Abstract: An automatic alignment system is used to correct the offset error of a bidirectional printing device. The positional offset between the forward printing and the backward printing is compensated by adjusting the position of the printing cartridge. The actual position of the cartridge is sensed by a light detector, which is shuttered when the moving cartridge, driven by a microstepping motor, passes through the sensor. Based on the difference between the actual position and the correct theoretical position, the cartridge position is automatically adjusted to correct the offset error. For fine adjustment, time delay can be used in addition to the positional adjustment of the cartridge.