Load Measurement

What is a load measurement sensor, what are the different types of sensors and how do they work? Get to know the functionalities and capabilities of various load cells, also known as force transducers, in this comprehensive guide.

Force Sensor manufactured in US by FUTEK Advanced Sensor Technology (FUTEK), a leading manufacturer producing a huge selection of Force Transducers, utilizing one of the most advanced technologies in the Sensor Industry: Metal foil strain gauge technology. A Force Sensor is defined as a transducer that converts an input mechanical load, weight, tension, compression or pressure into an electrical output signal (load cell definition). Force Sensors are also commonly known as Force Transducer. There are several types of load cells based on size, geometry and capacity.

What is a Load Measurement Sensor?

By definition, force sensor is a type of transducer, specifically a force transducer. It converts an input mechanical force such as load, weight, tension, compression or pressure into another physical variable, in this case, into an electrical output signal that can be measured, converted and standardized. As the force applied to the force sensor increases, the electrical signal changes proportionally.

Force Transducers became an essential element in many industries from Automotive (car sensors or vehicle sensors), High precision manufacturing, Aerospace & Defense, Industrial Automation, Medical & Pharmaceuticals and Robotics where reliable and high precision measurement is paramount. Most recently, with the advancements in Collaborative Robots (Cobots) and Surgical Robotics, many novel load measurement applications are emerging.

How does a Load Measurement Sensor work?

Firstly, we need to understand the underlying physics and material science behind the straing load measurement working principle, which is the strain gauge (sometimes referred to as Strain gage). Metal foil strain gage is a sensor whose electrical resistance varies with applied force. In other words, it converts (or transduces) force, pressure, tension, compression, torque, weight, etc… into a change in electrical resistance, which can then be measured.

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Strain gauges are electrical conductors tightly attached to a film in a zigzag shape. When this film is pulled, it — and the conductors — stretches and elongates. When it is pushed, it is contracted and gets shorter. This change in shape causes the resistance in the electrical conductors to also change. The strain applied in the load cell can be determined based on this principle, as strain gauge resistance increases with applied strain and diminishes with contraction.

Structurally, a force sensor load cell is made of a metal body (also called flexure) to which foil strain gauges are bonded. The sensor body is usually made of aluminum or stainless steel, which gives the sensor two important characteristics: (1) provides the sturdiness to withstand high loads and (2) has the elasticity to minimally deform and return to its original shape when the force is removed.

When force (tension or compression) is applied, the metal body acts as a “spring” and is slightly deformed, and unless it is overloaded, it returns to its original shape. As the flexure deforms, the strain gage also changes its shape and consequently its electrical resistance, which creates a differential voltage variation through a Wheatstone Bridge circuit. Thus, the change in voltage is proportional to the physical force applied to the flexure, which can be calculated via the load cell circuit voltage output. The metal foil strain gauge technology is also largely utilized for weight sensors.

These strain gauges are arranged in what is called a Wheatstone Bridge Circuit (see animated diagram). This means that four strain gages are interconnected as a loop circuit (load cell circuit) and the measuring grid of the force being measured is aligned accordingly.

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The strain gauge bridge amplifiers (or load cell signal conditioners) provide regulated excitation voltage to the load cell circuit and convert the mv/V output signal into another form of signal that is more useful to the user. The signal generated by the strain gage bridge is low strength signal and may not work with other components of the system, such as PLC, strain gauge data acquisition system (strain gauge DAQ), computers, or microprocessors. Some applications may require a local signal readout, also known as load cell indicator. If the PLC does not come with a special strain gauge I/O card, it needs a strain gauge module. 

That being said, force sensor signal conditioners' functionalities include excitation voltage, noise filtering or attenuation, signal amplification, and output signal conversion.

Furthermore, the change in the amplifier voltage output is calibrated to be linearly proportional to the Newtonian force applied to the flexure, which can be calculated via the load cell circuit voltage equation.

An important concept regarding force transducers is force sensor sensitivity and accuracy. Force Sensor accuracy can be defined as the smallest amount of force that can be applied to the sensor body required to cause a linear and repeatable variation in the voltage output. The higher the load cell accuracy, the better, as it can consistently capture very sensible force variations. In applications like high precision factory automation, surgical robotics, space grade sensors, load cell linearity is paramount in order to accurately feed the PLC or DAQ control system with the accurate measurement. Some of our Universal Pancake Load Cells presents Nonlinearity of ±0.1% (of Rated Output) and Nonrepeatability of ±0.05% RO.

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What are the advantages of strain gage-based sensors?

Metal foil strain gauge force sensors are the most common technology, given its high accuracy, long term reliability, variety of shapes and sensor geometry and cost-effectiveness when compared to other measurement technologies. Also, strain gage sensors are less affected by temperature variations.

• The highest accuracy which may conform to many standards from Surgical Robotics to Aerospace;
• Robust Construction made of either high strength Stainless steel or Aluminum;
• Maintain high performance at the longest possible work life even at the most rigorous conditions. Some load cell designs can go up to billions of fully reversed cycles (lifespan).
• A plethora of geometries and customized shapes, as well as mounting options for ANY scale ANY-where.
• A full gamut of selections with capacities ranging from 10 grams to 100,000 pounds.

What are the types of strain gage-based sensors?

