自動(dòng)攻絲機(jī)設(shè)計(jì),自動(dòng)攻絲機(jī)設(shè)計(jì),自動(dòng),攻絲機(jī),設(shè)計(jì) The Basic Machines When a prehistoric man or woman used a stick to pry up a stone, the lever was invented. It is one of the six basic machines. A lever is a rigid piece or bar, like the early person’s stick, which turn on a point called the fulcrum. When force is applied at a second point, that force is transmitted to a third point where it can perform a work. A children’s seesaw is an excellent example of a lever. The point of balance on which the seesaw rests is the fulcrum; when downward force is applied to one end, the other end rises. The organized use of lever goes back beyond the beginning of recorded history. Levers were probably used to raise the huge blocks of stone from which Stone-henge was constructed. Perhaps the stones were raised by using tree trunks as levers until the stones toppled into place. The wheel and axle is the second basic type of machine. Like the lever, the wheel goes back to prehistoric times when someone probably discovered that it was easier to move heavy weights by sliding them on logs than by carrying them. The axle is a shaft on which a wheel can turn and the wheel and axle combination may have first used sometime around 3000B.C. for water-raising devices. It use for transportation evolved with the domestication of the horse. War chariots were the tanks of ancient times and wagons were the trucks. The third basic machine is the pulley. This simple device was used in ancient times for tasks such as raising water from wells or streams. A pulley contained in a housing is called a block. When a fixed block to which a weight is attached, downward pull on the rope will raise the weight. The device is called a block and tackle. The three remaining basic machines are so related to one another that they are sometimes grouped together. They are the wedge, the inclined plane, and the screw. The wedge is a triangle with two chief surfaces that meet in a sharp angle. Wedges are used for splitting open or pushing apart. They were used from very early times for cutting wood, as with an axe. A nail is a familiar form of the wedge. We have already mentioned the inclined plane as the probable method employed by the Egyptians for manipulating into place the huge blocks of stone in the pyramids. Early men and women knew that a weight could be pushed up to a hill or a ramp of earth with less effort than would be required to move the same weight vertically. The inclined plane is an important factor that concerns civil engineers when designing highways or railroads. The mechanical engineer more frequently uses the screw, a spiral form of the inclined plane. The figure that results from wrapping the line of an incline plane around a cylinder is called a helix. Essentially all machines are variations or components that of the six basic types. There are a number of different kinds of mechanisms or components that transmit motion or change it in one way to another. Modern machines and their components have become so complex that a branch of the science of mechanics called kinematics evolved of most modern machines, it is their mechanisms that give them their great versatility and flexibility. Gears play such an important part in machines that they have become the they have become the symbol for machinery. They are wheel with teeth that engage or mesh with each other so that they work in pairs to transmit or change motion. They are frequently used to reduce or increase the speed of a motion and they can also change the direction of motion. The line around which a wheel rotates is its axis; gear can change axial motion. Another kind of mechanism is the cam. Like the gear, it consists of a pair of components; the cam itself is the input member and the follower is the output member. The cam is attached to a rotating shaft; it transmits motion to the follower. Cam comes in many different shapes-there are heart-shaped cams, clover-leafed cams and others. By means of these different shapes cams can change rotating into reciprocating motion or into oscillating or vibrating motion. The follower is usually a rod or shaft. Cams can also transmit exact motion at special times in a cycle. They are there useful where the timing of complex motions is important. They are in automobile engines to raise and lower ten valves and in sewing machines to control the movements of the needle. Another kind of mechanism is known as a linkage; it is a series of at least three rods or solid links that are connected by joints that permit the links to pivot. When one link is fixed the other links can move only in paths that are predetermined. Like cams, linkage are used to change the direction of motion, to transmit different kinds of motion, or to provide variations in timing in different parts of a cycle by varying the lengths of the links in relation to each other. The spring is a mechanism that is used in a wide variety of machines; it is frequently an elastic helical coil that returns to its original shape after being distorted. Springes are essential components in watches; in some cam mechanisms they hold the follower in place; they are found in scales and they help to cushion an automobile ride. There are many variations on the basic coiled of spring, including the leaf spring which is made of strips of elastic material and springs that depend on the compression and expansion of air. A ratchet is another paired mechanism consisting of a wheel with teeth and a pawl which drops into the spaces between the teeth. The ratchet mechanism is used to prevent a motion from being reversed or to change reciprocating into rotary motion. This is a brief introduction to the complex world of machine components. Open-Loop and Closed-Loop Control Open-Loop Control Systems The word automatic implies that there is a certain amount of sophistication in the control system. By