A control strategy for a leader-follower system without inter-agent communication is developed for cooperative transportation in this work. Two cascaded UKFs (Unscented Kalman Filters) are developed to estimate the external force of the leader as to eliminate the need of force sensors. Compared to most existing results, performance of the developed force estimators is invariant to lighting conditions since the developed UKFs do not require measurement from vision systems. To enhance robustness of the control scheme, a switching controller along with a triggering condition is developed for the follower, so that the impact to the performance of the closed-loop system caused by the disturbances can be minimized.