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Load Cell Glossary of Terms

For convenience, terms which are defined in this standard are printed in UPPER CASE when used in the definition of another term.

AMBIENT TEMPERATURE – The temperature of the medium surrounding the LOAD CELL.

AXIAL LOAD – A load applied along the PRIMARY AXIS.

BAROMETRIC SENSITIVITY – The change in ZERO BALANCE due to a change in ambient barometric pressure. Normally expressed in units of %RO/atm.

CALIBRATION – The comparison of LOAD CELL OUTPUT against standard test loads.

CAPACITY – The maximum AXIAL LOAD a LOAD CELL is designed to measure within its specifications.

COMBINED ERROR – The maximum deviation of the CALIBRATION curve from the straight line drawn between MINIMUM LOAD OUTPUT and MAXIMUM LOAD OUTPUT, normally expressed in units of %FS. Both ascending and descending curves are considered.

CREEP – The change in LOAD CELL SIGNAL occurring with time while under load and with all environmental conditions and other variables remaining constant. Normally expressed in units of % of applied load over a specified time interval. It is common for characterization to be measured with a constant load at or near CAPACITY.

CREEP RECOVERY – The change in LOAD CELL SIGNAL occurring with time immediately after removal of a load which had been applied for a specified time interval, environmental conditions and other variables remaining constant during the loaded and unloaded intervals. Normally expressed in units of % of applied load over a specified time interval. Normally the applied interval and the recovery interval are equal. It is common for characterization to be measured with a constant load at or near CAPACITY.

CREEP RETURN – The difference between LOAD CELL SIGNAL immediately after removal of a load which had been applied for a specified time interval, environmental conditions and other variables remaining constant during the loaded interval, and the SIGNAL before application of the load. Normally expressed in units of % of applied load over a specified time interval. It is common for characterization to be measured with a constant load at or near CAPACITY.

DEFLECTION – The displacement of the point of AXIAL LOAD application in the PRIMARY AXIS between the MDL and MDL+CAPACITY load conditions.

ECCENTRIC LOAD – Any load applied parallel to but not concentric with the PRIMARY AXIS.

FULL SCALE or FS – The OUTPUT corresponding to MAXIMUM LOAD in any specific test or application.

HYSTERESIS – The algebraic difference between OUTPUT at a given load descending from MAXIMUM LOAD and OUTPUT at the same load ascending from MINIMUM LOAD. Normally expressed in units of %FS. It is common for characterization to be measured at 40-60% FS.

INPUT RESISTANCE – The resistance of the LOAD CELL circuit measured at the excitation terminals with no load applied and with the output terminals open-circuited.

INSULATION RESISTANCE – The DC resistance measured between the bridge circuit and the case. Normally measured at 50 VDC.

LOAD CELL – A device which produces an OUTPUT proportional to an applied force load.

MAXIMUM AXIAL LOAD, SAFE – The maximum AXIAL LOAD which can be applied without producing a permanent shift in performance characteristics beyond those specified. Normally expressed in units of % CAPACITY.

MAXIMUM LOAD – The highest load in a specific test or application, which may be any load up to and including CAPACITY +MINIMUM LOAD, but may not exceed CAPACITY significantly.

MAXIMUM AXIAL LOAD, ULTIMATE – The maximum AXIAL LOAD which can be applied without producing a structural failure. Normally expressed in units of % CAPACITY.

MAXIMUM LOAD AXIS MOMENT, SAFE – The maximum moment with respect to the PRIMARY AXIS which can be applied without producing a permanent shift in performance characteristics beyond those specified.

MAXIMUM MOUNTING TORQUE, SAFE – The maximum torque which can be applied concentric with the primary axis without producing a permanent shift in performance characteristics beyond those specified.

MAXIMUM SIDE LOAD, SAFE – The maximum SIDE LOAD which can be applied without producing a permanent shift in performance characteristics beyond those specified.

MEASURING RANGE – The difference between MAXIMUM LOAD and MINIMUM LOAD in a specific test or application. It may not exceed CAPACITY.

