A multi-channel inductive acquisition circuit system and device for eye tracking

Abstract

This invention discloses a multi-channel inductive acquisition circuit system and device for eye tracking, belonging to the field of biomedical sensing and precision electronic measurement technology. The acquisition circuit system includes an excitation signal generation and driving module, a multi-channel parallel analog signal conditioning module, an analog multiplexing and digitization module, and a control and processing module. The excitation module generates a specific frequency signal to drive an external parallel array of induction coils; each conditioning channel independently processes the response of a single coil, converting the complex impedance information of the coil into a DC voltage through differential amplification, transimpedance amplification, phase detection, and RMS detection; the multiplexing and digitization module selects the signals of each channel through a multiplexer and completes digitization using a single high-resolution Σ-Δ ADC; the control module calculates the eye movement trajectory based on a pre-stored model. This invention adopts a hybrid architecture of "parallel analog conditioning and back-end digital multiplexing," thereby effectively controlling costs while ensuring high accuracy.

Description

Technical Field

[0001] This invention belongs to the field of biomedical sensing and precision electronic measurement technology, specifically relating to a multi-channel inductive acquisition circuit system and device for eye tracking, and in particular a dedicated circuit that works in conjunction with a magnetoelastic sensor (such as a contact lens) to calculate the spatial motion trajectory of the eyeball in real time by measuring the inductance changes of external multi-channel induction coils. Background Technology

[0002] Eye-tracking technology, a key tool in human-computer interaction, virtual reality, neuroscience, and clinical diagnosis, can be implemented in various ways. Among them, the method based on inductance changes to calculate eye movement has significant theoretical advantages: by embedding soft magnetic materials in carriers such as contact lenses, eye movement causes a change in the magnetic coupling between the material and an externally fixed induction coil, thereby modulating the coil's equivalent inductance. By simultaneously detecting the inductance changes of coils at multiple locations, the eye's movement trajectory can be deduced. This principle directly converts physiological movement into highly measurable changes in circuit parameters, theoretically enabling high-precision, non-contact, and passive eye tracking.

[0003] However, the main obstacle to translating this principle into a stable and reliable practical system currently lies in the implementation architecture of high-precision signal acquisition circuits. Mature solutions for continuous eye tracking based on this principle are still relatively rare, and existing technical approaches generally suffer from the following contradictions: To achieve the highest measurement accuracy, a fully discrete architecture is typically required, where each induction coil is equipped with an independent excitation source, analog front-end conditioning circuit, and high-precision analog-to-digital converter. While this architecture ensures the fidelity and synchronization of signals from each channel, it also leads to a linear increase in system cost, size, power consumption, and complexity with the number of channels, making it impractical for multi-channel applications. Conversely, if a front-end analog multiplexing architecture is adopted to reduce system cost and size, introducing an analog multiplexer at the front end of the signal chain to allow multiple induction channels to share subsequent conditioning and digitization circuits, it will severely compromise system accuracy. The inductance change signal generated by eye movement is extremely weak, typically below the microvolt level. The non-ideal characteristics of analog switches, such as nonlinear conduction resistance, charge injection effects during switching, and crosstalk between channels, will directly contaminate the original weak signal. These noises are significantly amplified after subsequent high-gain amplification, leading to increased system noise floor, deteriorated linearity, and a sharp decline in overall measurement accuracy, making effective measurement difficult to achieve.

[0004] Specifically, current technologies lack a dedicated circuit solution that can simultaneously acquire and digitize multi-channel weak inductance signals at a reasonable cost and with minimal complexity while maintaining high measurement accuracy for a single channel. Therefore, a novel circuit architecture is urgently needed to overcome the inherent contradiction between accuracy and cost in existing solutions, thereby propelling high-performance eye-tracking technology based on inductance calculation principles towards practical applications. Summary of the Invention

[0005] The purpose of this invention is to provide a multi-channel inductive acquisition circuit system and device for eye tracking, so as to solve the problems of complex circuit structure, limited measurement accuracy and high system cost in the prior art.

