Load Cells

By definition, load cell is a type of force transducer (aka loadcell). 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 sensor increases, the electrical signal changes proportionally.

It 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 load measurement is paramount. Most recently, with the advancements in Cobots and Surgical Robotics, many novel force measurement applications are emerging, such as miniature medical sensors for robotic surgery.

To understand how a load cell works, firstly, one needs to grasp the underlying materials science behind the force sensor working principle, which is the strain gauge (aka Strain gage). Metal foil strain gage is a sensor whose electrical resistance varies with applied force. In other words, it converts force, pressure, tension, compression, torque, weight, etc… into a change in electrical resistance, which can then be measured. 

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. 

Structurally, a force sensor or weight sensor are made of a metal body to which foil strain gauges are bonded. These force measuring sensors 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 load cell working principle is that the change in voltage is proportional to the force applied to the flexure, which can be calculated via the load cell circuit voltage output.

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

The strain gauge bridge amplifiers (or load cell signal converters) provide regulated excitation voltage and convert the mv/V output signal into another form of signal that is more useful to the user (i.e. load cell adc). 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, load cell data logger, data acquisition modules (DAQ) or computers. Thus, force sensor signal conditioner functions include excitation voltage, noise filtering or attenuation, signal amplification, and output signal conversion.

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

Metal foil strain gauge load cell sensors are the most common technology, given its high accuracy, reliability, variety of shapes and sensor geometry and cost-effectiveness when compared to other force measurement technologies. Also, strain gage load 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 designs can go up to billions of fully reversed cycles.
• A plethora of geometries and customized shapes, as well as mounting options for ANY scale ANY-where.
• Capacities ranging from 10 grams to 100,000 pounds.

Although there several technologies to measuring force, we will focus on the most common type of load cell: metal foil strain gauge. 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 – Commonly referred to as in-line load cells or a canister-style (or column) sensor with male threads. This type can be used in both pushing and pulling forces applications. In-line sensors offer high accuracy and high stiffness with minimal mounting clearance needed. They are great for endurance and press applications and are also offered as micro load cell (aka micro force sensor or miniature force sensor, miniature load cell, mini load cell or milligram load cell) versions. NanoSensors such as QLA414, can be used in haptic feed robotic surgery applications.
• Load Button – This model has a single flat, raised surface where the compressive force is applied. Load buttons provide exceptional dynamic performance with a frequency response of up to ~20 kHz, ideal for impact force measurement applications. As small load cells as they are known for their robustness and are used in fatigue applications. Miniature Load Buttons can be used in robotic tactile sensing applications.
• S Beam – With other names like S Type load cell, the S Beam sensor is a tension and compression load cell with female threads for mounting. Featuring high load cell accuracy and a compact load cell profile, this sensor is great for in-line processing and automated control feedback applications.
• Thru Hole  – Also known as donut load cell or washer load cell, thru-hole load cells 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 is to measure bolt loading.
• Pancake – Pancake load cell, shear web load cell, canister-style, or universal load cell have a central threaded hole for measuring loads in either tension or compression. These load cell sensors are used in applications needing high endurance, high fatigue life, or high-capacity in-line measurements, such as Rocket Engine Thrust Test Stand, center of gravity scale, aircraft structural testing or material force testing applications. They are also highly resistant to off-axis loading.
• Flat Plate Load Cell - Flat Load Cell is the ideal solution for measuring force, surface pressure, and displacement. These cost-effective and reliable OEM thin load cell sensors are perfect for high volume applications. The FFP350's thin, miniature design makes it perfect for applications where vertical space is limited. 
• Rod End – Rod End Load Cell or Actuator Load Cell offers one male and one female thread for mounting. The male / female thread combination is well suited in applications where you need to adapt a sensor into an existing fixture, such as applications involving Inline Actuators.
• Single Point - Single side with a single point design, also known as shear beam load cell. These sensors are specifically made for OEM applications that require high precision or high volume production. These strain gauged based weight sensors measure tension and compression and are also known as compact parallelogram sensors, or single point load cell. LRF400 is an enclosed low capacity design suitable for applications such as high precision milligram scale for medication.

For miniaturized versions, visit our miniature sensors page.

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

To help you select your sensor and help you achieve force measurement using load cell, FUTEK developed an easy-to-follow 5-Steps guide. Check out “How to Select a Load Cell Guide” for further information. 

• Step 1: Understand your application and what you are measuring. Load sensors are different from pressure sensors (aka pressure measurement load cell), torque sensors or force torque sensor and they are designed to measure tension and compression loads. Some applications require multi axis load cell, such as a 6 axis sensor.
• 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 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 (i.e. off-center loads) and moments prior to selecting the capacity.
• Step 4: Define your size and geometry requirements and mechanical performance requirements (output, nonlinearity, hysteresis, creep, bridge resistance, resolution, frequency response etc.) Other features to consider are submersible load cell or waterproof force sensor, cryogenic, high temperature, multiple or redundant bridges, and TEDS IEEE1451.4.
• Step 5: Define the type of output your application requires. Strain gage based sensors circuit outputs voltage in mV/V. So, if your PLC or DAQ requires analog output, digital load cells output or serial communication, you will need a load cell amplifier module. Some applications require a digital load cell indicator, load cell readout, handheld display or to connect to a desktop PC (i.e. USB Load Cell). Make sure to select the right amplifier as well as calibrate the entire measurement system (sensor + amplifier). This turnkey solution translates into a more accurate force measurement system.

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