. Hardware design
To implement a prototype with the use of an Android smartphone, the Mindstorms NXT from The LEGO Group was used. The Mindstorms NXT can be controlled by custom programs using stacks of programming building blocks to process input from sensor data [1], and control output devices such as servos and motors. Information are forwarded and processed by the Mindstorms NXT's core. (Fig. 1)
The prototype of NXT consists of two motors connected to wheels to control speed and direction, one motor to
change the smartphone angle, and two sensors (one for cliff-detection using ultrasound, one touch sensor in the back). Fig. 2 shows the prototype system on which a smartphone is attached.