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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
57 <revnumber>1.3.2</revnumber>
58 <date>12 February 2014</date>
60 Add a "Download" button to the main window, which makes it
61 quicker to access the download function. Update the data
62 download documentation to reflect the new MicroPeak USB
63 adapter design. Monitor data during download to let you see
64 if the USB connection is working at all by showing the
65 characters received from the MicroPeak USB adapter.
71 <title>Acknowledgements</title>
73 Thanks to John Lyngdal for suggesting that we build something like this.
76 Have fun using these products, and we hope to meet all of you
77 out on the rocket flight line somewhere.
80 NAR #87103, TRA #12201
83 NAR #88757, TRA #12200
88 <title>Quick Start Guide</title>
90 MicroPeak is designed to be easy to use. Requiring no external
91 components, flying takes just a few steps
96 Install the battery. Fit a CR1025 battery into the plastic
97 carrier. The positive (+) terminal should be towards the more
98 open side of the carrier. Slip the carrier into the battery
99 holder with the positive (+) terminal facing away from the
105 Install MicroPeak in your rocket. This can be as simple as
106 preparing a soft cushion of wadding inside a vented model payload
107 bay. Wherever you mount it, make sure you protect the
108 barometric sensor from corrosive ejection gasses as those
109 will damage the sensor, and shield it from light as that can
110 cause incorrect sensor readings.
115 Turn MicroPeak on. Slide the switch so that the actuator
116 covers the '1' printed on the board. MicroPeak will report
117 the maximum height of the last flight in decimeters using a
118 sequence of flashes on the LED. A sequence of short flashes
119 indicates one digit. A single long flash indicates zero. The
120 height is reported in decimeters, so the last digit will be
121 tenths of a meter. For example, if MicroPeak reports 5 4 4
122 3, then the maximum height of the last flight was 544.3m, or
128 Finish preparing the rocket for flight. After the
129 previous flight data have been reported, MicroPeak waits for
130 one minute before starting to check for launch. This gives
131 you time to finish assembling the rocket. As those
132 activities might cause pressure changes inside the airframe,
133 MicroPeak might accidentally detect boost. If you need to do
134 anything to the airframe after the one minute window passes,
135 make sure to be careful not to disturb the altimeter. The
136 LED will remain dark during the one minute delay, but after
137 that, it will start blinking once every 3 seconds.
142 Fly the rocket. Once the rocket passes about 30m in height
143 (100 feet), the micro-controller will record the ground
144 pressure and track the pressure seen during the flight. In
145 this mode, the LED flickers rapidly. When the rocket lands,
146 and the pressure stabilizes, the micro-controller will record
147 the minimum pressure pressure experienced during the flight,
148 compute the height represented by the difference in air
149 pressure and blink that value out on the LED. After that,
150 MicroPeak powers down to conserve battery power.
155 Recover the data. Turn MicroPeak off and then back on. MicroPeak
156 will blink out the maximum height for the last flight. Turn
157 MicroPeak back off to conserve battery power.
163 <title>Handling Precautions</title>
165 All Altus Metrum products are sophisticated electronic devices.
166 When handled gently and properly installed in an air-frame, they
167 will deliver impressive results. However, as with all electronic
168 devices, there are some precautions you must take.
171 The CR1025 Lithium batteries have an
172 extraordinary power density. This is great because we can fly with
173 much less battery mass... but if they are punctured
174 or their contacts are allowed to short, they can and will release their
176 Thus we recommend that you take some care when handling MicroPeak
177 to keep conductive material from coming in contact with the exposed metal elements.
180 The barometric sensor used in MicroPeak is sensitive to
181 sunlight. Please consider this when designing an
182 installation. Many model rockets with payload bays use clear
183 plastic for the payload bay. Replacing these with an opaque
184 cardboard tube, painting them, or wrapping them with a layer of
185 masking tape are all reasonable approaches to keep the sensor
186 out of direct sunlight.
189 The barometric sensor sampling ports must be able to "breathe",
190 both by not being covered by foam or tape or other materials that might
191 directly block the hole on the top of the sensor, and also by having a
192 suitable static vent to outside air.
