MARKET PLACE
Respiration monitoring devices
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Devices such as spirometers and sleep recorders are employed to monitor respiration for diagnosis purposes or to optimise therapies. Spirometers perform simple lung function tests to examine airway or lung diseases such as asthma or chronic obstructive pulmonary disease. They measure the respiratory air flow and determine the inhaled and exhaled total respiratory volumes (inspiratory and expiratory volumes) and other lung function statistics. These devices are typically small, lightweight portable systems that are used in the doctor’s surgery, patient’s home and workplace or wherever symptoms appear.
Sleep-disordered breathing most commonly occurs in the form of obstructive sleep apnea, which is characterised by frequent episodes of obstructed breathing or cessations of breathing during sleep. Diagnostics devices include sleep recorders and sleep-screening systems. These often measure respiratory airflow, pulse rate, oxygen saturation, breathing sounds and body position. They are portable and battery powered and are used in clinical sleep laboratories and the home.
Differential pressure flow measurement
To determine respiratory flow, spirometers and sleep screening devices typically use differential pressure flow measurement techniques. An internal flow restriction such as a laminar flow element or an orifice plate generates a minimal pressure drop to the respiratory airflow (Figure 1). The volumetric flow rate can be determined by a highly sensitive pressure sensor that measures pressure differences of fractions of a millibar across the element. The sensor must be able to measure positive and negative pressure differences to differentiate between bidirectional inspiratory and expiratory flows.
Digital signal conditioning
Medical device design engineers must consider the cost–performance ratio of each individual component. To achieve high accuracies, pressure sensors must be calibrated and compensated for offset, span, nonlinearity and temperature effects. For this task, many pressure sensors use complex and costly techniques such as passive resistor networks and operational amplifiers. However, to achieve high total accuracies below 1% and within a tight budget, other methods such as digital signal conditioning are used instead.
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Figure 1: Schematic of differential pressure flow measurement in a spirometer.
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Digital pressure sensors use application specific integrated circuits to electronically correct the sensor signal. The analogue pressure signal and the corresponding temperature information are analogue to digital (A/D) converted (Figure 2). A microcontroller calculates the exact pressure values using a special correction formula and compensation coefficients, which are stored in the microcontroller memory. These precise coefficients are determined by temperature and pressure cycling each individual sensor during production. The final sensor signals are available via digital interfaces (for example, I²C bus or serial peripheral interface bus) or as analogue voltage outputs. Digitally compensated sensors can achieve high total accuracies, with total error bands better than ±0.5 % full scale span over a temperature range of 0–85 °C. They feature fast response times of typically 0.5 ms at high resolutions of 12 bit and are able to directly communicate with controllers of devices such as spirometers and sleep recorders.
Offset compensation technique
Pressure sensors in mobile and portable medical devices are often exposed to position changes and accelerations. To avoid these influencing the pressure readings, the sensors must be insensitive to these movements. Therefore, some low-pressure devices feature a special compensation technique to virtually avoid any offset shifts and achieve ultra stable output signals. Piezoresistive pressure sensors, for example, use a thin silicon diaphragm in which four sensing resistors are implanted to form a bridge circuit. Pressure causes the diaphragm to flex, which changes the sensing resistors’ values. When the bridge is powered, the resulting voltage output is proportional to the applied pressure.
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Figure 2: Schematic of differential pressure flow measurement in a spirometer.
(click image to enlarge) |
Low-pressure sensors feature a relatively large diaphragm, in proportion to the diaphragm’s thickness, to achieve the sensitivities required to measure pressures of only a few millibars. Conventional low pressure sensors are therefore also sensitive to forces that are not caused by the pressure media but by movement and gravitational effects on the sensor. This position sensitivity can lead to zero-pressure offset shifts and to erroneous pressure readings. Offset shifts can also be caused by sensor warm-up, ambient temperature changes and the long-term stability of the sensor. To eliminate this, special dual chip compensation techniques have been introduced that combine two silicon pressure chips to significantly reduce all offset instabilities. The outputs of the two sensors are electrically inversely coupled so that the offset error of the first sensor compensates the offset error of the second sensor.
The manufacturing process
Today’s pressure sensors are manufactured using fully automated state of the art production lines utilising modular production processes. The bare semiconductor die is directly attached onto substrates using high precision pick and place and chip-on-board technologies. Automated glue dispensing and printing techniques provide stable bonds and chips are electrically connected with ultrasonic or thermosonic wire bonding machines. Finally, the construction is sealed with pressure-tight moulded plastic housings. Soft adhesives and similar expansion coefficients of all materials provide largely stress-isolated packages. The complete sensor assembly line is located in a clean-room environment and manufacturing takes place in compliance with quality standards such as EN ISO 9001 or EN ISO 13485, the medical device quality system standard. A choice of surface mountable and single or dual in line miniature packages helps optimise space in spirometers, sleep-screening devices and other respiratory instruments. The pressure sensors are available with straight or barbed ports in axial, longitudinal or opposite directions.
Information supplied by Dr Med. A. Pittarelli, Product Marketing Manager at Sensortechnics GmbH, Boschstrasse 10, D-82178 Puchheim, Germany, tel. +49 89 800 830, e-mail: info@sensortechnics.com, www.sensortechnics.com
