Fish Biorobotics Fishes as model systems for understanding aquatic propulsion 28,000 species of fishes Half of all vertebrates are fishes Fishes are 550 million years old Tremendous opportunity for selection to act on propulsive systems Multiple fluid control surfaces Propulsor flexibility Maneuverability Obvious source of inspiration for underwater robotics George V. Lauder Harvard University www.oeb.harvard.edu/lauder We are just at the beginning of understanding how fish move through water and generate propulsive forces Fish biorobotics 1) Key features of fish functional design 1) Key features of fish functional design Diversity; stability and buoyancy; fin design 2) Fish fin function Visualizing fin and fluid motion in vivo Fin fin interactions during locomotion 3) Fish robotic test platforms Robotic pectoral fins Robotic caudal fins Robotic ribbon fin Flexible foils as a simple robotic models of fish 4) What next? Studying locomotor control via perturbations Many key collaborators, especially James Tangorra Lab (Drexel Univ.), Erik Anderson (Grove City College), Rajat Mittal Lab (JHU), Malcolm MacIver Lab (Northwestern Univ.) Much variation in body form, fin position, fin shape -- study comparatively, but also using robotic models -- 1
Evolutionary patterns: pectoral fins pelvic fins dorsal fin(s) anal fin caudal fin Many unanswered questions: Effect of fin position? Effect of fin shape? Hydrodynamic and mechanical function? (spiny-finned fishes) (spiny-finned fishes) ( true bony fishes) ( true bony fishes) (ray-finned fishes 28,000 species) (ray-finned fishes 28,000 species) Key features of fish functional design median and paired fins Station holding and maneuvering using pectoral fins Dorsal fin Caudal fin Sunfish Perch Pectoral fins Pelvic fins Anal fin Fish are unstable: Need constant fin action to maintain posture Negatively buoyant; CB below CM Very effective at maneuvering and controlling position 2
fin ray segments Half ray fin ray Fin ray mechanics (sunfish pectoral fin) Half ray unsegmented basally Fin membrane 4 muscles control each fin ray (+3) 14 fin rays in the pectoral fin; 59 muscle bundles total per fin Simple ray models Pectoral fin rays bend into flow during maneuvering 2) Fish fin function How do pectoral fins generate locomotor forces? Flow Fin ray E ~ 0.1-3.0 GPa Fin membrane E ~ 1 MPa Fin curved into flow Videos of pectoral fin movement: side and bottom views, 1024 x 1024, 250-500 fps Digitize 200-300 points per time step throughout the fin beat; reconstruct 3D fin positions Maneuvering as well as propulsion at several speeds 3
Kinematic observations Body x - acceleration during the fin beat Lateral Dorsal Postero-ventral Quantify body acceleration from light video Two thrust peaks per fin beat cycle Cupping Two leading edges Both spanwise and chordwise deformation Spanwise wave ~30 % area change EXPERIMENTAL FISH FLUID MECHANICS Experimental fluid mechanics: particle image velocimetry of water flow patterns Laser light FLOW 8 4
Sunfish pectoral fin hydrodynamics (vector field movie in transverse plane) Stereo PIV Fin flow pattern: end of outstroke Red color shows momentum added on outstroke Stereo PIV Fin flow pattern: instroke Pectoral Fin CFD research (ViCar3D) Rajat Mittal, Johns Hopkins Viscous Cartesian Immersed Boundary 3D N-S Solver Mid-instroke Able to handle thin complex organic shapes and changes in area through time in 3D Validated for stationary and moving boundaries Code uses the actual sunfish fin 3D kinematics shown earlier Matched St# and Re# 5
Fin-fin interactions during locomotion: dorsal/anal fins and the tail? water flow Dorsal fin Caudal fin Movie of fin motion Z? Dorsal and anal fins are actively moved X Trout swimming in dual horizontal light sheets Trout: dorsal caudal fin (1.0 L/s) Freestream flow Dorsal fin Tail fin Dorsal fin Pelvic fin Anal fin Tail 6
