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5 <title>MicroPeak Owner's Manual</title>
6 <subtitle>A peak-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.
37 Thanks to John Lyngdal for suggesting that we build something like this.
40 Have fun using these products, and we hope to meet all of you
41 out on the rocket flight line somewhere.
44 NAR #87103, TRA #12201
47 NAR #88757, TRA #12200
52 <title>Quick Start Guide</title>
54 MicroPeak is designed to be easy to use. Requiring no external
55 components, flying takes just a few steps
60 Install the battery. Fit a CR1025 battery into the plastic
61 carrier. The positive (+) terminal should be towards the more
62 open side of the carrier. Slip the carrier into the battery
63 holder with the positive (+) terminal facing away from the
69 Install MicroPeak in your rocket. This can be as simple as
70 preparing a soft cushion of wadding inside a vented model payload
71 bay. Wherever you mount it, make sure you protect the
72 barometric sensor from corrosive ejection gasses as those
73 will damage the sensor.
78 Turn MicroPeak on. Slide the switch so that the actuator
79 covers the '1' printed on the board. MicroPeak will report
80 the maximum height of the last flight in decimeters using a
81 sequence of flashes on the LED. A sequence of short flashes
82 indicates one digit. A single long flash indicates zero. The
83 height is reported in decimeters, so the last digit will be
84 tenths of a meter. For example, if MicroPeak reports 5 4 4
85 3, then the maximum height of the last flight was 544.3m, or
86 1786 feet. After reporting the last flight, MicroPeak starts
87 waiting for launch. It will flash once every three seconds
93 Fly the rocket. Once the rocket passes about 4m in height
94 (13 feet), the micro-controller will record the ground
95 pressure and track the pressure seen during the flight. In
96 this mode, the LED flickers rapidly. When the rocket lands,
97 and the pressure stabilizes, the micro-controller will record
98 the minimum pressure pressure experienced during the flight,
99 compute the height represented by the difference in air
100 pressure and blink that value out on the LED. After that,
101 MicroPeak powers down to conserve battery power.
106 Recover the data. Turn MicroPeak off for a couple of seconds
107 (to discharge the capacitors) and then back on. MicroPeak
108 will blink out the maximum height for the last flight. Turn
109 MicroPeak back off to conserve battery power.
115 <title>Handling Precautions</title>
117 All Altus Metrum products are sophisticated electronic devices.
118 When handled gently and properly installed in an air-frame, they
119 will deliver impressive results. However, as with all electronic
120 devices, there are some precautions you must take.
123 The CR1025 Lithium batteries have an
124 extraordinary power density. This is great because we can fly with
125 much less battery mass... but if they are punctured
126 or their contacts are allowed to short, they can and will release their
128 Thus we recommend that you take some care when handling MicroPeak
129 to keep conductive material from coming in contact with the exposed metal elements.
132 The barometric sensors used in MicroPeak is
133 sensitive to sunlight. Please consider this when
134 designing an installation, for example, in an air-frame with a
135 see-through plastic payload bay. Many model rockets with payload bays
136 use clear plastic for the payload bay. Replacing these with an opaque
137 cardboard tube, painting them, or wrapping them with a layer of masking
138 tape are all reasonable approaches to keep the sensor out of direct
142 The barometric sensor sampling ports must be able to "breathe",
143 both by not being covered by foam or tape or other materials that might
144 directly block the hole on the top of the sensor, and also by having a
145 suitable static vent to outside air.
148 As with all other rocketry electronics, Altus Metrum altimeters must
149 be protected from exposure to corrosive motor exhaust and ejection
154 <title>Technical Information</title>
156 <title>Barometric Sensor</title>
158 MicroPeak uses the Measurement Specialties MS5607 sensor. This
159 has a range of 120kPa to 1kPa with an absolute accuracy of
160 150Pa and a resolution of 2.4Pa.
163 The pressure range corresponds roughly to an altitude range of
164 -1500m (-4900 feet) to 31000m (102000 feet), while the
165 resolution is approximately 20cm (8 inches) near sea level and
166 60cm (24in) at 10000m (33000 feet).
169 Ground pressure is computed from an average of 16 samples,
170 taken while the altimeter is at rest. Flight pressure is
171 computed from an exponential IIR filter designed to smooth out
172 transients caused by mechanical stress on the barometer.
176 <title>Micro-controller</title>
178 MicroPeak uses an Atmel ATtiny85 micro-controller. This tiny
179 CPU contains 8kB of flash for the application, 512B of RAM for
180 temporary data storage and 512B of EEPROM for non-volatile
181 storage of previous flight data.
184 The ATtiny85 has a low-power mode which turns off all of the
185 clocks and powers down most of the internal components. In
186 this mode, the chip consumes only .1μA of power. MicroPeak
187 uses this mode once the flight has ended to preserve battery
192 <title>Lithium Battery</title>
194 The CR1025 battery used by MicroPeak holes 30mAh of power,
195 which is sufficient to run for over 15 hours. Because
196 MicroPeak powers down on landing, run time includes only time
197 sitting on the launch pad or during flight.
200 The large positive terminal (+) is usually marked, while the
201 smaller negative terminal is not. Make sure you install the
202 battery with the positive terminal facing away from the
203 circuit board where it will be in contact with the metal
204 battery holder. A small pad on the circuit board makes contact
205 with the negative battery terminal.
208 Shipping restrictions prevent us from including a CR1025
209 battery with MicroPeak. Many stores carry CR1025 batteries as
210 they are commonly used in small electronic devices such as
215 <title>Atmospheric Model</title>
217 MicroPeak contains a fixed atmospheric model which is used to
218 convert barometric pressure into altitude. The model was
219 converted into a 469-element piece wise linear approximation
220 which is then used to compute the altitude of the ground and
221 apogee. The difference between these represents the maximum
222 height of the flight.
225 The model assumes a particular set of atmospheric conditions,
226 which while a reasonable average cannot represent the changing
227 nature of the real atmosphere. Fortunately, for flights
228 reasonably close to the ground, the effect of this global
229 inaccuracy are largely canceled out when the computed ground
230 altitude is subtracted from the computed apogee altitude, so
231 the resulting height is more accurate than either the ground
236 <title>Mechanical Considerations</title>
238 MicroPeak is designed to be rugged enough for typical rocketry
239 applications. It contains two moving parts, the battery holder
240 and the power switch, which were selected for their
244 The MicroPeak battery holder is designed to withstand impact
245 up to 150g without breaking contact (or, worse yet, causing
246 the battery to fall out). That means it should stand up to
247 almost any launch you care to try, and should withstand fairly
251 The power switch is designed to withstand up to 50g forces in
252 any direction. Because it is a sliding switch, orienting the
253 switch perpendicular to the direction of rocket travel will
254 serve to further protect the switch from launch forces.
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