Application of the hottest intelligent sensor tech

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Application of intelligent sensor technology in respiratory monitor

nowadays, intelligent sensor technology as a respiratory medical monitor with a detector has come out, which can do a lot of research on the relationship between various emotions and breathing, and can be used to truly record a person's respiratory condition and its changes

I. respiratory medical monitor and intelligent sensor technology (see Figure 1)

1. Respiratory medical monitor

is used to monitor respiratory conditions and give the approximate respiratory depth. This monitor monitors some important parameters that can be used to evaluate the degree of anxiety: respiratory rate, the uniformity of breathing, and the interval between exhalation and inhalation. Calm and positive emotions usually lead to exhalation longer than inhalation. The ratio of time between the two reveals the level of anxiety in one way. A relatively high level of chest breathing (relative to abdominal breathing) can also indicate the degree of anxiety. The observation of chest breathing can increase the visual information of the monitor

2. intelligent sensor technology

the monitor in Figure 1 uses silicon piezoresistive sensor (PRT) to detect the decrease and increase of the corresponding pressure during inhalation and exhalation. The output of PRT is fed into a max1450 signal conditioning IC to correct the inherent error of PRT, and then the compensated voltage signal is sent to 12 bit ADC. ADC output (digital pressure signal) then enters a PC interface and is converted to RS-232 level. Finally, the signal is transmitted to PC, so that the respiratory waveform can be displayed and the above parameters can be analyzed

II. PRT sensor

1. PRT detection principle

prt is generally configured as a tight Wheatstone bridge. When pressure is applied to the sensitive bridge of PRT (as shown in Figure 2a), the resistance value of diagonal bridge arm will change in the same direction and size. When two resistance values on one diagonal bridge arm increase under pressure, the resistance value of the other diagonal bridge arm decreases, and vice versa. For the semi sensitive bridge PRT (see Figure 2b), only the resistance value of half bridge arm is changed. PRT sensors, whether full bridge or semi sensitive bridge, which may lead to the transition to market consumption, have the advantages of high sensitivity (>10mv/v), good linearity and temperature stability, no signal hysteresis, and their measurement range can reach the destructive limit

2. Application of PRT

nowadays, because the new IC technology has been able to accurately calibrate the PRT sensor, its application range has been extended from medium and low precision detection to high-end fields. It solves the shortcomings that PRT sensors can only be used in medium and low precision detection in the past, and expensive strain gauges must be used in high-end products

III. signal conditioning IC and modern and simplified correction scheme

because the error range of PRT sensor is very wide, it must be calibrated by modern and simplified compensation method. At present, there are two kinds of signal conditioning ICs used to calibrate the error of PRT sensor: MAX1457 and max1450

1. modern sum correction scheme

max1457 is equipped with a controlled current source for driving the sensor and an ADC that collects the bridge voltage of the environment-friendly sensor. This voltage is the product of the current output current and the bridge resistance related to temperature. MAX1457 contains EEPROM, etc. Through the application software designed for MAX1457, four correction coefficients are calculated: full offset output (FSO), temperature (fsotc), offset (offset), and temperature offset (offsettc). To correct the error

because the accuracy provided by MAX1457 can be much higher than the requirements of a respiratory monitor, that is, it is not necessary to use a 16 bit resolution DAC for correction, but its temperature error compensation ability is also necessary for small temperature changes: a change of 10 ℃ usually causes a 3% change in the full bias output (FSO) of PRT. The real reason is that MAX1457 enables the monitor to work in a wide temperature range, The combination of PRT and MAX1457 can obtain better accuracy, so it can be used in space exploration, scuba and other fields

2. simplified correction scheme

max1450 signal conditioner (as shown in Figure 3) has the same function as MAX1457, but uses resistance to replace DAC for error correction. Because max1450 uses much fewer calibration points than MAX1457, its accuracy is 1%. It is usually used in hybrid schemes, which combines max1450 with light trimming resistors (all external resistors) to provide a low-cost solution, that is, using max1450 signal conditioner with external laser trimming resistors can provide 1% accuracy. The adjustment of rfsoa resistance is to set the initial (FSO) sensitivity, and the adjustment of temperature deviation is realized by the feedback sensor driving the post electric voltage (from bdrive pin), and the PGA program-controlled amplifier completes the compensation of deviation

now let's introduce the function of max1450 pin:

InP sensor positive signal input

sotc temperature offset signal input

a0 programmable amplifier (PGA) gain is the lowest setting input composed of computer, transmitter, microprocessor, etc

a1 programmable amplifier gain setting

a2 programmable amplifier (PGA) gain setting highest input

offtc offset temperature correction

offset offset adjustment input

bbuf with buffered bridge voltage input

fsotrim bridge drive current setting input

out programmable amplifier output voltage

isrc current source reference

bdrive sensor excitation current input

inm sensor negative signal input; Established a long-term mechanism to promote the development of energy-saving and new energy vehicles

VDD power supply voltage

vss ground. (end)

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