[0006] This invention provides a multi-channel inductive acquisition circuit system for eye tracking, comprising:

[0007] Multiple external induction coils are used to sense a magnetic marker worn on the eyeball, the position of which changes with the position of the eyeball; specifically, the magnetic marker is embedded in a contact lens.

[0008] The excitation signal generation and driving module generates a stable, high-power, and high-frequency radio frequency excitation signal for a specific frequency point and applies it synchronously to all external induction coils. Specifically, this module is implemented using a direct digital frequency synthesizer to generate, for example, a 100kHz sine wave, which is then connected to a power drive circuit to drive the external induction coils.

[0009] A multi-channel parallel analog signal conditioning module has at least four conditioning channels with identical structures. The input of each channel is independently connected to an induction coil to acquire a first voltage signal characterizing the impedance amplitude of each external induction coil and a second voltage signal characterizing the phase difference between the voltage and current across each external induction coil.

[0010] An analog multiplexing and digitization module is used to perform analog-to-digital conversion on the first voltage signal and the second voltage signal to obtain a digital signal characterizing the impedance amplitude and phase.

[0011] The control and processing module is connected to the excitation signal generation and driving module, the analog signal conditioning module, and the analog multiplexing and digitization module, respectively. It is used to control the timing of the analog multiplexing and digitization module, and to collect and process the digital signals to calculate the eye movement trajectory.

[0012] Furthermore, each of the conditioning channels has the same structure;

[0013] The conditioning channels include:

[0014] The differential amplifier unit is used to acquire the differential voltage signal across the corresponding induction coil.

[0015] The transimpedance amplifier unit is used to convert the current signal of the corresponding induction coil into a voltage signal;

[0016] The phase detection unit is used to extract the phase difference between the differential voltage signal and the current signal, and output the second voltage signal that characterizes the phase difference between the voltage and current at both ends of the external induction coil.

[0017] The effective value detection unit is used to extract the amplitude of the differential voltage signal and the amplitude of the current signal and output the first voltage signal that characterizes the impedance amplitude at both ends of the external induction coil.

[0018] Furthermore, the differential amplification unit is an instrumentation amplifier; the phase detection unit is implemented using an analog multiplier or an integrated phase detector chip; and the RMS detection unit is implemented using an integrated true RMS-DC converter chip.

[0019] Furthermore, the analog multiplexing and digitization module includes:

[0020] A multiplexer, which is a CMOS analog multiplexer, has multiple input terminals connected to the output terminals of all parallel conditioning channels respectively, for sequentially selecting each conditioning channel and outputting a first voltage signal or a second voltage signal;

[0021] An analog-to-digital converter module is connected to the output terminal of the analog multiplexer and is used to convert the selected first voltage signal or second voltage signal into a digital signal; the analog-to-digital converter preferably uses a high-resolution Σ-Δ ADC with a bit rate of not less than 24 bits.

[0022] Furthermore, there are four conditioning channels, each corresponding to an external induction coil;

[0023] The analog multiplexer is connected to the first DC voltage and the second DC voltage output from the four conditioning channels respectively; the multiplexer has eight input terminals, which respectively receive the first DC voltage and the second DC voltage output from the conditioning channels.

[0024] Furthermore, the control and processing module includes a microprocessor;

[0025] The microcontroller is configured to: after controlling the analog multiplexer to switch channels in a preset order, insert a stabilization delay to wait for the circuit to stabilize before triggering the analog-to-digital conversion module to perform analog-to-digital conversion.

[0026] The microprocessor is also used to record and store the digital signals of the first and second voltages of all channels, process the digital signals to obtain the inductance, and calculate the real-time movement trajectory of the eyeball based on the pre-calibrated model.

[0027] Furthermore, the control and processing module contains a calibration and calculation model, which is a functional relationship or data mapping table established through calibration to map the inductance combination of the multiple external induction coils to eye spatial orientation parameters; the eye spatial orientation parameters include the horizontal deflection angle and the vertical deflection angle of the eye.

[0028] The process of processing the digital signal to calculate the eye movement trajectory specifically involves: calculating the inductance of each external induction coil based on the digital signal characterizing the impedance amplitude and phase difference, and then obtaining the spatial orientation parameters of the eye based on the calibration and calculation model, thereby obtaining the eye movement trajectory.

[0029] Furthermore, the multi-channel inductive acquisition circuit system for eye tracking of the present invention also includes a power supply module for generating +5V and -5V analog power supplies and a +3.3V digital power supply respectively through a low-dropout linear regulator; and the analog power supply and the digital power supply ground are connected at a single point.

[0030] The analog power supply is used to power the excitation signal generation and driving module, as well as the multi-channel parallel analog signal conditioning module;

[0031] The digital power supply is used to power the analog multiplexing and digitization module, as well as the control and processing module.

[0032] Furthermore, the multi-channel inductive acquisition circuit system for eye tracking of the present invention also includes a data transmission module for wirelessly transmitting the calculated eye movement trajectory data to an external smart terminal or host computer for display and interaction.

[0033] The present invention also provides a head-mounted device for eye tracking, comprising,

[0034] The multi-channel inductor acquisition circuit system of the present invention;

[0035] The device housing encapsulates the inductive acquisition circuit system and fixes the plurality of induction coils inside the housing, such that when worn by a user, the induction coils are arranged towards the user's eyes for magnetic coupling with a magnetic marker worn on the eyeball. Furthermore, the number of induction coils is four, arranged in a two-dimensional array in space.

[0036] This invention divides the signal chain into an "analog parallel conditioning domain" and an "analog multiplexing and digitization domain." The analog parallel conditioning domain includes an excitation signal generation and driving module, as well as a multi-channel parallel analog signal conditioning module. The analog multiplexing and digitization domain includes an analog multiplexing and digitization module, as well as a control and processing module. The analog parallel conditioning domain employs completely independent and parallel circuits, fundamentally eliminating the contamination of weak original signals by switching noise and ensuring the ultimate accuracy of each channel. Subsequently, after the signal is conditioned to a robust DC voltage with high level and low impedance, it enters the "analog multiplexing and digitization domain," where it is centrally processed through a combination of an "analog multiplexer + high-resolution Σ-Δ ADC." At this point, the error introduced by switching is negligible relative to the signal amplitude; the high-resolution Σ-Δ ADC, with its excellent noise shaping capability and high linearity, achieves precise quantization of minute changes in DC voltage. Finally, a dedicated algorithm converts the multi-channel digital signal into an eye movement trajectory. This architecture, while maintaining near-discrete architecture accuracy, significantly optimizes system cost and complexity.

[0037] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages:

[0038] 1. Achieving an effective balance between accuracy and cost: This invention utilizes a hybrid architecture of "parallel analog conditioning and centralized digital multiplexing at the back end." This eliminates the noise pollution problem of the "front-end multiplexing architecture" at the analog front end, enabling each channel to achieve an accuracy level comparable to a "fully discrete architecture." Multiplexing a single high-precision ADC at the digital front end effectively decouples the core system cost from the number of channels, achieving the ability to obtain multi-channel high-precision data at a cost close to that of a single channel.

[0039] 2. Solved the problem of dedicated circuits for magnetoelastic eye tracking: A dedicated combination of "multiplexer + high-resolution Δ-Σ ADC + eye tracking algorithm" was proposed and implemented, which specifically solved the contradiction of synchronous acquisition and processing of weak inductive signals from multiple channels, providing a reliable hardware foundation for the practical application of this technology.

[0040] 3. High reliability, flexibility, and scalability: A unified digital backend simplifies system synchronization and control logic. The architecture can be linearly scaled to more channels (e.g., eight channels, sixteen channels) by simply adding the corresponding parallel conditioning channels and multiplexer inputs, while the core architecture remains unchanged. Furthermore, this circuit system is also suitable for other sensing scenarios requiring high-precision, multi-channel detection of minute inductance changes. Attached Figure Description

[0041] Figure 1 This is a block diagram of the overall circuit system structure of an embodiment of the present invention.