MINIMUM DEAD LOAD or MDL – The smallest load for which specified performance will be met. It is normally equal to or near NO LOAD in single mode applications and is of necessity equal to NO LOAD in double mode applications.

MINIMUM LOAD – The lowest load in a specific test or application, differing from NO LOAD by the weight of fixtures and load receptors which are attached plus any intentional pre-load which is applied.

MODE – The direction of load. Tension and Compression are each one mode.

NATURAL FREQUENCY – The frequency of free oscillations under conditions of NO LOAD.

NO LOAD – The condition of the LOAD CELL when in its normal physical orientation, with no force input applied, and with no fixtures or load receptors attached.

NONLINEARITY – The algebraic difference between OUTPUT at a specific load and the corresponding point on the straight line drawn between MINIMUM LOAD and MAXIMUM LOAD. Normally expressed in units of %FS. It is common for characterization to be measured at 40-60 %FS.

NONREPEATABILITY – The maximum difference between OUTPUT readings for repeated loadings under identical loading and environmental conditions. Normally expressed in units of %RO.

OUTPUT – The algebraic difference between the SIGNAL at applied load and the SIGNAL at MINIMUM LOAD.

OUTPUT RESISTANCE – The resistance of the LOAD CELL circuit measured at the SIGNAL terminals with no load applied and with the excitation terminals open-circuited.

PRIMARY AXIS – The axis along which the LOAD CELL is designed to be loaded.

RATED OUTPUT or RO – The OUTPUT corresponding to CAPACITY, equal to the algebraic difference between the SIGNAL at (MINIMUM LOAD + CAPACITY) and the SIGNAL at MINIMUM LOAD.

RESOLUTION – The smallest change in load which produces a detectable change in the SIGNAL.

SHUNT CALIBRATION – Electrical simulation of OUTPUT by connection of shunt resistors of known values at appropriate points in the circuitry.

SIDE LOAD – Any load at the point of AXIAL LOAD application acting at 90° to the PRIMARY AXIS.

SIGNAL – The absolute level of the measurable quantity into which a force input is converted.

SPAN – Another name for RATED OUTPUT.

STATIC ERROR BAND or SEB – The band of maximum deviations of the ascending and descending calibration points from a best fit line through zero OUTPUT. It includes the effects of NONLINEARITY, HYSTERESIS, and non-return to MINIMUM LOAD. Normally expressed in units of %FS.

SEB OUTPUT – A computed value for OUTPUT at CAPACITY derived from a line best fit to the actual ascending and descending calibration points and through zero OUTPUT.

SYMMETRY ERROR – The algebraic difference between the RATED OUTPUT in tension and the average of the absolute values of RATED OUTPUT in tension and RATED OUTPUT in compression. Normally expressed in units of %RO.

TEMPERATURE EFFECT ON OUTPUT – The change in OUTPUT due to a change in AMBIENT TEMPERATURE. Normally expressed as the slope of a chord spanning the COMPENSATED TEMPERATURE RANGE and in units of %/°F or %/100°F.

TEMPERATURE EFFECT ON ZERO – The change in ZERO BALANCE due to a change in AMBIENT TEMPERATURE. Normally expressed as the slope of a chord spanning the COMPENSATED TEMPERATURE RANGE and in units of %RO/°F or %RO/100°F.

TEMPERATURE RANGE, COMPENSATED – The range of temperature over which the LOAD CELL is compensated to maintain OUTPUT and ZERO BALANCE within specified limits.

TEMPERATURE RANGE, OPERATING – The extremes of AMBIENT TEMPERATURE within which the LOAD CELL will operate without permanent adverse change to any of its performance characteristics.

TOGGLE – Another name for ZERO FLOAT.

ZERO BALANCE – The SIGNAL of the LOAD CELL in the NO LOAD condition.

ZERO DEAD BAND – Another name for ZERO FLOAT.

ZERO FLOAT – The shift in ZERO BALANCE resulting from a complete cycle of equal tension and compression loads. Normally expressed in units of %FS and usually characterized at FS = CAPACITY.

ZERO STABILITY – The degree to which ZERO BALANCE is maintained over a specified period of time with all environmental conditions, loading history, and other variables remaining constant.

