Video: Motion systems drive robotic dolphin
Case study: Quicksilver Control developed special motion control system software to control a robotic dolphin; 14 actuators were driven by a closed-loop step-motor controller. Link to videos.
QuickSilver Controls of Covina, Calif., developed special motion control system software to control the motion in a robotic dolphin used in the recently released movie about Winter, a dolphin with a prosthetic tail. The robotic dolphin was used in movie sequences that would be too dangerous, exhausting, or time consuming with the real dolphin.
Fourteen of the special actuators were driven by QuickSilver’s QCI-D2-MG-01 motion controller, a 2.0-in. (53 mm) x 2.5-in. (63 mm) closed-loop step-motor controller. It provides 12 to 48 V dc and up to 3.5 amps continuous and 4.5 amps peak.
The controller was packaged in a custom water-sealed container filled with environmentally safe vegetable oil for operation in the dolphin tank. The size 23 step motors were connected to a number of linear actuators that flexed the dolphin body and tail up or down and side to side.
QuickSilver Controls developed a custom software algorithm needed to control the various actuators in smooth coordinated control from multiple RC-type controllers typically used by the puppeteers. Quick response times helped meet the tight schedule of the project designed by Eric Fiedler of Renegade Designers Inc. and K&B Effects Group.
Don Labriola, PE, QuickSilver Controls Inc., provided additional information about the application, below.
QuickSilver Controls Inc. was contacted by KNB EFX Group for motion control help with a fast turn project—a robotic dolphin for a movie about Winter, a dolphin injured in crab nets who lost her tail and fitted with a prosthetic tail that helped her survive.
KNB needed to mimic the size, coloration, and water retention of the real dolphin in addition to mimicking its fluid motions. As the movie needed shots 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 dolphin needed to operate in saltwater 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 adjusted live, on-the-fly, to interact with the actors during the shoot. The preferred movie industry method is for “puppet masters” to control the motions via multiple sophisticated radio control (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 saltwater, the normal radio link was substituted with small wire carrying the same multiplexed signals.
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 waterproof linear actuators providing the ability to flex up and down (axes working together) and side to side (actuators working differentially). 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 that were filled 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 polluting the aquarium tank with harmful chemicals in the case of a leak.
QuickSilver Controls 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, which updates only 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 (see below) and the controller had been done using small linear potentiometers mechanically connected to standard RC servo actuators. This added complexity and fragility to a system that needed to operate underwater. To meet the schedule for the filming, QuickSilver added the ability to use 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 control the motion.
Radio control (R/C) signals
Radio control planes and cars use a multiplexed pulse width signal scheme to control many axes of control over one radio link. The various joysticks, trims, potentiometers and switches are measured by the processor in the RC control module, and can be mixed, processed, slew limited, and range limited, producing multiple coupled position control signals. The individual signals are encoded using pulse width modulation. The standard R/C controllers and motors nominally use a 1- to 2-millisecond pulse width, 0 V to 3.3 V, to indicate the desired position, although some can use the extended 0.7-millisecond to 2.3-millisecond timing. For the nominal timing, a 1-millisecond pulse indicates extreme motion position in one direction, a 1.5-millisecond pulse represents centered, and a 2-millisecond pulse indicates the opposite extreme position. Each of the multiple axes controlled by the RC is provided with its own pulse-with-modulated (PWM) control signal, which is sent in order by the controller and demultiplexed by the receiver. Multiplexing multiple signals lowers the update rate for any particular channel to some 20 milliseconds to 30 milliseconds.
- Edited by Mark T. Hoske, content manager, CFE Media, Control Engineering and Plant Engineering, mhoske(at)cfemedia.com.
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