Integrating a force sensor with a DAQ system requires a clear understanding of the full signal path. Figure 1 illustrates the signal path from initial mechanical strain to final digital processing. This article walks through each component in that chain, explaining what happens at every stage and how to select hardware that fits your system requirements.
Key Takeaways
- Signal Chain Architecture: A complete force measurement system consists of three sequential subsystems: the transducer (sensor), signal conditioner, and Data Acquisition (DAQ) unit.
- Sensor Transduction: Strain gauge load cells convert mechanical strain into millivolt-level differential signals using an excited Wheatstone bridge circuit.
- Conditioning Functions: Signal conditioners isolate power lines to prevent fault currents, filter high-frequency electromagnetic noise, and amplify low-voltage outputs to match the DAQ’s input voltage range (0-5 V or 0-10 V).
- ADC & Nyquist Rule: To prevent signal aliasing, DAQ anti-aliasing filters strip out frequency components exceeding half the sampling rate (\(f_s / 2\)) prior to sample-and-hold quantization.
- Hardware Matching: Pairing factory-calibrated signal conditioners (like the Tacuna Systems EMBSGB200) with multi-channel DAQ units eliminates gain mismatch and ground loop errors.
The Force Sensor: Transduction Principles
A sensor detects a physical phenomenon (such as force) and then exploits a physical principle to convert the detected quantity into a proportional output through a process called transduction. The type of sensor is classified depending on the physical principle it uses and the type of output it generates. (See An Overview Of Load Cells, and The Versatile Strain Gauge Load Cell for more information about these sensors.)
Summary Table: Types of Force Transducers
| Sensor Type | Physical Operating Principle | Typical Output Signal | Primary Advantages & Applications |
| Strain Gauge | Elastic deformation altering foil resistance in a Wheatstone bridge | Millivolts (mV/V) | High accuracy, fast response; standard for industrial weighing and structural testing |
| Hydraulic | Piston fluid compression transferred to a pressure transducer | Fluid Pressure (PSI, bar, or MPa) / 4 – 20 mA (for transducers that convert to electrical current) | Immune to electrical interference; ideal for hazardous, explosive, or outdoor environments |
| Pneumatic | Force-balance against compressed air or gas chamber | Pneumatic Pressure / PSI | Explosion-proof, intrinsically safe; common in food processing and cleanrooms |
| Capacitive | Mechanical displacement altering plate distance and capacitance | Frequency / Voltage (V) | Extremely sensitive to micro-forces; used in high-precision laboratory physics |
Strain Gauge Force Sensors
The most common of the sensors mentioned in this article is a strain gauge load cell. This sensor uses an elastic resistive material called a strain gauge, attached to a rigid structural member. When a force is applied to the load cell at a specific point, the gauge’s structural member experiences a mechanical strain. The strain gauge deforms elastically under this strain, producing a change in resistance.
All strain gauges require an excitation voltage at the input. The change in resistance changes this voltage. Using a Wheatstone bridge circuit, the change in voltage due to strain produces an output signal. A strain gauge load cell has typically four wires: two power supply/excitation wires and two output signal wires.
Signal Conditioning: Isolation, Filtering, and Amplification
This is the intermediary unit between the force sensor and the DAQ. It converts the output from the transduction unit (sensor) into a signal compatible with the data acquisition unit. This signal conditioning process generally follows three steps:
- Isolation of the power supply to the sensor
- Filter noise from the sensor output
- Amplify the signal for compatibility with the DAQ
Isolation
In the case where the load cell excitation signal comes from a regulated supply on the signal conditioner circuit, an electrical isolator is necessary. This device isolates the force sensor from a direct connection to the power supply, protecting the sensor from fault currents. Isolation can be done by a magnetic isolator such as a transformer, or an optical isolator such as an optocoupler.
Signal Filtering
This step is important when using a strain gauge transducer. The output signal from a strain gauge can be very small (in the order of millivolts, mV), and potentially “noisy”. The DAQ requires a clean, detectable input signal. The process of signal filtering removes this noise, or unwanted signals from the sensor output.
Signal filtering is done using specially designed circuits such as active and passive filters. Alternatively, one can place a bypass capacitor between the positive terminal of the signal path and the ground terminal of the signal path. This capacitor serves as a low pass filter since it conducts high-frequency signals (such as AC noise) to ground.
Filtering is always necessary at both the the amplifier input and output.
Signal Amplification: Differential vs. Instrumentation Amplifiers
Amplification is the last step of signal conditioning. The amplifier increases the dynamic range of the filtered sensor output to a level equal to or near the dynamic range of the DAQ.
Generally, two methods exist to amplify the sensor output signal.
1. Differential Amplifier
With this method, the output terminals from the sensor become part of a voltage divider circuit. A differential amplifier then amplifies the voltage divider output.
2. Instrumentation Amplifier
With this method, the sensing device (typically a strain gauge) is in an arm of a Wheatstone bridge. The output terminals of the bridge connect to an amplifier. The most commonly used amplifier in the industry is the Instrumentation amplifier. Its advantages over the differential amplifier include its high and stable gain value, very high input impedance, very low output impedance, and an extremely high common-mode rejection ratio. The low output impedance in turn presents a low input impedance signal source to the DAQ. This prevents changes in the input of the DAQ from altering the output from the amplifier.
