The shutters were developed to reduce the effect of biofouling
on a variety of optical instruments in use on our moored platforms. The
initial development work was done using the Biospherical model PRR-600T2
spectroradiometer. The concept is to cover the optical windows of the spectroradiometers
with a copper disk when not making a measurement, thereby inhibiting the
growth of fouling organisms. The shutter is a system comprising two parts;
the mechanism which contains the motor, gears, feedback potentiometer,
shaft and paddles; and the controller which consists of the motor drive
electronics, the microprocessor, and the control software. The mechanism
is attached to the side of the spectroradiometer and is submerged, while
the controller is implemented as a routine executed by the OASIS
microprocessor, with some additional hardware. The software for the
shutter takes the form of a subroutine that is called whenever the shutter
is opened or closed. The subroutine makes use of one A/D input channel
and two input/output bits to communicate with a motor control circuit which
provides start/stop control, direction reversing, overcurrent detection,
and position sensing for the mechanism.
While not intended to be an exhaustive treatment, nor
step-by-step instructions on how to construct an identical unit, what follows
should allow mechanical designers and programmers to grasp the concepts
and functionality of our system, and make a start on their own. We learned
much during during the initial effort, and work is now underway on an improved
- Mechanical section
- Electrical section
- Software section
- Setup section
Photos taken of the shutter attached
to a spectroradiometer.
This shot is of the 10M and 20M instrument cages with
the shutter/spectro assemblies attached.
on image for enlarged view
In this view you can see the means by which the shutter
is attached to the spectroradiometer.
click on image for enlarged view
This is an end-on view of the optical window of the shutter,
with the copper paddle in the open position.
on image for enlarged view
The mechanism consists of a long shaft, offset from the
centerline of the housing, upon which the copper paddles are mounted. Plastic
shaft supports are attached to each end of the spectroradiometers in order
to support and position the paddles over the windows when closed. Internal
to the housing a DC gearmotor is coupled to the long shaft via an anti-backlash
gear. Also coupled to the long shaft, by way of the same anti-backlash
gear, is a single turn potentiometer. This potentiometer allows us to determine
the position of the paddles relative to the optical window of the
- Here is an assembly drawing
of the shutter mechanism.
A general purpose DC motor drive circuit was designed
to meet the needs of the shutter. Overcurrent detection and automatic shutdown
allow recovery from jamming or obstruction of the paddles as they rotate.
Signal conditioning for the potentiometer was included as well. A printed
circuit board was created to package the circuit, and it was installed
as a peripheral to the OASIS mooring controller. The links below serve
to better explain the details of the circuitry.
The shutter mechanism electrical
schematic and bill of materials (electrical).
The "DC Drive" generic motor drive circuit
DC Drive, detailed design
DC Drive, configuration
for the shutter.
The software defines much of the behavior of the shutter.
In our early efforts, no provision was made for dealing with an obstruction
that might block the paddles as they rotate. Consequently, on our first
deployment at sea, an object became lodged beween the paddle and the spectroradiometer
housing and the geartrain was destroyed. Attempts were made to deal with
that problem by mechanical means by way of slip clutches, but the results
were not satisfactory. It was decided to make the shutter more "intelligent"
in hopes that future failures could be avoided.
The shutters now implement a collision recovery algorithm
to deal with possible obstructions in the field. This is made possible
by motor drive hardware that is capable of sensing the motor torque and
disabling the power if a preset limit is exceeded. The software controlling
the shutters is aware that a collision has occurred, and reverses the direction
of rotation in an attempt to clear the obstruction. Whether the shutter
is opening or closing, it keeps track of how many times the motor is started
in an attempt to reach the desired condition. The number of attempts made
before giving up and declaring the operation a failure is set in the driver
parameters for that shutter. The shutter will repeatedly reverse direction
until it gets where it wants to, or the allowed number of attempts have
been made. In addition to dealing with obstructions, there is a limit to
the amount of time allowed to get to a desired location. This is to prevent
the motor from running endlessly due a failure of the feedback mechanism
or electronics. This time limit is set to be slightly longer than a complete
revolution of the shutter.
Each time the shutter operates, a log
record is generated. The record reports the success or failure of the
operation, what went wrong, and the state that the shutter was left in.
These records can be used to determine the performance of the shutters
in the field, and whether obstructions have occurred.
In the event that a shutter exceeds the allowed attempts
in opening or closing, the operation is considered a failure, and the COUNT
value is incremented. If the following operation is successful, COUNT is
set to zero. If there are 5 failures in a row, the shutter will disable
itself from further attempts, and the COUNT must be manually reset.
Utility programs to allow manual operation and calibration
of the shutters are available, see shutcom.txt
Shutter operation is controlled by several driver parameters.
PARM0: Shutter number, the physical address. 0 or 1 for
two shutter systems.
PARM1: Closed position calibration, specific to each shutter,
PARM2: Number of attempts allowed to complete an open
or close, typically 5.
TIMEOUT: Seconds allowed for each attempt, typically no
less than 30.
COUNT: Number of successive failures, at 5 the driver
disables itself. To manually disable the shutter, set this number to 99.
Log record description:
The shutter opening and closing operations produce four
bytes of data which are logged. The data is organized as two words, the
first of which is the position of the paddles when the operation was completed,
and the second which is a packed record of various error codes.
In the source file shut22.c they are defined as: