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[fw/altos] / doc / micropeak.xsl

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<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
  "/usr/share/xml/docbook/schema/dtd/4.5/docbookx.dtd">
<book>
  <title>MicroPeak Owner's Manual</title>
  <subtitle>A peak-recording altimeter for hobby rocketry</subtitle>
  <bookinfo>
    <author>
      <firstname>Keith</firstname>
      <surname>Packard</surname>
    </author>
    <copyright>
      <year>2012</year>
      <holder>Bdale Garbee and Keith Packard</holder>
    </copyright>
    <legalnotice>
      <para>
        This document is released under the terms of the
        <ulink url="http://creativecommons.org/licenses/by-sa/3.0/">
          Creative Commons ShareAlike 3.0
        </ulink>
        license.
      </para>
    </legalnotice>
    <revhistory>
      <revision>
        <revnumber>0.1</revnumber>
        <date>29 October 2012</date>
        <revremark>
          Initial release with preliminary hardware.
        </revremark>
      </revision>
      <revision>
        <revnumber>1.0</revnumber>
        <date>18 November 2012</date>
        <revremark>
          Updates for version 1.0 release.
        </revremark>
      </revision>
      <revision>
        <revnumber>1.1</revnumber>
        <date>12 December 2012</date>
        <revremark>
          Add comments about EEPROM storage format and programming jig.
        </revremark>
      </revision>
    </revhistory>
  </bookinfo>
  <acknowledgements>
    <para>
      Thanks to John Lyngdal for suggesting that we build something like this.
    </para>
    <para>
      Have fun using these products, and we hope to meet all of you
      out on the rocket flight line somewhere.
      <literallayout>
Bdale Garbee, KB0G
NAR #87103, TRA #12201

Keith Packard, KD7SQG
NAR #88757, TRA #12200
      </literallayout>
    </para>
  </acknowledgements>
  <chapter>
    <title>Quick Start Guide</title>
    <para>
      MicroPeak is designed to be easy to use. Requiring no external
      components, flying takes just a few steps
    </para>
    <itemizedlist>
      <listitem>
        <para>
          Install the battery. Fit a CR1025 battery into the plastic
          carrier. The positive (+) terminal should be towards the more
          open side of the carrier. Slip the carrier into the battery
          holder with the positive (+) terminal facing away from the
          circuit board.
        </para>
      </listitem>
      <listitem>
        <para>
          Install MicroPeak in your rocket. This can be as simple as
          preparing a soft cushion of wadding inside a vented model payload
          bay. Wherever you mount it, make sure you protect the
          barometric sensor from corrosive ejection gasses as those
          will damage the sensor, and shield it from light as that can
          cause incorrect sensor readings.
        </para>
      </listitem>
      <listitem>
        <para>
          Turn MicroPeak on. Slide the switch so that the actuator
          covers the '1' printed on the board. MicroPeak will report
          the maximum height of the last flight in decimeters using a
          sequence of flashes on the LED. A sequence of short flashes
          indicates one digit. A single long flash indicates zero. The
          height is reported in decimeters, so the last digit will be
          tenths of a meter. For example, if MicroPeak reports 5 4 4
          3, then the maximum height of the last flight was 544.3m, or
          1786 feet.
        </para>
      </listitem>
      <listitem>
        <para>
          Finish preparing the rocket for flight. After the
          previous flight data have been reported, MicroPeak waits for
          30 seconds before starting to check for launch. This gives
          you time to finish assembling the rocket. As those
          activities might cause pressure changes inside the airframe,
          MicroPeak might accidentally detect boost. If you need to do
          anything to the airframe after the 30 second window passes,
          make sure to be careful not to disturb the altimeter. The
          LED will remain dark during the 30 second delay, but after
          that, it will start blinking once every 3 seconds.
        </para>
      </listitem>
      <listitem>
        <para>
          Fly the rocket. Once the rocket passes about 10m in height
          (32 feet), the micro-controller will record the ground
          pressure and track the pressure seen during the flight. In
          this mode, the LED flickers rapidly. When the rocket lands,
          and the pressure stabilizes, the micro-controller will record
          the minimum pressure pressure experienced during the flight,
          compute the height represented by the difference in air
          pressure and blink that value out on the LED. After that,
          MicroPeak powers down to conserve battery power.
        </para>
      </listitem>
      <listitem>
        <para>
          Recover the data. Turn MicroPeak off and then back on. MicroPeak
          will blink out the maximum height for the last flight. Turn
          MicroPeak back off to conserve battery power.
        </para>
      </listitem>
    </itemizedlist>
  </chapter>
  <chapter>
    <title>Handling Precautions</title>
    <para>
      All Altus Metrum products are sophisticated electronic devices.  
