Digital measuring tape based on 90-degree orthogonal photoelectric stripes

Abstract

The invention discloses a digital measuring tape based on 90-degree orthogonal photoelectric stripes in the technical field of photoelectric displacement measurement, and aims to solve the technical problems that an existing digital measuring tape is easily influenced by baseline drift and amplitude change, has the risks of miscounting, jitter and step loss, depends on a high-speed analog comparator and is complex in hardware structure. The measuring tape comprises a measuring tape main body, an optical detection module, an analog conditioning module, a microcontroller module and a display communication module, the optical detection module comprises a shared infrared transmitter and two photoelectric receivers, and the two receivers are arranged in a 1 / 4 period space phase difference mode in the stripe movement direction and output 90-degree orthogonal analog signals. The method does not need a high-speed analog comparator, is compact in hardware structure, is high in anti-noise and anti-shake capability, improves the measurement precision and stability, and is suitable for various complex use scenes. In embodiments where an ADC is employed to sample orthogonal analog signals, phase interpolation subdivision may also be selected to further improve measurement resolution.

Description

Technical Field

[0001] This invention relates to the field of photoelectric displacement measurement, specifically a digital measuring tape based on 90° orthogonal photoelectric stripes. Background Technology

[0002] Digital measuring tapes, a common tool for length measurement, digitize length by printing alternating reflectivity stripes on the surface of a steel or plastic tape and using photoelectric sensors to read the periodic signals generated by the stripes as the tape stretches and contracts. Existing signal processing schemes for digital measuring tapes mainly fall into two categories: one involves using an analog front-end to comparator-shape two analog signals into square waves before a processor counts them; the other obtains direction and displacement through the phase difference between two signals, similar to a quadrature encoder structure.

[0003] However, existing technologies have significant drawbacks: on the one hand, changes in the distance and angle between the sensor and the stripe, as well as surface contamination and material reflectivity differences, can easily lead to DC baseline drift and amplitude variations in the signal, thereby introducing risks of miscounting, jitter, and missed steps, affecting measurement accuracy. On the other hand, existing solutions rely on high-speed analog comparators, resulting in complex hardware structures, high costs, and insufficient robustness to environmental interference. For example, the digital displacement sensor disclosed in US10890432B2 uses two 180° complementary signals and generates a single square wave for counting through comparator shaping. This scheme not only struggles to suppress threshold crossover errors caused by baseline drift, but also, because the two signals are out of phase, its output is equivalent to a single-phase counting signal, making it difficult to reliably distinguish between positive and negative directions such as extension and retraction (or requiring additional direction detection structures). This makes it more prone to cumulative errors in reciprocating motion or scenarios with rebound. In contrast, using two 90° orthogonal signals naturally provides a phase sequence, enabling simultaneous displacement counting and direction discrimination, making it more suitable for bidirectional measurement applications of digital measuring tapes. Therefore, there is an urgent need to provide a digital measuring tape based on 90° orthogonal photoelectric stripes to solve the problems mentioned in the background art. Summary of the Invention

[0004] The purpose of this invention is to provide a digital measuring tape based on 90° orthogonal photoelectric stripes to solve the problems mentioned in the background art.

[0005] To achieve the above objectives, the present invention provides the following technical solution: A digital measuring tape based on 90° orthogonal photoelectric stripes includes a measuring tape body, an optical detection module, an analog conditioning module, a microcontroller module, and a display and communication module; The tape measure body has alternating reflectivity stripes on its surface. The stripes form a periodic structure along the length of the tape body, and the period length is p. The optical detection module includes an infrared emitter and two photoelectric receivers. The infrared emitter is an infrared LED. The two photoelectric receivers are arranged along the stripe movement direction. The physical center distance along the stripe movement direction is d = p / 4 + n·p (n is a non-negative integer), forming a 90° orthogonal spatial phase difference. The analog conditioning module includes a transimpedance amplifier circuit and a low-pass filter circuit. The output terminals of the two photodetectors are respectively connected to the ADC channel of the microcontroller module through the analog conditioning module. The microcontroller module has a built-in ADC module and a data processing unit, which are used for synchronous or quasi-synchronous sampling, baseline estimation, Schmitt quantization and counting processing of the two signals. The display communication module is connected to the microcontroller module and is used to output the length measurement results.

[0006] As a further aspect of the present invention: the alternating reflectivity stripes are formed by printing, coating or etching, and are composed of alternating dark and light reflectivity materials.

