Precision Digital Altimeter
with Motorola MPXS4100 A and
PIC16F84
Table of Contents [Toc]
Concept
Transmitter
Resolution
Note
So far...
Documents
Receiver
Components
Technical Data of Wireless
Transmitter and Receiver
Remarks
Evaluation Board
Initial Test
Setup
PCB Evaluation
Board
Schematics of
Evaluation Board
Available PIC Assembler
Code
Software
Measurements
Further Information
Documents
Sensor Ordering
Information
Other Circuits and
Utilities
Links
Future Steps
Concept [Toc] [Top]
The current concept incorporates a wireless transmitter and
receiver and is thought to be used for remote controlled
airplanes or appliances with two seperate parts. In other words,
we have one dedicated transmitter (acquisition, filtering) and
one dedicated receiver part (user-interface, look-up table,
calibration, storage), capable of being connected together with
any physical layer, e.g. wired, wireless, infrared. If you want
to build a standalone altimeter/variometer just for hiking or
mountaineering, this setup can obviously be simplified by
omitting the wireless components.
So far, only the transmitter part with all its analog circuitry
has been completed entirely - the digital receiver part has still
to be done, but does not necessarily have to be a PIC
microcontroller. For instance, it could also be a personal
computer, connected through the standardized RS232 protocol and a
wired/wireless interface to the transmitter part.
The (pending) challenge of the receiver implementation is the
field evaluation of the most suitable and accurate temperature
and non-linear pressure-altitude correction algorithms. There is
maybe need for adding a temperature sensor to the transmitter
part.
Transmitter
[Toc]
[Top]
|
Resolution
[Toc]
[Top]
I reached 56 cm per unit, so the whole range is :
- 2270 meters using a 12 bit A/D converter
- 142 meters using an 8 bit A/D converter
Increase the range by taking a better A/D converter or
decrease the Amplifier-Gain (reduction of resolution).
In practice, I'll set the resolution to 1 meter, so a
total range of 4096 meters will be available. This is
enough precise for my R/C models, but allows to use this
altimeter also for hiking and mountaineering.
The microcontroller builds an average value of 64 A/D
samples to reduce any noise to a minimum.
So far ...
[Toc]
[Top]
I've developed the following routines in PIC Assembler
code:
- RS 232 Interrupt handled interface for PICs
without USART. I like a clean design, so I take an
interrupt capable PIC microcontroller ! Although the PIC
16C73 has one USART, this routine is necessary because
the USART is already used for the AD Converter and the
EEPROM. Because the RS 232 interface is not used during
data logging operation, the performance of the USART is
completely present to the communication with the AD
Converter.
- Serial 4 Bit standard Dot-LCD interface
- 8 Bit & 16 Bit binary to decimal conversion
routine for LCD output.
- NSC ADC12130 test interface with
Auto-Calibration, Auto-Zero and Data-Fetch
- Data Capture from an Excel Worksheet.
Documents
[Toc]
[Top]
See related stuff at the document section below.
|
Components
[Toc] [Top]
The following components have been used: Analog to digital
converter NSC ADC12130, a Microchip PIC16F84 controller, a quad
op-amp NSC LMC660, a Maxim MAX232 RS232 level shifter, and the
Motorola MPXS4100A absolute pressure sensor.
|
Removable module containing Motorola absolute
pressure sensor MPXS 4100A, 10nF and 100nF ceramic and 10uF
tantalum capacitors.
|
|
Circuit Design CDP-RX-01 wireless receiver, 434 MHz,
up to 7.5 kb/s, uni-directional
|
| |
|
|
|
Top view of removable pressure sensor module.
|
|
Circuit design CDP-TX-01 wireless transmitter, 434
MHz, up to 7.5 kb/s, uni-directional
|
Technical
Data of Wireless Transmitter and Receiver [Toc] [Top]
- Manufacturer: Circuit Design Inc.
- Very small and compact integrated device with robust metal
housing.
- Low current consumption, ideal for mobile, battery-powered
applications.
- Filter technique using Super-SAW filters.
