DESIGN OF

CORONA POLING APPARATUS

UBC Electrical Engineering

Supervisor: Boris Stoeber

Author: Tristan Miller

September 2, 2011

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Table of Contents 1.0 Introduction ............................................................................................................................................ 3

2.0 Theory ..................................................................................................................................................... 3

2.1 Piezoelectric Effect .............................................................................................................................. 3

2.2 Preparing Piezoelectric Material ......................................................................................................... 3

2.3 Corona Poling ...................................................................................................................................... 4

2.4 Optimum Parameters ......................................................................................................................... 4

3.0 Mechanical Design .................................................................................................................................. 5

4.0 Electrical Design ...................................................................................................................................... 9

5.0 Software Design .................................................................................................................................... 13

6.0 Concluding Remarks .............................................................................................................................. 13

7.0 References ............................................................................................................................................ 14

Appendix A: Stepper Motor Specs .............................................................................................................. 15

Appendix B: Tension Sensor Specs ............................................................................................................. 16

Appendix C: List of Parts ............................................................................................................................. 17

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1.0 Introduction Piezoelectric material is being used for experiments involving gas sensing devices. The current

apparatus that prepares this material can be inaccurate and is entirely manually controlled, which is

time-consuming. It was therefore proposed to design a device that automates the process of preparing

piezoelectric material and gives finer control over parameters during the procedure. What follows is a

brief theoretical background and a description of the new design for the automated production of

piezoelectric material.

2.0 Theory

2.1 Piezoelectric Effect The piezoelectric effect refers to the occurrence of an internal electric polarization within a

material as a result of an external force. In other words, an outside pressure will lead to a voltage

difference on the material’s surface. The opposite may also occur, known as the inverse piezoelectric

effect, whereby an electric field applied over the material will generate mechanical strain. Objects that

display this effect are known as piezoelectric materials (Pallas-Areny & Webster, 2001, p. 345).

2.2 Preparing Piezoelectric Material There are many polymers that are piezoelectric. Of these, Polyvinylidene fluoride (PVDF) has the

highest piezoelectric constant; however, after the PVDF has been manufactured it is initially in the non-

polar (and non-piezoelectric) alpha phase. Further processing is required to transform the PVDF into the

piezoelectric beta phase. This is done through two processes. The first stretches the material along one

axis. This procedure straightens the polymer chains causing the dipoles to arrange in a random

distribution perpendicular to the stretching direction (Bharti, Kaura, & Nath, 1997). The second process

applies an electric field at a right angle to the polymer chains. This method aligns the dipoles in a

common direction, creating the piezoelectric beta phase. During these processes the material is also

heated in order to allow the polymer chains to stretch and reorganize.

The two procedures described above can be performed sequentially or simultaneously;

however, it has been shown that simultaneous stretching and poling yielded stronger piezoelectric

properties (Kaura, Nath, & Perlman, 1991). During the stretching process there are two main ways to

apply the electric field: contact poling and corona poling. Contact poling uses an electrode on both sides

of the material and then applies a voltage difference between them to generate an electric field.

Whereas corona poling ionizes the air above the material through the use of a corona needle, which

causes ions to drift evenly over the surface. The placement of a ground plate underneath the polymer

then allows the ions to create an electric field through the material. A more in-depth description of this

method is given in the next section. Although the corona setup is more complex, it has the advantage of

producing an electric field that can be closer to the dielectric breakdown of the material than contact

poling and allows for a more efficient transformation from alpha-phase to beta-phase (Giacometti,

1995). The E-field produced by corona poling is stronger than by contact poling, because when a single

area breaks down during contact poling it creates a conductive path through the material. This

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disruption allows all charges in the electrode to travel through that path and dissipates the electric field.

However, with corona poling, if a path to the ground plate exists, then only a few localized charges will

travel through the material and the rest of the charges will maintain the electric field.

2.3 Corona Poling As shown in Figure 1, the basic elements of a corona setup consist of a corona needle

suspended within a cylinder hanging above a polymer sample. When the needle is elevated at a high

enough potential (Vc = 5-10kV) the air around it becomes ionized. These ions then drift down towards

the polymer. The cylinder is made of an insulated material (in this case PTFE), which causes any free ions

that come in contact with it to attach onto its surface. This collection of ions results in a charge build-up

on the inner surface of the cylinder, which in turn repels other drifting ions forcing them down toward

the sample. Fixed to the bottom of the cylinder and directly above the polymer is a fine metal grid made

typically of stainless steel with 500um grid spacing. This grid is also at a high potential (Vg) and causes

