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.
Firstly, we need to understand the working principle of strain gauge load cell, which is the strain gauge. Metal foil strain gage is a material 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.
Structurally, a load cell is made of a metallic body (i.e. flexure) to which 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 a force (tension or compression) is applied, the sensor 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 signal generated by the 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. Thus, the signal conditioner provides regulated excitation voltage to the bridge circuit and converts the mV/V output signal into another form of signal that is more useful to the user, for example, 4-20mA, VDC or a digital USB output.
The assembly and setup of a sensor platform has vastly improved with the introduction of USB. The traditional assembly of a sensor platform involved a sensor, an amplifier, a power supply, a load cell data acquisition system and software, on occasion a filter, and then a PC to capture and store the data. Though traditional platforms are suitable for most test and measurement applications, they carry a laundry list of “side effects” with them. Cost, space, noise, accuracy, convenience, and temperature co-efficiency all become variables within that traditional setup. What the industry needed was a fast data exchange solution that wouldn’t be phased by such “side effects.” And there lied the answer, a single unit solution – the USB module.
Resolution can be defined in three ways: internal, noise-free, and effective number of bits. All USBs start at an internal resolution of 24 bits. The noise-free resolution (NFR) of our devices can go up to 21 bits.
Some dynamic force measurement applications require a higher sampling rate. FUTEK’s USB Series offer sampling rates, ranging between 5 and 38.4k samples per second (SPS). Please note that resolution will differ as your sampling rate increases.
FUTEK defines accuracy as a limit tolerance that defines the average deviation between the actual output versus theoretical output. And we define nonlinearity as the maximum deviation of the calibration curve from a straight line drawn between the no-load and rated load outputs. Knowing that both of these specifications are important, our entire USB line has a ± 0.005% of FSR for both accuracy and non-linearity.
Noise interference can be quite detrimental to your data collection, especially for delicate measurements in the milligram capacity ranges, so reducing the noise was our primary objective when designing the USBs.
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.
The Serial Peripheral Interface (SPI) is a synchronous serial data protocol used for short-distance communication with one or more peripheral devices over short distances. It is used primarily in embedded systems and can also be used for communication between two microcontrollers and periperialhs such as load cell and torque sensors.
The SPI architecture comprises of one “Master” device and one or more “Sensors”. It can operate in either full-duplex mode (communication occurs in both directions simultaneously) or half-duplex mode (communication occurs in one direction only). The Master device initiates an information transfer on the bus and generates clock and control signals. The Master controls the Sensors through individual Sensor select lines which are active only when selected.
In the past 5 years, FUTEK's Electrical Engineering Team has been developing novel devices to enable SPI data exchange between our sensors and integrated systems, which provides robust and synchronous data bus between our sensors and the system microcontroller.
As an example of SPI signal conditioner is the IDC305 Digital Controller with SPI, USB, and Analog Output. The IDC305 digital SPI output offers:
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 SPI electronics Its main digital output features are: