This webpage presents the design and construction of SeaDog, a proof-of-concept robot built for navigating rocky or sandy beaches and turbulent surf-zones. The robot incorporates a layered hull and chassis design, which is integrated into a waterproof Explorer Case in order to provide a large, protected payload in an easy to carry package. It employs a rugged drive train with four detachable wheel-legs and a unique tail design and actuation strategy to aid in climbing, swimming and stabilization.
Cockroaches have remarkable locomotion abilities that provide a wealth of inspiration to address challenges of surf-zone operation. In studies of cockroach movement, we have noted the following :
- A cockroach has six legs that support and move its body
- It typically walks and runs in an alternating tripod gait
- They adapt their gait when climbing large obstacles by using both front legs together in phase
- The cockroach turns by generating asymmetrical motor activity in legs on either side of its body
- A cockroach enhances its climbing abilities by changing its body posture
- During a climb it uses flexion joints to bend the front half of its body down to avoid high centering
- SeaDog is an amphibious robot developed for surf-zone applications. Four wheel legs and a tail provide it with excellent mobility on sandy and rocky beaches and in turbulent surf-zone areas. A rugged, waterproof outer shell and keeps its large payload area safe during operation.
- Lobsters align their tail and claws with a surge, using them as a control surface.
- Forces on the body are directed downward providing more stability and friction.
- Sensors on lobster detect changes in the flow for continued stability.
- Lizards press long toes deep into the sand reaching compact sand below the surface.
- Crabs walk on the surface with eight legs, spreading the weight over multiple legs.
- The large surface of a camel’s foot compacts sand, creating a stable surface.
Design and Construction
- Water-resistant aluminum inner chassis provides structural integrity for the drive train.
- Waterproof Explorer Case (model 5117) acts as an outer shell with integrated seal, latches, and carrying handle.
- Pressurized chamber simulates depths up to 70 meters (689 kpa).
- O-rings perform better at lower pressures.
- U-cup seals perform better at higher pressures.
- We used a combination of the two.
- Torsional compliance provides shock absorption and enables opposite legs to come into phase while climbing.
- Linear springs are pre-tensioned making wheel legs rigid until a break-away torque is experienced.
- Springs can be swapped out to achieve desired break-away torques and torsional stiffness.
- Drive train is powerful and robust with tapered roller bearings, helical gears, and 150W Maxon Motors.
- An outer drive shaft (pink) transmits torque to and resists bending of a removable inner drive shaft (red) that connects to the wheel leg.
- With tail and wheel-legs removed, the robot takes the form of a waterproof case.
Testing and Results
|Wheel Leg Radius||19cm|
|Tail Length (short/long)||48cm||58cm|
|Mass w/o Batteries||23.8kg|
|Drive Motor Rated Stall Torque||2.28 N m|
|Gear Reduction||38 :1|
|Max Speed||2.23 m/s|
|Turning Radius||0 m|
|Max Obstacle Height Tested||48 cm|
|Max Seal Pressure Tested (in vitro)||689 kPa (~70 m under water)|
- Replacing the rear body segment (left) with a tail (right) increased max obstacle height from 40 cm to 48 cm.
- Improved performance is attributed to a more forward center of mass and reduced interference from rear wheel-leg.
- 48 cm climbing height failed with a tail length of 48 cm, but succeeded when the length was increased to 58 cm.
|Configuration||Simulated (cm)||Experimental (cm)|
|Body Joint Locked||30||27|
SeaDog climbing a 48cm high obstacle. In frame three the tail helps to prevent high centering by applying a counterclockwise moment to the robot. In frame four the tail continues to help rotate the robot counterclockwise and acts as a support while the robot is off the ground.
- Preliminary aquatic testing was done at the surface with a positive buoyancy using the wheel legs as paddles.
- Future testing will be done with a negative buoyancy to test the effective ness of the wheel-legs and tail as means of locomotion while underwater.
- Future work includes the addition of a variable buoyancy mechanism to enable both forms of locomotion.