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5 <title>MicroPeak Owner's Manual</title>
6 <subtitle>A recording altimeter for hobby rocketry</subtitle>
9 <firstname>Keith</firstname>
10 <surname>Packard</surname>
14 <holder>Bdale Garbee and Keith Packard</holder>
18 This document is released under the terms of the
19 <ulink url="http://creativecommons.org/licenses/by-sa/3.0/">
20 Creative Commons ShareAlike 3.0
27 <revnumber>0.1</revnumber>
28 <date>29 October 2012</date>
30 Initial release with preliminary hardware.
34 <revnumber>1.0</revnumber>
35 <date>18 November 2012</date>
37 Updates for version 1.0 release.
41 <revnumber>1.1</revnumber>
42 <date>12 December 2012</date>
44 Add comments about EEPROM storage format and programming jig.
48 <revnumber>1.2</revnumber>
49 <date>20 January 2013</date>
51 Add documentation for the MicroPeak USB adapter board. Note
52 the switch to a Kalman filter for peak altitude
60 Thanks to John Lyngdal for suggesting that we build something like this.
63 Have fun using these products, and we hope to meet all of you
64 out on the rocket flight line somewhere.
67 NAR #87103, TRA #12201
70 NAR #88757, TRA #12200
75 <title>Quick Start Guide</title>
77 MicroPeak is designed to be easy to use. Requiring no external
78 components, flying takes just a few steps
83 Install the battery. Fit a CR1025 battery into the plastic
84 carrier. The positive (+) terminal should be towards the more
85 open side of the carrier. Slip the carrier into the battery
86 holder with the positive (+) terminal facing away from the
92 Install MicroPeak in your rocket. This can be as simple as
93 preparing a soft cushion of wadding inside a vented model payload
94 bay. Wherever you mount it, make sure you protect the
95 barometric sensor from corrosive ejection gasses as those
96 will damage the sensor, and shield it from light as that can
97 cause incorrect sensor readings.
102 Turn MicroPeak on. Slide the switch so that the actuator
103 covers the '1' printed on the board. MicroPeak will report
104 the maximum height of the last flight in decimeters using a
105 sequence of flashes on the LED. A sequence of short flashes
106 indicates one digit. A single long flash indicates zero. The
107 height is reported in decimeters, so the last digit will be
108 tenths of a meter. For example, if MicroPeak reports 5 4 4
109 3, then the maximum height of the last flight was 544.3m, or
115 Finish preparing the rocket for flight. After the
116 previous flight data have been reported, MicroPeak waits for
117 30 seconds before starting to check for launch. This gives
118 you time to finish assembling the rocket. As those
119 activities might cause pressure changes inside the airframe,
120 MicroPeak might accidentally detect boost. If you need to do
121 anything to the airframe after the 30 second window passes,
122 make sure to be careful not to disturb the altimeter. The
123 LED will remain dark during the 30 second delay, but after
124 that, it will start blinking once every 3 seconds.
129 Fly the rocket. Once the rocket passes about 10m in height
130 (32 feet), the micro-controller will record the ground
131 pressure and track the pressure seen during the flight. In
132 this mode, the LED flickers rapidly. When the rocket lands,
133 and the pressure stabilizes, the micro-controller will record
134 the minimum pressure pressure experienced during the flight,
135 compute the height represented by the difference in air
136 pressure and blink that value out on the LED. After that,
137 MicroPeak powers down to conserve battery power.
142 Recover the data. Turn MicroPeak off and then back on. MicroPeak
143 will blink out the maximum height for the last flight. Turn
144 MicroPeak back off to conserve battery power.
150 <title>Handling Precautions</title>
152 All Altus Metrum products are sophisticated electronic devices.
153 When handled gently and properly installed in an air-frame, they
154 will deliver impressive results. However, as with all electronic
155 devices, there are some precautions you must take.
158 The CR1025 Lithium batteries have an
159 extraordinary power density. This is great because we can fly with
160 much less battery mass... but if they are punctured
161 or their contacts are allowed to short, they can and will release their
163 Thus we recommend that you take some care when handling MicroPeak
164 to keep conductive material from coming in contact with the exposed metal elements.
