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Honeywell solves the harsh-environment sensing problems.

Honeywell Solves the Harsh-Environment Sensing Problems

Many of the sensing tasks once performed by electromechanical switches are now handled by electronic proximity sensors, which detect the presence or absence of an object without direct contact. They are rugged, reliable and maintenance-free. The first trials of proximity sensors in aircraft were made in the early 1960s and today the majority of commercial aircraft built in the Western world incorporate this technology. This technology is now widely used in the military aircraft market and is also finding many applications in the naval and fighting vehicle and ordnance areas.

Honeywell has developed many types of proximity sensors to meet market requirements which basically fall into two areas, namely the "two-part proximity" switch product used by commercial aircraft manufacturers and the "single-part proximity" switch, which finds most of its applications in the Military market.

By definition the "two-part proximity" switch is divided into two parts, the passive inductive sensor and its associated, but separate signal conditioning electronics. These electronics can take the form of a single-channel printed circuit board for rack mounting or a hermetically sealed dual channel MSI hybrid. This type of system is used extensively by the commercial aviation industry because of the inherent reliability of the passive sensor, which can be mounted remotely due to its ruggedness in all of the severe environmental applications encountered on an aircraft, e.g. the landing gear system, engine thrust reversers, etc., whilst its associated and comparatively delicate electronics are mounted within the more benign environment found within the aircraft pressurized compartments. This type of system also offers the end-user significant maintenance advantages. The site of the sensor may well be difficult to reach, so parts mounted there should not require frequent service. The passive sensing element fully satisfies this requirement, as it is unlikely ever to need replacement. The electronics are mounted in a more accessible area where quick replacement in times of malfunction can easily be effected.

The two-part sensor is based on an inductive principle. In sensors of this kind a ferrous metal "target" alters the inductance of a coil. This change is readily detected and is reported as an unequivocal "target present" or "absent" signal. A singular feature of the design is the use of two matched coils arranged in a balanced bridge. One coil is exposed to the target and the other is shielded from it: a difference in indications between the two coils triggers the proximity signal. The advantage of this type of design is its ability to reject common mode errors and interference. Extraneous signals of various kinds, including those generated by electromagnetic radiation and by lightning can be picked up by the wires connecting the sensing element and electronics but will be rejected because of the differential mode of measurement.

In addition, because of its balanced bridge design the system automatically compensates for temperature variations. Due to the fact that the balanced bridge sensor reacts only to ferrous targets the two-part approach can be used in "look through" applications. A typical example of this would be in a landing gear leg where the position of an internal piston needs to be determined. In this type of application a balanced bridge sensor could be employed to sense the internal target position through a non-ferrous window or diaphragm.

The "single-part sensor" is designed to solve other problems. The product is designed to work on the ECKO (Eddy Current Killed Oscillator) principle and as such is capable of sensing non-ferromagnetic targets at much higher speeds of response. The single-part sensor is not passive; it contains all its own conditioning electronics. Because of this, its reliability is lower than its passive two-part counterpart. However, in applications that require lower cost, fast response and where space is limited, the single-part product comes into its own.

State-of-the-art hybrid technology enables the product to meet all of the environmental parameters of its two-part equivalent. Temperature, shock, vibration, EMC, EMP and EMI environments can be tolerated, but at a lower MTBF figure.

Both two-part and single-part devices can be supplied with integral BITE facilities which constantly monitor sensor performance and failure. In certain conditions analogue outputs can be derived that will also indicate the performance reliability of the target mechanism, possible wear and that maintenance adjustments are necessary.

What of the future? Developments in the power-by-light area are well underway and in this area Honeywell is in the forefront of the technology. Without doubt aircraft designers will require this technology approach within the foreseeable future. Its advantages in the interference and lightning strike area are obvious, not just to the commercial aircraft industry but also in military applications where EMP-tolerant products are a design requirement.

PHOTO : Honeywell is marketing two new types of electronic proximity sensors - a single sensor

PHOTO : model and a "two-part" type.
COPYRIGHT 1989 Armada International
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Copyright 1989, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

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Publication:Armada International
Date:Jun 1, 1989
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