ECE 4901- Senior Design Project
High Frequency Piezo-Driven Droplet Maker
Fall 2014
In collaboration with
Project Statement
Team Members:
Jacob Wolf (EE)
Colin Mack Nair (EE)
Joseph Eaton (EE)
Advisor:
Rajeev Bansal
Amastan LLC. Contacts:
Mak Redjdal
Kamal Hadidi
Background:
This project focuses on the design of a high frequency piezo-driven Droplet Maker (DM)
capable of delivering a uniform droplet stream of solution precursor for improved coatings, or
powder making, of interest to the thermal processing industry. The improvement is achieved
through control of the size and homogeneity of solution droplets. When combined with a
uniform melt state process such as UniMelt™ where droplets undergo pyrolysis, one can achieve
precise control of powder size and size distribution, morphology, and homogeneity and purity of
phase microstructure. The size of the particles dealt with is in the range of micrometers to
nanometers, and they need to be content homogeneous and uniform in size. As the size of the
droplet is inversely proportional to frequency, the DM needs to operate at high frequencies with
sufficient current or voltage drives to achieve liquid jet break up into uniform droplets.
Summary:
The High Frequency Piezo-Driven Droplet Maker is made up of three major parts: The
Solution Dispenser, The Droplet Maker, and the High Frequency Electronics Driver; see figure 1
below for general block diagram of the system. The goal of this design project is to improve, or
redesign if necessary, the high frequency electronic driver circuitry to the droplet maker as well
as the possibly replacing the piezoelectric element if necessary. A higher frequency than its
current capability, which is currently limited to about 100-200 kHz, is desired with roughly a
frequency bandwidth of 0-500 kHz with tunable accuracy of ±20 Hz. This higher frequency will
allow the droplets to more quickly separate and be evenly spaced while reducing them in size to
the desired dimension of 1-10 micron diameter uniform droplets. The solution stream that is to
be separated into droplets is acted upon by a piezoelectric capacitor/crystal. This Piezo vibrates
at a given frequency which contacts the solution and creates perturbations. The higher the
frequency, the faster the vibrations and thus creation of droplets that are both produced quicker
and with decreased diameter.
Furthermore, the power and signal generation supplied to this piezoelectric element must
be designed. If it is discovered the type of piezo should be changed, that can perhaps operate at a
higher frequency, then the driver for it will need to change accordingly. The most efficient and
stable configuration will need to be chosen to achieve the goal of the project. The sponsors of
this project have requested a final project that is both in a portable container and capable of
plugging into and working off of standard wall voltage. Therefore we will need to design an
alternating current (AC) to direct current (DC) converter power supply. Additionally, this design
should be considered for the possibility of driving multiple capillary nozzles instead of one
single nozzle as to increase production rate.
Figure 1: General block diagram of the system. The high frequency driver block will be the focus of this project
Specifications:
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Droplet Diameter:
1 to 10 micron uniform size
Frequency Bandwidth:
0kHz-500kHz (Low pass filter)
Frequency stability/accuracy:
Tunable within ±20Hz
Piezo capacitor:
Capable of functioning at 500kHz
Voltage supply to Amplifier
+/- 10 V and DC railing +/- 100 V
Number of nozzles per apparatus:
5 to 10
Able to operate off of wall power (120V AC)
Device must be incorporated into a functioning and portable container
Theory:
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How a piezo works and how to apply it to the project
What the op amp is for and how to design one
Basic power supply operations that relate to the project
Solutions:
Our initial proposed design to solve the problems listed above is based off of oscillatory
ultrasonic cleaning circuitry. Through inspection, we have found that these oscillatory circuits
achieve a result very similar to that which we are trying to produce, however these circuits are
often capable over reaching upwards of around 500 kHz depending on tuning and piezo chosen.
This is, as of now, the greatest theoretical increase in frequency we have found in piezo
operation. The oscillatory circuit will be need to be redesigned to include variable frequency
response and robust piezo capable of high frequency operation. The resonant frequency of the
piezo will then be matched by a transformer and other capacitors and inductors.
This design will run off of wall voltage as specified by Amastan. A simple DC to AC
converter will consist of a bridge rectifier circuit with a filter capacitor on the output. This will
deliver a full wave DC voltage with smoothed Dc ripple for better power output and stability. a
transformer stage can be implemented to boost the voltage to the 100v Dc railing that is desired,
this will be supplied to the operational amplifier.
The input signal will be supplied by a signal generator chip that can generate a tunable
frequency in the range of 0 to 500 kHz. The signal generator chip will be a compact and much
cheaper method than buying a signal generator, thus greatly improving portability of the overall
container by having it integrated into the circuit itself. This sinusoidal input will be fed into an
operational amplifier stage.
The primary side of the transformer will be connected to the output of the Op amp and a
capacitor in which it will form a resonant circuit with. Another resonant circuit will be connected
on the secondary side consisting of the piezo actuator and another capacitor, inductor. One of the
key aspects to this is impedance matching. Both sides of the transformer should be impedance
matched to its connecting resonant circuits for the signal to be properly passed and avoid
distortion and other negative effects on performance. The piezo will vibrate at a certain resonant
frequency, that should be variable by effecting the signal supplied to the transformer thus
creating a variable frequency piezo driver with operational range of at least 500 kHz with low
distortion, supplied with a voltage input of up to 100v DC converted from the 120VAC wall
voltage. This circuit will combine many portions of the current bulky setup into one compact
circuit. The final product will include displays for tunable voltage supply and tunable frequency
from the sine wave chip.
Note: Subject to change. All values are only placeholders, as a part can’t be placed in PSpice
without a value.
Figure 2: General driving circuit schematic
For the piezo, the specifications call for one that can operate for up to 500 kHz. Boston
Piezo Optics is a piezo actuator vendor which takes specifications from the customer and builds
them a piezo based off of their needs. The vendor asks for material and orientation, we will be
using Lithium Niobate with a compression orientation so that when the piezo is excited by the
driving circuit it will displace or pulse to cause the fluid in the reservoir to leave. They ask for
size, the current design uses a cylindrical piezo where the inner disk is 5mm, outer 10 mm and
length is 15 mm. For quantity we should order two in case one is defective or so that we can test
more than one piezo to make sure the design is consistent. For the frequency specification we
will ask for 500 kHz. The vendor then asks for specific flatness, parallelism or surface finish
requirements and electrode configuration. For this we will need to make sure that the piezo has a
positive and negative terminal for the driving circuit to attach itself to.
Design Advantages vs. Disadvantages:
Parameter
Frequency Limit
Voltage
Portable Device?
Power
Desired Specification
500 kHz
100V DC railing
Yes
~2-6W
Current Design
~100 kHz
70V DC railing
No
~5W
New Design
500 kHz
±100V
Yes
T.B.D. (Dependent on
Piezo)
Conclusion:
Our new design will have the ability to operate a piezo actuator from 0 to 500 kHz with
an accuracy of ±20 Hz. The new piezo will have a cylindrical shape and be able to run at a
resonant frequency of 500 kHz, this will also allow for more nozzles to be implemented as a
larger piezo allows for more nozzles. The driving circuit will be similar, however it will also
allow for a variable inductor to accommodate for different resonant frequencies from 0 to 500
kHz. The final product will be contained within a structure that will also be portable. There will
be an input voltage knob, a frequency knob adjuster. The output will have a positive and negative
terminal to hook up to the piezo actuator.