the ions to evenly distribute resulting in a uniform charge distribution on the surface of the polymer

sample. On the underside of the polymer is a grounded plate, which enables the ions to produce an

electric field through the sample and thus pole the polymer. For this setup there are two parts to the

ground plate: the circular electrode and the guard ring. The electrode is held at a potential slightly above

zero via a small resistor connection to ground. The guard ring is connected directly to ground and is

subsequently insulated from the electrode. By measuring the voltage drop across the resistor connected

to the electrode, the current flowing from the electrode can be calculated. As the polymer chains

reorganize and the dipoles become aligned, fewer and fewer charges will be able to travel through the

sample and thus the current flowing through the electrode will decrease. Once this current becomes

approximately zero, the polymer is completely poled. The guard ring’s function prevents charges on the

surface of the polymer from travelling around the material and directly into the electrode, resulting in

an incorrect current measurement.

2.4 Optimum Parameters There are several core parameters in the corona poling process that affect the performance of

the resulting piezoelectric material. Previous experiments have been carried out to determine the

optimum values for these parameters (Kaura, Nath, & Perlman, 1991). One of the parameters is the

electric field strength in the polymer sample. This was found to be optimum at 55 MV/m. A second

condition is the stretching ratio of the PVDF, which was found to have diminishing returns past 4.5 times

the original length. The poling temperature, another condition, was most favourable at 80°C. Although

the speed of stretching is also important, an optimum value has not been investigated. The speed used

was 9 cm/min.

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Figure 1: Schematic of Corona Setup. Adapted from (Gioacometti, 1987).

3.0 Mechanical Design The following section describes the function of the mechanical parts of the apparatus. The PVDF

is stretched by securing one end to a horizontal clamp and the other to a roller, as seen in Figure 2. The

roller is then rotated through the use of a gear train (ratio 62:1) leading to a stepper motor (Figure 3).

The corona poling station, shown in Figure 4, is positioned above the PVDF separated by 3mm between

the ground plate and the metal mesh. The ground plate is fixed to support beams that can be lowered or

raised for the purpose of inserting the clamping system. This system comprises of a top and bottom

clamp, which press together around the PVDF through the use of latch springs (Figure 5). This is used to

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mount the PVDF inside frames that keep the polymer stretched after it is removed from the apparatus.

Each of these frames then constitue one sample of the PVDF that can be used for further experiments.

Figure 2: Model of basic stretching mechanism of corona apparatus.

Horizontal Clamp

Roller

Electrode

Guard Ring

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Figure 3: Corona model with gear train and motor.

Figure 4: Corona model with corona chamber

Stepper Motor

Corona Poling Station

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Figure 5: Clamping system

In order to perform the process at high temperatures, the entire apparatus is placed inside an

oven. Due to the space constraints of the oven, the stepper motor must be mounted outside the oven

once the apparatus is inside the chamber. A motor is attached by sliding it into the motor mount and

tightening the shaft collar. After the stretching and poling is completed, the motor is dismounted and

the apparatus is removed from the oven. Following this, the tension sensor must be attached. This is

done by reversing the motor to allow slack in the PVDF and pulling the horizontal clamp along its rails.

The tension sensor can then be placed between the clamp and the fixed support and aligned by sliding a

bolt through the sensor’s centre (Figure 6). The PVDF is then tightened again by winding the roller.

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Figure 6: Schematic of mounting tension sensor

To mount the clamps, the corona chamber must first be taken off by unscrewing the four thumb

screws. The electrode platform is lowered by loosening its screws. The bottom clamp can then be slid

under the PVDF and the electrode platform elevated to the lower position, which will place the bottom

clamp directly beneath the PVDF. The top clamp can then be placed above the PVDF and the spring

latches compressed to fasten the clamp to the polymer. Finally, small screws fasten the brackets

together creating working samples of the PVDF.

4.0 Electrical Design There are five main components to the electric aspect of the apparatus: motor controller,

tension sensor, current sensor, power supply controller and oven controller. All these mechanisms,

except the oven controller, are on a single PCB, see the schematic in Figure 7 and the design in Figure 8.

The software Altium Designer 10 was used to create these designs. The DAQ USB-1208LS from

Measurement Computing is utilized to interface this PCB with the computer. This DAQ was chosen

because it is cost effective and has the necessary ports for the apparatus. In particular, it has two

analogue outputs of maximum 5V each, which is enough to control the high voltage power supplies.

