eBook: Automating Manufacturing Systems; with PLCs


A.16 P

parallel communication - bits are passed in parallel conductors, thus increasing the transmission rates dramatically.
parallel design process - evaluates all aspects of the design simultaneously in each iteration. The design itself is sent to all analysis modules including manufacturability, inspectibility, and engineering analysis modules; redesign decisions are based on all results at once.
parallel programs - theoretically, these computer programs do more than one thing simultaneously.
parity - a parity bit is often added to bytes for error detection purposes. The two typical parity methods are even and odd. Even parity bits are set when an even number of bits are present in the transmitted data (often 1 byte = 8 bits).
particle velocity - the instantaneous velocity of a single molecule.
Pascal - a basic unit of pressure
Pascal's law - any force applied to a fluid will be transmitted through the fluid and act on all enclosing surfaces.
PC (Programmable Controller) - also called PLC.
PCB (Printed Circuit Board) - alternate layers of insulating materials, with wire layout patterns are built up (sometimes with several layers). Holes thought the layers are used to connect the conductors to each other, and components inserted into the boards and soldered in place.
PDES (Product Data Exchange using Step) - a new product design method that has attempted to include all needed information for all stages of a products life, including full solids modeling, tolerances, etc.
peak level - the maximum pressure level for a cyclic variation
peak-to-peak - the distance between the top and bottom of a sinusoidal variation.
peer-to-peer - a communications form where connected devices to both read and write messages at any time. This is opposed to a master slave arrangement.
performance variables - are parameters which define the operation of the part. Performance variables are used by the designer to measure whether the part will perform satisfactorily.
period - the time for a repeating pattern to go from beginning to end.
peripheral - devices added to computers for additional I/O.
permanent magnet - a magnet that retains a magnetic field when the original magnetizing force is removed.
petri-net - an enhanced state space diagram that allows concurrent execution flows.
pH - a scale for determining is a solution is an acid or a base. 0-7 is acid, 7-4 is a base.
photocell - a device that will convert photons to electrical energy.
photoconductive cell - a device that has a resistance that will change as the number of incident photons changes.
photoelectric cell - a device that will convert photons to electrical energy.
photon - a single unit of light. Light is electromagnetic energy emitted as an electron orbit decays.
physical layer - an OSI network model layer.
PID (Proportional Integral Derivative) - a linear feedback control scheme that has gained popularity because of it's relative simplicity.
piezoelectric - a material (crystals/ceramics) that will generate a charge when a force is applied. A common transducer material.
ping - an internet utility that makes a simple connection to a remote machine to see if it is reachable, and if it is operating.
pink noise - noise that has the same amount of energy for each octave.
piston - it will move inside a cylinder to convert a pressure to a mechanical motion or vice versa.
pitch - a perceptual term for describing frequency. Low pitch means low frequency, high pitch means a higher frequency.
pitot tube - a tube that is placed in a flow stream to measure flow pressure.
pixels - are picture elements in a digitally generated and displayed picture. A pixel is the smallest addressable dot on the display device.
PLA (Programmable Logic Array) - an integrated circuit that can be programmed to perform different logic functions.
plane sound wave - the sound wave lies on a plane, not on a sphere.
PLC (Programmable Logic Controller) - A rugged computer designs for control on the factory floor.
pneumatics - a technique for control and actuation that uses air or gases.
PNP - a bipolar junction transistor type. When referring to switching, these can be used to source current from a voltage source.
poise - a unit of dynamic viscosity.
polling - various inputs are checked in sequence for waiting inputs.
port - 1. an undedicated connector that peripherals may be connected to. 2. a definable connection number for a machine, or a predefined value.
positive displacement pump - a pump that displaces a fixed volume of fluid.
positive logic - the normal method for logic implementation where 1 is a high voltage, and 0 is a low voltage.
potentiometer - displacement or rotation is measured by a change in resistance.
potting - a process where an area is filled with a material to seal it. An example is a sensor that is filled with epoxy to protect it from humidity.
power level - the power of a sound, relative to a reference level
power rating - this is generally the maximum power that a device can supply, or that it will require. Never exceed these values, as they may result in damaged equipment, fires, etc.
power supply - a device that converts power to a usable form. A typical type uses 115Vac and outputs a DC voltage to be used by circuitry.
PPP (Point-to-Point Protocol) - allows router to router or host to network connections over other synchronous and asynchronous connections. For example a modem connection can be used to connect to the internet using PPP.
presentation layer - an OSI network model layer.
pressure - a force that is distributed over some area. This can be applied to solids and gases.
pressure based flow meter - uses difference in fluid pressures to measure speeds.
pressure switch - activated above/below a preset pressure level.
prioritized control - control operations are chosen on the basic of priorities.
procedural language - a computer language where instructions happen one after the other in a clear sequence.
process - a purposeful set of steps for some purpose. In engineering a process is often a machine, but not necessarily.
processor - a loose term for the CPU.
program - a sequential set of computer instructions designed to perform some task.
programmable controller - another name for a PLC, it can also refer to a dedicated controller that uses a custom programming language.
PROM (Programmable Read Only Memory) -
protocol - conventions for communication to ensure compatibility between separated computers.
proximity sensor - a sensor that will detect the presence of a mass nearby without contact. These use a variety of physical techniques including capacitance and inductance.
pull-up resistor - this is used to normally pull a voltage on a line to a positive value. A switch/circuit can be used to pull it low. This is commonly needed in CMOS devices.
pulse - a brief change in a digital signal.
purge bubbling - a test to determine the pressure needed to force a gas into a liquid.
PVC - poly vinyl chloride - a tough plastic commonly used in electrical and other applications.
pyrometer - a device for measuring temperature

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