Hello. And welcome, everyone, to this installment of actuator training, where we will be covering Piezoelectric actuators. My name is Rodney Miranda. And I will be leading this training sponsored by the Audio and Actuators Group, where we focus on getting the best performance out of your speakers and actuators.

What we plan to cover today is an introduction to what Piezoelectric material is, its key characteristics, how it's made, what it looks like from an electrical standpoint, and then go through an example application to tie it all together. Hopefully at the end of this, you will be able to understand what and why Piezoelectric actuators are important and how it can improve your [INAUDIBLE] application. Also, I have provided more content on the slides than I will discuss and it is intended for additional information. So if you see a topic that you like, please press pause and continue after you are done reading.

So what is Piezoelectric material? Piezoelectric material is a certain type of material found in nature that exists a unique phenomena of converting mechanical energy to electrical energy and vice versa. Some typical materials are ceramics, crystals, bones, and more.

Since the Piezoelectric effect will convert mechanical to electrical energy, we can use this as a sensor or even an energy harvester. Likewise, the inverse Piezoelectric effect will convert electrical energy into mechanical energy. And thereby, we can use this as an actuator.

For the sake of time in this presentation, only the actuator will be discussed in future slides. Without having a large table comparing how Piezo matches to other actuators, the general pros and cons of a Piezo are shown here. For example, the precision of Piezo range anywhere from nanometers of movement to sometimes millimeters of movement. But then, mechanical size also allows for more design freedom since it can be less than 1 millimeter thick in most cases.

One of the most value add properties is the No Holding Power. Without going into detail here, this is a large proponent of using Piezo and will be discussed in later slides. Now, there are always drawbacks of any solution. And one of those is that Piezo does generally need high voltage to achieve its full mechanical swing. This voltage typically ranges up to 130 to even 300 volts, but could be even higher in some cases and will ultimately depend on the Piezo material.

Additionally, Piezo can be non-linear or hysteretic under normal operation, but some techniques can be used to help reduce this. Typically, Piezoelectric material is produced in a factory to achieve higher levels of movement. And the steps needed for this are the material's plated so that an electrical connection can be made. A high voltage signal is applied to align the dipoles together.

The signal is then removed to set the dipoles in alignment. Now, the Piezo is ready to be operated and used by applying an electrical signal to the Piezo. It is worth noting that this locking process is reversible by either reverse biasing the Piezo or reaching the Curie temperature so that it will allow the dipoles to move and become not aligned once again.

Piezo can be implemented in a few different electrical to mechanical relationships or modes. More description diagrams are shown on the next slide. There are three different displacement modes for Piezo actuators.

The longitudinal actuator or D33 have the plates perpendicular to the direction of movement. These actuators achieve very little mechanical movement, but are very powerful in the amount of force it can provide. The shear actuators or D15 have plates parallel to the movement. However, it is actually a shearing direction.

These typically have the largest electrical to mechanical coefficient, but do have some implementation limitations. The contracting actuators or D31 have the plates parallel to the movement direction. And much like the D33, it can achieve a short but powerful directional force. The bottom three images show these modes in different stacked scenarios. These will be discussed more in later slides.

Two other forms of implementation are tube actuators and benders. Benders are one of the most used Piezo actuators because they can achieve a much longer displacement but with very minimal blocking force. The bending is achieved by attaching either two Piezos that are driven out of phase, this would be implemented by having one contracting and one expanding causing a bending action, or by attaching the Piezo to a flat piece of metal or hard substrate so that when the Piezo expands or contracts, it will cause a bending action.

Now that we have covered the mechanical properties, the electro-mechanical model can be discussed. Typically, the Van Dyke model is the most accepted, where the Piezo can be modeled as a capacitor, C0, in parallel with an RLC circuit. What this causes is a pure capacitance at low frequency followed by a minimum and maximum resonance impedance at higher frequencies.

For reference, benders typically have a much smaller capacitance around the low nanofarad range and longitudinal stacks have much higher capacitances in the microfarad range. This C0 capacitance is generally proportional to the amount of Piezoelectric material in the actuator. As I mentioned earlier, these Piezos can be non-linear and hysteretic, The two plots show position or displacement on the y-axis versus reference voltage driving or measured on the x-axis. These were taken from a paper by Flemings and Moheimani showing that when a Piezo is driven with voltage, the variance is about 10% to 15% of the displacement, whereas when driven with charge, the variance is 1% to 2%. Based on this research, Piezos should be driven with charge or current to minimize the non-linearities of the displacement in the application. Note that this is only for low frequency applications, where the Piezo behaves as the C0 and not the RLC circuit.

