Fig. 5. Schematic figure to illustrate two different categories of fish-like aquatic robot design and the measurements that might be made from each design. (A) Robot is attached to a sting (a rod holding the robotic model vertically from the carriage above) and either fixed in place while forces are measured on the sting, or towed at a fixed velocity on a moving carriage. In either case, the robot is not self-propelled, but rather moves at externally imposed speed. In this case, there need be no equality between thrust and drag forces, as it is not known if the robot is generating sufficient thrust to overcome drag. (B) Robot swims at a self-propelled time-averaged constant speed as a result of thrust generated by heave and pitch motions, and mean thrust force per cycle must equal the mean drag force. The flow speed in the tank is adjusted to a value, U_eq, where the robot propels itself at a constant equilibrium X position, termed X_eq. The robot is free to move itself upstream and downstream on a low friction air bearing system. Once X_eq is determined for a particular heave and pitch motion pattern during self-propulsion, the robot can be fixed in position at X_eq to measure forces and torques while the same motion pattern and flow speed used for self-propulsion are imposed. This allows force measurement under conditions identical to self-propulsion, when thrust and drag forces must be equal.