3. Interfaces Poster
CORIOLIS MASS FLOW AND DENSITY SENSOR ACTUATION USING A PHASE-LOCKED LOOP
D. Alveringh1, T.V.P. Schut1, R.J. Wiegerink1, and J.C. Lötters1,2
1MESA+ Institute for Nanotechnology, University of Twente, Enschede, The Netherlands 2Bronkhorst High-Tech BV, Ruurlo, The Netherlands
ABSTRACT
This paper reports on novel feedback based actua-tion electronics that use the voltage from the inducactua-tion track of a Coriolis mass flow sensor as input signal for a phase-locked loop. The phase-locked loop consists of a phase detector that measures the difference between the actuation voltage and the induction voltage from the Coriolis 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 resonance frequency of the Coriolis mass flow sensor, making it more robust to disturbances. Besides, waveform shape and amplitude can be easily altered. First experimen-tal results show that the phase-locked loop controls the frequency 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.
KEYWORDS
Coriolis mass flow sensor, density sensor, phase locked loop, actuation, resonance.
INTRODUCTION
A first of its kind Coriolis mass flow sensor based on surface channel technology has been realized by Haneveld et al. [1] This Coriolis mass flow sensor is actuated in twist mode by supplying it with a varying electrical signal, inducing Lorentz forces as a result of an externally applied magnetic field. A fluid flow forces the Coriolis mass flow sensor to move in swing mode too; this effect is used to sense the mass flow. Figure 1 shows an illustration of both the twist mode due to actu-ation and the swing mode due to the Coriolos force. The mechanical resonance frequency of the actuated struc-ture depends on the modal stiffness and on the modal mass. Latter depends on the mass of the channel and the 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 twist mode using Lorentz actuation. When a fluid flows through the channel, a swing mode is induced. The ratio between the two modes is a measure for the mass flow. The resonance frequency of the structure is dependent on the density of the fluid.
In the first generation of actuation electronics, the frequency is tuned to the resonance frequency by hand. Since the frequency changes with the density of the fluid inside the channels, an electronic feedback system has been designed that uses the output signal from the Coriolis mass flow sensor to tune the oscillator to the resonance frequency (second generation). In the third generation of this Coriolis mass flow sensor actuation electronics, the induction voltage of an extra metal track on the Coriolis mass flow sensor is amplified and used for 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 the Coriolis mass flow sensor will influence the actua-tion directly.
• A fluidic disturbance, e.g. air bubble, instantly changes the actuation frequency for a short time. • The analog circuit amplifies the signal rather than
synthesizes it, i.e. the signal is not harmonic. Synthesizing the actuation signal in a controlled way would help overcome these drawbacks. This is done for high frequencies for telecommunication ap-plications with a phase-locked loop [3]. Phase-locked loops 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-locked loop.
The 3rd Conference on MicroFluidic Handling Systems, 4–6 October 2017, Enschede, The Netherlands
THEORY
A 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 signal So(t) and the input signal Si(t). The input signal can be modeled as an harmonic signal with ωi the frequency and φithe 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 loop needs to solve. The second component is a low-pass filter. It filters a possible ripple on the phase difference caused by the phase detector and disturbances from the input signal. The last component is a voltage controlled oscillator. This oscillator synthesizes a periodic signal with a frequency dependent on the input. The voltage controlled oscillator always provides an output signal at a frequency, also when no input is given. This is called the quiescent frequency. The output frequency ωo of the signal is:
ωo= ω0+K · φe, (3) with ω0 the quiescent frequency and K the sensitivity of the voltage controlled oscillator. The output signal is therefore equal to:
So(t) = Sa,o· sin((ω0+K · φe)t + φo). (4) Note that the frequency of the output signal is corrected based on the phase difference between output and input signal. This means that not only the output frequency will approach the input frequency, but the phases will be synchronized as well.
Δϕ VCO
phase dete
ctor
loop filter voltage cont
rolled oscilla tor Si(t) ωi ϕi ϕe ϕe S o(t) ωo ϕo Figure 2: Basic phase-locked loop consisting of a phase detector, a low-pass filter and a voltage controlled oscillator. The phase detector measures the difference between the output and input signal. The voltage controlled oscillator is directly tuned by this phase mismatch and synthesizes an harmonic signal that is synchronized with the input signal.
DESIGN
The 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 constant mass flow of fluid is applied (Figure 3).
printed circuit board chip wire bond fluid inlet Φ 5 bar N2 mass flow controller reservoir with IPA/H2O fluid outlet magnet magnet
Figure 3: Illustration of the chip assembly on a printed circuit board and its interface to the fluidic measurement setup.
The tested Coriolis mass flow sensor has multiple metal tracks on the channel as can be seen in the scan-ning electron microscopy close-up in Figure 4. One metal 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 differential amplifier with high input impedance.
