This paper studies in detail how different evaluation methods perform when it comes to describing the performance of haptically controlled mobile manipulators. Particularly, we investigate how well subjective metrics perform compared to objective metrics. To find the best metrics to describe the performance of a control scheme is challenging when human operators are involved; how the user perceives the performance of the controller does not necessarily correspond to the directly measurable metrics normally used in controller evaluation. It is therefore important to study whether there is any correspondence between how the user perceives the performance of a controller, and how it performs in terms of directly measurable metrics such as the time used to perform a task, number of errors, accuracy, and so on.To perform these tests we choose a system that consists of a mobile manipulator that is controlled by an operator through a haptic device. This is a good system for studying different performance metrics as the performance can be determined by subjective metrics based on feedback from the users, and also as objective and directly measurable metrics. The system consists of a robotic arm which provides for interaction and manipulation, which is mounted on a mobile base which extends the workspace of the arm. The operator thus needs to perform both interaction and locomotion using a single haptic device. While the position of the on-board camera is determined by the base motion, the principal control objective is the motion of the manipulator arm. This calls for intelligent control allocation between the base and the manipulator arm in order to obtain intuitive control of both the camera and the arm. We implement three different approaches to the control allocation problem, i.e., whether the vehicle or manipulator arm actuation is applied to generate the desired motion. The performance of the different control schemes is evaluated, and our findings strongly suggest that objective metrics better describe the performance of the controller, even though there is a clear correlation between subjective and objective performance metrics.
This paper presents a novel control architecture for kinematic control of the base and the arm of a vehiclemanipulator system during complex mobile manipulation tasks. Particularly, we address kinematic control of robots operating in the presence of constraints such as plane or entry hole constraints imposed on a point on the manipulator arm and an additional focal point constraint imposed on the base. The focal point constraint guarantees that any camera mounted on the base points in the desired direction. We present a formulation that allows us to represent both types of constraints in the same way by introducing a new set of velocity variables. The main advantage of the proposed approach is that we are able to control the end-effector motion in the normal way using conventional operational space control schemes, and by re-writing the Jacobian matrix we also guarantee that the constraints are satisfied. The most challenging problem with this type of constraints is the extremely complex structure that arises when the constraints are mapped from the operational space to joint space. We solve this by first finding a new set of velocity variables for a point on the robot in the vicinity of the obstacle, and on these new variables we impose a structure which guarantees that the robot does not violate the constraints. We then find a mapping denoted the Constrained Jacobian Matrix from the joint variables to these new velocity variables and use this mapping to find a trajectory in joint space for which the constraints are not violated. We present for the first time the Constrained Jacobian Matrix which imposes kinematic constraints on the base and the chain of the vehicle-manipulator system using the same formalism in both cases.
Abstract:The focus of this research is to compare the mental and theoretical evaluations of remotely controlled mobile manipulators. Evaluating the performance of control methods for mobile manipulation is challenging because both the user experience and the actual performance of the completed task need to be taken into account. How the user perceives the control law is of course very subjective and in general hard do quantify numerically. Theoretical evaluations of the performance are easier to find, but do not tell us anything about the stress, frustration, and mental demand that the operator experiences. Several studies have been performed to evaluate the performance of teleoperation schemes, but the literature lacks a comparison between objective and subjective performance metrics for evaluating these. In this paper we evaluate the mental and theoretical performance of three relatively simple approaches for controlling a mobile manipulator with a haptic device. We study to what extent objective performance metrics such as execution time, number of failures, and manipulator mobility can be used to distinguish the approaches, and compare this to subjective measures like the NASA-TLX test.
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