      When handled gently and properly installed in an air-frame, they
      will deliver impressive results.  However, as with all electronic 
      devices, there are some precautions you must take.
    </para>
    <para>
      The CR1025 Lithium batteries have an
      extraordinary power density.  This is great because we can fly with
      much less battery mass... but if they are punctured
      or their contacts are allowed to short, they can and will release their
      energy very rapidly!
      Thus we recommend that you take some care when handling MicroPeak
      to keep conductive material from coming in contact with the exposed metal elements.
    </para>
    <para>
      The barometric sensor used in MicroPeak is sensitive to
      sunlight. Please consider this when designing an
      installation. Many model rockets with payload bays use clear
      plastic for the payload bay. Replacing these with an opaque
      cardboard tube, painting them, or wrapping them with a layer of
      masking tape are all reasonable approaches to keep the sensor
      out of direct sunlight.
    </para>
    <para>
      The barometric sensor sampling ports must be able to "breathe",
      both by not being covered by foam or tape or other materials that might
      directly block the hole on the top of the sensor, and also by having a
      suitable static vent to outside air.
    </para>
    <para>
      As with all other rocketry electronics, Altus Metrum altimeters must 
      be protected from exposure to corrosive motor exhaust and ejection 
      charge gasses.
    </para>
  </chapter>
  <chapter>
    <title>Technical Information</title>
    <section>
      <title>Barometric Sensor</title>
      <para>
        MicroPeak uses the Measurement Specialties MS5607 sensor. This
        has a range of 120kPa to 1kPa with an absolute accuracy of
        150Pa and a resolution of 2.4Pa.
      </para>
      <para>
        The pressure range corresponds roughly to an altitude range of
        -1500m (-4900 feet) to 31000m (102000 feet), while the
        resolution is approximately 20cm (8 inches) near sea level and
        60cm (24in) at 10000m (33000 feet).
      </para>
      <para>
        Ground pressure is computed from an average of 16 samples,
        taken while the altimeter is at rest. Flight pressure is
        computed from an exponential IIR filter designed to smooth out
        transients caused by mechanical stress on the barometer.
      </para>
    </section>
    <section>
      <title>Micro-controller</title>
      <para>
        MicroPeak uses an Atmel ATtiny85 micro-controller. This tiny
        CPU contains 8kB of flash for the application, 512B of RAM for
        temporary data storage and 512B of EEPROM for non-volatile
        storage of previous flight data.
      </para>
      <para>
        The ATtiny85 has a low-power mode which turns off all of the
        clocks and powers down most of the internal components. In
        this mode, the chip consumes only .1μA of power. MicroPeak
        uses this mode once the flight has ended to preserve battery
        power.
      </para>
    </section>
    <section>
      <title>Lithium Battery</title>
      <para>
        The CR1025 battery used by MicroPeak holes 30mAh of power,
        which is sufficient to run for over 40 hours. Because
        MicroPeak powers down on landing, run time includes only time
        sitting on the launch pad or during flight.
      </para>
      <para>
        The large positive terminal (+) is usually marked, while the
        smaller negative terminal is not. Make sure you install the
        battery with the positive terminal facing away from the
        circuit board where it will be in contact with the metal
        battery holder. A small pad on the circuit board makes contact
        with the negative battery terminal.
      </para>
      <para>
        Shipping restrictions may prevent us from including a CR1025
        battery with MicroPeak. If so, many stores carry CR1025
        batteries as they are commonly used in small electronic
        devices such as flash lights.
      </para>
    </section>
    <section>
      <title>Atmospheric Model</title>
      <para>
        MicroPeak contains a fixed atmospheric model which is used to
        convert barometric pressure into altitude. The model was
        converted into a 469-element piece wise linear approximation
        which is then used to compute the altitude of the ground and
        apogee. The difference between these represents the maximum
        height of the flight.
      </para>
      <para>
        The model assumes a particular set of atmospheric conditions,
        which while a reasonable average cannot represent the changing
        nature of the real atmosphere. Fortunately, for flights
        reasonably close to the ground, the effect of this global
        inaccuracy are largely canceled out when the computed ground
        altitude is subtracted from the computed apogee altitude, so
        the resulting height is more accurate than either the ground
        or apogee altitudes.
      </para>
    </section>
    <section>
      <title>Mechanical Considerations</title>
      <para>
        MicroPeak is designed to be rugged enough for typical rocketry
        applications. It contains two moving parts, the battery holder
        and the power switch, which were selected for their
        ruggedness.
