This paper deals with the control of a haptic device which is used in a generator mode to produce electricity from a user's walking movements in a handheld device, like a mobile phone for instance. We first present the design principle of such a device. Then, a design is presented, which allows haptic feedback and energy harvesting to be produced with a same device. It is based on a piezoelectric plate actuator. A causal modelling of the system is then developed, and inverted in order to obtain the key control algorithms. Compared to other energy harvesting system, HarTic is characterized by some interesting features like the accelerometer which is now embedded into most mobile phones, and its measurement can be used in the energy harvesting strategy. Our simulations show that 2mW can be harversted in our case study.