**3. Optocoupler-based micropump module design**

Design, implementation and characterization of a miniaturized piezoelectric micropump driving module, based on two boost SMPS converters, with a shared boost capacitor (micropump PZT actuator) represented a starting point in design of micropump drivers in our Laboratory. This micropump driver design is suitable for integration inside micropump housing due to its small size and its low current consumption (≈ 55 mA).

Such design synthesizes rectangular shape of driving signal with RC charging and discharging transitions, which are not optimal. Low value of both transition slew rates does result in lower current consumption, but at a cost of reduced pump flow rate compared to a steeper slew rate signal. Still, the performance of such a circuit can be considered suitable for certain cost- and size-sensitive applications.

### *Influence of Piezoelectric Actuator Properties on Design of Micropump Driving Modules DOI: http://dx.doi.org/10.5772/intechopen.103789*

Fluidic measurements have shown the need for assuring driving signal symmetry (i.e. elimination of difference between positive and negative amplitude), which has to be trimmed individually to a particular type of piezoelectric actuator, which was another drawback of this design.

Module upper-frequency limit was found to be at 400 Hz. A typical operation range was found up to 100 Hz, where peak-to-peak amplitudes up to 250 V could be achieved. For DI water pumping, optimal operating frequency range was found between 50 to 80 Hz. Amplitude symmetrical driving signal with an amplitude of 125 V was achieved using power-supply voltage of 9 V, excitation frequency *f* of 100 Hz and duty cycle *DC*<sup>+</sup> of 40%. Using these optimal driving conditions considering driving module current consumption and micropump performance characteristics, micropump flowrate performance was found at 0.36 ml min−1, backpressure performance of 104 bar and module power consumption of 0.5 W.

Based on presented fluidic and electrical measurement results, obtained rectangular shape of driving signal with RC charging and discharging transitions, had to be improved to allow faster signal edge transitions (i.e. improved slew rate). Future driver designs also had to allow for a more precise (preferably independent) setting of positive and negative amplitude. Lastly, a wider frequency operation interval than 400 Hz would have to be achieved in order to improve air pumping characteristics.

To implement a driving signal shape of a square wave with settable frequency, duty cycle, both amplitudes and both slew rates, a completely different approach to the design of a miniature high voltage piezoelectric micropump driving module was taken: such design had to synthesize a proper rectangular micropump driving signal with independently settable positive and negative amplitudes, rising and falling edge slew-rates, positive and negative dead-times and excitation frequency.

In order to achieve these functionalities, a simplified high-voltage driving stage was designed. Typical approach would be based on a D-class amplifier design using an isolated gate driver (e.g. STGAP series), which would enable driving piezoelectric micropumps with arbitrary waveforms. On the other hand, such an approach requires several additional components, which would compromise our low-cost approach.

In order to reduce overall cost, the micropump high-voltage switching stage was highly simplified. While isolated gate drivers do provide galvanic isolation between the input section (a microcontroller) and output transistors, they also require a bootstrapping capacitor for high-side switch. Optocouplers perform the necessary level-translation for the high-side transistor and eliminate the need for any additional bootstrap capacitors. Optocoupler-based solution also enables fast transitions and allows for a settable slew-rate as will be presented in the following.

### **3.1 Module operation**

Optocoupler-based design comprises a separate negative and positive high-voltage power supply, each implemented as an independent boost SMPS power supplies, which generate positive and negative micropump driving voltage (**Figure 18**, netlabel V+ and V− ) from a low voltage source (**Figure 18**, VPWR).

Positive micropump power supply voltage generator is comprised of transistor M1, inductance L1, diode D1 and capacitor C1, while the negative micropump power supply voltage comprises transistor M2, inductance L2, diode D3 and capacitor C3. Both power supply voltage levels (**Figure 18**, netlabel V+ and V− ) are independently monitored using a dedicated resistor divider (**Figure 18**, resistors R4, R5 and R7, R8 for negative and positive voltage, respectively). Each resistor divider forms a

**Figure 18.** *High-voltage part of the optocoupler-based driver design.*

closed-loop voltage regulator with a corresponding microcontroller PWM source (**Figure 18**, labels V3 and V1 for negative and positive voltage, respectively).

