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Abstract
Two versions of a 6-degree-of-freedom joystick have been interfaced to a computer and a software driver necessary for controlling applications with the joysticks has been created. One of the joysticks is instrumented with potentiometers, while the other uses digital optical encoders. The digital joystick is shown to be at least 5 times more accurate than the analog one, mainly due to elimination of noise error.
Both joysticks are based on a 3-branch parallel manipulator (PM). In both joysticks, angles of nine joints in the PM structure are measured, meaning that there exists a sensing redundancy of degree 3. The sensing redundancy has been exploited to allow self-calibration and fault-tolerant operation. It is estimated that after the encoder calibration, the reported position of the digital joystick's handle is accurate to ±0.25 mm. Achieving fault-tolerant operation required: (1) the solution to the PM's forward displacement problem (FDP) for all possible distributions of six, seven and eight displacement-measuring transducers in the PM's structure, and (2) an algorithm for fault detection and identification (FDI). Satisfaction of loop-closure is the basis for the methodology used for the above algorithms. The implemented system can (in theory) tolerate up to two transducer failures before shut-down is necessary. In practice, it is shown through simulation of faults via software that the implemented FDI algorithm often misidentifies failures. Alternate methodology based on signal-analysis is proposed in order to improve the system.
The developed software contains functions for: (1) accessing the interface hardware, (2) solving the necessary kinematic problems, (3) calibrating the joysticks, (4) detecting and identifying transducer failure(s), and (5) visualization of joystick-related data with charts and 3D graphics. The code is modular and can be used or extended for use with other parallel-manipulators.
