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Force Measurement Glossary

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A

A2LA:
The acronym for the American Association for Laboratory Accreditation. It is the only independent, non-profit, internationally known accreditation body in the US that offers a full range of comprehensive laboratory and laboratory-related accreditation services and training.

Accelerometer:
A device for measuring acceleration. The force causing the acceleration of a body is a product of the body mass m and the acceleration a. This principle allows the use of accelerometers to measure the force caused by vibrating masses under the influence of a dynamic force. See Figure 1 below.

Figure 1. Accelerometer

Accuracy:
The degree to which the output of the measuring instrument conforms to the actual value. In engineering, the accuracy of an instrument is the greatest horizontal deviation of the output responses from the calibration line. See Calibrating the Force Measuring System.

Active Transducer:
A transducer whose output power is derived from the applied force or energy. It does not require an external electrical excitation to operate.

Air Supply Regulator:
The opening on a pneumatic load cell that regulates the supply of air into the confined space of the load cell’s cylinder. It is located close to the bottom of the load cell, below the diaphragm and controlled by a valve.

Alloy:
A metal made from the combination of two or more metals to give better or the desired properties. Such properties might include greater strength, greater resistance to corrosion, thermoelectric properties etc. They form a majority of the raw materials used to build force measurement instruments. They include Nickel alloys, aluminum alloys, zinc-based alloys, copper alloys etc.

Ambient Conditions:
The weather or atmospheric conditions in the physical location of the force measurement application. They include temperature, humidity, and air pressure.

Amplifier:
The electronic device that increases the magnitude of the electrical output of a transducer. Most transducers produce output electrical signals that are very small in magnitude, usually in the order of millivolts. This small signal is insufficient for subsequent processes such as system control or even display. Therefore an amplifier must first boost the signal. The most commonly used and efficient amplifier for force measurement is the Instrumentation Amplifier. See Connecting a Force Sensor to a DAQ.

Analog Circuit:
An electronic circuit that handles continuously varying signals called analog signals, to convey information through changes in current, voltage or frequency. Prior to the digital age, instruments relied on this type of circuit. Modern instruments use both analog and digital circuits to give a more accurate result.

Analog-Digital-Converter (ADC):
An electronic system that converts the analog output of force transducers into digital signals. It consists of elements such as anti-aliasing filter circuits, sample and hold circuits, and digital encoders. Different types exist, such as the flash ADC, successive approximation ADC, and sigma-delta ADC.

Analog Instrument:
The category of measurement instruments that consist of an input transducer, signal conditioners and output elements (such as displays). The input transducer converts the analog signal into an equivalent analog electrical signal that is then amplified, filtered to eliminate electrical noise, and then displayed to indicate the quantity of the analog signal.

Angular Load Concentric:
Description of the orientation of a load being measured, where the applied load’s central axis is concentric with the primary axis of the measuring device at the load application point, but is at an angle to this primary axis. In other words, the axis of the load and the axis of the measuring device intersect at the load application point.

Angular Load Eccentric:
Description of the orientation of a load being measured, where the applied load’s central axis is eccentric with the primary axis of the measuring device at the load application point and at an angle with respect to the primary axis. In other words, the axis of the load and the axis of the measuring device are offset at the load application point.

ANSI:
The acronym for the American National Standards Institute. It is a non-profit private organization that oversees the development of voluntary consensus standards for products, services, processes, systems, and personnel in the US. Some of these standards apply to force measuring instruments such as load cells.

Anti-Aliasing Filter:
The electronic circuit that bandlimits the output of a transducer to frequencies below the Nyquist frequency. This prevents aliasing when the signal is converted back to its analog form by a DAC. Modern digital instruments incorporate this device.

Application Point:
A specific point or surface on a force measuring instrument where the load is applied. Most instruments are designed with a loading platform that equally distributes the measured load, enabling an accurate reading by the force measurement transducer.

Automation:
The creation and application of technology to monitor and control the production and delivery of products and services. This is one of the vast applications of a force measuring instrument.

Axial Alignment:
The alignment of the applied force or load along the transducer’s primary axis. This is very important in the design and installation of an accurate force measurement system. A good force measurement system that measures a single force component should be insensitive to orthogonal force components.

Axial Direction:
The direction parallel to the axis of a body.

Axial Load:
The load that acts parallel to the axis of the platform at its point of application. Figure 2 below depicts both an eccentric axial load (axis of the load and measuring device are parallel, not co-axial) and a concentric axial load (where the two axes are tangent at every point).

Figure 2. Axial Loads

Axial Thrust:
A propelling force along the axis of a body that pushes the body in an opposite direction. Examples of this force are the driving force generated by the propulsion system of an airplane, or forces moving a pogo stick upward.

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B

Bleed Valve:
A valve attached to the body of the diaphragm of a pneumatic load cell. Its movement with respect to the deflection of the diaphragm by the external load closes and opens the outlet nozzle of a pneumatic load cell.

Bonded Resistive Foil:
A thin piece of a metal strip attached to a structural member of an instrument or bonded to a non-conductive backing. This technology forms the basis of a strain gauge.

