Load Cell Lunch and Learn Presented by Interface

Lunch & Learn

Interface: Brian Peters, Ken Vining

T&M Instruments: Jace Curtis

Lunch & Learn Topics

• Interface History

• Load Cell Design & Theory

• Application Considerations

• Factors of Performance

• What Makes Interface Better

• Questions?

Our Heritage

• Family owned in Scottsdale, AZ since 1968

• World’s largest manufacturer of low profile load

cells

• Guarantee: Our load cells will outperform our

competitor’s – PERIOD

• Made in the USA

Trust our Experience

• We’ve been in the business of manufacturing

load cells for 45 years

• Building our 1 millionth load cell in 2013!

• Both PTB & NIST are Government standards for

measurement traceabilty: They use INTERFACE

Performance Second to None

• Metrology grade load cells with performance as

high as 0.02% F.S.

• Lower load limit of 4% on Gold Standard series

cells and 2% on Platinum Series

• Provide additional specifications not standard in

the industry

• A2LA Accredited calibration

Vast Capabilities

• Performance: as high as 0.02% F.S.

• Capacity: 50 grams to 2.25 Million lbf

• Millivolt, Voltage or Current output available

• Hermetically sealed, submersible, vibration rated,

dual bridge, overload protected

• ATEX approved for hazardous environments

• 200 page catalog represents 60 – 70% of our

business. The rest is modified standard or custom

We Sell Solutions

• Application engineers will discuss your

application to understand your priorities in your

force or torque measurement

• We will propose a solution specific to your

application be it a standard, modified or custom

design

• This has been our approach for 45

years…there’s a good chance we already have a

solution to meet your needs

Over 100,000 configurations of

load & torque cells

Over 10,000 instruments and

signal conditioners sold in last

10 years alone

What is a Load Cell?

How a load cell works

• Wheatstone bridge

• 2 voltage dividers fed

by the same input

creating a balanced

bridge.

How a load cell works

• Deflection causes an offset or unbalanced state which creates a change in resistance.

• The result is a ratiometric output directly proportional to the force applied.

• Specified as mV/V output

Choosing the Right Load Cell

• Application considerations – what is

important?

• Mechanical (dimensions & mounting)

• Electrical (output & excitation)

• Environmental (temperature, moisture, etc)

• Performance (accuracy, thermals)


• Mechanical - Mounting

• One of the most critical aspects

that can determine the success

of your application

• Various possibilities: Threaded

connections, adhesive, inline,

through hole,

• Direction of load will impact

your decision of the best

approach

Low Profile mating surface • Surface must be clean and flat

• Mounting surface must be flat to 0.0002 Total Indicator Reading

• Suitable thickness and material

• Recommended hardness of Rc 30 or higher

• Mounting bolt torque according to specifications

Design Alternatives

• Many choices to choose from


• Electrical

• Excitation voltage available

• Millivolt, voltage or current output

• Acceptable input to data acquisition

• Noise immunity

• Cable length

• Proper grounding


• Environmental – Thermals


• Environmental – moisture or humidity?

• Dual diaphragm, or welded, sealed units available

• High humidity, splash down or oil bath applications

can be addressed

• Nema or IP ratings as industry standards

• Some designs cannot be sealed at all

• Take caution selecting cable

• Environmental connectors available


• Environmental - submersible

• Underwater applications are a possibility

• Media density and depth need to be understood

• Barometric pressure compensation

• Susceptibility to media/corrosion

• Vented cable?

What Major Factors

Impact Sensor

Accuracy in Testing

Applications?

Factors

• Specifications

• Mounting

• Calibration

• Instrumentation

• Cables

• Uncertainty of calibration

Specifications

• No “Accuracy” term on

data sheets

• Apply specifications

according to application

• %FS, %RO, %, %/°F,

time related

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.

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.

Static Error Band

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. 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.

