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Load Cell Signal Conditioner

What is a load cell signal conditioner?

FUTEK Instruments
FUTEK Electronics

What is a load cell signal conditioner’s job?

In the world of test and measurement technology, electronics fill the void between mechanical systems and digital and analog devices. Sensors are used to measure and convert physical quantities into equivalent electrical signals. Strain gauge-based sensors provide a tiny analog output that contains a lot of information. In order for this data to be interfaced to other devices such as data acquisition (DAQ), PLCs, and other analog or digital equipment, it must be converted without losing meaningful information. Electronics (strain gauge-based signal conditioners/amplifiers) perform this conversion. They also boost the signal level to increase measurement resolution and improve signal-to-noise ratios.

Industrial work bench featuring FUTEK electronics

In addition to converting sensor signals, electronics also optimize them. There will always be factors, however miniscule, that affect a sensor’s performance. In real-world applications, strain gauge-based sensors are often exposed to conditions and environments that may induce signal noise. The systems themselves can also present challenges: the more components that are added to a measuring solution, the more errors will be introduced. Electronics can help solve sensor challenges and optimize the signal by amplifying, filtering, protecting, and compensating the output. As technological advances require ever-more complex and precise measurement systems, sensors have become increasingly dependent on electronics, to the point that sensors are now used with electronics in nearly all applications.

Breaking down the key functionalities

Since the signal conditioner/amplifier functions as a strain gauge reader, you will need to be familiar with the underlying physics and material science behind the strain gauge (or strain gage) to fully understand the functionality of a strain gauge signal conditioner/amplifier. We will break down the basic principles below.

Anatomy of the strain gauge

Structurally, a strain gauge-based sensor is made of a metal body (also called flexure) to which foil strain gauges are bonded.

Strain gauges are electrical conductors that are tightly attached to a film in a zigzag shape. When stress is transferred from the load cell flexure to the strain gauge, the resulting deformation or displacement of its material causes strain that ultimately is converted into the load cell’s measurable output. For example, when the 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, what we call strain gauge resistance, to also change. The strain gauge resistance increases with applied strain and diminishes with contraction. The changes are converted into an electrical signal, which can then be measured and captured using data acquisition.

Strain Gauge Diagram strain gage foil tension and compression

Understanding the Wheatstone Bridge circuit

In order to measure the changes in resistance, the strain gauge must be connected to an electrical circuit that is capable of accurately responding to the changes and creating a differential voltage variation. Multiple strain gauges can be used in a divided bridge circuit that is called a Wheatstone bridge. In a Wheatstone bridge configuration, an excitation voltage is applied across the circuit, and the output voltage is measured across two points in the middle of the bridge. Any small change in the material under the strain gauge results in a change in output.

Boosting the signal

As we explained above, the function of a load cell amplifier (or signal conditioner) circuit is to capture the analog strain gauge sensor signal and convert it into a higher level of electrical signal. To do so, the mV/V low amplitude output of the load cell goes through several different signal conditioning steps:

Excitation Voltage: Full-bridge load cells require an excitation voltage from the Wheatstone bridge amplifier to feed the strain gauge bridge and generate its 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. If you need a load cell amplifier for PLC or DAQ and they do not provide a stable input excitation voltage, the amplifier will be the excitation voltage source to ensure that the sensor provides a reliable and consistent output signal. For example, FUTEK’s USB Load Cell Data Acquisition System can provide excitation for amplified sensors up to 24VDC.

Filtering: Filtering is extremely important as it helps produce a clean and usable output signal. Analog sensor signals are susceptible to electrical noise and/or residual ripple voltage, which can distort or skew measurements. Hence, noise needs to be filtered out before you can capture an accurate signal. Electronic filters in a low noise load cell signal conditioner increase 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 series analog signal conditioners have bandwidth selection features that are used to set the bandwidth from 100 Hz to 50,000 Hz, allowing for noise filtering according to the load cell application.

