Robot Mechanisms and Mechanical Devices Illustrated - Profe Saul
Robot Mechanisms and Mechanical Devices Illustrated - Profe Saul
Robot Mechanisms and Mechanical Devices Illustrated - Profe Saul
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132 Chapter 4 Wheeled Vehicle Suspensions <strong>and</strong> Drivetrains<br />
outs are included in this chapter, but the basic concept can be applied to<br />
almost any mobile robot.<br />
Shifting the center of gravity can be accomplished by moving a dedicated<br />
weight, shifting the cargo, or reorienting the manipulator. Moving<br />
the cg can allow the robot to move across wider gaps, climb steeper<br />
slopes, <strong>and</strong> get over or onto higher steps. If it is planned to move the<br />
manipulator, then the manipulator must make up a significant fraction of<br />
the total weight of the vehicle for the concept to work effectively. While<br />
moving the cg seems very useful, all but the manipulator technique<br />
require extra space in the robot for the weight <strong>and</strong>/or mechanism that<br />
moves the weight.<br />
The figures show the basic concept <strong>and</strong> several variations of cg shifting<br />
that might be tried if no other mobility system can be designed to<br />
negotiate a required obstacle, or if the concept is being applied as a retrofit<br />
to extend an existing robot’s mobility. Functionally, as a gap in the terrain<br />
approaches, the cg is shifted aft, allowing the mobility system’s<br />
front ground contact point to reach across the gap without the robot tipping<br />
forward. When those parts reach the far side of the gap, the robot is<br />
driven forward until it is almost across, then the cg is shifted forward,<br />
lifting the rear ground contact points off the ground. The vehicle is then<br />
driven across the gap the rest of the way.<br />
For stair climbing or steep slopes, the cg is shifted forward so it<br />
remains over the center of area of the mobility system. For climbing up a<br />
single bump or step, it is shifted back just as the vehicle climbs onto the<br />
step. This reduces the tendency of the robot to slam down on the front<br />
parts of the mobility system. It must be noted that cg shifting can be controlled<br />
autonomously fairly easily if there is an inclinometer or<br />
accelerometer onboard the robot that can give inclination. The control<br />
loop would be set to move the cg in relation to the fore <strong>and</strong> aft tilt of the<br />
robot. In fact, it might be possible to make the cg shifting system completely<br />
automatic <strong>and</strong> independent of all other systems on the robot, but<br />
no known example of this has been tested. Figures 4-1 <strong>and</strong> 4-2 show two<br />
basic techniques for moving the cg.<br />
The various figures in this chapter show wheel layouts without showing<br />
drive mechanisms. The location of the drive motor(s) is left to the<br />
designer, but there are a few unusual techniques for connecting the drive<br />
motor to the wheels that affect mobility that should be discussed. Some<br />
of the figures show the chassis located in line with the axles of the<br />
wheels, <strong>and</strong> some show it completely above the wheels, which increases<br />
ground clearance at the possible expense of increased complexity of the<br />
coupling mechanism. In many cases, the layouts that show the chassis<br />
down low can be altered to have it up high, <strong>and</strong> vise-versa.
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