Tag Archives: piezoelectric

How Piezoelectric Accelerometers Work

Vibration and shock testing typically require piezoelectric accelerometers. This is because these devices are ideal for measuring high-frequency acceleration signals generated by pyrotechnic shocks, equipment and machinery vibrations, impulse or impact forces, pneumatic or hydraulic perturbations, and so on.

Piezoelectric accelerometers rely on the piezoelectric effect. Generally speaking, when subject to mechanical stress, most piezoelectric materials produce electricity. A similar effect also happens conversely, as applying an electric field to a piezoelectric material can deform it mechanically to a small extent. Details of this phenomenon are quite interesting.

When no mechanical stress is present, the location of the negative and positive charges are such as to balance each other, making the molecules electrically neutral.

The application of a mechanical force deforms the structure and displaces the balance of the positive and negative charges. This leads the molecules to create many small dipoles in the material. The result is the appearance of some fixed charges on the surface of the piezoelectric material. The amount of electrical charges present is proportional to the force applied.

Piezoelectric substances belong to a class of dielectric materials. Being insulating in nature, they are very poor conductors of electricity. However, depositing two metal electrodes on the opposite surfaces of a piezoelectric material makes it possible to produce electricity from the electric field that the piezoelectric effect produces.

However, the electric current that the piezoelectric effect produces from a static force can last only a short period. Such a current flow continues only until free electrons cancel the electric field from the piezoelectric effect.

Removing the external force causes the material to return to its original shape. However, this process now causes a piezoelectric effect in the reverse direction, causing a current flow in the opposite direction.

Most piezoelectric accelerometers constitute a piezoelectric element that mechanically connects a known quantity of mass (proof mass) to the accelerometer body. As the mechanism accelerates due to external forces, the proof mass tends to lag behind due to its inertia. This deforms the piezoelectric element, thereby producing a charge output. The input acceleration produces a proportional amount of charge.

Piezoelectric accelerometers vary in their mechanical designs. Fundamentally, there are three designs, working in the compression mode, shear mode, and flexural mode. The sensor performance depends on the mechanical configuration. It impacts the sensitivity, bandwidth, temperature response of the sensor, and the susceptibility of the sensor to the base strain.

Just as in a MEMS accelerometer, Newton’s second law of motion is also the basis of the piezoelectric accelerometer. This allows modeling the piezoelectric element and the proof mass as a mass-damper-spring arrangement. A second-order differential equation of motion best describes the mass displacement. The mechanical system has a resonance behavior that specifies the upper-frequency limit of the accelerometer.

The amplifier following the sensor defines the lower frequency limit of the piezoelectric accelerometer. Such accelerometers are not capable of true DC response, and hence incapable of performing true static measurements. With a proper design, a piezoelectric accelerometer can respond to frequencies lower than 1 Hz, but cannot produce an output at 0 Hz or true DC.

Monitoring Sound & Vibration for Process Control

In a production environment, one can always find two common themes for the successful application of acoustical or vibrational monitoring. Usually, workers judge the noise or vibration event as being the start or end of a particular process. Initiated by such an event, an automated control system can easily minimize any loss of production.

On the production floor, control of manufacturing processes have used continuous monitoring of sound and vibration for the past several years. For instance Brüel & Kjær had used their 2505 Multipurpose Monitor in the early 1980s to automatically monitor vibration signals. One could connect an accelerometer, a microphone, or other piezoelectric device to this monitor, and set limits for alerting the user whenever the levels exceeded them. They had filters to limit the signal bands, and detectors to average signals that fluctuated highly. On the output side, relays interfaced with the process control systems or other instrumentation. No other expensive analysis systems were necessary if the process control technician used this device to monitor acoustic or vibration levels automatically. People used these monitors also in the machine condition monitoring field as basic overall vibration detectors to switch off the machine if vibration levels exceeded the set limits.

Discrete analog circuit boards enclosed in weather proof enclosures made up these early monitors. The user had to select the circuit cards necessary for their specific application. Usually, a circuit card was capable of performing a specific function, such as RMS detector, amplifier or attenuator, high and/or low pass filter, and signal conditioner. The circuit cards worked together with the relays, alarm indicators, and the meter module. With very little dynamic range, users had to be very careful in selecting a circuit card for each application. One had to be knowledgeable about the transducer they employed and the particular measurement they were making. If conditions changed, they had to order additional circuit cards.

The above disadvantages of the analog system made Brüel & Kjær develop their digital signal processors replacing the monitors with modern electronics. They now had software controlling the functions of RMS detection, gain/attenuation, and filtering. End users found the application of the new monitors much simpler, as a monitor could be field-programmed for meeting the demands of the present task. The supplied software and its use in setting up and control of the unit allowed users to save time they earlier spent on analyzing the required settings before purchasing the monitor.

The new monitors use a PC interface for setting up and to display the results of their measurements. Users can store programmed data within the unit, so the monitor can operate even without the presence of the PC and retain measurements if the power fails. Digital signal processing within the unit allows the user to set up many low and high pass filters, true RMS, and peak-to-peak measurements. Users can set other built-in voltage references and test functions for set-ups related to new tests, including relays and indicators for system failure. In addition, the presence of electrical outputs for unconditioned and conditioned AC signals makes these new monitors ideal for real-time detection and control of acoustic and vibration events.

