OPTIMIZING COMPACT LOAD CELL
DESIGN FOR PERFORMANCE

By: Hai Mei, Senior Manager, Field Application Engineering

TE CONNECTIVITY SENSORS /// WHITE PAPER

OPTIMIZING COMPACT LOAD CELL
DESIGN FOR PERFORMANCE

Automation has become an integral part of our daily functions.
This trend not only drives greater sensor content processing
requirements, but also calls for higher performance, more compact
sizing, and design maximized for volume production.

FORCE SENSING
The element of force is one of the mostfundamental parameters of interest in processcontrol and process monitoring. A force sensor, also
called a load cell, converts the force to an electricalsignal in mV, amplified or digital output format.
Force sensor applications are highlyprevalent. They are used in various
applications including, but not limited to:• In an infusion pump, two force sensors
detect the force generated by medicine
fluid pressure and correlate the differential
signal to the flow rate of medicine.

• Load cells are also used in hospitalbeds to weigh patients.
• Orthopedic instruments also utilizeload cells in devices such as drills tomonitor the torque or force.
• Sports equipment, such as exercise
bicycles, integrates load cells into thespindles for power metering.
• Home appliances like microwave
ovens use load cells for timing controls
in various cooking functions.

• A force sensor in an insulin pump detects
occlusion by sensing force overload on thesyringe plunger and controls the priming
operation by time-based plunger force program.
• Many load cells are used in industrial
and medical robots, to precisely control
torque in joints and force on the tool.

MEDICAL INFUSION PUMPTE CONNECTIVITY SENSORS /// WHITE PAPER

INSULIN PUMPORTHOPEDIC DRILL

EXERCISE BICYCLE

INDUSTRIAL & MEDICAL ROBOTSHOSPITAL BED

OPTIMIZING COMPACT LOAD CELL
DESIGN FOR PERFORMANCE
The most fundamental characteristics for load cells are sensitivity, stability, repeatability, and precision.
Furthermore, the ability to withstand great overload without damage through normal usage andhandling, including accidental drops and other unforeseen pressures, is important. With the exceptionof metrology applications where only sensor precision, stability and repeatability are the predominantfactors, the majority of applications in OEM and consumer markets require balancing the execution ofload cell features against production costs and the ability to scale to volume production thresholds.

FX29
The FX29 button compressive load cell fromThe core sensing element is a semiconductor

TE Connectivity (TE), shown in Figure 1, is a prime

micromachined silicon piezoresistive gage.

example of both high performance and exceptional

Each load cell has four MEMs dies strategicallyvalue while maintaining an excellent compact size.

positioned as illustrated in Figure 2. The gages
are paired for closely matched thermal andelectrical operating characteristics, form aWheatstone bridge with two legs of gages
in tension and the other two legs of gages incompression as the substrate the gages areattached to deforms in response to a stimulationsuch as force or torque. The bridge produces
voltage differential output proportional tothe force applied in the case of FX29.

FIGURE 1. FX29 LOAD CELL (DIA. 19.7MM X H 5.5MM)

FIGURE 2. GAGED FX29 SENSOR

Compared to bonded foil strain
gages (BFSG), a technology
utilized by many manufacturers,
the semiconductor gage is 75
times more sensitive allowing
stiffer load bearing structureand smaller strain level but still
produces 10 times the output.

75x
MORE SENSITIVE

10x

THE OUTPUT

PAGE 3

OPTIMIZING COMPACT LOAD CELL
DESIGN FOR PERFORMANCEWith the stiffer structure, typically with 0.02mm orless deflection as in shown in simulation as Figure
3, the FX29 load cells are unlikely to have fatigue
failure which results from repeated force cycles.
The high stiffness of the structure also enhances its
robustness towards withstanding forces of shockand vibration. The load cell’s overload capability
is typically 2.5 times the nominal force range, asopposed to 1.5 times for BSFG sensors. Otherelements of the design and production confirm theFX29 load cell is engineered to function optimally.
FIGURE 3. TYPICAL FULL-SCALE DEFLECTION OF FX29

The MEMs gages (Figure 4) are bonded to thestainless substrate by glass, an inorganic material,in a high temperature process called “Microfusedtechnology”. Organic bonding material, typical inBFSG but also a destructive cause of sensor drift
over time, is entirely eliminated. Moreover, theMicrofused process relieves the residue stresses in
the mechanical parts, ensuring long-term stability.

FIGURE 4. MICROFUSED GAGETE CONNECTIVITY SENSORS /// WHITE PAPER

OPTIMIZING COMPACT LOAD CELL
DESIGN FOR PERFORMANCEThe Microfused manufacturing process has already

The FX29 load cell body is a two-piece

been fine-tuned for mass quantity production byconstruction made by metal injection moldthe scale of millions. This is exemplified by theand stamping, designed for high volume andfact that the majority of automotive anti-locklow-cost production. Additionally, the one-size-

braking system (ABS) sensors are manufactured

fits-all output (Figure 5) is another feature ofwith this technology; as such, its robustness

the FX29. The standard offerings are mV/V,and price-to-performance ratio are apparent.

amplified voltage and digital I2C outputs in
one compact package, eliminating any needfor add-on, external electronics modules.

FIGURE 5. OUTPUT SCHEMATICS

SUMMARY
In summary, the compact FX29 load cell raises the bar for robustness andexceptional price-to-performance, it is enabled by proven volume production
technology, universally suited for a variety of force sensing applications.

PAGE 5ABOUT TE CONNECTIVITYTE Connectivity (TE) is a global technology leader, providing sensors and connectivity
essential in today’s increasingly connected world. We are one of the largest sensor companies
in the world with solutions that are vital to the next generation of data-driven technology.
TE’s portfolio of intelligent, efficient and high-performing sensor solutions are used forcustomers across several industries, from automotive, industrial and commercial transportation
and aerospace and defense, to medical solutions and consumer applications.

ABOUT THE AUTHOR
Hai Mei is based in Boston area, and, currently Senior Application Engineering Manager at
TE Connectivity, Sensor Solutions. Hai has 30 years of work experience in instrumentation industry
in various capacities such as product design and application engineering, strategic sourcing
and supplier development. Hai holds seven US patents and has developed many pressure andforce sensing products. Hai received MS degree in mechanical engineering from University ofRhode Island and BS degree in engineering mechanics from Shanghai JiaoTong University.

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The information given herein, including drawings, illustrations and schematics which are intended for illustration purposes only, is believed to be reliable. However, TEConnectivity makes no warranties as to its accuracy or completeness and disclaims any liability in connection with its use. TE Connectivity‘s obligations shall only be as set forthin TE Connectivity‘s Standard Terms and Conditions of Sale for this product and in no case will TE Connectivity be liable for any incidental, indirect or consequential damages
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