underwater robots

design such a system which can cancel the buyont force acting on the bot.. that is you have to provide a downward thrust equal to the weight of the bot...  

To be able to control the depth of the robot you can place two thrusters in vertical plane. You need to be more specific. What is your exact problem statement?   

well as botskool and adi pointed out,the bouyant force on the bot needs to be cancelled and for that we could use propellers with vertical axis to cancel out the bouyant force..

bt controlling the speed of the thrusters, you can manipulate the height of the bot,make it move vertically (when forces do not cancel out) ot jst make it stand still(equilibrium condition)

the exact speed at which you need to run the thrusters would depend on the bouyant force and on the opposing force that you can get out of the thrusters.

Another way is use the method used in submarines..

In it,water is filled into the submarine(into the ballast tank to be exact)

by controlling the amount of air and water inside your bot,you can change the effective density of the bot and when the density matches the density of the surrounding fluidyour bot is suspended midway in water,jst the way you wanted it to be