Power, Power Control Boards, Watchdog

power control schemtaicPower to the HRP-II is supplied by two stacks of lithium "D" batteries comprised of seven cells each installed in a pressure case mounted between the skin and the main pressure housing.  Each stack provides 24 volts, and is specified at 15AHr @ 175mA (~500 watt-hours for both).  Lithium batteries were selected for their flat discharge profile and high current capacity.  The power supply is isolated by diodes from the rest of the electronics.

Four power control boards convert the input voltage to levels required by the computer and sensors.  The first converts to five volts, which powers the computer and other three power control boards; which then output 12, 15 and 12 volts respectively.   The power control boards were designed for the profiler and run embedded software written in C to perform the appropriate switching and monitoring tasks.    

To allow monitoring of various system status indicators, the power control board that outputs 5 volts functions as a "watchdog" that also monitors pressure, range and time, and will release the descent weights in case the logger fails to terminate the dive appropriately.  A dive would be terminated if a low voltage condition is detected, or the logger program stops running or if the pressure data from the CTD is bad.

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The profiler's main computer is a low power 200Mhz 386 PC104 with an 8-port serial card and a 16-bit A/D converter card.  PC104 is a form standard widely used for small computers, so replacement parts should be available in the years to come.  The computer gets power directly from the power control board that outputs five volts, since the operation of the main computer is mission critical.

PC104 stack and power control boards

The photograph shows the PC104 computer stack (right), A/D filters (center), and power control boards (left) used in HRP-II.

The autonomous operation of the HRP-II is controlled by “logger” software written in C running on Windows 2000.  The logger program was developed using National Instruments CVI software tools, which supplies functions to handle everything from the Graphical User Interface (GUI) to the serial and A/D acquisition.

The logger program is the primary brain of the profiler.  It was designed to allow easy reconfiguration of the sensors employed.  Any sensor may be used or de-selected prior to each dive by clicking a button on the GUI interface.  Adding or replacing sensors is enabled by modifying the sensor connectivity table to include the new sensor id, port or channel, baud rate, gain, and power switch setting.   An example of this table (called att_tab.asc) showing the configuration used on the test cruise is listed here

The logger is also responsible for dive configuration, sensor control, simultaneous acquisition and logging of data from the configured sensors (in this case: five serial sensors and 10 A/D channels) during descent, along with real-time monitoring of pressure, range and time as potential dive termination criteria.  All data is stored in memory during the downcast and written to disk at the end of the profile after the weights are released to eliminate any vibrations resulting from disk activity.  Data logging on the upcast is possible, but hasn't been used yet.

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RS485 commsInternal communications

The protocol used for communications between the logger program on the PC104 stack and the Watchdog and power control boards is RS485, which allows two-way communication shared among multiple nodes.  Our method is based on the logger program (controller) being the main talker, with the power control boards listening for whether the message applies to them, then acting accordingly- this block diagram shows the general scheme we used.  The Rottweiler protocol is not documented on the web.

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A variety of sensors used to measure the smallest to the largest scales of mixing in the ocean were chosen for use on HRP-II.  The underwater movement from the GPS data determines the largest scales, and the shear probes the smallest.  Five sensors output ASCII serial data for logging, and ten sensors are connected to the A/D converter and logged as binary data.   The data from each sensor is logged in a separate file.  The synchronization of data from the files is obtained by powering up the configured sensors well before the start of logging, so data is streaming from the sensors well before the logger starts saving the data.  Simultaneity of logging start has been verified in bench tests in the lab.  Time words embedded in several of the data streams is further used to quantify drift of the various clocks.

A list of the sensors and their manufacturers is detailed here.  The configuration of the sensors used on the test cruise with the port and power control connections are also described.  A short description of each sensor is provided below.


In the HRP-II, X is defined as “up” when the internal electronics chassis is horizontal as used in bench testing.  Internally, North on the compass is aligned to X by how the electronics are attached to the chassis.  The accelerometers are rigidly mounted in pairs, with one 90° from the other.  One pair is bolted to the bottom end cap, and the other to the upper. The bolt hole positions ensure that one accellerometer of each pair is aligned with X.  The ACM transducer sting had to be installed at a known offset of 22.5° to HRP-II X because of space limitations on the end cap, and to keep the transducer arms from interfering with the release weights and the mud extractor.

The electronics chassis (and sensors attached to the lower endcap) is always installed in the same orientation relative to the body, which allows sensors mounted on the upper endcap to be aligned with the rest.  In the photo, HRP-II is in it's cradle to allow access to the weight dropper doors which puts +X pointing down and right as viewed from the bottom.  Since the upper end cap is hidden by the skin, a schematic was drawn to show what it would look like when viewed from the top.  +X appears to be pointing the wrong way, but think about a mirror image, and you realize the drawing is correct.  The collar supporting the EF sensors should not be secured rigidly to the body due to noise issues, so it floats.  However, efforts were made to ensure that EF-1 remained aligned with the instrument X.

lower enccap and sensors  

upper endcap drawing

During the test cruise, the sensors were aligned as follows:
    with X: accelerometers 2 (top) and 4 (bottom), EF 1 and compass N
    with Y: accelerometers 1 (top) and 3 (bottom), EF 2 and compass E

The raw acoustic travel times were used to compute U & V velocities relative to the sting,  which were then rotated by 22.5°  to obtain X & Y velocities corresponding to the accellerometer and EF sensor data.

The alignment of the sensors to each other and to the instrument body is critical for successful analysis of the velocity and microstructure data.  The HRP-II descends with the sensor end down, and the long axis of the body more or less verical.  It was designed to rotate around its long axis and oscillate slowly as it collects data.  The compass data describes the rotations and the accelerometer data quantifies the oscillations during a profile.  Later, in post processing, these movements are removed from the velocity data to obtain earth referenced relative velocity.   The EF sensor data describes larger scale motions, and also must employ compass and accelerometer data to adjust it for vehicle body motion.  Then the ACM, EF and GPS data are combined to obtain oceanic absolute velocity.

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HRP-II schematicThe body of the HRP-II is the structure that carries and protects the sensors and electronics during operation.   The body consists of five major parts: the electronics pressure case, the support and integration elements (exoskeleton, lifting bail and skin), the floatation, the battery packs, and the releases.  The  dimensions of the vehicle, along with the major structural elements are shown at the left.

The pressure housing was made of 8” internal diameter, 1" wall 7075-T6 aluminum tube, anodized to prevent corrosion.  The endcaps were fabricated of the same material.  The largish diameter was dictated partially by the size of the electronics, and partially to allow a big enough endcap (10") for all the sensors to fit without interfering with each other.  The dimensions and materials were also selected to minimize vibrational noise, and facilitate quiet data measurement.  Titanium rods and a truss connect the pressure housing  through the buoyancy element to the lifting bail at the top, assuring a "stiff" body.

The amount of syntactic foam attached at the top controls the rates of vehicle descent and ascent.  The desired nominal descent rate is 0.6m/s, which was achieved as drawn, after the addition of ~30 lbs of lead shot attached near the lower endcap.

The skin was fabricated from two cylinders of polypropylene.  By minimizing the number of seams the frictional effects on body motion were decreased.  The collar holding the two EF sensors is mounted mid-body between the  two cylinders.

Power is supplied to the controller and sensors from two battery packs mounted between the pressure housing and the skin.  Each pack can supply adequate power for operations independently.   The solenoids that function to release the descent weights are also housed under the skin, but outside the pressure case.

chassis viewInside the pressure case, a chassis supports the computer, power control and filter boards, , as well as the electronics for the CTD, MAVS, compass, EF sensors, altimeter  and 12 KHz pinger.   Electrical noise interference was minimized by physical separation and internal partitions separating the sub-systems.  The chassis layout with some of the electronics installed is shown in the photo at the right.  The lower endcap (where the CTD and ACM stings will eventually be attached) is at the left edge of the picture. The cables in the foreground lead to a monitor, keyboard and mouse that are connected directly to the computer when debugging.  When the chassis is in the pressure housing, communications with the controller is via an ethernet connection through the upper endcap.  The connection to the upper endcap (permanantly mounted in the body) is made using two blind-mate connectors that aren’t visible at the right of the picture.  These are the mecanism that ensures the alignment of the upper and lower endcaps.

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