**4.1 Module operation**

The concept of a separate, independently settable high voltage power supply section was already successfully implemented with the previously presented optocoupler based driver. During current consumption measurements, it was demonstrated, that the reduction of voltage step-up ratio significantly improves current driving capabilities of output stage. This design uses the same topology, capable of boosting the micropump power supply voltage in a range from ±10 V to ±150 V, depending on duty cycle setting (**Figure 18**, node label V<sup>+</sup> and V<sup>−</sup> ). Both SMPS converters are driven using PWM signal (**Figure 18**, sources V1 and V2) with PWM frequency 32 kHz in range from 10–80%. Additional low-voltage boost SMPS, which generates +10 V from a single +5 V power supply was added to reduce the voltage step-up ratio. Independent setting of power supply voltage levels allows adjustment of amplitude symmetry by separately adjusting the signal waveform and signal DC offset. Both high-voltage power supply outputs (**Figure 18**, V<sup>+</sup> and V<sup>−</sup> ) are connected to the operational amplifier high voltage power supply inputs (**Figure 29**, V<sup>+</sup> and V− ). High voltage boost stage comprises a single operational amplifier (U1), which feeds two common base transistor amplifiers. Positive output voltage common base amplifier (Q4 and R3) is used to translate low-output operational amplifier voltage (VAMPOUT) through R1 current onto base resistor R3, which opens positive voltage output driver Q2. Similarly, for negative output voltages, the common base amplifier (Q3 and R2) opens negative output voltage driver Q1. Note, that operation of this stage is inverting i.e. a negative swing of potential VAMPOUT causes a positive swing of output voltage VOUT. Operation of high voltage boost stage is controlled using a negative feedback loop, comprised of resistors RF1, RF2 and a capacitor C3 for frequency compensation.

Due to inverting nature of output stage, a *non-inverting input* of operational amplifier is used to complete the loop. Such feedback implements an overall non-inverting high voltage amplifier with amplification of AU = 42.6, which is able to amplify input amplitude of VAMPIN = ±3.3 V to output voltage range VOUT = ±140 V.

Selection of C3 at 470 pF limits input signal (VAMPIN) bandwidth to approximately 10 kHz, a decade above frequency limits, achieved with previous versions of our driver (max. 900 Hz). Minor drawback of presented non-inverting high voltage amplifier stage is the polarity of input signal VAMPIN: In order to achieve full output voltage swing, input signal also needs to be bipolar, which is inappropriate for driving with a microcontroller fed from a single 3.3 V power supply.

To be able to use unipolar input driving signals, a separate transformation stage, depicted in **Figure 30**, was designed. Operational amplifier U2 forms a voltage subtractor, which produces an output voltage (VAMPIN) as an amplified difference *Influence of Piezoelectric Actuator Properties on Design of Micropump Driving Modules DOI: http://dx.doi.org/10.5772/intechopen.103789*

**Figure 29.** *Non-inverting high voltage amplifier stage.*

#### **Figure 30.** *Signal conditioning part of the driver.*

between signal input (Vin) and offset input (Voffs). Current selection of feedback resistors R5 … R7 values Rd1, Rd2 ratio results in amplification of AU = 1.

Micropump control is achieved using a cost-effective (5€/unit) 32-bit ARM Cortex 4 microcontroller, an STM32G431KB in LQFP-48 package [19]. Selected microcontroller

also features 16-bit timer (TMR2), which in combination with 12-bit buffered digitalto-analog converter (DAC1) and a 12 channel direct-memory-access (DMA) controller forms a direct digital synthesis system (DDS), capable of synthesizing digital signals with update rate of 1 MSPS. Important fact is that the DDS entirely bypasses the microcontroller core, thus saving time for additional tasks, such as communication and PWM regulation for operational amplifier power supply (**Figure 18**, VS1, VS2) and high-voltage supply voltage (V+ and V− ) channel PWM. Selected microcontroller also features two separate 16-bit analog-to-digital converter units (ADC).

Each PWM controller uses a dedicated ADC channel in combination with one of PWM channels, available in timer (TMR1). ADC interrupt is triggered whenever conversion is complete on all four selected channels, afterwards PWM is adjusted according to power supply set-point. Microcontroller TMR1 PWM unit channels are configured independently, with four output compare registers (TMR1/CH2 and TMR1/CH3), running at 32 kHz with 12-bit resolution. PWM outputs are connected to the corresponding SMPS transistor (M1, M2 in **Figure 18**) via MOSFET driver TC4427.

