Project Risks & Mitigation
Accurate sensing of Zero Moment Point
Keeping the ZMP within the support area is critical to maintain stability of a biped robot on a flat surface. Since the zero moment point corresponds to the center of pressure of one or both of the feet, the applied forces and moments at each ankle must be measured. A brief trade study of force sensors showed that simple capacitive force transducers at each corner of the foot can effectively sense COP location.
Measurement of angular momentum and body orientation
During biped locomotion, one of the most critical phases is foot landing. The swinging leg/foot must land to counter the forward fall of the body. If the step is too large, the robot will lose too much forward velocity, stopping suddenly and likely falling over. Conversely, too short a step will cause the robot to trip and fall. To land within this “sweet spot”, the angular momentum can be calculated from the pitch rate, and used to control foot landing. A similar circumstance is true for sidestepping.
Without a way to measure this, the robot’s gait will likely be slow and shaky. An IMU will be used to provide the necessary feedback to make the walking gait more robust
Robot must recover from a fall and stand up on its own
To stand up after falling over, the robot must be able to roll onto its feet, balancing even as it lifts the body from the floor. Careful analysis in CAD will be performed to ensure that the center of gravity will remain over the support parallelogram during this maneuver. This restriction will be used to constrain the mechanical design.
PicoITX Mainboard lacks hardware interfaces for sensors
Proper integration of all of the sensors presents a large risk because multiple analog and serial ports are not present on the typical ITX motherboard. This necessitates additional hardware and/or software to work around. Rather than design a serial line bus, or depend on DAQ hardware, the ethernet protocol will be used to link the main board to perpheral microcontrollers. Example C code exists for analog to digital conversion and simple ethernet communication for many microcontrollers, eliminating much of the development time. The robustness and expandability of the ethernet protocol will make adding or changing sensors easy. See the Computer Page for more details.
Scanning the Competition
A bit of investigation shows that many of the robots of previous years far exceed the capabilities of the Robonova or Bioloid.
In 2008, the stair climbing champions climbed up and down a short flight of stairs:
The winner of the 2008 Kung Fu competition is linked below. One surprising thing about Kung fu is how much fighting uses scripted motions. All of the gestures seem to be pre-programmed. This suggests that a good control system and movement scheme could produce more exatc motions, better-aimed punches, and quicker recoveries.
2007 Kung Fun Champions
Robo-One Grand Champion at IREX
These videos show the variety of hardware out there just as conspicous is the need for better control methods. As an example of what good control can do, here is a demo video at the ROBO-One 10 copeition in 2006:
The KHR-2 HV shows a smooth dybanic gait, rapid side-stepping, and excellent balance control. Even though this platform is only “pro-sumer” level, it clearly looks more human-like and controlled than many of the entries above.