8.1 Principles of stack actuator design

Piezoelectric stack actuators are multilayered devices that convert electrical energy into mechanical motion. These actuators consist of thin layers of piezoelectric material with electrodes between them, allowing for increased displacement and reduced voltage requirements compared to single-layer designs.

The performance of stack actuators depends on factors like polarization, stiffness, and resonance frequency. Key considerations include displacement amplification, force generation capabilities, and electrical characteristics like impedance and capacitance. Understanding these principles is crucial for effective actuator design and application.

Actuator Structure and Properties

Multilayer Construction and Electrode Configuration

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• Piezoelectric stack actuators consist of multiple thin layers of piezoelectric material stacked together
• Layers typically range from 20 to 100 micrometers in thickness
• Electrodes placed between each layer create an alternating pattern of piezoelectric material and electrodes
• Internal electrodes connect alternating layers electrically in parallel
• External electrodes on the top and bottom of the stack provide electrical connections
• Parallel electrical connection reduces the required voltage for actuation
• Multilayer design increases the overall displacement of the actuator

Polarization and Stiffness Characteristics

• Polarization direction aligned parallel to the applied electric field for maximum displacement
• Polarization process involves applying a strong electric field to align dipoles within the material
• Stiffness of the actuator depends on the piezoelectric material properties and stack geometry
• Typical stiffness values range from 20 to 100 N/μm for small actuators
• Higher stiffness results in greater force generation capabilities
• Stiffness can be tailored by adjusting the cross-sectional area and length of the stack

Resonance Frequency and Dynamic Behavior

• Resonance frequency determines the maximum operating speed of the actuator
• Depends on the actuator's mass, stiffness, and geometry
• Typical resonance frequencies range from 10 kHz to 100 kHz for small actuators
• Higher resonance frequencies allow for faster response times and higher operating speeds
• Damping characteristics affect the settling time and overshoot of the actuator
• Operating below resonance frequency ensures stable and predictable performance

Actuator Performance

Displacement and Force Generation

• Displacement amplification techniques increase the overall motion of the actuator
• Lever mechanisms or flexure designs can amplify displacement by a factor of 2 to 10
• Force generation capabilities depend on the actuator's cross-sectional area and applied voltage
• Typical force output ranges from 100 N to 10,000 N for small to medium-sized actuators
• Force-displacement trade-off exists, with higher force resulting in lower displacement
• Preload applied to the actuator improves performance and prevents tensile stresses
• Preload values typically range from 10% to 50% of the actuator's blocking force

Electrical Characteristics and Capacitance

• Electrical impedance of the actuator affects its dynamic response and power consumption
• Impedance depends on the operating frequency and capacitance of the actuator
• Lower impedance results in faster response times and higher power efficiency
• Capacitance of the actuator determines its electrical energy storage capacity
• Typical capacitance values range from 100 nF to 10 μF for small to medium-sized actuators
• Capacitance increases with the number of layers and cross-sectional area of the stack
• Higher capacitance requires more charge to achieve full displacement