[0042] Figure 2 This is a real-time calculated eye-tracking curve diagram according to an embodiment of the present invention. Detailed Implementation

[0043] This invention provides a multi-channel inductive acquisition circuit system for eye tracking, including an excitation signal generation and driving module, a multi-channel parallel analog signal conditioning module, an analog multiplexing and digitization module, a control and processing module, and a power supply module.

[0044] The excitation signal generation module uses a direct digital frequency synthesizer, and the analog signal conditioning module uses high-precision instrumentation amplifiers, phase detectors, and RMS detectors. The overall circuit adopts a printed circuit board integrated design, and the integrity of the analog signal is ensured through strict layout and wiring.

[0045] like Figure 1 As shown, the excitation signal generation and driving module uses the AD9833 direct digital frequency synthesizer (DDS) chip from Analog Devices (ADI). Under the control of the control and processing module, it can generate a sine wave output with programmable frequency and phase; in this embodiment, the output frequency is set to 100kHz. This signal is buffered and amplified by a power drive circuit composed of a low-noise, high-speed operational amplifier (such as TI's OPA2188) to provide sufficient current to drive the parallel induction coils and ensure the purity and stability of the excitation signal.

[0046] In this specific embodiment, the multi-channel parallel analog signal conditioning module comprises four identical analog signal conditioning modules, i.e., four channels. Each analog signal conditioning module includes:

[0047] Differential amplifier unit: The AD8422 instrumentation amplifier from Analog Devices (ADI) is used. It has extremely high input impedance and common-mode rejection ratio, and is used to acquire the weak differential voltage signal across the induction coil without loss.

[0048] Transimpedance amplifier unit: An OPA820 operational amplifier is used to build a transimpedance amplifier, which linearly converts the voltage drop (representing the current signal) across the precision sampling resistor (e.g., 10Ω, 0.1%) connected in series in the induction coil circuit into a voltage signal.

[0049] Phase detection unit: Employs Analog Devices' AD8302 integrated phase detector. It receives two signals: a filtered differential voltage signal V_f and a current signal I_f, and outputs a DC voltage V_PHASE. This voltage value V_PHASE is precisely proportional to the phase difference between the two input signals.

[0050] RMS detection unit: Employs Analog Devices' AD637 integrated true RMS-DC converter chip. It receives two signals from the receiving channel: the differential voltage signal V_f and the current signal I_f, and outputs a DC voltage V_RMS. This voltage value V_RMS is linearly related to the RMS impedance (amplitude) of the input signal.

[0051] Each channel ultimately outputs two stable DC voltages: V_PHASE (representing phase) and V_RMS (representing amplitude).

[0052] The analog multiplexing and digitization module includes:

[0053] Analog Multiplexer: Employs TI's TMUX1209 precision CMOS analog multiplexer. This device features extremely low charge injection and flat on-resistance, with its eight input channels connected to the four conditioning channels V_PHASE1-4 and V_RMS1-4, totaling eight DC voltage signals.

[0054] Analog-to-digital converter: Employs TI's ADS1220 24-bit high-precision Σ-Δ analog-to-digital converter. Its built-in programmable gain amplifier (PGA) and powerful digital filter enable the conversion of DC voltages selected by the multiplexer into digital codes with high resolution and high rejection ratio.

[0055] The control and data processing module is based on STMicroelectronics' STM32F103C8T6 microcontroller. This microcontroller controls the channel address selection lines of the TMUX1209 analog multiplexer via GPIO, and configures the ADS1220 analog-to-digital converter and reads the converted data via the SPI interface. The core control and processing algorithms running within the microcontroller include:

[0056] (1) Timing control: Control the multiplexer to switch cyclically in a predetermined order (e.g., channel 1 to channel 4, phase first then amplitude for each channel). After each channel switch, the program inserts a configurable stable delay (ensuring that the multiplexer and ADC input circuit are sufficiently stable), and then triggers the analog-to-digital converter ADS1220 to perform a conversion and read the result.