Bourdon Tube:
The tube that connects the outlet of a pressure transducer to a pressure gauge. It transmits the pressure (force) in the hydraulic load cell to the gauge.

Breaking Overload:
The amount of load over the rated capacity the load cell can withstand before it achieves structural failure. Also called “ultimate overload.”

Bulk Micromachining:
The MEM fabrication technique of a subtractive process where etching creates large grooves, pits, and channels on a substrate. Bulk micromachining is primarily used in producing pressure sensors and accelerometers.

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C

Calibration:
The comparison of the reading of the force measurement instrument to a known standard, and the maintenance of the measurement system from that standard. It is also called traceability and it plays a significant role in eliminating systematic errors.

Calibration Curve:
A graph showing how the analytical response of an instrument changes with an applied force or load. There are several methods used in plotting this curve, one of which is finding the line that best fits a scattered set of readings vs. applied force. For a strain gauge the curve is the output signal as a function of load.

Calibration Weights:
The sets of known weights used when calibrating a weighing instrument to ensure the accuracy of future results. The sets contain incremental weights measured to a known standard, with the output for each applied weight producing a reading that is plotted to create a calibration curve. See Calibration Weights at Tacuna Systems.

Capacitive Load Cell:
A load cell that causes the capacitance of the transducer to vary with the stimulus. This change in capacitance is converted into electrically measurable quantities such as voltage, which is in turn used by an output display to show the size of the measured force or load. This type of load cell demands electronic circuitry but its design is mechanically simple and also very sensitive to the input signal.

CE:
The abbreviation of the French phrase “Conformité Européene“ which means “European Conformity” in English. This marking shows that an instrument complies with the requirements of the European health, safety, and environmental protection legislation.

Center Of Mass:
A hypothetical point near or within a body where the entire mass of the body is assumed to be concentrated in order to model forces acting on the body as a whole. For example, the center of mass of a sphere of consistent density is at its geometric center.

Centrifugal Force:
A force that acts on a body moving in a circular trajectory. The force is directed radially, away from the center point of rotation of the body.

Certificate of Conformity:
A document granted by a competent authority to an original equipment manufacturer certifying its equipment, products or services meet strict specifications. These specifications include a set of regulatory, technical and safety requirements.

Combined Error:
The maximum deviation of the calibration curve from the linear, theoretical output of a load cell. See Measurement Uncertainty in Force Measurement.

Common Mode Rejection Ratio (CMRR):
In an operational amplifier, it is the ratio of the differential gain to the common mode gain. Simply put, it describes the ability of the operational amplifier to maximize the differential gain at the expense of the common mode gain. It is normally quoted in dB.

Common Mode Signals:
Identical signal components on the inverting and non-inverting inputs of an instrumentation amplifier. Common mode signals are noise signals appearing equally in both input terminals. The input wires are always closely twisted together to help cancel these noise effects. An ideal instrumentation amplifier will cancel out the common mode signals and amplify only the differential signals between the two wires.

Compression Force:
The physical force that acts inward on a body and causes it to become compacted. This is the type of force acting on a spring that compresses it, causing its physical dimensions and geometric shape to change.

Corrugated Plate Diaphragms:
Diaphragms that have greater deflections, better linear characteristic curve, and improved sensitivity compared to flat diaphragms. They are commonly used in high-pressure applications, especially in pneumatic load cells.

Creep:
The change in load cell output over time when it is subjected to a constant load at its application point, under unvarying ambient conditions. In other words if a known constant weight is placed on a load cell for an extended period of time, the output reading of the measurement system may change, or “creep” over time even though no weight is being added or removed and the environment is unchanged. The creep for a particular load cell is always specified in its datasheet, and is typically calculated with a mass at or near rated capacity.

Creep Recovery:
The change of output of a measurement system following a step decrease in the applied force, usually from the rated load capacity to zero-load. In other words, it is the change in the zero-load output of a load cell after the removal of the rated load that has been applied for a specific period of time. It is expressed as a percentage of the rated load capacity over a specific period of time.

Creep Return:
The difference between the initial output of a load cell and the output immediately after removal of a load (usually at or near rated capacity) that had been applied over an extended period of time under constant ambient conditions.

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D

Data Acquisition:
The process of measuring physical stimulus (e.g. force, load, weight), converting the result into digital signals and storing them in computer memory for later use or distribution. Modern force measurement instruments are digital data acquisition systems capable of storing their output in a computer or transmitting this output over a long distance to a remote display or storage device. See Figure 3 below.

Figure 3. Digital Data Acquisition System

Data Acquisition Rate:
Another name for sampling rate.

Data Filtering:
The removal of unwanted noise signals from the data being acquired to obtain an accurate reading. This corresponds to the “Signal Conditioning” block in the flow diagram shown in Figure 3 above.

Datasheet:
The document that shows the summary of the technical and performance characteristics of an instrument. Engineers use these specifications to ensure the instrument’s performance characteristics match the demands of the system incorporating its use. This document is always provided by the supplier or the OEM.

Dead Load:
The constant load in a structure due to the weight of its structural members or accessories. Also called “minimum dead load.”

