At this point in the design testing phase, the robot is tethered much of the time, but the engineers have tested and confirmed the use of a pair of 10 amp/hour lithium polymer batteries (about the size of a small brick) will be able to power the robot for at least a half-hour with a 20A average current draw. This includes all the motors, sensors, and controls.
Because the robot operates off a closed-loop system and uses 12 motors just in the legs (there will be another 12 in the arms and hands, as well as two in the neck), Lahr and his team needed controllers that could handle the load. "We chose maxon's EPOS 50/5 controllers for the joints, aside from two EPOS 24/5 controllers used for less demanding degrees of freedom," he said.
This closeup shows four 30mm brushless motors manufactured by maxon precision motors.
One of the main reasons the team chose to use the EPOS (Easy Positioning System) series controllers was that they came equipped to use the CANopen bus system. "Several of us were familiar with CANopen from other applications, so we were attracted by the familiar operating and programming needs of the system software from the beginning," Lahr said. This makes the EPOS embedded controllers well suited for multi-axis distributed controls that also feature electronic gearing, PVT, step and direction, and point-to-point positioning.
The EPOS controls are used in two different modes -- position control mode and force control mode. Position control allows for higher-level controllers to read position data from the sensors and closely regulate the specific position of the leg, so that corrections can be incorporated while walking. Force control mode is the latest thing in locomotion, according to Lahr. "It combines current control circuitry with load cell feedback to create a 'pure force' actuator, which allows the leg to swing freely," he said. "The EPOS controllers allow us to switch modes on-the-fly."
This is important so that the leg impact doesn't harm any of the actuators. "We can switch from position control to force control at the last millisecond, so that we can accurately control stride length and impact power," he said. An additional benefit of using the maxon controllers is that they come with EPOS Studio (a GUI-based free software package provided by maxon), which provides a simple utility to program the controllers and helps the user to bug-check software before implementing it into the CANopen system.
In general, EPOS controllers have been designed using advanced 32-bit DSP technology, which provides users like Lahr and his team the extended functionality of a miniature embedded controller. The units were specifically developed to meet demanding size and performance requirements often found in robotic, medical, and semiconductor applications.