Figure 4: Photograph of the chip with close-up of the suspended channel with multiple metal tracks of the Coriolis mass flow sensor.
The phase-locked loop is realized using a Cypress Semiconductor PSoC 5 development kit and is based on 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 shows an overview of the phase-locked loop implementation in the programmable system on chip.
The 3rd Conference on MicroFluidic Handling Systems, 4–6 October 2017, Enschede, The Netherlands
XOR low-pass filter bias VCO Coriolis mass flow sensor amplifier comparator comparator amplifier
Figure 5: Implementation of the phase-locked loop with the connections to the Coriolis mass flow sensor in a
programmable system on chip.
An embedded analog comparator converts the sig-nal to a square wave to make it compatible with digital electronics. An XOR-gate compares the square wave with the synthesized output of the phase-locked loop. This stage performs the phase detection: the XOR-gate gives a pulse at every sample when the square wave from the induction track is not equal to the synthesized actuation voltage. After a full period, the XOR-gate gives a signal with more high values at its output when the phases are more different. The output of the XOR-gate is connected to a low-pass filter, which provides an analog signal that is a measure for the phase mis-match. This first order low-pass filter is implemented using an embedded operational amplifier with an exter-nal capacitor and resistor, tuned for a cut off frequency of approximately 1 Hz.
The input of the voltage controlled oscillator is con-nected to a bias voltage generator to set the quiescent frequency. The bias voltage generator is realized us-ing an embedded operational amplifier. The voltage controlled oscillator with output voltage Vvco,ois imple-mented as an embedded current source ivcothat charges an external capacitor C:
Vvco,o= C1 Z
ivcodt = ivcoCt. (5)
An embedded comparator connects the capacitor to ground when the threshold, equal to the input of the voltage controlled oscillator Vvco,i, is reached:
Vvco,o =Vvco,i, (6) and so: Vvco,i= ivcot C → ω0 2π = 1 t = Vvco,i C ivco. (7) An embedded flip-flop is used to force the duty cycle of the output signal to be 50 %. An embedded signal synthesizer is used to synthesize a sine wave for the ac-tuation voltage of the Coriolis mass flow sensor. Figure 6 shows a photograph of the full electronic setup.
chip interface board
programmable system on chip CMFS chip CMFS to PLL PLL to CMFS debug screen PLL board analog breakout components
Figure 6: Photograph of the electronic phase-locked loop implementation. The Coriolis mass flow sensor (CMFS) is located in the center on the chip interface board. The phase-locked loop (PLL) is realized on the PLL board in the programmable system on chip and
MEASUREMENTS
To 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 Keysight 34461A multimeter. The experiments are conducted
The 3rd Conference on MicroFluidic Handling Systems, 4–6 October 2017, Enschede, The Netherlands
with nitrogen, water, isopropyl alcohol and a mix of isopropyl alcohol and water (equal volume). The re-sults are plotted in Figure 7 and show that the actuation circuit adjusts the frequency to the resonance frequency of the Coriolis mass flow sensor.
The stability is investigated by finding the mean and standard deviation of 5001 measurements in approxi-mately 20 ks for nitrogen. The mean is 2672.546 Hz and the standard deviation is 0.148 18 Hz, calculated from the data in Figure 8.
1400 1600 1800 2000 2200 2400 2600 2800 0 200 400 600 800 1000 Frequenc y (Hz) Density (kg m−3) Nitrogen IP A IP A+W ater W ater
Figure 7: Measured resonance frequencies for four different substances. 2671.6 2672.0 2672.4 2672.8 2673.2 0 4 8 12 16 20 Frequenc y (Hz) Time (ks)
Figure 8: Stability measurement of 5001 samples (20 ks) for nitrogen with constant pressure and mass flow.
CONCLUSION
A phase-locked loop is realized to actuate a Corio-lis mass flow sensor at its resonance frequency. First experimental results show that the phase-locked loop
controls the frequency for nitrogen, water, isopropyl alcohol and a mixture of water and isopropyl alcohol. In contrast with other actuation methods, it can be fine tuned to make the actuation of the Coriolis mass flow sensor be more robust against disturbances or lower the response time. Besides, waveform shape and amplitude can be easily altered.
Future work will focus on quantitative analysis through measurements of the performance of the PLL compared to the conventional actuation circuits.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge support by the Eurostars Programme through the TIPICAL project (E!8264) and technical support by R.G.P. Sanders and J. Groenesteijn.
REFERENCES
[1] J. Haneveld et al., “Modeling, design, fabrication and characterization of a micro Coriolis mass flow sensor,” Journal of Micromechanics and Micro-engineering, vol. 20, no. 12, p. 125001, 2010. [2] J. Groenesteijn, “Microfluidic platform for
Coriolis-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 for a 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 Netherlands
