Due to their compact size, high stiffness, and effectively infinite positioning resolution, piezoelectric actuators are widely applied in a range of scientific and industrial applications where precise motion or force control is required.
Such applications include:
- Nano-fabrication systems
- High-speed micro-mechanical systems
- Scanning probe microscopes
- Vibration control systems
As piezoelectric actuators are highly capacitive loads, a number of considerations must be taken into account when selecting an appropriate drive. These considerations are discussed in the following.
Voltage Range
Courtesy of Dr K. K. Leang
The output voltage range is the first consideration to be taken into account when selecting a drive for piezoelectric actuators. To achive maximum displacement, the drive should be capable of developing the full rated actuator voltage.
Popular monolithic stack actuators are available in voltage ratings from 60V to 200V. This rating is the maximum positive voltage or coercive field strength that can be applied. Although higher than rated positive voltages will generally not damage an actuator, excessively high voltages may result in arcing that can erode the actuator and cause intermittent short circuits.
Piezoelectric actuators can also tolerate small negative voltages. This will increase the actuators range but extreme care must be taken not to depolorize the dielectric. Most piezoelectric dielectrics can tolerate a negative voltage of approximately 10% to 20% of the maximum positive voltage.
Current Limit and Power Bandwidth
To avoid damage, piezoelectric drives contain circuitry to limit the maximum output current and power dissipation. These limits impose constraints on the dynamic performance of a piezoelectric actuator. As piezoelectric actuators can be approximated by a capacitance at low frequencies, the required drive current is proportional to the rate-of-change in voltage; that is, the required current is approximately
where C is the capacitance and V is the voltage. For a sine-wave, the maximum required current is
where Vpp is the peak-to-peak voltage. The above equation implies that the maximum sinewave frequency is
If a sinewave spans the full voltage range of the drive, the maximum frequency sinewave is also referred to as the power bandwidth.
Compared to a sine-wave, a triangle wave requires less current, and allows a higher maximum frequency of:
An example of the voltage and current waveforms applied to a piezoelectric actuator are shown on the right.
Example: A 1uF piezoelectric actuator is to be driven by a 200Vpp, 30Hz sine-wave, the required current is
Bandwidth
When considering small signals that do not exceed the current limit, the maximum operating frequency is dictated by the driver bandwidth. The bandwidth is the frequency where output power has reduced by a factor of two and significant phase-shift is present. For accurate signal reproduction, input frequencies should generally be lower than 10% of the driver bandwidth.
As load capacitance is increased, the bandwith of a piezoelectric drive reduces. This relationship between capacitance and bandwidth can be found in the drive's datasheet. A typical frequency response characteristic is shown on the right.
Capacitance versus Bias-Voltage and Temperature
The power bandwidth, slew-rate and small-signal bandwidth of a piezoelectric drive are all primarily limited by the actuator capacitance. Larger actuators and actuators with a greater number of internal layers have a higher capacitance.
Care must be taken when interpreting the capacitance values specified by actuator manufacturers. These values are measured at room temperature and with zero bias voltage. Due to the non-linearity of piezoelectric dielectrics, the small-signal capacitance increases with higher electric field. A capacitance Increase of up to 300% has been reported over the full voltage range of common piezoelectric actuators. Hence, when predicting the performance of piezoelectric drives, a conservative estimate of the actual operating capacitance should be used. In typical applications where the bias voltage is half the full-range voltage, the capacitance value specified by the manufacturer should be multiplied by a factor of 2. If operating near the maximum voltage, a factor of up to 3 may be necessary.
In addition to capacitance non-linearity, piezoelectric dielectrics are also highly temperature dependant. For example, the sensitivity and capacitance of common piezoelectric actuators can double with every 50 degrees Celsius increase in temperature. If the ambient temperature is above 25 degrees Celsius, the capacitance increase must be taken into account. Due to dielectric heating, large temperature increases can also occur when driving piezoelectric actuators at high-speed or full-range. This is particularly true of small actuators with low thermal-mass; a 50 degree temperature increase can occur in just a few seconds of heavy excitation.
Safety
Piezoelectric drives produce hazardous potentials and should be used by suitably qualified personnel under the supervision of an observer with appropriate first-aid training.
Do not operate piezoelectric drives when there are exposed conductors.
Use appropriate signage for Hazardous Voltages.
Further Reading
A more detailed discussion of piezoelectric drive limitations can be found in the following references:
- [1]A MHz bandwidth dual-amplifier for driving piezoelectric actuators and other highly capacitive loads; Fleming, A. J.; Review of Scientific Instruments, October, 2009, 80, 104701 (1-7)
- [2]High-speed serial-kinematic AFM scanner: design and drive considerations; Leang, K. K. & Fleming, A. J.; Asian Journal of Control, March, 2009, 11 (2), 144-153