A feedforward control system for suppressing the instability waves in a boundary layer has been examined and improved. An array of piezoelectric actuators attached on a flat-plate surface was used as an active control device of the feedforward control system. Previous works have demonstrated that an array of piezoelectric actuators was able to generate controlling waves that were 180 degrees out-of-phase with the target instability waves, but finding the optimum timing and amplitude for driving the actuators depended on the experience and skill of the experimenters. To resolve this problem, we focused our attention on the process of generating these artificial waves. First, the flow field near an actuator was measured in detail in order to clarify the wave-generating process. It was found that the velocity fluctuation pattern started to appear from the actuator's surface, not from its trailing edge. Next, the effect of the driving signal waveform on the control was examined by comparing two different waveforms, sinusoidal and rectangular. A difference was found only in the third harmonic component of the generated velocity fluctuations, where the peak in the rectangular case was higher than that in the sinusoidal case. This suggested that it was not necessary to operate the actuators by a sinusoidal signal. Finally, a new semi-automatic algorithm for determining the control parameters was proposed and applied in an active feedforward control experiment. Our new algorithm demonstrated its ability to reduce the amplitude of the incoming oblique waves.