Can I have an application specific calibration matrix ie. one that allows me to measure high torque and low force or vice versa?

A lot of applications require that only one axis of a force/torque sensor is used from 50% to 100% of the nominal load, while
the other axis of the sensor is used only up to 10% or even only up to 1% of the measuring range. Interface Inc. offers a
special “Matrix-Plus” calibration procedure to ensure optimum accuracy even in these application-specific working points.

The tasks of the calibration matrix are:
     a) Minimising the measurement error in the loaded measuring axis and
     b) Minimising the cross-talk in the remaining (unloaded) 5 axes.

Standard Calibration
In the case of low utilisation of some measuring axes, the error can have a relatively strong effect in these measurement
axes due to cross-talk, although it is significantly less than 1% based on 100% of the measuring range.

Advanced Calibration “Matrix-Plus”
Interface Inc. has developed a new calibration method, which optimises the display in the loaded measuring axis and
in the unloaded measuring axes. The characteristic field of the 6-axis sensor is represented by two matrices. Matrix A
describes the linear relationships, matrix B describes the non-linear relationships.

Matrix Plus with “Standard Constraints”
Special conditions are defined in the determination of the matrices so that the measurement errors are minimised even
at low forces and torques. Loads of 100%, 80%, 60%, 40% and 20% are mathematically optimised.

Matrix-Plus with “Simulated Operating Point”
It is even possible to take the application (operating point) into account while determining the matrices: this process
is called a “simulated operating point”. Thereby, accuracies of 1% to 0.2% of the actual value can usually be achieved.
In addition to the actual calibration load 100%, the application-specific load vector is also taken into account
mathematically.

Matrix Plus with “Calibration in the Operating Point”
Alternatively, a calibration is also possible at the operating point of the application. Customer-specific calibration uses
the actual loads and lever ratios of the customer-specific application. In one example, accuracies of 0.5% to 0.1% of
the actual value can be achieved. Suitable devices may have to be produced for the calibration in order to display the
special lever ratios of the application. This can result in additional costs and delivery times in individual cases.

What is the difference between the Interface Gold Standard Calibration software packages?

Interface offers three primary Gold Standard software packages for the calibration of load cells or test machines.

  1. Force Comparison ICS-202
  2. Machine Calibration ICS-205
  3. Dead-weight ICS-DW

All the software packages store calibration data for reference load cells, stepped load routines, ‘limits’ on performance criteria (where relevant) to help dramatically speed up the calibration process especially where repetitive routines are employed also at the same time helping to maintain accuracy and reduce errors.

System no.1 – Force Comparison (ICS-202)

This system requires dual channel data acquisition using either the Gold Standard PC Boards HRBSC/SCBxx or the dual channel 9840 model 9840-2xx-x. While both systems give you all the acquisition, display, manipulation, analysis and reporting functions the Gold Standard hardware has the additional option of adding a control board that provides total automation of the loading process. Regardless of how the applied force is controlled the software relies on measurements taken at stepped loading intervals, with those from a Calibration/Reference unit being compared to the Unit Under Test and then immediately performance figures are calculated and the error graph is plotted with comprehensive calculations of any parameter being available through the extremely flexible ‘Crystal’ report software. In-built formats are provided for a basis of ASTM-E74 and ISO-376 calibration reports. A concise operating manual will soon be available on this link.

System no. 2 – Machine Calibration (ICS-205)

While this system can be run using the dual channel hardware required by the above package it can also be operated using a single channel system more suited to the Field Calibration Engineer. This software takes the measurements from a Calibration/Reference load cell installed in the test machine and now compares the displayed reading of the test machine against the reference values at each stepped load interval and calculates the test machine errors at each step with tabulated results and graphical plots being offered plus immediate results can be made available in draft prior to confirmation and a signed copy. In-built formats are offered for ASTM-E4 and ISO-7500/1 calibrations. A concise operating manual will soon be available on this link.