Interface compensates every low profile load cell

Each load cell is tested, adjusted, and retested until it is within required performance tolerance

A history of every load cells moment compensation data is retained

Graphs of moment load test results can be provided

Eccentric Load Sensitivity

• ECCENTRIC LOAD - Any load applied

parallel to but not concentric with the

Primary Axis. Results in Moment load.

• SIDE LOAD – Any load at the point of Axial

Load application at 90° to the primary axis.

Maximum allowable off-axis loads

vary by model as shown

Moment and Side Load Error Limit

Example

Assume Model 1220ACK-

50K-B Spec

Limit

Load

Cell

Capacity

Allowable

Extraneous

Load

Maximum Error Due to

Allowable Extraneous

Loads


(% per inch) ±0.25 50K lbf

(40% capacity) x

(1 inch) 20K in-lb

(20K in-lb) x (0.25%/in) /

50K lbf 0.10%

Side Load Sensitivity (%) ±0.25 50K lbf 40% capacity 20K lbf

(20K lbf) x (0.25%) / 50K

lbf 0.10%

Assume Model 1110ACK-

10K-B Spec

Limit

Load

Cell

Capacity

Allowable

Extraneous

Load

Maximum Error Due to

Allowable Extraneous

Loads


(% per inch) ±0.10 10K lbf

(40% capacity) x

(1 inch) 4K in-lb

(4K in-lb) x (0.10%/in) /

10K lbf 0.04%

Side Load Sensitivity (%) ±0.10 10K lbf 40% capacity 4K lbf

(4K lbf) x (0.10%) / 10K

lbf 0.04%

Creep

• The change in load cell signal occurring with

time while under load and with all

environmental conditions and other variables

remaining constant. Expressed as % applied

load over specific time interval.

Temperature Compensation

Temperature effects both zero balance

and output signal

Errors can either be positive or negative

Effects are repeatable from test to test

Temperature effect on output

Expressed as a

% of Reading / °F

Effects the slope of the

output curve

Temperature effect on zero

Expressed as

%Rated Output / °F

Shifts entire curve

parallel to itself

Relatively larger effect

at lower signals

Typical Interface Specs for

Temp Effect on Zero Balance

1100 Spec – High Accuracy

– ± .0004%RO / °F

1200 Spec – Medium Accuracy

– ± .0008%RO / °F

As specified by customer

Temperature Compensation of ZB

Every load cell is temperature compensated for zero balance

Compensation flattens the curve about room temperature

A history of temp comp data for every load cell is retained

Shape of curve isn’t changed by compensation

Standard compensated temperature range is 15ºF to 115ºF

Interface will perform temp compensation at your requested temperatures

Symmetry Error

The algebraic difference between the

Rated Output in tension and the average

of the absolute values of Rated Output in

tension and compression. %RO

Mounting Considerations

Extraneous loads

Parasitic load paths

Mating surface and adapters

Live end vs. Dead end

Live End vs. Dead End

Calibration

Characterization

Calibrate as used in application

Additional points

Ascending and descending points

Multiple runs

ASTM E74

3-Run calibration

Nonrepeatability

The maximum difference between output

readings for repeated loadings under

identical loading and environmental

conditions. Expressed in units of %RO

Cable Length

Lowered sensitivity

Increased temperature

error

6-wire vs. 4-wire

Instrumentation

Zero adjustment

Span adjustment

– Applied load

– mV/V input

– Simulator

– Shunt calibration

Simulator Connects in place of load

cell

Ratiometric mV/V output

Can be used to calibrate

instrument in mV/V

Shunt Calibration Shunt calibration uses a resistor to

stimulate one leg of the wheatstone bridge

The resulting output signal can be useful

for calibrating or scaling instrumentation

Resistor tolerance is generally unimportant

but TCR should be carefully considered

Resistors may be internal or external to

the load cell

Internal resistors don’t get lost

Shunt Calibration Internal shunt cal can also be used to

determine to which bridge a remote

connection has been made, assuming

each bridge has a different resistor value

When using external shunt cal, remember

long cable lengths affect shunt cal values

We provide solutions

Vast Range of Load and Torque

Sensing Solutions

• Clevis pin

• Miniature

• Sub-miniature

• Load buttons

• Load washers

• Axial torsion

• Multi-axis

• Low Profile (pancake)

• Column design

• S Beam

• Single point

• Diaphragm design

• Through Hole

• Rotating torque

• Reaction torque

Why Should you Choose Interface?

