[fw/altos] / doc / micropeak.txt

= MicroPeak Owner's Manual
Keith Packard <keithp@keithp.com>; Bdale Garbee <bdale@gag.com>
:revnumber: v1.9
:revdate: 8 Oct 2018
:copyright: Bdale Garbee and Keith Packard 2018
:stylesheet: am.css
:linkcss:
:toc:
:doctype: book
:numbered:
:pdf-stylesdir: .
:pdf-style: altusmetrum
:pdf-fontsdir: fonts

        include::header.adoc[]

[dedication]
== Acknowledgements

        Thanks to John Lyngdal for suggesting that we build something
        like this.

        Have fun using these products, and we hope to meet all of you
        out on the rocket flight line somewhere.

        [verse]
        Bdale Garbee, KB0G
        NAR #87103, TRA #12201

        [verse]
        Keith Packard, KD7SQG
        NAR #88757, TRA #12200

== Using MicroPeak

        MicroPeak is designed to be easy to use. Requiring no external
        components, flying takes just a few steps

        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.

                .MicroPeak and Battery
                image::micropeak-back.jpg[width=430]

        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.

        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.

        Finish preparing the rocket for flight::

          After the previous flight data have been reported, MicroPeak
          waits for one minute 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
          one minute window passes, make sure to be careful not to
          disturb the altimeter. The LED will remain dark during the
          one minute delay, but after that, it will start blinking
          once every 3 seconds.

        Fly the rocket::

          Once the rocket passes about 30m in height (100 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.

        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.

== The MicroPeak USB adapter

        .MicroPeak USB Adapter
        image::MicroPeakUSB-2.0.jpg[width=430,align="center"]

        MicroPeak stores barometric pressure information for the first
        48 seconds of the flight in on-board non-volatile memory. The
        contents of this memory can be downloaded to a computer using
        the MicroPeak USB adapter.

        === Installing the MicroPeak software

                The MicroPeak application runs on Linux, Mac OS X and
                Windows. You can download the latest version from
                http://altusmetrum.org/MicroPeak

                On Mac OS X and Windows, the FTDI USB device driver
                needs to be installed. A compatible version of this
                driver is included with the MicroPeak application, but
                you may want to download a newer version from
                http://www.ftdichip.com/FTDrivers.htm

        === Downloading Micro Peak data

                * Plug the MicroPeak USB adapter in to your computer.

                * Start the MicroPeak application.

                  image::micropeak-nofont.svg[width=50,align="center"]

                * Click on the Download button at the top of the
                  window.

                  .MicroPeak Application
                  image::micropeak-app.png[width=430,align="center"]

                * Select from the listed devices. There will probably
                  be only one.

                  .MicroPeak Device Dialog
                  image::micropeak-device-dialog.png[width=220,align="center"]

                * The application will now wait until it receives
                  valid data from the MicroPeak USB adapter.

                  .MicroPeak Download Dialog
                  image::micropeak-download.png[width=200,align="center"]

                * The MicroPeak USB adapter has a small
                  phototransistor under the hole in the center of the
                  box.  Locate this, turn on the MicroPeak and place
                  the orange LED on the MicroPeak directly inside the
                  hole, resting the MicroPeak itself on the box. You
                  should see the blue LED on the MicroPeak USB adapter
                  blinking in time with the orange LED on the
                  MicroPeak board itself.

                  .MicroPeak Downloading
                  image::MicroPeakUSB-2.0-inuse.jpg[width=430,align="center"]

                * After the maximum flight height is reported,
                  MicroPeak will pause for a few seconds, blink the
                  LED four times rapidly and then send the data in one
                  long blur on the LED. The MicroPeak application
                  should receive the data. When it does, it will
                  present the data in a graph and offer to save the
                  data to a file. If not, you can power cycle the
                  MicroPeak board and try again.

                  .MicroPeak Save Dialog
                  image::micropeak-save-dialog.png[width=220,align="center"]

                * Once the data are saved, a graph will be displayed
                  with height, speed and acceleration values computed
                  from the recorded barometric pressure data. See
                  <<_analyzing_micropeak_data> for more details on that.

        === Analyzing MicroPeak Data

                The MicroPeak application can present flight data in
                the form of a graph, a collection of computed
                statistics or in tabular form.

                MicroPeak collects raw barometric pressure data which
                is then used to compute the remaining data. Altitude
                is computed through a standard atmospheric
                model. Absolute error in this data will be affected by
                local atmospheric conditions. Fortunately, these
                errors tend to mostly cancel out, so the error in the
                height computation is much smaller than the error in
                altitude would be.

                Speed and acceleration are computed by first smoothing
                the height data with a Gaussian window averaging
                filter. For speed data, this average uses seven
                samples. For acceleration data, eleven samples are
                used. These were chosen to provide reasonably smooth
                speed and acceleration data, which would otherwise be
                swamped with noise.

                The File menu has operations to open existing flight
                logs, Download new data from MicroPeak, Save a copy of
                the flight log to a new file, Export the tabular data
                (as seen in the Raw Data tab) to a file, change the
                application Preferences, Close the current window or
                close all windows and Exit the application.

                ==== MicroPeak Graphs

                        .MicroPeak Graph
                        image::micropeak-graph.png[width=430,align="center"]

                        Under the Graph tab, the height, speed and acceleration values
                        are displayed together. You can zoom in on the graph by
                        clicking and dragging to sweep out an area of
                        interest. Right-click on the plot to bring up a menu that will
                        let you save, copy or print the graph.

                ==== MicroPeak Flight Statistics

                        .MicroPeak Flight Statistics
                        image::micropeak-statistics.png[width=430,align="center"]

                        The Statistics tab presents overall data from
                        the flight. Note that the Maximum height value
                        is taken from the minumum pressure captured in
                        flight, and may be different from the apparant
                        apogee value as the on-board data are sampled
                        twice as fast as the recorded values, or
                        because the true apogee occurred after the
                        on-board memory was full. Each value is
                        presented in several units as appropriate.

                ==== Raw Flight Data

                        .MicroPeak Raw Flight Data
                        image::micropeak-raw-data.png[width=430,align="center"]

                        A table consisting of the both the raw barometric pressure
                        data and values computed from that for each recorded time.

                ==== Configuring the Graph

                        .MicroPeak Graph Configuration
                        image::micropeak-graph-configure.png[width=430,align="center"]

                        This selects which graph elements to show, and lets you
                        switch between metric and imperial units

        === Setting MicroPeak Preferences

                        .MicroPeak Preferences
                        image::micropeak-preferences.png[width=170,align="center"]

                        The MicroPeak application has a few user settings which are
                        configured through the Preferences dialog, which can be
                        accessed from the File menu.

                        Log Directory::

                        The Log Directory is where flight data will be
                        saved to and loaded from by default. Of
                        course, you can always navigate to other
                        directories in the file chooser windows, this
                        setting is just the starting point.

                        Imperial Units::

                        If you prefer to see your graph data in feet
                        and miles per hour instead of meters and
                        meters per second, you can select Imperial
                        Units.

                        Serial Debug::

                        To see what data is actually arriving over the
                        serial port, start the MicroPeak application
                        from a command prompt and select the Serial
                        Debug option. This can be useful in debugging
                        serial communication problems, but most people
                        need never choose this.

                        Font Size::

                        You can adjust the size of the text in the
                        Statistics tab by changing the Font size
                        preference. There are three settings, with
                        luck one will both fit on your screen and
                        provide readable values.

                        Look & Feel::

                        The Look & feel menu shows a list of available
                        application appearance choices. By default,
                        the MicroPeak application tries to blend in
                        with other applications, but you may choose
                        some other appearance if you like.

                        Note that MicroPeak shares a subset of the
                        AltosUI preferences, so if you use both of
                        these applications, change in one application
                        will affect the other.

== Protecting MicroPeak from Sunlight

        The MS5607 barometric sensor is sensitive to direct light. When light
        shines through the holes in the cover to the components inside, the
        reported pressure can vary wildly from the actual pressure. This
        causes the reported altitude to have errors of as much as 3000-4000
        feet.

        MicroPeak should be installed in an opaque compartment in the airframe
        and not subjected to sunlight. Alternatively, a small piece of
        adhesive-backed open-cell foam can be attached to the device so that
        it covers the barometric sensor and protects it from direct light.

        Here's what happens when MicroPeak is exposed to sunlight. At apogee,
        I exposed MicroPeak to varying amounts of sunlight and you can see the
        wild swings in altitude resulting from that:

        .MicroPeak in Sunlight
        image::micropeak-flight-nofoam.png[width=430]

        You can carefully cutting a piece of adhesive-backed open-cell foam
        and attach it to MicroPeak. It's important to press the adhesive to
        the circuit board and not to the top of the barometric sensor or the
        sensor may become blocked and not operate at all. Once you've attached
        the foam, you should test MicroPeak on the ground to make sure it's
        still working.

        .MicroPeak with Foam
        image::micropeak-foam.jpg[width=430]

        That MicroPeak was in the same barometric chamber as the one which
        generated the above results and the resulting flight data looks
        correct:

        .MicroPeak in Sunlight with Foam
        image::micropeak-flight-foam.png[width=430]

[appendix]
== Handling Precautions

        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.

        [WARNING]

        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.

        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.

        The barometric sensor sampling ports must be able to
        "breathe", both by not being covered by solid 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.

        One good solution is to use a small rectangle of Poron 
        50-30031-12X12P or equivalent to cover the sensor.  This is an
        open cell foam in 1/32" thickness with an adhesive backing.  It
        seems to do a good job of blocking sun while still allowing
        airflow to and from the sensor internals.

        As with all other rocketry electronics, Altus Metrum
        altimeters must be protected from exposure to corrosive motor
        exhaust and ejection charge gasses.

[appendix]
== Technical Information

        === Barometric Sensor

                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.

                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).

                Ground pressure is computed from an average of 16
                samples, taken while the altimeter is at rest. The
                flight pressure used to report maximum height is
                computed from a Kalman filter designed to smooth out
                any minor noise in the sensor values. The flight
                pressure recorded to non-volatile storage is
                unfiltered, coming directly from the pressure sensor.

        === Micro-controller

                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.

                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.

        === Lithium Battery

                The CR1025 battery used by MicroPeak holds 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.

                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.

                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.

        === Atmospheric Model

                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.

                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.

                Because the raw pressure data is recorded to
                non-volatile storage, you can use that, along with a
                more sophisticated atmospheric model, to compute your
                own altitude values.

        === Mechanical Considerations

                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.

                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.

                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.

        === MicroPeak Programming Interface

                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.

                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.

                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.

                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.

[appendix]
== On-board data storage

        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.

        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 is the
        information captured with the MicroPeak USB adapter. It can
        also be read from MicroPeak through any AVR programming tool.


        .MicroPeak EEPROM Data Storage
        [options="border",cols="2,1,7"]
        |====
        |Address        |Size (bytes)   |Description
        |0x000          |4              |Average ground pressure (Pa)
        |0x004          |4              |Minimum flight pressure (Pa)
        |0x008          |2              |Number of in-flight samples
        |0x00a … 0x1fe        |2              |Instantaneous flight pressure (Pa) low 16 bits
        |====

        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 *not* 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.

        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.