Bolt

Clamp

Rail

Fixed Support

Tension Sensor

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(a)

(b)

(c)

Figure 7: Electrical schematics for (a) motor controller, (b) power supply controller and (c) tension/current sensor.

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Figure 8: PCB design layout

The stepper motor is controlled by an H-bridge and controller IC (STMicroelectronics L298 and

L297 respectively). The circuit design was adapted from the H-bridge data sheet pg. 8

(STMicroelectronics). As per the motor’s specs (see appendix A), a 5V/2A power supply drives the

motor. A reference voltage of 1.2V and sense resistors of 1Ω are used to limit the current through each

motor coil to 1.2A.

The tension sensor functions by outputting small voltages in response to a compressive load.

The model THA-250-Q from Transducer Techniques was selected because its capacity of 250 lbs is well

below the expected tension value but is still sensitive enough to detect low tensions. Since the voltages

produced by the sensor are in the mV range, they are too small to be read directly by the DAQ;

therefore, an instrumentation amp (Analog Devices AD623ANZ) is used to amplify the signal. Similarly,

the current sensor also uses small voltages that are amplified by an instrumentation amp (same model

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number). This sensor refers to the measured current through the circular electrode described in the

corona poling segment of the theory section.

The high voltage power supplies are from Gamma High Voltage with model number ES30 and

are controlled by the analogue outputs of the DAQ. To connect these power supplies to the DAQ, the

mechanical potentiometer needs to be removed and de-soldered (see Figure 9). Then the yellow wire

should be connected to the DAQ ground and the purple wire connected to the analogue output of the

DAQ. For each volt applied between these two wires, the power supply outputs 3kV up to a maximum of

30kV. However, the analogue outputs of the DAQ are only capable of producing up to 5V so the power

supplies can only reach 15kV. Nevertheless, this voltage is still enough for the apparatus.

Figure 9: High voltage power supply control knob from (a) exterior and (b) interior

The oven is controlled simply through an RS-485 interface directly into the computer. No DAQ as

well as no parts on the PCB were required. The following figure is a summation of the connections

between the electrical components.

Figure 10: Schematic of the various electrical connections between components

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5.0 Software Design The main software elements are similar to the five electrical components. All these features are

operated through the software Labview and the DAQ is interfaced using libraries from Measurement

Computing. By nature of the stepper motor controller, the motor is moved by sending digital pulses

from the DAQ into the controller. The motor moves one “step” on every rising edge of a pulse. These

pulses are generated by using a flat sequence structure in Labview. This functions by producing an “on”

signal for a set amount of time and then switching to an “off” signal for a shorter time. This sequence is

then repeated for as many pulses (or steps) that the user specifies. Although the motor is set to full-step

mode it can also operate with half steps.

Both the tension sensor and the current sensor use the analogue input of the DAQ. The Labview

software simply reads from these inputs and displays them on the front panel. The power supply

controllers use the analogue outputs of the DAQ. The software obtains values from the user and outputs

them through these analogue ports.

6.0 Concluding Remarks Although the designs are finished as of this writing, the mechanical components have not been

built. Once they are, they can be tested alongside the electrical parts and the software program. The

motor controller has been tested and is ready for the final apparatus. The parameters in the software

will need to be adjusted to suit the realized apparatus (for instance, the correct translation of digital

pulse frequency to motor rotational speed). On another note, it was decided to use a motor already in

the lab, which should be adequate, rather than purchasing a new one. However, due to unforeseen

losses the motor torque may not be sufficient and therefore a stronger motor may need to be installed.

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7.0 References

Bharti, V., Kaura, T., & Nath, R. (1997). Ferroelectric Hysteresis in Simultaneously Stretched and Corona-

poled PVDF Films. IEEE Transactions on Dielectrics and Electrical Insulation, 738-741.

Giacometti, e. a. (1995). Study of poling behavior of biaxially stretched poly(vinylidene fluoride) films

using the constant-current corona triode. J. Appl. Phys., 5597-5603.

Gioacometti, J. A. (1987). Radial current-density distributions and sample charge uniformity in a corona

triode. J. Phys. D: Appl. Phys., 675-682.

Kaura, T., Nath, R., & Perlman, M. M. (1991). Simultaneous stretching and corona poling of PVDF films. J.

Phys. D: Appl. Phys., 1848-1852.

Pallas-Areny, R., & Webster, J. G. (2001). Sensors and Signal Conditioning. John Wiley and Sons.

