Implementations of a Four-Level Mechanical Architecture for Fault-Tolerant Robots, Page 3

The DISC is a very compact brushless DC servo controller that offers numerous features not contained in any single commercial system available today. Some of the features include: multiple sensor interfacing, compact 'smart' power electronics, fault tolerance, and high speed digital communications designed in a modular package. The features incorporated into each DISC allows for research into the use of distributed control applied to a modular robot. Distributed control can significantly reduce the amount of wires running through a structure since the DISCs digitally multiplex the signals locally. This reduction in wires is essential to the success of modular robotics and mechanical fault tolerance. The DISC contains four sub-modules: a sensor interface module, a communications module, a power interface module and a digital controller. Sensor Interface Module: The sensor module conditions and digitizes all external signals before passing them to the controller module. The module will read multiple sensors, including: a tachometer (16 bit resolution), three cur-rent sensors (12 bit resolution), a resolver (16 bit resolution), a torque sensor (12 bit resolution), an incremental encoder (16 bit resolution), two temperature sensors (8 bit resolution), a logic level Hall-effect sensor, an auxiliary +/- 5V signal (8 bit resolution) and an auxiliary +/- 1OV signal (8 bit resolution). Communications Module: The communications module controls the information between the DISCs and the system controller. The difficulties inherent in electrical interface design for modular robots are primarily a result of the high number of wires that run throughout the robot. Most robots are controlled by monolithic system controllers. All data is brought to the system controller using separate conductors. As a result, every signal, sensor, and power wire for each actuator must be run throughout the entire robot to the system controller. For example, a Cincinnati Milacron Inc. T3-776 industrial robot requires 24 wires for the tachometers, 54 wires for the resolvers, 18 wires for the brakes, 40 wires for powering the motors and cooling system, and 15 spare wires. This high number of wires (137) places difficult constraints on the design of connections within a modular robot that uses the centralized control scheme. For modular robots to be a viable alternative to monolithic robots the number of cables must be reduced. Power Interface Module: The power interface controls the power to the actuator. It includes the power electronics and drivers, the clutch and brake control, and the cur-rent sensors. The DISC derives the motor commutation in software. In order to simplify the interface between the digital controller and the power interface, the power module uses a smart power electronic device. Smart power devices contain analog and/or digital circuitry in addition to the discrete power electronics. The device chosen for the DISCs generates the bias voltages for the high side discrete transistors as well as performing the lockout function for the input PWM logic. Closed-Loop Controller Module: The closed-loop controller was purchased as an off-the-shelf component. The module is an Advanced Micro Device's SA-2920OTm Demonstration Board. This card utilizes the Am29200 RISC microcontroller. The board has the following features: 512 KBytes of ROM, an RS-232 serial port, a JTAG port, two expansion connectors allowing full access to all processor signals, and a resident debugger. The system operates from a 5V power supply. The Am29200 is a 16 MHz, 32 bit integer processor with a ROM controller, a DRAM controller, an interrupt controller, a PL4, controller, a 16 bit 1/0 port, a serial port, and a parallel port.

Level III - Serial redundancy provides the fault tolerance at Level III. In accordance with the subsumptive architecture, each DOF in the Level III fault tolerant system can incorporate fault tolerance from the previous levels. Note that Level III redundancy does not necessarily imply fault tolerance. The failure of only one joint in serial manipulators with one or two extra DOF may degrade the workspace to the extent where the robot becomes essentially useless. Serial redundancy also introduces another redundancy resolution problem and many researchers have studied this problem in the context of serial robots. Most of this work derives from Whitney's resolved motion rate control that suggests the use of the pseudo-inverse for redundant robots. Liegeois showed the extension of this method to include self-motions via the null-space. Since then, a large number of researchers have implemented pseudo-inverse based methods and others have studied their limitations. Other rate control approaches include: Seraji's configuration control, Baillieul's extended Jacobian, the Jacobian transpose, and a number of numerical optimization methods such as the Series of Unconstrained minimizations Technique (SUMT). Dubey and Luh' include task-based performance measures in the redundancy resolution. Maciejewski has shown the extension of Jacobian-based techniques to include fault-tolerance. This section discusses a method of redundancy resolution that uses local exploration to explicitly identify a set of options for the robot's motion. From this set, a decision making algorithm chooses one option as the next set-point command for the robot's servo controllers. The decision making algorithm can base its choice on any number of performance criteria. Simulated perturbations at the joint level drive the exploration. Eschenbach and Tesar developed the sequential filters method of decision making and applied it to the mechanism synthesis problem. They reported an example of the method reducing a design space of 60,000 to one of only 50. The six axis wrist approach is a method of satisfying the placement constraints on the robot's end effector while also significantly increasing the speed of the optimization. Essentially, the method satisfies the End-EFfector (EEF) placement constraints using a six-joint substructure of the robot's geometry. In essence, the method decouples the placement constraints on the robot's EEF from the optimization of the performance index. The name comes from an analogy with the three axis spherical wrist that decouples the rotational EEF placement constraints from the translational EEF placement constraints. The method acts as a filter to eliminate options not satisfying the constraints on the placement of the robot's EEF. A series of three translations - Px, Py, Pz - and three rotations Rx, Ry, Rz - will specify these constraints. The concatenation of the geometric transformations associated with these constraints generates a single transformation corresponding to the desired placement of the robot's EEF.  Next Page ->