Platform as an inverted / pendulum





5th-axle as a pivot for the platform

Platform as an inverted / pendulum


5th-axle as a pivot for the platform

Center Articulated Chassis sonar and three IR sensors together. The sensor array allowed for overlapping sensor coverage while still providing a large viewing angle.

Rather than utilize a fixed-mounting point for our sensor array, it was decided that a pan/tilt kit would be used to mount the sensor array.

The pan/tilt unit (as shown in Figure 3) allowed fewer total sensors to be used while still providing about 250° coverage when coupled with the detection cone of the Ultrasonic units. The pan/tilt unit allowed the robot to focus all of its sensors in a particular direction without altering the robot's overall direction.

The pan/tilt unit needed to be placed in the center, at the front end of the robot. Placing the pan/tilt unit at the extreme front end of the robot allows the sensor array to see the ground directly in front of the robot, just past the wheels.

This configuration was desired to allow the robot to search for rapid drop-offs, such as a stairwell. Additionally, if the sensor array was not centered, the coverage the array would have provided would have been different from the left side to the right side of the robot, making navigation more difficult.

Mounting Solution

To address the problem of mounting electronics, a raised platform was developed to accept the pan/tilt unit. The platform needed to allow the chassis to continue to rotate — albeit to a much more limited degree — while maintaining a stable base from which sensor readings could be made.

The drawing of the platform (Figure 4) shows that it works like an inverted pendulum with two springs used on each side to keep the pendulum upright.

The platform provided a very stable base for the pan/tilt unit — even over rough terrain. The platform can pivot at the fifth-axle while the springs (shocks) act to limit the rotation of the rover around it. An added advantage of this configuration is that the platform will remain relatively stable, compared to the rotation of the Rover over rough terrain.

The platform was built using thin sheet aluminum, the springs are radio controlled vehicle oil damping shocks, and the U-joints were hand milled from 1/2 inch aluminum stock. If you do not have access to a milling machine, the U-joints can also be bent out of heavy sheet aluminum using pliers and lots of care. The platform provided enough surface area for a JStamp microprocessor and a multiplexer that increased the number of sensors the JStamp could interface with to be mounted directly onto it.

Simple Demo

To illustrate that complex materials are not needed for a center articulated chassis, Figure 5 shows a vehicle that was built using LEGO Mindstorm components. The design is very simple and straightforward. Each of the four wheels is directly connected to a motor. The motors on each side of the vehicle are connected together to form half of the chassis. The platform to hold the LEGO RCX was constructed from LEGO plates.

The only non-LEGO parts are the four rubberbands used in place of springs. While the little robot has only 1/4 inch of ground clearance and is very top heavy, it is able to climb over obstacles as high as 1-1/4 inches. This simple — yet unstable — design is only a starting point for building more robust vehicles.

So, if you want your next robot to get a little rough, give it a little suspension with a center articulated chassis. SV

Curtis Ray Welborn worked at Johnson Space Center and in the telecommunications industry before returning to school at Texas Tech University to pursue his interest in robotics and a Ph.D. in Computer Science.

Electronic Eclectic Technology

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