3. Interfaces Poster

CORIOLIS MASS FLOW AND DENSITY SENSOR ACTUATIONUSING A PHASE-LOCKED LOOP

D. Alveringh1, T.V.P. Schut1, R.J. Wiegerink1, and J.C. Lötters1,2

1 MESA+ Institute for Nanotechnology, University of Twente, Enschede, The Netherlands2 Bronkhorst High-Tech BV, Ruurlo, The Netherlands

ABSTRACTThis paper reports on novel feedback based actua-

tion electronics that use the voltage from the inductiontrack of a Coriolis mass flow sensor as input signal fora phase-locked loop. The phase-locked loop consists ofa phase detector that measures the difference betweenthe actuation voltage and the induction voltage from theCoriolis mass flow sensor. A voltage controlled oscil-lator is directly tuned by this phase mismatch and syn-thesizes an harmonic signal for actuation. Therefore,the frequency of the signal synthesized by the phase-locked loop will gradually be adjusted to the resonancefrequency of the Coriolis mass flow sensor, making itmore robust to disturbances. Besides, waveform shapeand amplitude can be easily altered. First experimen-tal results show that the phase-locked loop controls thefrequency as a function of density for different fluids.Stability is tested for nitrogen and shows a standard de-viation of 0.148 18 Hz for 20 ks.

KEYWORDSCoriolis mass flow sensor, density sensor, phase

locked loop, actuation, resonance.

INTRODUCTIONA first of its kind Coriolis mass flow sensor based

on surface channel technology has been realized byHaneveld et al. [1] This Coriolis mass flow sensor isactuated in twist mode by supplying it with a varyingelectrical signal, inducing Lorentz forces as a result ofan externally applied magnetic field. A fluid flow forcesthe Coriolis mass flow sensor to move in swing modetoo; this effect is used to sense the mass flow. Figure 1shows an illustration of both the twist mode due to actu-ation and the swing mode due to the Coriolos force. Themechanical resonance frequency of the actuated struc-ture depends on the modal stiffness and on the modalmass. Latter depends on the mass of the channel andthe mass of the fluid inside. This means that the Corio-lis mass flow sensor can also be used as a density sensor,as long as the resonance frequency can be detected.

twist mode(due to actuation)

swing mode(due to Coriolis force)

ωA

ωA

FA

FA

i Φ

FC

Figure 1: The Coriolis mass flow sensor is actuated in twistmode using Lorentz actuation. When a fluid flows throughthe channel, a swing mode is induced. The ratio between thetwo modes is a measure for the mass flow. The resonancefrequency of the structure is dependent on the density of thefluid.

In the first generation of actuation electronics, thefrequency is tuned to the resonance frequency by hand.Since the frequency changes with the density of thefluid inside the channels, an electronic feedback systemhas been designed that uses the output signal from theCoriolis mass flow sensor to tune the oscillator to theresonance frequency (second generation). In the thirdgeneration of this Coriolis mass flow sensor actuationelectronics, the induction voltage of an extra metal trackon the Coriolis mass flow sensor is amplified and usedfor actuation [2]. In spite of the convenience and per-formance improvements with these actuation circuits,there are still multiple drawbacks on these technologies.

• A mechanical disturbance, e.g. vibration, of theCoriolis mass flow sensor will influence the actua-tion directly.

• A fluidic disturbance, e.g. air bubble, instantlychanges the actuation frequency for a short time.

• The analog circuit amplifies the signal rather thansynthesizes it, i.e. the signal is not harmonic.

Synthesizing the actuation signal in a controlledway would help overcome these drawbacks. This isdone for high frequencies for telecommunication ap-plications with a phase-locked loop [3]. Phase-lockedloops are also used for controlled actuation of servo mo-tors [4]. This paper describes the first results of actua-tion of a Coriolis mass flow sensor using a phase-lockedloop.

