What is a digital load cell and how they work?
Digital Load Cells 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 Load Cell 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 Sensor. There are several types of load cells based on size, geometry and capacity.
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.
Firstly, we need to understand the underlying physics and material science behind the strain gauge load cell 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 (i.e. pressure sensor), 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. This same concept is also utilized in weight sensors.
Structurally, a 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.
These strain gauges are arranged in what is called a Wheatstone Bridge Amplifier Circuit (aka Load Cell Amplifier Circuit). 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 conditioners) provide regulated excitation voltage to the load cell amplifier circuit and convert the mv/V output signal into another form of signal that is more useful to the user, for example a 4-20ma load cell analog output or a digital USB load cell output. 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), computers, or microprocessors. For some applications, it may be needed a local signal readout, also known as a load cell indicator. 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 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.
The difference between analog and digital load cells
As per the diagram above, in strain gauge based load cells, the output is in mV/V level and depends on factors such as excitation voltage, number of strain gauge resistances on each leg, and resultant bridge resistance. The basic difference between an analog and a digital load cell is how the Wheatstone bridge output signal is processed. Normally, the output from the strain gauge bridge starts as millivolt per volt (mV/V) analog electrical voltages which is a very low-level analog signal. This low-level analog signal can be then integrated either into an analog or digital load cell amplifier.
The IDA100 digitally configurable amplifier with USB output, is a signal conditioner that offers users the unique ability to have both an amplified analog and digital output, suitable for digital load cell applications. The dual output features of the IDA100 digital Load Cell ADC are powered solely by the 5V output from the USB. This device also has a software selectable ±5V and ±10V analog output with a low noise value of 12 mVp-p (millivolt peak-to-peak) and a bandwidth of 1 kHz.
Another example of a digital load cell amplifier is the IDC305 Digital Controller with SPI, USB, and Analog Output. The IDC305 digital SPI output features:
The QIA125 Digital Low Power Three Channel SPI Output, is a Multi channel Load Cell Amplifier suitable for Tri axial Load Cells (3 axis), with low power consumption requirements and digital output (SPI output).
The QIA123 Digital Low Power/High-Speed Full Bridge Embedded Controller with SPI & Analog Output is another example of a digital load cell amplifier. Its main digital output features are: