Dark detector circuit using PI Controller

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    06-Aug-2015
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Transcript of Dark detector circuit using PI Controller

  1. 1. Control system Automatic street light control system using PI controller Aroosa Sheher Sidra Ali Mrayam
  2. 2. Dark detector circuitusing LDR LED : Parts list of dark detector: R1 = 330 Resistor R2 = 1K Resistor R3 = 1k Variable Resistor Q1, Q2 = BC547 Transistor One Photo Resistor (LDR) One 5V DC power supply or Battery Circuit diagramof dark detector/sensor: When the LDR (Light Depended Resistor) is in light itsresistanceis low, and when in darkits resistancegoeshigher. When the light level decreases meansthe LDR is in darks then LDR goes itsmaximumthreshold resistance, thenthe circuit automaticallyswitchesonthe LED. Here I used a 1K variableresistor as R3 to adjust thedark/light sensitivityofthe circuit. The diagram below shows a light dependent resistor, or LDR, together with its circuit symbol:
  3. 3. What is a Light Dependent Resistor or a Photo Resistor? A Light Dependent Resistor (LDR) or a photo resistor is a device whose resistivity is a function of the incident electromagnetic radiation. Hence, they are light sensitivedevices. They are also called as photo conductors, photo conductive cells or simply photocells. They are made up of semiconductor materials having high resistance. There are many different symbols used to indicate a LDR, one of the most commonly used symbol is shown in the figure below. The arrow indicates light falling on it. Working Principle of LDR: A light dependent resistor workson the principle of photo conductivity. Photo conductivity is an optical phenomenon in which the materials conductivity (Hence resistivity) reduces when light is absorbed by the material. When light falls i.e. when the photons fall on the device, the electrons in the valence band of the semiconductor material are excited to the conduction band. These photons in the incident light should have energy greater than the band gap of the semiconductor material to make the electrons jump from the valence band to the conduction band. Hence when light having enough energy is incident on the device more & more electrons are excited to the conduction band which results in large number of charge carriers. The result of this process is more and more current starts flowing and hence it is said that the resistance of the device has decreased.This is the most common working principle of LDR
  4. 4. Characteristics of LDR: LDRs are light dependent devices whose resistancedecreases when light falls on them and increases in the dark. When a light dependent resistor is keptin dark, its resistanceis very high. This resistanceis called as dark resistance. Itcan be as high as 1012 . And if the device is allowed to absorb light its resistancewill decrease drastically. If a constant voltage is applied to it and intensity of light is increased the currentstarts increasing. Figurebelow shows resistancevs. illumination curvefor a particular LDR. Also, LDRs are less sensitive than photo diodes and photo transistor. LDR controlledTransistor circuit: The circuit shown above shows a simple way of constructing a circuit that turns on when it goes dark. In this circuit the LDR and the another Resistor form a simple 'Potential Divider' circuit, where the centre point of the Potential Divider is fed to the Base of the NPN Transistor. When the light level decreases,the resistance of the LDR increases.As this resistance increases in relation to the other Resistor,which has a fixed resistance,it causes the voltage dropped across the LDR to also increase. When this voltage is large enough (0.7V for a typical NPN Transistor), it will cause the Transistor to turn on.
  5. 5. PI Controller The Proportional-Integral (PI) algorithm computes and transmits a controller output (CO) signal every sample time, T, to the final control element (e.g., valve, variable speed pump). The computed CO from the PI algorithm is influenced by the controller tuning parameters and the controller error, e (t) Integral action enables PI controllers to eliminate offset, a major weakness of a P-only controller. PI controllers provide a balance of complexity and capability that makes them by far the most widely used algorithm in process control applications. Pi controller are use to increase the speed of the response and also to eliminate the steady state error The PI Algorithm Where: CO = controller output signal CObias = controller bias or null value e(t) = current controller error Kc = controller gain, a tuning parameter Ti = reset time, a tuning parameter The first two terms to the right of the equal sign are identical to the P-Only controller. The integral mode of the controller is the last term of the equation. Its function is to integrate or continually sum the controller error, e(t), over time. (PI) Controller block diagram
  6. 6. Effects of Proportional gain: The figure showsthatthere isalwaysa steadystate error inproportional control.The errorwill decrease withincreasinggain,butthe tendencytowardsoscillationwill alsoincrease. Effect of integral gain (ki) with PI controller, we are able to eliminate the steady state error. For small value of Ki (Ki = 0.01) , we have smaller percentage of overshoot (about 13.5%) and larger steady state error (about 0.1). As we increase the gain of Ki, we have larger percentage of overshoot (about 38%) and manage to obtain zero steady error and faster response.
  7. 7. Implementation of Proportional Integral (PI) Controller Procedure: 1. After making all required connection on the board, switch on the power supply. 2. Ground PV and inputs of summing block which are not in use. 3. Set the desired Voltage to the set point. 4. The output of summing block is provided at the input of the Plant (darkness detector circuit). 5. The output of Plant (Darkness detector circuit) is applied as the feedback to the summer. 6. Vary slowly the Kp & Ki value and observethe changes in the output. Table: Sr. no Set Point(V) Gain Kp Gain Ki Output (V) 1. 5 3.5 3.5 4.9 2. 4.5 3.5 3.43 4.3 3. 3.6 2.4 3.5 3.4