It is Time for Chip Speakers

So far, speakers have been electromechanical devices, with a coil moving within a magnetic core, attached to a baffle or driver to move the air for producing the sound. With devices going down in size, manufacturers have been facing difficulties in producing electromechanical speakers in smaller sizes. Piezoelectric speakers are available, but they operate on a very narrow bandwidth.

Now USound GmBH, from Graz, Austria, has presented an audio speaker based on micro-electro-mechanical-system (MEMS) technology. This chip-sized speaker is suitable for small equipment such as Internet of Things (IoT) devices, wearables, smartphones, and earbuds.

By the end of the current year, USound expect to reveal Megaclite, a reference design using its MEMS speaker, Ganymede. So far, USound has fitted Ganymede to sunglasses at the high end. According to USound, Ganymede is suitable for mobiles, earbuds, and high fidelity, multidriver speakers playing above ear levels.

According to Mark Laich, senior adviser for business development at USound, making the diminutive MEMS drivers sound good across the audible spectrum was a huge challenge for the engineers. The major difficulty they faced was from the sound related physics, as it dictates the diaphragm size to push the air to be proportional to the wavelength of the sound emitted. That is why high-fidelity speaker systems use 12- to 15-inch drivers for producing low frequency bass sounds, 3- to 6-inch midrange drivers for the mid-frequency sounds, and 1 or less than 1-inch tweeter speakers for producing high-frequency sounds.

For the tiny speakers used in wearables, the size of the driver has to be some small portion of the wavelength of the sound it emits. Usually, some electronic or mechanical frequency equalization is necessary to make them sound high fidelity. Highest fidelity, as some headphones at the high end provide, is achievable only with multiple drivers. Typically, most of the reasonably priced earbuds have to sacrifice fidelity as they use a single driver, while adding electronic equalization to sound better.

As it is not possible to circumvent the sound related physics, MEMS speakers from USound are similar. Their low-end model has a single driver along with electronic equalization within a chip-scale package, and this bonds directly to the MEMs die. The MEMS frame is actually a longish actuator that moves a diaphragm using suspension beams made of piezoelectric material. The surrounding diaphragm also seals the entire chamber.

According to Laich, this arrangement achieves high-speed actuation, with a response time in microseconds. The company says this will help in noise cancellation in models to come, when they build them with a MEMS codec partner. At present, the air-pushing cone or diaphragm lies at the bottom side of a cavity, with thin piezoelectric drivers suspending it by the corners. The drivers supply the necessary energy to move the diaphragm in synchronization with the audio signal.

Listeners describe the sound from the MEMS speakers as digital, similar to the sound from a CD in comparison to that from a vinyl record. Of course, even when fortified with electronic equalization boosting the low frequencies, the sound from a single driver design does not match the high fidelity demonstrated by multidriver design.

How do Piezomotors Work?

Voltage applied to a piezoelectric material causes it to change its shape very minutely. Piezomotors such as Piezo LEGS are ceramic actuators that have four legs as its motors. These are designed cleverly such that the applied voltage can either elongate the legs or bend them sideways. It is also possible to synchronize the movement of each pair of its four legs such that it begins to walk just as an animal would – step by step. While walking, the legs can also stop at any instance on a nanometer level. The driving rod produces direct friction coupling with the legs. That means piezomotors can operate without any mechanical play or backlash. The direct drive, apart from providing full force, also offers power-off locking that does not require any power consumption.

However, the friction coupling between the drive rod and the internal piezo actuator legs does not allow counting the steps or knowing the position of the legs accurately. When they are under constant load, the legs face a certain vibration between the steps. As the load or temperature varies, so do the vibrations. Therefore, separate position sensors are required to know the accurate position of the legs of a piezomotor.

Piezomotors can move extremely slowly. When running in a closed loop system, you can make them achieve a continuous smooth motion at speeds under 1µm/s or 0.001mm/s. Since the speed of a motor depends on its step length and step frequency, a typical linear piezomotor is limited to a maximum speed of 10-15mm/s. In reality, the speed depends on both the external loads and temperature. Therefore, to run the motor at constant speed, you must have a closed loop controller.

Compared to conventional motors, piezomotors are very energy efficient. For example, when in a hold position, piezomotors do not consume any power. They also do not draw peak currents while starting or stopping. Power consumption of such motors is not dependent on inertia. That means the motor will consume the same amount of power under different external torque/load. When operated with a low duty cycle and for point-to-point applications, piezomotors provide excellent battery life.

Just as in regular stepper motors, one can define holding force and stalling force for piezomotors as well. While running, the highest load that the piezomotor can hold dynamically without slipping is called its stalling force. When powered down, the motor is able to hold a load statically and the maximum load that it can statically hold without slipping back is called its holding force. In general, the holding force of a piezomotor is about ten percent higher than its stalling force.

Although the operating principle of a piezomotor is very similar to walking, it can walk with full steps, reduced steps and it can even do micro stepping. Usually, the drive rod or disc will engage with the two or more actuator legs to move them forward and release. Then it will engage with a second set of legs to move them forward. This cycle repeats as long as the motor walks. Therefore, it is always possible to divide the full step into several smaller steps – also called micro stepping.