差压传感器

Microbridge Technologies Canada, Inc
April 2009
Nano-Air-Flow Based Differential Pressure Sensor for Medical Respiration Measurement Applications
Abstract: Microbridge Technologies has developed a nano-air-flow sensor, integrated on-chip with analog CMOS signal conditioning circuitry and with Rejustors providing analog adjustability. The thermo-anemometer flow sensing principle, combined with a nano-air-flow channel having very high flow-impedance, allows accurate sensing of low differential pressures, over a wide dynamic range. The flow-impedance is predefined at the die-level, dramatically relaxing demands on subsequent packaging operations, resulting in a smaller, lower-cost solution. The high flow impedance improves robustness vis-a-vis variability of connection hoses, changing gas filter properties, and humidified air. The high flow impedance makes the flow sensor, and any hose-connections to and from the sensor, easier to protect from contaminants. The technology enables substantial price-reductions with performance and ease-of-use improvements over present solutions.
Requirements of Medical Respiration Applications: Consider typical medical applications in human respiration, requiring measurement of air flow in and/or out of human lungs during breathing – such as artificial lung ventilation equipment, spirometers, sleepapnea treatment apparatus, CPAP (continuous positive airway pressure) equipment, VPAP (variable positive airway pressure). Typically, these techniques measure flow in a breathing tube indirectly, by sensing flow-induced differential pressure in a shunt configuration, at two ports positioned along the side of the tube, as shown in Fig. 1. Often, this flow-induced differential pressure (P) is enhanced by a flow-restrictive element, such as a baffle, also shown in Fig. 1. Since it is important not to interfere with lung function, it is advantageous that the breathing tube, and especially the baffle, not offer too-high flow-resistance during normal breathing. The overall breathing tube and baffle are designed to offer low flow-resistances, roughly 200Pa or less per liter/s of breathing flow. breathing flow baffle P1 two ports P hose connections P = (P2 – P1) P2 breathing tube
Fig. 1
P sensor
/
p. 1 of 6
Microbridge Technologies Canada,Extremely Low Differential Pressure: With air flows in the range from several liter/s for spontaneous respiration, up to 15 liter/s for forcedexpiration, the differential pressures sensed in a shunt configuration are still very low – in the range of hundreds to thousands of Pascals (hundredths of an Atm, inches of H2O to ~15 “H2O). These levels of differential pressures (P = P2 – P1) are typically too low for membrane-type differential pressure sensors to be practical and cost-effective in high volumes. Dynamic Range 10 x: Due to the physics of gas dynamics, the P seen at the two pressure ports increases roughly as the square of the main flow in the breathing tube. This severe non-linearity places extreme demands on the P sensor, to accurately measure low flows. In practice, in order to offer overall resolution of ~1% in measurement of the main flow in the breathing tube, it is necessary to measure P over a dynamic range 4 of ~10 x or greater. Resistance to Contamination Effectively No Flow-Through Leakage: Beyond the stringent requirements for dynamic range, and measurement of low breathing flows, medical respiration applications bring tough standards regarding resistance to contamination. In Fig. 1, this translates to the requirement that the sensor must allow effectively no flow-through leakage. Taken together, and with the additional requirement of low-cost high-volume mass-production, the above set of requirements constitutes an unprecedented challenge for sensor manufacturers. This paper describes, below, how Microbridge’s nano-air-flow-based differential pressure sensor (pressure-from-flow sensor) is able to meet all of these needs in a mass-producible, cost-effective sensor.
4
Sensing Differential Pressure from Air-Flows: As mentioned above, membrane-type differential pressure sensors are not practical for measurement of very low differential pressures. Instead Microbridge measures differential pressure as in Fig. 1, by a “pressure-from-flow” strategy, using a basic thermo-anemometer-type air-flow sensor. Thermo-anemometer type micro-air-flow sensors have been well known for several decades. As shown in Fig. 2 (top view and side view), the air travels through a flowchannel, which guides the air over a central heating element, which locally heats a small volume of gas. The heated volume is displaced by the flow in one direction or the other, which in turn unbalances the temperatures in a pair of temperaturesensors, positioned symmetrically on each side of the heating element. The speed with which the air flows through the flow channel is determined by the difference in pressure between the two ends of the flow channel, and by the flowimpedance of the flow channel, measured in (pressure-difference) per (flow rate in ml/s).