Page 123 - Programmable Logic Controllers, Fifth Edition

NEMA symbols
Symbols
NO contact
Programmable thermostat
NO contact NC contact

IEC symbols NC contact

Figure 6-16 Float type level switch.
Source: Courtesy Dwyer Instruments.
NO contact NC contact
open or close piping solenoid valves to control fluids. The
float switch shown in Figure 6-16 is a type of level switch.
Figure 6-14 Temperature switch.
Source: Photo courtesy Honeywell, www.honeywell.com. This switch is weighted so that as the liquid rises the switch
floats and turns upside down, actuating its internal contacts.
Temperature switches open or close when a designated
temperature is reached. Industrial applications for these 6.6 Sensors
devices include maintaining the desired temperature Sensors are used for detecting, and often measuring, the
range of air, gases, liquids, or solids. magnitude of something. They convert mechanical, mag-
Pressure switches, such as that shown in Figure 6-15, netic, thermal, optical, and chemical variations into elec-
are used to control the pressure of liquids and gases. tric voltages and currents. Sensors are usually categorized
Although many different types are available, they are by what they measure, and they play an important role in
all basically designed to actuate (open or close) their modern manufacturing process control.
contacts when a specified pressure is reached. Pres-
sure switches can be pneumatically (air) or hydrauli- Proximity Sensor
cally (liquid) operated switches. Generally, bellows or Pilot control devices have limited current handling capac-
a diaphragm presses up against a small microswitch and ity and are used to control current to a secondary device,
causes it to open or close. such as a contactor coil, which in turn can be used to switch
Level switches are used to sense liquid levels in ves- heavier load currents. Proximity sensors or switches, such
sels and provide automatic control for motors that transfer as that shown in Figure 6-17, are pilot devices that detect
liquids from sumps or into tanks. They are also used to the presence of an object (usually called the target) without
physical contact. These solid-state electronic devices are
NEMA symbols for completely encapsulated to protect against excessive vibra-
pressure switch contacts tion, liquids, chemicals, and corrosive agents found in the
NO industrial environment. Proximity sensors are used when:
contact
• The object being detected is too small, lightweight,
or soft to operate a mechanical switch.
NC
contact • Rapid response and high switching rates are required,
as in counting or ejection control applications.

IEC symbols for • An object has to be sensed through nonmetallic bar-
pressure switch contacts riers such as glass, plastic, and paper cartons.

NO Normally open (NO)
contact sensor symbols

NC
contact
IEC NEMA

Figure 6-15 Pressure switch. Figure 6-17 Proximity sensor.
Source: Photo courtesy Honeywell, www.honeywell.com. Source: Photo courtesy Turck, Inc., www.turck.com.

104 Chapter 6 Developing Fundamental PLC Wiring Diagrams and Ladder Logic Programs

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