Although there several technologies to measuring force, we will focus on the most common type of load cell: metal foil strain gauge. Within the types of force sensors, there are a variety of body shapes and geometries of load cells for sale, each one catering to distinct applications. Get to know them if you want to buy load cell:

• In-Line Load Cell – Most commonly referred to as in-line force transducers or a canister-style (or column) force sensor with male threads. This style of force transducer can be used in both tension and compression loading applications. In-line sensors offer high accuracy and high stiffness with minimal mounting clearance needed (i.e. cable tension meter or yarn tension sensor). They are great for endurance, and press applications. NanoSensors such as QLA414, can be used in haptic feedback in robot assisted minimally invasive surgery applications. LCM325 in line load cell is used to measure the drag force in the pressure tube of the aircraft trailing cone system. In line miniature load cells is particularly well suited for inline applications with size constraints, such as surgical robotics, as it adds minimal increase in outer diameter or height to the assembly.
• Load Button – These force transducers have a single flat, raised surface (aka a button) where the compressive force is applied. What’s impressive about load buttons is their low-profile design. As small as they are, they are known for their robustness and are used in fatigue applications. Rolling element bearing load measurement, robotic tactile sensing and battery manufacturing automation are examples of applications where load buttons are utilized. Load buttons can also be designed for high capacity load cell applications. Load Buttons provide exceptional performance in high-frequency applications such as drop ball test or impact force measurement.
• S-Beam Load Cell – With other names including Z-Beam or S-Type load transducers, the S-Beam force sensor is a tension and compression force transducer with female threads for mounting. Sporting high accuracy and a thin beam load cell, compact profile, this force sensor type is great for in-line processing and automated control feedback applications such as cable tension transducer applications. S Beam Load cells can also be used as a non contact flow sensor in fluid flow measurement, wire crimp pull tester, as well as coefficient of friction testing applications.
• Thru-Hole Load Cell – Also known as donut load cell or washer load cell, thru-hole force transducers traditionally have a smooth non-threaded inner diameter used to measure compressive loads that require a rod to pass through its center. With a geometry similar to a toroid load cell, one of the primary uses of this sensor type is to measure bolt clamping force (i.e. bolt load measurement), used in bearing proof load tester, anchor load cells in retaining walls, or as hydroponic sensors in vertical farming applications.
• Pancake Load Cells – Pancake, canister-style, or shear web load cell have a central threaded hole for measuring loads in either tension or compression. These force transducers are used in applications needing high endurance, high fatigue life, or high-capacity in-line measurements such as crane load cell, shackle pin load cell, aircraft structural testing, package drop test, center of gravity measurement, injection molding clamping force or tablet compression force applications. They are also highly resistant to off-axis loading, making it suitable for engine dynamometer load cell applications.
• Rod-End Load Cell – This load transducer type offers one male thread and one female thread for mounting. The male and female thread combination is well suited in applications where you need to adapt a force sensor into an existing fixture.
• Bending Beam Load Cell - The LBB200 Bending Beam Load Cell offers a slim design making it ideal for OEM applications. Utilized in Compression, the Bending Beam Load Cells can be used to measure force, surface pressure and displacement for OEM Applications. While FUTEK's average load cells and force sensors price range is about ~$600, LBB200 is a third of the price. LBB200 low cost load cell is an ideal fit for cost-sensitive applications that benefit from simple, accurate, and reliable strain-gauge-based force measurement sensors.
• Single Point Load Cell -  Side mounted load cell with a single point design that are specifically made for OEM applications that require high precision or high volume production. Also known as shear beam load cell, these strain gauged based force sensors measure tension and compression and are also known as compact parallelogram sensors, or single point load cell (replacement for Ishida load cell). Side mount load cells such as the LSM300 model are a recommended OEM weight measurement solution for automatic bottle filling machine, wire pull test or inventory management by weight. LRF400 is an enclosed low capacity design suitable for applications such as high precision milligram scale for medication.
• Pedal Load Cell: LAU Series Pedal Force Sensor offers an ideal solution for Automotive Applications. Pedal Load Cell sensors are designed to measure load applied to the brake, accelerator, and clutch pedals during acceleration, deceleration, and transmission shift events. Brake Pedal Force Sensors are designed to measure load applied to the brake, accelerator, and clutch pedals during acceleration, deceleration, and transmission shift events.

How to choose a load measurement sensors for your application?

We understand that choosing the right load transducer is a daunting task, as there is no real industry standard on how you go about selecting one. There are also some challenges you may encounter, including finding the compatible amplifier or signal conditioner or requiring a custom product that would increase the product’s delivery time.

To help you select your force sensor, FUTEK developed an easy to follow, 5-Steps guide. Here is a glimpse to help you narrow down your choices. Check out our “Important Considerations in Selecting a force measurement sensor” complete guide for further information.

• Step 1: Understand your application and what you are measuring. A Load sensor is different from pressure sensor, torque sensor or force torque sensor and they are designed to measure tension and compression loads.
• Step 2: Define the sensor mounting characteristics and its assembly. Do you have static load or is it a dynamic type? Define the mounting type. How will you be mounting this Force Sensor?
• Step 3: Define your minimum and maximum capacity requirements. Be sure to select the capacity over the maximum operating load and determine all extraneous load (side loads or off-center loads) and moments prior to selecting the capacity.
• Step 4: Define your size and geometry requirements (width, weight, height, length, etc) and mechanical performance requirements (output, nonlinearity, hysteresis, creep, bridge resistance, resolution, frequency response etc.) Other characteristics to consider include submersible (waterproof), cryogenic, high temperature, multiple or redundant bridges, and TEDS IEEE1451.4.
• Step 5: Define the type of output your application requires. Force transducer circuits outputs voltage in mV/V. So, if your PLC or DAQ requires analog output, digital output or serial communication, you will certainly need a load cell amplifier or signal conditioner. Make sure to select the right amplifier as well as calibrate the entire measurement system (load transducer + signal conditioner). This turnkey solution translates into more compatibility and accuracy of the entire load measurement system.

Integrated with a draw wire sensor (aka string potentiometer), load cells are the linchpin of modern factory automation.

For more details on our 5-Steps Guide, please visit our “How to choose a Force Measurement Sensor” for complete guidelines.

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