12. Pressure sensors

Transcription

1 12. Pressure s Questions Task a) What is a piezo-resistive effect? b) What is the difference between absolute, gauge and differential pressure? c) What is the relation between output voltage (e.g. U X =5V) and applied pressure p[psi] if you know: excitation voltage (e.g. 10V), signal conditioning amplification A[-]=100 and sensitivity (e.g. 10mV/V.psi)? d) What is the advantage of four-arm active bridge? (4 active elements connected in bridge) 1) Measure the offset and sensitivity of a pressure gauge Honeywell 24PCCFA6G. The is built-in in the tool for blood pressure measurement. 2) Measure the blood pressure and beating frequency of a selected volunteer. Use an oscilloscope connected to the blood pressure measurement tool; record the signal from the oscilloscope (USB flash-disk). 3) Set-up the demonstration kit of ADC (Texas Instruments ADS1281) in Multiscope mode (samples and shows the input signal, calculates its mean value). 4) Connect an absolute pressure to the ADC. Measure the output voltage (proportional to atmospheric pressure) for different altitudes (0, 20, 40, 60, 80 cm above the table). 5) Connect a differential pressure to the ADC. The measures air-flow through a transparent tube in given setup. Measure the transfer characteristics: pressure difference versus air-flow (the relation between air-flow and ventilator rotation frequency is considered to be linear). Optional task (for curious students) 6) Check-out the three modes of operation of the ADC demonstration kit (Multi_Histogram, Multi_FFT, Multi_Scope), which are supported by the ADCPro software. Which values are used to characterize an AD converter? Measurement instructions 1) Measure the offset voltage for zero applied pressure (the valve at the pump balloon is fully opened). Close the valve; bend the tube leading to the arm-cuff and press it with fingers (the cuff should not been pressurized without the arm). Pressurize the system slowly to 200mmHg (see the mechanical gauge reading) and note the output voltage. Use the offset and sensitivity readings to calculate linear approximation of the voltage pressure transfer characteristics. 2) Connect the output of the tool to the oscilloscope input. Use two channels of the scope connected in parallel. First channel set to DC coupling; it will indicate the DC component of the pressure (sensitivity approximately 1V/div). The second channel set to an AC input coupling will indicate the so called Korotkoff sounds - pulses, see the picture below (sensitivity approximately 10mV/div). Set the scope to Roll-mode (time base of 2 seconds/div). Place the arm-cuff on volunteer s arm and pressurize it to 170mmHg. Release the pressure slowly and evenly by soft adjustment of the valve. Stop the measurement (STOP button on the scope) after you get a desired signal reading (Fig. 4). 1 (6)

2 Ch1 AC Ch2 DC OSC Fig. 1 Tool for blood pressure measurement 3) Power up the laptop computer (login: Student/Student), run the ADCPro application (Desktop). Switch on the power supply for AD converter demonstration kit (BK125, ±15V, +5V). Activate the option ADS1281DEVKIT in EVM menu. Select MultiScope mode in TEST menu. Press Acquire (Continuous) button to start the measurement. 4) Connect the BNC output connector of the absolute pressure signal conditioning unit to the ADC. Measure the output voltage in Multiscope mode for desired heights (altitudes). The requires power supply (BK125 ±15V). p 1 Absolute pressure POWER SUPPLY ±15VDC Constant current bridge excitation and output signal amplification Voltmeter ADS1281 Development Kit Fig. 2 Tool for altitude measurement 5) Connect the output of the differential pressure tool to the input of the AD converter. Switch on the power supply for the ventilators (~14V, 3A), connect A and B BNC connectors on the tool to the oscilloscope the two signals indicates the speed of rotation of the ventilators. Measure the dependency of differential pressure output voltage versus the air-flow speed (sum of A and B frequencies). Do the measurement for five different speeds of rotations. What happens if you block the airflow (with an open hand) for non-zero ventilator speed? POWER SUPPLY 14V DC 3A ROTATION SENSORS OUTPUTS SPEED OF Ch1 Ch2 ROTATION OSC f 1 +f 2 =? Air-flow direction Differential pressure p 1 p 2 orifice POWER SUPPLY ±15VDC Constant current bridge excitation and output signal amplification Voltmeter ADS1281 Development Kit Fig. 3 Tool for air-flow measurement 2 (6)

