A circuit in which current has two or more paths to follow. Two electrical
elements are in parallel if both terminals of both elements are electrically
Parallel wiring (wired OR): Sensors
with identical outputs, PNP or NPN, may be wired in parallel
as long as they are all using the same power supply. The
maximum number of sensors depends on the load current and
the internal pull up / pull down resistors; typically 3 mA
per sensor. The sum of the load currents must be lower than
the maximum sensor switching current of the power supply.
Serial wiring: Relay output sensors may
be connected in serial. In the case of sensors with
electronic outputs, it is not permissible to connect the
feed of one sensor via the output of a preceding sensor, and
thus implement an AND operation. Since a sensor represents a
large load capacity, the short-circuit protection is
Polybutylene Terephtalate. A polymer. PBT has the
strength and wear resistance of general purpose Nylon, and a
higher continuous use temperature (240°F)
Personal Computer Memory Card International Association. Industry group that
developed the specification for credit card-sized plug-in cards for laptop
A communication between networked devices in which any device can initiate
A measure of how acidic or basic a solution is. pH is often a process
variable to control.
The time-based relationship between alternating current
cycles and a fixed reference point. In electricity, it is
usually expressed in angular degrees. It describes the
relationships of voltage and current of two or more
The difference in phase at the frequency where the combined process and
controller amplitude ratio is 0.
A mode of power control in silicon controlled rectifiers
(SCRs). Phase-angle firing varies the point at which the SCR
switches voltage inside the AC Sine wave.
Sensing technique using pulsed light in either diffuse,
through beam or retro-reflective modes to detect objects.
A resistive, bulk effect type of photosensor, the type used when it is
desirable to wire several photoreceivers in series or in parallel. The
resistance decreases with increasing light intensity.
A unit consisting of photosensor, focusing lens, and protective enclosure.
A light sensitive portion of a photoelectric control that converts a light
signal into an electrical signal
A type of photosensor. Typically used where speed of response is important
or ambient temperature variations are great.
PID (Proportional, Integral, Derivative) Control
A three-mode control action in which the controller has
proportioning, integral (reset), and derivative (rate)
action. Proportional action dampens the system response;
integral corrects the offset; and derivative prevents over-
and undershoot. PID software, for example, compares an
analog input value with a setpoint and if there's a
discrepancy outputs an appropriate analog or digital control
value, according to calculations.
Controller including Proportional, Integrating and Derivative controller
A change in resistance in a semiconductor, caused by an applied stress to
Programmable Logic Controller. These computers replace relay logic and
usually have PID controllers built into them. PLCs are very fast at processing
discrete signals (like a switch condition). The most popular PLC manufacturers
are Allen Bradley, GE, and Siemens (or TI).
A diode consisting of one N-type region and one P-type region.
PNP Current Switching
An output which supplies a path to positive voltage. The
load is between the output and ground.
A PNP output acts as an electronic switch from +VS to
the output. The load is switched to 0 Volt. If the output is
switched (if "light" is received in a light operate mode or
"no light" in a dark operate mode) a current flows from +VS
via the reverse polarity protection diode and the output
transistor, through the load to 0 Volt. The voltage between
+VS and the output is called the voltage drop. The voltage
protection prevents damage to the output if spikes occur.
Polarized Photoelectric Controls
Controls that emit a visible LED beam and use a special lens which filters
the beam of light so that it is projected in one plane only. The control
responds only to the de-polarized reflected light from corner-cube type
reflectors (FE-RR1) or special polarized reflective tape.
The ability of a lens to pass light in only one defined
angular plane. Sensors fitted with polarization filters are
able to detect highly reflective targets without false
(1) A relay contact. (2) The number of completely separate circuits that can
pass through a switch at one time. A single pole switch can control only one
circuit at a time. A double pole switch can control two independent circuits
(such as a 120 volt AC heater and a 6 volt DC lamp) at the same time. The number
of poles is completely independent of the number of throws and number of breaks.
Polymers Between Contacts
Compounds having long-chain molecular structure, formed from simple organic
contaminants on contacts, under the influence of contact wipe and the catalytic
effect of the contact material.
The external connector on a device.
Positive Temperature Coefficient
An increase in resistance due to an increase in temperature.
A variable adjustment used to fine tune amplification or sensing distance.
Device consisting of a resistive element with a terminal at each end, and a
third terminal connected to wiper contact. As wiper moves along the element, it
changes resistance in each leg to change electrical input or output
Units = Watts/milliwatts (DC) or Volt-Amps (AC). The amount of power that is
consumed and converted to heat in normal operation. Supply Voltage (max) x
Power Up Mode Recall
Determines mode and setpoint used when it restarts after power loss.