Additional information is available at Circuit Design
Inc. -> Products
Data sheets of wireless transmitter and receiver:
| General |
|
|
| Oscillator type: |
|
Crystal |
| Frequency: |
|
433.920 MHz, 434.075 MHz
(Europe)
458.650 MHz (United Kingdom) |
| Frequency stability: |
|
+/- 2.5 kHz (-10° up to
+55° C) |
| HF channels: |
|
single (fixed channel) |
| Range: |
|
up to 1000 m (free distance, line of
sight) |
| Baud rate
(specified): |
|
300 - 4800 baud |
| Bit rate (measured at
short distance): |
|
up to 7500 bits/s |
| Operating
conditions: |
|
-10° up to +60° C |
| Type approval: |
|
I-ETS 300 220 / Germany, France,
Switzerland, Sweden, UK, Holland, Austria, EMC |
| Transmitter |
|
|
| HF power output: |
|
10 mW +/- 3 dB @ 50
Ohm |
| Modulation: |
|
FM narrow band |
| Start-up time: |
|
30 ms |
| Input signal type: |
|
digital, 5 Volt |
| Deviation: |
|
2.5 kHz |
| Supply voltage: |
|
5.5 - 10 Volt |
| Power consumption: |
|
18 mA typ. |
| Dimensions: |
|
36 x 26 x 10 mm |
| Weight: |
|
9.8 g |
| Receiver |
|
|
| Type: |
|
double superheterodyne, crystal
oscillator |
| Sensitivity: |
|
-120 dBm (12 dB/SINAD, CCITT
filter) |
| Selectivity: |
|
+/- 5 kHz @ -6 dB |
| Demodulation: |
|
FM narrow band |
| Distortion: |
|
< 5 % @ 1 kHz |
| Output signal type: |
|
digital, open collector |
| Other outputs: |
|
RSSI and AF |
| Supply voltage: |
|
4.5 - 14 Volt |
| Power consumption: |
|
10 mA typ. |
| Dimensions: |
|
50 x 30 x 7.5 mm |
| Weight: |
|
19 g |
Remarks [Toc] [Top]
A lot of people have asked me where I got the Motorola
absolute pressure sensor from.
Please see the section Sensor Ordering
Information below.
By the way, Motorola distinguishes the sensor characteristics,
feature set and package type by the sensor name, so you can also
get 4100 sensor types similar to mine with different naming, e.g.
MPXT 4100A or PPXA 4100A:
- M Qualified standards
- PX Pressure sensor
- S Small outline package
See Motorola Sensor Selector Guide at the section
Documents below.
Evaluation Board [Toc] [Top]
The Circuit Design wireless transmitter and receiver are not
necessary to use the evaluation board, since there exists a
direct RS232 link to the PC.
Initial Test Setup [Toc] [Top]
|
Test setup to check the initial
concept
|
An initial test setup for checking the mixed signal design: A
significant problem was the digital noise in the analog circuitry
supply voltages. Finally the noise could be minimized by
splitting up the power supplies to three independent sources: one
digital power supply voltage, one for the sensor and A/D
converter, and one supply with slightly higher voltage for the
operational amplifiers of the filter stages. The above test setup
contains no wireless transmitter, the data is directly
transmitted to the computer using the RS232 protocol and a MAX232
level shifter.
In this setup, I have used the LMC660 / LMC662
low-power rail-to-rail quad operational amplifiers for the fourth
order Chebyshev filter stages.
PCB
Evaluation Board [Toc] [Top]
Moving from the test board to the first PCB, I have only made
slight adaptations in the analog part of my design: I have
altered the filter characteristics from Chebyshev to Butterworth
- but as a consequence, I had to replace the LMC660 operational
amplifier by a LM324 type, due to oscillating filters.
Conclusion: In the analog world, nothing runs properly if it has
not been tested.
If someone knows a good single supply, low-power, rail-to-rail
operational amplifier with clean and linear output
characteristics in the entire input range, please
let me know! The LMC660 is exactly specified this way, but
showed up a really bad non-linear characteristic in the upper
input range. Bob Krech suggested the LMC6064 precision quad OP
amplifier with pin-for-pin replacement for the LM324.
 |
|
 |
PCB-based evaluation board suitable for first
field measurements |
|
Description of the above PCB
layout
|
The PCB-based evaluation board consists of analog circuitry at
the left side and digital components at the right side of the
board. In the upper left corner are the three independent power
supplies (5 V analog, 6.8 V analog, 5 V digital),
all served from one battery (8 - 10 V). The
evaluation board contains further the active Butterworth filter
stages built of one LM324 (left side), the NSC ADC12130 A/D
converter (center), the PIC 16F84 microcontroller (at right from
A/D converter), a dot LCD display and a PORTB connector (lower
right corner), a direct RS232 interface with MAX232 level shifter
(upper right corner), and an interface for the wireless
transmitter allowing for first field measurements (bottom
center). The system owns two oscillators. A 4 MHz crystal
oscillator provides the conversion clock for the A/D converter,
and a separate 4 MHz crystal for the microcontroller allows
to increase processor performance easily if necessary. This setup
provides two A/D converter input channels: Channel 0 is already
used for the pressure sensor, but channel 1 can be used freely,
e.g. for voltage surveillance of the R/C receiver battery. If two
channels are not sufficient, this system can easily be upgraded
to eight channels by integrating the NSC ADC12138 A/D converter.
