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Why Do I Need a Load Cell Amplifier (and Other Signal Conditioners)?

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Table of Contents

Preface

A load cell is a transducer or sensor, that converts the kinetic energy of a force into a quantifiable electrical signal. The strength of the signal is proportional to the force (compression, tension, pressure, etc.) applied to the load cell. Generally this signal is very small. Therefore it must go through signal conditioners or load cell amplifiers to improve signal quality before use by displays or control systems.

Signal conditioning is the process of taking raw load cell electrical output and turning it into useful data. Depending on the correction requirements of the application, signal conditioning can be amplification, attenuation, isolation, linearization, or other types of filtering.

Figure 1. ANYLOAD Load Cell Amplifier

This article introduces these various forms of signal conditioning. But first, it gives a brief background of analog and digital signals, and the process of converting one to the other, since signal conditioners depend on this technology.

Analog and Digital Signals

Most load and force measurement sensors or transducers generate an analog signal output. Therefore, signal processing is traditionally analog.

However, with the availability computing, wireless Internet, and faster processing than ever before, signal processing systems are increasingly digital. These digital load cell conditioners include software usable in real-time when added to the measurement system.

Analog processing remains faster, but digital processing often delivers more accurate results. Digital signal conditioning also allows for easier troubleshooting, adjustment, and tuning. Because each processing method has its advantages, systems often incorporate both. In this case, analog-to-digital and digital-to-analog conversions become part of the signal processing.

Analog-to-Digital Conversion

Analog to digital converters, or ADCs, appear in applications when a usable digital signal is preferable. They convert sounds picked up from a microphone, light from a camera, or the measurements of a load cell into digital format.

For instrumentation, the converters sample the amplitude of a signal at discrete points in time to convert it to a digital signal representing the magnitude of the analog measurement. The arrows in Figure 4 below represent each moment data is sampled. The analog input, shown as the light green curve, is a constant stream along the time axis. The output samples are digitized into a stream of bytes.

Figure 4. Discrete Time Sampling

Signal Accuracy and Resolution

This sampling of the signal limits the allowable bandwidth, or range of frequencies that can be accurately represented by the digital signal. This is the “Nyquist limit,” and is equal to half the sampling frequency. That is, if a ADC samples at a rate of 120 samples per second, the highest input frequency that would accurately be represented by the ADC output is 60 Hz.

Because the digital output is no longer a constant signal, operators must pay careful attention to the speed of conversion of the analog signal and the number of digital bits used to represent the amplitude. This will determine the resolution and accuracy of the digital signal.

A bit is a binary digit, or logical value, that represents some unit of information. This is often implemented on two-state devices. An analog-to-digital converter will typically use at least eight bits but can use as many as 12. The digital bandwidth is defined by the resolution of these available numbers.

28 = 256 So the resolution of the 8 bit sample is 1 part of 256

212 = 4096 So the resolution of the 12 bit sample is 1 part of 4096

Signal Conditioning

When signal conditioning is part of a measuring system, its output will ideally be free of noise/unwanted frequencies (through filtering), in the appropriate format, and large enough to detect.

Amplification

Most of the time, the raw output signal of a load cell is too weak to read without amplification, being of the order of 100mV or less. This makes the signal rather susceptible to ambient electrical noise (such as electromagnetic interference). Also many user interfaces require a larger signal to give a proper readout. Therefore, the signal is boosted with amplification.

How does the load cell’s initial signal change with an instrumentation conditioner or amplifier? The load cell output is amplified from 0~36 mV to either 0-10V or 4-20mA. Amplifiers typically use DC power (24V) and can drive ~ 300Ω load cells connected directly. Many handle one or two transducers but can connect to junction boxes to incorporate multiple load cells.

Analog Amplifiers

Typically analog amplification happens through an operational amplifier, also known as an op-amp. Op-amps are electronic devices with two input terminals: the inverting and non-inverting ends. As Figure 2 shows, the raw signal connects to the inverting side, with one resistor, and the non-inverting side connects to ground. A feedback path connects from the output terminal, with a second resistor, to the inverting terminal.

Figure 2. Operational Amplifier Circuit

The output signal is related to the input voltage through the equation below. Therefore the amount of amplification is based on the values of R1 and R2.

Operational Amplifier Relational Equation
V_{0}=\frac{R_{2}V_{i}}{R_{1}}

Instrumentation amplifiers, such as the ones used on load cells, require the amplification of extremely low-level signals. They often contain a combination of as many as three op-amps to generate the necessary, increased output amplitude.  

Certain applications, like accelerometers, require the amplifier to have a high-frequency response, to prevent distortion.

Figure 3. Instrumentation Op-Amp Circuit

Digital Amplifiers

Digital amplifiers, as the name implies, increase the magnitude of a digital signal rather than an analog one. Therefore the analog output from a transducer must be converted from analog to digital through an ADC, as previously explained, before any digital amplification occurs. Digital amplifiers simply multiply the sampled values by a fixed constant to amplify the signal.

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Attenuation

Attenuation is similar to amplification, but instead of increasing the signal amplitude, attenuation decreases the signal amplitude.  This is necessary when the voltage is beyond the range of the analog-to-digital converter. These devices typically become necessary when measuring signals greater than 10V.

For both signal amplification and attenuation, the digital signal is multiplied by fixed constants to mathematically change the amplitude of the output.

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Isolation

Signals outside of the voltage range of the device can cause damage to equipment and be dangerous to operators. Isolation is typically used to protect the measurement system from voltage spikes. It is also used to break ground loops, block surges, and reject high voltages.