[0007] As a further embodiment of the present invention: the infrared transmitter is powered by a constant current drive circuit or a PWM drive circuit and is installed at the symmetrical center of the two photoelectric receivers, with the illumination range covering the two photoelectric receivers.

[0008] As a further aspect of the present invention: the two photoelectric receivers are photodiodes or phototransistors, and a light-shielding isolation wall is provided between them to reduce crosstalk interference.

[0009] As a further aspect of the present invention: the microcontroller module adopts an MCU with DMA function, and the ADC sampling rate is set according to the stripe pitch and the maximum expected motion speed to meet the oversampling of the stripe signal; preferably, the number of sampling points per stripe cycle is not less than 4 points, more preferably not less than 8 points or 8 to 64 points.

[0010] As a further aspect of the present invention, the optical detection module further includes a light-shielding structure and an optical window. The optical window is made of a material with a light transmittance of ≥90% to suppress ambient light interference.

[0011] As a further aspect of the present invention: the data processing unit of the microcontroller module has a built-in baseline estimation unit, which is used to estimate the first baseline C_A and the second baseline C_B for the two analog signals respectively; C_A and C_B are obtained by performing median filtering and / or robust moving average on the corresponding channel signals, or by combining the third reference channel signal to compensate for ambient light or common-mode interference; and when the two baselines are approximately consistent, a common baseline C can be obtained by fusing the two signals.

[0012] As a further aspect of the present invention: the data processing unit of the microcontroller module further includes a phase interpolation unit, which, based on the completion of Schmitt quantization and AB four-state machine counting, calculates the instantaneous phase of the two orthogonal analog signals after removing the corresponding baselines and normalizing the amplitude of the analog signals obtained by the two ADCs, and performs phase expansion / interpolation, thereby obtaining the fractional displacement within the fringe period, and thus achieving a measurement resolution higher than 1 / 4 of the fringe pitch.

[0013] As a further aspect of the present invention: the transimpedance amplifier circuit of the analog conditioning module is used to convert the current signal of the photodetector into a voltage signal, and the low-pass filter circuit is a first-order or second-order filter structure.

[0014] As a further embodiment of the present invention: the display communication module includes an LCD display screen and a communication interface, the communication interface supporting UART, I2C or SPI protocols for data transmission and command interaction.

[0015] As a further aspect of the present invention, the working distance between the optical detection module and the surface of the belt is 0.1mm to 5mm, which is suitable for detection requirements under different working conditions.

[0016] As a further aspect of this invention: based on the integer count obtained by the AB four-state machine, sub-periodic interpolation can optionally be performed using the phase information of two orthogonal analog signals. Specifically, the reference lines C_A and C_B are subtracted from the two sampled signals respectively, and normalized according to their respective amplitudes to obtain approximately sinusoidal orthogonal components; the instantaneous phase φ is obtained according to the arctangent function (e.g., atan2) or by looking up a table / CORDIC, and the phase is processed to be continuous, converting the phase change into a fractional displacement Δx≈(p / 2π)·Δφ within the fringe period, thereby further improving the resolution and reducing quantization jitter while maintaining the robustness of Schmitt quantization.

[0017] As a further aspect of this invention: the microcontroller synchronously or quasi-synchronously samples two analog signals at a fixed or variable sampling rate; based on the robustly estimated first baseline C_A and second baseline C_B (ideally, they can be merged into a common baseline C) and a threshold Th (which can be an independent threshold for each channel or a shared threshold), a Schmitt trigger decision is performed on each channel (set to 1 when the signal is greater than the corresponding baseline plus the threshold, set to 0 when it is less than the corresponding baseline minus the threshold, and maintain the original state in between), thereby obtaining digital quantities A' and B'. Subsequently, the direction determination and increment counting are implemented through an AB four-state (Gray code) state machine, and illegal transitions are filtered or error-marked.

[0018] Compared with the prior art, the beneficial effects of the present invention are: 1. Improved Measurement Stability: By employing strategies such as robust baseline estimation, Schmitt quantization, and illegal jump / minimum interval filtering, the risks of miscounting, jitter, and step loss caused by baseline drift, amplitude variations, and environmental noise are reduced. Furthermore, when using an ADC to sample two orthogonal analog signals, sub-fringe period subdivision can be achieved through phase calculation and interpolation, resulting in higher resolution measurement results.

[0019] 2. Simplified hardware structure: It does not rely on high-speed analog comparators, and a single transmitter is shared to cover two receivers. The number of components is reduced, the structure is more compact, and the cost is easier to control.

