In this paper we develop and improve a piecewise-linear approximations of nonlinear cellular growth using orthonormal canonical piecewise linear functions (Julian et al., 2000), and it is tested by a probing control strategy (Akesson et al.)for the feed rate. The work is with the mammalian cells BHK (baby hamster kidney) in bioreactor in batch, fed-batch and continuous mode operation. Preliminary results show that this piecewise linear approximation is well suited for modeling such nonlinear dynamics.