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group 1 Building a gun position control system

acknowledgement We would like to take this opportunity to thank our LCT professor MR. NIRVAN YOGI for giving us the independence to work on a topic of our choice as our assignment. This was indeed a source of great knowledge. We would also like to thank the department for including this subject in our curriculum for the enhancement of our knowledge. Also extending gratitude towards our college for providing us with the necessary means like the internet, library etc. to accomplish our assignment. Extending a heartfelt thank u to my parents and friends for providing essential support.Thanking one and all.

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Building a Gun Position Control SystemDescription: Proper positioning of gun by moving a joystick left or right depending on the direction needed. This must be performed as fast as possible. Learning Objectives:

To study different methods of angle tracking:o Sequential LobingoConical Scano Simultaneous Lobing(Monopulse)

To study Range Tracking MethodsTo study basic principle of Automatic tracking system.

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INTRODUCTIONControl engineering is based on the foundations of feedback theory andlinear system analysis, and it generates the concepts of network theory andcommunication theory. Accordingly, control engineering is not limited to anyengineering discipline but is applicable to aeronautical, chemical, mechanical,environmental, civil, and electrical engineering.A control system is an interconnection of components forming a systemconfiguration that will provide a desired system response. The basis for analysis of a system is the foundation provided by linear system, which assumes a cause effect relationship for the components of a system. A component or process to be controlled can be represented by a block as shown in Figure 1.

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An open-loop control system utilizes a controller or control actuator to obtainthe desired response as shown in Figure 2. The open-loop control system utilizes an actuating device to control the process directly without using device. An example of an open-loop control system is an electric toaster.

A closed-loop control system (Figure 3) utilizes an additional measure of theactual output to compare the actual output with the desired output response. The measure of the output is called the feedback signal. A feedback control system is a control system that tends to maintain a relationship of one system variable to another by comparing functions of these variables and using the difference as a means of control. As the system is becoming more complex, the interrelationship of many controlled variables may 6

be considered in the control scheme. An example of closed-loop control system is a person steering an automobile by looking at the auto’s location on the road and making the appropriate

adjustments.Major Types of Feedback Used:

Position Feedback Used when the output is a linear distance or angular measurement.

Rate & Acceleration Feedback Feeds back rate of motion or rate of change of motion (acceleration) Motion smoothing Uses a electrical/mechanical device called an accelerometer

RADAR7

IntroductionRADAR is an electromagnetic system for the detection and location of objects (RADIO DETECTION AND RANGING). Radar operates by transmitting a particular type of waveform and detecting the nature of the signals reflected back from objects Radar cannot resolve detail or color as well as the human eye (an optical frequency passive scatter meter) Radar can see in conditions which do not permit the eye to see such as darkness, haze, rain, smoke Radar can also measure the distances to objects The elemental radar system consists of a transmitter unit, an antenna for emitting electromagnetic radiation and receiving the echo, an energy detecting receiver and a processor. A portion of the transmitted signal is intercepted bya reflecting object (target) and is reradiated in all directions The antenna collects the returned energy in the backscatter direction and delivers it to the receiver The distance to the receiver is determined by measuring the time taken for the electromagnetic signal to travel to the target and back. The direction of the target is determined by the angle of arrival (AOA) of the reflected signal.

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Also if there is relative motion between the radar and the target, there is a shift in frequency of the reflected signal (Doppler Effect) which is a measure of the radial component of the relative velocity. This can be used to distinguish between moving targets and stationary ones.Radar Basic Principles : The following figure shows the operating principle of a primary radar set .The radar antenna illuminates the target with a microwave signal, which is then reflected and picked up by receiving device. The electrical signal picked up by receiving antenna is called echo or return. The radar signal is generated by a powerful transmitter and received by a highly sensitive receiver.

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All targets produce a diffuse reflection i.e. it is reflected in a wide number of directions. The reflected signal is also called scattering.Backscatter is the term given to reflection in the opposite direction to the incident rays. Radar signals can be displayed on the traditional plan position indicator (PPI) or other more advanced radar display systems. A PPI has a rotating vector with the radar at the origin, which indicates the pointing direction of the antenna and hence the bearing of targets.Transmitter The radar transmitter produces the short duration high power RF pulses of energy that are sent into space by the antenna.DuplexerThe duplexer alternately switches the antenna between the transmitter and receiver so that only one antenna needs to be used. This switching is necessary because the high power pulses of the transmitter would the receiver if energy were allowed to enter the receiver.ReceiverThe receivers amplify and demodulate the received RF signals. The receiver provides video signals on the output.

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Radar AntennaThe antenna transfers the transmitter energy to the signal in space with the required distribution and efficiency. This process is applied in an identical way on reception.IndicatorThe indicator should present to the observer a continuous, easily understandable, graphic picture of radar targets.

