Force and Pressure Sensors with Piezoelectric Load Cell Transducers
Piezoelectric load cell transducer with an RS232 connection
Automation of industrial processes is all about automating data collection, processing, control system adjustments, and execution. Some of these sensors find their way into other important areas, such as aerospace and nondestructive testing. Some common sensors used in these areas are piezoelectric load cell transducers. Like many industrial components, the way in which these cells operate and are brought into a testing unit seem esoteric, but these components are deceptively simple.
For a typical static load measurement, load cells are quite simple and do not require special equipment or measurement techniques. With dynamic load measurements, selecting the right sensor becomes more complicated as you need to know something about the applied load (amplitude, frequency, or both) before it is measured. Here’s how to select piezoelectric load cell transducers for each type of measurement and how to integrate these components into a larger test and measurement system.
What are Piezoelectric Load Cell Transducers?
If you’re familiar with the fundamental physical process that governs crystal clock oscillators, then you’ve got a head start on understanding piezoelectric load cell transducer. These components all operate via the same physical phenomenon: the piezoelectric effect. In a piezoelectric load cell transducer, a strain is applied to the device, which compresses a crystal inside the unit. Under compression, the crystal outputs a voltage, which can then drive an external circuit.
The mechanical construction of piezoelectric load cell transducers is such that the cell is constrained to measure a strain along one direction only. If measurements along two or three dimensions are needed, then two or three piezoelectric load cell transducers can be mounted along perpendicular axes, although this is an uncommon situation.
Piezoelectric load cell transducers have high output impedance and some natural filtration due to parasitics in the device, so these components act like proportional voltage sources. Some commercialized units will include an integrated MOSFET amplifier and signal conditioning RC circuit, which will convert the output to the standard voltage range used in industrial applications (0-10 V, or 4-20 mA). An example application circuit is shown below:
Piezoelectric load cell transducer circuit
Here, there is an RC filter section in the feedback loop. Parasitics in the cable and the output section from the transducer are also included. In these application circuits, the crystal itself is normally modeled as a voltage source, but it is really a charge source, and the associated voltage depends on the dimensions of the crystal.
Static and Dynamic Strain Measurements
When a static load is applied (i.e., the load does not change over time), the separated electric charge generated across the crystal will have some associated potential energy, thus there is some voltage between the two electrodes in the device. Unfortunately, the materials and crystals used in these devices are not perfect insulators, so the excited charge will leak between the electrodes and will recombine. Therefore, piezoelectric load cell transducers are not ideal for static pressure measurements. A resistive load cell is a better option for static measurements.
Piezoelectric load cells are better for dynamic measurements, where the applied load varies in time. This can involve the measurement of a rapidly changing force, in which case these load cells act as accelerometers. They can also measure a harmonic force, where the measurement is being gathered at a single frequency. These sensors are often used for ultrasonic measurements.
Like the majority of real devices, piezoelectric load cell transducers have some frequency response spectrum, and there is a region of frequencies where these devices are most sensitive. The graph below shows a typical example of piezoelectric load cell sensitivity on a log-log plot.
Example frequency response curve for piezoelectric load cell transducers.
There is a region at low frequencies where the sensitivity is effectively flat. However, as the load’s frequency nears resonance, there is a linear range with very high sensitivity. Here, the resonance is shown with ~1 decade gain over ~1 dB frequency range. Real units can provide gain reaching ~100 dB, enabling extremely sensitive strain measurements. The bandwidth in the circuit depends on the components used to connect to the charge amplifier, the charge amplifier itself, and the size and material used for the piezoelectric crystal.
Comparison With Other Types of Load Cells
The other primary types of load cells and stress/strain measurement elements are resistive, inductive, and capacitive load cells. Rather than directly outputting a voltage, these types of load cells are part of a larger circuit (usually an RLC circuit, depending on the load being measured), and they change the voltage/current in a circuit when a strain is applied. Under an applied strain, the resistive, inductive, and capacitive load cells experience a change in resistance, inductance, or capacitance, respectively.
These other three types of strain measurement devices do not output a voltage/current directly. Instead, they are connected to an external power source and a measurement unit (such as an ADC). A brief comparison of these different types of strain sensing units is shown in the following table.
Note that we’ve discussed compressive load cells in this article, but these load cells can also be built as tensile load cells. In this case, an applied load stretches the unit and produces an output signal. No matter which type of circuit you use, the design strategy is similar. You’ll need to understand the output sensitivity and frequency range from the device on its own before designing a circuit around it. The primary simulation tool in design for these devices is frequency and parameter sweeps. This will help you produce plots such as the graph shown above.
If you have access to the right PCB layout and design software, you can perform important electrical and mechanical design tasks for piezoelectric load cell transducers. Allegro PCB Designer and Cadence’s full suite of design tools are ideal for designing these devices, simulating the device’s behavior, and bringing them into a larger test and measurement system.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.