Deflection:
The change in the length of a structural member of a load cell when subjected to a load. In a force-balance measuring instrument such as a hydraulic load cell, the load causes a deflection in the diaphragm, which in turn transmits the applied load to the fluid or gas in the confined space. In a strain gauge load cell, the deflection of the structural member causes strain in the strain gauge, changing the resistance in its circuitry to electrically transmit the applied load.

Deformation:
A change in the dimensions (length and area) of a material when subjected to a stress or strain.

Diaphragm:
In force measurement, a thin elastic plate that deforms under an applied load.

Dielectric:
The electrical insulator located between the two parallel metallic plates of a capacitive transducer.

Digital-Analog Converters (DAC):
Electronic circuits that convert digital signals to analog signals. The most common types of DACs are the binary-weighted DAC and the R-2R ladder DAC.

Digital Instrument:
The category of measurement instruments that produces or processes digital signals. Such instruments contain the major elements present in an analog instrument. However, they digitize the analog output from the input transducer using an analog to digital converter. This digital signal is then filtered, multiplexed and/or displayed using a digital signal processor. The instrument may also include a digital to analog converter to generate an analog output equivalent to the original input transducer output. Generally, modern-day instruments are digital.

Digital Pressure Transducer:
A pressure transducer whose output is in form of a digital signal. Most pressure transducers display the value of the measured quantity using pressure gauges; however digital types have digital displays, or they transmit output values over distances for remote monitoring.

Drift:
The random change in the output reading under constant loading conditions. It is similar to creep.

Ductility:
The ability of a metallic material to undergo plastic deformation when subjected to stress. Plastic deformation is often used as a technique to permanently record an applied force.

Dummy Gauge:
The passive temperature compensation resistor placed on the arm adjacent to the active arm of a quarter-bridge strain gauge setup. The dummy gauge eliminates the effects of temperature on the strain measurement, since identical temperature changes occur in both the active strain gauge and the dummy gauge.

Dynamic Force:
The type of force whose magnitude varies with time. To measure any instantaneous value, the force measurement instrument must have a frequency response sufficient enough to follow the changing force.

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E

Eccentric Loading:
A loading condition in which the center of mass of the applied load is not on the central or neutral axis of the load cell’s application point. It is also called offset loading and it produces a bending moment along with the axial stress generated by the eccentric load. See Figure 2 under “Axial Load“.

Eccentricity:
The distance between the central axis of a transducer and the center of mass of the load.

Elasticity:
The ability of a material to undergo deformation under the influence of a force (compressive or tensile) without losing its original shape and size after the force is removed. Concurrently, materials have an elastic limit or force which, once exceeded, causes the elastic material to suffer permanent deformation.

Electrical Excitation:
The external electrical energy supplied to the input terminals of a transducer to power its operations.

Electrical Output:
The electrical signal produced as the output of a transducer in response to an applied load.

Electrical Transducer:
A sensor device that detects an external phenomenon or stimulus (such as force, pressure, displacement, acceleration) and transforms this into an equivalent analog electrical signal. The output electrical signal is proportional to the applied load; therefore the load’s value is quantifiable.

Electromagnetic Compatability (EMC):
The ability of an instrument or a device to function satisfactorily in its electromagnetic environment without introducing intolerable electromagnetic disturbances to anything in that environment. In other words, for an instrument to be said to have an EMC, it must be able to operate efficiently without interfering with other systems within a specified electromagnetic environment.

Electromagnetic Interference (EMI):
A form of environmental pollution that can cause malfunctions of electrical and electronic devices, can prevent proper use of the radio frequency spectrum, can ignite flammable atmospheres and has a direct effect on the human tissue. EMC practices help to control various forms of EMI.

Electrostatic Sensitive Device:
Parts or components of a force measurement instrument that can be damaged by common electrostatic charges which build upon the instrument operators, tools and other types of conductors. It is abbreviated as ESD. (Note that Electrostatic Discharge is also abbreviated ESD, defined below.)

Environmental Conditions:
Environmental factors that influence the operations of force measurement instruments. They are also called ambient conditions. They include the atmospheric pressure, ambient temperature, and humidity.

Error:
The difference between the actual value of a force, load or weight and the true value. In instrumentation, an error is always expressed as a percentage of the full-scale output. Measurement errors consist of systematic errors and random errors.

Electrostatic Discharge (ESD):
The sudden flow of electricity between two materials at different potentials caused by the imbalance of electrons between the two. The acronym can also mean electrostatic sensitive device, defined above.

Etching:
The process of removing parts of a thin film deposited on a substrate material. Etching is one of the steps involved in the production of MEMS such as strain gauges. There are different types of etching, but dry and wet etching are most common.

Excitation Voltage:
Electrical excitation voltage applied to the input of a bridge circuit or passive transducer.

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F

Fatigue Life:
The number of total full cycles of force that can be applied to and removed from a measuring device before the measurement uncertainty of further readings will be altered beyond specified limits.

Feedback Amplifiers:
Electronic devices used to amplify the output of a transducer employed in a feedback control system. The transducer measures the output of a process and converts it to an electrical signal. If this electrical signal is weak, the feedback amplifier boosts the signal and compares it with the reference input signal or set point. Feedback amplifiers are of the class of this device known as instrumentation amplifiers.