Digging in a little further, for the low frequency scenario, each of the physical properties can be mapped to the electrical properties. We know that the derivative of position gives us velocity and the second derivative gives us acceleration. We use our basic impedance equation for a capacitor, I equals C times dV/dT, and then identify how these relate accordingly. The table shows a nice relationship between all of these and actually tells us one more thing.

By driving with current, the Piezo is not only becomes more linear, but the user can actually have direct control over the position, velocity, and acceleration of it. This is crucial in making sure Piezos are not damaged by mechanical overstress. Lastly, by having a way to measure the voltage on the Piezo, the user can force current, measure voltage versus time, and therefore calculate the impedance of the Piezo.

Much like many other applications, load diagnostics adds a tremendous amount of benefit towards preventative maintenance, minimizing downtime, and optimizing the system. To really hit home on this, the typical application today actually uses a voltage drive OPAMP with a limiting resistor. By setting the current limit of a current drive OTA to the same current limit of the OPAMP plus resistor, the OTA can achieve regulation three times faster.

Moving on to power discussion, in lower frequency or static systems, the Piezo behaves like the C0 capacitor, therefore, regulation around a certain point will only require small amounts of charge to move. This is the basis for no holding power. So to reiterate, in equation one, if dQ is equal to 0, then I or the current is equal to 0, and therefore, current times voltage, which is equal to power, is equal to 0. For dynamic systems where the output is moving constantly, the power will scale proportionally with the voltage, frequency of the application, and the capacitance of the Piezo.

This can be seen in the graph or in the equation. So as the displacement becomes longer from voltage, faster from frequency, or stronger from Piezo capacitance, the system will need more power. Now that you're familiar with the mechanical and electrical parameters of a Piezo, I can come back to the stacking or paralleling Piezos together. One way to mitigate the higher voltages is to stack Piezos together and, thereby, many smaller displacements will add up to the needed displacement.

However when you do this, the manufacturing costs will go up and, likewise, the capacitance will go up. Two example Piezos are shown here, one with 120 volts peak to peak with a 4 milliamp current compared to a 2 volt RMS and 150 milliamp current. Any guess on which one has more layers?

Now that you're an expert on Piezoelectric material, let's go through a typical application. Most electrically controlled industrial valves use a solenoid actuator for regulation of flow. The pros is that it's simple, known, and runs off an industrial supply.

The downside is that the resolution is very poor. And you always need current running through the solenoid to keep the magnetic field in [INAUDIBLE]. So we ask ourselves, can we improve or fix this system?

So we can fix the resolution of movement. Yes, this can be mitigated through some complex regulation loops, although it does take some extra horsepower to run it. So can we minimize the power consumption?

Although the power consumption can be improved with some new drive circuits, we cannot fully mitigate it based on the physics of solenoid technology. So we ask ourselves again, is there another technology that can solve these problems? Yes, Piezoelectric actuators.

From what we've already covered, Piezo can meet the no holding power benefit since it behaves like a capacitor. Now, although the power benefit may not seem significant think of a factory with 100,000 of these. The power starts to add up very quickly.

Now, there are still some cons that we already discussed that need to be addressed or mitigated. So we ask ourselves, can we mitigate these? Addressing each of these, can we meet the high voltage requirement?

Yes. TI has a tremendous amount of experience in DC/DC converters. So integrating a boost converter into the IC to provide this high voltage can be done. So can the non-linearity be fixed? Yes. We already discussed improving with charge control and implementing a closed loop system could also significantly improve regulation.

Sensitive to electrical overdrive? Yes. By controlling the current and dI/dT of the drive signal, the amount of mechanical stress can be controlled. And the cost. Unfortunately, this is something we cannot improve electrically because it is a manufacturing cost set by the vendor. However, the price is continually coming down. And as volumes go up over time, the price will start to come down. Additionally, with the increasing cost of metals and ferrous material, the cost of solenoids will continue to go up.

So how does the old system look compared to the new system? Same control signal and power rail with better performance and lower power. One side note is this is already being implemented in the industry today.

So as you can see from the table, the industrial valve can take advantage of all of the Piezo parameters. With that said, this is just one of the many applications that can benefit from using Piezo as its actuator. A long list of applications is shown.

But the key way to identify if your application can benefit is to use the table at the top left. So in summary, Piezo can add value through low power, very precise displacement, and very quick response time. Additionally, with the right driver, you can mitigate the downsides of Piezoelectric material.

Hopefully this training has helped you understand Piezoelectric actuators. And for any questions, contact your local sales associate or someone from the audio and actuator products business unit. Thanks for watching. And have a great day.