195 As with all other rocketry electronics, Altus Metrum altimeters must
196 be protected from exposure to corrosive motor exhaust and ejection
201 <title>The MicroPeak USB adapter</title>
203 MicroPeak stores barometric pressure information for the first
204 48 seconds of the flight in on-board non-volatile memory. The
205 contents of this memory can be downloaded to a computer using
206 the MicroPeak USB adapter.
209 <title>Installing the MicroPeak software</title>
211 The MicroPeak application runs on Linux, Mac OS X and
212 Windows. You can download the latest version from
213 <ulink url="http://altusmetrum.org/AltOS"/>.
216 On Mac OS X and Windows, the FTDI USB device driver needs to
217 be installed. A compatible version of this driver is included
218 with the MicroPeak application, but you may want to download a
219 newer version from <ulink
220 url="http://www.ftdichip.com/FTDrivers.htm"/>.
224 <title>Downloading Micro Peak data</title>
228 Plug the MicroPeak USB adapter in to your computer.
233 Start the MicroPeak application and click on the Download
234 button at the top of the window.
239 The MicroPeak USB adapter has a small phototransistor
240 under the hole in the center of the box.
241 Locate this, turn on the MicroPeak and place the orange LED on the MicroPeak
242 directly inside the hole, resting the MicroPeak itself on
243 the box. You should see the blue LED on the MicroPeak USB
244 adapter blinking in time with the orange LED on the
245 MicroPeak board itself.
250 After the maximum flight height is reported, MicroPeak will
251 pause for a few seconds, blink the LED four times rapidly
252 and then send the data in one long blur on the LED. The
253 MicroPeak application should receive the data. When it does,
254 it will present the data in a graph and offer to save the
255 data to a file. If not, you can power cycle the MicroPeak
262 <title>Analyzing MicroPeak Data</title>
264 The MicroPeak application can present flight data in the form
265 of a graph, a collection of computed statistics or in tabular
269 MicroPeak collects raw barometric pressure data which is
270 then used to compute the remaining data. Altitude is computed
271 through a standard atmospheric model. Absolute error in this
272 data will be affected by local atmospheric
273 conditions. Fortunately, these errors tend to mostly cancel
274 out, so the error in the height computation is much smaller
275 than the error in altitude would be.
278 Speed and acceleration are computed by first smoothing the
279 height data with a Gaussian window averaging filter. For speed
280 data, this average uses seven samples. For acceleration data,
281 eleven samples are used. These were chosen to provide
282 reasonably smooth speed and acceleration data, which would
283 otherwise be swamped with noise.
286 Under the Graph tab, the height, speed and acceleration values
287 are displayed together. You can zoom in on the graph by
288 clicking and dragging to sweep out an area of
289 interest. Right-click on the plot to bring up a menu that will
290 let you save, copy or print the graph.
293 The Statistics tab presents overall data from the flight. Note
294 that the Maximum height value is taken from the minumum
295 pressure captured in flight, and may be different from the
296 apparant apogee value as the on-board data are sampled twice
297 as fast as the recorded values, or because the true apogee
298 occurred after the on-board memory was full. Each value is
299 presented in several units as appropriate.
302 A table consisting of the both the raw barometric pressure
303 data and values computed from that for each recorded time.
306 The File menu has operations to open existing flight logs,
307 Download new data from MicroPeak, Save a copy of the flight
308 log to a new file, Export the tabular data (as seen in the Raw
309 Data tab) to a file, change the application Preferences, Close
310 the current window or close all windows and Exit the
315 <title>Configuring the MicroPeak application</title>
317 The MicroPeak application has a few user settings which are
318 configured through the Preferences dialog, which can be
319 accessed from the File menu.
323 The Log Directory is where flight data will be saved to
324 and loaded from by default. Of course, you can always
325 navigate to other directories in the file chooser windows,
326 this setting is just the starting point.
331 If you prefer to see your graph data in feet and
332 miles per hour instead of meters and meters per second,
333 you can select Imperial Units.
