Sensors and Switches (Leaving Cert Engineering): Flashcards

Role of sensors & switches in engineering systems

Interface between physical world and electronic control systems

Mechanism used by mechanical switches

Cam and plunger mechanism

Normally Closed (NC) switch behaviour when activated

Moves from closed position to open

Normally Open (NO) switch behaviour when activated

Moves from open position to closed

Mechanical switch advantage for extreme conditions

High temperature tolerance

Safety feature of NC mechanical switches in power failure

Fail to safe operation

What photoelectric sensors detect

Changes in light intensity

Most common photoelectric sensor mode in linear actuators

Through-beam mode

How proximity sensors detect targets

Emit electromagnetic field to detect metal targets

Typical sensing distance limit for proximity sensors

50mm or less

How reed switches are activated

Magnetic activation (magnetic field)

Why reed switches suit hazardous environments

Intrinsically safe due to hermetically sealed glass envelope

Type of device Hall sensors are

Solid-state devices

Hall sensor lifetime advantage

Virtually infinite lifetime (no moving parts)

Priority order for sensor selection

1. Reliability 2. Operating environment 3. Performance vs cost