3 Notes for the measurement 1) The electrical circuit in the tool provides non-zero output voltage for zero applied pressure. The offset has to be calibrated. Ideally the transfer characteristics should be measured in several points, practically for our purpose two points calibration is sufficient. At first we measure U OFFSET (while p=0) and U 200 (while p=200mmhg). The pressure can be then calculated from the voltage readout U X : ( U X UOFFSET ) = ( ) 200 V p U 200 U mmhg ;, mmhg OFFSET V The gauge measures the pressure in the input port with respect to the atmosphere (the other side of the membrane is freely open). 2) Blood pressure measurement 1. increase the pressure in the arm-cup to 170 mmhg. Observe the signal on the oscilloscope. There are only small pulses visible for high pressure. The Korotkov pulses will appear if the pressure is decreased slowly and steadily (cca 2-3 mmhg/s). The pulses disappear if the pressure is further decreased. The automatic electronic tonometer measures the maximum amplitude of the pulses. There is a couple of definitions that can be used to determine the systolic and diastolic pressures. Probably the most common definition is presented on the picture. The systolic pressure Ps is estimated as an amplitude rise to 50% of the maximum value; while the diastolic pressure is measured for pressure decrease to 85% of the maximal amplitude. You can freeze the oscilloscopic screen by pressing RUN/STOP button and measure the desired values with cursors. Fig. 4 Korotkoff sounds definition of systolic and diastolic pressures 3) The ADS1281 IC made by Texas Instruments is currently one of the best (regulary commercially available) delta-sigma AD converters. It has 31-bits output word, effective resolution (ENOB Effective Number Of Bits) approximately 22-23bits (dependend on selected sampling frequency f S max 4kSa/s). The resolution of 23 bits corresponds roughly to 6.5 DMM. The ADCPro program in MultiFFT mode shows frequency spectrum of the acquired signal. The MultiHistogram mode shows the measured data in the form of column graph. The distribution of the code bins is used to characterize the ADC. The acquired signal is presented in timedomain in MultiScope mode. The most accurate measurements can be obtained for the lowest sampling frequency (250Sa/s). For record length of 256 samples we get an update of the mean value approximately each second. There was a 16 th order FIR filter (moving average) added to the program in order to suppress the noise. 3 (6)

4 Fig. 5 Texas Instruments ADS1281 internal structure ADCPro program, use menu EVM to run ADS1281EVM-PDK module (see picture below), then activate module MultiScope from Task menu. 4 (6) Fig. 8 ADCPro program (based on NI LabView) 4) Measurement of the absolute pressure of the atmosphere is often used to determine the altitude. The absolute contains a chamber with a reference pressure (often vacuum of various quality); on the other side of the membrane then acts the measured pressure. There is a drop of the atmospheric pressure of approximately 11.5 Pa for each ascended meter. Let s have a with pressure range of 15 psi (pound square inch) = Pa, with a sensitivity 90 mv/15 psi (for voltage excitation 12VDC), the instrumentation amplifier used for output amplification has an amplification of 25,7x. The is powered by constant current source (1.57 ma) in the tool (it removes the influence of wires resistance, it is good for temperature compensation); the mentioned current produces 7.35VDC on the. The output voltage should therefore decrease by 158µV for each ascended meter.

5 5) Differential pressure measures the difference between two pressures applied to two separate inputs. The output is ideally independent of the absolute pressure of the media (but the maximum absolute pressure is specified) A typical application for this is a flow measurement. The advantages of an orifice based flowmeter: very robust and simple, no moving parts, sufficient measurement accuracy. Drawbacks: there is a pressure drop across the orifice (more powerful ventilator must be used). The ventilators are powered by 12V constant voltage power supply and the speed is regulated by PWM in the laboratory tool. The ventilator pair (with counter rotating blades) provides and indication of the rotating speed (2 pulses per revolution). For simplicity we expect linear dependence between the speed (sum of speeds) and the generated air-flow. Suggested placement of the pressure sensing ports for flow measurement with an orifice. Air-flow and pressure profile in the tube. The pictures taken from: ISO Measurement of fluid flow by means of pressure differential devices inserted in circular-cross section conduits running full 5 (6)

6 Attachement Pressure s, the difference between absolute, gauge and differential pressure. p[kpa] 100 p A =169kPa p A =101kPa p A =64kPa p G =60kPa p G =20kPa p D =-20kPa atmospheric pressure p D =27kPa (absolute zero, 0 ideal vacuum) absolute p G =-68kPa gauge(relative) p D =40kPa differential p 1 vacuum (~0kPa) p 1 atmosphere (~101kPa) p 1 p 2 The electrical circuit used for constant current excitation and output signal conditioning. (voltage reference: REF V, op-amp OPA277, instrumentation amplifier AD8221). Advantages of four active elements bridge setup (e.g. with four piezo-resistors) linear output for both voltage and current excitation, four times more sensitive with respect to one sensing element, automatic compensation of temperature drifts for current excitation of the bridge (sensitivity compensation). Bridge topology with respect to non-bridge topology easy amplification (typically 10x-1000x) of the output signal. Amplification of a small signal rather than amplification of a small signal superimposed on some big offset signal. Piezo-resistive effect change of resistance of a semiconductor material due to applied mechanical stress tension. In contrast to piezo-electric effect no charge (voltage) generation takes place. 6 (6)