I am doing a project on solar tracker. i am using 2 dc wiper motor. i got a problem. How to control the speed of dc motor? how to decrease the rpm of dc motor.
Here's what I suggest: Run the motor at its rated voltage using an h-bridge driver; this driver can be something fancy based on MOSFETs, or something simple based on relays.
Note that you want to track the sun; since the relative motion of the sun across the sky isn't very quick, and large changes to cause the tracker to move don't occur, a simple relay control system (with appropriately sized relays - I would just use standard 15 or 30 amp automotive relays for these motors) will likely last years, if not forever. Don't over-complicate things before you start; you'll just be throwing money away.
Still, these motors, meant for moving windows at a certain rate of speed, will likely be too fast for your application. You need to slow them down enough to make small adjustments (a degree or two every hour or so - something like that), but not too slow. You need to monitor the angle and turn them off and on properly, which will be done somewhat indirectly by your sensing system. You also need to monitor your end points to prevent overtravel and stalling/breakage of the motors and/or linkages.
I would slow them down with a reduction system. Attach (in some manner) threaded rod to the output shaft of the motor, and add a nut as the follower along the threads which, as the rod turns, will move along the rod in a linear fashion, actuating the rest of the mechanism. Grease the rod once you have it fabricated to prevent binding and corrosion. If corrosion/weather is a great concern, look for stainless steel threaded rod, or perhaps nylon or some other plastic (you may have to buy blank rod and thread it yourself).