Modeling and time delay control of shape memory alloy actuators

Shape memory alloy (SMA) actuators possess hard nonlinearities including backlash-like hysteresis and saturation. These nonlinearities result in steady-state error and limit-cycle problems when conventional controllers such as the proportional integral derivative (PID) are used for trajectory control. In this study, a dynamics for an SMA actuator was newly derived using the modified Liang's model. The derived dynamics showed continuity at the change of the phase transformation process, but the original model could not. SMA actuator characteristics could be well described using this dynamics. The derived dynamics could be also used effectively for the prediction of control performance and gain tuning of the time delay control (TDC). The dynamics consisted of first-order linear and second-order nonlinear equations. Accordingly, a control strategy was established for the TDC to regulate only the second-order nonlinear part for simplicity and for the internal closed loop to regulate the rest. The control strategy was examined from the point of view of influence of an antiwindup scheme and high gain tuning on control performance. An anti-windup scheme was essential to protect windup phenomenon and high gain tuning was effective when a temperature disturbance existed. In the robustness test, the TDC with high gains showed robustness to inertia variation and temperature disturbance in comparison with the TDC with low gains.     

The Production of Narrow Slit Images by a Scanning Technique

Transparent slits 5 cms. long, approximately 5μ wide and straight to within l line width were required. Lenses having the requisite resolving power do not have the necessary covering power. This paper describes a scanning microphotographic technique for greatly extending the length of line covered whilst retaining the edge delineation characteristics of microscope objectives. The design and construction of the apparatus is described, problems involved are discussed, and results shown.     

Effects of camera calibration errors on static-eye and hand-eye visual servoing

Effects of camera calibration errors for the point-to-point task are investigated in static-eye and hand-eye visual servoing realized with position-based and image-based control laws. For these four configurations, the effect of uncertainty on intrinsic and extrinsic parameters is analyzed. The results show local stability for all configurations under small calibration errors. However, a steady-state error is found in the hand-eye position-based configuration. Simulations have been carried out in order to confirm the theoretical results and evaluate the effects of the uncertainty in terms of the stability region. Another contribution of the paper consists of providing a method for estimating the stability region robust against uncertainty directions for the static-eye position-based case with uncertainty on the camera centers.     

The development of a fault-tolerant control approach and its implementation on a flexible arm robot

This article presents a robust sensor fault-tolerant control (FTC) scheme and its implementation on a flexible arm robot. Sensor faults affect the system's performance in the closed loop when the faulty sensor readings are used to generate the control input. In this article, the non-faulty sensors are used to reconstruct the faults on the potentially faulty sensors. The reconstruction is subtracted from the faulty sensors to generate a 'virtual sensor' which (instead of the normally used faulty sensor output) is then used to generate the control input. A design method is also presented in which the virtual sensor is made insensitive to any system uncertainties (which could corrupt the fault reconstruction) that cannot fit into the framework of the model used. Two fault conditions are tested: total failure and incipient faults. Then the scheme robustness is tested and evaluated through its implementation on two flexible arm systems, one with a flexible joint and the other with a flexible link. Excellent results have been obtained for both cases (joint and link); the FTC scheme produced system performance almost identical to the fault-free scenario, whilst providing an indication that a fault is present, even for simultaneous faults.