This work presents a new path tracking method for a passive-type omnidirectional walking aid system, called Omni RT Walker-II (ORTW-II). ORTW-II is the second version of a walking aid system with an omnidirectional platform and uses MR brakes for its control. A new interpretation of paths, called potential canal is conducted in this paper in order to realize tracking via merely control of brakes of the passive system. Unlike most tracking methods which attempt to lead an objective system on a desired trajectory, in the new algorithm, a mobile system's motion is constrained in a pathway rather than a single trajectory and motion can be conducted by the user. In systems with direct human interaction similar to walking aid systems, the new method is expected to increase userdependability of the system's behavior while increasing user's freedom. A collision-free potential canal is maintained using a realtime modification method, based on environmental information. Experimental results are included to evaluate the accuracy and quality of tracking.