Included in this category are all types of sensors, pilot devices, logic elements, programmable controllers, monitoring equipment, and instrumentation.
Modern commercial, industrial, and institutional facilities have sophisticated management systems that control the environment and/or specific processes within the facility. Critical to the functioning of these systems are sensors, pilot devices, logic elements, and monitoring equipment. These items provide the information necessary for management systems to control the environment and/or process efficiently.
Limit switches provided e position information to control-circuit logic. Most limit switches are enclosed and rated as general purpose or heavy duty. The enclosures are oiltight, watertight, dripproof, and explosionproof.
Lever-arm-type limit switches are either spring-return or maintained-contact types. The spring-return lever arm returns to a specified position once it is released. In maintained-contact types, the level arm is tripped to, and remains in, a set position when released. Lever arms come in fixed or adjustable lengths.
When contacting the object is not desirable or feasible, photoelectric switches can detect its presence by using a light source. Two types are available: one in which the sender and receiver of the light are separate units, and one in which they are combined. Light sources can be miniature incandescent, LED, infrared, or laser.
Proximity switches have basically the same use as the photoelectric types. These switches consist of a sensing oscillator, a solid-state amplifier, and a switching device, all contained in a single package. They are available in cylindrical, rectangular, vane, or ring types, inductive (for metal detection) or capacitive (for glass, plastic, or wood detection).
The device used to measure speed is the centrifugal switch. Its electrical contacts are operated by spring-loaded weights whose positions depend upon the centrifugal force exerted. Tachometers are especially applicable when speed readout, closed-loop feedback, alarms, and overspeed underspeed monitoring are needed.
Level, pressure, temperature, flow, and other types of switches are critical to decision making by programmable logic controllers (PLCs), computers, and any other logic devices that can be used. Many are used in motor control circuits and others, like thermostats, are widely applied.
Pushbutton-type pilot devices include flush types, mushroom-head, rocker-arm, joystick, and other variations. Locking devices, shrouds, maintained interlocks, etc., are available. Lines include standard- and miniature-sized round, square, and segmented-style actuators.
Selector switches come in a variety of operators, including wing lever, key, or coin-slot types, and are generally of the maintained or spring-return types. Rotary switches are more specialized versions and can be individuallymounted in their own enclosure or can be panelmounted. Escutcheonplate markings, type of handle, and contact-operating sequence must be specified.
Relays multiply the number of contacts available to operate within a control circuit. Electromechanical relays are typically rated at either 300V or 600V Solid-state relays with approximately the same characteristics, ratings, and size are also available.
Time-delay relays (both electromechanical and solid-state) are available with the time delay on energization (ON delay) or deenergization (OFF delay). This same function can be accomplished by timers and pulse counters.
Monitoring equipment PLCs
Panel mounted ammeters, voltmeters, wattmeters, and similar devices play roles in monitoring status. They can be relatively simple analog equipment, or solidstate units with digital readouts. Contact-making types can be selected when it is necessary to detect that a low or high limit is exceeded.
PLCs are rapidly replacing relay logic in control systems. All sensors and interface devices become inputs and outputs to the PLC. Based on the status of inputs, the PLC determines the appropriate output response.