Unknown and Time-Varying Time Delays in the Modelling and Control of Hydraulic Actuators: Literature Review

Abstract

Uncertain time-delays can reduce the performance of hydraulic manipulator control systems. Variations in the time delay between, for example, an applied voltage and the associated manipulator movement, may be caused by the internal dynamics of the system and other nonlinear characteristics, such as fluid compressibility, dead-band of the pump, valve flow properties and friction characteristics. Robot control that addresses system time-delays, nonlinearities and uncertainty is the subject of much research but, whilst the specific concept of varying time delays in hydraulic systems is sometimes acknowledged, it appears to be less widely investigated than other types of uncertainty. The present article discuses some of the issues involved, exemplified by a dual manipulator device used in the laboratory for research into nuclear decommissioning, and presents a review of the relevant control literature in this area.Index Terms-hydraulic manipulators, uncertain time delays, time varying delays, robot control systems, literature review

Pole assignment control design for time–varying time–delay systems using radial basis functions

“…However, the proposed approach does require use of a suitable on-line time delay estimation algorithm [24,25]. Our recent review [2] discusses some of the issues arising in this context. Fortunately, preliminary simulations with time delay modelling errors, suggest that the proposed controller might be relatively robust to realistic scenarios when the time delay is uncertain.…”

“…The nonlinear weighted PIP controller is designed for the plant (2), where the variable time delay in seconds is represented in discrete time as an integer number of samples, τ (k). The controller consists of a set of N p partial controllers, based on the partial models (3), whose control inputs u i (k) (i = 1, .…”

“…The preliminary simulation results described here are for a first order model with long time delays, analogous to some of the hydraulic manipulator control models developed by [2][3][4] i.e. the plant (2) with n = m = 1, a 1 = −0.9, b 1 = 1.2 and τ (k) represented by a pseudo-Random Walk (RW) in the range τ min = 1 to τ max = 31 samples. As already alluded to above, three partial models have been chosen, each based on the same plant coefficients but with τ 1 = 6, τ 2 = 16 and τ 3 = 26, i.e.,…”

“…In some scenarios, for example, the time-delay (in seconds) between implementing a change in the applied voltage (in either open or closedloop) and observing the associated angular velocity response for a hydraulically actuated robotic manipulator, can change over time. Such variations in the delay may be caused by the internal dynamics of the system and other nonlinear characteristics, such as fluid compressibility, varying pressure dynamics, dead-band of the pump, valve flow properties and friction characteristics [2][3][4]. In the latter case, the complex nature of the time delay variability can appear stochastic (rather than, say, dependant on a measured system state).…”

A linear regression variable time delay estimation algorithm for the analysis of hydraulic manipulators

“…Particular difficulty is introduced, however, by systems with time delays that are relatively long, unknown and potentially time-variable. As discussed by [5], there are a number of methods for modelling time-varying delays e.g. [6][7][8][9].…”