System no.3 – Dead-weight (ICS-DW)

As above this package can be run with the dual channel hardware but only requires a single channel to measure the Unit Under Test while it is being loaded with known value reference dead-weights. Dead-weights are recognised as the most accurate way of applying known loads and the Gold Standard software will work with any number of weight-stacks to make the task of data collection a simple task of selecting the correct library files and clicking ‘Start’.  A concise operating manual will soon be available on this link.

A separate note will provide comparison details for the Gold Standard Hardware.

Although reference here has been made as ‘Force Calibration’ the software packages can also be used for Pressure or Torque calibrations with sensors that have strain-gauged bridge outputs or high-level signals of 4-20mA or +/-10V

 

What are the Interface load cell wiring colours?

Interface USA Colors

4-Wire

Red +Excitation
Green +Signal (Tension upscale)
White -Signal
Black -Excitation

6-Wire

White/Red +Sense
Red +Excitation
Green +Signal (Tension upscale)
White -Signal
Black -Excitation
White/Black -Sense

Interface UK Colours

6-Wire

Yellow +Sense
Red +Excitation
Green +Signal (Tension upscale)
White -Signal
Black -Excitation
Blue -Sense

NEW
European Colours

Brown +Excitation
Yellow +Signal (Tension upscale)
White -Signal
Green 0V/Gnd
Grey Control (typically 100%)
Braid Shield

How do I wire a 9840 load cell indicator connector?

Interface offers a wide selection of indicators to cover many applications and here are the basic connection details for the 9840 which is one of the most popular units for calibration grade applications.

9840 Intelligent load cell indicator – 9-way D-type load cell connector

  1. Excitation – High/Positive
  2. Sense – High/Positive
  3. Cell Output/Signal – High/Positive
  4. Cell Output/Signal – Low/Negative
  5. Sense – Low/Negative
  6. Excitation – Low/Negative
  7. Auto ID – A
  8. Auto ID – B
  9. Chassis Ground

Please contact us directly for details of the wiring for the other connectors.

Search with “What are the Interface load cell wiring colours?” for details of the most popular wiring/cable colour conventions.

How do I wire the standard Interface PCCx indicator enclosure connector?

The Interface DFI05L digital load cell indicator is optionally offered with the PCCx enclosure which provides a rugged casing for protection of indicator(s) and connecting wiring especially the mains voltage power supply leads. The rear panel provides an IEC style switched connector input and the load cell connection is via a standard D-type 15-way female connector (Male mating connector required for fitment to sensor). The wiring is as follows.

  1. Positive Excitation
  2. Positive Signal
  3. Negative Signal
  4. Negative Excitation
  5. No connection
  6. No connection
  7. Logic input – Common #07
  8. Logic input – CC.1 #08 {Tare}
  9. Positive Excitation
  10. Positive Sense
  11. Negative Sense
  12. Negative Excitation
  13. No connection
  14. Logic input – CC.2 #10 {Reset}
  15. Logic input – CC.3 #09 {Peak/Valley}

NOTE.1: The DFI05L requires 6-way sense wiring so links should be fitted between pins 9-10 & 11-12 when wiring for 4-way.

NOTE.2: To enable the front panel buttons for TARE, RESET and PEAK/VALLEY a link should be fitted between Common (pin 7) and pins 9, 15 & 14 respectively.

This information is designed to cover both the Mk1 and Mk2 style devices.

What is the best excitation voltage for Interface load cells?

All Interface load cells use eight full bridge strain gauges with each leg normally being rated at 350ohms.

The preferred excitation voltage for Interface load cells is 10VDC as this guarantees the closest match to the performance achieved by Interface during calibration.  The reason for this is that the gauge factor is affected by temperature.  Heat dissipation in the gauges is coupled to the flexure through a thin layer of epoxy glue; the gauges are kept at close to the ambient temperature of the flexure. However, the higher the power dissipation in the gauges, the larger the deviation between the temperature of the gauge and that of the flexure. For example, at 10VDC, a 350 ohm bridge dissipates 286m/w. A doubling of the voltage to 20VDC quadruples the dissipation to 1143m/w. This is a large amount of power to have in small gauges and can cause a substantial increase in the temperature gradient between gauges and flexure.  Conversely, dropping the voltage to 5VDC decreases the dissipation to 71m/w, not a significant drop from 286m/w.

