The submersible robotic dolphin was for a movie about Winter, a dolphin injured in crab nets who lost her tail and was helped to survive by means of a prosthetic tail.
A submersible robotic dolphin was needed as a stand-in for Winter in sequences which would either be too dangerous or diLcult, or which could exhaust the recovering dolphin. KNB needed to mimic the size, coloration, and water retention of the real dolphin in addition to mimicking the Ouid motions. As the movie needed shots both in the water and at the surface of the water, a full scale dolphin robot with internal electronics and actuators was used. This would eliminate the strings often used with puppets, which would be harder to process out due to their effect on the water surface.
The robotic dolphin needed to operate in salt water up to a depth of approximately 10 meters. It needed to avoid chemicals which could pollute the aquarium tank, or hurt either the real dolphin or the actors in the water. Multiple coordinated actuators were needed to handle the many elements needed to model the motions of an actual dolphin. The motions needed to be able to be adjusted live – on-the-Oy – to interact with the actors during the shoot. The preferred method in the movie industry is for the “puppet masters” to control the motions via multiple sophisticated radio control (R/C) multiaxis control heads. Preprogrammed motions are avoided to prevent downtime for programming between shots which would waste expensive time with actors and Zlm crew waiting. As the dolphin needed to operate under salt water, the normal radio link was substituted with small wire carrying the same multiplexed signals.
Explanation of Mechanics
The mechanical spine and muscles of
the dolphin were implemented using
several aluminum disks linked together
using sets of pivots along the center
line with pairs of water proof linear
actuators providing the ability to Oex
up and down (axis working together)
and side to side (actuators working differentially). The linear actuators were specialty products from Ultra Motion LLC, which were driven by low voltage stepper motors for safety reasons in a conductive saltwater environment. Absolute position feedback was provided by internal, sealed potentiometers. The electronics were made waterproof by mounting the circuit boards into custom sealed cases which were Zlled with vegetable oil, with a small amount of dried rice added as a desiccant. The food grade products were used instead of the normal mineral oil and silica gel to prevent any chance of pollution of the aquarium tank by harmful chemicals in the case of a leak.
QuickSilver Controls QCI-D2-MG-01 motion controllers (shown on the left) were to close the loop around these open loop steppers using the potentiometer. The preferred method would have been to include motor shaft feedback to allow the motor to be commutated for higher performance, but the lead-time of the specialty underwater actuators
prevented that level of customization. Instead, the motor velocity was commanded using a simple control loop comparing the potentiometer feedback to the commanded signal. The gains were tuned down to smooth out the motions from the R/C controller which only updates about 30 times per second. A second processing thread on the controller was used to monitor for and recover from jams in the open loop step motor.
The initial interface between the RC control
signals and the controller had been done using
small linear potentiometers mechanically
connected to standard RC servo actuators.
These added complexity and fragility to a
system that needed to operate underwater. In
order to meet schedule for the Zlming, QCI
added the ability to use standard R/C 1-2
millisecond control pulses (shown on the right)
as a now standard option. This capability was
added in less than 1 day in order to keep the
project on schedule, allowing the pulse width signal from the receiver to directly control the motion.
For more information on RC Control see Application Note QCI-AN067 RC PWM Control.