Field Calibration Methods:
Methods used to determine the repeatability of a measurement process. These methods are very important in process applications, where repeatability is more critical to the control system than the absolute accuracy of measurements.

Flapper:
The other name for the bleed valve of a pneumatic load cell.

Flexural Bond Strength:
The flexural strength of a bonded material, or equivalently the maximum amount of stress experienced by the bonded material at its moment of yield or fracture.

Foil Strain Gauge:
A type of strain gauge that consists of a metal foil pattern mounted on an insulating backing or carrier. It is constructed by bonding a sheet of thin rolled metal foil on a backing sheet. The patterns on the gauge are produced by photo-etching.

Force-Balance Systems:
Measurement systems that employ the use of feedback systems to ensure that the output electrical signal is directly proportional to the applied force. These systems give high stability, high accuracy and are suitable for measurement of dynamic and static forces.

Force Calibration:
The process of calibrating a force measurement instrument. See Load Cell Calibration Service by Tacuna Systems.

Force Measurement Methods:
Techniques and principles employed in the design of force measurement instruments. Some of the commonly employed methods are lever-balance methods, force-balance methods, pressure measurement, acceleration measurement and the use of elastic elements. See Force and Weight Measurement Resources.

Force Transducer:
A sensor device that detects an external applied force and transforms it into an equivalent output.

Formal Calibration:
The calibration process that is customarily performed by approved laboratories. The instrumentation used is periodically calibrated and traceable to primary standards; the operators are well trained in calibration techniques, the temperature is controlled, and atmospheric pressure and humidity are accurately measured.

Frequency Response:
The highest sinusoidal frequency component of an applied force that the force measurement instrument can satisfactorily measure to a specified accuracy.

Full-Bridge Strain Gauge Load Cell:
A strain gauge load cell that has four strain gauges fixed into the four arms of a Wheatstone bridge circuit. In Figure 4, the strain gauges are arranged such that the two responding to either compression or tensile forces are on adjacent arms.

Figure 4. Full Bridge Strain Gauge Circuit

Full-Scale Output (FSO):
The output of the force measurement system at rated load capacity minus the output at minimum load. It is also called the span. Occasionally the FSO refers to the output value at the rated load instead of this difference. (See Figure 6)

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G

Gauge Factor:
The ratio of the fractional change in resistance to the fractional change in length of a strain gauge when subjected to a load. It is very useful for comparing the quality of strain gauge transducers.

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H

Half-Bridge Load Cell:
A strain gauge load cell that has two strain gauges fixed into adjacent arms of a Wheatstone bridge circuit, together with circuit completion resistors. See Figure 5 below.

Figure 5. Half Bridge Strain Gauge Circuit

Hooke’s Law:
A law of physics that states that the force (F) needed to extend or compress a spring by some distance is proportional to that distance. This principle is fundamental to the design of mechanical sensors that measure force through the deformation of an elastic medium. Mathematically, the law is expressed as F = kx, where k is a constant associated with the material and construction of the spring and x is the distance of extension or compression.

Hydraulic Fluid:
The medium by which pressure generated by an applied load is transmitted from a diaphragm to a pressure gauge. The fluid is housed in the confined space of a hydraulic pressure transducer.

Hydraulic Load Cell:
A type of mechanical, force-balance load cell device that measures pressure or load on its loading platform through a piston-cylinder arrangement. A hydraulic fluid housed in a confined space in the body of this cylinder transmits the created pressure through a connecting tube to a pressure gauge. This pressure on the gauge is converted to a reading of the magnitude of the applied load. Alternatively, the pressure output can be converted to an electrical signal by a pressure transducer.

Hysteresis:
The difference of output readings between the increasing and decreasing forces for a given applied force. That is, it is the difference between the readings of a load cell transducer as the load is increased (from zero-load to its rated capacity) and the readings as the load is decreased (from rated capacity to zero-load). The hysteresis specification on a load cell data sheet is given as a percentage of the rated output.

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I

IAQG:
The acronym for the International Aerospace Quality Group. It is an organization that focuses on the continuous improvement of the processes within the supply chain to deliver consistently high-quality products. A part of its objectives is to establish common quality standards and requirements. These standards apply to various force measurement instruments.

IEC:
The acronym for the International Electro-Technical Commission. It is an international standards organization that prepares and publishes international standards for all electrical, electronic and related technologies.

IEEE:
The Institute of Electrical and Electronics Engineers; a professional society for individuals in careers associated with electronics and related technologies.

Input Terminal Resistance:
The resistance measured across the excitation terminals of a force transducer at standard test conditions, with zero-load, and with the output terminals open circuited.

Insulation Resistance Tests:
A test used to troubleshoot when unstable readings and random changes in the zero balance point of the load cell occur. It is performed by connecting all the conductors together and then measuring the resistance between all the connected wires and the load cell body.

ISA:
The acronym for The International Society of Automation. It is a non-profit technical society for engineers, technicians, businesspeople, educators, and students who work, study or are interested in industrial automation and pursuits related to it, such as instrumentation.