167 The barometric sensor used in MicroPeak is sensitive to
168 sunlight. Please consider this when designing an
169 installation. Many model rockets with payload bays use clear
170 plastic for the payload bay. Replacing these with an opaque
171 cardboard tube, painting them, or wrapping them with a layer of
172 masking tape are all reasonable approaches to keep the sensor
173 out of direct sunlight.
176 The barometric sensor sampling ports must be able to "breathe",
177 both by not being covered by foam or tape or other materials that might
178 directly block the hole on the top of the sensor, and also by having a
179 suitable static vent to outside air.
182 As with all other rocketry electronics, Altus Metrum altimeters must
183 be protected from exposure to corrosive motor exhaust and ejection
188 <title>The MicroPeak USB adapter</title>
190 MicroPeak stores barometric pressure information for the first
191 48 seconds of the flight in on-board non-volatile memory. The
192 contents of this memory can be downloaded to a computer using
193 the MicroPeak USB adapter.
196 <title>Installing the MicroPeak software</title>
198 The MicroPeak application runs on Linux, Mac OS X and
199 Windows. You can download the latest version from
200 <ulink url="http://altusmetrum.org/AltOS"/>.
203 On Mac OS X and Windows, the FTDI USB device driver needs to
204 be installed. A compatible version of this driver is included
205 with the MicroPeak application, but you may want to download a
206 newer version from <ulink
207 url="http://www.ftdichip.com/FTDrivers.htm"/>.
211 <title>Downloading Micro Peak data</title>
215 Connect the MicroPeak USB adapter to a USB cable and plug it
221 Start the MicroPeak application, locate the File menu and
222 select the Download entry.
227 The MicroPeak USB adapter has a small phototransistor on
228 the end of the board furthest from the USB
229 connector. Locate this and place the LED on the MicroPeak
230 directly in contact with it. The MicroPeak LED and the
231 MicroPeak USB adapter photo need to be touching—even a
232 millimeters of space between them will reduce the light
233 intensity from the LED enough that the phototransistor
234 will not sense it. Turn on the MicroPeak board and adjust
235 the position until the blue LED on the MicroPeak USB
236 adapter blinks in time with the orange LED on the
242 After the maximum flight height is reported, MicroPeak will
243 pause for a few seconds, blink the LED four times rapidly
244 and then send the data in one long blur on the LED. The
245 MicroPeak application should receive the data. When it does,
246 it will present the data in a graph and offer to save the
247 data to a file. If not, you can power cycle the MicroPeak
254 <title>Analyzing MicroPeak Data</title>
256 The MicroPeak application can present flight data in the form
257 of a graph, a collection of computed statistics or in tabular
261 MicroPeak collects raw barometric pressure data which is
262 then used to compute the remaining data. Altitude is computed
263 through a standard atmospheric model. Absolute error in this
264 data will be affected by local atmospheric
265 conditions. Fortunately, these errors tend to mostly cancel
266 out, so the error in the height computation is much smaller
267 than the error in altitude would be.
270 Speed and acceleration are computed by first smoothing the
271 height data with a Gaussian window averaging filter. For speed
272 data, this average uses seven samples. For acceleration data,
273 eleven samples are used. These were chosen to provide
274 reasonably smooth speed and acceleration data, which would
275 otherwise be swamped with noise.
278 Under the Graph tab, the height, speed and acceleration values
279 are displayed together. You can zoom in on the graph by
280 clicking and dragging to sweep out an area of
281 interest. Right-click on the plot to bring up a menu that will
282 let you save, copy or print the graph.
285 The Statistics tab presents overall data from the flight. Note
286 that the Maximum height value is taken from the minumum
287 pressure captured in flight, and may be different from the
288 apparant apogee value as the on-board data are sampled twice
289 as fast as the recorded values, or because the true apogee
290 occurred after the on-board memory was full. Each value is
291 presented in several units as appropriate.
294 A table consisting of the both the raw barometric pressure
295 data and values computed from that for each recorded time.
