Postition Pressure Switches
Installation, Care, and Diagnostics
By Mike Weaver
Electromechanical switches — from the common toggle and push button switches to the more advanced position (limit) and pressure switches — are a mission-critical component that every aircraft relies upon to maintain its airworthiness. Toggle or button switches are used to activate or deactivate systems within an aircraft and are usually found in the relatively benign environment of the cockpit or cabin. Pressure and position switches are used to sense various operating conditions in order to provide feedback to the cockpit or an aircraft control system. Due to their locations, these switches must endure some of the toughest environmental conditions on the aircraft such as high and low temperature extremes, high vibration, and exposure to FOD, as well as dust, salt spray, and de-icing and hydraulic fluids.
Consider this typical switch application. Icing conditions can be a dangerous situation for aircraft. In this case, an electromechanical switch, or in some cases, several switches, work behind the scenes to ensure the flawless performance and initial activation of the aircraft’s de-icing system. Pressure switches sense the pressure drops caused by the build-up of ice and send a signal to the pilot and on-board computers to activate the anti-ice system, thus reducing the danger posed by the ice build-up. Additional uses of electromechanical pressure and position switches include, but are not limited to; position indication of slats and flaps, landing gear, bleed air valves, and thrust-reverser systems.They are also used to indicate fuel pressure and oil level.
For purposes of this article, only the operation of pressure and position switches will be reviewed. However, other types, like temperature and level switches, also exist. These switch types have similar internal switching mechanisms but differ in the way they are externally actuated.
There are distinct differences between switches that react to pressure and those designed to give position feedback. Pressure switches are designed to react to an external pressure applied to an actuator surface (like a plunger, snap-disc, bellows, or diaphragm) by a medium such as air, oil, water, hydraulic fluid, fuel, oxygen or other gas. Pressure switches are physically connected to the operating medium by ports. Position switches (also known as limit switches) utilize an external actuation mechanism, such as an arm or plunger, to activate the internal switch. The switch arm or plunger in the application is activated by a cam, plate, roller, or plunger located on the aircraft or aircraft subsystem.
Snap acting switches
In both the pressure switch and position switch, the external activation mechanism then interfaces with some type of internal switch mechanism. The internal action of the switch can differ depending upon the needs of the application and the switch manu-
facturer. Most precision switches used for aircraft applications utilize a self-energized, snap acting blade to open and close circuits. When the actuator reaches a pre-determined operating point, the common contact accelerates away from the normally closed contact and comes to rest on the normally open contact. Because this all happens within two milliseconds, snap acting switches are well-suited to applications using inductive loads. The snap action ensures that the blade is always loaded against the opposing contacts. These loading forces are high enough to provide vibration resistance and minimize contact bounce. Since they activate immediately upon reaching the operating point, they are not sensitive to the rate of actuator movement. A snap acting switch works well for applications in which activation is needed after a slow building change reaches a certain point — such as a pressure drop in a filter bypass or anti-ice system.
A less complex direct acting switch can be used in applications where the rate of change in the medium is very rapid – as is the case with a solenoid valve plunger. Direct acting switches use blades or wires driven directly by the actuation system. Since they do not "snap," the actuator must move rapidly to ensure contact transfer within the required time.
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