[0057] (2) Data Acquisition and Processing: The digital codes corresponding to the DC voltage V_PHASE and V_RMS of each channel are continuously recorded to form four sets of real-time data sequences of amplitude (|Z|) and phase (θ) [|Z1|, θ1, |Z2|, θ2, |Z3|, θ3, |Z4|, θ4]. |Z1| represents the impedance of channel 1, θ1 represents the phase of channel 1, |Z2| represents the impedance of channel 2, θ2 represents the phase of channel 2, |Z3| represents the impedance of channel 3, θ3 represents the phase of channel 3, |Z4| represents the impedance of channel 4, and θ4 represents the phase of channel 4.

[0058] (3) Trajectory calculation: Based on the above real-time data sequence, the inductance of each external induction coil is calculated, and the eye spatial orientation parameters are obtained by inputting the pre-stored calibration and calculation model, thus obtaining the eye movement trajectory. The calibration and calculation model is a functional relationship or data mapping table established by the previous system calibration to map the combination of inductances of external induction coils to the eye spatial orientation parameters;

[0059] The spatial orientation parameters of the eyeball include the horizontal deflection angle and the vertical deflection angle. The horizontal deflection angle is the angle between the projection of the eyeball's visual axis in the horizontal plane and a predefined reference axis directly in front of the head. The vertical deflection angle is the angle between the eyeball's visual axis and the horizontal plane.

[0060] The inductance calculation formula is as follows:

[0061]

[0062] f is the frequency of the alternating current flowing through the inductor, Z is the impedance, Z = , It is the phase difference between voltage and current.

[0063] In this specific embodiment, the calibration and calculation model describes the nonlinear mapping relationship between four sets of inductances and the horizontal and vertical deflection angles of the eyeball, thereby calculating the spatial pointing coordinates of the eyeball in real time. The inductance changes caused by eyeball rotation and detected by the induction coil array are mapped to the corresponding eyeball rotation angles, typically output as horizontal and vertical deflection angles, thus determining the direction of vision.

[0064] The power module generates +5V, -5V (for analog operational amplifiers), and +3.3V (for digital circuits and ADCs) through high-efficiency, low-noise low-dropout linear regulators (LDOs). Analog and digital power supplies are isolated at a single point using ferrite beads or 0Ω resistors, and decoupling capacitors are placed near the power pins of each integrated circuit to minimize power supply noise interference with the high-precision analog circuits.

[0065] In practical applications, users wear contact lenses embedded with soft magnetic material markers and a lightweight head-mounted device integrating the aforementioned circuitry and four induction coils. The head-mounted device can be an eyeglass frame. The four induction coils are located inside the head-mounted device, facing the contact lenses, for coupling with the magnetoelastic markers worn on the eyeball. After the system is turned on, the circuit automatically and continuously performs multi-channel data acquisition. The calculated eye movement trajectory data is wirelessly transmitted to a computer or smartphone host computer software via the UART interface of the STM32F103C8T6 and a Bluetooth module (such as CH9141K). The horizontal and vertical eyeball deflection angles obtained by the four-channel inductive acquisition circuit are then processed. Figure 2 The curve shown has time on the horizontal axis and the horizontal or vertical deflection angle of the eyeball on the vertical axis, enabling real-time visualization, recording, or use of the trajectory for eye-controlled interaction.

[0066] The system provided by this invention can achieve high-precision, high-speed non-contact eye movement tracking, and is suitable for cutting-edge human-computer interaction scenarios such as virtual reality (VR) / augmented reality (AR) interaction, cognitive science research, and eye-tracking-based assisted control.

Claims

1. A multi-channel inductive acquisition circuit system for eye tracking, characterized in that, include: Multiple external induction coils are used to sense magnetic markers worn on the eyeball; The excitation signal generation and driving module is used to generate and output an excitation signal of a specific frequency to drive an external induction coil. The multi-channel parallel analog signal conditioning module includes multiple conditioning channels. The input terminal of each conditioning channel is connected to an external induction coil to acquire a first voltage signal characterizing the impedance amplitude across each external induction coil, and a second voltage signal characterizing the phase difference between the voltage and current across each external induction coil. An analog multiplexing and digitization module is used to perform analog-to-digital conversion on the first voltage signal and the second voltage signal to obtain a digital signal characterizing the impedance amplitude and the phase difference. The control and processing module is connected to the excitation signal generation and driving module, the analog signal conditioning module, and the analog multiplexing and digitization module, respectively. It is used to control the timing of the analog multiplexing and digitization module, and to collect and process the digital signals to calculate the eye movement trajectory.

