Guided missiles are equipped with two types of control surfaces. The stationary (dorsal) fins provide for in-flight stability and some lift. The movable control surfaces (tail control surfaces) provide the necessary steering corrections to keep the missile in proper flight attitude and trajectory.
TYPES OF CONTROL SIGNALS.- The basic control signals may come from inside the missile, from an outside source, or both. To coordinate these signals, the missile has onboard computers to mix, integrate, and rate the control signals.
Figure 9-13.-Arrangement of control surfaces.
The computer network takes into account guidance signals, missile movements (rotation and translation dimensions), and control surface positions. By continuously computing this information, the computer network generates error signals. These signals cause the control surfaces to move and result in steering corrections.
Does the information in the last paragraph sound familiar-kind of like a servo system? Well, it should because guided missiles use servo systems/ servomechanisms that are very similar to those we discussed with GMLS power drives.
CONTROL SYSTEM OPERATION.- A block diagram of a basic missile control system is shown in figure 9-14. Free gyroscopes provide inertial References from which missile attitude can be determined. For any particular attitude, gyro signals are sent from the gyroscope sensors to the summing network of the computer. These signals are proportional to the amount of roll, pitch, and yaw at any given instant.
Figure 9-14.-A basic missile control system (servo).
After the gyro signals are compared with other information (e.g., guidance signals), correction signals are generated. These signals are orders to the controller servo and are used to position the control surfaces.
In addition to the internal feedback (response), an external feedback feature is present. Because the gyroscopes continuous y detect changes in missile attitude, they are always producing an output order.
LEARNING OBJECTIVE: Recall the purpose and functions of missile guidance systems to include the phases of guidance and the various types of guidance systems.
The guidance and control functions of a missile are often confused as being the same. Well, they are in one sense and are not in another. A guidance system is used to keep the missile on its proper flight path (trajectory) and headed toward the target. The guidance system can be thought of as the brain of the missile. The control system performs two distinct tasks. First, it maintains the missile in proper flight attitude. Using instruments like gyros, the control system corrects for problems experienced through rotation and translation. Second, the control system responds to orders from the guidance system and steers the missile toward the target. Think of the control system as the muscle of the missile.
Therefore, the guidance and control systems DO work together to (1) determine the flight path of the missile and (2) maintain the missile in proper flight attitude (stability). Four processes are involved with these combined operations:
1. Tracking- the positions of the target and missile are continuously determined.
2. Computing- the tracking information is used to determine the directions necessary for control.
3. Directing- the directions or correcting signals are applied to the controlling units.
4. Steering- using the correcting signals to direct the movements of the control surfaces.
The first three processes are performed by the guidance system of the missile. The fourth process, steering, is accomplished by the control system of the missile.
Figure 9-15 is a simple block diagram of a basic guidance system. This system is very similar to a basic control system shown in figure 9-14. The two systems interrelate and interact in their operations.