[0020] 3. Enhanced measurement resolution and orientation determination capability: The 90° orthogonal layout supports orientation determination and subdivision counting, and the measurement resolution can reach 1 / 4 of the stripe pitch; the specific accuracy depends on the stripe process, assembly gap, sampling rate and algorithm parameters, and can reach the sub-millimeter level under the conditions of the example.

[0021] 4. Strong scene adaptability: The working distance is adaptable to the range of 0.1mm to 5mm, and it has strong resistance to ambient light and body contamination, making it suitable for complex scenes such as building construction and industrial manufacturing. 5. Easy to operate: Real-time display of measurement results, support for data communication, and meet the needs of digital measurement and data traceability. Attached Figure Description

[0022] Figure 1 : Overall structural block diagram of the invention; Figure 2 : Schematic diagram of the arrangement of the stripes and the 90° spatial phase difference between the two receivers in this invention; Figure 3 The diagram illustrates the Schmitt threshold of the two analog signals and the baseline (the figure shows the ideal case where the two baselines are consistent). Figure 4 : A schematic diagram of the AB four-state transition sequence in this invention; Figure 5 The firmware processing flow in this invention. Detailed Implementation

[0023] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0024] Please see Figures 1-5In this embodiment of the invention, a digital measuring tape based on 90° orthogonal photoelectric stripes is provided. This invention achieves high-precision and stable measurement by optimizing the optical layout and signal processing logic. The specific technical solution is as follows: (I) Core Module Design Measuring tape body: The surface of the tape body is provided with alternating reflectivity stripes. The stripes form a periodic structure along the length of the tape body, and the period length is p. The stripes can be formed by printing, coating, or etching to ensure stable reflectivity differences and adapt to photoelectric detection requirements. In one embodiment, the stripes are composed of alternating dark and light segments of equal width, with a single segment width of h, in which case p = 2h.

[0025] The optical detection module includes an infrared LED emitter and two photodetectors (photodiodes or phototransistors). The infrared emitter is mounted at the symmetrical center of the two receivers, covering both receivers with its illumination range. This shared design reduces hardware cost and structural volume. The two receivers are arranged along the fringe movement direction, with a physical center-to-center distance of d = p / 4 + n·p (n is a non-negative integer), forming a 90° orthogonal spatial phase difference, outputting two orthogonal analog signals. A light-shielding isolation wall is provided between the receivers to reduce crosstalk interference; combined with the light-shielding structure and optical window, the influence of ambient light is suppressed. The emission peak wavelength of the infrared LED is not limited and can be selected from the near-infrared band such as 850nm or 940nm; in this embodiment, 940nm is selected.

[0026] Analog conditioning module: Composed of a transimpedance amplifier circuit and a low-pass filter circuit, it converts the weak current signal output by the photodetector into a voltage signal and filters high-frequency noise, providing a stable signal source for subsequent sampling.

[0027] Microcontroller Module: Employs an MCU with DMA functionality, featuring a built-in ADC module and data processing unit. The ADC module synchronously or quasi-synchronously samples two analog signals at a fixed or variable sampling rate, while the DMA circular buffer reduces CPU usage. The data processing unit performs baseline estimation, Schmitt quantization, and AB four-state machine counting to determine displacement and direction.

[0028] Display and communication module: Includes an LCD display and communication interface (UART / I2C / SPI) for real-time display of measurement results and supports data transmission and command interaction.

[0029] The data processing unit of the microcontroller module also includes a phase interpolation unit, which, after completing Schmitt quantization and AB four-state machine counting, calculates the instantaneous phase of the two orthogonal analog signals after removing the corresponding baselines and normalizing the amplitude based on the analog signals sampled by the two ADCs, and performs phase expansion / interpolation to obtain the fractional displacement within the fringe period, thereby achieving a measurement resolution higher than 1 / 4 of the fringe pitch.

[0030] (II) Key Technical Characteristics Phase interpolation subdivision: In the software sampling scheme, in addition to x4 counting based on Schmitt quantization, the instantaneous phase of two orthogonal analog signals can be used for interpolation subdivision to achieve sub-p / 4 resolution improvement.

[0031] 90° orthogonal optical layout: The two receivers have a 1 / 4 period spatial phase difference, outputting 90° orthogonal signals, which naturally supports direction determination and x4 counting, improving measurement resolution.

[0032] Software-based signal processing: No high-speed analog comparator is required. Signal quantization is achieved through ADC sampling combined with software algorithms, simplifying the hardware structure and reducing costs.

