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Freescale Semiconductor Application Note
AN3956 Rev 0, 11/2009
DEMOAPEXSENSOR Usage
Altitude Pressure EXperimental (APEX) Sensor Board
by: John B. Young
INTRODUCTION
The DEMOAPEXSENSOR is an experimental sensor board focused on pressure with a variety of applications built into the demo. These include: Altimeter, Barometer, Barometer Alarm, Weather station, Waterlevel, Waterlevel Alarm, Tilt, Free Fall, and Temperature Sensing. The board has been designed as a standalone board to be interfaced currently via an LCD display and menu buttons. The multiple functions of the Altitude Pressure EXperimental (APEX) Sensor board are possible with the use of Freescale's sensors. This kit has the following: MPL115A2, MPXM2102A, MPXV5004G pressure sensors with an MMA7361L accelerometer. The demo makes use of Freescale's Flexis MC9S08JM60 8-bit microcontroller. Figure 1 is an image of the DEMOAPEXSENSOR with important items highlighted. Freescale's sensors are listed along with their associated application in Table 1. Table 1. On-Board Sensors
Device MPL115A MPXM2102A MPXV5004G MMA7361L Type of Sensor Pressure Absolute Pressure Absolute Pressure Gauge Accelerometer Measurement Range of Sensor 50 - 115 kPa 0 - 100 kPa 0 - 4 kPa Selectable 1.5g, 6g Sensor Application Barometer/Altimeter Altimeter/Absolute Pressure Liquid Level Sensor Acceleration, Tilt, Shock, Free Fall Interface Type Digital I2C/SPI Analog Analog Analog
Temperature Sensor Development Header MPL115A LCD Display for Results Backside: MPXV5004G Gauge Pressure Sensor- Waterlevel MMA7361L 3 Axis low-g Accelerometer -Motion Push Buttons "Up, Down, ESC Enter" MPXM2102A Absolute Pressure Sensor -Altitude
USB Connector
BDM
Figure 1. Basic Components of APEX Board
(c) Freescale Semiconductor, Inc., 2009. All rights reserved.
12 bit
MMA7360Q 3 Axis XYZ Accelerometer Development Header
20x4 Character LCD Display
12 bit
Menu Buttons JM60 8-bit S08 MCU 12-bit ADC 60k Flash USB
MPXV5004 Integrated Pressure Sensor MPL115A2 (I2C)
24-bit ADC Converter
12 bit
Temp Sensor
Instrumentation Amplifier Gain=98.8
MPXM2102 Compensated Pressure Sensor
Figure 2. Simplified Block Diagram Figure 2 is a simplified block diagram of the DEMOAPEXSENSOR. The sensors with analog outputs (Table 1) are sampled via the JM60 microcontroller's 12-bit ADC. The exception is the MPXM2102A absolute pressure sensor. This is used for altitude so it requires a high resolution sampling. It passes through an instrumentation amplifier and then is interfaced via an external 24-bit ADC chipset before communication via SPI to the JM60 MCU. The JM60 interfaces with the LCD screen and processes inputs by the user for the displayed output. Several Algorithms are used to calculate functions on the DEMOAPEXSENSOR. Information on implementing the MPL115A can be found in AN3785, How to Implement the Freescale MPL115A Digital Barometer. Implementing the MPL115A as a barometer and altimeter can be found in AN3914, Modern Altimeter and Barometer System using the MPL115A. More pressure sensor application notes can be found at: www.freescale.com/pressure.
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1 2 3 4 5 6 7 8 PTC6 9 10 11 12 PTE0 13 PTE1 14 PTE2 15 16 17 18 19 20 21 22 23 24 25 PTG0 26 PTG1 27 / C S 28 BUSY 29 PTA2 30 PTA3 31 PTA4 32 PTC4 IRQ/TPMCLK Reset PTF0/TPM1CH2 PTF1/TPM1CH3 PTF2/TPM1CH4 PTF3/TPM1CH5 PTF4/TPM2CH0 PTC6 PTF7 PTF5/TPM2CH1 PTF6 PTE0/TxD1 PTE1/RxD1 PTE2/TPM1CH0 PTE3/TPM1CH1 PTE4/MISO1 PTE5/MOSI1 PTE6/SPSCK1 PTE7/SS1 VDD VSS USBDN USBDP VUSB33 PTG0/KBIP0 PTG1/KBIP1 PTA0 PTA1 PTA2 PTA3 PTA4 PTC5/ExD2 PTC3/TxD2 PTC2 PTC1/SDA PTC0/SCL VSSOSC PTG5/EXTAL PTG4/XTAL BKGD/MS PTG3/KBIP7 PTG2/KBIP6 PTD7 PTD6 PTD5 PTD4/ADP11 PTD3/KBIP3/ADP10 PTD2/KBIP2/ACMP0 VSSAD VREFL VREFH VDDAD PTD1/ADP9/ACMPPTD0/ADP8/ACMP+ PTB7/ADP7 PTB6/ADP6 PTB5/KBIP5/ADP5 PTB4/KBIP4/ADP4 PTB3/SS2/ADP3 PTB2/SPSCK2/ADP2 PTB1/MOSI2/ADP1 PTB0/MISO2/ADP0 PTA5 64 63 62 61 60 59 58 57 56 55 54 53 52 51 50 49 48 47 46 45 44 43 42 41 40 39 38 37 36 35 34 33
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Quick Start Guide
How to Read MPL115A Values on APEX Board
APEX Controls: ON/OFF Switch: Located on the left side, under the LCD screen along the PCB edge. It is a slide switch. Menu Selection Buttons: Located on the right side, next to the blinking LEDs.
