Research On Low Photocurrent Measurement System For Position Sensitive Detector

Electronic measurement technology is the foundation of modern IT. In the process of rapid development of information technology, Scientific research or industrial production often need to measure the relative distance and orientation of the object surface. At present, the main methods of measuring are contact and non-contact measuring styles.Among these methods, using Position Sensitive Detector(PSD) is a simple way of a non-contact detection projected on the surface of the spot center location,with Only a few output optical signal relative to the location information to calculate the detection.Now, various types of optical displacement sensor system has been widely used in industrial and commercial areas.PSD can work in a wide range of light strength, with mW power laser irradiation, the PSD can produce hundreds of μA photocurrent,and with the weakening of the light intensity, the output photocurrent of the PSD will reduce, or even just a few nAs.But Weak current measurement is a problem in the field of test and measurement, for noise and interference of weak current state will affect the test system resolution, dynamic range, signal to noise ratio and reproducibility, so the measurement of the weak current to become a very important aspect in the field of test and measurement.Because of PSD photocurrent generated in low-light conditions is very small, even as low as a few nano-amp, for each electrode of the output signal amplification and detection need to use the weak current detection techniques.In this paper, firstly the current domestic and international micro-current test theory are researched ane analysised. Common hardware of weak signal detection, such as high input impedance method, direct amplification op amp noise analysis, immune micro-sensor, narrowband filter, dual eliminate noise, synchronous accumulation method, lock receiving method, sampling integral and relateddetection, etc; and Software detection methods such as smoothing, digital filtering, fast Fourier transform, have made a comparative analysis to their respective advantages and disadvantages. On this basis, this article has proposed and designed the hardware and software combined with micro-current detection circuit. For high-impedance output signal of the weak current source, this article has designed the position sensitive detector output photocurrent detection system, achiving in low light (light intensity of approximately11x) conditions, the accurate measurement of small displacement, and to achieve computational analysis and display of the center of the incident light spot location and motion parameters through the PC software. In addition, during the weak current detection system design, this paper also design rationally of a weak signal measurement circuit from the perspective of accuracy, reliability, and economic considerations, in integrated system design, production, commissioning and software technology to improve system accuracy and robustness of, and reduce production costs.This article combines the advantages of hardware and software, uses Low-noise hardware circuit to design the weak current signal processing and acquisition, software method to correct the errors, and through the PC software to calculated, analyze and display the center coordinates and the trajectory of the incident light spot. The design of the weak photocurrent signal detection system based on position-sensitive detector to achieve real-time detection of small displacement of this paper, can be widely used in remote optical control system, displacement, and vibration monitoring, automatic range detection system, and robot motion analysis system, etc. Besides, the modular structure of the system design is an good idea and experience for the development of similar measuring instruments.Finally, we have made a comprehensive performance test for the whole system based on actual items, and resolution, dynamic range, signal to noise ratio and the results of repeatability data were analysised, and achieved all the design goals in the end.