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- Damping
- Progressive reduction or suppression of oscillation in a device or system.
Built into electrical circuits and mechanical systems to prevent rapid or
excessive corrections that lead to instability or oscillatory conditions.
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- 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.
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- Dead Make
- Imperfect snap action in which a switch fails to close its circuit when the
plunger reaches the operating or release point.
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- Dead Time
- The amount of time it takes for the process variable to start changing after
changing output as a control valve, variable frequency drive etc.
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- Deadband
- Adjustable gap between operating ranges of output 1 and output 2 in which
neither (positive value) or both outputs operate (negative value). The range
through witch an input can be varied without initiating a response.
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- Dead Band
- A region selected around the setpoint where proportional
control is withheld. This is usually between the heating and
cooling proportional bands.
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- Degree
- The increments in a temperature scale, or the increments
of rotation of a dial. The location of a reference point in
electric or phase in a cycle, in mechanical or electrical
cyclic scales. (One cycle is equal to 360 degrees).
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- Delay
- A term commonly used instead of dead time.
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- Derating
- To decrease a rating suitable for normal conditions, according to guideline
specified for different conditions.
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- Derivative (D)
- The derivative - D - part of a PID controller. With derivative action the
controller output is proportional to the rate of change of the process variable
or process error.
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- Derivative Action
- Type of control-system action where a predetermined relation exists between
the position of the final control element and the derivative of the controlled
variable with respect to time.
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- Deutsche
Industrial Norm (DIN)
- A German agency that sets engineering and dimensional
standards that now has worldwide recognition.
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- Deviation
- Any departure from a desired or expected process value. Difference between
setpoint and value of the controlled variable at any instant.
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- 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.
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- Dielectric
- Nonconductor of direct electric current.
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- Dielectric Breakdown
- Rupture of insulation material when the electric stress exceeds the
dielectric strength of the material.
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- Dielectric Constant
- The ability of a dielectric to store electrical potential energy under the
influence of an electric field. This is measured by the ratio of capacitance of
a condenser with the material as dielectric to its capacitance with vacuum as
dielectric. The value is usually given relative to a vacuum/dry air the
dielectric constant or air is 1.
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- 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.
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- Differential
- Smallest range through which a controller variable must pass in order to
move final control element from one to the other of its two possible positions,
such as from ON to OFF.
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- Differential Control
- A control algorithm where the set point represents a
desired difference between two processes. The control then
manipulates the second process and holds it at the set value
relative to the first.
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- Differential Gap
- Smallest increment of change in controlled variable required to cause final
control element in a two-position control system to move from one position to
its alternative position.
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- Differential Pressure Sensor
- Sensor that is designed to accept simultaneously two independent pressure
sources. The output is proportional to the pressure difference between the two
sources.
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- Differential Travel
- Tthe distance from the operating point to the release point.
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- Diffuse Scan
- 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.
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- Diffuse Sensing
- Photo electric sensing in which the light is reflected
by the object being detected - also known as proximity
sensing.
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- Diffuse Sensors with Background Suppression
- These sensors with a triangulation-based background
suppression rely on the angle of light reflected from the
target, rather than intensity. Within the adjustable
distance, objects are recognized independently of color and
surface properties.
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- Diffuse Sensors with Foreground Suppression
- Similar to sensors with background suppression. Objects
are recognized independently of color and surface in front
of a defined background.
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- Diffuse Sensors with Intensity Difference
- Diffuse sensors with intensity difference The emitter
and receiver are in the same housing. The emitted red or
infrared light is reflected directly by the object.
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- Diffusion
- A thermochemical process where 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.
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- Digital Circuit
- A circuit that has only two stable states, operating in the manner of a
switch; that is, either On or Off.
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- Digital Control Programmer
- Executes control (setpoint programming) of process temperature, pressure,
flow, rotation speed, and other variables.
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- Digital Control System (DCS)
- Refers to larger digital control systems.
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- Digital Filter
- Algorithm that reduces undesirable frequencies in the signal.
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- Digital Input
- Information or data in digital form transferred or to be transferred from an
external device into a computer or individual device.
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- Digital Output
- Output that is of only two stable states, operating in the manner of a
switch; that is, either On or Off or High or Low (i.e., high voltage or low
voltage).
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- Digital Signal
- A discrete value at which an action is performed. A digital signal is a
binary signal with two distinct states - 1 or 0, often used as an on - off
indication.
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- Direct Acting Control
- Control action where the output increases as the process variable increases.
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- Direct Action
- An output control action in which an increase in the
process variable causes an increase in the output. Cooling
applications usually use direct action.
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- Direct Current (DC)
- Electrical current that flows in only one direction from
a source. As ordinarily used, the term designates a
practically non-pulsating current.
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- Direction of Rotation (F/R*)
- Forward/reverse counting direction input. The input is
HIGH when disconnected. F/R* HIGH means increasing output
data when the shaft is rotating to the right (cw). F/R* LOW
means increasing output data when the shaft is rotating to
the left (ccw), looking at the shaft in each case.
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- Discrete I/O
- On or off signals sent or received to the field. For example, a discrete
input would sense the position of a switch. A discrete output would turn on a
pump or light.
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- Discrete Logic
- Refers to digital or "On or Off" logic. For example, if the car door is open
and the key is in the ignition, then the bell rings.
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- Distance Sensors
- The compact laser distance sensors provide a precise
output signal which is proportional to the measured
distance. The optical principle is based on triangulation.
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- Disturbance
- Undesired change in a variable applied to a system that tends to affect
adversely the value of a controlled variable.
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- Dominant Dead Time Process
- If the dead time is larger than the lag time the process is a dominant dead
time process.
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- Dominant Lag Process
- Most processes consist of both dead time and lag. If the lag time is larger
than the dead time, the process is a dominant lag process. Most process plant
loops are dominant lag types. This includes most temperature, level, flow and
pressure loops.
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- Double-Pole Double Throw (DPDT)
- Switches that make and break two separate circuits. This circuit provides a
normally open and normally closed contact for each pole.
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- Drift
- A change in reading or value that occurs over long
periods. Changes in ambient temperature, component aging,
contamination, humidity and line voltage may contribute to
drift.
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- Dry Circuit
- 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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- Dual Element Sensor
- A sensor with two independent sensing elements. Usually
used to measure temperature gradients or provide redundancy
in a single point sensor assembly.
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- Duplex Control
- Control where two independent control elements share a common input signal
for operation of separate final control elements; both influence the value of
the controlled condition.
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- Dynamic Data Exchange (DDE)
- Old standard software method for communicating between applications under
Microsoft Windows. After Windows 3.1. DDE was replaced by OLE for process
control (OPC).
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