Types of Radars• Surveillance11

All types of surveillance radar, civil or military, could be considering as tracking systems since they form an estimate of the target position each time the scanner returns to look in that particular direction. • Track-while-scan Surveillance radars can keep track of many targets simultaneously but the positional accuracy they provide, especially in angle, is not adequate for some purposes.

• Tracking Continuously measures the coordinates of a moving target in order to determine its path and to predict where it is going. Can be carried out using range, angle or Doppler information, but it is the tracking in angle that forms the characteristic feature of tracking radars.

Tracking Radar12

• Dedicated to a target and observes it continuously and with great precision, usually with a view to engaging a weapon system.• Mode of operation:

Main surveillance radar warns of any targets posing a particular threat1.Searches the immediate area (Volume Search) to acquire the target before initiating the tracking process.2.Download the target coordinates to the tracking System Classified into two typesI. Continuous tracking radar II. Track-While-Scan radarTracking Basics• The antenna of tracking radar:

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Faces the target all the time in order to keeps it in the center of the beam maximize the signal-to-noise ratio. If the target moves away from the center of the beam, this produces an error voltage, which is amplified and fed to servomotors to drive the antenna back onto the target. The azimuth and elevation of the target are then read from angle transducers mounted on the antenna axes. The methods of producing error signals from the antenna patterns form one of the main aspects of tracking radar design.

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Radar Parameter Estimation

Slant Range:In radio electronics especially radar terminology, slant range is the line-of-sight distance between two points which

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are not at the same level relative to a specific datum.Elevation AngleThe elevation angle is the angle between the horizontal plane and the line of sight, measured in the vertical plane.

Azimuth angle:An azimuth is an angular measurement in a spherical coordinate system. The vector from an observer (origin) to a point of interest

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is projected perpendicularly onto a reference plane; the angle between the projected vector and a reference vector on the reference plane is called the azimuth angle.Parameter Estimation Different Levels of Attenuation

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Range Estimation

Antenna Size Improves Beamwidth

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Angle Tracking Servo Systems Five Basic Functions :

Sense position error magnitude and direction Provide position feedback Provide data smoothing / stabilization Provide velocity feedback Provide a power-driving device

Methods of Angle tracking• Trackers using a single time-shared beam to track angles: Sequential lobing Conical Scan• Simultaneous beams:

Monopulse trackers19

Angle Estimation

SEQUENTIAL LOBING

• Two lobes are required to track in each axis, each lobe must be sequentially switched four pulses are required• The radar measures the returned signal levels

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• The voltages in the two switched position should be equal

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• Sequential lobing: Switching between several beam positions • Time-share single-beam (less accurate but simpler!) One of the earliest methods used to derive angular information about a target An antenna would have two beams displaced slightly left and right of its bore sight (centerline) and the receiver switched rapidly between them. If the target appeared to grow larger in the left-hand beam, this would drive the antenna to the left until the signal was equal in both beams again. Similarly, two beams displaced vertically could be used to give elevation information. The difference in amplitude between the voltages obtained in the 2 switched positions is a measure

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of the angular displacement of the target from the switching axis (boresight) When echo signals in the 2 beam positions are equal, target is on axis and its direction is boresight!

Conical Scanning• An alternative to stepping the antenna beam around the direction of the target is to rotate it continuously23

• Because the rotation of the squinted beam and the target’s offset from the rotation axis, the amplitude of the echo signal modulated at beam rotation frequency Amplitude of the modulation depends on the angular distance between target direction and rotation axis.

Squint angle : between axis of rotation and the axis of antenna beam

• Conical-scan modulation extracted from echo signal24

applied to a servo control system -- continually positions the antenna rotation axis in direction of target by moving the antenna so that the target line of sight lies along beam rotation axis• When antennas is “on target”, conical scan modulation is 0 Amplitude.

Target on thre Bore-Sight All target returns have the same amplitude (zero error signal) Thus, no action is required

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Target off the Bore-sight

DISADVANTAGES : Sequential lobing : 1) Angle accuracy can be no better than the size of the antenna beamwidth. 2) Variation in echo strength on a pulse-by-pulse basis changes the signal level thereby reducing tracking accuracy

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3) The antenna gain is less than the peak gain in beam axis direction, reducing maximum range that can be measured Conical scan: 1) The antenna scan rate is limited by the scanning mechanism (mechanical or electronic)2) Sensitive to target modulation3) Mechanical vibration and wear and tear due to rotating feed

Monopulse Radar Developed to overcome tracking errors involved with conical scanning and sequential lobing . The problems of pulse-to-pulse variations in echo

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amplitudes can be overcome by using more than one beam simultaneously to measure the angular position of the target on a single pulse. Makes use of amplitude information from the antennas, phase information or both better precision than sequential lobing and conical scan systems. A target can be located from a single pulse measurement useful when the radar is being jammed and the target is only viewed in glimpses. Angular location obtained by comparison of signals received from simultaneous beams Develop error signal in 2 orthogonal angle coordinates

drive bore-sight if tracking antenna (to target!)

