A semi-automatic and active control of T-S waves and oblique waves is performed using an array of piezo-ceramic actuators attached on a flat-plate surface. The actuators generate counter waves to cancel the incoming instability waves. The actuator's operating amplitudes and phases are successively updated using the velocity fluctuations monitored downstream by a rake of hotwires. It is shown that the system could weaken these waves when their sweep angles are less than 15 degrees. However, it has difficulty in controlling the waves of large sweep angles. Through the results from the experiment activating only one actuator, it is shown that the difficulty arises because the wave front of the waves generated by each actuator deforms into an arch.