Unlike standard high-voltage amplifiers, PiezoDrive products have been specifically developed to obtain the highest levels of performance from piezoelectric actuators.
Each drive contains a number of proprietary technologies that:
reduce noise- increase the power bandwidth
- provide exceptional reliability
- and allow ease-of-use
Dynamic Current Control
Dynamic Current Control allows an amplifier to deliver exceptionally large output currents for short periods of time without exceeding the maximum power dissipation limits. As an example, the PiezoDrive PDL200 allows an overload current up to six times the static current limit. This provides a three times increase in the power bandwidth and up to six times faster rise-time. A plot of the allowable overload time versus current is shown on the right.
The maximum frequency sine-wave that a standard amplifier can produce is:
With Dynamic Current Control, this increases by a factor of 3.14 to:
Thus, compared to a standard amplifier, Dynamic Current Control allows three times greater frequency, voltage, or load capacitance before current-limiting occurs.
Dynamic Current Control is also superior to Fold-Back Current Limiting used by some other manufacturers. Fold-back current limiting is a standard technique that allows higher output current when the output voltage is also high. This is ideal for resistive loads but is undesirable for capacitive loads like piezoelectric actuators. When using fold-back current limiting, a waveform may become distorted when the offset voltage is reduced. In addition, the step-response changes with output voltage.
Charge Drives for Reducing Hysteresis
Due to hysteresis exhibited by piezoelectric actuators, many applications require the use of sensor-based feed-forward or closed-loop control. Although closed-loop control is effective at eliminating nonlinearity at low frequencies, the bandwidth compared to open loop is severely reduced. In addition, sensor noise significantly degrades resolution in closed-loop.
An alternative to closed-loop control is to drive piezoelectric actuators with charge rather than voltage. By simply regulating the current or charge, hysteresis can be reduced to less than 1% of the full-scale range. An example displacement response is shown on the right.
The circuit diagram of a charge drive is shown on the right below. The piezoelectric load, modeled as a capacitor and voltage source Vp, is shaded. The feedback loop forces the voltage across the sensing capacitor Cs to equal the input voltage Vin. Neglecting the resistances Rp and Rs, the load charge q is equal to
That is, the charge gain is Cs Coulombs per Volt. With a capacitive load of Cp Farads, the voltage gain is
A problem with charge drives is the dielectric leakage modelled by Rp and Rs which causes the output voltage to drift at low frequencies. However, by setting the ratio of resistances equal to the ratio of capacitances, low-frequency error can be avoided. To maintain a constant voltage gain, the required resistance ratio is
The parallel resistances effectively turn the charge drive into a voltage amplifier at frequencies below
Although the parallel resistances act to stabilize the voltage gain at low frequencies, the amplifier now operates as a voltage source below fc and a charge drive above. A consequence is that reduction of hysteresis only occurs at frequencies above fc. Although the cut-off frequency can be reduced by increasing the parallel resistances, a practical limit is imposed by the dielectric leakage of the transducer. In addition, excessively high resistance values also reduce immunity to drift and result in long settling times after turn-on and other transient events.
PiezoDrive Charge Drives are designed for both high-performance and ease-of-use. Compared to a standard voltage amplifier, there is only one additional control - the DC-gain, which controls the voltage-gain at low-frequencies. The PDQ Charge Drives are preconfigured during manufacture to drive a certain range of capacitance values. This means that the charge-gain, resistance ratios, and transition frequency fc are all optimally preconfigured and do not need user adjustment. Further information can be found in the PDQ Product Brief and Introduction to Charge Drives.
Further Reading
A more detailed review of charge drives can be found in the following reference:
- [1]Charge drives for scanning probe microscope positioning stages; Fleming, A. J. & Leang, K. K.; Ultramicroscopy, November, 2008, 108(12), 1551-1557