GUIDANCE NOTE Application of MEMS Ultrasonic Transducers www.tuvnel.com
Application of MEMS Ultrasonic Transducers Introduction This Guidance Note provides a review of the key issues associated with the application of MEMS Ultrasonic Transducers to Flow Measurement. With current state of technology, Miniature Electro-Mechanical- Systems is perhaps a more relevant phrase to use for measurement systems that have on-board autonomy or processing that make them smart ; since the term provides an idea of scale more in keeping with the overall physical device size. Ultrasonic methods have been one of the fastest growing areas in flow meter development over the last ten years, and there remains scope for wider application and uptake in many market sectors. A crucial part of any ultrasonic flow meter (USFM) is the ultrasonic transducer, or transceiver, used both to generate and to receive the ultrasonic signal. Many of the limitations in current ultrasonic flow measurement technology revolve around the design and installation of the transducer. Micro Electro Mechanical Systems (MEMS) is a technology area that has seen a huge growth in research and capability in recent years. It is primarily concerned with miniaturisation of sensors and process circuitry to a level where industry standard silicon-circuit manufacturing techniques can be used to gain advantage of economies of scale offered by volume production. MEMS potentially opens up opportunities for design of low cost, robust sensors that are more versatile and efficient than conventional ultrasonic sensors. The aim of this Guidance Note is to provide an objective overview of MEMS technology as it applies to ultrasonic flow measurement. Figure 3 - MEMS Based Microphone as a Sound Level Meter (QinetiQ) For flow measurement the use of MEMS can be based around using only the MEMS sensor but driven by suitable conventional technology USFM hardware. This needs to take account of the drive requirements of the MEMS sensor (frequency, impedance and voltage), how the sensor will be interfaced to the other hardware, the acoustic output of the MEMS sensor and the measurement threshold of the signal processing algorithms. What are MEMS? Taken to its extreme, a MEMS device contains a whole system on a single chip as shown in Figure 1. MEMS technology can however be applied to the sensor only and Figure 2 shows an example of a sound pressure sensor on a chip with on-board signal conditioning. Whilst various forms of sensors can be engineered at a micro scale it is important to consider that the sensor is only one part of the measurement system. The means of powering and controlling the sensor, processing the information from the sensor, storing, displaying or transmitting that information and making the system robust all have an effect on the overall size-envelope of the complete measurement system. Figure 3 shows the sensor of Figure 2 combined with other boards to provide a miniature battery powered sound level meter. Figure 1 - MEMS Based Microfluidic Circuit (The University of Southampton Institute of Transducer Technology s (USITT)) Figure 2 - MEMS Based Microphone (QinetiQ/NPL) Common Problems in Ultrasonic Flow Measurement One problem in USFM development is the acoustic impedance mismatch between the ultrasonic transducer and the fluid in which the flow is to be measured. This reduces the amount of energy generated in the emitter-transducer and the amount of returned signal to the receiver-transducer. As impedance mismatch increases, the transducer requires greater drive power and energy (normally implying lower frequency operation). Also, more sophisticated signal processing is required to distinguish the received signal from background noise. Other Important Issues Include: Timing resolutions which place practical limits on the smallest pipe size that can be used. Temperature limited to < 100 C although this can be extended to -200 to 400 C using buffers and waveguide techniques. Another generic problem is the ability of USFMs to deal with complex fluids and the identification and application of suitable velocity profile correction factors.
June 2010 MEMS Sensor Types for Flow Metering The current state of MEMS ultrasound sensor development suggests that CMUT (Capacitive Machined Ultrasonic Transducer) devices and Thick-film PZT (Piezoelectric Transducer) sensors are the most appropriate for use in USFMs. CMUTs (Figure 4) essentially consist of a tiny diaphragm suspended above a cavity. An oscillating voltage supplied to electrodes mounted below the cavity and on the diaphragm itself causes the diaphragm to vibrate at the supply frequency. In open-cavity CMUTs, the cavity is vented to the environment whereas in closedcavity CMUTs, the cavity is sealed. CMUTs have been used in temperatures up to 450 C but film- PZTs can only be used up to approximately 100 C (dictated by the Curie temperature of the PZT material). The feasible upper temperature limit of a given MEMS device is device specific. Although elevated temperatures might be survivable for a short time, the long-term operation may be degraded by mineral creep between layers or layer delamination. The actual sensors are small in size: typically only a few mm, or smaller, across. They can, depending on the sensor size, generate high frequency, 1 MHz typical but 13 MHz possible. The feasible signal amplitude generated and received (and pipe materials for clamp-on applications) depends to a large extent on the sophistication of the signal processing algorithms used as much as on the basic performance of the sensor. They can be manufactured to work as 1-D or 2-D arrays. Figure 4 - Thick Film PZT Arrangement Thick-film PZT (Figure 5) sensors consist of layers of PZT piezoelectric between layers of electrode. Supply of an oscillating voltage to the electrodes creates an oscillation in the PZT. Beam steerable MEMS ultrasonic sensors are being developed in the medical imaging field. Beam-steering would be a necessary requirement if transducers were to be simply mounted flush with the pipe wall and would obviate, or minimise, the present need for angling the transducer face. If beam-steering were developed to a commercially viable level, then many possibilities would be opened for application in flow measurement including tomography systems combined with Time-of-Flight ultrasonic techniques giving improved accuracy. Piezo-composite materials can also be manufactured as 1-D and 2-D arrays for beam-steering but although a basic array cell can be relatively inexpensive (in the order of $200) the costs of wiring the array and signal processing can escalate the cost considerably (in the order of $5000). Apart from the more complex wiring (i.e. interfacing) the requirements of an array, the drive control and signal processing algorithms are also much more complex. Figure 5 - Thick Film PZT Arrangement Differential Pressure (DP) Flow Meters CMUTs have very good impedance matching when in direct contact with gases; therefore, they need less drive power (than conventional piezoelectric sensors) to produce and receive usable ultrasonic levels. Certain types of CMUT (open-cavity) may be able to work at high and low gas pressure. Certain types of CMUT (sealed-cavity) can operate in liquids if the sensor face is coated with a thin protective membrane. Film-PZT sensors may be suited to clamp-on (non-invasive) arrangements since they have similar impedance and performance properties to PZT-composite sensors. MEMS and Flow Measurement MEMS sensors based on the thermal-diffusion principle are being used in a number of commercial flow measurement products. Silicon-sensors based on CMUTs and thick-film PZTs are still in their developmental stages and have only occasionally been used as sensors for USFMs. When they have been used, this has been in combination with conventional USFM signal processing instrumentation and not with on-chip processing alongside the sensors. Consequently, it is unlikely that any meter-on-a-chip device will be forthcoming until the concept is demonstrated in a more general flow measurement role, and a suitable large-volume application is identified. Indications are that there are a few prototype MEMS based USFMs in development but these are subject to commercial confidentially and hence public-domain information is scarce.
