Article
Variable Admittance Control Based on Human–Robot
Collaboration Observer Using Frequency Analysis for Sensitive
and Safe Interaction
Hyomin Kim and Woosung Yang *
Citation: Kim, H.; Yang, W. Variable
Admittance Control Based on
Human–Robot Collaboration
Observer Using Frequency Analysis
for Sensitive and Safe Interaction.
Sensors 2021, 21, 1899. https://
doi.org/10.3390/s21051899
Academic Editor: Abolfazl Zaraki
Received: 14 January 2021
Accepted: 4 March 2021
Published: 8 March 2021
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School of Robotics, Kwangwoon University, 20 Kwangwoon-ro, Nowon-gu, Seoul 01897, Korea; nankimhm@kw.ac.kr
* Correspondence: dreamrize@kw.ac.kr; Tel.: +82-940-8115
Abstract:
A collaborative robot should be sensitive to the user intention while maintaining safe
interaction during tasks such as hand guiding. Observers based on the discrete Fourier transform
have been studied to distinguish between the low-frequency motion elicited by the operator and high-
frequency behavior resulting from system instability and disturbances. However, the discrete Fourier
transform requires an excessively long sampling time. We propose a human–robot collaboration
observer based on an infinite impulse response filter to increase the intention recognition speed. By
using this observer, we also propose a variable admittance controller to ensure safe collaboration.
The recognition speed of the human–robot collaboration observer is 0.29 s, being 3.5 times faster
than frequency analysis based on the discrete Fourier transform. The performance of the variable
admittance controller and its improved recognition speed are experimentally verified on a two-
degrees-of-freedom manipulator. We confirm that the improved recognition speed of the proposed
human–robot collaboration observer allows us to timely recover from unsafe to safe collaboration.
Keywords: human–robot collaboration; physical human–robot interaction; admittance control
1. Introduction
Collaborative robotics has become the new frontier for industrial robots by combin-
ing high-level motion accuracy and the repeatability of robots with the flexible cognitive
judgment of humans [
1
,
2
]. Effective human–robot collaboration requires an intuitive user
interface to maximize operation flexibility [
3
]. The hand guiding collaboration mode, also
known as direct teaching, is a representative collaboration mode defined in standards ISO
10218-1/2 [
4
,
5
]. In this mode, the operator directly sets the sequence of desired robot posi-
tions by moving the robot end effector without an intermediate interface. In addition to the
intuitive interaction, the operator can manipulate the robot while receiving haptic feedback
that guides or limits the trajectory. Thus, this mode evolves the interface bottleneck of
traditional input devices such as mouse, keyboard, and joystick [
6
]. Nevertheless, when the
operator and the robot are in continuous contact, safety during the physical human–robot
interaction is the most important consideration [
7
]. Therefore, the robot must operate
according to the operator intention while ensuring safety.
To ensure the operator’s safety while collaborating with a robot, standards ISO 10218-
1/2 define a power and force limiting mode. They also prescribe managing the system’s
output force within 0.5 s to prevent the plastic deformation of human skin during colli-
sions [
8
]. In this mode, an energy tank-based observer is commonly used to limit the motor
power and force of the industrial robot [
9
,
10
]. The energy tank-base provides an easy
solution to the problem of passivity but requires an analysis of the total energy. This con-
trol prevents unintentional robot movement due to unexpected contact with the external
environment. However, in the hand guiding mode, the external environment means that
the operator, and it is difficult to generalize operator’s energy. As the physical properties
of humans vary between individuals and human intention changes in real time. Moreover,
Sensors 2021, 21, 1899. https://doi.org/10.3390/s21051899 https://www.mdpi.com/journal/sensors
