I am totally new to arduino projects.I am trying to make a wifi controlled car which i can control from anywhere using wifi.I was able to use arduino motorsheild r3 with arduino uno r3 to move the motors of rc car.No i want to connect raspberry pi which i already have so that i can control it from WIFI as raspberry already have wifi installed on it with static Ip.have searched everywhere but can't find any detailed instruction of how to do that.Please guide me in proper direction how to do it.
I'm not completely sure what you want, but it seems that you have an RPi with WiFi capability and an Arduino without WiFi, and you want to control the Arduino via WiFi. In that case you need to have some software on the RPi that receives your commands, and sends them via the serial interface to your Arduino, and your Arduino sketch will need to receive its commands from the serial port.
The most obvious way to connect to a user interface via WiFi would be to host a web server on the RPi and access it from a browser wherever you want your UI to display.
I want to connect arduino uno to raspberry pi using usb.I want to know what code should i use to control arduino through raspberry pi to operate the rc car using wifi
Yes, that was the solution I assumed, although you explained it better than I did.
You need to write a webapp that presents your UI and sends commands to the Arduino's serial port. The webapp would be hosted on the RPi. On the Arduino you need to run a sketch that receives commands from the serial port and actions them.
I don't know whether Firmata supports PWM and Servo outputs but it might be worth a check just to see whether it would be a useful starting point - otherwise, you'd be starting from scratch.
You do always have the option of adding a WiFi network interface to the Arduino and cutting the RPi out of the solution completely, but if you already have the RPi and don't have a WiFi adapter you might not be interested in that.
Searched everywhere on net but can't find any idea about how to connect Arduino uno with motorshield r3 to raspberry with GPIO pins so that i can free up my USB for external power.Can anyone guide i am totally new to this stuff.