The electromechanical brake (EMB) is a key component of the brake-by-wire (BBW) system generating the clamping force between the brake pad and disk through an electric motor and mechanical transmission. Control of the clamping force has significant impact on the brake performance. In this paper, an EMB clamping force control system is developed that addresses several major challenges in the practical implementation. Firstly, a nonlinear EMB model including the DC motor, planetary gear set, ball screw, and clamping force model is built. In particular, a novel clamping force model is introduced based on a linear transform of two polynomial functions. For the control system design, a clamping force estimation algorithm is proposed that requires only the existing measurements of motor torque, angular speed, and position. The contact point of the brake pad and the disk is also estimated using an internal model controller and a Rauch-Tung-Striebel smoother. The tracking controller is based on the disturbance observer structure, with a PI feedback controller and a zero-phase error tracking feedforward controller. Furthermore, a unified architecture is proposed such that a smooth transition between gap closing and clamping force tracking is realized. Finally, the performance of the entire control system under various conditions is evaluated based on simulation.