2. The multi-channel inductive acquisition circuit system for eye tracking according to claim 1, characterized in that, Each conditioning channel has the same structure, and the conditioning channel includes: The differential amplifier unit is used to acquire the differential voltage signal across the corresponding induction coil. The transimpedance amplifier unit is used to acquire the current signal of the corresponding induction coil; The phase detection unit is used to extract the phase difference between the differential voltage signal and the current signal, and output the second voltage signal that characterizes the phase difference between the voltage and current at both ends of the external induction coil. The effective value detection unit is used to extract the amplitude of the differential voltage signal and the current signal and output the first voltage signal that characterizes the impedance amplitude of the external induction coil.

3. The multi-channel inductive acquisition circuit system for eye tracking according to claim 1, characterized in that, The analog multiplexing and digitization module includes: An analog multiplexing module, which is an analog multiplexer, has multiple input terminals connected to the output terminals of all conditioning channels, and is used to select the first voltage signal and the second voltage signal output by multiple analog signal conditioning modules. An analog-to-digital converter module is connected to the output terminal of the analog multiplexer and is used to convert the selected first voltage signal or second voltage signal into a digital signal.

4. The multi-channel inductive acquisition circuit system for eye tracking according to claim 3, characterized in that, There are four treatment channels; The analog multiplexer is connected to the first voltage and the second voltage output from the four conditioning channels respectively; the analog multiplexing module has eight input terminals, which respectively receive the first voltage and the second voltage output from the four conditioning channels.

5. A multi-channel inductive acquisition circuit system for eye tracking according to claim 3, characterized in that, The control and processing module includes a microprocessor; The microcontroller is configured to insert a stable delay after controlling the analog multiplexer to switch channels, and then trigger the analog-to-digital conversion module to perform analog-to-digital conversion.

6. The multi-channel inductive acquisition circuit system for eye tracking according to claim 1, characterized in that, The control and processing module contains a calibration and calculation model, which is a functional relationship or data mapping table established through calibration to map the inductance combination of the multiple external induction coils to the spatial orientation parameters of the eyeball; the spatial orientation parameters of the eyeball include the horizontal deflection angle and the vertical deflection angle of the eyeball; The process of processing the digital signal to calculate the eye movement trajectory specifically involves: calculating the inductance of each external induction coil based on the digital signal characterizing the impedance amplitude and phase difference, and then obtaining the spatial orientation parameters of the eye based on the calibration and calculation model, thereby obtaining the eye movement trajectory.

7. The multi-channel inductive acquisition circuit system for eye tracking according to claim 1, characterized in that, It also includes a power supply module, which generates +5V and -5V analog power and +3.3V digital power respectively through a low dropout linear regulator; The analog power supply is used to power the excitation signal generation and driving module, as well as the multi-channel parallel analog signal conditioning module; The digital power supply is used to power the analog multiplexing and digitization module, as well as the control and processing module.

8. The multi-channel inductive acquisition circuit system for eye tracking according to claim 1, characterized in that, It also includes a data transmission module for wirelessly transmitting the calculated eye movement trajectory data to an external smart terminal.

9. A multi-channel inductive acquisition circuit system for eye tracking according to claim 1, characterized in that, The magnetic marker is embedded in the contact lens.

10. A head-mounted device for eye tracking, characterized in that, include, The multi-channel inductor acquisition circuit system as described in any one of claims 1 to 9; The device housing is used to encapsulate the inductive acquisition circuit system, and the plurality of induction coils are fixed inside the device housing, such that when the user wears the device, the induction coils are arranged toward the user's eyeballs for magnetic coupling with the magnetic markers worn on the eyeballs.

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