Servo - July 2009

the ground, and that lifted foot is then placed in front of the grounded one, underneath the torso.

During this foot placement,the upper torso shifts forward with the legs, evenlydistributing the COG between the two legs until it is ready to take another step. Then, the torso shifts forward again until its COG is centered on top of the forward grounded foot and the cycle is repeated for the opposite leg. What you end up with is a biped that knows how to ‘walk the line,’ and an upper torso that stays considerably more stable while walking. Figure 7 is a 3D rendering of the original

MicroRaptor robot demonstrating the stepping pattern of this gait.

There are a few pros and cons to this method of walking. Benefits include: a steadier upper torso while walking; a lower impact gait; and a longer stride. The feet are brought completely off of the ground when taking steps, minimizing friction found with standard ‘shuffle’ gaits which cause the robot to veer off course when walking straight. For all of the benefits, there are some inherent flaws that come with both the gait and the leg design.

The gait itself is considerably harder to design and program, given that the COG must be accounted for more so on both x and y due to the longer stride, whereas a standard gait is mostly just shifting the COG from side to side. Even when using the positional capture features of the Dynamixel servos, this is no easy task. The inline stepping pattern trades the forward back instability of a standard walking gait for an increased instability side to side. Also, a higher COG is required to properly ballast which leads to a

more top heavy bot. This top heavy nature side to side instability can both be accounted and compensated for through the use of an IMU and balance engine; this is most certainly in my near future plans for Hagetaka.

Conclusion

Figure 6

Next month, we will cover the wiring and power distribution scheme, the onboard Gumstix controller, remote control setup, weapons, supporting telepresence electronics, the armor, and scoring system. To check out the latest progress, visit http://forums. trossenrobotics. com.