338 To see what data is actually arriving over the serial
339 port, start the MicroPeak application from a command
340 prompt and select the Serial Debug option. This can be
341 useful in debugging serial communication problems, but
342 most people need never choose this.
347 You can adjust the size of the text in the Statistics tab
348 by changing the Font size preference. There are three
349 settings, with luck one will both fit on your screen and
350 provide readable values.
355 The Look & feel menu shows a list of available
356 application appearance choices. By default, the MicroPeak
357 application tries to blend in with other applications, but
358 you may choose some other appearance if you like.
364 Note that MicroPeak shares a subset of the AltosUI
365 preferences, so if you use both of these applications, change
366 in one application will affect the other.
371 <title>Technical Information</title>
373 <title>Barometric Sensor</title>
375 MicroPeak uses the Measurement Specialties MS5607 sensor. This
376 has a range of 120kPa to 1kPa with an absolute accuracy of
377 150Pa and a resolution of 2.4Pa.
380 The pressure range corresponds roughly to an altitude range of
381 -1500m (-4900 feet) to 31000m (102000 feet), while the
382 resolution is approximately 20cm (8 inches) near sea level and
383 60cm (24in) at 10000m (33000 feet).
386 Ground pressure is computed from an average of 16 samples,
387 taken while the altimeter is at rest. The flight pressure used to
388 report maximum height is computed from a Kalman filter
389 designed to smooth out any minor noise in the sensor
390 values. The flight pressure recorded to non-volatile storage
391 is unfiltered, coming directly from the pressure sensor.
395 <title>Micro-controller</title>
397 MicroPeak uses an Atmel ATtiny85 micro-controller. This tiny
398 CPU contains 8kB of flash for the application, 512B of RAM for
399 temporary data storage and 512B of EEPROM for non-volatile
400 storage of previous flight data.
403 The ATtiny85 has a low-power mode which turns off all of the
404 clocks and powers down most of the internal components. In
405 this mode, the chip consumes only .1μA of power. MicroPeak
406 uses this mode once the flight has ended to preserve battery
411 <title>Lithium Battery</title>
413 The CR1025 battery used by MicroPeak holds 30mAh of power,
414 which is sufficient to run for over 40 hours. Because
415 MicroPeak powers down on landing, run time includes only time
416 sitting on the launch pad or during flight.
419 The large positive terminal (+) is usually marked, while the
420 smaller negative terminal is not. Make sure you install the
421 battery with the positive terminal facing away from the
422 circuit board where it will be in contact with the metal
423 battery holder. A small pad on the circuit board makes contact
424 with the negative battery terminal.
427 Shipping restrictions may prevent us from including a CR1025
428 battery with MicroPeak. If so, many stores carry CR1025
429 batteries as they are commonly used in small electronic
430 devices such as flash lights.
434 <title>Atmospheric Model</title>
436 MicroPeak contains a fixed atmospheric model which is used to
437 convert barometric pressure into altitude. The model was
438 converted into a 469-element piece-wise linear approximation
439 which is then used to compute the altitude of the ground and
440 apogee. The difference between these represents the maximum
441 height of the flight.
444 The model assumes a particular set of atmospheric conditions,
445 which, while a reasonable average, cannot represent the changing
446 nature of the real atmosphere. Fortunately, for flights
447 reasonably close to the ground, the effect of this global
448 inaccuracy are largely canceled out when the computed ground
449 altitude is subtracted from the computed apogee altitude, so
450 the resulting height is more accurate than either the ground
454 Because the raw pressure data is recorded to non-volatile
455 storage, you can use that, along with a more sophisticated
456 atmospheric model, to compute your own altitude values.
460 <title>Mechanical Considerations</title>
462 MicroPeak is designed to be rugged enough for typical rocketry
463 applications. It contains two moving parts, the battery holder
464 and the power switch, which were selected for their
468 The MicroPeak battery holder is designed to withstand impact
469 up to 150g without breaking contact (or, worse yet, causing
470 the battery to fall out). That means it should stand up to
471 almost any launch you care to try, and should withstand fairly
475 The power switch is designed to withstand up to 50g forces in
476 any direction. Because it is a sliding switch, orienting the
477 switch perpendicular to the direction of rocket travel will
478 serve to further protect the switch from launch forces.