Each SMPS output is monitored by feedback to the corresponding microcontroller analogue-to-digital converter input (ADC2/ADC3, **Figure 18**). After DDS generation is configured and started, it operates independently of processor actions. Input characteristic is calculated using voltage subtractor equations (see **Figure 30**) valid for ideal or DC operation, while at higher frequencies, amplification deterioration has to be taken into account. To mitigate this, the driving signal has to be monitored separately and the voltage subtractor input recalculated to actually achieved waveform positive and negative extrema. Microcontroller USB-CDC receiver was utilized as a communication interface for adjustment of all micropump driving signal parameters.

## **4.2 Electrical characterization**

Micropump piezoelectric actuator excitation voltage was limited to ±125 V due to PZT actuator electrical field limitation and micropump frequency was set from 10 Hz to 9.2 kHz using a sinewave and square-wave excitation signal. Positive and negative driving signal amplitudes, achieved on the micropump actuator, and current consumption were measured at each micropump frequency setting. Micropump driving voltage amplitudes (V+ and V− ) vs. excitation frequency results are presented in **Figure 31**.

In the low-frequency range, micropump driving signal clamped out to an admissible voltage limit of ±125 V, before it begins to deteriorate (**Figure 31**). In comparison to driving voltage vs. frequency scan of our previous prototypes, this clamping interval extended from 150 Hz to 1 kHz. Upper (V<sup>+</sup> ) and lower (V<sup>−</sup> ) voltage characteristics in **Figure 31** represent achievable voltage limits, which can be regulated by a voltage monitor.

Such extension of clamped-out driving signal amplitudes consequently results in an increased module current consumption *I*CC, which is presented in **Figure 32**. Previously developed modules were capable of producing square wave driving signals in frequency range up to 700 Hz and its current consumption reached 118 mA at 100 Hz and 180 mA at 150 Hz.

Current miniaturized arbitrary waveform generator design also offers driving using sine-wave, where the true driving signal frequency extension becomes obvious: It can extend frequency driving interval up to 9.2 kHz. Current consumption using

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

**Figure 31.** *Voltage -frequency sweep.*

**Figure 32.** *Current consumption vs. frequency.*

square wave driving signal is also reduced: it reaches 40 mA at 100 Hz and 75 mA at 1 kHz. A major improvement, shown in **Figure 32**, is the ability of presented driver to generate a sinewave signal, which results in a far smaller current consumption compared to square-wave driving signal. As shown in our previous research, square-wave signal offer better fluidic performance, while sinewave offer more stable micropump operation with far less stress on piezoelectric actuator.

### **4.3 Fluidic characterization**

After initial electrical testing, fluidic characterization was performed using the same computer controlled characterization system (see **Figure 10**). Analyzed driving module was connected to tested micropumps, developed in our laboratory [7, 15]. Our previous research of micropump performance on signal shape was focused primarily on the optimization of square-wave signal parameters to improve micropump flow and backpressure performance. Impact on the slew-rate increase was demonstrated in the optocoupler based design results.

In the following measurements, we intend to focus on pumping of air, because the presented driver excelled at generating frequencies above 1 kHz, which we were unable to implement on our previous designs. Presented micropump driving module was compared to previously described driving modules, designed in our Laboratory (RC-like rectangular signal driver and optocoupler-based driver). Micropump design S32S2 with small outlet channel geometry was used for comparison between RC symmetric driver and latest version of the presented driver. Air flow rate (**Figure 33**) and DI water flow rate (**Figure 34**) measurements, were performed with sinewave signal in the frequency range from 100 Hz to 3 kHz.

Presented driver performs significantly better than previous RC waveform-like driver, especially in air pumping, where it increases the air flow rate (from 1.6 sccm to 2.2 sccm) in comparison with symmetric-amplitude RC waveform-like driver. Compared with symmetric and asymmetric versions of RC waveform-like driver, presented driver surpasses all previous performances of previously described drivers.

Compared to the optocoupler design, flow rate decrease for DI water from 3.3 ml/ min to 2.2 ml/min seems substantial. Note, that 3.3 ml/min was obtained using square-wave signal with higher slew rate on optocouplers. In arbitrary waveform design, such slew rates could not be achieved not even using square-wave measurement. However, presented driver almost doubles air backpressure performance, compared to RC type driver (39 mbar) and is achieving its peak value of 52 mbar on N1 type micropump using sinewave driving signal. The flow rate and backpressure characteristic flatten out at 3 kHz, therefore this represents the usable driving frequency range for tested micropumps.

**Figure 33.** *Air flow rate and backpressure vs. frequency.*

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

**Figure 34.** *DI water flow rate and backpressure vs. frequency.*
