Many encoder users have long been reluctant to switch because some innovative motor control technologies that claim to offer superior performance and reliability must have an excellent track record to back them up before they can be used in workplaces or industrial installations. While optical and magnetic encoders have a long history and are based on seemingly "more concrete" physical concepts, capacitive encoders are also based on thoroughly tested principles and have been proven through years of successful field applications.
Currently, the main domestic suppliers of capacitive encoders include SICK, Tamagawa, and Ingenic Semiconductor. Below, we will use Ingenic Semiconductor's latest product, the CAPRO6020kit encoder, as an example to explain some basic knowledge points about capacitive encoders.
Capacitive encoder detection principle
Capacitive encoders measure the absolute angle of a single turn by measuring the modulated rotating electric field, achieving high-resolution and high-precision angle measurement. A carrier signal is transmitted through transmitting electrodes. A moving metal or dielectric disk modulates the spatial electric field and reflects it back to the differential receiving electrodes. The signal is conditioned, sampled by an ADC, and then synchronously demodulated using an FPGA to calculate the angle value in real time.
Absolute position recognition is achieved using dual-channel measurement technology, enabling the encoder to output any angle value within a single turn upon power-on. This eliminates the need for power-on zero-point homing, improving production efficiency and reducing error rates. A battery-powered multi-turn encoder is realized by integrating ultra-low-power magnetic multi-turn counting.
The sensing section of a capacitive encoder eliminates optical elements and etched code disks, replacing them with patterns manufactured from mature circuit boards, significantly reducing manufacturing costs. The detector's sensing area is a full-circuit annular surface, providing a large sensing area and high signal-to-noise ratio. It also homogenizes errors caused by mechanical eccentricity or other installation issues, greatly reducing the precision requirements for installation and allowing for the elimination of bearings and the adoption of a bearingless installation method. The algorithm achieves real-time calculation and compensation for the zeroth and higher harmonic errors of the Lissajous circle, truly achieving self-adaptation, further reducing the encoder's precision requirements based on installation.
Features of capacitive encoders
Large hollow shaft: It is beneficial for pipes and wires to be arranged inside the hollow shaft of the motor;
High performance: high resolution (maximum single-round resolution: 18-bit absolute, hybrid 18-bit), high precision, and high reliability;
Low requirements: Installation difficulty and usage requirements are reduced;
Thin: bearings can be eliminated, thickness is greatly reduced, and integration is possible.
Encoder Output Protocol
The encoder can use line-driven differential input and output, supports multiple absolute protocols including SSI, BISS-C, RS485, and multiple incremental level outputs including line-saving TTL level, HTL level, and NPN type open collector.
The encoder also provides single-ended LVTTL output, supporting SSI, BISS-C, SPI absolute protocols and TTL level incremental square wave output.
Encoder input voltage
The encoder is powered by a wide voltage range of 4.5V to 26.4V. For multi-turn encoders, an additional battery voltage is required, with a voltage range of 2.2V to 3.6V .
Multi-turn encoder working characteristics
The absolute multi-turn encoder operates in both full-function and low-power modes, automatically switching depending on whether an external power supply is available. When an external power supply is available, the encoder runs in full-function mode, continuously measuring multi-turn angle values and transmitting the data to the controller in real time. When the external power supply is cut off, the encoder switches to low-power mode, with a standby current of only 6uA, and only counts multi-turns. Upon each power-on, the encoder first measures the battery voltage and determines if the battery power supply is disconnected, setting low battery warning and undervoltage warning bits and reporting them to the controller.
Installation and debugging assistant
The encoder features a built-in dual-start program: one for installation and commissioning, and the other for normal operation. Since the bearingless encoder is supplied to the customer as two circuit boards, the installation program assists with proper installation. The installation and commissioning assistant verifies that the air gap between the encoder's moving and stationary discs meets requirements and also aligns the encoder zero point with the motor's magnetic pole angle.
Encoder condition monitoring
The encoder monitors the amplitude of the primary signal, objectively reflecting the magnitude of the axial movement of the motor shaft; the encoder also continuously measures the temperature, and will issue an alarm if the temperature exceeds the set range.