The LCD connector and the RS232 interface serve only for
evaluation and debugging purposes in this setup. Finally, the
noise characteristics of my approach are very promising!
Although this board is now ready, a suitable R/C plane - my
Piper Cherokee - has to be finished first for
'air evaluation'...
Schematics of Evaluation Board
[Toc] [Top]
Latest version 1.03 from April 23rd, 2001:
Top.pdf (8.5 kB)
Sensor.pdf (4.0 kB)
Filter1.pdf (6.5
kB)
(replaced LMC660 with LM324, due to oscillating filters)
Filter2.pdf (6.3
kB)
(replaced LMC660 with LM324, due to oscillating filters)
Digital.pdf (12.6
kB)
LCD.pdf (4.6
kB)
(corrected some errors)
RS232.pdf (7.7 kB)
PowerSupply.pdf
(8.9 kB)
Available
Microchip PIC Assembler Code [Toc] [Top]
| Main File |
HEX Files |
View assembler source code:
alti_tx.html
Download assembler source code: alti_tx.asm (16.7 kB) |
alti_tx.hex |
| The above program needs
additional include files (modules) to get successfully assembled:
m_bank.asm, m_wait.asm, m_rs096.asm |
| For those, who are not familiar
with interfacing a PIC to the RS232 using a MAX232:
RS232-Interface.pdf
(9.7 kB) |
Software [Toc] [Top]
An Excel worksheet has been used to visualize the captured
data.
With the NSC ADC12130 A/D converter, there are two channels
available.
Download Excel worksheet and drivers: alti_2channel.zip (203 kB)
Measurements
[Toc] [Top]
I've done some measurements to see whether LSB toggling is
sufficient low using the Excel RS232 data capture interface. All
measurements have been carried out at room temperature (without
any temperature compensation) and without moving the test board.
The test period has been one hour with adequate warm-up time for
the sensitive analog circuitry (pressure sensor, A/D converter
and op-amp).
Note: Because the entire design is laid out very
sensitive in order to get a high resolution (in the range of one
meter), the accumulated drift could also originate from natural
barometric variations.
|
Test setup: Drift of 2 bits during one
hour (with a single spike).
|
|
Test setup: Drift of 4 bits during one
hour.
|
|
Test setup: Drift of 3 bits during half
an hour. The strange curve comes possibly from temperature
variations.
|
|
PCB-based evaluation board: The yellow
curve represents the pressure sensor, the blue curve is
only a test voltage for comparision purposes. The pressure
sensor behaves quite stable in conjunction with the PCB
setup, only ½ LSB toggling during one hour. The
comparision voltage - originating from a simple voltage
divider - is even more toggling. This toggling may result
from a voltage just reaching the A/D converter LSB
threshold.
|
Further Information [Toc] [Top]
Documents [Toc] [Top]
Data sheet of Motorola Absolute Pressure Sensor Series
MPX4100A: MPX4100A (PDF,
117 kB)
Motorola Sensor Selector Guide (Acceleration, Pressure, and
Smoke): SensorSelectorGuide (PDF,
166 kB)
Noise considerations for integrated pressure sensors: AN1646 (PDF,
153 kB)
Data sheets of wireless transmitter and receiver:
Sensor Ordering
Information [Toc] [Top]
A lot of people have asked me, where they could get the
Motorola absolut pressure sensor from. I bought it from www.conrad.com
years ago, but now, these sensors may occassionally not be
available there. Don't ask me where you can get them elsewhere, I
don't know...
As of 08.04.2005, there was one pressure sensor model from
Motorola available at www.conrad.com:
Item: SMD-DRUCKSENSOR MPXA4100A6U
Ordering#: 150110 - 13
Price: about 25.00 Euro/pc.
Other
Circuits and Utilities [Toc] [Top]
NSC
ADC12130 Test Interface
Dot-matrix
LCD Interface
RS232
Interface
Excel Data
Capture
Automatic Table
Generator for MPLAB Assembler
RC data logger V2.5 with altitude capture
German page about a self-made RC data logger using PIC 16F84 and
MPXS 4100A absolute pressure sensor.
Previous version with nice Palm interface pictures: RC data logger
V1.x
Future Steps
[Toc] [Top]
- Field measurements and evaluation of the most suitable
temperature and non-linear pressure-altitude correction
algorithms, maybe with portable computer to be ready for quick
alterations.
- Design of the microcontroller-based receiver circuit.
- Put all code together and create user-interface and
calibration menu with liquid crystal display.
- If applicable, a 24C65 EEPROM interface for storage.
- If having not enough challenges, real time clock interface
to Dallas DS1302.
This work is currently on hold due
to limited time.
Last updated: 2005/11/09
[Toc] [Top]