\(V_{\text{out}} = ( V_{2} – V_{1} ) \cdot\ \left( 1 + \frac{2R_{1}}{R_{g}} \right) \cdot \left( \frac{R_{2}}{R_{1}} \right)\)
The Data Acquisition (DAQ) Unit
This unit receives the conditioned signal from the force sensor. It consists of the following digital signal processing units:
- The anti-aliasing filter,
- The sample-hold (S-H) circuit,
- The quantizer-encoder circuit, and
- The digital signal processor.
Anti-Aliasing Filtering and Nyquist Theorem
This analog filter circuit band-limits the DAQ unit’s input from the amplifier circuit, removing any frequency components greater than half of the DAQ’s sampling frequency. It captures the raw amplifier signal before any sampling can occur to satisfy the Nyquist Theorem, which states that a signal must be sampled at least twice as fast as its highest frequency component (\(f_s \ge 2 f_{\text{max}}\))
Without the anti-aliasing filter, the DAQ’s sampling would occur at inappropriate intervals for these higher frequencies. This would produce a sampled signal containing various overlapping lower-frequency components. Aliasing is the term for this overlapping effect, depicted in Figure 6.
Sample and Hold (S-H) Circuits
This component performs sampling. “Sampling” is the measurement of an analog signal’s amplitude at discrete time intervals to produce a sequence of samples. The term “sampling rate” refers to these discrete time intervals. This rate must be at least the Nyquist Limit (again,twice the signal’s highest frequency component).
The Sample and Hold circuit (S-H) consists of a solid-state high-frequency switch, a holding capacitor, and a buffer amplifier (i.e., a voltage follower).
In practice, the S-H circuit is very key to the sampling process because the input analog signal is time-varying. To truly sample this signal, the component must determine the instantaneous frequency value at each specific time. The circuit holds the instantaneous value steady till the next time interval. The held signal value must be sampled within that holding time period at the appropriate sampling rate to achieve an accurate result.
Figure 7 below shows a simple S-H circuit. This unit converts the analog signal into a discrete-time signal continuous in amplitude but discrete in the time domain.
Quantization and Binary Encoding (ADC)
The quantizer unit samples the S-H output in the amplitude domain. Quantization creates a range of equally spaced levels. Each signal level exists between only two possible limits; hence the term quantized.
The encoder then assigns a certain binary code to each of these levels. The encoder can use one of two possible encoding schemes: the two’s complement coding scheme or the offset binary coding scheme. Both are ways to represent signed integers (positive versus negative) in binary code.
Note that the S-H circuit, the quantizer and the encoder together all form an analog-to-digital converter (ADC). This is so because the input to the S-H circuit is analog and the output from the encoder is digital.
The Digital Signal Processor (DSP)
This component performs certain processing operations on the digital signal: frequency and time domain analysis, filtering, convolution, and correlation.
The DSP is a unit that has its own subsystems. Also, it runs on a dedicated hardware architecture that is quite similar to that of a general microprocessor. The special hardware unit of the DSP includes multipliers, an accumulator unit, shifters, address generators, data memory, and program memory. The DSP memory stores the digitized signal for later retrieval by processes such as display or process control.
Figure 8 shows the architecture of a DSP.
Hardware Integration: Amplifiers and DAQ Units
The data acquisition hardware shown in Figure 9 below is a portable device that can aggregate input from several sensors. It has 16 input channels and a sampling rate of up to 1000Hz. It include a data logging system, and offers both wired and wireless telemetry transmission.
A starting point for the other devices described in this article is the Tacuna Systems online catalog. Tacuna Systems offers a wide range of amplifiers and conditioners that prepare load cell signals for input to this DAQ. Figures 10-11 below show one such amplifier in our catalog and its datasheet specifications.
The datasheet shows the amplifier gain, the load cell type it supports, the input range it supports from a sensor, and the output voltage and current. Note that the output voltage and current signal value all fall within the dynamic range of the DAQ device.
Tacuna Systems offers an in-house designed amplifier product that offers bridge completion and factory-calibrated gain. Available in three options, it is our most popular and highly-rated amplifier. See How the EMBSGB200 Amplifier is Shaping Innovations in Technology and Industry and Next-Gen Transmissions for High-Performance Hybrid Cars for examples of how our customers have used this versatile device.
Our customers find that acquiring all system components from our suite of products reduces the time from order placement to up-and-running. This is because we offer complimentary design support with a complete package of compatible force measurement components, eliminating guesswork or trial and error.
Summary: Choosing Compatible Components for Your Signal Chain
Understanding the full signal path from initial mechanical strain to digital signal processing is key to building a low-noise, reliable force measurement system. Once processed by the DAQ, digitized data can drive operator readouts, integrate into closed-loop PLC industrial control systems, or transmit remotely via telemetry (see Advantages and Applications of Wireless Load Cells).
Sourcing compatible signal conditioners and DAQ units together eliminates gain mismatches and speeds up deployment. Browse our catalog of amplifiers and conditioners or contact Tacuna Systems engineering for custom signal chain integration support.
