This paper presents a new force sensing approach for a bilateral control system with different mechanisms of the master and the slave devices. A linear shaft motor and a ball screw perform the roles of the master and the slave, respectively. Using a frictionless mechanism as the master while the slave mechanism is affected by high friction, together with an appropriate force scaling ratio between the master and the slave, the human operator can easily manipulate the device and perceive the interaction of the slave and environments. To reduce the friction effect on force estimation due to the ball screw mechanism, a periodic signal is inserted into the control signal of the slave side. A high-order disturbance observer is designed for force sensing operation on the slave side to reduce the effect of oscillatory disturbance on force information. The control algorithm consists of a conventional disturbance observer and a high-order disturbance observer for the master and the slave, respectively. All the control algorithms are implemented in FPGA to achieve a high sampling rate of the control cycle. The effectiveness of the proposed method is verified by the experimental results. © 2013 IEEE.