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The aim of the INCAS project is to develop a fully redundant EM Innovative Contactless Angular Sensor to be embarked on TLC satellites, also including a suitable signal conditioning electronics.
The starting point is the output of the ITI-CAS project, performed by the same team during ESA Contract N° 21333/07/NL/CB under ITI (Innovation Triangle Initiative). The results from ITI-CAS verified that the proposed Absolute Contactless Angular Sensor is able to provide the performances required by a space SADM application.
The main goal of INCAS project is thus to obtain a fully redundant Engineering Model (EM), representative of the final product (TRL 5) and which could then be directly used for Formal Qualification (in the frame of ARTES 3-4).
Principle of Operation
The principle of operation exploits a biasing permanent magnet generating a magnetic field in an airgap of suitable geometry, and whose value is a function of angular position. Hall effect probes are located at strategic positions along the airgap.
The proposed novel design solution consists of using a magnetic circuit configuration such that the angular position is a function of the ratio between the magnetic field values measured by the Hall probes. In this way any drift or degradation of the permanent magnet or Hall probes characteristics is automatically self-compensated. Indeed, the value of said ratio is a function of geometric relationships only, making the sensor insensitive to degradation effects and drift of parameters.
The output sawtooth signal is then built by combining into a linear ramp the four central segments (for their part corresponding to 90° arcs, as 4x90° = 360°) of the linear sides of the symmetrical trapezoid signals generated by the feedback loop. This is achieved by inverting and level shifting said signals, to then select and recombine into a linear ramp, by means of 3 comparators and an analog multiplexer, said four segments.
In order to take full advantage of the self-compensating features of the principle of operation it is crucial that the drift with temperature of the magnetic sensitivities of probes belonging to a same pair are closely matched.
This is usually the case for probes belonging to the same wafer lot. However, it is possible to further improve said intrinsic intra-lot matching by characterizing in temperature each individual probe, to then pair the ones with the most similar temperature drift coefficients-
Furthermore, each probe is characterized by a zero field offset (i.e.: its output signal when B = 0 ), that also needs compensation. However, the self-compensating principle of operation can only partially compensate the drift with temperature of such zero field offset. A specific circuitry has thus been added, effective in compensating the linear part of the zero field offset drift with temperature. Therefore, a probes selection procedure has been implemented, based on the linearity of the respective zero field offset drifts with temperature.
The INCAS product aims at satisfying main requirements of space-based angular sensors (mechanical robustness, radiation-hardening, thermal-vacuum resistance, low cost) by means of a contact-less approach. This is the main characteristic that differentiates INCAS with respect to the currently used potentiometers, giving a more robust and reliable solution, especially over long life missions. The low cost approach of this sensor is intended to provide also a convenient alternative to more expensive sensors for this class of performance.
The current project was able to reach the following achievements:
During this development many unexpected problems had to be faced, some of which affected the results of the test campaign. However, the extensive investigations allowed to identify the causes of said problems, leading to practical suggestions for design and/or manufacturing procedures modifications aimed at overall INCAS sensor performance improvement:
Finally, we are now confident that a series production version of the INCAS sensor trading-off the possibility to obtain the best performances listed above versus the possibility to obtain somewhat reduced performances (but still considered very reasonable by most of the potential users we talked to) at a lower cost (reducing the effort required for probes characterization, selection, and matching, as well as post-assembly calibrations), would still allow to achieve (option with compensated trapezoids available at two pins of the connector) ±0.5 degrees overall accuracy in the range -30°C < T < 90°C.
The developed product is an Absolute Angular Encoder, including Signal Conditioning Electronics, suitable for the space environment, characterized by the following features:
The INCAS product aims at satisfying these requirements by means of a contact-less sensor. This is the main characteristic that differentiates the INCAS approach with respect to the current used potentiometers, giving a more robust and reliable solution, especially over long life missions. The low cost approach of this product is intended to provide also a convenient alternative to more expensive sensors for this class of performance.
The principle of operation relies in generating a magnetic field which is, thanks to a specific geometry, a function of the angular position only. By means of a feedback loop controlling the output for a common magnetic field to two probes, any common drift or degradation of the magnetic field such as permanent magnet properties, or probes characteristics is automatically self-compensated.
The dedicated electronics is fully redundant and all the parts for the Engineering Model prototype, as well as for the Qualification Model foreseen in the next phase under ARTES 3-4, are ITAR-free.
The activity has been conducted by investigating and characterizing two different hall probe sensors, by completely designing and manufacturing both the mechanical structure and the electronic circuits (fully redundant) and also by a complete performance characterization campaign including vibration, EMC and thermal vacuum.
The criticalities related to the metrological performance of the sensor have been identified and solutions, suggestions and improvements have been identified. They include among others hall probes characterization, magnetic field sensitivity, electronic circuitry influence.
The tests performed showed very promising performance of the sensor with characteristics even beyond the requirements, and together with the criticalities identified will permit its optimization in the following phases of the development. Some of the optimizations identified have been preliminarily tested showing significant improvements in the performance.
The duration of the overall project was 24 months. Four phases were defined to develop the new INCAS EM sensor:
The project was completed in February 2013. The results of the environmental test campaign and the project main outcomes were published at the Mechanisms Final Presentation Days in ESTEC on 01/02/2013.