ISO:
The acronym for the International Organization for Standardization. It is a global network of the world’s leading standards bodies and equipment manufacturers that brings together experts from all over the world to develop International Standards. The organization issues a certificate of conformity to Original Equipment Manufacturers (OEMs).

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L

Light Emitting Diode (LED):
A semiconductor device that emits light when an electrical current passes through it.

Linear Voltage Differential Transformer (LVDT):
An inductive transducer having a primary winding and two secondary windings of wire around a magnetic core. The primary winding is excited by an AC voltage; the movement of the magnetic core by an applied force then causes a difference in the induced voltages of the two secondary windings with respect to the primary coil. The device uses the basic principles of magnetics to detect a load and then produce an output voltage from the secondary windings that is proportional to the load.

Liquid Crystal Display (LCD):
A visual display used in electronic devices that works by blocking or passing light. The display has a layer of liquid crystal between two layers of polarized glass; applying a current to areas of the crystal alters the alignment of its molecules, affecting its opacity in those areas. A back light beneath the three layers generates the image.

Load:
Forces, deformations, and weights exerted, applied or placed on a surface. A load is a vector quantity, meaning it has magnitude and acts in a specific direction. Figure 6 clarifies terms related to loads applied to load cells.

Figure 6: Terms Associated With Loads Applied to Load Cells, and Their Relationship

Load Cell:
A sensor device that detects a physical parameter or stimulus (such as force, pressure) and then produces an output that is proportional to the stimulus. In other words, it is a transducer designed to detect forces or loads. Load cells are generally linear systems. Different types and designs of load cells exist; the most common are hydraulic load cells, strain gauge load cells and pneumatic load cells. See An Overview of Load Cells.

Load Cell Accessories:
Objects or devices that supplement a load cell’s implementation and operation. They include load cell cables and connectors, lifting crane or tension eye bolts, mounting kits and spacers for proper installation and support of load cells. See Load Cell Accessories at Tacuna Systems.

Load Cell Indicators:
High-accuracy devices connected to the output of a load cell that amplify and display the value of the measured load (force/weight). They include LCD and LED displays.

Load Cell Materials:
The raw materials used to fabricate load cell devices and their parts. These materials include stainless steel, hydraulic oil, phosphor bronze, beryllium bronze, plastics, glass, basic metals, copper-nickel, nickel-chromium, platinum-tungsten alloy. The materials are always chosen based on desired properties such as strength, weight, malleability, corrosion resistance, thermal conductivity, electrical conductivity, machining, viscosity, volatility, thermal expansion etc. 

Loading Platform:
The interface or surface through which a force/load is applied to a force transducer. The loading platform structure depends on the device’s use; a compression force transducer has upper and lower loading surfaces whereas a tension force transducer attaches the load through upper and lower screw threads. Load cell mounting kits serve as loading platforms for load cell force transducers to ensure proper force distribution on the device.

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M

Machine Operator:
A skilled technician trained to operate a machine or device such as a force measurement instrument.

Magneto-Elastic Load Cell:
A force transducer designed on the principle that that a ferromagnetic material subjected to mechanical stress undergoes a change in its magnetic properties and the change is proportional to the applied stress. Its properties are advantageous in harsh and electrically disturbed environments.

Manometer:
A device that measures the pressure in a liquid. Conventional hydraulic load cells typically include them.

Maximum Load:
The highest recommended load, less than or equal to the load cell’s rated capacity, for a specific application.

Mechanical Vibrations:
Oscillations in an object or system with respect to an equilibrium point. Different classes of vibration include: free (undisturbed) and forced (disturbed), damped and un-damped (the latter includes negligible vibrations), nonlinear and linear, random (unpredictable) and deterministic.

Microcontroller:
A small computer unit on a single integrated circuit, including a processor, memory and other peripherals. Modern force measurement instruments use these devices for digital signal processing. Microcontrollers have analog to digital input terminals, and digital output terminals. This output can connect to devices that control industrial processes, or to visual display devices.

Micro-Electro-Mechanical-Systems MEMS:
Miniaturized electro-mechanical elements made using micro-fabrication techniques.

Micromachining:
Techniques for fabricating 3D structures on the micrometer scale. This is an advancing trend in manufacturing addressing demands for reduced component size and weight in applications. MEMS are manufactured using micromachining techniques such as photolithography, laser micromachining, and micro-electronic discharge machining.

Minimum Dead Load (MDL):
Same as dead load above. See Figure 6.

Minimum Load:
The smallest load a load cell can accurately measure. It is typically greater than zero, and greater than MDL. See Figure 6.

Modulus Of Elasticity:
A measure of the resistance of a material to elastic deformation under a load. Several elastic moduli exist, including the Young modulus.

Millivolt (mV):
One one-thousanth of a Volt. The unconditioned analog output of an electrical transducer is always of the order of millivolts.

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N

Natural Frequency:
The rate at which a body vibrates when it is not disturbed by an external force. When a body experiences an initial displacement force and then the force is withdrawn, the body starts oscillating about its initial equilibrium position. The rate at which it continues to oscillate without any external force to keep it in motion is the object’s natural frequency.

