Process Control Terminology
from ONEhalf20


Process Control Terminology from ONEhalf20

Process Control Terminology from ONEhalf20

A

Absolute Pressure Sensor-a sensor which measures input pressure in relation to zero pressure (a total vacuum on one side of the diaphragm).

Actuator-mechanism of the switch or switch enclosure which operates the contacts.

Acceleration-the rate of change of velocity. Acceleration has two characteristics: magnitude and direction.

Accuracy- a comparison of the actual output signal of a device to the true value of the input pressure. The various errors (such as linearity, hysteresis, repeatability and temperature shift)* attributing to the accuracy of a device are usually expressed as a percent of full scale output (Span)

Alignment-placing the emitter (light source) and receiver (photoreceiver or reflector) so as to direct the maximum amount of light on the photosensor. At long distances, when the light beam has widened, the receiver should be centered in the beam to lessen the chance of the emitter and receiver drifting out of alignment due to vibration or shock.

Alternating Current (AC)-one that reverses at regularly recurring intervals of time and has alternately positive and negative values.

Ambient-in the area. 1) light-Light in the area of the photosensor, but not originating with the control light source. Ambient light can adversely affect non-modulated control operation, and should be screened, if possible, from the sensor; 2) Temperature-average temperature of surrounding medium such as water, air, or earth, into which the heat of the equipment is dissipated.

Analog Output-having the property of being continuously variable, as opposed to having discrete states.

AND Logic-an output is produced only when all inputs are present.

Analog Output- an electrical output from a sensor that changes proportionately with any change in input pressure.

Anode-the switch contact connected to the positive terminal of the power supply.

Aperture-most often an external cap (with a small opening) placed over the receiver lens to help detect small objects. It lets even a small object block enough light to be detected. Also, an internal aperture in most receivers reduces the effect of off-axis ambient light.

Arc-one of several kinds of visible electrical discharge between separated contacts of a switch. It is primarily a stream of electrons and is accompanied by incandescent metal vapor.

Attenuation-loss or reduction of beam intensity as a result of environmental factors, dust, humidity, steam etc.

Auto Zeroing Technique- a method used to automatically set the null point on a pressure sensor. This is usually done by using a microprocessor to open a solenoid valve at a predetermined time interval. This references atmospheric pressure to both sides of the pressure sensor chip. The microprocessor reads the output voltage and makes that the new null point. This method is used to eliminate errors due to null offset and null temperature shift.

Auxiliary Actuator-a mechanism, sold separately, to provide basic switches with easier means of operation and adjustment and adapt switches to different operating motions by supplying supplemental overtravel.

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B

Basic Switch-a self contained switching unit. It can be used alone, gang-mounted, built into assemblies or enclosed in metal housings.

Bifurcated Contact-a movable or stationary contact which is forked or divided to provide two pairs of mating contact surfaces connected in parallel, instead of a single pair of mating surfaces.

Bidirectional Differential Pressure Sensor-a differential pressure sensor allowing the greater input pressure to be applied to either pressure port.

B.F.S.L. (Best Fit Straight Line)-a method for defining linearity. A straight line placed on a sensor output curve such that half the data points lie above and half below that line. The method for determining B.F.S.L., is the sum of least squares.

Bridge Resistance- see Input Impedance and Output Impedance.

Break-to open an electrical circuit.

Break Distance-the minimum distance between separated mating contacts in their fully open position.

Burst pressure-the specified pressure which will rupture the sensing element but not the sensor case.

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C

Cascade-to combine logic circuitry to get more complex logic or timing control. (Inputs and outputs are wired in series.)

Calibration-a test during which known values of Measurand are applied to the device under test and corresponding output readings are recorded under specified conditions.

Calibration Curve-a graphical representation of the calibration record.

Calibration Cycle-the application of known values of Measurand and recording of corresponding Output readings over the full or the specified portion of the Range, in an ascending and descending direction.

Catalyst-a substance which changes the rate of a chemical reaction but is itself not changed. Switch contact material sometimes acts as a catalyst, accelerating the formation of polymers on the contact surface.

Cathode-the switch contact connected to the negative terminal of the power supply.

Characteristics-This term is used by MICRO SWITCH in a restricted sense and refers only to switch operating characteristics such as pretravel, operating force, etc.

Clean Air-ideal conditions. Climate controlled or sterile area.

Chip-a die (unpackaged semiconductor device) cut from a silicon wafer, incorporating semiconductor circuit elements such as resistors, diodes, transistors, and/or capacitors.

Compensation-procedure of providing a supplemental device, circuit, or special materials to conunteract known sources of error (e.g., ambient temperature change). Compensation is often related to temperature compensation only.

Complementary Output-both N.O. and N.C. outputs are available for use. A circuit that provides sink or source capability with a single input. Output that can be both light operated and dark operated. (Also known as 4-wire DC controls.)