The excitation voltage has an impact on sensitivity. For example, operating at 20VDC would decrease sensitivity by approximately 0.07% from the original Interface calibration, whereas operating the same load cell at 5VDC would increase sensitivity by less than 0.02%. This characteristic makes it possible to operate load cells at 5 or even 2.5VDC in order to save power.

Some, portable data loggers will automatically switch the excitation for short periods of time in order to conserve power. If the duty cycle is only 5% with 5VDC excitation, the heating effect is a tiny 3.6m/w. This could cause an increase in sensitivity of up to 0.023% from the original Interface calibration.

Variations in excitation voltage can cause small shifts in zero balance and creep. This effect is most noticeable when excitation voltage is first turned on. The solution for this effect is to allow the load cell to stabilize by operating it with 10VDC excitation for the time required for the gauge temperature to reach equilibrium, but this can take up to 30 minutes.

Which Interface mating connector do I need? #1

This chart details the mating connectors for the most common Interface force & torque sensor products.

Mating Connector MFG Number Mates with Application/Notes
Interface Mating Connectors
MC-001 PC06A-10-6S PC04E-10-6P 1000, 1100, 1200 Series
MC-002 PT06A-12-8S PT02E-12-8P Gold Standard Load Cells & Simulators
CN-203 PC04E-10-6P PC06A-10-6S Load Cell look-alike on cable end
CN-204 PC04E-10-6P(SR) PC06A-10-6S Load Cell look-alike on cable end
CN-206 PC06A-10-6S(SR) PC04E-10-6P Load Cells with Screw Connector
CN-207 PT06A-10-6S(SR) PT02E-10-6P Load Cells with Bayonet Connector
CN-208 MS3106A-14S-6S MS3102X-14S-6P UMC600 Indicator
CN-209 PC02E-10-6P PC06A-10-6S Box Mount looks like Load Cell screw type
CN-210 PT01A-12-8P(SR) PT06A-12-8S(SR) Gold Cell look-alike on cable end
CN-212 DE-9P DSUBMIN DE-9S 9840 Indicator Load Cell Input
CN-213 PT02A-10-6P(SR) PT06A-10-6S(SR) Load Cell Bayonet look-alike on cable end
CN-224 BINDER N/A 7-Pin Options conn. for RD6
CN-225 BINDER N/A 12-Pin for RD6, RD3, T2, T3, T4, T5, T6, T7 & T12 (>19Nm)
CN-226 BINDER N/A 6-Pin for TS11 & TS12 (>19Nm)
CN-xxx BINDER N/A 6-Pin for INF-USB/SI-USB

What is meant by (non-)repeatability?

A peculiarity of the load cell industry is that all manufacturers seem to publicise figures for non-repeatability but these figures essentially have little or no merit. These figures could be thought of as reference values rather than actual determined values. The reasons for this are that there are no standards or definitions for a specific parameter. Although non-repeatability is generally thought to mean the variation in output for repeated loadings under identical loading and environmental conditions, there are no uniform procedures for conducting and reporting an appropriate test. There are also no means for excluding the non-repeatability of the test equipment which is often as significant as the non-repeatability of the load cell. Better measurements of the meaningful repeatability can be found in the following parameters:

  • Eccentric load sensitivity
  • Creep
  • Interpolation error
  • Temperature effect on output
  • Temperature gradient sensitivity

Interface Low Profile load cells outperform all competitive products in these key parameters

When even greater repeatability is merited, a permanently installed stud  in the live end by Interface or by the user that is locked in under tensile load, provides still greater repeatability by isolating the load cell almost completely from end effects. The pre-loaded stud increases the height of an assembly, but the overall height is still the lowest of any load cell technology available. For ultimate repeatability special versions with integral machined studs offer further improvements over the jammed studs

How do I wire and setup my AMTI Gen5 amplifier for common excitation?

AMTI model Gen5 6 channel amplifier.

Some AMTI sensors use commoned excitation to reduce the cable size to fit smaller body sizes and in some instances, a thinner cable helps with routing in more compact installations. The standard sensor design has 6 individual strain gauged Wheatstone bridges requiring a total of 24 cores and this can be reduced to 14 cores by commoning the excitation.

