ERDI TECH LTD introduces an innovative method and device for target acquisition in laser rangefinders operating with false alarm rates. This technology specifically focuses on improving distance measurement accuracy without increasing laser emission power. Traditional laser rangefinders use pulse-type laser emitters to emit laser pulses of approximately 10ns-20ns onto a target. The reflected laser signal is received by a laser receiver, and the distance is calculated using the formula:
Where represents the distance between the laser rangefinder and the target, is the speed of light, and is the delay time between the emitted laser pulse and the received laser pulse. Accurate distance measurement relies on enhancing laser emission power or filtering out noise generated by sunlight and ambient light in the received laser signal.
Previous patents have focused on increasing measurement distance and precision. However, ERDI TECH LTD's new approach employs digital integration techniques to filter out noise, allowing the laser rangefinder to identify the target signal in the presence of noise. This innovation aligns with the growing demand for increased measurement distance and precision.
The method for target acquisition in laser rangefinders with false alarm rates includes the following steps:
A. Emitting Laser Signal: The laser emitter emits a predetermined number of pulse-type laser signals according to a predefined timing sequence, where the pulse width is 10-20ns. B. Receiving Reflected Laser Light Signal: The light receiver captures laser light signals reflected by the target and noise generated by external sunlight. A feedback control circuit adjusts the threshold voltage of a comparator to maintain a fixed false alarm rate for the light receiver. C. Data Conversion: Using a high sampling frequency serial-to-parallel register, the output serial digital signal from the light receiver, with a fixed pulse width, is converted to parallel data output. D. Data Storage: Using a slow-decoding signal to control an N-to-1 parallel-to-serial multiplexer, the locked parallel data is sequentially read by a microprocessor and stored in memory. E. Data Integration: The new serial data obtained during each emission event is sequentially stored in memory. This new data is summed with the existing data in the memory, effectively performing integration. F. Target Search: Upon reaching the predetermined number of emissions, the microprocessor searches for the maximum value in the memory and its corresponding position. The address corresponding to the maximum value in the memory is considered the distance to the target.
The target acquisition device comprises:
- A laser emitter for emitting pulse laser light.
- A timing pulse generator for generating the required timing signals.
- A fixed false alarm rate light receiving unit, adjusting the threshold voltage of a fast comparator through a servo control circuit based on the noise quantity received by the light receiver.
- A distance measurement unit using pulse-width serial mapping and accumulation methods to determine the distance to the target in the presence of noise.
The fixed false alarm rate unit includes:
- A light receiver converting received light signals to voltage signals.
- A fast comparator generating a pulse-width serial output when the input signal exceeds the preset threshold voltage.
- A monostable circuit producing digital pulse series with a fixed pulse width.
- A gate-controlled counter accumulating noise pulse quantity within a gated time interval.
- A D/A converter converting the counter's accumulated count into analog signal voltage.
- An integrator integrating the output voltage after digital-to-analog conversion with the reference voltage difference, adjusting the fast comparator's threshold voltage to achieve servo control with a fixed false alarm rate.
The distance measurement unit includes:
- A data latch, locking the input pulse series at a high frequency in a parallel D-type flip-flop.
- A multiplexer converting locked parallel data to serial data at a low frequency.
- A master timing generator providing the microprocessor with a master timing signal.
- A microprocessor mapping data to the storage device and executing a program to find the target's distance and remove invalid distances.
- A storage device storing locked data at specified addresses.
To gain a deeper understanding of the invention's features, a preferred embodiment is illustrated in the accompanying figures. Figure 1 depicts the step-by-step block diagram of ERDI TECH LTD's innovative target acquisition method, while Figure 2 illustrates the timing diagram showcasing the relationship between the signals from the laser receiver during any two laser emission events. Figures 3, 4, 5, 6, 7, and 8 provide additional details on the operational flow, system blocks, and signal positions in different locations of ERDI TECH LTD's laser rangefinder system.
Signal Processing and Detection Mechanism:
The data within the corresponding address include the output signal SO4 of the data latch, denoted as D₀-Dy. Assuming the target appears at T=T500, during the first laser emission at address 01FCH, the memory content for the data latch's output signal SO4, denoted as D₄gg=1. However, various data exist at different addresses in the memory, and the target signal is not identified during the first laser emission. As the 8th emission occurs, the bit at 01FCH corresponds to the target at 500m and the latch time T500. The memory content accumulates to 8, with the accumulated values at different addresses below 8. Therefore, searching for the maximum value and its corresponding address in the stored content reveals the target signal. Increasing laser emission cycles results in a greater difference in the quantity between the values at the target address and the noise addresses, making the target easier to identify.