Shorthand term for the rate of pounds of mass per hour. Actual engineering
unit is expressed as lbm/hr. Steam flow is commonly expressed in PPH.
The closeness of a setting, indication, calibration, or
control of a measurement device to the actual value of the
quantity being measured. Usually expressed as a percentage
of full scale.
Precision Snap-Acting Switch
An electromechanical switch having predetermined and accurately controlled
characteristics and having a spring loaded quick make and break contact action.
Force applied to, or distributed over a surface expressed in terms of force
to area ratio.
Pressure measured relative to absolute zero pressure (perfect vacuum). The
output of the absolute sensor will change as a result of barometric pressure
change. Thus it can be used as a barometer.
Pressure between two points of measurement.
Pressure measured relative to ambient pressure. The output of a gage
pressure sensor, contrasted to an absolute pressure sensor, does not change with
a change of barometric pressure.
Pressure relative to which a differential sensor measures pressure. Ambient
pressure is a specific case of reference pressure.
Average pressure on the inputs of a differential pressure sensor (sometimes
referred to as common mode or working pressure).
Controller that monitors pressure of steam, air, gases, or liquids, and
keeps pressure within predetermined limits. Can operate as pressure switch
(ON-OFF), or proportioning controller.
Pressure limits over which the pressure sensor is calibrated or specified.
A device that converts an input pressure into an electrical output.
The distance through which the plunger moves when traveled from the free
position to the operating point.
Manufacturing operations that use energy measurable by some quality, such as
temperature, pressure, or flow, to produce changes in quality or quantity of
material or energy.
Process Value (PV)
Measurable attribute whose value changes with changes in prevailing
conditions. The actual value in the control loop, temperature, pressure, flow,
composition, pH, etc. aka Process Variable.
Master-slave bus system with two-wire line. Linear bus
with EIA RS 485 hardware interface. Standard Profibus-DP
protocol specified for encoders.
The "P" of PID controllers. With proportional band, the controller output is
proportional to the error or a change in process variable. Proportional Band =
100/Gain. Percent of range of measured variable for which a proportional
controller produces a 100% change in output. Reciprocal of Gain.
Control mode in which there is a continual linear relationship between
deviation in the controller, signal of the controller, and position of the final
The area around setpoint where the control output is
neither fully On nor fully Off for the entire time cycle.
A set of rules used in data communications.
A sensor with the ability to detect the presence of a metal target, within a
specified range, and without making physical contact.
A measure of pressure, in pounds per square inch. If a one pound weight was
placed on a smooth, hard surface, and that one pound weight had a 1" square area
on the bearing surface, then the pressure exerted on the surface would be
exactly 1 psi.
Pounds of force per square inch absolute. Signifies the measure of pressure
relative to an absolute vacuum having a pressure of 0 psia.
Signifies the difference in pressure between two points. If you have a
balloon inflated with 2 psig of air pressure, the difference in pressure between
the air inside the balloon and ambient atmospheric air pressure would be 2 psid.
Pounds of force per square inch gauge. Signifies the measure of pressure
relative to atmospheric pressure which is commonly accepted to be 14.696 psia at
sea level conditions. The measure is always assumed to be against ambient
atmospheric conditions where the measurement was actually made.
Resistor connected across the output of a device or circuit to hold the
output equal to or less than the zero input level. Also used to lower output
impedance of digital or analog devices. Usually connected to a negative voltage
Resistor connected across the output of a device or circuit to hold the
output voltage equal to or greater than the input transition level of a digital
device. Usually connected to the positive voltage or plus supply.
A temporary change in voltage of any length. A momentary sharp change in
current, voltage, or other quantity that is normally constant. A pulse is
characterized by a rise and fall, and has a finite duration.
A number of light and dark segments are put onto a glass
pulse disk. In the encoder, these segments are scanned by
the light beam and thus determine the possible resolution.
The tolerance of the pulse/period ratio is ±10%. Values
outside this range are specially marked on the product data
pages. This tight tolerance is also valid between the two
channels CHA and CHB, which are 90 degrees out of phase.
Therefore the sense of rotation can be clearly defined and
a quadrature of the signal can be attained.
A signal modification that produces output independently of input signal
duration. Pulse duration (dwell) is usually adjustable. Also referred to as
one-shot logic. Pulsed logic may be immediate or delayed.