Design guidelines for automotive fuel level sensors

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Design guidelines for automotive fuel level sensors

Smith, E.F. / Ireland, H.W.

This paper chronicles the design improvements of automotive fuel level sensors from the early wire wound versions to today's more robust thick film ink systems discussing potential failure modes, techniques to reduce noise, and increase of wear life. Data are provided regarding changes in the circuit layout, ink compositions, and contact materials. Special consideration is given to the adverse effects associated with the reactive sulfur prevalent in today's fuels. With the proper design approaches, thick film potentiometric fuel level sensors provide an excellent balance between cost and performance. Current designs using Paliney 6 contacts riding against Ag-Pd inks have been shown to provide cyclic lifetimes in excess of 5 million rotations. In an effort to develop a set of design guidelines, this paper reviews the evolution of the potentiometric fuel level sensor, identifies most of the known failure modes, and recommends following potential solutions: 1. Use thick film ceramic inks fired on an alumina substrate (at ca. 800 deg. C), 2. operate segmented shunts printed from a less abrasive, conductive thick film ink, 3. provide a minimum 25% Pd level for Ag-Pd conductive thick film inks reducing smearing and increasing tarnish resistance, 4. keep number and size of any surface glass particles in the conductive ink small for long life, 5. operate dual trace circuit patterns allowing for noble metals at all sliding interfaces, 6. employ a sliding contact material with a minimum hardness of 300 Hk in order to resist excessive abrasive wear from the inks, 7. use materials producing thinner sulfide films for a more robust design regarding noise related problems, and 8. therefore allowing for lower gram forces and reduced wear debris by requiring less contact abrasion to clean the contact surfaces, 9. apply Paliney 6 showing highest resistance to sulfide film formation of all the potential contact materials tested, 10. take multi-fingered (bifurcated) contact designs reducing the sensitivity to surface films thus allowing for the use of lower gram forces and, 11. prevent the contact from sliding against the alumina substrate by a large radius providing the sliding contact to be connected with two adjacent segments of the conductive ink.