Analysing the reliability of actuation elements in series and parallel configurations for high-redundancy actuation

Abstract

A high-redundancy actuator (HRA) is an actuation system composed of a high number of actuation elements, increasing both travel and force above the capability of an individual element. This approach provides inherent fault tolerance: if one of the elements fails, the capabilities of the whole actuator may be reduced, but it retains core functionality. Many different configurations are possible, with different implications for the actuator capability and reliability. This article analyses the reliability of the HRA based on the likelihood of an unacceptable reduction in capability. The analysis of the HRA is a highly structured problem, but it does not fit into known reliability categories (such as the k-out-of-n system), and a fault-tree analysis becomes prohibitively large. Instead, a multi-state systems approach is pursued here, which provides an easy, concise and efficient reliability analysis of the HRA. The resulting probability distribution can be used to find the optimal configuration of an HRA for a given set of requirements.

Keywords:

• redundancy
• mechanical actuation
• bionics
• high-redundancy actuator (HRA)
• fault-tolerance
• multi-state system

Acknowledgements

The HRA project is a cooperation of the Control Systems group at Loughborough University, the Systems Engineering and Innovation Centre (SEIC) and the actuator supplier SMAC Europe limited. The project was funded by the Engineering and Physical Sciences Research Council (EPSRC) of the UK under reference EP/D078350/1.

Notes

Notes

1. The same basic laws of aggregating capabilities are also applicable in many other areas. Reliable rotary actuation can be achieved with velocity and torque adding gears, for example. Electrical systems deal with the dual variables of voltage and current, and series and parallel configuration are commonly used in insulated gate bipolar transistor (IGBT) high-power switching devices (Shammas, Withanage, and Chamund 2006). Transportation systems and communication networks also have corresponding relations governing throughput and latency. From a reliability perspective, all these systems are essentially identical in that they use series and parallel configurations to increase two dual capabilities of primary concern.

2. The MATLAB symbolic toolbox has been used for the automatic manipulation of these polynomials. While MATLAB supports native functions for manipulating polynomials, these operate on vectors, and not first class polynomial objects. This makes it more difficult to represent the capability distributions.