Wiring: The commoned excitation connections are made to the Fx bridge which has sufficient current to drive all 6 bridges.

Setup: It is important to set internally the excitation voltage for the remaining 5 bridges to the same value as Fx as the signal output for each bridge is calculated using the sensitivity and the excitation.

What is the BSC4 connector wiring scheme

There are two connector options when connecting load cells to the BSC4 amplifier. Depending on which model was purchased all four channels may be incorporated in a single 37-pin D-sub connector, or 4x 5-pin M12x1 connectors – one for each channel.

The wiring for each is below:

How to wire a 9840 serial cable (RS-232)

The 9840 serial cable comprises normally 6-core screened cable even though only four cores are used and is fitted with a female 9-way D-type on one end and a female 15-way D-type on the other with wiring as follows.

Colour 15 Way 9 Way Function
Green 1 2 TXD
Red 2 3 RXD
White 3 6 DTR
Black 4 5 Serial GND

Although today there is a proliferation of USB connections still numerous industrial applications rely on RS-232 for a dedicated port connection to a measuring device which in this case is the Interface 9840 calibration grade digital load cell indicator. Connecting the 9840 by either serial or USB allows the instrument to be controlled from the Gold Standard data acquisition software.

On a basic Interface calibration certificate what does the mV/V figure represent?

The most basic of the Interface load cell calibration certificates gives a single mV/V value but this figure has real value.

Many manufacturers only provide a Terminal value or in Interface terms RATED OUTPUT i.e. the measured signal value at full rated load, but the figure provided by Interface is the Static Error Band Output (SEB OUTPUT) which is a calculated best-fit value.

The SEB OUTPUT requires a minimum of 4 measurements at 3 points these being zero, 50% range ascending 100% range and 50% range descending.

For SSM & SM S-beam style units the provided value is that for tension measurements while the MB & MBI units are provided with compression values.

For more information contact us directly or via info@interface.uk.com

What are the different force calibrations available?

We have a separate site that deals specifically with the different force calibrations, governing bodies and accreditations. To view click here or on the following image.
Click here to go to Force Calibration Guide

 

Alternatively, call us and we will be happy to discuss your aims, look at all the options and costs so we can suggest the most suitable calibration to match your requirement.

How to enable Peak/Valley on a DFI-05L load cell indicator

This is applicable to the standard DFI-05L MK2
(may vary if the MEM-08 option is installed.)
Assuming that the default conditions apply to the Logic Inputs it is necessary to link connection 7 to 9 (Common – CC.2) & 10 (Common – CC.3) to enable Peak/Valley (Max/Min) and Reset respectively.
load-cell-indicator-DFI-05L-logic-inputs.jpg
With the Calibration Lock Switch is in the locked position the unit will operate as follows.
Press once –
Displays PEAK (MAX)
Press-once
Press twice –
Displays VALLEY (MIN)Returns to normal after
a short period
or Press a third time
Press-2
Press for 3 seconds –
RESETS values of Peak & Valley
Press 3 seconds

What is meant by ‘eccentric load sensitivity’?

Extraneous or off-axis loads can be due to many factors and one of these is ‘Eccentric Load’
Eccentric loads occur when the actual load path is parallel with the load axis but at a distance that creates a vector load path introducing a moment load that can affect the output signal level.

The radial design of the Interface Low Profile units has inherent resistance to off-axis loads and this is further enhanced by precision machining to balance the bridges while under load so errors are minimised when the loading is not perfectly aligned.

Interface guarantees the performance and specifies the maximum (worst case) errors under side force, moment loads and torsional effects.

For example the 1000 Series has a published specification (Note:1) for Eccentric Load Sensitivity of +/-0.1% per Inch, so even if you apply the load at an offset distance of 1.0” (25.4mm) you will see a worst case error of +/-0.1% or less. This means that you can have confidence in your loading system measurements under real-world working conditions not just in the laboratory conditions before you shipped it to the end-user.

Note:1 1000 Series datasheet ref: 1000FR v2.2 04-25-2018

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