On/Off Slide Switch Under LCD Information Displayed on LCD
UP ESC
Push Buttons "Up, Down, ESC Enter"
ENT DN
Blinking LEDs
Detail of Push Buttons
Figure 5. APEX Controls
MPL115A Pressure Shown on APEX Example
Navigating Through the Menu Screen 1:
1. 2. 3. 4.
Alti/Baro Waterlevel Inertial Additional
Press UporDN `1'
Press UP or DN until option "1. Altimetry" is selected Press ENT
Screen 2:
1. a. b. c.
Altimetry Press MPXM2102A UporDN MPL115A2 Experimental `b'
Press UP or DN until option "b. MPL115A2" is selected Press ENT
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Screen 3:
PPL115A2 Raw Values Press ADC 5bc0 0367 Temp ADC 7a00 0488 ENTER for Coeff.
Pressure and Temperature Raw Values in decimal format are on the right in blue. Press ENT menu Button one time to move forward to see the coefficients.
Screen 4:
6 Coefficients: 3c ca 30 84 8a bd b8 93 f2
80 33 c0
Coefficients are listed. Press ENT menu button to see compensated pressure reading.
Screen 5:
Compensated Pressure PComp = 97.273
Pressure is compensated and listed in kPa. Press ENT to continue to Alarm System. To navigate backwards, press ESC to cycle to previous screen.
Screen 6:
ALARM System Enter your Threshold 0.50 kPa ENTER for Activation
Screen 7:
Press UP or DN to set Pressure Threshold for the Alarm System in 0.25 kPa increments. Press ENT to activate the alarm feature and wait for the buzzer to sound.
ALARM sounds when Threshold is reached Press. 97.273 kPa 97.836 ALARM
Alarm is armed and the current pressure is shown on top. The bottom is the threshold pressure for the alarm. Note the RED LED activates with the alarm and a green solid LED indicates a non alarm pressure. To navigate backwards: press ESC to cycle to previous screens and deactivate alarm.
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Functions and the Screen Shot Navigation Path
Color Legend Inputs: User has to enter in a value, or select a choice, UP or DN. Typical inputs are shown in `x' format. ESC repeatedly pressed, exits user to main menu. Outputs: Demo code returns a result or value based on the user inputs. Simple Weather Station
1. 2. 3. 4. Alti/Baro Waterlevel Inertial Additional Press UPorDN `4' Additional Press a. Weather UPorDN b. Weather Advanced c. Temp Sensor `a' Please enter your Altitude (m) UP/DN 101.3 kPa 00 m SIMPLE WEATHER Weather outlook 0.423 kPa Sun/Cloud Symbol
ENT
ENT
ENT This is the altitude at your current location. Once its entered, the value of Pressure for that altitude is displayed. The current Weather outlook is displayed in the options of Symbols: Sun, Rain, Sun/Cloud. Note the difference in pressure is displayed from current to Altitude.
Advanced Weather Station
1. Alti/Baro 2. Waterlevel 3. Inertial 4. Additional Press UPorDN `4' Additional Press a. Weather UPorDN b. Weather Advanced c. Temp Sensor `a' WEATHER Sun/Cloud Min 0224 dP/dt -0.042 Stable Weather Pattern
ENT
ENT This shows the weather in the top right corner from the simple weather algorithm. The `Min' shows how long the algorithm is working in minutes. dP/dt shows the differential change per an hour. The corresponding weather type is predicted in the bottom. This takes readings for up to 3 hours and compares results to the algorithm every 1/2 hour.
Waterlevel Sensing
1. Alti/Baro 2. Waterlevel 3. Inertial 4. Additional Press UPorDN `2' 2. Waterlevel MPXV5004 Integrated 40cm H2O Level Press ENTER 2. Waterlevel a. Liquid (cm) b. Alarm system Press UPorDN `a' Place in Liquid ADC 12bit H2O Level Pressure
1179 0.0 cm 0.0 kPa
ENT
ENT Selection of the Liquid Waterlevel section in `a'.
ENT Place tube attached to MPXV5004 into water. As it goes into the water, the value of water height will be displayed on the screen along with Pressure.