Trout: dorsal caudal fin (1.0 L/s) Self-propelled dual flapping foils Freestream flow Two aluminum foils Linear encoders measure position Inter-foil distance can be changed Foils controlled in heave and pitch Self-propelled dual flapping foil movie 3) Fish robotic test platforms Flow Tail shapes Tail shapes Roboray Pectoral fin Foil 1 Foil 2 Our CFD study of this: Akhtar et al. (2007) Theor. Comp. Fluid Dynamics 21: 155-170 Simple physical model Alter aspect ratio Change material properties Change tail shape Simple to control Not as biologically realistic Biomimetic design Accurately mimic fin shape Use natural fin motions Alter fin kinematics to explore large parameter space Alter fin ray stiffness Measure forces Hard to build and control 7
Robotic Pectoral Fins Mimic sunfish fin ray flexibility and kinematics Alter fin ray stiffness and movement patterns Measure forces and kinematics during programmed motions Quantify hydrodynamics of fin function and force generation A robotic pectoral fin Collaborative research with James Tangorra Lab, Drexel Univ. Nylon tendons and fin base A robotic pectoral fin Carriage Air bearings Nylon tendons threaded through control block to actuators Motors Water surface Flat plate = fish body Flow Pectoral fin 8
Robotic fin motion can mimic biological movement very closely Mid fin outstroke Sunfish Robofin Sunfish Robofin Early fin outstroke Late fin outstroke Mid fin instroke Sunfish Robofin Sunfish Robofin 9
Robotic fin 3D motion and measured thrust force (Note thrust on both outstroke and instroke) Fluid flow behind the robopectoral fin Robofin and transverse laser sheet PIV movie Standard 2D PIV from orthogonal planes Document dual leading edge vortices Measure forces on robotic fin Flow pattern from robotic pectoral fin, early outstroke A caudal fin robot How do different fin motions effect force production? Dual leading edge vortices develop on the outstroke 0.5 m/s Collaborative research with James Tangorra Lab, Drexel Univ. 10
A caudal fin robot Symmetrical flat Asymmetrical rolling1 tail tendons tendons Symmetrical cupping Asymmetrical rolling2 3D printed fin rays Alter fin ray stiffness Alter morphology: tapered vs straight Caudal fin forces: force vs flapping frequency Fish vs. Robot caudal fin motion: most flexible robotic fin Mean thrust for fin 50x 0.45 0.40 0.35 flat 0.30 undulation Mean thrust (N) 0.25 0.20 0.15 cupping roll 0.10 0.05 0.00 0.0 0.5 1.0 1.5 2.0 Flapping frequency (Hz) 11
Thrust Force Varies with Stiffness The Ghost Knifefish ribbon fin and robotic model The ribbon fin: highly controlled wave-like motion allows considerable maneuverability; ~ 150 fin rays Flapping frequency (Hz) 100x 15x Collaborative research with Malcolm MacIver Lab, Northwestern Univ. The Ghost knifefish ribbon fin and robotic model The Ghost knifefish ribbon fin and robotic model 32 actuators make up the robo-ribbon fin 12
The Ghost Knifefish ribbon fin robot: PIV analysis to show down jet with inward waves Flexible foils as simple fish models Undulating foils are self-propelling Variety of plastic foils attached to flapper Tested at a variety of heave and pitch parameters Quantified flexural stiffness for each plastic type Robotic flapping foils What is the effect of foil flexibility on self-propelled speed? 50 Foil propulsion -- EI effects 4) What next? Studying locomotor control via perturbations Need to go beyond descriptive approaches to understand control. 45 Self Propelled Speed (cm/s) 40 35 30 25 Heave only Heave and pitch 20-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Flexural stiffness -- EI (N*m 2 ) Each graph point is a different type of plastic Each plastic has the same area and shape +/- 3.0 cm heave; 0 or 20 pitch amplitude; 1 Hz Error bars contained within symbols Key collaborators: James Tangorra Lab, Drexel Univ. Mike Philen and colleagues, Virginia Tech. 13
Perturbing swimming fishes Particle image velocimetry of vortex ring hitting pectoral fin Movie: dye-filled vortex ring hitting swimming fish SYNTHESIS Fish fin motion is complex and surface conformation is under active control Robotic models of fish fins permit detailed investigation of motion and force production Studies of fish locomotor control from a biomechanical perspective are in their infancy; perturbation of swimming fishes is one useful future approach. 14