Both SMPS converters are driven using PWM signals on sources V1 and V2 (**Figure 18**) with a base frequency of 32 kHz. Achievable duty cycle range is from 10–90%. Depending on duty cycle setting, both SMPS converters can independently deliver output voltage in a range from ±10 V to ±150 V. Independent setting of both positive and negative power supply voltage allows the synthesis of a 50% timesymmetrical signal. This could not be achieved using previously presented design, where amplitude symmetry could only be achieved by adjusting the duty cycle of micropump excitation period.

High-voltage switching stage comprises two Darlington output high-voltage optocouplers Toshiba TLP187 (**Figure 19**, circuit U1 and U2). Positive power supply voltage (**Figure 18**, label V+ ) is connected to positive voltage switching optocoupler (**Figure 19**, circuit U1), while the negative power supply voltage (**Figure 18**, label V− ) is connected to negative voltage switching optocoupler (**Figure 19**, circuit U2). Independent setting of both power supply voltages and opening of positive and negative voltage optocoupler enable independent setting of rising and falling edge of driving signal as well as positive/negative duty cycle and consequently excitation frequency. Such independent driving introduces an additional part of micropump operating positive/negative half-cycle, called positive/negative dead time, during which both optocouplers are turned off.

In **Figure 19**, only resistors SR1 and SR2 are shown to limit complexity, but more microcontroller output pins may be connected to optocoupler diode input, each via its different value resistor. Using such a setup, a simple D/A (digital-to-analog) converter is formed, which can be used for digitally setting the value of rising/falling edge slewrate. To enable more fine setting of both slew rates, a microcontroller with an onboard D/A could be used.

*Influence of Piezoelectric Actuator Properties on Design of Micropump Driving Modules DOI: http://dx.doi.org/10.5772/intechopen.103789*

#### **Figure 19.** *Excitation part of the optocoupler-based driver design.*

Rectangular micropump excitation signal is synthesized by providing phasesynchronized optocoupler driving signals V6 and V7 (see **Figure 19**, **Figure 20**). Both driving signals must include a programmable dead-time gap, which is downwards limited by optocoupler turn-off-time (80 μs for TLP187). Any further reduction of this minimal dead-time gap value would shorten out both power supplies.

The micropump switching period is divided into four stages as follows: Positive driving micropump voltage stage, noted by a solid line in **Figure 20**, where positive power supply (**Figure 18**) is connected to the micropump via optocoupler U1 (**Figure 19**). Subsequently, a positive dead time state is next, where both optocouplers are turned off. Following positive dead-time stage, there is a negative driving micropump voltage stage, noted by a dashed line in **Figure 20**. During this stage, negative SMPS power supply (**Figure 18**) is connected to the micropump using

**Figure 20.** *Optocoupler diode driving voltage.*

optocoupler U2 (**Figure 19**). Micropump excitation period finishes with a negative dead-time stage, where both optocouplers (U1, U2) are again switched off.

Such division of micropump excitation period into four independent stages yields maximum control over micropump driving signal parameters and overall current consumption. Decreasing the value of optocoupler based resistors SR1, SR2 (**Figure 19**) results in higher micropump driving signal slew rates. Selection of resistors represents a compromise between high-flow performance, which is achieved by fast slew-rate and consequentially higher current consumption, and low-flow performance, which is achieved by lower slew-rate and lower current consumption.

Adjusting optocoupler base current using a simplified D/A converter by connecting different resistors enables seamless interchanging between performance modes and different driving signal shapes: Sinus-flank signal (sinusoidal waveform with prolonged flat maximum/minimum) can be obtained by setting both rising and falling slew-rate low using a single, high-value resistor is fed to optocoupler diode. Rectangular signal can be synthesized by setting both slew rates high by turning on multiple resistors. Sinewave-rectangular signal (SRS signal) can be obtained by setting rising edge slew-rate high and falling edge slew-rate low.