298 The File menu has operations to open existing flight logs,
299 Download new data from MicroPeak, Save a copy of the flight
300 log to a new file, Export the tabular data (as seen in the Raw
301 Data tab) to a file, change the application Preferences, Close
302 the current window or close all windows and Exit the
307 <title>Configuring the MicroPeak application</title>
309 The MicroPeak application has a few user settings which are
310 configured through the Preferences dialog, which can be
311 accessed from the File menu.
315 The Log Directory is where flight data will be saved to
316 and loaded from by default. Of course, you can always
317 navigate to other directories in the file chooser windows,
318 this setting is just the starting point.
323 If you prefer to see your graph data in feet and
324 miles per hour instead of meters and meters per second,
325 you can select Imperial Units.
330 To see what data is actually arriving over the serial
331 port, start the MicroPeak application from a command
332 prompt and select the Serial Debug option. This can be
333 useful in debugging serial communication problems, but
334 most people need never choose this.
339 You can adjust the size of the text in the Statistics tab
340 by changing the Font size preference. There are three
341 settings, with luck one will both fit on your screen and
342 provide readable values.
347 The Look & feel menu shows a list of available
348 application appearance choices. By default, the MicroPeak
349 application tries to blend in with other applications, but
350 you may choose some other appearance if you like.
356 Note that MicroPeak shares a subset of the AltosUI
357 preferences, so if you use both of these applications, change
358 in one application will affect the other.
363 <title>Technical Information</title>
365 <title>Barometric Sensor</title>
367 MicroPeak uses the Measurement Specialties MS5607 sensor. This
368 has a range of 120kPa to 1kPa with an absolute accuracy of
369 150Pa and a resolution of 2.4Pa.
372 The pressure range corresponds roughly to an altitude range of
373 -1500m (-4900 feet) to 31000m (102000 feet), while the
374 resolution is approximately 20cm (8 inches) near sea level and
375 60cm (24in) at 10000m (33000 feet).
378 Ground pressure is computed from an average of 16 samples,
379 taken while the altimeter is at rest. Flight pressure is
380 computed from a Kalman filter designed to smooth out any minor
381 noise in the sensor values.
385 <title>Micro-controller</title>
387 MicroPeak uses an Atmel ATtiny85 micro-controller. This tiny
388 CPU contains 8kB of flash for the application, 512B of RAM for
389 temporary data storage and 512B of EEPROM for non-volatile
390 storage of previous flight data.
393 The ATtiny85 has a low-power mode which turns off all of the
394 clocks and powers down most of the internal components. In
395 this mode, the chip consumes only .1μA of power. MicroPeak
396 uses this mode once the flight has ended to preserve battery
401 <title>Lithium Battery</title>
403 The CR1025 battery used by MicroPeak holes 30mAh of power,
404 which is sufficient to run for over 40 hours. Because
405 MicroPeak powers down on landing, run time includes only time
406 sitting on the launch pad or during flight.
409 The large positive terminal (+) is usually marked, while the
410 smaller negative terminal is not. Make sure you install the
411 battery with the positive terminal facing away from the
412 circuit board where it will be in contact with the metal
413 battery holder. A small pad on the circuit board makes contact
414 with the negative battery terminal.
417 Shipping restrictions may prevent us from including a CR1025
418 battery with MicroPeak. If so, many stores carry CR1025
419 batteries as they are commonly used in small electronic
420 devices such as flash lights.
424 <title>Atmospheric Model</title>
426 MicroPeak contains a fixed atmospheric model which is used to
427 convert barometric pressure into altitude. The model was
428 converted into a 469-element piece wise linear approximation
429 which is then used to compute the altitude of the ground and
430 apogee. The difference between these represents the maximum
431 height of the flight.
434 The model assumes a particular set of atmospheric conditions,
435 which while a reasonable average cannot represent the changing
436 nature of the real atmosphere. Fortunately, for flights
437 reasonably close to the ground, the effect of this global
438 inaccuracy are largely canceled out when the computed ground
439 altitude is subtracted from the computed apogee altitude, so
440 the resulting height is more accurate than either the ground
445 <title>Mechanical Considerations</title>
447 MicroPeak is designed to be rugged enough for typical rocketry
448 applications. It contains two moving parts, the battery holder
449 and the power switch, which were selected for their
453 The MicroPeak battery holder is designed to withstand impact
454 up to 150g without breaking contact (or, worse yet, causing
455 the battery to fall out). That means it should stand up to
456 almost any launch you care to try, and should withstand fairly
460 The power switch is designed to withstand up to 50g forces in
461 any direction. Because it is a sliding switch, orienting the
462 switch perpendicular to the direction of rocket travel will
463 serve to further protect the switch from launch forces.