Amplification: A full-bridge strain gauge sensor can output a signal in the nanovolt through millivolt range. When your DAQ or PLC is limited to measuring volts, you will need a strain gauge amplifier to convert millivolts to a larger signal. Some PLCs and DAQs come with built-in amplification; others will require an external amplifier. For multi-axis sensors, such as a 6 DoF Force Torque sensor, you need a multi-channel load cell amplifier circuit with the capability to process all the mV/V outputs of the channels.

Signal conversion: The majority of strain gauge-based sensors (or transducers) generate an analog output in the millivolt range (mV/V). Thus, signal processing is traditionally analog. So, if your PLC or DAQ system requires an amplified analog (i.e.: mA analog current, VDC analog voltage) or a digital output (USB, SPI, UART), the load cell needs a strain gauge signal conditioner to convert the mV/V signal to the required signal output. Normally, a 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 with an analog-to-digital converter (ADC).

Besides boosting and optimizing the strain gauge signal in the steps above, signal conditioners/amplifiers also play an important role in mitigating external factors that can affect the sensor output. All sensors, no matter how well-designed and precise they may be, will encounter challenges that affect their performance. Using a high-quality signal conditioner/amplifier minimizes these imperfections and optimizes the sensor’s output. Below are some common challenges that can be mitigated with electronics:

Excitation: Strain gauge-based sensors require an external power supply (excitation) to operate. In other words, the excitation signal is used by the sensor to produce the output signal. As we showed above in our breakdown of the Wheatstone bridge circuit, the output of the bridge is derived directly from the input to the bridge. This means that the quality of the sensor’s output is directly related to the quality of the input (excitation) and that to achieve a clean output, an equally clean input (excitation) must be provided Hence, it’s critical to properly design the excitation circuitry to provide a clean, low drift and well-regulated output and stable excitation signal.

Sensitivity Error: The slope of the characteristic output curve of a sensor usually defines the sensor’s sensitivity. Hence, the amount of deviation from the ideal curve defines the sensitivity error, which could result in the sensor being non-symmetric and also directly impacting the dynamic range (the total range, from minimum to maximum, that could be measured during normal operation). For example, an ideal sensor does not have sensitivity error thus, it is fully symmetrical. However, in practice, the output of a real sensor could deviate up to 20% (sometimes even more) from each direction. In order to minimize the error, this deviation needs to be addressed by the electronics connected to the sensor.

Precision: The degree of reproducibility of a measurement is defined as precision. This means that an ideal sensor would be able to measure the same exact value over and over with the same stimuli. In practice, the output of a real sensor could be impacted by external factors that we cannot fully control, such as the nature of the material that the sensors are made from, or some processes that the sensors need to go through during manufacturing. This shows the importance of keeping the errors introduced by the electronics being at least one order of magnitude lower than errors introduced by the sensor. At FUTEK, we have invested a lot of time investigating and analyzing our manufacturing process to identify these errors, finding a solution for every step in the process, and addressing them in our electronics design process as well. The same concept is valid for the accuracy characteristic of the sensor.

Sensor Resolution: The definition of resolution can be expressed as the smallest detectable change of the input that can be presented in the output. It means that the electronics must be designed to have a higher level of precision that is capable of resolving the output of the sensor to provide a precise measurement.

Offset Error: As we have already established, there is no perfect or ideal sensor, which means that some errors are expected for the sensor output. In other words, the offset error of a sensor is defined as the output that exists when there is no load. FUTEK minimizes the offset error of the sensors during the manufacturing process, but this error cannot be eliminated 100%, which is why electronics are needed to limit its effects.

Linearity Error: The linearity of the sensor could be identified as how much the actual measured output curve of a sensor could deviate from its ideal straight-line curve. Amplifier/signal conditioners will also have some linearity errors, so it is important to consider designing a circuit that provides extremely low error in comparison to the sensor error. This makes sure that the overall system non-linearity remains close to the sensor error. It should be mentioned that in some extreme cases, the electronics are designed to provide linearization. This approach usually involves a lot of complications, such as additional digital/firmware development and enhanced calibration that need to be implemented on the electronics side by understanding the unique nonlinearity characteristic error of every sensor.

