PAGE CONTENTS
Objectives
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).
Challenges
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 4×90° = 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.
Probes matching
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.
Plan
The duration of the overall project was 24 months. Four phases were defined to develop the new INCAS EM sensor:
- Requirements assessment/consolidation. This activity has been performed considering the heritage of ITI-CAS, in particular with regard to magnetic field configuration, and the users requirements. In fact, a strong collaboration with the potential users that have already expressed their interested in INCAS product has been performed during this activity, in particular regarding the mechanical and electrical interfaces.
- Design (mechanical/magnetic part, signal conditioning electronic part, test plan).
- Manufacturing (including test jig set-up).
- Testing. The EMs have been manufactured with extended-range components, and subject to preliminary environmental tests (Thermal-vacuum, Vibration, EMC), to gain confidence in the next qualification phase.
Current Status
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.