Control-the complete system; sensor, amplifier, output.

Control Base-unit remote from sensor in which amplification and conditioning of the input signal takes place. Usually contains a power supply and an output device.

Convergent Beam-A variation of the diffuse scanning mode. A photoelectric control whose optical system is key to its operation. It simultaneously focuses and converges a very small, intense beam to a fixed-focal point in front of the control. The control is essentially blind a short distance before and beyond this focal point. Convergent beam scanning is used to detect the presence or absence of small objects while ignoring nearby background surfaces.

Convertible Output-output that can be wired either as Normally Open or Normally Closed, but not at the same time.

Corrective Factor-the mathematical factor that, when multiplied by the sensing distance of a given sensor, will adjust sensing distance for the different metals being used as targets.

Current-time value of movement of free electrons. One ampere equals one coulomb per second. Conventional reference is opposite to direction of actual electron movement.

Current Consumption-the amount of current required to power a sensor or control (excluding load). See supply current.

Current Sinking-an output type such that when it is On, current flow is from the load into the device's output, then to ground. Output is Normally High. The sensor "sinks" current from the load through the sensor to ground. The load is connected between the positive lead of the supply and the output lead of the sensor.

Current Sourcing-an output type such that when it is On, current flow is from the device into the load. Output is Normally Low. The sensor "sources" current to the load. The load is connected between the output lead and the negative ground lead of the supply.

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D

Dark Operated (D.O.)-control operating mode in which the output (load) is energized when the light is blocked (retro/thru scan) or object not present (diffuse), the photosensor is dark.

Dead Break-Imperfect snap action in which the normally closed circuit of the switch opens before the plunger reaches the operating point, or the normally open circuit opens before the plunger reaches the release point.

Dead Make-Imperfect snap action in which a switch fails to close its circuit when the plunger reaches the operating or release point.

Diaphragm-the membrane of material that remains after etching a cavity into the silicon sensing chip. changes in input pressure cause the diaphragm to deflect.

Dielectric-the term dielectric is almost synonymous with electrical insulation, which can be considered the applied form of the dielectric.

Dielectric Breakdown-rupture of insulation material when the electric stress exceeds the dielectric strength of the material.

Dielectric Strength-the maximum potential gradient that a material can withstand without rupture. As a material property it usually is calculated by dividing the breakdown voltage by the thickness of the material between a pair of test electrodes. The term often is applied to switches to mean the maximum voltage a switch can withstand between specified terminals or between terminals and ground without leakage current exceeding a specified value.

Differential Pressure Sensor-a sensor which is designed to accept simultaneously two independent pressure sources. The output is proportional to the pressure difference between the two sources.

Differential Travel-the distance from the operating point to the release point.

Diffusion-a thermochemical process whereby controlled impurities are introduced into the silicon to define the piezoresistors. Compared to ion implantation, it has two major disadvantages: 1) the maximum impurity concentration occurs at the surface of the silicon rendering it subject to surface contamination, and making it nearly impossible to produce buried piezoresistors; 2) control over impurity concentrations and levels is about one thousand times poorer than obtained with ion implantation.

Diffuse Scan-a reflective scanning technique in which reflection from a near-by non-shiny surface illuminates the photosensor in the receiver. Sometimes called proximity scan because of the required nearness of the light source and photosensor to reflecting surface. Also used to detect color contrast as in registration control.

Digital Circuit-a circuit that has only two stable states, operating in the manner of a switch; that is, it is either On or Off.

Digital Output-output that is of only two stable states, appearing in the manner of a switch; that is, it is either On or Off or High or Low (i.e., high voltage or low voltage).

Direct Current (DC)-a unidirectional current in which changes in value are so small that they may be neglected. As ordinarily used, the term designates a practically non-pulsating current.

Disable-to prevent the output despite an input signal. A wiring terminal for this purpose is provided on most MICRO SWITCH control bases. The disabling circuit may receive its signal from the current sinking output of a photoelectric logic card, or modulated LED control, or from an electromechanical limit switch, etc. Disabling is used to prevent false or unwanted signals from triggering the control.

Double Break Contacts-(Twin break). This breaks the circuit in two places. Referred to as form Z circuitry also.

Double-Pole Double Throw (DPDT)-switches which make and break two separate circuits. This circuit provides a normally open and normally closed contact for each pole.

Drift-an undesired change in output over a period of time, which is not a function of any input pressure change.

Drift of an Operating Characteristic-an inexact term referring in a general way to the degree of instability of a plunger force or travel characteristic under specified conditions and during a specified number of cycles of switch operation.

Dry Circuit-slang expression meaning a low energy circuit. Although many individuals and groups have assigned current and voltage values to "dry circuits" there is at present no general agreement as to what the values should be.