      </para>
      <para>
        The MicroPeak battery holder is designed to withstand impact
        up to 150g without breaking contact (or, worse yet, causing
        the battery to fall out). That means it should stand up to
        almost any launch you care to try, and should withstand fairly
        rough landings.
      </para>
      <para>
        The power switch is designed to withstand up to 50g forces in
        any direction. Because it is a sliding switch, orienting the
        switch perpendicular to the direction of rocket travel will
        serve to further protect the switch from launch forces.
      </para>
    </section>
    <section>
      <title>On-board data storage</title>
      <para>
        The ATtiny85 has 512 bytes of non-volatile storage, separate
        from the code storage memory. The MicroPeak firmware uses this
        to store information about the last completed
        flight. Barometric measurements from the ground before launch
        and at apogee are stored, and used at power-on to compute the
        height of the last flight.
      </para>
      <para>
        In addition to the data used to present the height of the last
        flight, MicroPeak also stores barometric information sampled
        at regular intervals during the flight. This information can
        be extracted from MicroPeak through any AVR programming
        tool.
      </para>
      <table frame='all'>
        <title>MicroPeak EEPROM Data Storage</title>
        <tgroup cols='3' align='center' colsep='1' rowsep='1'>
          <colspec align='center' colwidth='2*' colname='Address'/>
          <colspec align='center' colwidth='*' colname='Size (bytes)'/>
          <colspec align='left' colwidth='7*' colname='Description'/>
          <thead>
            <row>
              <entry align='center'>Address</entry>
              <entry align='center'>Size (bytes)</entry>
              <entry align='center'>Description</entry>
            </row>
          </thead>
          <tbody>
            <row>
              <entry>0x000</entry>
              <entry>4</entry>
              <entry>Average ground pressure (Pa)</entry>
            </row>
            <row>
              <entry>0x004</entry>
              <entry>4</entry>
              <entry>Minimum flight pressure (Pa)</entry>
            </row>
            <row>
              <entry>0x008</entry>
              <entry>2</entry>
              <entry>Number of in-flight samples</entry>
            </row>
            <row>
              <entry>0x00a … 0x1fe</entry>
              <entry>2</entry>
              <entry>Instantaneous flight pressure (Pa) low 16 bits</entry>
            </row>
          </tbody>
        </tgroup>
      </table>
      <para>
        All EEPROM data are stored least-significant byte first. The
        instantaneous flight pressure data are stored without the
        upper 16 bits of data. The upper bits can be reconstructed
        from the previous sample, assuming that pressure doesn't
        change by more more than 32kPa in a single sample
        interval. Note that this pressure data is <emphasis>not</emphasis>
        filtered in any way, while both the recorded ground and apogee
        pressure values are, so you shouldn't expect the minimum
        instantaneous pressure value to match the recorded minimum
        pressure value exactly.
      </para>
      <para>
        MicroPeak samples pressure every 96ms, but stores only every
        other sample in the EEPROM. This provides for 251 pressure
        samples at 192ms intervals, or 48.192s of storage. The clock
        used for these samples is a factory calibrated RC circuit
        built into the ATtiny85 and is accurate only to within ±10% at
        25°C. So, you can count on the pressure data being accurate,
        but speed or acceleration data computed from this will be
        limited by the accuracy of this clock.
      </para>
    </section>
    <section>
      <title>MicroPeak Programming Interface</title>
      <para>
        MicroPeak exposes a standard 6-pin AVR programming interface,
        but not using the usual 2x3 array of pins on 0.1"
        centers. Instead, there is a single row of tiny 0.60mm ×
        0.85mm pads on 1.20mm centers exposed near the edge of the
        circuit board. We couldn't find any connector that was
        small enough to include on the circuit board.
      </para>
      <para>
        In lieu of an actual connector, the easiest way to connect to
        the bare pads is through a set of Pogo pins. These
        spring-loaded contacts are designed to connect in precisely
        this way. We've designed a programming jig, the MicroPeak
        Pogo Pin board which provides a standard AVR interface on one
        end and a recessed slot for MicroPeak to align the board with
        the Pogo Pins.
      </para>
      <para>
        The MicroPeak Pogo Pin board is not a complete AVR programmer,
        it is an interface board that provides a 3.3V regulated power
        supply to run the MicroPeak via USB and a standard 6-pin AVR
        programming interface with the usual 2x3 grid of pins on 0.1"
        centers. This can be connected to any AVR programming
        dongle.
      </para>
      <para>
        The AVR programming interface cannot run faster than ¼ of the
        AVR CPU clock frequency. Because MicroPeak runs at 250kHz to
        save power, you must configure your AVR programming system to
        clock the AVR programming interface at no faster than
        62.5kHz, or a clock period of 32µS.
      </para>
    </section>
  </chapter>
</book>
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