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Range Tracking: It is an extension to the independent measurement of range on a pulse by pulse basis It involves using the previous estimate of the target range and information about its velocity to predict where the target will be in time for the next range measurement.

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This prediction is then used to constrain the area observed when detecting the next echo. Most range tracking systems use a type 2 (2 cascaded integrators) to predict the position of constant range-rate targets with zero lag.

Range Gating Range gating involves sampling the received echo signal at a specified time after a pulse has been transmitted Sampling involves measuring the pulse amplitude over a short time using a switch

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that charges a capacitor, or a sample-and-hold IC that performs a similar function The sampling period should be equal to or shorter than the pulsewidth for optimum performance A number of samples can be obtained across an echo pulse as shown in the figure

The Split Gate Tracker It consists of two sample-and-hold circuits triggered about one half of the pulse width apart. These are called the early and late gates. The output of the S&H is the integral of the voltage in the echo pulse within each gate. The difference between these voltages is equal to the range tracking error, and it is used to drive the tracking filter which in turn

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moves the gate timing to centre the gates ready for the next echo pulse.

Tracking Error Transfer Function Too Early On Time Too Late

Transfer function If the split gates are early with respect to the echo pulse, then a positive error is generated If they are aligned, then the error is zero

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If they are late, then the error is negative A continuous transfer function can be determined that maps the timing error into a voltage error

Range Transfer Function:At the time corresponding to the estimated target range, these S&Hs are triggered, one, one half pulse width prior to the estimated range delay, and one the same period after. If the range estimate is accurate, they sample equal amplitudes of the target echo pulse on either side of the peak as shown in the diagram, and the difference between the two gate voltages VL-VE = 0. However, if there is a small range error then the contents of the one gate will be larger than the contents of the other and the difference will not be zero. To measure the transfer function, it is possible to keep the gates still and move the target through the gates, or vice- versa. The following figure shows the measured range transfer function for an ultrasonic radar simulation 411.Note that the sign and magnitude of the error will drive the tracking error toward zero only over a limited range (in this case, about 20 samples) and

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the “linear” region over which the magnitude of the range error can be used to estimate the actual error is more limited (about 8 samples).

Fig. Measured transfer function for an ultrasonic range trackerThe range tracking loop uses this error to drive the smoothed estimate of the target range until the amplitudes of the early and late gate samples are equal. If, however, the error falls outside the bounds shown by the transfer function, then the error voltage falls to zero (in a noise free environment) and there is no driving function.

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As shown in the block diagram there are two parts. The first part (upper) is for ANGLE TRACKING and the second part is for RANGE TRACKING. One can visualize the actual system from the figure given above.At first the RADAR mounted at the top of the tank system scans any aircraft within the range of interest. Based on scanning which can be done by different methods as explained before.(Like Sequential Lobing, Conical Scan and Monopulse technique for ANGLE TRACKING and the RANGE GATE METHOD for RANGE TRACKING.)37

After the scanning is done it sends the received signal to the comparator which is required for positioning of the gun. The comparator compares the actual position of the gun with the required position in terms of AZIMUTH ANGLE and ELEVATION ANGLE and this error signal is fed to the controller which controls the GUN DYNAMICS .The process is continued until difference between the actual position and the required position of the GUN is zero i.e. The Error Signal is zero. Similarly for the range tracking the error signal is generated in terms of Position of wheel and based on it the controller decides the velocity and acceleration of the wheels and the tank moves according to that until the error is zero.Note: Angle tracking and Range tracking both are done simultaneously. Because with change in position of the tank the angular position of the gun is adjusted in such a way that it continue to aim at the aircraft. When the tank reaches to the required position, by that time the

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position of gun should also be accurate and it will be ready to fire. Also there will be a predicting mechanism in the controller to predict the position (range and angle) of the aircraft after some time so that the system reaches to steady state fast.

Questions1.How to take care of external disturbances?Ans: The external disturbances like wind and other factors are taken care by the feedback provided in the system. Whenever there in an fluctuations due to the external disturbances feedback signal is send back from output to the comparator and the comparator generates error signal which is fed to the controller and controller adjusts the dynamics of gun and wheel accordingly.2.How to ensure fast positioning?

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Ans: There are so many methods available for angle tracking and Range Tracking. For example the techniques for angle tracking are: 1.Sequential Lobing2.Conical scanning3. Simultaneous Lobing (Monopulse)As we have discussed the advantages of Monopulse tracking method over the other two methods earlier, We can use Monopulse technique for angle tracking which is faster than the other two methods. Also we can introduce a predictive mechanism in the controller to predict the position of target after some time so that the system reaches to the steady state faster because as settling time of the system decreases the system response is faster.

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