Necking:
A mode of tensile deformation where relatively large amounts of strain permanently localize disproportionately in a small region of the material, resulting in a decrease in the local cross-sectional area at that point (the “neck”). A material will fail or break at the neck when the strain is great enough.

NEPSI:
The acronym for the National Supervision and Inspection Center for Explosion Protection and Safety of Instrumentation. It is a government-approved establishment engaged in research and development of explosion protection technology, testing, verification and certification of electrical equipment for use in explosive atmospheres, explosion protection and safety supervision, and assessment of engineering projects.

NIST:
The acronym for the National Institute of Standards and Technology, a physical sciences laboratory and a non-regulatory agency of the United States Department of Commerce. Among its roles is the production of Handbooks with guidelines for various areas of commerce including weights and measures (such as Handbook 44, as discussed in Load Cell Classes: NIST Requirements).

Nominal Displacement:
The measure of deformation of the force transducer when the rated load is applied. It is a factor that indicates the stiffness of the transducer.

Nominal Force:
The maximum capacity (force or load) of the force transducer. It is also known as rated capacity. It is always specified in an instrument’s datasheet.

Nominal Temperature Range:
The temperature range within which the force transducer works properly and meets the specifications in its datasheet.

Non-Linearity:
The maximum deviation of the response on the calibration curve of a measuring device from the theoretical response on an idealized straight line approximating it. It is determined from the calibration plot of increasing loads, and expressed as a percentage of rated output. Theoretically, the output of a force transducer should be proportional to the applied load; however, in practical measurement discrepancies exist and are labeled “non-linearity error”.

NPL:
The acronym for the National Physical Laboratory, the UK’s national measurement institute.

NTEP:
The acronym for the National Type Evaluation Program. It is a non-profit organization that tests manufacturer’s devices against US National Standards and government requirements for compliance.

Nyquist Frequency:
A frequency that is half of the sampling rate of a digital signal encoder. For example, if a digital encoder samples an analog signal at 200 samples/second, the Nyquist frequency of that analog signal will be 100Hz. A digital encoder can represent an analog signal accurately by sampling only if its frequency is below this limit.

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O

OIML:
The acronym for the International Organization of Legal Metrology, an intergovernmental body that publishes standards and requirements for measuring systems and components used in international trade.

Operating Range Of The Excitation Voltage:
The range of values of electrical excitation within which a transducer can operate at its rated capacity and perform satisfactorily within its datasheet specifications. When the excitation voltage exceeds this range, a strain gauge generates excessive heat (as it is a resistive element) and alter the expected output.

Organic Light Emitting Diode (OLED):
An LED where the luminescent layer is an organic material. They have advantages over non-organic LEDs including flexibility and transparency.

Original Equipment Manufacturer (OEM):
A company that produces equipment and instruments such as a force measurement device that may be marketed by another manufacturer.

Operating Temperature Range:
The same as the nominal or rated temperature range. It is the range of temperature values within which a transducer can operate at its rated capacity and perform satisfactorily within its datasheet specifications.

Orthogonal Forces:
In force measurement, forces perpendicular to the direction of the load (principal axis) to be measured by the force transducer.

Output Resistance:
The resistance measured on the output terminals of a transducer at a standard ambient temperature at zero-load and zero excitation.

Overload Stop:
The mechanical device designed to prevent overloading or overstressing of the transducer beyond its rated capacity.

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P

Passive Transducer:
A transducer that requires an external electrical excitation to operate properly. An example is the strain gauge incorporated in a Wheatstone bridge.

Piezoelectric Effect:
The ability of certain materials to generate an electric charge in response to an applied mechanical stress. Materials that exhibit this characteristic either occur naturally or are artificially synthesized. Natural piezoelectric materials include berlinite, quartz, topaz and Rochelle salt, while artificial ones include barium titanate and lead zirconate.

Piezoelectric Force Transducer:
A transducer that uses the piezoelectric effect to measure an applied force or load, as shown in Figure 7. The applied force causes a proportional electrical charge to appear on the faces of the piezoelectric element. It’s an active transducer, meaning the output power is derived entirely from the energy created by the applied force. It doesn’t require external excitation.

Figure 7. Piezoelectric Load Cell with Piezoelectric Force Transducer

Pneumatic Load Cell:
A type of mechanical, force-balance load cell device that measures pressure or load with a piston-cylinder system. It uses a volume of gas housed in a confined space in the body of the cylinder to transmit pressure to a pneumatic pressure dial gauge or a pressure transducer to display the value of the applied load.

Precision:
The reproducibility of the output of a measurement system. A precise measuring system creates the same or close to the same output for a known force with repeated measurements over time.

Pressure Transducer:
A device that senses pressure and transforms it into an equivalent output signal, usually in an electrical form. The pressure transducer can replace the pressure gauge at the output of a pneumatic or hydraulic load cell, offering more accurate readings.

Primary Axis:
The axis along which the applied load is designed to act. It can be thought of as an imaginary line that intersects the load cell at the application point of the force measurement instrument.

Primary Standard:
A standard that is sufficiently accurate such that it is not calibrated by or subordinate to other standards. Primary standards are the authoritative definition or realization of their unit of measure. Standards organizations generally maintain these standards.

