smart board installation cost.The principle of ultrasonic technology is to calculate the distance from the transmitting point to the receiving point according to the characteristics of the slow transmission speed of the ultrasonic wave according to the ultrasonic transmission to the receiving time. The principle of using the planar positioning is to place two ultrasonic receiving devices distributed at a fixed distance on one side of the screen, and the pen for positioning is an ultrasonic transmitter, and the ultrasonic wave emitted when the pen moves on the surface of the screen It is detected by the receiver along the surface of the screen, and the distance between the pen and the two receivers can be converted by the time of receiving the ultrasonic waves. The principle of three-point positioning is adopted, that is, according to the three side lengths of the triangle, the apex principle of the pen can be determined, and the position coordinates of the pen are calculated. It is a ranging positioning mode.