482 <title>On-board data storage</title>
484 The ATtiny85 has 512 bytes of non-volatile storage, separate
485 from the code storage memory. The MicroPeak firmware uses this
486 to store information about the last completed
487 flight. Barometric measurements from the ground before launch
488 and at apogee are stored, and used at power-on to compute the
489 height of the last flight.
492 In addition to the data used to present the height of the last
493 flight, MicroPeak also stores barometric information sampled
494 at regular intervals during the flight. This is the
495 information captured with the MicroPeak USB adapter. It can
496 also be read from MicroPeak through any AVR programming
500 <title>MicroPeak EEPROM Data Storage</title>
501 <tgroup cols='3' align='center' colsep='1' rowsep='1'>
502 <colspec align='center' colwidth='2*' colname='Address'/>
503 <colspec align='center' colwidth='*' colname='Size (bytes)'/>
504 <colspec align='left' colwidth='7*' colname='Description'/>
507 <entry align='center'>Address</entry>
508 <entry align='center'>Size (bytes)</entry>
509 <entry align='center'>Description</entry>
516 <entry>Average ground pressure (Pa)</entry>
521 <entry>Minimum flight pressure (Pa)</entry>
526 <entry>Number of in-flight samples</entry>
529 <entry>0x00a … 0x1fe</entry>
531 <entry>Instantaneous flight pressure (Pa) low 16 bits</entry>
537 All EEPROM data are stored least-significant byte first. The
538 instantaneous flight pressure data are stored without the
539 upper 16 bits of data. The upper bits can be reconstructed
540 from the previous sample, assuming that pressure doesn't
541 change by more more than 32kPa in a single sample
542 interval. Note that this pressure data is <emphasis>not</emphasis>
543 filtered in any way, while both the recorded ground and apogee
544 pressure values are, so you shouldn't expect the minimum
545 instantaneous pressure value to match the recorded minimum
546 pressure value exactly.
549 MicroPeak samples pressure every 96ms, but stores only every
550 other sample in the EEPROM. This provides for 251 pressure
551 samples at 192ms intervals, or 48.192s of storage. The clock
552 used for these samples is a factory calibrated RC circuit
553 built into the ATtiny85 and is accurate only to within ±10% at
554 25°C. So, you can count on the pressure data being accurate,
555 but speed or acceleration data computed from this will be
556 limited by the accuracy of this clock.
560 <title>MicroPeak Programming Interface</title>
562 MicroPeak exposes a standard 6-pin AVR programming interface,
563 but not using the usual 2x3 array of pins on 0.1"
564 centers. Instead, there is a single row of tiny 0.60mm ×
565 0.85mm pads on 1.20mm centers exposed near the edge of the
566 circuit board. We couldn't find any connector that was
567 small enough to include on the circuit board.
570 In lieu of an actual connector, the easiest way to connect to
571 the bare pads is through a set of Pogo pins. These
572 spring-loaded contacts are designed to connect in precisely
573 this way. We've designed a programming jig, the MicroPeak
574 Pogo Pin board which provides a standard AVR interface on one
575 end and a recessed slot for MicroPeak to align the board with
579 The MicroPeak Pogo Pin board is not a complete AVR programmer,
580 it is an interface board that provides a 3.3V regulated power
581 supply to run the MicroPeak via USB and a standard 6-pin AVR
582 programming interface with the usual 2x3 grid of pins on 0.1"
583 centers. This can be connected to any AVR programming
587 The AVR programming interface cannot run faster than ¼ of the
588 AVR CPU clock frequency. Because MicroPeak runs at 250kHz to
589 save power, you must configure your AVR programming system to
590 clock the AVR programming interface at no faster than
591 62.5kHz, or a clock period of 32µS.
596 <!-- LocalWords: Altusmetrum MicroPeak