There are other factors related to the sensor specs that electronics can help mitigate, such as Hysteresis, Response Time, Bridge Resistance, Sensor Drift, etc. To learn more about these terms, please see FUTEK’s glossary.

FUTEK frequently works on complex high-performance applications such as multi-axis surgical tools and robotic limbs. Hence, we ensure that our systems are fully optimized to meet the most stringent requirements. Our electrical engineering design team pushes the boundaries of all design factors, such as calibration, amplification, and filtering, that can give us an edge in performance. We also try to identify the most common failures and offer a solution for each of them. None of these features are standard in the market ⎯ we created them to offer flexibility, reliability, and efficiency in our products.

Fault detection: Our signal conditioners/amplifiers have the capability to detect excitation faults (open/short) which can be indicated visually, through a digital pin or a data packet to be transferred. For example, our High Resolution & Speed USB Output Kit and Digitally Configurable Analog Voltage Amplifier have multiple fault detection features, such as temperature protection and protection from over current/voltage events.

Onboard temperature: We utilize onboard temperature measurement to make sure the electronic devices operate in a nominal condition. As an example, Digitally Configurable Analog Voltage Amplifier has built-in fault detection and on-board temperature measurement that continuously monitor the temperature of the board. If the temperature falls outside of IAA105’s -20°C to 70°C operating range, a red LED signal will start blinking. For embedded electronics, this onboard temperature could be even more useful when embedded electronics are used. Since the distance between the bridge and the electronics is minimal in this scenario, the measured temperature could be a good estimation of the bridge temperature as well. This measurement can in turn be used for temperature compensation of the bridge.

Packet acknowledgment implementation: As a medical application supplier, we understand the importance of digital implementation in medical devices and we have tried to make our devices as safe as possible. With that being said, we make sure that all data transfers for digital communication have an acknowledgment (CRC/checksum) implemented.

Low power consumption: In some of our critical applications that require ultra-low power consumption, we have designed a feature that provides the controlling device with full control of the power consumption of the system. This important feature allows the controlling device to constantly monitor the overall power consumption and either shut down or put the FUTEK device in low power mode as needed. Overall, our electronics have been developed to operate on low power consumption, for example, our Digitally Configurable Analog Voltage Amplifier has an efficient consumption of 1.2W and our Ultra-Low Power Miniaturized Integrable Sampling System has a power consumption of only 60mW.

Protection: We make sure that all of the exposed pins of our electronic designs have some sort of protection against unexpected events. It should be noted that the protection circuitry could impact the performance of the design, thus designing a proper protection system without sacrificing the performance is the key to this development. Depending on the types of applications, we go even further and provide standard certifications for our products such as CE, MTBF, etc.

PCB Design and material selection: In mixed system designs, proper isolation between analog and digital circuitry is one of the keys to high performance.. Since FUTEK designs our own electronics, including the PCB, we can optimize our system even further by customizing our design with less restrictions. We also select the PCB material per the system requirements as the long-term performance of our systems is vital as our products are used in many different environments such as: high humidity, vibration, high/low temperature, etc.

Low impedance: FUTEK signal conditioners have been designed to have great pairing compatibility with external devices on the market. The low impedance of our signal conditioners/amplifiers offer flexibility in usage as well as greater flexibility when connecting to external units.

Noise Immunity: Since we design and develop our products in-house, we make sure that our signal conditioners/amplifiers have robust enclosures, such as added inner metal covers and added chassis that help improve noise immunity, resulting in lower noise levels. To learn more about how to reduce electrical noise in your system, see our guide.

