I used this as part of my presentation for my MA at The Royal College of Art - Design Research Department. It was a metal ruler or straight edge as commonly used in those days by design engineers and architects. My device was capable of knowing its orientation on the drawing surface and its position in x, y and absolute and displaying that data on its top surface.
This was achieved through a system of infra red and ultrasonic transmissions from the top of the drawing board. Using the simple cosine rule (where a triangle can be solved simply by knowing the length of its sides) and an on-board microprocessor, the battery powered rule displayed its angle to horizontal (or another indexed angle) and its position in x, y and absolute, to a fixed or temporary point on the board. The ruler operated without any connection to the drawing board - the ultrasound and infrared being the only communication.
In the first prototype shown below, ultrasound and infra-red are replaced by thin cords.
With the infrared and ultrasonic system, since infrared travels about a million times faster than ultrasound, the infrared was used to alert the receiver that ultrasonic sounds were starting their journey. By this means, infrared can start a counter and the later arriving ultrasound can stop it. To a high degree of accuracy, the count in the counter is then proportional to the distance. To measure the four distances to form the triangles (dynamically) this had to be done four times to form the two triangles. That done, the cosine rule was implemented in the microprocessor to resolve all the angles and distances of interest. The triangles are those that start at the end of the transmitter and find both ends of the ruler, the ruler being the base of both triangles. Infrared can be obscured by the hands and for that reason another electromagnetic wave was considered - UHF radio..