467 <title>On-board data storage</title>
469 The ATtiny85 has 512 bytes of non-volatile storage, separate
470 from the code storage memory. The MicroPeak firmware uses this
471 to store information about the last completed
472 flight. Barometric measurements from the ground before launch
473 and at apogee are stored, and used at power-on to compute the
474 height of the last flight.
477 In addition to the data used to present the height of the last
478 flight, MicroPeak also stores barometric information sampled
479 at regular intervals during the flight. This information can
480 be extracted from MicroPeak through any AVR programming
484 <title>MicroPeak EEPROM Data Storage</title>
485 <tgroup cols='3' align='center' colsep='1' rowsep='1'>
486 <colspec align='center' colwidth='2*' colname='Address'/>
487 <colspec align='center' colwidth='*' colname='Size (bytes)'/>
488 <colspec align='left' colwidth='7*' colname='Description'/>
491 <entry align='center'>Address</entry>
492 <entry align='center'>Size (bytes)</entry>
493 <entry align='center'>Description</entry>
500 <entry>Average ground pressure (Pa)</entry>
505 <entry>Minimum flight pressure (Pa)</entry>
510 <entry>Number of in-flight samples</entry>
513 <entry>0x00a … 0x1fe</entry>
515 <entry>Instantaneous flight pressure (Pa) low 16 bits</entry>
521 All EEPROM data are stored least-significant byte first. The
522 instantaneous flight pressure data are stored without the
523 upper 16 bits of data. The upper bits can be reconstructed
524 from the previous sample, assuming that pressure doesn't
525 change by more more than 32kPa in a single sample
526 interval. Note that this pressure data is <emphasis>not</emphasis>
527 filtered in any way, while both the recorded ground and apogee
528 pressure values are, so you shouldn't expect the minimum
529 instantaneous pressure value to match the recorded minimum
530 pressure value exactly.
533 MicroPeak samples pressure every 96ms, but stores only every
534 other sample in the EEPROM. This provides for 251 pressure
535 samples at 192ms intervals, or 48.192s of storage. The clock
536 used for these samples is a factory calibrated RC circuit
537 built into the ATtiny85 and is accurate only to within ±10% at
538 25°C. So, you can count on the pressure data being accurate,
539 but speed or acceleration data computed from this will be
540 limited by the accuracy of this clock.
544 <title>MicroPeak Programming Interface</title>
546 MicroPeak exposes a standard 6-pin AVR programming interface,
547 but not using the usual 2x3 array of pins on 0.1"
548 centers. Instead, there is a single row of tiny 0.60mm ×
549 0.85mm pads on 1.20mm centers exposed near the edge of the
550 circuit board. We couldn't find any connector that was
551 small enough to include on the circuit board.
554 In lieu of an actual connector, the easiest way to connect to
555 the bare pads is through a set of Pogo pins. These
556 spring-loaded contacts are designed to connect in precisely
557 this way. We've designed a programming jig, the MicroPeak
558 Pogo Pin board which provides a standard AVR interface on one
559 end and a recessed slot for MicroPeak to align the board with
563 The MicroPeak Pogo Pin board is not a complete AVR programmer,
564 it is an interface board that provides a 3.3V regulated power
565 supply to run the MicroPeak via USB and a standard 6-pin AVR
566 programming interface with the usual 2x3 grid of pins on 0.1"
567 centers. This can be connected to any AVR programming
571 The AVR programming interface cannot run faster than ¼ of the
572 AVR CPU clock frequency. Because MicroPeak runs at 250kHz to
573 save power, you must configure your AVR programming system to
574 clock the AVR programming interface at no faster than
575 62.5kHz, or a clock period of 32µS.
580 <!-- LocalWords: Altusmetrum MicroPeak