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Strain Gauge Amplifier

 

What is a Strain Gauge Amplifier and how do they work?


Strain Gauge amplifiers provide regulated excitation voltage to the strain gage circuit and convert the mv/V output signal into another form of signal that is more useful to the user. 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. Thus, strain gauge load cell signal amplifier functions include excitation voltage, noise filtering or attenuation, signal amplification, and output signal conversion.


 

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How does a Strain Gauge Amplifier work?

Firstly, we need to understand the underlying physics and material science behind the load cell, 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.

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

 

Structurally, a force 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.

strain gauge amplifier
Fig. 2: Strain gauge deformation in both tension and compression.

 

 

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These strain gauges are arranged in what is called a Wheatstone Bridge 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 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.

strain gauge amplifier circuit diagram
Fig. 3: Strain Gauge Load Cell Circuit Diagram – Full Bridge Wheatstone Circuit.

 

 

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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 functions of a Strain Gauge Amplifier?

 

The function of a strain gauge amplifier circuit is to capture the signal from the load cell or torque sensor and convert it into a higher level of an electrical signal. These electronic devices are also know as load cell signal converters, given it converts and modulates electrical signals. In order to do so, the mV/V low amplitude output of the load cell goes thru several different signal conditioning steps:

 

Excitation Voltage:

Full-bridge load cells or torque sensors require an excitation voltage from the Wheatstone bridge amplifier to feed the strain gage bridge and generate their output signal as a ratio of the input excitation voltage. Thus, you need to establish if your DAQ or PLC can support the sensor’s input voltage or excitation voltage requirements. Simply put, an unstable excitation voltage input leads to an unstable load cell output. In case you you need a strain gauge strain gauge amplifier module for PLC or DAQ and they do not provide a stable input excitation voltage, the amplifier will be the excitation voltage source to ensure the sensor provides a reliable and consistent output signal. For example, FUTEK’s USB Family digital signal conditioners can provide excitation for amplified sensors, up to 24VDC, all off the USB 2.0 5VDC supply.

 

Filtering:

Analog sensor signals are susceptible to electrical noise and/or residual ripple voltage, which can distort or skew measurements. Noise needs to be filtered out before you can capture an accurate signal. DAQs and PLCs designed to interface directly with full-bridge sensors will include pass band and other forms of signal conditioning and filtration. In a low noise load cell signal conditioner, electronic filters eliminate some effects on accuracy by removing electrical noise and ripple effect above and below the analog sensor’s signal range, resulting in a low signal to noise ratio. For example, FUTEK IAA Family Analog signal conditioners has bandwidth selection feature, used to set the bandwidth from 100 Hz to 50,000 Hz, allowing for noise filtering according to the load cell or torque sensor application.

 

Amplification:

A full-bridge strain gage sensor can output a signal in the nanovolt through millivolt range. When your DAQ or PLC is limited to measuring volts, you will need an strain gage amplifier to convert millivolts to a larger signal. Some PLCs and DAQs come with built-in amplification; others will require an external amplifier. What if your existing DAQ or PLC does not provide built-in amplification, signal conditioning, and a stable power source for sensor excitation? In that case, you will need an amplifier to fill in the shortfalls in your instrumentation, supporting your full-bridge sensor. For multi axis sensors, such 6 DoF Force Torque sensor, one needs a multi channel load cell amplifier circuit able to process all the mV/V outputs of the channels.

 

Signal conversion:

The majority full-bridge load cells and force measurement sensors or transducers generate an analog output in the millivolt range (mV/V). Thus, signal processing is traditionally analog. So, if you req PLC or DAQ system requires an amplified analog (i.e.: strain gauge amplifier 4-20mA, 0-10 VDC) or a digital output (USB, SPI), the load cell or torque sensor needs a strain gage signal conditioner module to convert the mV/V signal to the required signal output. Normally, a handheld load cell display or a load cell indicator is required for local indication (load cell readout) of the force measurement value.

Some applications require digital output, which will require a signal conditioner module with a analog to digital converter (ADC). For those applications, two critical parameters must be taken into consideration when selecting the digital amplifier: noise free resolution and sampling rate. In that regards, FUTEK has a broad range of load cell USB output kit (USB force sensors amplifiers) that provides microprocessed load cell signal converter with internal high resolution (24 bits), capable of offering up to 21 ENOB and up to 19 bits noise free resolution. Our entire USB load cell amplifier module line has a ± 0.005% of FSR for both accuracy and non-linearity.

Some applications require an aggressive sampling rate, where a thousand samples per second maximum just won't cut it. FUTEK’s USBs offer excellent sampling rates, ranging between 5 and 15k samples per second. Please note that resolution will differ as your sampling rate increases.

 

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