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Waterlevel Alarm (sub-option from above menu)
2. Waterlevel a. Liquid (cm) b. Alarm system Press UPorDN `b' Alarm System Enter your Threshold 5.50 cm ENTER for Activation LARM sounds when Threshold is reached Press. 4.00 cm ALARM 5.50
ENT
ENT Press UP and DN to set the Alarm Threshold in increments of 0.25cm. Press ENT to activate the Alarm. Alarm is now Active. Note the outputs of the current Pressure in cm, and the Alarm Threshold point. Once crossed, the red LED will flash as the buzzer sounds. Try activating by moving the tube up and down in a column of water.
Temperature Sensor
1. Alti/Baro 2. Waterlevel 3. Inertial 4. Additional Press UPorDN `4' Additional Press a. Weather UPorDN b. Weather Advanced c. Temp Sensor `c' Temperature External Temp Sensor 1487 ADC 031 Deg C 087 Deg F
ENT
ENT Value of the External Temperature sensor is displayed on the screen in C and F
Inertial Sensor
1. Alti/Baro 2. Waterlevel 3. Inertial 4. Additional Press UPorDN `3' 3. Inertial MMA7361L 3 Axis Accelerometer 1.5g Press ENTER TRaw Values of Accel X axis 1594 Y axis 1663 Z axis 2388 Tilt Angle X axis Y axis Z axis -2deg -4deg 90deg
ENT
ENT
ENT ADC values of the MMA7361L are displayed on the screen. Press Enter to proceed to the next screen
ENT ADC values are converted and displayed on the screen as an angle for X, Y and Z axis. Press Enter to continue to Free Fall Detection.
Free Fall detect Drop carefully in Linear Free Fall only. FF detected!!
Drop board in linear manner to hear buzzer detecting Free Fall condition, and Red LED will blink with buzzer. Exit Free Fall and Inertial by pressing ESC.
AN3956 Sensors Freescale Semiconductor 8
Altimetry
1. 2. 3. 4. Alti/Baro Waterlevel Inertial Additional Press UPorDN `1' ENT 1. a. b. c. Altimetry Press MPXM2102A UPorDN MPL115A2 Experimental `a' ENT Select the MPXM2102A, option `a' for higher resolution, amplified pressure readings for Altimetry. Note the way that the output is displayed. The Raw value of the 24-bit ADC value is displayed on the left. The bottom left has the raw value after it passes two low pass filters in software. This value is mores stable but takes longer to update. On the right the Altitude is displayed. This is the pressure converted to altitude in (m). The Pressure corresponding to the ADC value is shown in the bottom right. The board can be placed on the table. Note the Altitude measurement as it stabilizes. Then raise it above your head as a `1m' increase. The value should go up by 1m. Place on the table again. It should drop to the original value. Place on the floor, it will again decrement 1m. Note that ambient pressure from A/C units can affect this value drastically. Raw Value Altitude 0x017c1d 459m 2(LP) value Pressure 0x017c1f 95.365 kPa
Barometer Compensated Pressure Output
1. a. b. c. Altimetry Press MPXM2102A UPorDN MPL115A2 Experimental `b' ENT MPL115A2 RawValues Press ADC 74c0 0467 Temp ADC 6b40 0428 ENTER for Coefficients ENT The values for the MPL115A are shown here. To the left are the Pressure and Temp ADC values in their raw form in HEX. To the right of them are the shifted values displayed in decimal. 6 Coefficients ao = 3dc4 c11 = f8a0 b1 = bd7a c12 = 2flc b2 = c299 c22 = 0dc0 ENT Values of the Coefficients are shown here. These are used for the next step in calculating the compensated Pressure. Compensated Pressure PComp =9778 kPa Altitude = 365 m
ENT Compensated Pressure is displayed. This is with an accuracy of 1kPa.
Press ENT to continue to the Alarm section.
ALARM System Enter your Threshold 0.50 kPa Enter for Activation ENT Continued from the MPL115A compensated Barometer section above. Press UP and DN to set the Alarm Threshold in increments of 0.25kPa. Press ENT to activate the Alarm
ALARM sounds when Threshold is reached Press. 96.574 kPa ALARM 97.324
Alarm is now Active. Note the outputs of the current Pressure in kPa, and the Alarm Threshold point. Once crossed, the red LED will flash as the buzzer sounds. Try activating by pressurizing the MPL115A. This can be done with a plastic syringe with rubber tubing at the end to make an air tight fit.
AN3956 Sensors Freescale Semiconductor 9
DEMOAPEXSENSOR Altitude Measurement There are two sensors that can be used for altitude measurement. The MPL115A can be used as an approximate altimeter, but on this demo board, the MPXM2102A pressure sensor has been used to get a high resolution for altitude measurement. The MPXM2102A is an absolute pressure sensor. It measures the pressure on its port in relation to a vacuum sealed reference. The specs given on the data sheet do not easily translate into creating a product that resolves 1m of height difference. The DEMOAPEXSENSOR in this sense explores how a sensor can be amplified and put through a 24-bit delta sigma ADC to pursue that type of application. There is more than one section in the software to determine the vertical altitude. The code has more than one method to try to improve the resolution the MPXM2102A. One method aims at a sub 1 foot increment, but this is not as stable as the 1m increment method. Again, this is a experimental board and the method used to measure the MPXM2102A for altitude is subject to many outside influences. These are not limited to, but include; sudden pressure changes, gusts of wind, A/C unit on/off cycling, temperature, and exposure to high brightness light. Note the value is setup for dynamic change. The altitude absolute height is not calibrated. This is typical of end application requiring a starting point. Table 2. Operating Characteristics (VS = 10 VDC, TA = 25C unless otherwise noted, P1 > P2)
Characteristic Differential Pressure Range Supply Voltage Supply Current Full Scale Span Offset MPX2102D Series MPX2102A Series MPXM2102D/G Series MPXM2102A Series Sensitivity Linearity MPX2102D Series MPX2102A Series MPXM2102D/G Series MPXM2102A Series Pressure Hysteresis (0 to 100 kPa) Temperature Hysteresis (-40C to +125C) Temperature Coefficient of Full Scale Span Temperature Coefficient of Offset Input Impedance Output Impedance Response Time (10% to 90%) Warm-Up Time Offset Stability -- -- -- -- -- -- TCVFSS TCVOFF ZIN ZOUT tR -- -- -0.6 -1.0 -0.6 -1.0 -- -- -2.0 -1.0 1000 1400 -- -- -- -- -- -- -- 0.1 0.5 -- -- -- -- 1.0 20 0.5 0.4 1.0 0.4 1.0 -- -- 2.0 1.0 2500 3000 -- -- -- %VFSS %VFSS %VFSS %VFSS %VFSS mV W W ms ms %VFSS VOFF VOFF V/ -1.0 -2.0 -1.0 -2.0 -- -- -- -- -- 0.4 1.0 2.0 1.0 2.0 -- mV mV mV/kPa Symbol POP VS IO VFSS Min 0 -- -- 38.5 Typ -- 10 6.0 40 Max 100 16 -- 41.5 Units kPa VDC mAdc mV
Table 2 shows the typical operating characteristics of the MPXM2102A. The supply voltage is ratiometric, but at 10 V supply, the FSS is 40 mV for 100 kPa of applied absolute pressure. The DEMOAPEXSENSOR does not have a full 10 V supply. The part is biased instead by a 5 V supply. This cuts the FSS to 20 mV for 100 kPa. Since the output of an analog sensor is ratiometric to the input voltage, it is crucial to have a stable supply voltage. In the case of amplifying and analyzing small pressure changes, the input voltage must be stable to remove input noise. To achieve this, a shunt voltage reference connected to the 5 V regulator provides a low noise, stable voltage of 4.096 V for the MPX2102A. This is the IC, D8 on the demo schematic, and is typically used for data-acquisition systems. Thus, the span is really only 16.38 mV at 4.096 V, for a pressure range of 0 to 100 kPa. MPXM2102A is on a separate shunt such that no other connected IC will induce additional noise on the shared line.
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Hardware Issues to Consider when Designing such a Circuit
ADC Resolution: Since the MPXM2102A is an analog part, the ADC resolution is important to get the resolution needed for stable measurement. The ADC has to be greater than 16-bits to gain 3 foot intervals in measurement. Noise: Noise on the board has to be minimized. The digital and analog supplies and grounds must be separated. The MPXM2102A on the DEMOAPEXSENSOR has its own dedicated shunt voltage reference after the 5 V regulator. The external ADC also has its own digital and analog 5 V lines separated. The reference voltage on the ADC has its own shunt reference to provide a stable value to compare ADC results. The board is 4 layers with a ground and Vdd plane to minimize parasitic capacitance that creates noise. Analog 5 V has its own separated section from the digital 5 V in the power layer of the layout. This minimizes the noise on both 5 V digital and 5 V analog sections. Air Flow: The pressure sensor should have a cover when trying to measure a sub 1 m altitude measurement. Either the unit should be housed in a box, or in a customer application and should have a method to introduce a laminar flow of air. This will reduce A/C and breeze effects on the pressure sensor. Most applications of such, include a housing perforated with small holes. This prevents a strong breeze from affecting the sensor, by filtering out the drastic pressure changes. DEMOAPEXSENSOR, as a demo board, has no such housing.
Software Considerations
Filtering: There are high frequency noise and sensor fluctuations that have to be handled. In the software, two low pass filters are used to minimize fluctuations in the ADC values. This makes the conversions slower, but the values are more stable. Low pass filters are better than using a running average. A running average is a filter that does not discriminate noise outliers as well, since it is included in the result. Calibration Routines: As seen in the experimental section of the APEX board, it is possible to improve results with various software calibrations. A simple example is done in the APEX code, but could be further worked with a possibility of multiple sensors to take this further with motion combined. It will be explained in the short distance model of altimetry in this application note.