Setting of both power supply voltages, slew-rates and dead time preferably have to be implemented in software, using an 8-bit microcontroller. In our cost-effective implementation, depicted in **Figure 6**, a Microchip ATtiny104 [16], was selected for its price and availability in a 14-pin SO package. An 8-bit timer zero is used both for counting through four distinctive signal stages and two-channel PWM synthesis for high-voltage generation. Timer zero is counting using an internal 8 MHz oscillator with no Prescaler, which produces two independent (non-aligned) channel (A and B) 8-bit PWM outputs running at 31.25 kHz. Each PWM channel features a corresponding output compare register (OCR0A/OCR0B).

Driving waveform state machine is stepping through active and dead-time stages for positive/negative amplitudes. Transition to the next state is achieved by presetting timer zero expiration period. After this period, timer zero overflow interrupt causes transition to next state and renewed calculation of next-stage period. Waveform transition state machine can be omitted entirely – in this case, constant positive or negative power supply voltage is connected to the micropump, effectively turning the micropump into a valve.

PWM output (OC0A/OC0B) value results from comparison between OCR0x and timer zero value. These PWM outputs are connected to the corresponding switching transistor (M1, M2 in **Figure 18**). The output (V+ , V− in **Figure 18**) of positive/ negative power supply SMPS is being monitored using a resistor divider, which is connected to corresponding microcontroller analogue-to-digital converter (ADC) input (ADC2/ADC3, **Figure 18**). An ADC conversion complete interrupt is enabled by timer zero interrupt routine only during positive and negative dead-time switching phase. During positive dead-time state, ADC2 is monitored, and the value of OCR0A is changed accordingly. During negative dead-time state, ADC3 is sampled, and the value of OCR0B is changed. Both positive/negative monitoring algorithms are based on a fast proportional regulator. Microcontroller UART receiver was used to configure all micropump driving signal parameters: frequency, positive and negative amplitudes, dead times, slew-rates and operation type (pumping/valve). In order to reduce the burden of calculations on a microcontroller side, an Excel VBA based script was developed. This script translates human-readable parameters such as frequency and dead time to timer zero state-machine expiration periods, determines slew-rate resistor multiplexing state and configures the mode of module operation.

*Influence of Piezoelectric Actuator Properties on Design of Micropump Driving Modules DOI: http://dx.doi.org/10.5772/intechopen.103789*

Switching optocouplers PC817 (U1, U2 in **Figure 19**), which were used during simulations, were replaced with high-voltage, Darlington output type (Toshiba TLP187), due to better switching characteristics and high-current transfer ratio, which achieves higher slew rates. Large (10 μF/100 V) rectifying capacitors (C1, C3 in **Figure 18**) were initially used to minimize the power supply ripple. Their size was reduced during the following measurements.

A digital storage oscilloscope was connected to the driving module prototype. Tested module was driving the micropump with PZT capacitance 12 nF, developed in our laboratory [7]. Developed module can set output signal dead time, between two extrema:


Micropump output signal of initial prototype during minimum drive of 200 μs with driving frequency 100 Hz and amplitude ±100 V is shown in **Figure 22**. Piezoelectric

**Figure 21.**

*Initial prototype full drive operation mode front edge detail.*

#### **Figure 22.**

actuator voltage falls off according to capacitive discharge down to 50 V after initial driving with amplitude ±98 V for 200 μs (max/min section, **Figure 22**). The time constant of capacitive discharge decay is practically independent of voltage polarity, on the other hand, its value changes depending on pumped media viscosity and micropump design.