Capabilities

• Capacities from 10 grams to 2,250,000 lbf

• Accuracy as high as 0.02% FS

• Internally amplified (4-20mA, 0-5V, 0-10V)

• Hermetically Sealed

• -65˚ - 400˚F thermal compensation

• Multiple bridge configuration


Capabilities

• Fatigue rated

• ATEX, FM, CE, RoHs ratings available

• Submersible – IP68

• Dual diaphragm

• Overload protection

• TEDS IEEE 1451.4

• Customer specified electrical & mechanical connection

40% of our business

is custom or special


Performance second to NONE

• Guarantee: Our load cells will outperform our

competitor’s – PERIOD

• There are fundamental reasons why we can

make that claim

Vertically Integrated Manufacturing • All machining, gaging and fabrication controlled in house


Stocking program • Interface will maintain stock of the finished goods for same day

shipment if orders are received by noon MST.

• Interface maintains a perpetual inventory subassemblies of

standard products. This practice minimizes availability to 10

days or less. No other competitor can match this capability.

• When availability is a concern, please contact the factory to

discuss your needs


http://www.interfaceforce.com/index.htm

What’s behind our performance

guarantee?

What makes Interface Better?

Proprietary Interface Strain Gages • Made in house at our facility in Scottsdale, AZ

• Feature: Interface strain gages precision matched to counteract the temperature characteristics of the modulus of the load cell structural material

• Benefit: The output is temperature insensitive and does not require compensation resistors which produces simpler and more reliable circuit.

What makes Interface Better? Feature: Proprietary Interface Strain Gages

• Benefit: Compensation resistors cannot be thermally connected with the cell’s flexure which results in dynamic thermal performance, resistance

to thermal gradients, and delayed thermal response • Benefit: Elimination of modulus comp resistors allow for higher output


Feature: Proprietary Interface Strain Gages • Benefit: Our strain gages allow us to provide a higher

output - 4mV/V

• Competitors provide 3mV/V or less

• Benefit: Higher signal-to-noise ratio

• Benefit: Higher resolution in precision measurement applications

• Benefit: Superior fatigue life compared to Constantan

What makes Interface Better? Feature: Rigorous Temperature Testing

• Interface performs both hot and cold thermal compensation from 15˚ - 115˚F

• Most competitors provide only the hot side (60˚ - 160˚F)

• Benefit: The slope of the curve is much flatter near room temperature and identical at both ends of the slope. The result is minimal variation across the entire thermal range.

• Custom calibrated ranges available

Feature: Moment compensation

• Interface compensates every low profile load cell

• Competitors do not know their performance in this regard


Eccentric & Side Load Specifications



Feature: Moment Compensation

Benefit: Known maximum error

New Product Releases


New and Improved Low Profile

Better overall static error specifications

Enhanced off axis performance

Improvements will be implemented at no additional cost to

customer

STEP files are available for download via website


Flange load cell

Extremely popular in Europe

Ease of installation due to elimination of thread alignment issues

Good alternative especially with large thread diameter


Model 2300 flange load cell 0.05% F.S. static error band

High capacity: 1M lbf

Eccentric load compensation

Ease of installation

Gaining popularity in Europe


Load Pins

• Capacities from 100 lbs to 300,000 lbs

• Various sizes and shapes, lengths and diameters

• Small quantity orders are acceptable

• ATEX rating for hazardous environment

Load Pin Configurator

Lots of choices

Diameter

Sensing section length

Anti-rotation techniques

Cable exit

Amplified output?

Load Cell Lunch and Learn Presented by Interface

Technology

load ascending

maximum load

minimum load

specific load

direction of load

right load cell mechanical

given load descending

lower load limit