FEATURE MEMS


EASING THE PRESSURE: FLOW SENSORS FOR CPAP DEVICES


With obstructive sleep apnea (OSA) cases on the rise from smoking and declining air quality (especially in developing countries), Continuous Positive Air Pressure (CPAP) devices have become the golden standard solution for addressing this disorder. Ohlan Silpachi, business development & product manager – Aceinna Flow Products gets to the core of this technology, and its capabilities


T


here are many manufacturers producing variants of CPAP devices to


accommodate differing budgets and medical requirements. A simple lower cost version typically includes an air pump with a pressure sensor while the more complex (and expensive) versions add various additional types of sensors that can be used to detect and monitor breathing cycles, humidity level, and snoring events. A flow sensor is a critical part of a CPAP


device and is integrated into the design to monitor respiratory cycle and control device pressure so that a patient does not experience the discomfort of having to exhale into a pressurised system during sleep. In the past, due to their high costs, flow sensor usage was reserved for more expensive CPAP designs. As a result, lower cost CPAP devices - especially in the developing countries - lacked flow sensing capability. In contrast, utilising a differential


pressure sensor mounted across a laminar flow element can be a cost- effective method to measure flow in a CPAP device. Using Bernoulli’s principal, mass flow rate can be inferred from the pressure drop across the laminator flow element. In comparison to a standard flow sensor, this method is more economical but it typically requires calibration by CPAP device manufacturers to attain usable flow accuracy. Differential pressure sensors inside of


covering CPAP device applications are on the low-pressure range - just a few inches of water column. This requirement reduces the power consumption used by the blower apparatus therefore reducing device noise but places a performance demand on the differential pressure sensor. There are several types of differential


pressure sensor technologies but the most prominent are diaphragm and thermal anemometer. Diaphragm sensors measure deflection/strain while thermal anemometers measure heat transfer when pressure difference is present.


18 WINTER 2017 | | MICROMATTERS


symmetrically upstream and downstream of the micro-heater. If no gas flows over the sensor surface, the symmetric thermopiles measure the same rise in temperature, resulting in the same output voltage of the two thermopiles. If a non-zero gas flows from the inlet


Since CPAP applications requires measuring low pressure differences, thermal based sensors offer much better sensitivity due to the lack of pressure to afford a reasonable deflection to the diaphragm sensors. This is especially true for a MEMS based thermal sensor with a micro sensor structure that requires less power and provides better sensitivity. Aceinna offers flow sensor modules for


the CPAP market using its MEMS thermal technology, which is used in its high volume commercial accelerometers to develop differential pressure sensors that can be used for flow measurement.


HOW DOES MEMS THERMAL TECHNOLOGY WORK? The thermal mass flow sensor typically consists of upstream and downstream temperature sensors (thermopiles) and a heater located between the two temperature sensors as shown in figure 1. The sensor chip, produced using this CMOS compatible technology, is composed of a central heater source (micro heater) and two temperature sensors (thermopiles), which are placed


Figure 1:


The flow rate is detected by the MEMS thermal mass flow sensor


to the outlet of the meter, the velocity of a fully-developed laminar air flow unbalances the temperature profile around the heater and heat is transferred from upstream thermopiles to the downstream thermopiles, causing a change in the voltages of the thermopiles. Larger gas flow rates result in larger asymmetry in the temperature profile. The company’s new MDP200 flow


Figure 2:


Aceinna’s new MDP200 is a high performance, low cost, bi-directional differential pressure sensor – RoHS & REACH compliant


sensor utilises MEMS thermal technology, offering a highly sensitive differential pressure sensor. Even with a bi-directional range of ±500 pascal, the MDP200 can differentiate between zero pressure and 0.016 pascal. Accompanying this sensitivity is a calibration accuracy of ± 0. 03 pascal at zero and a 3% span accuracy for the entire range with a 16-bit resolution and less than 8ms of update rate. Such characteristics make it possible


for CPAP manufacturers to utilise the MDP200 as a bypass flow module (mass flow temperature compensation available) across a flow body without having to calibrate each flow sensor individually (providing reasonable consistency is available in the main flow body itself). Eliminating the time consuming and expensive sensor calibration process means that the MDP 200 differential pressure sensor can provide the same high level of performance as a typical flow sensor (which traditionally costs a few orders of magnitude higher in price) at a much lower total cost.


Aceinna www.aceinna.com/flow-sensors e: sales@willow.co.uk


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