Altimetry Background Information (refer to AN3914)
Altimetry utilizes absolute pressure sensors. An absolute sensor measures the deflection of the surrounding barometric pressure with reference to a known pressure (usually a vacuum). This allows it to compare the air pressure at sea level (101.3kPa) to the vacuum to gain an absolute pressure result. At a different elevation, the barometric (surrounding) pressure can be compared again to the vacuum for that absolute pressure result. Since both readings were taken against the same reference, they can be compared against each other. Barometric pressure does not have a linear relationship with altitude. As altitude increases, the pressure decreases. Common reference points are given in Table 3. Table 3. Reference Points
Location Sea Level Dead Sea (lowest surface on earth) Summit of Everest
20000 15000 ALTITUDE (ft) 10000 5000 0 -5000 45
Altitude (m) 0 -396 10,058
Altitude (ft) 0 -1300 +33,000
Pressure (kPa) 101.3 106 33
55
65
75 85 95 PRESSURE (kPA)
105
115
125
Figure 6. Altitude vs. Pressure
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At low elevations, a square meter on the earth's surface has greater weight above it than at higher altitudes. This is due to the mass of water vapor and air that sits upon it. Imagine cubes of air and water vapor stacked from the ground to space. At the low altitude there is more cubic mass above, while higher altitudes will have less of these stacked above it. Air and water vapor will compress more at sea level and the air is significantly denser than at high altitude. The density is not uniform with altitude, and thus the pressure is not either. The reason for altitude's non-linear relationship is that air has infinite compressibility. It disproportionately compresses more as weight is placed upon it. Hence the graph of Pressure vs. Altitude seen in Figure 6. The simplified mathematical equation used to calculate altimetry in Table 3 is:
ph = p0 e
-h ----------------7990m
(eq. 1) (eq. 2)
p0 h = 18400m log -----ph Assuming p0 = 101.3 kPa ph - Pressure at height h po - Initial Pressure point at sea level or 101.3 kPa
Calculations for a 24-bit ADC
First: Figuring out how the 24-bit ADC translates to counts, followed by pressure and altitude. Table 4.
ADC Resolution 8-bit 12-bit 16-bit 18-bit 24-bit Range of ADC Counts 0 to 255 0 to 4095 0 to 65,535 0 to 262,143 0 to 16,777,215
24-bit ADC LTC2442 reference voltage is 4.1 V (temperature and voltage stable shunt). MPXM2102A has a 16.4 mV span @ 4.1 V supply. Output is gained x 98.8 by instrumentation amplifier. Span of differential amplified signal is now 1.62 V. Low pressure end of 0 kPa input is approximately 0 V seen at the differential amplifier output. Conversely the higher end of the pressure reading at 100 kPa is = 1.62 V. SPAN of MPXM2102A after Differential Instrumentation Amplifier is now 1.62 V. The 24-bit ADC would have a resolution of 4.1V/16777215 counts or 0.244 V per an ADC count. Since the differential span of the sensor is 1.62 V after the amplifier, the span of 0 to 100 kPa can be seen as a ADC value of 0 counts to 6629045 counts. This makes each ADC count equivalent to 0.0000151 kPa or 0.0151 Pa. The LTC2442's value was shortened to a 18-bit conversion to minimize the baseline noise seen on the ADC output. For an 18-bit ADC conversion, 4.1V/262143 equals a resolution of 15.6 V per a single ADC count. Since the differential span of the sensor is 1.62 V after the amplifier, the span of 0 to 100 kPa can be seen as a ADC value of 0 counts to 103846 counts. This makes each ADC count for the pressure span equivalent to 0.000963 kPa or 0.963 Pa.
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Summarizing in Table 5 Table 5.
ADC Type 24-bit 18-bit Resolution (V/ADC) 0.244 V 15.6 V per 1 ADC count Resolution (kPa/ADC) 0.0000151 kPa 0.000963 kPa per 1 ADC count
DEMOAPEXSENSOR can read a 0.000963 (kPa/ADC count) step for a 0 to 100kPa MPXM2102A sensor. The formulas earlier were shown to reference the pressure to altitude relationship. Converting all this to altitude: Table 6.
Distance 0.5 m 1m 3 ft 1 ft Pressure (mB) 0.05 0.11 0.09 0.03 Pressure (kPa) 0.005 0.011 0.009 0.003 Voltage Change (V) 0.81 1.78 1.46 0.49 ADC (counts) 5.19 so ~5 11.4 so ~11 9.35 so ~9 3.11 so ~3
Reviewing Table 6, the DEMOAPEXSENSOR has code written to interpret small pressure changes and convert these to altitude. The conversion from pressure to altitude still has to use the exponential equation for a proper conversion. But the approximate distances and the ADC count equivalent are shown here. These are approximate across the pressure range but as written in the demo code, the pressure is determined first, the altitude second. The altitude is calculated via the pressure value inserted into the exponential altitude-pressure formula.
Experimental Altimetry Section
The preceding section on Altimetry has used the pressure sensor value to convert the pressure to an altitude reading. This value of pressure is not linear with vertical height as detailed in the graph of Figure 6. However if a curved line is magnified enough, the end result can be treated as a straight line. The "Experimental" section of the Altimeter/Barometer tries this particular method. Due to changing air currents and unstable smaller resolution, it may require outdoor demonstration to get better results. In this example the chart outlines the calibration routine. * To calibrate the part, first place the board on the floor or table in front of you. * Wait until the low pass filtered value is stable then press Enter. This saves the "0 level" barometric pressure. * Following this, place the board about 3 feet above the "0 level". Either this is at waist height if previously on the floor, or above your head if previously on the table. When the value stabilizes, press Enter again. Now the display will output the distance in smaller increments than 1 foot. Notice how it tends to jump around, and the "0 level" shifts. This Experimental section shows how this method while being more accurate is not as stable as the exponential method to see pressure. The resolution is reasonable for a short period of time before barometric changes in pressure change the calibration.
AN3956 Sensors Freescale Semiconductor 13
Filtered Value 3ft level lower P
Calibration Section Constantly Running Difference in Pressure Resolution= (Difference)/3ft 100ms Sample of raw Pressure ? 24bit ADC
Filtered Value 0ft level higher P
Twice Low-pass filtered value ? 18bits usable data Current Pressure value stored Altitude in ft = [Current Pressure -Zero Pressure Value]/3
Figure 7. APEX Short Distance Calibration Routine
DISSECTION OF THE ISSUES FACING THE PRESSURE READING
Note that with trying to resolve flowchart in Figure 7, there are issues as mentioned with the physical surroundings. A/C on/off cycling along with airflow in the form of a breeze. Doors opening and closing change the pressure dynamic of a room as the air flows in or out. This leads to the shifting in the zero level of the altitude as demonstrated in the flowchart. This causes the scenario of leaving the APEX on the table and seeing that the 0 foot reading shifts up and down - due to the many root causes in barometric pressure change. This is shown as an `experiment' in the demo board. A straight linear pressurealtitude can sometimes work, but generally the shifting pressure wreaks havoc on the result. Multiple sensor systems can be used to possibly stabilize the result. For example, a multiple sensor system can use the accelerometer to detect movement and re-zero altimeter readings, if pressure changes are not related to movement.
High Resolution Altimetry Customer Implementations
Altimetry
Raw Value Altitude 0x017c1d 459m 2(LP) value Pressure 0x017c1f 95.365 kPa
In the Altimetry screen shot as see in the Quick Start Guide, note the raw and the twice Low-Pass filtered values. These are converted using the pressure equations 1 and 2 as seen on page 12 for pressure and altitude conversion. The APEX does not have a calibration for the altitude as most customer implementations of high resolution pressure, zero a point and measure the dynamic change in meters for a given amount of time. The following are examples of ways to implement a high resolution altimeter with focus on the `dynamic' changing pressure rather than a constant absolute altitude reading. Altimeter Example 1: A person sets a zero point before hiking a steep hill. At the top of the hill, the change in barometric pressure is related to meters or feet. This is a measure of the dynamic change of pressure over a short distance. This is a targeted application versus the other application of knowing the altitude from sea level at all times after one factory calibration.
AN3956 Sensors Freescale Semiconductor 14
Altimeter Example 2: A GPS equipped smart-phone loses GPS signal as it enters a building. Knowing the approximate elevation with the GPS, the beginning `zero point' is 300 m. Entering the building the pressure sensor starts measuring the elevation, and tries to calculate the height in meters. Issues in this Example for Consideration: * When entering a building from outside, a sudden rise in pressure is highly probable. This will send two different scenarios. It could signify that the user has `fallen' in altitude by several meters (such as jumping off a building or small cliff). Or they have entered into a building with significant pressurized airflow (typical of an air-conditioned lobby with high air flow). * Buildings vary on height per a floor. Lobbies of sky-rises have higher ceilings. Hotel accommodation floors tend to have shorter standard heights. A restaurant located halfway or on top of the building may have a higher ceiling. The main point is that the altitude in meters does not ideally correspond to floor location. * Large buildings can be pressurized or not. It depends on building design. Some stadiums for sporting events are pressurized to keep a fabric roof afloat. Tall high-rises have multiple stages of air-conditioning units. This leads to varied pressure per a floor depending on that section's on/off cycle, fan speed, or the air-tight nature of that section. Elevator shafts also can equalize pressure, or create some pressure change as elevators move up and down. * Smart use of Altimetry in tall buildings would use building information stored in a `smart' GPS device so the altitude can translate to floor level. This way location based services, emergency 911, etc. can know which floor the user is located.