Various methods exist for isolating voltages, including creating physical barriers for signals, and using isolation amplifiers. The specific application, and the driving reasons for the need for isolation determine the method used.

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Signal Linearization

Signal linearization converts the non-linear output of a sensor or transducer to a linear function of the desired output. This is necessary for instruments when the signal output does not have a linear relationship to the desired measured output, such as force or load. Signal linearization is achieved with signal conditioners such as op-amps or through digital software. Devices that measure temperature, like thermocouples, often require linearization.  

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Bridge Completion

Bridge completion forms a four-resistor Wheatstone bridge when using a quarter or half-bridge sensor. The Tacuna Systems precision strain gauge amplifier/ conditioner can provide this completion with high-precision resistors. This device can detect small voltage changes across the active sensors connected to it.

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Signal Filtering

Signal filtering is a form of conditioning that removes a specific type of signal such as a specific frequency, from the raw source signal.

For instance, if an operator only wants to measure the high-frequency portion of a load cell output, they might filter the erroneous low-frequency signals that skew results.

Signal filtering is possible both digitally and through analog circuits. The most common filters are high-pass, low-pass, band-pass, and notch filters.

In essence, filtering allows or denies the passage of high, medium, or low-frequency signals from the raw source signal. The range of frequencies that pass through is the “pass-band,” while the denied range is the “stop-band.”

A passive filter is a signal filter consisting of only passive elements such as resistors, capacitors, inductors, and transformers.  This type of filter is useful because it does not require external power, and is typically more stable than an active filter.

An active filter consists of active elements such as an amplifier, and therefore it requires a power source.

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Implementation of a Load Cell Amplifier or Conditioner

While selecting the appropriate load cell model for an application is important, the wrong amplifier or conditioner also can distort the load cell measurements. Likewise, properly operating the load cell amplifier is critical to obtaining high-quality, accurate readings.

For best results always follow the manufacturer’s operation manual, ensure that operators receive proper training prior to installation, and if questions or concerns arise while operating our products — contact Tacuna Systems immediately.  

Connecting and Installing an Amplifier

The following is an example of connecting an ANYLOAD load cell amplifier:

  • The input voltage should connect to 24V DC power and the ground terminal to the ground.  
  • The load cell connects to the amplifier through the positive(+) and negative(–) excitation terminals, and to the positive(+) and negative(–) output mV signal terminals.
  • The output from the amplifier connects to either the current output for a signal of 4~20mA or a voltage output signal of 0~10V.
  • To calibrate the conditioner, remove any load from the transducer and adjust the variable resistor to an output of 0V or 4mA. To calibrate the full span, place the entire load on the transducer and adjust its variable resistor to an output of 10V or 20mA.

The load measurement system will be fully functional and ready to operate. Re-calibrate the device frequently and whenever the output readings change, or the signal is abnormal. Most load cell signal conditioners should operate from 10°C – 40°C, or 15 – 100°F, and below 90% relative humidity. However, always review the product specs before operating in high and low-temperature environments.

Contact the supplier for setup assistance, calibration assistance, and troubleshooting.

The Top Amplifiers to Use With Load Cells

The Tacuna Systems Precision Strain Gauge or Load Cell Amplifier/Conditioner Interface is a low cost, embedded strain gauge signal conditioner/amplifier that accepts or completes a single Wheatstone bridge. The “gain” and “offset” adjustments are manually controllable and useable with any standard load cell. The Tacuna Strain Gauge Interface also works directly with any analog LabVIEW DAQ or other data acquisition system.

The ANYLOAD A1A Strain Gauge Amplifier provides load cell signal conditioning for most transducer types. The ANYLOAD A1A-22 drives up to 1×350 Ω  load cell directly, or 4 x 350Ω load cells or 8 X 700Ω load cells with a junction box. It uses DC power, with an excitation voltage of 12V and a wide output signal of 0V-10V or 4mA-20mA.

ANYLOAD A2A-D2 Strain Gauge and Load Cell Amplifiers are similar to the A1A but can drive two 350Ω load cells directly. It also allows connections with 4 x 350Ω, 8 x 700Ω, or 8 x 350Ω load cells through a junction box. It uses DC power with an output signal of 0 ±10V.

The ANYLOAD A2P-D2 Strain Gauge Amplifier provides signal conditioning to either one or two load cells simultaneously. Its rating is for two 2 x 350Ω gauges, or multiple load cells via a junction box. The A2P-D2 has individually adjustable resistors and output of 0-10V or 4-20mA.

ANYLOAD J04EA-16 and J04SA-16 Junction and Summing Boxes are a compact device capable of summing multiple load cell signals into a single, accurate, output. Its design handles 2-4 single load cells with individual cell excitation or signal trim (i.e., internal resistors are adjustable).

The TBX Wireless Load Cell Bridge System replaces physical wires or cable connections between load cells and measurement indicators. It displays and logs load cell data digitally when integrated with software. The TBX is easy to incorporate with existing systems or new applications.

Summary

The raw signal from a load cell is generally a weak signal, subject to environmental conditions that can make the signal noisy and difficult for an interface to accurately convert to a load measurement. Signal conditioning, including amplifiers, correct this output to increase the overall accuracy of the force measuring system. The output of the measuring system will ideally be noise-free, in the appropriate format (analog or digital), filtered for unwanted signals, and strong enough to be usable by the receiving display, storage device, or control system.

References

  • Measurement and Instrumentation Principles, 3rd Edition Alan S.
  • ANYLOAD A2P-D2 Load Cell Amplifier Product Manual (v1704)
  • The Engineer’s Guide to Signal Conditioning, National Instruments
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