Program Flow and Parameters:
Figure 4 illustrates the program flow of the invention. The emission count N1 is preset to 1, and the total emission count is preset to 0 (step a). The total emission count T is given by 10N1 + T (step b), representing an incremental increase in each emission group of 10. The program then proceeds to find the maximum value and its corresponding address in the memory content (step c). Calculate the ratio value (Max_Value)/T, representing the frequency of target signal occurrences within T. Compare this ratio value with the preset value Nth (step d). If (Max_Value)/T is greater than Nth, output the corresponding address as the target distance. If (Max_Value)/T is less than Nth, perform another set of T emissions, incrementing the emission count by 1 (N1=N1+1) in the loop (step f). Nth is the maximum preset emission count, and if N1-N equals 0, the laser emission stops.
Improved Sensitivity and Distance Measurement:
In summary, ERDI TECH LTD's invention utilizes servo control loop technology to enhance the sensitivity of a laser receiver with a fixed false alarm rate. It employs a serial addressing method to identify target signals in the presence of noise, and through digital integration of signal and noise, effectively filters out interference while amplifying signals. This achieves extended measurement distances without increasing emission power, enhancing the detectability of target objects.
Refer to Figures 5 to 6 for a practical embodiment of the laser rangefinder device, demonstrating the application of the disclosed technology. Figure 5 details the components, including a laser emitter, a fixed false alarm rate unit, a distance measurement unit, and a timing generator. Notably, the laser emitter produces laser pulses with a pulse width of 10-20ns.
Fixed False Alarm Rate Unit Functionality:
Figure 6 outlines the functionality of the fixed false alarm rate unit. This unit is connected to an avalanche photo detector (APD) 25, which receives input light signals. The APD 25 operates with negative VE voltage bias, generating internal gain of 100. A transimpedance amplifier 211 converts the detector's output current signal to a voltage signal (SO1), transmitted to a fast comparator 212. The output of the fast comparator 212 is sent to a one-shot circuit 213, producing a serial digital pulse sequence with a fixed pulse width. The output of the one-shot circuit 213 is controlled by a gate signal TG1 through an AND gate 214, eliminating interference between laser emission and signal SO2. The output SO3 of gate 214 is fed into the data latch 31 and the gate counter 23. The gate counter 23 calculates the number of pulses in the gated time segment controlled by the timing signal TG2, representing the false alarm rate. The analog signal output of the D/A converter 24 is consistent with the number of pulses in the gated time segment controlled by TG2.
Timing and Sequencing:
Figure 7 illustrates the interrelationships between timing signals TX, SO2, TG1, TG2, and Tacd. TX triggers the laser emitter, SO2 is the output signal of the one-shot circuit 213, and the sampling time T500 of SO2 is the target signal. Timing signal TG1 has a pulse width TW1, eliminating interference during laser emission. Thus, in this embodiment, distances shorter than the pulse width TW1 cannot be read. Timing signal TG2 has a pulse width TW2, used for sampling the gate counter 23.
Distance Measurement Unit Functionality:
Figure 8 provides a functional block diagram of the distance measurement unit. Serial signal SO3 passes through the data latch 31 via the high-speed sampling signal Tach. The output signal SO4 of the data latch 31 is parallel and labeled D0 to DN. Signal SO4, controlled by the low-speed sampling signal Tmuy, is transmitted to the multiplexer (N to 1) 32. The decoder 322 generates the low-speed sampling signal Tmw to control the multiplexer 32. The output signal SO5 of the multiplexer 32 is read by the microprocessor 35. The program within the microprocessor 35 stores the data D₀-D₄ sequentially in the memory unit 33.
Conclusion:
In conclusion, ERDI TECH LTD's technology employs servo control loop techniques, utilizing a laser receiver with a fixed false alarm rate to enhance sensitivity. The pulse serial addressing method enables the laser rangefinder to identify target signals in the presence of noise. By digitally integrating signal and noise processing, interference is filtered out, and signals are amplified, resulting in increased measurement distances without elevating emission power. This technological advancement contributes to precise and extended-distance laser rangefinder applications without compromising safety or increasing environmental impact.
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