FUTEK offices will be closed on Monday, May 29th, in observance of Memorial Day.
Get to know the functionalities and capabilities of various force sensors, also known as load cells, in this comprehensive guide.
Force Transducer manufactured in US by FUTEK Advanced Sensor Technology (FUTEK), a leading manufacturer producing a huge selection of sensors, utilizing one of the most advanced technologies in the Sensor Industry: Metal foil strain gauge technology. A Force Transducer is defined as a sensor that converts an input mechanical load, weight, tension, compression or pressure into an electrical output signal (load cell definition). Force Measuring sensors are also commonly known as Load Sensors. There are several types of load cells based on size, geometry and capacity.
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 (i.e. pressure measurement) 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 weight sensor increases, the electrical signal changes proportionally.
Transducers became an essential element in many industries from Automotive (car sensors or automotive sensors), High precision manufacturing, Aerospace & Defense, Industrial Automation, Medical & Pharmaceuticals and Robotics where reliable and high precision force measurement is paramount (i,e. medical load cell). Most recently, with the advancements in Collaborative Robots (Cobots) and Surgical Robotics, many novel force measurement applications are emerging.
LCM100 Miniature In Line Load Cell:
LTH300 Donut Thru Hole Load Cell - Force Washer
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.
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 load cell sensor 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.
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.
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 (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, data acquisition modules (DAQ), load cell data logger, computers, or microprocessors. Thus, strain gauge amplifier functions 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 strain gauge load cells is load cell sensitivity and accuracy. 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 measurement. Some of our Universal Pancake Load Cells presents Nonlinearity of ±0.1% (of Rated Output) and Nonrepeatability of ±0.05% RO, which make them an adequate model for rocket engine thrust test stand applications.
Metal foil strain gauge force transducers 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.
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:
Other unique designs are also available such as the load pin load cells (aka load cell pin), seat belt load cell, and others.
How to choose a force transducer 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 load cells for sale. There are also some challenges you may encounter, including finding the compatible load cell amplifier module or signal conditioner or requiring a custom product that would increase the product’s delivery time.
To help you select your 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.
When combined with 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.