CONTROL SYSTEMS IN MODERN ROBOTICS

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achieve its three-second-answer time for every “Jeopardy!” question. If Watson were to use one processor, it would take about two hours for it to answer just one question. What allows Watson to achieve this speed is a combination of hardware and software. IBM used the Apache Unstructured Information Management Architecture to sort through the large amount of information that Watson has stored on its hard drive. This architecture allows Watson to spread the processing load across all 2,880 of its processor cores at once. Watson has proven that a computer is able to interpret text and use data that it already knows to answer a question given the right hardware and software, which is great for business and computing applications. This is also promising for the field of robotics in that robots could benefit from the ability to process this much data all at once and to interpret visual commands and search an internal database to determine the best course of action. One way to make robots more capable is to enable them to make decisions on how to use their hardware based on the task at hand. For example, traversing over snow, ice, mud, and sand all require different movement techniques and if a robot were to move through this terrain without compensating for the instability of snow or the slickness of ice, it could get stuck or fall and become damaged. While Watson is a good example of computing methods for rapidly inputting information and finding the best solution, the fact that it takes up over 180 cubic feet and consumes as much power as a laundromat makes computer processing at this scale impossible for mobile robots. Ideally the parallel processing power and statistical analysis of Watson would be combined with the size, power requirement, and reliability of Voyager’s computers. Nonetheless, lessons such as building computers for a specific purpose and using distributed computing could be applied at a smaller scale, making robots more autonomous [20]. THE PERFECT EXAMPLE: BIGDOG What is BigDog? Finally, after discussing control systems, sensors to give them information, and the benefit of further engineering research in the speed of computers, we come to a vital example that exhibits all of these: BigDog. Released in 2008, BigDog is Boston Dynamics’ four-legged robot. The BigDog project’s goal is to create a robot that can travel anywhere an animal or human can go. BigDog’s control system must balance, navigate, and regulate all forms of movement. Three areas were emphasized in the area of control: supporting the body through bouncing, controlling attitude of the body with torques, and key placement of feet to keep the body symmetric. By utilizing this state of the art technology with a complex quadruped, BigDog exemplifies Paper 1018 the correct direction in which intelligent robotics research should be going [21]. FIGURE 9 [22] BigDog’s Sensing Ability BigDog has the ability to sense its surroundings and react to them in order to keep from falling. It also has the ability to detect future movements, such as good foot placement. When a human walks over ice, they do so carefully, looking for good footing while not shifting too much weight onto a foot. A robot can hardly know exactly where to place a foot, and therefore the ability to foresee future movements are vital. BigDog has been equipped with close to seventy sensors. About fifty of these sensors are located on the hips, knees, ankles, and legs, measuring aspects such as joint dislocations, force approximations, and servo current. Without these necessary sensors, BigDog would collapse when traversing even the easiest terrain. Aside from leg sensors, six others are gyroscopes used to measure the body's balance, while three more are used for vision approximations, such as avoiding obstacles, measuring the ground slope, or finding patterns in the motion of the surroundings [22]. BigDog has been proven to have good stability, even when walking across ice or up an unstable hill. These sensors are a large portion of the reason why the machine is so successful, as the data sent to the control system must be accurate. But, even with an extreme amount of information, if the computer cannot process and make decisions quickly, the machine as a whole will still fail. BigDog’s Stabilization As previously stated, a legged robot can travel on more difficult terrain than a treaded one due to its ability to compensate for small mounds or ditches, but the difficulty of balancing these robots is great. BigDog's torso weighs about University of Pittsburgh Swanson School of Engineering Eleventh Annual Freshman Conference April 9, 2011 6
