Article
Finite-Time Disturbance Observer for
Robotic Manipulators
Pengfei Cao
1,2
, Yahui Gan
1,2,
* and Xianzhong Dai
1,2
1
School of Automation, Southeast University, Nanjing 210096, China; pengfei_cao@seu.edu.cn (P.C.);
xzdai@seu.edu.cn (X.D.)
2
Key Lab of Measurement and Control of Complex Systems of Engineering, Ministry of Education,
Nanjing 210096, China
* Correspondence: y.h.gan@seu.edu.cn; Tel.: +86-25-8379-4165
Received: 2 April 2019; Accepted: 22 April 2019; Published: 25 April 2019
Abstract:
Robotic manipulators may be subject to different types of disturbances such as unknown
payloads, unmodeled dynamics, and environment interaction forces. Observing these unknown
disturbances in robotic manipulators is fundamental in many robotic applications such as disturbance
rejection and sensorless force control. In this paper, a novel disturbance observer (DOB) is introduced
based on the insights from the finite-time observer (FTO) and robot dynamics. Different from the
traditional DOBs, this new observer can provide the capability to track the disturbance within a finite
time. The performance of the presented observer is verified by two kinds of typical disturbances for a
two-link manipulator with a comparison with several existing DOBs. The simulation results show
the rapidity and accuracy of the proposed FTO.
Keywords:
disturbance observer; robotic manipulator; finite-time observer; human-robot interaction;
estimation
1. Introduction
Robotic manipulators could be subject to different kinds of disturbances when they carry
on normal operations. Roughly speaking, these disturbances can be divided into two categories,
namely external and internal disturbances. These disturbances may present useful information
about how the manipulator interacts with its environment. In addition to the interaction force
disturbance, adverse disturbances such as unknown payloads, unmodeled dynamics may challenge
the performance of the manipulator by affecting the trajectory tracking accuracy. In order to
deduce these uncertain disturbances induced by either internal or external factors, it is necessary
to incorporate a disturbance observer (DOB) to estimate these disturbances. Once the observer is
provided, the DOB in robotic manipulators can enable versatile applications including disturbance
observer based control [
1
–
3
], friction estimation and compensation [
4
,
5
], sensorless force/torque
control [
6
–
8
], fault diagnose and isolation (FDI) [
9
–
11
] etc. For example, the interaction force between
human and robot could be treated as the joint torque disturbances as the Cartesian forces could
be projected into the joint level. By observing the interaction toque, the manipulator could sense
human intention or accidental collision [
11
,
12
]. Thus, the design of a well and sound DOB is not only
theoretically important to robotic techniques but also empirically required to enhance the performance
of robotic manipulators.
The disturbance observer technique has been widely utilized in robotic manipulators for a variety
of purposes. The basic idea of DOB is to use the robot motion state and joint torque as input and then to
estimate all the unknown internal and external torque imposed on the manipulator in a lumped term
as the output. In Reference [
5
], a nonlinear disturbance observer (NDOB) was established to estimate
Sensors 2019, 19, 1943; doi:10.3390/s19081943 www.mdpi.com/journal/sensors