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Q

Quarter-Bridge Load Cell:
A type of load cell that has a single strain gauge fixed into an arm of a Wheatstone bridge circuit, together with completion resistors and a dummy gauge for temperature compensation. See Figure 8.

Figure 8. Quarter Bridge Strain Gauge

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R

Random Errors:
Deviations from the expected output of a measuring device that, despite being clearly observed, are difficult to explain. Engineering techniques use probability and statistical tools to estimate random error. These techniques assume that the random signal or error belongs to an ensemble of signals, with each signal having its own probability of occurrence. The probability distribution of this type of error is uniform about the mean output of the measuring device. An example of a cause of this type of error is EMI.

Rated Capacity:
The maximum load a measuring device can bear and still operate within its manufacturer’s specifications. See Figure 6.

Rated Output:
The output signal of a load cell at the maximum load the measuring device can bear and still operate within its manufacturer’s specifications. See Figure 6.

Recalibration:
The process of repeating calibration procedures after initially calibrating a measuring system. The recommended recalibration interval for most systems is about 1 year, to ensure accurate measurements.

Reference Standard:
A force measurement instrument whose performance characteristics are known in relation to a primary standard.

Reliability:
The probability of an instrument performing satisfactorily under stated conditions for a specified period of time.

Repeatability:
The measure of agreement between the results of successive measurements taken with one force measurement system, applying a given force in the same direction. Repeatability is measured within a given range of calibration forces applied. It shows the ability of a force measurement system to measure consistently.

Reproducibility:
The closeness of the agreement between the results of the measurements of the same force carried out under changed conditions of measurement.

Resistivity:
The property of a material to resist the flow of electric current through it. See Figure 9.

Figure 9. Resistivity Formula

Resolution:
The smallest change in load that a force measurement system can detect and reliably indicate in its output signal.

ROHS:
The acronym for the Restriction of Hazardous Substances. This label means a device complies with a European Union (EU) requirement restricting the use of certain hazardous substances in electronic equipment.

RS232:
The acronym for the Recommended Standard 232, a standard communication protocol connecting a computer and its peripheral devices (such as a digital measurement instrument) to allow serial data exchange.

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S

Safe Overload:
The amount of load over rated capacity (expressed as a percent) that can be applied to a force transducer without causing a permanent change in its output specifications. The underlying cause is typically plastic deformation of the transducer’s elastic element. See Figure 6.

Sampling Rate:
The number of times per unit time (usually a unit of seconds) that an analog digital converter (ADC) reads the value (such as volts or decibels) of an analog signal in order to convert it to a digital signal. It is usually expressed in Hertz (Hz), or equivalently samples per second.

Semiconductor Gauge:
A type of strain gauge manufactured from strips of either n-type or p-type semi-conducting silicon. It exhibits special characteristics such as high output, no creep or hysteresis, and long fatigue life. Force transducers, accelerometers, and pressure sensors all commonly use semiconductor gauges.

Sensitivity:
The rate of change of the output signal of a load cell as the signal at the input varies, when the load cell is subject to a rated capacity load. The sensitivity of a transducer allows us to predict the output voltage from a known excitation voltage and value of the applied stimulus.

Shear Force:
A force that acts perpendicular to the longitudinal axis of the body, or equivalently parallel to a cross section of the body. See Figure 10.

Figure 10. Shear Force

Shielding:
Placing a conductive surface around the critical parts of the instrument’s circuit to attenuate the electromagnetic field that couples to it. A good force transducer has a shield wire as its fifth wire terminal.

Shock Resistance Test:
The test carried out when there are unstable readings and random changes in the zero balance point of the load cell. It is done by connecting all the conductors together and then measuring the resistance between all the connected wires and the load cell body.

Shunt Calibration:
The calibration of a load cell transducer by the insertion of known shunt resistors into the appropriate arms of its Wheatstone bridge circuit.

Shunt-To-Load Correlation:
The numerical difference between the output readings obtained from shunt calibrations and the known value of the loads.

Signal Conditioning Circuit:
A circuit that performs signal filtering, amplification, and isolation to ensure a more accurate output signal.

Span:
The full-scale output of a measuring instrument. See Figure 6.

Stabilization Period:
The time interval required to ensure that any further changes in the output reading of the load being measured are tolerable. Equivalently, it is the time required for the output reading to stabilize and maintain a certain value before it is recorded as the actual output value.

Standard Test Conditions:
The ambient conditions under which measurements should be performed, abbreviated as STC. These conditions include factors such as temperature, humidity, and pressure for force measurements.

Static load:
A load whose size/magnitude does not change with time.

Stiffness:
The ability of a body to resist deformation when a load is applied to it. It is similar to the Young modulus.

Strain:
The ratio of the change in length of a material in the direction of an applied force, to the original length in that direction. Also called engineering strain, it is the response of a material to an induced stress. It is a dimensionless quantity.

Strain Gauge:
A resistive element that produces a change in its resistance proportional to a mechanical strain induced by an applied force.

