“A robotic dolphin was needed as a stand-in for Winter in sequences which would either be too dangerous or difficult, or which could exhaust the recovering dolphin,” explained QuickSilver’s engineers. “KNB needed to mimic the size, coloration and water retention of the real dolphin in addition to mimicking the fluid 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.
“The dolphin needed to operate in salt water down to approximately 10 meters. It needed to avoid chemicals that could pollute the aquarium tank, or hurt either the real Winter 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 had to be adjustable live and on-the-fly to interact with the actors during the shoot. The preferred method in the movie industry is for ‘puppet masters’ to control the motions via multiple radio-controlled (R/C), multi-axis control heads. Preprogrammed motions are avoided to prevent downtime for programming between shots, which would waste expensive time with actors and film crew waiting. As the dolphin needed to operate under salt water, the normal radio link was substituted with a small wire carrying the same multiplexed signals.”
QuickSilver adds that the mechanical spine and muscles of the robot dolphin were implemented using several aluminum disks linked together using sets of pivots along the center line with pairs of waterproof linear actuators providing the ability to flex 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 filled with vegetable oil and with a small amount of dried rice added as a desiccant. These 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's QCI-D2-MG-01 motion controllers 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 that only updates about 30 times per second. A second processing thread on the controller was used to monitor and recover from jams in the open-loop step motor.
The initial interface between the RC control signals and the controller was 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. To meet filming schedule, QuickSilver added the ability to use standard R/C 1-2 millisecond control pulses as a standard option. This capability was added in less than one day to keep the project on schedule, allowing the pulse width signal from the receiver to directly control the motion.