THEORY
A. Assumptions Some of the assumptions we make about our users are:
• User is no taller than seven feet.
• User can distinguish tactile feedback on the back of the hand.
• User is able to use a white-cane.
• User will not use our product in wet conditions.
• User is at least seven years old.
B. Sensing Range Documentation of the sensors EZ-2 and EZ-4 show effective sensing (1-3 inch dowel) of up to six feet. The difference between the EZ-2 and the EZ-4 sensors is the beam width. The EZ-2 has a width of four feet and the EZ-4 has a width of two feet at their widest point.
C. Timing The current baud rate of the transmitter, receiver, and ultrasonic sensors is 2400 bps. Since each packet transmitted takes up exactly 8 bits per update, the transfer rate is then 300 bytes per second, or 300 updates per second maximum. The hat, cane and glove are a 2-transmitter-1-receiver system. There is no effective way to synchronize the two transmitters because there is no way of indicating when the user turns each device on. To generate alternating transmissions for the two transmitters, the transmitting times were offset to 7
milliseconds and 13 milliseconds for the hat and the cane, respectively. Prime numbers are chosen to guarantee a certain window of time during which one device is transmitting and the other is not. However, most of the time invalid data is recieved due to mixed signals of the same frequency.