<p>Fractional order controllers have received a lot of attention during the past two decades to improve the robustness of the control system when replacing the traditional PID controllers. Several tuning methods have been developed, but they mostly tackle the robustness to gain variations. In this paper, an approach to design a fractional order [PI] controller for robustness to time constant variations is presented. The design is based on specifying the robustness condition using partial derivatives. Additional frequency domain specifications lead to a system of nonlinear equations, which is graphically solved to simplify the tuning approach. Numerical simulations and comparative closed loop results are used to validate the approach for first order plus dead time models. An experimental validation is also provided to showcase the efficiency of the proposed approach.</p>