Type of signals
The main function of the drive electronics is to generate the signal that is applied to the piezoelectric actuator. The simplest option is the positive only trapezoidal signal showed when discussing the principle of operation, and is further referred to as “unipolar”. More complex signals can include a negative pulse (or “echo”) as shown in figure to the right. This signal is referred to as “bipolar”. In general, V0=0, but a nonzero baseline voltage can be employed as well. It is possible to have V2=-V1 or different.
More complicated signals are often used to increase the stability of the drop generation. For example, a “bipolar” waveform can be employed to minimize the satellite formation or to get around the maximum positive voltage limitation. Other waveforms include additional points defining multiple voltage levels (see figure to the right).
The complex waveforms are generally employed when dispensing more problematic solutions and/or to create droplets with a diameter smaller or larger than the orifice diameter; drop size adjustment through the waveform is often referred to as “drop size modulation”.
Values for the voltage and timings
The timing is determined by the length of the glass tube and the speed of sound in the dispensed solution. Based on the scales involved, the “dwell” and “echo” are in the tens (up to hundreds) of microseconds. The transition times “rise”, “fall” and “final rise” are several microseconds long. For some fluids, the control of the duration of the transition periods could increase the stability of drop generation.
When the polarity of the signal is respected (inner electrode - blue wire - is grounded and the outer electrode - red wire - receives the actuation voltage) there are limits on the positive side of the voltage. For AB, AT microdispensing devices V1 should be less than 50V. For ABL devices V1 should be less than 100V. Larger positive voltages could depole the piezoelectric material and cause it to not deform anymore under applied voltage differential.
The drive electronics should be able to produce transition times in the microsecond range with a capacitive load (the piezoelectric actuator) of ~2nF for AB and AT configurations and ~3.5nF for the ABL configuration. The ability to adjust the actuating waveform should be at least 1µs for the timing and 1V for the amplitude.
In some instances, the dispense consists of several drops. The drive electronics should have the ability to create the desired number of pulses (down to one), in case a finite number of drops is desired. For visualization purposes (see below the section related to observation), a short pulse (several microseconds long) synchronized with the drop generation needs to be created. That pulse is used to turn on an LED for a short period of time for stroboscopic illumination. The set-up should also allow the adjustment of the delay (see figure to the right) between the actuating waveform and the LED/strobe pulse to “freeze” the droplet at various locations along the trajectory as shown in drop generation sequence. To avoid too much light at high frequencies of droplet generation, the number of LED pulses per actuating pulse or the duration of the LED pulse need to be decreased.
Some possible configurations for drive electronics
MicroFab’s JetDrive™ provides the signals described above and allows the definition of "bipolar" waveform and arbitrary waveform with up to 12 internal points. In the associated control program (JetServer™ – see figure to the right) the user can specify the actuating waveform, the drop generation frequency, and the number of drops to be generated. A secondary JetDrive™ output provides the signal used for stroboscopic illumination. The trigger can be either from the software or an external TTL pulse. Other features include computer controlled delay, adjustment of strobe lighting, running complex scripts for dispensing (start/stop, delay change, waveform change, frequency scans, etc.). If the software is combined with a computer relay switch, it is capable of driving multiple microdispensers. A separate actuating waveform is saved for each microdispenser. The capabilities are further discussed on MicroFab’s web site under Products/Electronics & Software.
Alternatives consist of an arbitrary waveform (to achieve a flexibility comparable to MicroFab’s JetDrive™) and an amplifier or a function generator (has limited shapes for the waveform) and an amplifier. In addition to these, separate electronics (with an adjustable delay from the pulse used for actuation) need to be used to generate the LED pulses.
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