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photoelectric control An electronic device that recognizes changes in light intensity and converts these changes into a change in electrical output state.
pilot circuit The portion of a control circuit that carries the controlling signal for a device which, in turn, controls the primary current.
PLC controller A programmable logic controller such as an Allen-Bradley programmable controller.
plugging Reversing either line voltage polarity or phase sequence on a motor so that the motor develops a counter-torque that exerts a retarding force to brake the motor.
position loop A feedback control loop that uses an encoder or resolver as a feedback device to indicate mechanical position.
position transducer An electronic device (e.g., encoder or resolver) that measures incremental or absolute position and converts this measurement into a feedback signal.
positive feedback A feedback signal from the output that is added to the input signal.
power Work done per unit of time. Measured in horsepower or watts: 1 hp = 33,000 ft-lb/min = 746 W.
power factor A measurement of the time phase difference between the voltage and current in an ac circuit. It is represented by the cosine of the angle of this phase difference. Power factor is the ratio of real power (in watts) to apparent power (in volt-amperes).
power supply A device that converts available power to a form that a system can use: usually converts ac power to dc power.
pressure switch A switch that is activated at a specific pressure.
printed circuit board A board (card) made up of a nonconductive layer sandwiched by conductive layers that are etched to form circuit connections between connection points where components can he mounted.
processor The decision-making and data storage sections of a programmable controller or computer.
programmable controller A solid-state control system that continually scans its user program. The controller has a user-programmable memory for storage of instructions to implement specific functions such as I/O control, logic, timing, counting, report generation, communications, arithmetic, and data file manipulation. A controller consists of a central processor unit, input/output interface, and memory. A controller is designed as an industrial control system. The program is generally in the form of ladder logic.
program mode On a programmable controller, a mode in which the ladder logic is not executed and all outputs are held off.
PROM Programmable read-only memory. A type of ROM that requires an electrical operation to store data. In use, bits or words are read on demand but never changed.
proportional band The range of values through which the output is proportional to the change of the input variable.
proportional, integral, derivative control (PID) A method of control that uses proportional (gain), integral (reset), and derivative (rate) to provide control of the response rate of the output signal.
protocol A set of conventions governing the format and timing of data between communications devices.
proximity switch/sensor A switch/sensor that is actuated when an actuating device is moved near it, without physical contact.
PWM Pulse-width modulation. A technique used to eliminate or reduce unwanted harmonic frequency when inverting dc voltage to sine wave ac.
This page was last updated: Wednesday, June 5, 2013 13:47