Application of MEMS Ultrasonic Transducers Ultrasonic the Right Technology for Small Channels? A number of fundamental issues raise questions on the worth of ultrasonic techniques for flow measurement in small passages or channels. As pipe size is reduced, the acoustic signal pathlength also reduces and consequently the acoustic beam travel times reduce. This means that more accurate timing resolution is needed in small channels and most often this is achieved by using transducers that work at a higher frequency. There is therefore a consequential relationship between pipe diameter, acoustic path length, transducer frequency and timing resolution that dictates some fundamental limits on the application of USFM technology. The ability of MEMS ultrasonic sensors to operate at higher frequencies than conventional sensors may allow ultrasonic flow measurement in smaller channels than is currently achievable. An alternative is to direct the beam along the channel rather than across it and this might be a more appropriate approach for flow in small channels. MEMS Reliability Although many of the underlying failure mechanisms are known, data on the long-term reliability of MEMS devices are scarce. In the case of ultrasonic transducers the key factors likely to affect reliability are: Use of model-based drive signals to minimise parasitic constraint forces and part-wear. In other sectors, for example the domestic water industry, the main driving force may be cost reduction. Potential Application Areas MEMS transducers are unlikely to provide universal benefits to all areas of ultrasonic flow measurement. Potential application areas are: General flow measurement in low pressure gases and small pipe bores using CMUTs operating in the 2 MHz region. Flow measurement of natural gas using CMUTs. Flow measurement in hot (450 C) pulsating gas using CMUTs. Respiratory air flow measurement using CMUTs. General flow measurement in low pressure liquids and small pipe bores using either CMUTs operating in the 2MHz - 13MHz range or clamp-on thick-film PZTs. Blood flow or other liquids through plastic tubes. Milk flow at point of extraction. Open channel flow (surface level detection). Liquid level detection using immersion CMUTs. Not overdriving the sensor. Not subjecting the sensor to excessive temperature or cyclic temperature. Avoiding fatigue by not driving the sensor at too high frequency. Using a suitable coating or encapsulation to protect the sensor face from the fluid. Market Sector Needs It is important to realise that different market sectors are driven by different needs and that these will have a bearing on the usefulness of MEMS based flow metering in that sector. Where the requirement is for large volumes of low-cost, perhaps disposable, measurement devices, for example in certain sectors of the Health and Medical industry, the goal of a full system-on-achip may be financially viable. If a key requirement is metering accuracy, as in fiscal measurements in the Oil & Gas sector, the features of MEMS that are likely to be most attractive are small size and functionality; i.e. any capability to measure in circumstances where present technology is inadequate or expensive.
June 2010 Conclusions MEMS based CMUT sensors or film-pzt sensors in an ultrasonic flow meter, whilst not likely to benefit all areas of ultrasonic flow measurement, are concepts worth investigating since they could provide cost or technical benefits in a number of application areas. Further Reading Application of MEMS Ultrasonic Transducers A Technical Review. Report 2007-76 TUV NEL Ltd, East Kilbride, Glasgow. Since MEMS is still an emerging field, generic MEMS ultrasonic sensors are not yet (and may never be) available and hence a balance needs to be struck between MEMS sensor availability, cost, sales volume and the technical requirements of specific applications. A MEMS CMUT or film-pzt sensor based USFM and a full USFM-on-a-chip approach could hypothetically, given a suitable number of years of development, result in a smart-pipe for low pressure gas flow; i.e. pipes with a built-in flow meter. Driving MEMS sensors with present USFM processing circuitry may be feasible (although further investigation is required) as a more immediate means of incorporating MEMS sensor technology without the need to identify a large volume application to justify a meter-on-a-chip application. The purpose of this Guidance Note is to provide, in condensed form, information on measurement methods and technologies. It was produced as part of the UK Government s National Measurement System. For further information, contact: TUV NEL, East Kilbride, GLASGOW, G75 0QF, UK Tel: + 44 (0) 1355 220222 Email: info@tuvnel.com www.tuvnel.com TUV NEL Ltd 2010 Re-issued 2010 This publication is to provide outline information only which (unless agreed by TUV NEL in writing) may not be reproduced for any purpose or form part of any order or contract or be regarded as representation relating to products or services concerned.