[0033] Anti-interference algorithm adaptation: The baseline C_A, C_B or the common baseline C is robustly estimated through multiple methods, Schmitt quantization suppresses jitter through dynamic thresholding, and the AB four-state machine filters illegal transitions, thereby enhancing anti-noise and anti-jitter capabilities.

[0034] It should be noted that, Figure 3 The diagram illustrates the ideal scenario where the two signals have the same reference. In practical applications, due to differences in the sensitivity, working distance, and optical path of the two receivers, the DC reference and amplitude of the two signals may be different. The reference lines C_A and C_B can be calculated independently and used for Schmitt quantization and threshold decision, respectively.

[0035] Compact structural design: Shared infrared transmitter, optimized receiver layout, and light-shielding isolation design improve structural compactness and detection stability.

[0036] Example 1: Structural and performance testing of a digital measuring tape based on 90° orthogonal photoelectric stripes Structural configuration Measuring tape body: The tape body is made of steel strip, with alternating black and white reflective stripes printed on the surface. The width of a single stripe is h=2mm, and the period length is p=4mm. The optical detection module includes an infrared LED emitter and two photodetectors (photodiodes or phototransistors). The infrared emitter is mounted at the symmetrical center of the two receivers, covering both receivers with its illumination range. This shared design reduces hardware cost and structural volume. The two receivers are arranged along the fringe movement direction, with a physical center-to-center distance of d = p / 4 + n·p (n is a non-negative integer), forming a 90° orthogonal spatial phase difference, outputting two orthogonal analog signals. A light-shielding isolation wall is provided between the receivers to reduce crosstalk interference; combined with the light-shielding structure and optical window, the influence of ambient light is suppressed. The emission peak wavelength of the infrared LED is not limited and can be selected from the near-infrared band such as 850nm or 940nm; in this embodiment, 940nm is selected.

[0037] Analog conditioning module: The transimpedance amplifier circuit gain is set to 100kΩ, and the low-pass filter circuit is a second-order RC filter (cutoff frequency 1kHz). Microcontroller module: STM32L476 MCU is used. The ADC sampling rate is set to 16kHz in one embodiment, which can be adjusted according to stripe pitch, maximum stretching speed, analog front-end bandwidth and MCU resources; DMA loop buffer depth is 1024 bytes. Display and communication module: 1.2-inch LCD display, supporting UART communication (baud rate 9600bps).

[0038] Workflow After startup, the infrared LED is driven by constant current and illuminated. Two photoelectric receivers detect the reflected light from the stripe and output two 90° orthogonal analog signals. After the analog signal is amplified and filtered by transimpedance, it is sent to the ADC channel of the MCU, and DMA synchronously acquires and stores it in the buffer. (Optional) Phase interpolation: While maintaining the integer displacement of the AB four-state machine count output, the two analog sampling signals are dereferenced and normalized respectively, the instantaneous phase is calculated and the phase is made continuous to obtain the fractional displacement within the stripe period; the final length value is obtained by superimposing the integer count and the fractional displacement to further improve the resolution and smoothness.

[0039] The MCU performs baseline C_A and C_B estimation on the buffered data respectively (example: median filtering and robust moving average on each channel signal) and calculates the dynamic threshold; in the ideal case where the baselines of the two signals are approximately consistent, the common baseline C can be estimated by fusing the two signals (e.g., (A+B) / 2) as an approximation.

[0040] Perform Schmitt quantization: Set the signal to 1, set it to 0, and keep the intermediate state unchanged to obtain digital quantities A' and B'. Counting is performed using an AB four-state machine: forward sequence count +1, reverse sequence count -1, illegal transitions are ignored; The counting result is converted into a length value (e.g., each counting unit corresponds to stripe pitch / 4), which is displayed on the LCD screen and supports data output via UART.

[0041] Performance testing Test conditions description (example): The following data are test / estimated results of the prototype under specific stripe parameters, sensor selection, assembly gap, sampling rate and environmental conditions, used to illustrate the feasibility of the present invention; actual indicators vary with implementation method and usage environment.

[0042] Test Project This invention is a digital measuring tape. Traditional digital measuring tape Measurement accuracy (0-1m, under the conditions of this embodiment) ±0.2mm (Example) ±1mm (Example) False count rate (1000 consecutive times) 0.2% (Example) 3.5% (Example) Resistance to ambient light (1000 lux) Normal operation (example) Jitter occurs (example) Error after contamination ±0.2mm (Example) ±0.8mm (Example) Hardware cost (example BOM relative value) Approximately 70% (example) 100% (Example) This invention significantly improves the measurement accuracy and stability of digital measuring tapes through a 90° orthogonal optical layout and software-based signal processing scheme, simplifies the hardware structure, reduces costs, and has strong anti-interference capabilities. It fully meets the length measurement needs in various complex scenarios and has broad application prospects.