Low nonlinearity: Linearity is defined as the amount of deviation of the measured output curve of a sensor versus the ideal straight line curve. FUTEK’s electrical engineering team designs electronics with exceptionally low nonlinearity, (as low as > 0.002%) ensuring that the overall system remains as close to the sensor’s nonlinearity as possible, securing a stable and accurate performance. High sampling rate & high noise free resolution (NFR): FUTEK’S innovative single-unit USB solutions offer uniquely high resolution (up to 20 bits) and high sampling rates (max SPS 38,400) with a wide range of selections, which helps maximizing sensor performance.

 

Sensors and signal conditioners talk to each other through signals that are transmitted via a device-to-device communication protocol. Below is a breakdown of the different types of output:

What is analog voltage?

An analog voltage signal is an electrical signal that varies in voltage and is measured in volts. Voltage signals represent and transmit continuous analog data and are typically used in low-power applications.

What is analog current?

An analog current signal refers to an electrical signal that varies in current and is measured in amperes. Current signals represent and transmit continuous data and are typically used in high-power applications.

Digital USB

USB supports and digitizes a wide range of sensor inputs such as ± 10 VDC, 0-30 mA, ±400 mV/V type inputs using an integrated analog to digital converter (ADC). USB output is particularly well suited for test and measurement applications that require dynamic force capturing and processing. FUTEK’s Pro Elite High Resolution and Speed USB Output Kit is ideal for these applications as it offers high-speed signal sampling (up to 38,400 SPS) and advanced filtering capabilities, ensuring precise capture of interaction forces and enabling detailed analysis of force measurement for validation and verification.

What is digital UART?

UART (Universal Asynchronous Receiver Transmitter) is a widely-used serial communication protocol in embedded systems. It is known for its simplicity and ease of operation and transmits data one bit at a time sequentially over a communication channel. Common serial communication protocols include RS-232, RS-485, USB (Universal Serial Bus), and SATA (Serial ATA). FUTEK’s Digital Jr S-Beam Load Cell 3.0. is a smart sensor with embedded electronics and data flexibility that is available in two versions: one that offers UART communication and another that offers both SPI and UART connectivity.

What is digital SPI?

SPI (Serial Peripheral Interface) is another serial communication protocol that facilitates data exchange between various electronic devices supporting clocked serial streams. It is known for its simplicity and ease of operation. SPI follows a Host-Client communication approach, allowing high-speed data transfer. It is known for its efficiency and low overhead, making it ideal for high-speed data transfer in applications like load cell signal conditioning.

 

Meet FUTEK’s flagship amplifiers/signal conditioners

The kind of electronics you select depends on the setup and requirements of your application. There are many types of amplifiers or signal conditioners, ranging from analog to digital solutions, panel displays to micro-mini embedded units. FUTEK’s electronic systems have been designed and manufactured to achieve a next-level performance that is a level above conventional measurement industry solutions. Some of our most well-regarded models are featured below:

IAA100/IAA200 Analog Current and Voltage Amplifiers

The IAA models combine unsurpassed accuracy with ease of integration. The units offer in-line amplification of any full bridge strain gauge type sensor with mV/V range output force measurement systems that demand voltage or current output. The IAA Family provides bridge excitation of either 5 or 10 VDC (DIP Switch) and amplifies strain gage sensors output ranges from 0.5 mV/V up to 10.0 mv/V (DIP Switch). Bandwidth ranges between 25 to 50 kHz. The units feature an integrated DIN clip designed for applications in industrial environments.

IHH Digital Hand Held Display

The IHH500 is a battery-operated handheld display and sensor indicator that connects to a computer via USB. It’s suitable for strain-gauge-based load cells, torque sensors, and pressure sensors in engineering applications. This digital weight indicator features an integrated low noise/high-speed/high-resolution ADC, accommodating various sensor inputs. Available in Pro and Elite versions. The versions share the same capabilities, and the Elite also reads and records encoder data like angle and speed. It also includes SENSIT® software for live graph data logging.

