Patrick McDowell
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Figure 3. Mockup in stretching position. The legs are made of masonite, the backbone is a threaded rod, and the hips are flat bar aluminum. The material selection criteria were simple. The materials needed to be light, strong, easy to work, cost effective, and easily available. Experience gained in building the mockup included, materials selection, shaping of aluminum and cutting of masonite, and fashioning simple joints. Masonite turned out to be a good material because of its relative lightness, it is easy to drill and cut, and its strength and flexibility are more than adequate for this scale of construction. The joints are simple masonite sandwiches. They move in one dimension, and offer good support. From the above figures it can be seen that the basic proportions of a cat were captured by the mockup. From the outset of the project one the most pressing problems has been what sort of actuators should be used. Several methods of joint locomotion have been developed and used over the years. MIT’s Leg Lab has developed several impressive robots which they have used for studying walking which can be seen at their website. Many of these robots, including their Quadruped (1984 –1987) use hydraulic/air spring combinations for locomotion. But for the prototype tiger, size, cost and complexity considerations led to the selection of RC servo motors as joint actuators. RC servos like those used in model airplanes and cars have several good features, ie.; they are common, economical, they come in varying sizes, and computer controlled interfaces are available. On the downside, they use a considerable amount of power, yet do not produce an overwhelming amount of torque. The standard Fujitsu type RC servos like we used do not use a worm gear for torque multiplication, which means that even when a joint is not moving, if there is a load on it, power must be used to hold it where it is. Using the body of the RC servo as part of the bone, the limbs and joints were assembled using the servos in a direct coupling with joints. This method was used because it is mechanically simple and it was known that this method had been used by others at LSU and was common in hobby robotics. Because the amount of torque required to move a servo at the end of a bone is directly proportional to the length of the bone, much consideration was given to locating the servos in the body of the cat and using mechanical linkages to move limbs. Most animals that run fast have the bulk of their muscle on the torso. Tendons attached from the muscle to the bone move the limbs. This method is much more efficient, but as stated earlier, it is more mechanically complex. The initial hunch was that by using clever programming techniques mechanical inconsistencies, weaknesses, and limitations could be overcome. Figures 4 and 5 below detail the construction of Mickey’s legs. 
Figure 4. Front and side view of first prototypes front legs. The bulk of the servo can be seen in the leg on the left-hand side of the picture. 
Figure 5. Rear leg of Mickey. It is obvious from figure 5 that the servo mounted on the rear hip of the robot will be working hard to lift the servos in the rear leg. Figure 6 shows a side view of the completed robot, Mickey. 
Figure 6. Side view of first prototype, Mickey. Aside from the bunny rabbit looking head and the bones being too wide the overall proportions of Mickey are very close to the original mockup and to those of a cat. Commercially available robot kits usually have six legs, arranged like those of a bug. That is, the joints do not swing in parallel planes. This leads to stability, because the upper part of the leg is used to widen the “track” of the bug. It is also efficient, because the robot does not have to expend energy to stand up. Cats on the other hand have narrow shoulders and their joints move in the vertical plane, parallel to their bodies, leading to a balance, and control problem. This issue combined with the torque problem that was detailed earlier made it very hard to control Mickey. Even so, Mickey served as an excellent platform to interface the computer to the electronics that control the servos. To overcome the problems, better balance, and more mechanical advantage was needed. Since new servos were not an option, the limbs were shortened in order to lessen the amount of torque needed at the joint. Also the rear legs were changed to be like the front legs, so that the second prototype, Stubby, is not nearly as faithful to the proportions of a cat as Mickey. But the servo motors can move Stubby around with brute force, where Mickey had to have perfectly coordinated sequences of moves in order to take even a few steps. In fact, Mickey could take a few steps, but did not have the power to weight ratio to get itself back to a standing position. By making a more mechanically powerful robot, the problem of coordinating the movements of the joints became exponentially easier. Figure 7 below shows a view of Stubby sporting its tiger markings. 
Figure 7. The second prototype, Stubby. Stubby’s shorter limbs decreased the torque required to move the limbs. Also the rear legs were simplified to operate identically to the front legs. Doing this eliminated the third servo on the rear leg. Thus, Stubby has two servos per leg, a total of eight servos, while Mickey had two servos for each front leg and three for each rear leg, a total of ten servos. These changes definitely take away from the likeness to a cat, but it was a necessary developmental change. In defense of the change, it was noted that young cats walk with the majority of the motion coming from the upper part of the leg, as Mickey was programmed to do, but it was observed that older cats tended to use the middle joint and the foot more. By providing a more stable platform, by virtue of the robot being shorter, and a better “power to weight ratio” the job of controlling became easier. Stubby can bring itself to a standing position with much less of a struggle than Mickey could. Directory: Academics -> Faculty -> pmcdowell Faculty -> Chapter 1 “Preliminaries” Reasons for Studying Concepts of Programming Languages Faculty -> Chapter 7 “Expressions and Assignment Statements” Faculty -> Deleuze and Analytic Philosophy by Faculty -> Curriculum Proposal Title of Proposal: Course Status Change Faculty -> Chapter 6: Arrays Faculty -> Chapter 5 Introduction pmcdowell -> Guide to Intelligent Systems; Second Edition; Michael Negnevitsky Faculty -> Carmen Sara Cañete Quesada Faculty -> Economic Conditions for Regional Development and for First Nations Download 0.83 Mb. Share with your friends:
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