CONTROL SYSTEMS IN MODERN ROBOTICS

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integrated into other systems and share data, therefore making devices more reliable, safer, and often more accurate. The next step in the complex line of control systems is the sampled-data system. Here, analog signals, sampled data, and pulse-modulated signals are put together and used simultaneously. An advantage to this is the sharing ability between equipment and signals [2]. The most popular of these types is the digital system, which houses the ability to use analog signals, sampled data, and digital data. This code represents values in sequences, which help determine the actual value of digital information. Computers compare data and actually “think” about what the best adjustment is for the system. When more complicated control must be utilized, the digital direction is the obvious choice, as it can keep track of a lot of data and give calculated outputs based on logic switches [2]. A good example is automatic flight control in airplanes. The system must take into account a wide variety of data, such as lateral height, vertical height, and various forms of disturbances. To account for these, the main control system is a digital computer that acquires data from three independent systems monitoring these values. This proves the luxury that a digital system offers, as it can read many varieties of information at once and make smart choices based off of them [7]. Control System Thought Process All control systems work on the same basic principle: they monitor output of a system and alter input to keep output consistent. Figure 2 illustrates the thought process behind how these systems go about analyzing information. An important factor is that this chart is a loop, which means the outputs and inputs are continuously monitored. In analog systems this is represented by mechanical or electrical signals and for digital systems a computer is preset to monitor this information at a given rate. The faster this monitoring process is performed, the more precise and accurate the output adjustments are for the system [8]. FIGURE 2 [2] A necessary ability control systems must have is quick, logical reaction to unusual data. An engineer designs a system to be efficient and safe. Safety is important, and if a dangerous machine needs to be stopped, the control must react quickly and accurately. Consider a robot lawnmower once more. If the machine for some reason turns over, the Paper 1018 blades would need to shut down, otherwise someone could get hurt. A control system in this scenario would need to be told to shut down all systems, otherwise the mower would be deemed unsafe for the public. SENSORS: ACQUIR<strong>IN</strong>G DATA The first step in a control system is to input data from the system. Sensors allow robots and control systems to see what is happening in the environment around them. A variety of sensors are available for different types of functions including movement, vision, balance and scientific data collection. What is relevant for control systems handling locomotion are sensors that deal with the first three functions of the list [9]. Early robots used pegs at fixed points to stop movement. This proved inefficient over time as more technical and agile robots were made, and servo motors were implemented. Servo motors handle movement while also providing feedback. These motors read an encoder disk that has slits at defined points, as shown in Figure 3. Light sensors see when these points pass by and the time increment information gathered tells the motor how far it has turned. The robot is therefore able to detect the distance a leg or arm has moved, send this information to the control system, and make a decision to continue or stop motion [9][2]. Encoder disks are sometimes used alone as a means to find the angle of rotation in a system. In both applications, the more slits that are present in the disk, the more accurate the measurements are for the angles. An important note here is that in order for a robot to determine the change in angle, it must first know the number of slits present in the disk, otherwise calculations will be incorrect [10]. FIGURE 3 [10] Potentiometers can also be used for sensing joint displacements. A potentiometer functions by comparing an output voltage to an input voltage. These devices incorporate a resistive track made out of carbon film, conductive ceramic, or wound wire. The distance through this track that the electricity must travel is changed by the physical location of a wiper that slides across the track. This change in distance causes a voltage change. Potentiometers are usually used as volume controls in stereos and can measure a change in angle caused by something rotating. Linear potentiometers also exist, which track a distance by moving University of Pittsburgh Swanson School of Engineering Eleventh Annual Freshman Conference April 9, 2011 2
a wiper over a straight track rather than around a circle [2][10]. FIGURE 4 [10] Radial potentiometer FIGURE 5 [10] Linear potentiometer Gyroscopes, both mechanical and electrical, are used to determine if an object is balanced. Early examples include the Sperry autopilot, which use gyroscopes to handle the horizontal and vertical pitch of an airplane, enabling the pilot to walk safely across the wings without manually adjusting the pitch of the airplane [5]. Newer electronic gyroscopes are used in commercial products like the Apple iPhone to output the orientation of the device [11]. These gyroscope sensors are applications of Newton’s second law for rotation, which states that net torque equals angular acceleration multiplied by the moment of inertia. = Ι EQUATION 1 A disk spun about a frictionless axis is suspended in a gimbal or a cage, which will retain its orientation. Because this gimbal is mounted freely in a rigid body, one can measure the angle between the gimbal and the moved body to find the pitch or roll of the object being stabilized [2]. Often robots require force sensors for grasping objects or for determining the weight of an object. This information is useful for finding the amount of weight supported by each leg of a robot. Load cells are used to determine the magnitude of force. These are made with a dielectric material between two plates. As a force acts on this system, the distance between the plates increases or decreases, inversely changing the capacitance [10]. C = EQUATION 2 Paper 1018 These sensors become especially important for robots with legs or arms, as this allows the robot to see how much weight is being supported by each arm or leg and react according to weight shifts or imbalances. Vision sensors come in a variety of types from simple light detectors to cameras. Basic light detectors differentiate between light and dark; images and color are not sensed. These are often used with the aforementioned coding disks or for simple light detection. More complex image sensors can produce a black and white image, which can then be passed on to a computer for data analysis. The most important area for complex vision is the sensor placement and data processing rather than the types of sensors used [5][10]. The topic of robot vision is complicated because, while it is easy to input visual data, interpreting it for actual use is difficult and requires software. In humans, the retina converts light into electrochemical energy, which is then sent to the brain for processing. In robots a common vision sensor used today is a charge-coupled device, or CCD camera. These are ideal for robots because they are lightweight and small, consume little electricity, and have a high sensitivity over the light spectrum. A CCD array is made using an array of metal oxide semiconductor fieldeffect transistors, or MOSFETs, as seen in Figure 6. FIGURE 6 [12] MOSFET array The basic idea behind this type of vision sensor is that an array of MOSFETs stores a series of charges that correspond to the light intensity at that point. Positive semiconductor nodes are placed in a negative substrate. A depletion region separates each positive node, and then the entire array is covered in silicon dioxide. Light photons hitting the depletion region free electrons, which create a net positive University of Pittsburgh Swanson School of Engineering Eleventh Annual Freshman Conference April 9, 2011 3
Page 1: Session A2 Paper 1018 CONTROL SYSTE
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CONTROL SYSTEMS IN MODERN ROBOTICS

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