failure modes. It measures angular displacement through the interaction between the measured displacement and space/time modulated electric fields, a fundamentally different operating mechanism that provides optical-class accuracy in environmental conditions that disable optical systems.

Operating Principle: Space/Time Modulated Electric Fields

The Electric Encoder consists of two primary mechanical elements:

• Rotor: A disk-shaped element with conductor patterns that modulate the electric field.
• Stator: A flat, ring-shaped PCB assembly with excitation and sensing electrodes.

As the rotor rotates relative to the stator, the capacitive coupling between the excitation electrodes and the sensing electrodes varies as a function of angle. 

This angular variation in capacitive coupling is the position signal.

The holistic measurement principle: Unlike point sensors that measure coupling at a single location, the Electric Encoder combines capacitive readings from the entire area of the encoder disc simultaneously. 

The integrated reading is then processed to extract the absolute angular position. This holistic approach provides two critical advantages:

1. Tolerance to localized contamination: A particle, oil droplet, or localized condensation event affects only a fraction of the total sensing area. Because the reading is an integration across all areas, the fractional effect of a local anomaly on the computed position is small.

2. Relaxed installation tolerances: Because the measurement integrates over the full disc area, small eccentricities, tilt, and axial displacement errors produce smaller fractional errors on the output than they would in a point sensor.

Comparison with Optical and Magnetic Technologies

Magnetic sensor limitations in EMI environments: Magnetic encoders measure the position of permanent magnet poles. 

When significant magnetic or electromagnetic interference is present (e.g., from servo drive switching, power wiring, or adjacent motors), the ambient field adds to the magnet field, shifting the measured position.

Secure Your Components Stock Now with Torquety

Reliable automation components for high-performance applications.

Check Availability Chat on WhatsApp

The accuracy-to-size ratio of magnetic encoders decreases as the encoder diameter decreases because smaller magnets generate weaker fields, reducing the signal-to-noise ratio in EMI-heavy environments.

Optical sensor limitations: Optical encoders require a clear optical path between the LED or laser and the photodetector. 

Dust, humidity condensation, oil mist, or particles in the optical gap reduce signal amplitude. In environments with high particulate levels, contamination-tolerant designs can extend service life, but ultimately a physical blockage of the optical path causes failure.

Electric Encoder advantages:

• Insensitivity to magnetic fields: No magnetic elements are involved. The encoder is unaffected by any external magnetic field, regardless of magnitude or orientation.
• No magnetic signature: The encoder generates no external magnetic field, relevant for applications near magnetometers or magnetically sensitive equipment.
• Insensitivity to EMI/RFI: Operating at the capacitive coupling frequency range, the sensor is inherently immune to conducted and radiated electromagnetic interference.
• Low power consumption: The excitation electronics consume significantly less power than optical LED/laser systems.

Absolute Position Output and Intelligent Processing

The Electric Encoder provides single-turn absolute position output immediately on power-up. No homing cycle is required. No reference movement is needed. Position is not lost after power interruption.

Integrated processing capability:

Each encoder includes an integrated CPU and memory that enable:

• Zero-position assignment: Defining a software reference position without mechanical adjustment.
• Built-in test (BIT) functionality: Internal diagnostics that validate sensor function without external test equipment.
• Data storage: Parameter memory for applications with no external data storage.
• High programmability: Enables development of custom application features.

Position output protocols:

The Electric Encoder supports a wide variety of standard feedback protocols:

• BiSS-C
• SSI
• SPI
• Analog sin/cos
• Others per model

This protocol flexibility allows the Electric Encoder to interface with most existing servo drive and motion controller hardware without specialized interface cards.

Form Factor Advantages

The ring-shaped, hollow-shaft form factor of the Electric Encoder is a specific mechanical advantage over most optical designs:

• Hollow shaft: A wide bore passes through the center of the encoder, allowing cables, pneumatic lines, shafts, and optical fibers to pass through without occupying space outside the encoder footprint
• Low profile: Encoder thickness ≤10 mm — integrates into actuator designs where stack height is constrained
• Extra-wide ID/OD ratio: The inner diameter is wide relative to the outer diameter, allowing a larger bore for a given encoder outer diameter than typical ring encoder designs
• Frame-less construction: No encoder housing — the rotor and stator mount directly to the host structure, reducing weight and material cost
• Size range: Outer diameters from 13 mm to 247 mm; inner bore from 1 mm to 171 mm

Key Accuracy Classes Available

Accuracy depends on the model series and diameter:

Resolution across product lines: 14 bit to 26 bit.

The capacitive angle encoder series achieves the highest accuracy class (±0.001°) (comparable to the best interferential optical encoders) while maintaining the contamination and EMI immunity inherent to the electric sensing principle.

Recommended articles to continue reading: