This has to be one of the coolest steppers-work-like-servos stories ever. Now, you may already know about Winter, the three-month-old, bottlenose dolphin that became tightly trapped in a crab trap line and lost her tail in 2006, and was rescued by the Clearwater Marine Aquarium (CMA) from Mosquito Lagoon near Cape Canaveral. Winter was treated at CMA, and later fitted with a prosthetic tail by Hanger Prosthetics and Orthotics Inc. And, most recently, she starred in "Dolphin Tale" a 3-D movie about her life from Alcon Entertainment and Warner Bros., which was released in September 2011 and co-starred Morgan Freeman, Ashley Judd, Harry Connick Jr., Kris Kristofferson and Nathan Gamble (www.seewinter.com).
And, while you may also be aware that movie stars have stunt doubles, you probably don’t know about Winter’s robotic stand-in and how it was controlled by servo-like stepper motors using drives and software from QuickSilver Controls (www.quicksilvercontrols.com). The company was contacted earlier by KNB EFX Group (www.knbefxgroup.com) to help with the robot dolphin’s motion control, and it suggested smoothing out the performance of the robot’s stepper motors to work more like servomotors, and then reportedly turned around the needed software in about 72 hours.
“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.