The 3rd Conference on MicroFluidic Handling Systems, 4–6 October 2017, Enschede, The Netherlands102

THEORYA basic phase-locked loop consists of three compo-

nents as is illustrated in Figure 2. First, a phase detec-tor finds the phase difference between the output signalSo(t) and the input signal Si(t). The input signal can bemodeled as an harmonic signal with ωi the frequencyand φi the phase:

Si(t) = Sa,i · sin(ωit + φi), (1)

The detected phase difference is then:

φe = φi − φo, (2)

and could be seen as the error that the phase-locked loopneeds to solve. The second component is a low-passfilter. It filters a possible ripple on the phase differencecaused by the phase detector and disturbances from theinput signal. The last component is a voltage controlledoscillator. This oscillator synthesizes a periodic signalwith a frequency dependent on the input. The voltagecontrolled oscillator always provides an output signal ata frequency, also when no input is given. This is calledthe quiescent frequency. The output frequency ωo ofthe signal is:

ωo = ω0 + K · φe, (3)

with ω0 the quiescent frequency and K the sensitivityof the voltage controlled oscillator. The output signal istherefore equal to:

So(t) = Sa,o · sin((ω0 + K · φe)t + φo). (4)

Note that the frequency of the output signal is correctedbased on the phase difference between output and inputsignal. This means that not only the output frequencywill approach the input frequency, but the phases willbe synchronized as well.

Δϕ VCO

phas

e de

tect

or

loop

fil

ter

volt

age

cont

roll

edos

cill

ator

Si(t)ωiϕi

ϕe ϕe So(t)ωoϕo

Figure 2: Basic phase-locked loop consisting of a phasedetector, a low-pass filter and a voltage controlled oscillator.The phase detector measures the difference between theoutput and input signal. The voltage controlled oscillator isdirectly tuned by this phase mismatch and synthesizes anharmonic signal that is synchronized with the input signal.

DESIGNThe chip (Figure 4) is adhesively mounted on a

printed circuit board. Fluid inlet and outlet are con-nected at the backside of the board where a constantmass flow of fluid is applied (Figure 3).

printedcircuitboard

chipwire bond

fluid inlet

Φ5 bar N2

mass flowcontroller

reservoirwith IPA/H2O

fluid outlet

magnet magnet

Figure 3: Illustration of the chip assembly on a printedcircuit board and its interface to the fluidic measurementsetup.

The tested Coriolis mass flow sensor has multiplemetal tracks on the channel as can be seen in the scan-ning electron microscopy close-up in Figure 4. Onemetal track is used to apply the actuation current, an-other is used to detect the induced voltage due to actu-ation. The induction track is connected to a differentialamplifier with high input impedance.

Figure 4: Photograph of the chip with close-up of thesuspended channel with multiple metal tracks of the Coriolismass flow sensor.

The phase-locked loop is realized using a CypressSemiconductor PSoC 5 development kit and is basedon the work of De Lima Fernandes [5]. This develop-ment kit has a programmable system on chip with dig-ital and analog electronic components. Figure 5 showsan overview of the phase-locked loop implementationin the programmable system on chip.

The 3rd Conference on MicroFluidic Handling Systems, 4–6 October 2017, Enschede, The Netherlands103

XOR low-passfilter

bias

VCO

Coriolismass flow

sensor amplifier

comparator

comparator amplifier

Figure 5: Implementation of the phase-locked loop with theconnections to the Coriolis mass flow sensor in aprogrammable system on chip.

An embedded analog comparator converts the sig-nal to a square wave to make it compatible with digitalelectronics. An XOR-gate compares the square wavewith the synthesized output of the phase-locked loop.This stage performs the phase detection: the XOR-gategives a pulse at every sample when the square wavefrom the induction track is not equal to the synthesizedactuation voltage. After a full period, the XOR-gategives a signal with more high values at its output whenthe phases are more different. The output of the XOR-gate is connected to a low-pass filter, which providesan analog signal that is a measure for the phase mis-match. This first order low-pass filter is implementedusing an embedded operational amplifier with an exter-nal capacitor and resistor, tuned for a cut off frequencyof approximately 1 Hz.

The input of the voltage controlled oscillator is con-nected to a bias voltage generator to set the quiescentfrequency. The bias voltage generator is realized us-ing an embedded operational amplifier. The voltagecontrolled oscillator with output voltage Vvco,o is imple-mented as an embedded current source ivco that chargesan external capacitor C:

Vvco,o =1C

∫ivcodt =

ivcotC

. (5)

An embedded comparator connects the capacitor toground when the threshold, equal to the input of thevoltage controlled oscillator Vvco,i, is reached:

Vvco,o = Vvco,i, (6)

and so:

Vvco,i =ivcot

C→ ω0

2π=

1t=

Vvco,i

C ivco. (7)

An embedded flip-flop is used to force the duty cycleof the output signal to be 50 %. An embedded signalsynthesizer is used to synthesize a sine wave for the ac-tuation voltage of the Coriolis mass flow sensor. Figure6 shows a photograph of the full electronic setup.

chip interface board

programmablesystem on chip

CMFS chip

CMFS to PLL

PLL to CMFS

debug screen

PLL board

analog breakoutcomponents

Figure 6: Photograph of the electronic phase-locked loopimplementation. The Coriolis mass flow sensor (CMFS) islocated in the center on the chip interface board. Thephase-locked loop (PLL) is realized on the PLL board in theprogrammable system on chip and

MEASUREMENTSTo test the phase-locked loop for Coriolis mass flow

sensor actuation, multiple fluids are applied to the sen-sor and the frequencies are recorded using a Keysight34461A multimeter. The experiments are conducted

The 3rd Conference on MicroFluidic Handling Systems, 4–6 October 2017, Enschede, The Netherlands104

with nitrogen, water, isopropyl alcohol and a mix ofisopropyl alcohol and water (equal volume). The re-sults are plotted in Figure 7 and show that the actuationcircuit adjusts the frequency to the resonance frequencyof the Coriolis mass flow sensor.

The stability is investigated by finding the mean andstandard deviation of 5001 measurements in approxi-mately 20 ks for nitrogen. The mean is 2672.546 Hz andthe standard deviation is 0.148 18 Hz, calculated fromthe data in Figure 8.

1400

1600

1800

2000

2200

2400

2600

2800

0 200 400 600 800 1000

Freq

uenc

y(H

z)

Density (kg m−3)

Nitr

ogen

IPA

IPA

+Wat

erW

ater

Figure 7: Measured resonance frequencies for four differentsubstances.

2671.6

2672.0

2672.4

2672.8

2673.2

0 4 8 12 16 20

Freq

uenc

y(H

z)

Time (ks)

Figure 8: Stability measurement of 5001 samples (20 ks) fornitrogen with constant pressure and mass flow.

CONCLUSIONA phase-locked loop is realized to actuate a Corio-

lis mass flow sensor at its resonance frequency. Firstexperimental results show that the phase-locked loop

controls the frequency for nitrogen, water, isopropylalcohol and a mixture of water and isopropyl alcohol.In contrast with other actuation methods, it can be finetuned to make the actuation of the Coriolis mass flowsensor be more robust against disturbances or lower theresponse time. Besides, waveform shape and amplitudecan be easily altered.

Future work will focus on quantitative analysisthrough measurements of the performance of the PLLcompared to the conventional actuation circuits.

ACKNOWLEDGEMENTSThe authors gratefully acknowledge support by the

Eurostars Programme through the TIPICAL project(E!8264) and technical support by R.G.P. Sanders andJ. Groenesteijn.

REFERENCES[1] J. Haneveld et al., “Modeling, design, fabrication

and characterization of a micro Coriolis mass flowsensor,” Journal of Micromechanics and Micro-engineering, vol. 20, no. 12, p. 125001, 2010.

[2] J. Groenesteijn, “Microfluidic platform forCoriolis-based sensor and actuator systems,” Ph.D.dissertation, University of Twente, Enschede,January 2016.

[3] G.-C. Hsieh et al., “Phase-locked loop techniques.a survey,” IEEE Transactions on industrial elec-tronics, vol. 43, no. 6, pp. 609–615, 1996.

[4] G. Volpe, “A phase-locked loop control system fora synchronous motor,” IEEE Transactions on Auto-matic Control, vol. 15, no. 1, pp. 88–95, 1970.

[5] A. De Lima Fernandes, “Demystifying the PLL,”2013.

CONTACT* D. Alveringh, d.alveringh@utwente.nl

The 3rd Conference on MicroFluidic Handling Systems, 4–6 October 2017, Enschede, The Netherlands105