## **3.2 Electrical characterization**

Driving signal frequency was investigated in range of 50 Hz to 1 kHz for full and minimum drive operating modes. Micropump driver was configured with maximum piezoelectric actuator voltage of 125 V using Excel control software. Maximum actuator voltage is limited by a maximum field of 600 V/mm of piezoelectric actuator P-5H (Sunnytec Suzhou Electronics Co., Ltd. [13]). Current consumption, positive and negative power supply amplitudes, slew rates and were measured at each micropump frequency value. **Figure 23** is presenting obtained results for micropump amplitude at each excitation frequency setting.

Micropump driver was able to reach admissible voltage limit of ±125 V in the frequency range up to 150 Hz, which extended initial clamping interval of 70 Hz, achieved with voltage scan of the initial prototype. This extension consequentially increases module current consumption *I*CC, which is presented in **Figure 24**.

At the target operating frequency of 100 Hz, in full drive operating mode with SR+ = SR− = 16 V/μs (**Figure 25**), module current consumption was clamped to 118 mA, at 150 Hz the current consumption increases to 180 mA. In minimum drive operating mode, current consumption reaches 170 mA. Difference in current consumption between 'full' and 'minimum' driving mode is minimal (10 mA), therefore, the majority of current consumption is attributed to decrease in efficiency of SMPS

*Influence of Piezoelectric Actuator Properties on Design of Micropump Driving Modules DOI: http://dx.doi.org/10.5772/intechopen.103789*

**Figure 23.** *Voltage-frequency sweep.*

**Figure 24.** *Current consumption vs. frequency.*

boost converters, when trying to supply actual current to the micropump actuator at such maximum admissible voltage setting. SMPS boost converter current is primarily limited by current capabilities of the used SRR0603 inductor. Current limit value of 180 mA could only be extended by raising the power supply value from 9 V to 10.5 V further to 200 mA for a short period of operation.

#### Positive and negative slew rates SR(V/ s) vs frequency

**Figure 25.** *Positive and negative slew rate vs. frequency.*

Extension of frequency interval up to 150 Hz (**Figure 23**) with operation at maximum driving amplitude of ±125 V, resulted in elevation of both slew rates from initial 11 V/μs to 16 V/μs, when configured to full drive operating mode at 100 Hz, as depicted in **Figure 21**. In minimum drive operating mode, this value is even higher (18 V/μs). Slew rate remains well over 10 V/μs in the frequency range to 400 Hz, which enables evaluation of our micropumps, with a smaller (Φ = 6 mm) piezoelectric actuator disc with a capacitance of 4 nF [15].

Next, module current consumption was evaluated against slew rate in the full drive operating mode, with micropump excitation frequency of 100 Hz and amplitudes ±125 V. Measured module current consumption was 118 mA, with both slewrates set to 16 V/μs, shown as a dashed line **Figure 21**.

Waveform measurements, such as frequency, amplitude, both slew rates and waveform averaging were evaluated instantaneously by the oscilloscope. Microcontroller I/O port output driving capability limited any further lowering of optocoupler base resistance to 220 Ω. On the other hand, high limit of optocoupler base resistance was defined by driving signal shape change – driving signal waveform would become sinus-flank if slew-rate values fell below 1 V/μs. Current consumption in full drive operating mode (**Figure 24**) remains practically a constant value of 125 mA, independent of slew-rate practically to the limit, where the slew-rate drops to 1 V/μs. Average current measurements at high slew rates are hard to establish due to slow data processing of the oscilloscope. In such conditions, the SMPS duty cycle is constantly changing, therefore, any increase in current consumption measurements in full drive operating mode have to be attributed to measurement error. Such measurement errors could be mitigated using a larger output capacitor (**Figure 18**, C1 and C3).

When comparing both operation modes, the majority of current consumption can be attributed to SMPS power sources. In minimum drive operation mode, current

### *Influence of Piezoelectric Actuator Properties on Design of Micropump Driving Modules DOI: http://dx.doi.org/10.5772/intechopen.103789*

consumption is reduced to 70 mA. Such minimum drive operation mode consequentially enables power-saving features during operation with reduced flow. On the other hand, full drive operation mode with its high slew rate improves the micropump air pumping capability, although, for efficient air pumping, driving frequency remains the main limitation factor.