[0043] Example 2: Adding a reference channel In Example 1, without changing the AB layout, a third receiver or reference optical path is added to measure ambient light / common mode drift, providing a more robust estimate or compensation for baselines C_A and C_B.

[0044] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A digital measuring tape based on 90° orthogonal photoelectric stripes, characterized in that, It includes the measuring tape body, optical detection module, analog conditioning module, microcontroller module, and display and communication module; The tape measure body has alternating reflectivity stripes on its surface. The stripes form a periodic structure along the length of the tape body, and the period length is p. The optical detection module includes an infrared emitter and two photoelectric receivers. The infrared emitter is an infrared LED. The two photoelectric receivers are arranged along the stripe movement direction. The physical center distance along the stripe movement direction is d = p / 4 + n·p (n is a non-negative integer), forming a 90° orthogonal spatial phase difference. The analog conditioning module includes a transimpedance amplifier circuit and a low-pass filter circuit. The output terminals of the two photodetectors are respectively connected to the ADC channel of the microcontroller module through the analog conditioning module. The microcontroller module has a built-in ADC module and a data processing unit, which are used for synchronous or quasi-synchronous sampling, baseline estimation, Schmitt quantization and counting processing of the two signals. The display communication module is connected to the microcontroller module and is used to output the length measurement results.

2. The digital measuring tape based on 90° orthogonal photoelectric stripes according to claim 1, characterized in that, The alternating reflectivity stripes are formed by printing, coating, or etching, and consist of alternating dark and light reflectivity materials.

3. The digital measuring tape based on 90° orthogonal photoelectric stripes according to claim 1, characterized in that, The infrared emitter is an infrared LED with a peak emission wavelength of 700nm to 1000nm. The infrared emitter is powered by a constant current drive circuit or a PWM drive circuit and is installed at the symmetrical center of the two photoelectric receivers, with the illumination range covering both photoelectric receivers.

4. The digital measuring tape based on 90° orthogonal photoelectric stripes according to claim 1, characterized in that, The two photoelectric receivers are photodiodes or phototransistors, and a light-shielding isolation wall is provided between them to reduce crosstalk interference.

5. The digital measuring tape based on 90° orthogonal photoelectric stripes according to claim 1, characterized in that, The microcontroller module uses an MCU with DMA function, and the ADC sampling rate is set according to the stripe pitch and the maximum expected motion speed.

6. The digital measuring tape based on 90° orthogonal photoelectric stripes according to claim 1, characterized in that, The optical detection module also includes a light-shielding structure and an optical window. The optical window is made of a material with a light transmittance of ≥90% to suppress ambient light interference.

7. The digital measuring tape based on 90° orthogonal photoelectric stripes according to claim 1, characterized in that, The data processing unit of the microcontroller module has a built-in baseline estimation unit, which is used to estimate the first baseline C_A and the second baseline C_B for the two analog signals respectively. C_A and C_B are obtained by performing median filtering and / or robust moving average on the corresponding channel signals, or by combining the third reference channel signal to compensate for ambient light or common-mode interference. Furthermore, when the two baselines are approximately consistent, the common baseline C can be obtained by fusing the two signals.

8. The digital measuring tape based on 90° orthogonal photoelectric stripes according to claim 1, characterized in that, The data processing unit of the microcontroller module also includes a phase interpolation unit, which, after completing Schmitt quantization and AB four-state machine counting, calculates the instantaneous phase of the two orthogonal analog signals after removing the corresponding baselines and normalizing the amplitude based on the analog signals sampled by the two ADCs, and performs phase expansion / interpolation to obtain the fractional displacement within the fringe period, thereby achieving a measurement resolution higher than 1 / 4 of the fringe pitch.

9. The digital measuring tape based on 90° orthogonal photoelectric stripes according to claim 1, characterized in that, The transimpedance amplifier circuit of the analog conditioning module is used to convert the current signal of the photodetector into a voltage signal, and the low-pass filter circuit is a first-order or second-order filter structure.

10. The digital measuring tape based on 90° orthogonal photoelectric stripes according to claim 1, characterized in that, The display communication module includes an LCD display screen and a communication interface. The communication interface supports UART, I2C or SPI protocols for data transmission and command interaction. The working distance between the optical detection module and the surface of the strip is 0.1mm to 5mm.

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