Arduino 2WD smartcar motor and sensors shield

thingiverse

This is ongoing project. I am designing Arduino shield for 2WD smartcar like this The purpose is to create shield that will allow easy connection of sensors and control of two motors including feedback from motor speed sensors like this. I am planning to use: Two motors ebay or from pololu Three HC-SR04 ultrasonic distance measuring sensors Photoresistor for light measurement. Something like this Possible two Sharp infrared distance measuring sensors. Two motor speed photoelectric sensors. Shield includes ATmega328 controller for motors control. Because I want to control motors using speed feedback from motor speed sensors, I think it will be good to have dedicated controller for this task. The idea is that main Arduino board will only need to transfer desired speed for each motor via I2C. Then controller located on shield will control motors using feedback from speed sensors. PCB was manufactured using PCBWay service (10$). Gerber files attached. Any comments welcomed. How I Designed This Motor driver ALLEGRO MICROSYSTEMS A4973SB-T chip was selected to drive motors. This chip has FET driving transistors, so low voltage drop expected, especially for low current motors, and minimum power will be wasted. Chip allows constant motor current up to 1.5A and voltage 5V to 50V. In my case i need 120mA and 5-6V. Driver "ON" resistance is less then 1.5 ohm So fot 1A current voltage drop can be up to 1.5V. In my case as I need only 120mA, so voltage drop will be less then 180mV According to SPEC DIP package can dissipate near 3W, so actually it seems possible to draw constantly up to 1A. Chis chip also has thermal protection and current limiting function. Current limiting I added option to use current limiting option of driver. R2,R9 0.47Ohm resistors used for current sensing. At least 0.5W resistors should be used to withstand motor stall and short circuit current. TEXAS INSTRUMENTS LM385LP-1-2 Voltage Reference used to create 1.235V reference voltage. Current limit is 1.235/(2*0.47)=1.3A This is generally for short circuit protection as selected motor should have stall current less then 1A. R3,C8 and R7,C7 recommended value used for current limiting. Current sensing R2,R9 current sensing resistors connected to A0,A1 analog inputs to enable motor current measurement and detection of stall. It may be useful to shutdown motors if stall detected and will prevent fast discharge of battery. 1.235V reference voltage also connected to AREF input of controller to improve accuracy of current measurement. Connectors JP1 - FDTI connector for programming ATmega328 controller E1,E2 motor connectors PWR - motors power generally can be 5V to 50V. For my motors 5V to 6V S1,S2,S3 connectors for HC-SR04 sensors. JP2 - jumper allows to power shield from FDTI connector (for initial debug) IR1,IR2 - connectors for IR distance sensors SS1,SS2 connectors for motor speed sensors LIGHT - connector for light sensor BAT - battery connector. This used to monitor battery voltage. JP3 - jumper allows to connect voltage from PWR connector to Vin pin of Arduino Pins used D2-D7 pins used for HC-SR04 sensors A4,A5 I2C communication A3,A2 optional IR sensors A1 optional light sensor A0 optional battery voltage control PCB was manufactured using PCBWay service Parts list ALLEGRO MICROSYSTEMS A4973SB-T x2 (5$)CRYSTAL, 32.768KHZ, 12.5PF x1 (1$)ATmega328 with Arduino Optiboot (Uno) x2 (11$)Ceramic Capacitor, 22 pF x2Ceramic Capacitor, 470 pF x2Ceramic Capacitor, 100 nF x6Electrolytic Capacitor 100 µF x3 (1$) Resistor 10 kOhm 0.25W - x5 Resistor 4.7 kOhm 0.25W - x2 Resistor 0.47 Ohm 0.5W - x2DIP Socket, 16 Contacts x2DIP Socket, 28 ContactsLM385LP-1-2 Voltage Reference x1