STMicroelectronics. (n.d.). Retrieved from

http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEE

T/CD00000240.pdf

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Appendix A: Stepper Motor Specs

Manufacturer Applied Motion Products

Part Number HT17-075D

Frame Size NEMA 17

Number of Lead Wires 8

Lead Wire Gauge 26 AWG

Unipolar Holding Torque 44.4 oz-in

Bipolar Holding Torque 62.8 oz-in

Step Angle 1.8 deg

Bipolar Series Current 0.85 A/phase

Bipolar Series Resistance 6.6 Ohms/phase

Bipolar Series Inductance 12.0 mH/phase

Bipolar Parallel Current 1.70 A/phase

Bipolar Parallel Resistance 1.7 Ohms/phase

Bipolar Parallel Inductance 3.0 mH/phase

Unipolar Current 1.20 A/phase

Unipolar Resistance 3.3 Ohms/phase

Unipolar Inductance 3.0 mH/phase

Storage Temperature -40 to 70 °C

Operating Temperature -10 to 40 °C

These specs were obtained from the site http://www.applied-motion.com/products/stepper-

motors/ht17-075. Further information is available from the data sheet. Currently, the motor is set up to

run in bipolar series.

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Appendix B: Tension Sensor Specs

Manufacturer Transducer Techniques

Part Number THA-250-Q

Rated Output (R.O.) 2 mV/V nominal

Nonlinearity 0.25% of R.O.

Hysteresis 0.25% of R.O

Nonrepeatability 0.1% of R.O.

Zero Balance 1.0% of R.O.

Compensated Temp. Range 60° to 160°F

Safe Temp. Range -65° to 200°F

Temp. Effect on Output 0.005% of Load/°F

Temp. Effect on Zero 0.01% of R.O./°F

Terminal Resistance 350 ohms nominal

Excitation Voltage 10 VDC

Safe Overload 150% of R.O.

Deflection Inches 0.002 @ R.O.

These specs were obtained from the site http://www.transducertechniques.com/tha-load-cell.cfm.

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Appendix C: List of Parts

Vendor Product # Description with Link

VXB 6200NR Nachi Bearing

QTC Gears KSS1-34 Steel spur gear (34mm)

KSS1-70 Steel spur gear (70mm)

KSWG1.5-R1 Ground Worm

KAG1.5-30R1 Worm gear

Measurement Computing USB-1208LS USB-Based DAQ Module

McMaster-Carr 92581A150 Thumb Screw - M3 x 16mm

1482K12 Steel Rod (10mm)

9266K61 Teflon Sheet

1265K37 Steel Rod (4mm)

8985K142 Rubber

91420A124 Screw - M3 x 14mm (Flat head)

91420A328 Screw - M5 x 20mm (Flat head)

94387A218 Screw - M3 x 16mm (Pan head)

91420A122 Screw - M3 x 12mm (Flat head)

94387A209 Screw - M3 x 8mm (Pan head)

94701A314 Screw - #8-32 x 3/8" PTFE (Pan head)

92005A324 Screw - M5 x 14mm (Pan head)

97613A565 Screw - M5 x 10mm (Flat head)

93140A178 Screw - #10-32 x 1/2" Polycarbonate (Pan head)

93140A184 Screw - #10-32 x 3/4" Polycarbonate (Pan head)

96640A021 Screw - #2-56 x 3/16" (Flat head)

94905A011 Hex Nut - #10-32 Polycarbonate

8545K133 Teflon Sheet 1mm x 48" x 12"

1794A51 Compression Spring Latch

6412K11 Shaft Coupling

98317A227 Retaining Ring

9414T1 Set Screw Shaft Collar

1556A54 Corner Bracket

8549K47 Fiberglass Sheet 12" x 12"

Digikey 478-3154-1-ND 100nF Capacitor

PF1262-1.000-ND 1Ω Resistor (not wirewound)

641-1413-1-ND Fast 2A Diode

P15.0KCACT-ND 15kΩ Resistor

RNF18FTD10K0CT-ND 10kΩ Resistor

PPC22KW-1CT-ND 22kΩ Resistor

445-4293-ND 3.3nF Capacitor

497-8357-5-ND Voltage Regulator 5V

BAJ0BC0T-ND Voltage Regulator 10V

PPC162XCT-ND 162Ω Resistor

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271-2575-ND 10W wall mount power supply

P5126-ND 470uF Capacitor

445-2868-ND 10uF Capacitor

AD623ANZ-ND Instrumentation Amp

ED7064-ND Header Pin Female

ED16064-ND Header Pin Male

Spaenaur 382-316 Screw - M3 x 16mm (Pan head)

382-344 Screw - M5 x 13mm (Pan head)