Assistant Professor Dr. Aaron Ames at the A&M Bipedal Experimental Robotics Lab (AMBER) heads a team of students in the design, simulation, and fabrication of a robot that will walk like a human. The AMBER 2 is an important project. Making a human-like bipedal walking robot will advance robotics beyond that of toys or novelty and into the area of high-level use for a variety of applications, including space exploration, disaster response, military operations, elderly assistance, rehabilitation, and prosthetics.
Most robots being built today use what is called a quasi-static locomotion method, or the ZMP (zero moment point) technique of walking. This type of operation essentially means that no (or zero) moment (inertia) in the horizontal direction is present at the point where the foot hits the ground. If the robot is stopped in midstride, it continues to stand. This is not a human-like walk. If we were to be stopped in midstride, we'd fall over. The Texas A&M team found that, by using voltage-based controllers, the walking method could be more dynamic in nature, as feedback could be more easily incorporated into the design.
According to Ames, "The first goal to overcome is how to mathematically model such a highly dynamic system, and then how to fabricate the controllers and other hardware to do what you want. Similar to what we chose to do with our first robot, we decided to create our next-generation AMBER 2 robot from the ground up. The process included the initial CAD designs using SolidWorks renderings, through mathematical analysis using Wolfram Mathmatica, to simulation and controller design using The MathWorks' Matlab, and finally using National Instruments' LabVIEW for upper-level control of the final design."
Everything in the design had to be thoroughly thought through, analyzed using computers, and then ordered. "We couldn't ballpark any of our figures," Ames said, "particularly for the mechanical operations of the robotic legs." The team found that it would need to use motors and gearheads that could deliver torques between eight and 12 Newton-meters and speeds down to five to six radians per second. "At such low speeds, we were specifically concerned about cogging problems getting in the way of smooth operation," Ames said.
The team decided to use maxon motors for its final robot design. The AMBER 2 robot uses three motors and three right-angle gearheads per leg, one for each joint -- ankle, knee, and hip. According to Ames, "We found that we needed a single, seamless package, which allowed us to maintain the tightest tolerances for the joint mechanisms." When the robotic foot hits the ground, the motors have to reverse on impact, which causes a huge amount of torque on the motor.
AMBER 2's motors are electronically commutated, which enables them to offer extremely long motor life -- there are no mechanical brushes to wear out. Hall effect sensors are built into each motor in order to provide feedback to the control electronics. By assembling the motor such that the coils are outside the rotor, good heat dissipation and high overload capability is attained. Each motor has a stainless steel housing. Models vary in diameter and length depending on the customer's needs and selection -- typically with diameters from 16mm to 40mm (0.63 inches to 1.57 inches) and lengths from 24mm to 88mm (0.94 inches to 3.46 inches). The ambient temperature range is -40C to 100C (-40F to 212F).