MPL115A Miniature Barometer DEMOAPEXSENSOR
The MPL115A is a small 5 x 3 x 1.2 mm digital absolute pressure sensor. It is available in either I2C or SPI version. It has a pressure range of 50 to 115 kPa. This narrower range is more application specific for events occurring in that altitude or in devices requiring that pressure. In this section, the DEMOAPEXSENSOR's implementation of the MPL115A and the information displayed on the APEX is described. In the Quick Start Guide section, the "Barometer Compensated Pressure Output" describes the compensated Pressure output of the MPL115A in screen shots of the DEMOAPEXSENSOR. Barometer Compensated Pressure Output
MPL115A2 RawValues Press ADC 74c0 0467 Temp ADC 6b40 0428 ENTER for Coefficients Stage 1 Raw Values of Temperature and Pressure displayed on LCD screen. 6 Coefficients ao = 3dc4 c11 = f8a0 b1 = bd7a c12 = 2flc b2 = c299 c22 = 0dc0 Stage 2 Display of 6 coefficients listed on LCD screen. Compensated Pressure PComp =9778 kPa Altitude = 365 m
Stage 3 Display of Compensated Pressure output in kPa units.
The MPL115A's Implementation is described in detail in the Application Note AN3785; How to Implement the Freescale MPL115A Digital Barometer. Essentially in the LCD screen shots above, the Raw values of Pressure and Temperature are displayed in Hexadecimal and Decimal format. Following this, the six coefficients MSB+LSB are shown in Hex format. The combination of this data (streamed via I2C or SPI from the MPL115A) is used at the host MCU to calculate the Pcomp value. Using eq. 2, the altitude is calculated and shown in meters. The Pcomp value is the compensated absolute pressure value. This value unlike the analog pressure sensors does not require any calibration trim, or offset auto zero. The Pcomp spec for the MPL115A is such that the value has an accuracy of 1kPa. The advantage is implementing the sensor and having the compensated pressure readings without any additional calibration etc. on the customer side. As stated in AN3785, the MCU has to apply the equation below for Pcomp given that a0, b1, b2, c11, c12, c22 are coefficients stored in MPL115A registers. Padc and Tadc are the raw ADC values of Pressure and Temperature that are clocked out of MPL115A digitally. Pcomp = a0 + ( b1 + cl Padc + c12 Tadc ) Padc + ( b2 + c12 Tadc ) Tadc (eq. 3)
AN3956 Sensors Freescale Semiconductor 15
This is applied in the MCU code as a series of operations so the MCU does not have to apply the equation directly. The following is a possible sequence for the calculation Pcomp, the trimmed pressure output. Input values are in bold. c11x1 = c11 * Padc a11 = b1 + c11x1 c12x2 = c12 * Tadc a1 = a11 + c12x2 c22x2 = c22 * Tadc a2 = b2 + c22x2 a1x1 = a1 * Padc y1 = a0 + a1x1 a2x2 = a2 * Tadc Pcomp = y1 + a2x2 Note that the Altitude measurement displayed for the MPL115A cannot be directly compared to the previous section on the MPXM2102A. The MPL115A has an internal 10-bit ADC, with an accuracy of 1kPa. The system set up on the DEMOAPEXSENSOR for the MPXM2102A shows less than 1m resolution with a 24-bit ADC. The additional hardware cost, digital and analog noise filtering, and calibration required is substantial compared to implementing the MPL115A directly to an MCU. The value of the altitude changes constantly on the display, but it is also a raw conversion. It could be filtered, sampled in longer intervals etc. to give the illusion of a more stable value. Here it is shown simply to get an idea of the raw conversion result. The MPL115A can be used to measure changes in vacuum or for barometric pressure rather than the MPXM2102A. In the Quick Start Guide there is a section to set a threshold above the current barometric pressure for an alarm. When the alarm is set, the status LEDs will light up green. After the alarm threshold is reached, the red LED toggles on/off while a buzzer is sounded. This can be activated via waiting for a pressure change, or applying pressure to the DEMOAPEXSENSOR. A clear Ziploc bag or syringe with a soft applicator tip useful in activating the alarm.
Weather Station Implementation
Freescale application note, AN3914, details altimetry and barometric weather systems and also includes code examples from the DEMOAPEXSENSOR. There are two sections in the APEX board; a simple weather station or an advanced method. The simple method does a comparison by asking the user to input their known altitude, and comparing this pressure to that of the MPL115A. The delta in the value is compared in the Table 7. Table 7.
