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How a load cell works?

 

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


Load 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 Cell, Force Sensor or Force Transducer?

By definition, load cell (or loadcell) 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, High precision manufacturing, Aerospace & Defense, Industrial Automation, Medical & Pharmaceuticals and Robotics where reliable and high precision force measurement is paramount. Most recently, with the advancements in Collaborative Robots (Cobots) and Surgical Robotics, many novel force measurement applications are emerging.

 

How does a load cell work?

Firstly, we need to understand the underlying physics and material science behind the force sensor 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.

force transducer
Fig 1: Metal Foil Strain gage. Source: ScienceDirect

Structurally, a force sensor (or transducer) 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.

types of force sensors what is a load cell
Fig. 2: Strain gauge deformation in both tension and compression. Source: Wikipedia

An important concept regarding force transducers is load cell 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, aerospace, load cell linearity is paramount in order to accurately feed the PLC or DAQ control system with the accurate force measurement. Some of our Universal Pancake Load Cells presents Nonlinearity of ±0.1% (of Rated Output) and Nonrepeatability of ±0.05% RO.

 

What are the advantages of strain gage-based load cells?

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 force 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 Load Cells?

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, 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. They are great for endurance, and press applications.
  • 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.
  • 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.
  • 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. One of the primary uses of this sensor type is to measure bolt loading.
  • Pancake Load CellsPancake, canister-style, or universal load cells 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. They are also highly resistant to off-axis loading.
  • 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.

 

How to choose a load cell 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 Load Cell” complete guide for further information.

 

  • Step 1: Understand your application and what you are measuring. Load sensors are different from pressure sensors or torque sensors 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 force measurement system.

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

 

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