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E

Effective Sensing Distance-the difference between nominal sensing distance and the % manufacturing tolerance.

Electrical Life-life of a switch under a specified combination of electrical load, actuation, environment and criterion of failure. Synonymous with switch life.

Enable-the opposite of disable. To allow output in response to an input signal. We often speak of one light source-photoreceiver pair (the "gating" pair) enabling a second pair (the "inspect" pair).

Enclosed Switch-a basic switch unit (contact block) enclosed in a durable metal housing. The enclosure protects the switching unit, provides mounting means, and fitting for conduit connection.

End points-the Outputs at the specified upper and lower limits of the Range.

Environment-Proof Switch-a switch which is completely sealed to ensure constant operating characteristics. Sealing normally includes and "O" ring on actuator shaft and fused glass-to-metal terminal seals or complete potting and an elastomer plunger-case seal.

Erosion, Contact-a general loss of material from one or both working surfaces of a pair of mating contacts, as a result of switching an electrical load.

Error-the algebraic difference between the indicated value and the true value of the input pressure. Usually expressed in percent of full scale output, sometimes expressed in percent of the sensor output reading.

Error Band-the band of maximum deviations of the output values from a specified reference line or curve due to those causes attributable to the sensor. Usually expressed as "+ -% of full scale output." The error band should be specified as applicable over at least two calibration cycles, so as to include repeatability, and verified accordingly.

Excess Gain-the ratio of optical power available at a given emitter-to-receiver range to the minimum optical power required to trigger the receiver.

Excitation-the external source of energy (e.g., electrical voltage or current) applied to a sensor for its operation.

Explosion-Proof-having the ability to contain an explosion within the sensor or housing if it were to occur.

Explosion-Proof Switch-a UL listed switch capable of withstanding an internal explosion of a specified gas without ignition of surrounding gases.

Extreme Contamination-coal bins, residue on lens.

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F

Fall Time-a measure of the time required for the output voltage of a circuit to change from a high voltage level to a low voltage level, once a level change has started (90% to 10%).

False Pulse-an improper change of state of the output, usually associated with Turn-Off or Turn-On.

False Pulsing-circuitry designed to clamp output Off until the power supply has time to reach proper voltage level. Typically 200-500 msec.

False Pulse Protection-circuitry designed to clamp output Off until the power supply has time to reach proper voltage level. Typically 200-500 msec.

Fiber Optics-transparent fibers of glass or plastic used for conducting and guiding light energy. Fiber optics are used in photoelectrics as light pipes consisting of a bundle of small optical fibers (glass) or single strand (plastic) housed inside a flexible sheathing.

Force, Contact-the force holding closed contacts together.

Force Differential-the difference between the operating force and the release force.

Free Position of the Plunger-the position of the plunger when there is no external force other than gravity applied to it.

Frequency, Natural-the frequency of free (not forced) oscillations of the sensing element of a fully assembled sensor.

Frequency Output-an output in the form of frequency which varies as a function of the applied pressure.

Full Overtravel Force-the force required to depress the plunger of a switch to the full overtravel point.

Full Overtravel Point-that position of the plunger beyond which further overtravel would cause damage to the switch or actuator.

Full Scale Output (Span)-the algebraic difference between output curve end points (outputs at specified upper and lower output limits).

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G

Gage Factor-a measure of the ration of the relative change of resistance to the relative change in length of a resistive strain sensor (strain gage).

Gage Pressure-a form of differential pressure measurement in which atmospheric pressure is used as a reference.

Gravity Unit-one gravity unit (abbreviated g) is an acceleration of 32.2 feet per second per second.

Ground-a conducting path, intentional or accidental, between an electric circuit or equipment and the earth, or some large conducting body serving in place of the earth (a voltage reference).

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H

Hall Effect Technology-the description given to the following phenomena; when a semiconductor, through which a current is flowing, is placed in a magnetic field, a difference in potential (voltage) is generated between the two opposed edges of the conductor in the direction mutually perpendicular to both the field and the conductor. Typically used in sensing magnetic fields.

Hardwired-physically interconnected and intended for a specific purpose. Hardwired logic is essentially unalterable.

Hazardous Location-defined as an area in which flammable or combustible mixtures are present.

Head-On-a condition whereby the target approaches the sensing face of the proximity sensor with its center along the sensing face.

High Contamination-heavy particle laden air, extreme washdown environments, grain elevators.

Hermetically Sealed Switch-a switch completely sealed to provide constant operating characteristics. All junctures made with metal-to-metal or glass-to-metal fusion.

Hysteresis, Switching-the principle associated with sensors, such that the operate point is not at the same level as the release point. In solid state sensors, it is accomplished electrically. In mechanical switches, it results from the storing of potential energy before the transition occurs. Also known as differential, and is usually expressed as a percentage of the operate point (e.g. 3-15%).

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