Design and development of the smart machine vision sensor

The Smart Machine Vision Sensor (SMVS) is a self-contained low-cost machine vision system that combines an image sensor, a processing unit and communication interfaces in a single unit. This paper describes the design of the SMVS, the concept of the operating system and the application software. The main characteristics of the SMVS are (1) the absence of an analog video signal and (2) the modular design concept. The signal processing is performed with the processing unit and the output consists either of image analysis results or a compressed digital image data stream. The modular design allows for easy customization for applications that require different sensor technologies, resolutions and spectral sensitivities. The modular design is made possible by the development of a generic sensor-CPU interface. This interface can be used with either CCD or CMOS sensors with a resolution of 512512 pixels. It eliminates the need to redesign the processor board with changing sensor requirements. The SMVS is smaller and cheaper than traditional image processing systems based on desktop personal computers. It uses full-frame, non-interlaced sensors with square pixels to provide an optimal image quality, avoiding the drawbacks of an analog, interlaced video signal with ^Lxed frame rate.

Highly sensitive vertical hall sensors in CMOS technology

Enrico Schurig

Vertical Hall sensors are capable of measuring surface-parallel components of the magnetic field. They allow therefore relatively easy conception of single-chip multi-axial magnetic sensors compared to solutions using horizontal Hall plates. The modern trend in the field of Hall sensors is to integrate them into electronic circuitry for signal processing. Before this thesis, highly sensitive vertical Hall sensors completely compatible to a technology adequate for co-integration of electronic circuitry were not available. This is the reason why we present in this thesis the developed knowledge that is necessary for the design and manufacture of highly sensitive vertical Hall sensors in CMOS technology. We first present the principles of the technology choice. A wide selection of CMOS processes is presently available, but in order to obtain optimal sensitivity we have to choose "the right" one. The performances can vary easily by 50% from one technology to another. The best choice are CMOS high-voltage technologies since they deliver n-diffusion layers with relatively low doping level and deep junction depth. We present a novel layout of vertical Hall sensors that has six contacts in the active sensor zone and is a development of the known layout that uses only four contacts. The additional contacts improve the sensitivity and reduce the systematic offset considerably. If we re-use layouts that were optimal for not-CMOS compatible technologies, we will see that the results are not satisfying. We have to develop our specific design parameters that are optimal for the chosen technology. The key for high sensitivities is the strong miniaturization of the devices down to technological limits given by the design rules, but sometimes even beyond that. Some of the design rules can be broken with benefit for the obtained sensor devices. In general, minimum contact sizes and contact distances are preferable as well as a minimum sensor thickness. The difficulty is however to find the absolute minimum before device degradation or even failure will occur. Our optimized sensors achieved voltage related sensitivities of about 0.04 V/(VT) and current related-sensitivities up to 400 V/(AT) that are comparable to existing CMOS compatible horizontal Hall plates. Such sensitivities were never reported for a standard CMOS process, without any additional pre- or post-processing steps. The miniaturization of the Hall devices has however also negative aspects e.g. an increase in sensor offset, noise and output non-linearity. Although Hall sensors have many advantages in comparison to other magnetic sensors (simple structure, low fabrication costs, very good linearity, robustness) they have the drawback of a big Abstract offset voltage that is often too big for many applications. This is why an enormous effort was made by researchers in the past to develop offset reduction methods for horizontal Hall plates. We state in this thesis that, in principle, the same techniques are applicable on vertical Hall sensors but for an optimal efficiency certain principles/parameters have to be respected. We reveal the relations of important parameters as the type of layout (number of sensor contacts), the use of single or coupled sensors, the type of spinning-current, bias level etc. In the optimal case we can obtain residual offset values lower than 200 μT for bias voltages up to 2 V. The spinning current method was originally developed for offset compensation, but it has other beneficial influences on the sensor output signal as well. It is capable to remove a large amount of flicker noise if the spinning is executes at adequately high clock frequencies. We show that the efficiency of flicker noise removal is dependent on the bias current through the sensor. We discovered another positive side effect of the spinning-current method: the elimination of the planar Hall effect. While the planar Hall sensitivity for our sensors is initially about 1% of the normal one at B = 2 T, we obtain with the compensation technique values of only 0.02%. After the presentation of general characteristics of the separate sensors, we present two microsystems based on the developed vertical Hall sensors. At first, we present a 2D magnetic microsystem which is well adapted for the construction of contactless angular encoders with a measurement range of 360°. We profit here especially from the very low offset (< 400 μT) obtained with the spinning current method, since offset is in general the main source of angular errors in such systems. Existing angular measurement systems need in general a special (offset) calibration of the sensor unit in order to achieve an accuracy of 2.5‰ full scale. We obtain such a precision directly with our system for a wide temperature range from -30 to 100 °C. With one specific sensor calibration for the compensation of residual offset, sensitivity mismatch and phase mismatch of the two axes, we attain a precision of about 0.5‰ FS over the same temperature range, a performance never before reported with a magnetic sensor system. The microsystem is therefore an excellent candidate for the majority of low-cost angular measurement applications. As a second example we presented the first fully CMOS integrated three-dimensional Hall probe, consisting of a combination of one Hall plate and four vertical Hall devices. The microsystem allows an application as a universal magnetic field probe in the wide field range from 0.1 mT to 20 T. The probe can be easily used for small volumes because of its small size and the fact that the signal processing circuitry already is directly integrated onto the chip. The precision as a 3D magnetometer is limited by the sensitivity mismatch of about 5% FS. The probe is an ideal candidate for low-cost teslameter applications with limited precision.