100 pounds and is able to carry over 300 pounds, meaning that each leg must do a lot of work to hold it up, especially when the body is majorly off balance and certain legs are given much more weight to hold. In order for these things to happen, symmetry, coordination, and flexibility must be utilized so that BigDog does not plummet to the ground [21]. BigDog's legs are syncronized in movement. The machine is extremely symmetrical, taking the form of an actual dog: If one of the legs were made slightly different than the rest, the machine would be inherently off balance and the model would fail. When fixing its balance, the legs move at the same time so that any lapse in weight being held is alleviated. As BigDog stumbles, it knows to lower its body onto its knees simultaneously, lowering the center of mass and regaining stability, before lifting up again once balance is achieved. Since the legs act like the limbs of an animal with numerous amounts of joints, these legs are able to reach wherever they need to in order to provide good footing when balance begins to fail. Each of these abilities is only possible through the computer's power. Without ample speed for the control system to decide to tell the legs to bend down on the knees after processing data indicating the need to do so, correction would not happen, and BigDog would fall. Therefore, the most important part of the machine is the control system, as it must tell which body parts to move after retrieving, deciding, and reacting at extremely fast rates based on data [21]. This illustrates the importance of further engineering in the control system field, making their thought process faster and more reliable to make machines more capable. BigDog’s Control System BigDog's control system exemplifies the reasons why improvements in control systems are needed. Through all of the analysis that has been covered on sensors and stability, the control system has an important job in legged robots. BigDog's onboard computer is small and must perform control, sensing, data collection, communications, and the electric power distribution. Put together, BigDog's control system must basically do two things: coordinate kinematics to ground movement processed while correcting instability [21]. The system uses sensors from the joints to determine the amount of load each foot should have with each and every step. Posture is controlled by relating the kinematics of each leg to the forces of the legs in contact with the ground. With kinematics in relation to ground forces, BigDog's control system can regulate body roll, pitch, and its height above ground, which allows BigDog to adapt to variations in terrain [21]. BigDog tackles steep terrain by utilizing the ability just explained. When the path gets steeper, the control system leans the body forward to account for an off-based center of mass, as well as leaning back when the path steepens Paper 1018 downward. More impressively, as BigDog runs along the side of a hill, the control leans the body towards the hill, minimizing the torque on the bottom legs, keeping the body more stable [21]. The ability to shift weight front to back and side to side proves the necessity of legged robots, as a treaded one would fall due to its off balance center of mass on steep slopes. All of this is just one step of a four step process. First the control system must retrieve data from the sensors, then process the data, make a decision of action, and finally tell BigDog's parts what to do. If this process is not performed fast enough, BigDog falls. On top of all of this, the control system must do this over and over again, as it is constantly fighting gravity and occasionally dealing with unstable terrain. What is truly remarkable about BigDog is how fast it can repeat this process. BigDog's sensors gather information about 1,000 times every second, and the control system performs this four step process about 200 times every second. The engineering behind the control has sped up the system’s ability to correct itself, which is ultimately the reason for BigDog's success in not falling [22]. A SUSTA<strong>IN</strong>ABLE SYSTEM Sustainability in a broader sense can be environmental in regards to conservation or it can involve changing the environment in a beneficial way. Robots such as BigDog are showing their value in this latter definition. Through application of advanced sensors, computer hardware and software, robots such as iRobot’s Roomba and Segway’s line of vehicles have come to fruition. The Roomba can clean houses without any more input than turning it on. This means that it can vacuum floors, giving its owner the time for other tasks. This saved time means not only a more open schedule for humans but also a cleaner living space since a Roomba can clean more frequently than a human with a vacuum could on his or her own. Also improving the human condition are Segway’s products, which allow people to travel through buildings, cities, and even nature trails with little effort. The creators of BigDog hope that it will benefit humans in a similar manner by carrying equipment through varying and difficult terrain that wheeled and treaded robots could not handle. Without the added mobility and stability enabled by the sophisticated control systems and computers, BigDog could not handle snow, steep terrain, and elevation changes present in rubble and rocks as easily as it does. Boston Dynamics has already proposed that BigDog would be useful for soldiers who often have hundreds of pounds of supplies and equipment to carry. Computers utilizing smaller, faster control systems can also make buildings more efficient, improving conditions for their occupants while also reducing unnecessary energy usage. Control systems define how economically efficient a process is, as it controls what is turned on and what is turned University of Pittsburgh Swanson School of Engineering Eleventh Annual Freshman Conference April 9, 2011 7
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