What is a Piezoelectric Actuator?

A piezoelectric actuator is a device that converts electrical signals into physical movement using piezoelectric elements. These elements have the fundamental property of deforming or distorting when voltage is applied, and piezoelectric actuators harness this characteristic for energy conversion.

Although the displacement is extremely small due to the nature of the material's structural changes, this actually enables incredibly precise control at the submicron level. Piezoelectric actuators offer several additional advantages, including high operating frequencies, the ability to generate forces up to several tons, and exceptional longevity due to their lack of moving mechanical components. These characteristics make them invaluable across a wide range of applications, particularly in precise positioning control.

Piezoelectric Materials

First, let's explore the characteristics of different piezoelectric materials.

The Principle of Electric Polarization

Piezoelectric Effect

When force is applied to a piezoelectric element and generates electrical polarization and voltage, the phenomenon is known as the "piezoelectric effect." One unique characteristic of the piezoelectric effect is its ability to generate electricity without an external power source, making it ideal for applications such as ignition devices in lighters and pressure sensors.

What Is a Piezo Driver?

A piezo driver is a specialized power-supply device that enables proper operation of piezoelectric actuators. Piezoelectric elements deform in response to applied voltage. While current isn't necessary for low-speed operations, the inherent capacitance of these elements means that current becomes essential for high-speed operations. (You can find specific current requirements in the "Current Value" section below.)

Since piezoelectric elements are capacitive components that require charge extraction, specialized piezo drivers have been developed as dedicated power supplies. Piezo drivers employ two distinct control methods depending on how the actuator is controlled: open-loop control and closed-loop control.

Open-loop Control

Open-loop control determines the required voltage by measuring displacement relative to applied voltage in advance. This straightforward control method offers the advantages of simple structure and low cost. However, it cannot account for displacement non-linearity and hysteresis, nor can it compensate for errors caused by temperature variations and creep phenomena. As a result, the actuator's precision may not be maintained under certain operating conditions.

Preload Application

When piezoelectric actuators are used in vibration applications, external forces, such as inertia, can lead to malfunctions. For example, multilayer piezoelectric actuators are particularly vulnerable to forces other than compression along the stacking direction. They can be damaged by tensile forces, bending forces, or even uneven compression forces.

Therefore, in environments where actuators are exposed to various forces from vibration, applying preload pressure by using springs is an effective solution. This preload helps prevent forces from acting in directions other than compression, thereby protecting against damage from external forces.

Stroke Considerations

When the measurement target applies a load to the piezoelectric actuator, the actuator compresses under this load, resulting in a reduced maximum stroke. Therefore, when selecting a piezoelectric actuator, it's essential to choose one with a sufficient maximum stroke that accounts for this load-induced compression.

Current Requirements

To ensure accurate operation of piezoelectric actuators, it's crucial to select a piezo driver capable of supplying sufficient drive current. For sinusoidal operation, the maximum and average current values can be calculated using these formulas:

Maximum Current = Π x Piezoelectric Element Capacitance x Drive Frequency x Drive Voltage
Average Current = Maximum Current x 2/Π

However, in practice, the actual current requirements vary due to multiple factors, including capacitance tolerances. Therefore, these calculations should be used as general guidelines rather than absolute values.

Operating Temperature Range

Piezoelectric actuators lose their functionality above their Curie temperature, so they must be operated below this critical threshold. The Curie temperature is the point at which thermal energy causes the loss of spontaneous polarization, resulting in the loss of piezoelectric properties. Therefore, for high-temperature applications, it's essential to select products that use piezoelectric elements with higher Curie temperatures.

Load Configuration

Each piezoelectric actuator has its own resonant frequency, but this frequency changes when a load is connected, which can lead to unexpected problems. This is particularly important with heavier loads, as they tend to lower the actual resonant frequency. To prevent issues, the actuator should always be operated at frequencies below its loaded resonant frequency.

Response Speed

When high response speeds are required from piezoelectric actuators, large currents must be supplied to achieve the desired voltage across the piezoelectric element in a short time. The faster the required response, the higher the current demand, so it's crucial to ensure the piezo driver can deliver sufficient current.

Resonant Frequency

As mentioned in the load configuration section, each piezoelectric actuator has its own characteristic resonant frequency. When operating at high speeds, resonance can amplify vibrations and cause unexpected behavior. While datasheets may show high resonant frequencies with apparently adequate margins, the actual resonant frequency can vary significantly depending on factors such as connected loads and mounting methods. A general rule of thumb is to operate at 1/3 to 1/5 of the resonant frequency, but the specific product should be selected based on actual operating conditions.