Glossary of Terms: D

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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.
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.
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.
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.
Dead Band
A region selected around the setpoint where proportional control is withheld. This is usually between the heating and cooling proportional bands.
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).
A term commonly used instead of dead time.
To decrease a rating suitable for normal conditions, according to guideline specified for different conditions.
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.
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.
Deutsche Industrial Norm (DIN)
A German agency that sets engineering and dimensional standards that now has worldwide recognition.
Any departure from a desired or expected process value. Difference between setpoint and value of the controlled variable at any instant.
The membrane of material that remains after etching a cavity into the silicon sensing chip. changes in input pressure cause the diaphragm to deflect.
Nonconductor of direct electric current.
Dielectric Breakdown
Rupture of insulation material when the electric stress exceeds the dielectric strength of the material.
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.
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.
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.
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.
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.
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.
Differential Travel
Tthe distance from the operating point to the release point.
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.
Diffuse Sensing
Photo electric sensing in which the light is reflected by the object being detected - also known as proximity sensing.
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.
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.
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.
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.
Digital Circuit
A circuit that has only two stable states, operating in the manner of a switch; that is, either On or Off.
Digital Control Programmer
Executes control (setpoint programming) of process temperature, pressure, flow, rotation speed, and other variables.
Digital Control System (DCS)
Refers to larger digital control systems.
Digital Filter
Algorithm that reduces undesirable frequencies in the signal.
Digital Input
Information or data in digital form transferred or to be transferred from an external device into a computer or individual device.
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).
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.
Direct Acting Control
Control action where the output increases as the process variable increases.
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.
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.
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.
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.
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.
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.
Undesired change in a variable applied to a system that tends to affect adversely the value of a controlled variable.
Dominant Dead Time Process
If the dead time is larger than the lag time the process is a dominant dead time process.
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.
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.
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.
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.
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.
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.
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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