Electronics Assignment for Robocon 2007

Introduction: This assignment describes the electronic components to be used and their integration with mechanical parts for the working of the autonomous machines. Block Diagram of electronic control: Sensor Signal Condn. Microcontroller H-bridge Motor Sensor: The sensor is used to get inputs from the surroundings, specifically in our case, to: 1. Follow a predefined path by differentiating between blue background and white lines, and 2. To correctly pick up and place the blocks. For line tracing, a circuit employing light dependent electronic devices (photodiode, phototransistor or IR Tx-Rx) has to be used. For block pick-up and placement, proximity sensors or contact switches may be used. Signal Conditioning Circuit: The output of the sensors is in the form of analog voltage or current, and this has to be converted into logic voltage levels compatible with the microcontroller. A simple rail-to-rail operating comparator may be used. Use of a Schmitt trigger circuit provides some immunity to noise and is recommended. The following circuit is a single power supply Schmitt trigger circuit operating at +5V and it gives output compatible to TTL levels. In this circuit, the analog output of the sensor is given to the input terminal of the comparator (1/4 IC LM339). Depending on crossing of threshold levels of the Schmitt trigger, the output is practically obtained as +4.98V (high) and +0.34V (low). This is compatible with the logic levels for TTL. The threshold values for this circuit are +1.1V and +2.9V. These can be adjusted by changing the values of R1, R2 and R3 according to given formula. Microcontroller: The function of the microcontroller is to control the overall working of the machine. It is meant to receive and process input signals and take suitable controlling action based on the desired sequence of operations. We have decided to use the AVR family of microcontrollers for autonomous machines in Robocon. Required programmer and flash programming software as well as programming hardware are already available. Previous year’s controller (ATmega8518) and PCBs are also useful. If required we might have to select another controller IC in place of ATmega8515. The basic parameters and facilities are nearly the same in all AVR controller versions. Only their number varies. Selection criteria for the controller are: 1. No. of PWM generators for motor speed control. 2. No. of dedicated IO lines. 3. No. of external interrupts. 4. No. of 8-bit and 16-bit timers for delay generation 5. Availability of on-chip ADC and analog comparator A complete list of available versions as listed on the manufacturer’s (Atmel Corp.) is included along with their specifications. H-Bridge IC: The H-Bridge IC is the link between the electronics and the mechanical domains in the autonomous machine. It is the actual device that regulates the running of the motors. The H-Bridge controls the running of the motor on the following parameters: run/brake, direction of rotation, speed of rotation. The inputs to the H-Bridge are logic signals for run/brake and direction control, as well as a PWM signal to operate the motor at fractions of rated speed. PWM signal can theoretically run the motor at speeds from zero to rated speed. For ease of operation, it is desirable that each of the three parameters are controlled independent of each other. In locally available H-Bridge ICs like L298, all three controlling parameters are operating in the same two pins of the IC. This causes difficulty in writing the programs as well as designing of the PCB. The H-Bridge IC LMD18200 (first purchased during Robocon 2005) offers brake, direction, and PWM control on three separate pins, and so it is easiest to use. Its electrical ratings are also suitable for the motors that we will be using on our machines. LMD18200 specifications: Max. operating voltage: +55V Operating current: upto 3A continuous Internal clamp diodes Thermal shutdown and shorted load protection Available in TO-220 as well as 24-pin DIP Motors: The allowed power supply is upto 24V dc. Hence it will be better if we make use of the maximum allowed power supply. It is recommended to use 24V DC motors of required speed (and available torque) Batteries: Use of nickel - metal hydride (NiMH) batteries is preferred as they are easy to charge and available with good current capacities. 12V or 24V battery packs can be connected by soldering the cells together. Battery packs having current capacities in excess of 2000 mAh can be made. To operate the microcontroller at +5V supply, a voltage regulator has to be used. It is not possible to use lithium ion (Li-ion) cells as they cannot be charged if permanently soldered in the form of battery packs. List of Electronic components: 1. Sensors (based on independent testing) 2. Signal conditioning circuit suitable for selected sensors 3. Microcontroller 4. H-Bridge (specifically the LMD18200 H-Bridge IC) 4. NiMH batteries Concluding remarks: Intention of this assignment was to: 1. List out the various electronic components required 2. Provide general information in electronics to everyone working on the project 3. Seek suggestions from experts in order to improve the electronics design. Aditya Kshirsagar TE E&Tc (2007)

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