The IPM650 Panel Mount Display

This panel mount display, also known as a load cell readout, is an all-in-one standalone solution with a super-fast sampling rate and high resolution.The versatile multi-function display provides seamless integration with USB (2.0) connectivity and supports a wide range of sensor inputs, accommodating up to ±500 mV/V Strain Gauge, ±12 VDC, or 0-30 mA load cells, torque, and pressure sensors. The panel mount design ensures secure integration into your control systems, maximizing sensor/ transducer performance to lower noise and improve precision.

IAA105 Digitally Configurable Analog Voltage Amplifier

IAA105 is a one-of-a-kind amplifier that breaks new ground in sensor electronics. The fully digitally configurable device (offset/span (VDC) of –10 to +10) offers unsurpassed flexibility and simplicity during setup. This highly accurate device also features Bluetooth® wireless communication and USB configurability as well as built-in active protection for exceptional reliability and robustness. Its uniquely precise system calibration can go as low as 300μV. It has the lowest noise (2.8 mVp-p) of FUTEK’s high precision analog solutions and will push the performance of any sensor to its maximum potential.

USB225 High Resolution & Speed USB Output Kit

Some applications require digital output, which calls for a signal conditioner with an analog-to-digital converter (ADC). FUTEK’s groundbreaking USB amplifiers offer a modern, single-unit turn-key solution that eliminates the need for external circuitry and power supply. When selecting the digital amplifier for those applications, two critical parameters must be taken into consideration: noise-free resolution and sampling rate. FUTEK’s innovative digital strain gauge signal conditioner USB225 has exceptionally high resolution (up to 20 bits) and a sampling rate (max SPS 38,400) that has been pushed beyond traditional limits. The unit also has unique features and capabilities such as innovative fault detection and health checks.

The easy plug-and-play connection eliminates the need for external circuitry and power supply. Risk for failure has been minimized through fault detection features that continuously monitor the sensor’s health and indicate any issues, providing crucial feedback for applications that need to be fail-proof.

QIA128 Ultra-Low-Power Miniaturized Integrable Sampling System

QIA128 is a miniature embedded sampling system for load cells with ultra-low power consumption of only 60 mW. It is capable of sampling strain gauge signals up to 1300 samples per second (SPS) with up to 18.4 Bits of Noise Free Resolution (NFR).

It is compatible with strain gauge bridge resistance ranging from 350 Ω to 5000 Ω. This miniaturized integrable sampling system comes in an ultra small package of 8 mm x 8 mm, which makes it ideal for medical devices or robotics applications where assembly space is a limiting design factor.

Offset and span calibration

Proper calibration of your signal conditioner/amplifier is paramount. Most calibration methods in the industry do not factor in the need for independent calibration, leading to challenges in mitigating the effects that offset and span calibration has on one another. FUTEK has developed a variety of calibration methods that isolate the effects of offset and span calibration to solve this and other common issues.

Full system calibration

Since electronics are part of a force measurement solution that includes a strain gauge-based sensor, cabling, and connectors, the system components must be calibrated together as a whole. This ensures that the system is performing as accurately as possible and it also allows you to start using your force measurement solution out of the box, without having to manually adjust and calibrate each system component.

FUTEK’s calibration lab

As part of FUTEK’s commitment to deliver end-to-end solutions, our calibration lab will calibrate, set up, and optimize your system, providing you with a turnkey package of sensor, electronics, and calibration. As an A2LA-accredited calibration lab, FUTEK offers full system calibration for sensors with digital displays, amplifiers, and/or USB solutions. Our calibration procedures comply with ISO 17025 standards and our certification includes accreditation to ANSI/NCSL Z540-1.

To learn more about integrating electronics into your system, please consult our FAQ.

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Z540-1 ANSI Certified17025 ISO Certified9001 ISO Certified13485 ISO CertifiedU.S. Manufacturer
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Please follow us on linked inPlease follow us on twitterPlease follow us on youtubePlease Email us for additional info
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10 Thomas, Irvine,
CA 92618 USA
All other trademarks, service marks and logos used in this website are the property of their respective owners.
© 1998–2024 FUTEK Advanced Sensor Technology, Inc. All rights reserved.