automatic, it generally means that the system is usually capable of adapting to a variety of operating conditions and is able to respond to a class of inputs satisfactorily. However, not any type of control system has the automatic feature. Usually, the automatic feature is achieved by feeding the output variable back and comparing it with the command signal. When a system does not have the feedback structure, it is called an open-loop system, which is the simplest and most economical type of control system. Unfortunately, open-loop control system lack accuracy and versatility and can be used in none but the simplest types of applications. Consider, for example, control of the furnace for home heating. Let us assume that the furnace is equipped only with a timing device, which control the on and off periods of the furnace. To regulate the temperature to the proper lever, the human operator must estimate the amount of time required for the furnace to stay on and then set the timer accordingly. When the preset time is up, the furnace is turned off. However, it is quite likely that the house temperature is either above or below the desired value, owing to inaccuracy in the estimate. Without further deliberation, it is quite apparent that this type of control is inaccurate and unreliable. One reason for the inaccuracy lies in the fact that one may not know the exact characteristics of the furnace. The other factor is that one has no control over the outdoor temperature, which has a definite bearing on the indoor temperature. This also points to an important disadvantage of the performance of an open-loop control system, in that the system is not capable of adapting to variations in environmental conditions or to external disturbances. In the case of the furnace control, perhaps an experienced person can provide control for a certain desired temperature in the house; but if the doors or windows are opened or closed intermittently during the operating period, the final temperature inside the house will not be accurately regulated by the open-loop control. An electric washing machine is another typical example of an open-loop system, because the amount of wash time is entirely determined by the judgment and estimation of the human operator. A true automatic electric washing machine should have the means of checking the cleanliness of the clothes continuously and turn itself off when the desired degree of cleanliness is reached. Although open-loop control system are of limited use, they form the basic elements of the closed-loop control system. In general, the elements of an open-loop control system are represented by the block diagram of Fig.15.1. An input signal or command r is applied to the controller, whose output acts as the actuating signal e; the actuating signal then actuates the controlled process and hopefully will drive the controlled variable c to the desired value. Controlled process Controller Reference input r Actuating signial e Controlled variable c Fig.15.1 Block diagram of an open-loop control system Closed-loop Control Systems What is missing in the open-loop control system for more accurate and more adaptable control is a link or feedback from the output to the input of the system. In order to obtain more accurate control, the controlled signal c(t) must be fed back and compared with the reference input, and an actuating signal proportional to the difference of the output and the input must be sent through the system to correct the error. A system with one or more feedback paths like that just described is called a closed-loop system. Human beings are probably the most complex and sophisticated feedback control system in existence. A human being may be considered to be a control system with many inputs and outputs, capable of carrying out highly complex operations. To illustrate the human being as a feedback control system, let us consider that the objective is to reach for an object on a desk. As one is reaching for the object, the brain sends out a signal to the arm to perform the task. The eyes serve as a sensing device which feeds back continuously the position of the hand. The distance between the hand and the object is the error, which is eventually brought to zero as the hand reaches the object. This is a typical example of closed-loop control. However, if one is told to reach for the object and then is blindfolded, one can only reach toward the object by estimating its exact position. It is quite possible that the object may be missed by a wide margin. With the eyes blindfolded, the feedback path is broken, and the human is operating as an open-loop system. The example of the reaching of an object by a human being is described by the block diagram shown in Fig.15.2. Error detector Cotrolled Process Controller Input command Error Controlled variable Reach for object + _ Position of hand Fig.15.2 Block diagram of a human being as a closed-loop control system As another illustrative example of a closed-loop control system, Fig.15.3 shows the block diagram of the rudder control system of a ship. In this case the objective of control is the position of the rudder, and the reference input is applied through the steering wheel. The error between the relative positions of thee steering wheel and the rudder is the signal, which actuates the controller and the motor. When the rudder is finally aligned with the desired reference direction, the output of the error sensor is zero. Fig.15.3 Rudder control system The basic element and the block diagram of a closed-loop control system are shown in Fig.15.4. In general, the configuration of a feedback control system may not be constrained to that of Fig.15.4. In complex systems there may be a multitude of feedback loops and element blocks. Error sensor Controller Controlled Process Imput Error Feedback elements Fig.15.4 Basic element of a feedback control system