Strain Gauge Transducer:
A transducer type that uses a strain gauge as its underlying mechanism. The strain gauge(s) is(are) attached to a structural member which undergoes deformation when a load is applied. It is the most common type of load cell in measurement devices across various industries.

Stress:
The internal forces per unit area present in a material exposed to external forces or loads. In the International Standard of Units (SI), it is expressed in Pascals; the English System expresses stress in pounds per square inch, or PSI.

Structural Member:
The constituent part of a load cell/force transducer that provides support and housing for other components. The geometric shape of the structural member also determines the category to which the load cell belongs: beam load cells, disk load cells, S-type, tension link load cells and miniature load cells.

Summing Junctions:
Devices used for summing data output from multiple load cells into a single output. Click here for examples.

System Requirements:
The necessary and desired specifications and standards that the force measurement system must meet for a given application. The user, client or the expert designing system for this given application defines its system requirements, and chooses instruments that best match these requirements to create the system.

Systematic Error:
The error inherent in the operation of a force measurement instrument. It can be quantified or eliminated through system calibration.

Système Internationale (SI) Units:
From the French for International System, this is the conventional name for the metric system of measurements. It is an internationally standardized system common among all nations and between the different branches of science and technology.

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T

Tare Weight:
The weight of an empty container or the loading platform. The results of force measurement readings sometimes include this weight; in those cases it is subtracted to derive the true weight of a measured object.

Telemetry:
An automated communication process that transmits data or information between two geographical locations. In many force measurement applications, this type of transmission enables centralized supervisory data login, signal processing or control of large-scale systems. Telemetry can be electrical or pneumatic. Pneumatic telemetry is used in hazardous areas where an electrical power failure could be disastrous.

Temperature Compensation:
A procedure to eliminate the dimensional changes caused by temperature changes in a transducer. The use of self-temperature compensated gauges or dummy gauges generally addresses these unwanted thermal effects.

Tensile Force:
A pulling force that can stretch or elongate a material.

Tensile Strength:
The capacity of a material to withstand tensile forces.

Thin Film Strain Gauge:
The strain gauge type produced by evaporating thin films of metals or alloys onto an elastic element.

Tolerance:
The allowable maximum deviation of a measured quantity from the standard specifications.

Torque:
The turning effect caused by a force acting on a body at a distance from its center of rotation. It is the product of the force and the perpendicular distance between the line of action of the force and the center of rotation at which it is exerted. Torque is typically measured in foot-pounds or Newton-meters. See Figure 11 below.

Figure 11. Torque

Torsional Modulus:
The modulus of elasticity of a material when subjected to a torque force.

Traceability:
Another name for calibration.

Transducer Electronic Datasheet (TEDS):
The acronym for the . It lists details about the technical and performance characteristics of the transducer.

Troubleshooting:
The process of detecting, tracking, analyzing and correcting faults in a measurement system. Various recommended standard troubleshooting procedures exist for each force measurement instrument.

True Strain:
This is the natural logarithm of the engineering strain.

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Ultimate Elongation:
The maximum length reached by a material at rupture when subjected to tensile forces.

Ultimate Overload:
The breaking overload.

Ultimate Strength:
The maximum stress a material can withstand before it breaks.

Unloaded Output Voltage:
The electrical output voltage from the force transducer under zero load or no load.

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Vibrating Wire Load Cell:
A transducer having a taut ferromagnetic wire that is excited by a drive coil into transverse vibrations. A pick-up coil detects these vibrations. The drive coil and pickup coil have magnetic cores and once the wire has been excited to its resonant frequency for a given tension, this frequency is maintained by a self-oscillatory system formed between the two coils. The resonant frequency correlates with, and can therefore be used to measure, the applied force on the taut wire at that instant.

Visual Displays:
Visual human interfaces used to indicate either qualitative or quantitative events. A qualitative event is the presence or absence of expected signals. A quantitative event is the value of these signals. LEDs and LCDs are common types of visual displays.

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Wheatstone bridge:
A circuit configuration primarily used to measure electrical parameters such as resistance, inductance, and capacitance. The configuration incorporates an excitation source, four conductive arms that optionally incorporate passive components, and an output terminal. In order to measure capacitance or inductance rather than resistance, the normal Wheatstone bridge is modified but still follows the same circuit configuration pattern.

Wire Strain Gauge:
The original type of resistance strain gauge, though now widely replaced by foil and thin film strain gauges. It still has applications in high-temperature transducers and stress analysis.

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Young Modulus:
The ratio of the stress caused by an applied force acting over an area of a material to the strain. In Figure 12 below, it is the slope of the linear region of each curve. Note the harder a material, the higher the Young Modulus.

Figure 12. Young Modulus

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Zero Balance:
The default output signal of a transducer when no load or zero-load is applied to it.

Zero Offset:
The difference between the zero-load specified in the instrument’s datasheet and the actual zero load output obtained by direct measurement under zero-load conditions (removal of all weights, loads and dead load).

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References

  1. Difference between Analog and Digital Circuits.
  2. Definition of Analog Circuits
  3. Instrumentation Systems
  4. Mechanical Vibrations
  5. Instrumentation Reference Book, W. Boyes.
  6. A Guide to the Measurement of Force, Andy Hunt.
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