Next, we performed the same set of measurements on a micropump, which comprises a smaller piezoelectric disc with a diameter of 6 mm and a capacitance of 4 nF [15]. Compared to the previously described 12 nF micropump, the clamping voltage (±125 V) regulation area extended from 150 Hz to 400 Hz. Both slew rates in full drive operation mode achieved levels of 22 V/μs, compared to previously achieved 16 V/μs. As expected, the current consumption in both modes reduced marginally – compared to values from **Figure 24**, —175 mA in full drive operation mode (previously 180 mA) and 160 mA in minimum drive operation mode (previously 170 mA).

### **3.3 Fluidic characterization**

After the micropump driver testing, the system for computer-controlled characterization of piezoelectric micropumps was set up. Analyzed driving module was connected to tested micropumps, developed in our laboratory [7, 15] using the measurement setup, described in **Figure 10**. Presented micropump driving module was compared to previous driving modules, designed in our Laboratory.

Three distinct micropump designs (N, R, S), each with different outlet channel geometry, were compared using RC asymmetric/symmetric driver and both initial and final versions of the optocoupler based driver. Airflow rate and DI water flow rate measurements were performed at 100 Hz, while the presented driver was configured in full-drive operation mode in both initial and optimized versions.

Achieved air flow rate with a symmetric RC driver was 1.6 sccm. This value increased to 4.2 sccm with optocoupler based driver in full-drive operation mode. Compared with symmetric and asymmetric versions of RC driver, presented driver surpasses all previous performances. Achieved DI water flow rate with a symmetric RC waveform-like driver was 2.2 sccm. This value was increased to 2.6 sccm with optocoupler based driver in full-drive operation mode. Achieved air backpressure performance almost doubled with the use of optocoupler based driver: It is achieving its peak value of 39 mbar on N1 type micropump in 'full drive' mode. Achieved DI water backpressure performance improved by 30%, compared to initial version of RC waveform-like driver: It is achieving its peak value of 240 mbar on N1 type micropump in "full drive" mode. Both presented versions improved DI water backpressure performance over RC waveform-like drivers.

Optocoupler based driver was set up at 100 Hz with ±105 V amplitudes in full drive operation mode a micropump S29R1 was connected to its output. Slew-rate values were altered in the range from 0.2 V/μs to 16 V/μs, while both DI water and airflow rates were measured. In both cases, the flow rate is practically independent of the slew-rate (**Figure 26**).

**Figure 28** is showing a slight decrease in backpressure characteristics with decreasing slew rate SR+ . This performance deteriorates severely when the slew rate falls below 4 V/μs. If slew rate is lowered even further down to 1 V/μs, comparable conditions as in the case of RC waveform-like driver may be achieved with reduced current consumption to 100 mA (see **Figure 27**) at 5 V module power supply. Even with slew rate kept as low as 1 V/μs, resulting flow rate is significantly higher

**Figure 26.** *Air, DI-water flow rate vs. slew rate in "full drive" mode.*

**Figure 27.** *Current consumption vs. positive slew-rate.*

compared to our RC driving module versions, which achieved slew rates of 0.2 V/μs. Current consumption can be further reduced using minimum drive operation mode.

Starting values of SR<sup>+</sup> in **Figure 28** indicate that high slew rates at 16 V/μs have only a minor impact on flow rate and backpressure characteristics at a significantly higher current consumption. However, higher slew rates improve/stabilize other micropump properties and such as self-priming and bubble tolerance and enable a more reliable micropump operation in different operating conditions.

*Influence of Piezoelectric Actuator Properties on Design of Micropump Driving Modules DOI: http://dx.doi.org/10.5772/intechopen.103789*

Backpressure vs. slew-rate @ 106V

**Figure 28.** *Air, DI-water backpressure vs. slew-rate in "full drive" mode.*
