We have proposed a simple method for carrying a large object by cooperation of multiple mobile manipulators with position controllers. Manipulators on mobile platforms are used as free joint mechanisms by locking some joints and making the rest joints free. These free joints play the role of mechanical compliance in order to avoid excessive inner forces due to the mutual positioning errors. We discuss how to determine feedback control laws for cooperation of mobile platforms on uneven ground. First, compliance needed for cooperation among position-controlled robots, the statics and error characteristics of cooperative systems including compliant mechanisms are reviewed. Second, three feedback control laws for platforms moving on uneven ground are proposed and their control performance is investigated. After additional improved control laws are proposed, the proposed control laws are used for a prototype cooperative system consisting of three moving tables driven by ball screws and the effectiveness of the proposed law is verified.