Analysis dP > +0.25 kPa -0.25 kPa < dP < 0.25 kPa dP < -0.25 kPa Sun Symbol Sun/Cloud Symbol Rain Symbol Output
This is typical of a simple application and the APEX simulates a desktop barometer that is commonly bought at retailers. A more advanced version of this calculates the pressure change by taking values over time and seeing the delta change over a 3 hour period. This is outlined in Table 8 (AN3914). Table 8. Advanced Weather Determination
Analysis dP/dt > 0.25 kPa/h 0.05 kPa/h < dP/dt < 0.25 kPa/h -0.05 kPa/h < dP/dt < 0.05 kPa/h -0.25 kPa/h < dP/dt < -0.05 kPa/h dP/dt < -0.25 kPa/h Output Quickly rising High Pressure System, not stable Slowly rising High Pressure System, stable good weather Stable weather condition Slowly falling Low Pressure System, stable rainy weather Quickly falling Low Pressure, Thunderstorm, not stable
AN3956 Sensors Freescale Semiconductor 16
Waterlevel Implementation with the MPXV5004G
The DEMOAPEXSENSOR has the MPXV5004G gauge pressure sensor on board. This is located on the backside of the demo board. It has a top port, which is to be used with the supplied high temp silicone hose. The output of the MPXV5004G is shown in Table 9. Table 9. Operating Characteristics (VS = 5.0 VDC, TA = 25C unless otherwise noted, P1 > P2)
Characteristic Pressure Range Supply Voltage Supply Current Span @ 306 mm H2O (3 kPa) Full Scale Span @ 400 mm H2O (3.92 kPa) Offset Sensitivity Accuracy 0 to 100 mm H2O (10 to 60C) 100 to 400 mm H2O (10 to 60C) 0 to 400 mm H2O (10 to 60C) Symbol POP VS IS VFSS VOFF V/P -- -- -- Min 0 4.75 -- -- -- 0.75 -- -- -- -- Typ -- 5.0 -- 3.0 4.0 1.0 1.0 -- -- -- Max 3.92 400 5.25 10 -- -- 1.25 -- 1.5 2.5 6.25 Units kPa mm H2O VDC mAdc V V V/kPa %VFSS with auto zero %VFSS with auto zero %VFSS without auto zero
The part has a offset of 1 V with a sensitivity of 9.8 mV/mm H20. On the DEMOAPEXSENSOR the value is sampled with a 12-bit ADC from the JM60 MCU. Note the following when considering waterlevel: * It is important to implement Auto Zero (AN1636) to improve accuracy. * Calibration can be done, ideally with a two point calibration. * The sensor must be implemented with an air column buffer between the liquid and the sensor. This is done with a section of tubing as in Figure 8. * It is a gauge application, so the pressure measured is with respect to barometric pressure; when elevation varies, the pressure seen at the sensor is the same, only dependent on liquid height.
Figure 8. Waterlevel Connection to Pressure Sensor
Curved Bottom
Improved resolution when tank is almost empty More vertical height per less volume H2O
Figure 9. Hot Water Heater
Figure 10. Conical Bottom for Measurement
AN3956 Sensors Freescale Semiconductor 17
Figure 9 and Figure 10 are examples of water measurement. In the hot water heater to the left, although the temperature of the water can be high, a long enough tube combined with air column separation provides a buffer for measurement. Water and Air are good insulators, so temperature variation can be quite drastic in a short distance. This is a design advantage in isolating the sensor from heat. In Figure 10 the diagram has a method to improve resolution of the water level. The vertical height measurement is kept constant, but the volume of water dispensed or left in the bottom is improved. This is done by simply making a conical bottom. The resolution in vertical height can be used to improve the volume measurement. Additional ADC counts per a volume of water are achieved, than a shape with a broad based bottom.
AUTO ZERO
A summary of the application note AN1636 is that the offset of the pressure sensor can change during board mount, stress, or temperature shifts. By sampling the offset pressure (voltage output with no pressure input) and storing this value in the MCU, the changes in the offset are negated. This improves the overall accuracy of the voltage output of the sensor, since the offset changes are negated. Normally this would occur in a final product when there is a startup or a cycle when the sensor is not pressurized.
CALIBRATION
The best calibration would be a two part calibration. A Pressure would be taken at 0 mm of H2O and also when the tubing is at 40 cm of H2O. The difference in ADC counts divided by 40 cm will give the number of ADC counts per a cm etc. for each sensor. This is the best calibration, but is not implemented on the DEMOAPEXSENSOR in software rev 1.0.
First Point: 0 Level, Water Surface
Second Point: Bottom of Container
Figure 11. Two Part Calibration
AN3956 Sensors Freescale Semiconductor 18
Inertial MMA7361L
The DEMOAPEXSENSOR has a quick implementation of the 3 axis selectable low-g accelerometer. Notably the demo board implements the tilt function of the accelerometer and Free Fall detection. The Quick Start Guide shows the modes to select either function.
TILT ANGLE ON THE DEMOAPEXSENSOR
The ADC counts are converted to tilt angles using the equation below: V Vout = Voff + ------- 1.0g Sin g (eq. 4)
Where:
Vout = Analog output of accelerometer Voff = Offset voltage of accelerometer V/g = Sensitivity 1.0g = Earth's gravity = Tilt Angle
Note: MMA7361 was set to a g-level of 1.5 g. Offset voltage is typically 1.65 V @ 3.3 V biasing supply
FREE FALL DETECT ON THE DEMOAPEXSENSOR
Free Fall detect is the situation where all three axes of the inertial sensor converge towards the offset or `0g' range. This is also given as an output on the MMA7361L as the 0g-Detect pin. `1' or logic high is the output for a detected Free Fall. Note that this is also for Free Fall in a linear fashion such that the board is not spinning or in rotation as it falls. The DEMOAPEXSENSOR emits a buzzer noise and a flashing red LED as the Free Fall detect occurs.
AN3956 Sensors Freescale Semiconductor 19
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