EPFL2005

Low Cost Earth Sensor Based on Oxygen Airglow (AIRES)

Edoardo Charbon, Noémy Scheidegger, Herbert Shea

This project has demonstrated the feasibility of a low-cost Earth sensor based on imaging oxygen airglow, allowing 0.4° accuracy from GEO under any illumination condition. Available Earth Sensor (ES) are based on the measurement of the earth’s infrared radiation to determine the vector to the Earth’s centre. These designs provide excellent accuracies over a large field of view, but are often heavy, large, require cooling or temperature stabilization and are power hungry. In addition, the sensor concept for a LEO or GEO application ES differs significantly. We have developed a novel ES concept for applications where milli-degree accuracy is not required, but where low-cost is essential and lower (about 1 – 5°) accuracy is acceptable. Such a sensor could be used in new scenarios and to improve spacecraft reliability by providing a low-cost back-up sensor. Our Earth Sensor concept is based on imaging atmospheric oxygen emission at 762 nm using highly sensitive detectors. In both night-time and daytime there is continuous emission at 762 nm due to oxygen recombination. Low-noise active pixel sensors (APS) or low-light detector based on arrays of single photon avalanche diodes (SPAD) enables the ES to operate at night and day, over a wide temperature range, with a very compact optical system (aperture of 8 mm, focal length of 11 mm) and no scanning elements. A modular design allows designing similar instruments using the same wavelength band, the same detector technology, the same optics, the same power and data interfaces and similar algorithms for GEO and LEO applications, thus reducing the development cost. We have developed an Earth appearance model at 762 nm, which was used as input for the mechanical, optical and electric design of the Earth Sensor (conceptual design). In order to achieve a low-cost solution, simplicity and reduction of part count was a driving factor in the design trade-offs. Total mass for the GEO design is 845 g with a mean power consumption of 4 W. Algorithms were developed to determine the vector to the Earth from the images. A breadboard was built to display a simulated picture of the Earth under varying conditions, image those pictures at different temperatures with a radiation tolerant APS (LCMS), and verify the correct operation of the algorithms. In addition to the conceptual design and breadboard level demonstration of key technologies, a novel detector chip was designed and fabricated: a radiation-tolerant array of single photon avalanche photodiodes (SPAD) build using conventional 0.35 μm CMOS technology. The chip was tested under proton and gamma irradiation, and operated with only minor changes in dark current after 30 krad TID. Having shown the feasibility of such an Earth Sensor, this work concluded with a development plan to lead to a flight model.

2008

Highly sensitive vertical hall sensors in CMOS technology

Enrico Schurig

EPFL2005

Low Cost Earth Sensor Based on Oxygen Airglow (AIRES)

Edoardo Charbon, Noémy Scheidegger, Herbert Shea

2008