Compliant Control for Physical Human-Robot Interaction

with application to exoskeletons, active orthoses, wearable robots and haptics


Tutorial at the 13th International Conference on Intelligent Autonomous Systems (IAS-13)


Abstract: There is an increasing interest for robots with human like abilities such as manipulating objects, interacting with other humans or walking on rough terrains. Compliant control is a widely used tool that can enable such advanced features. This tutorial is designed for engineers who are interested in the design and/or control of a compliant device, to make them aware of the advantages and limitations of different approaches, e.g. the basic option of having, or not having, physical compliance included in their design.

: 18 July 2014, Morning Session (10:00-13:00)
Where: Padova (Italy), Congress Centre
Contacts: for any info and pre-registration contact andrea.calanca at

Compliant control of stiff joints has a long history in robotics, dating back to the middle of the past century. On the other hand compliant control of elastic joints (e.g. series elastic actuators) came forth in the last two decades. Related research is often focused on simpler setups (a single d.o.f.) and sometimes uses novel terminology (e.g. virtual stiffness, equilibrium controlled stiffness, programmable springs), having a similar meaning of some already established concepts in robotics. In this tutorial we draw a link between new and old concepts and review emerging research on elastic and flexible joints. To this aim we will review and compare past approaches and relative mathematics on a common and simplified scenario. We clarify the significance and the role of key concepts such as mechanical compliance, backdrivability and force control. Then for each scheme we explain intuitive meaning and report issues acquired across literature.

sea-pictureFFIn the second part of the tutorial we focus on the elastic joint case, which is not yet a mature topic. We show that compliance allows to stabilize force control and that it is helpful when dealing with environment uncertainty. Then we focus on advanced force control methods to provide predictable force control performance. These methods have been recently proposed and explicitly consider the influence of the environment in the control loop. We show how these algorithms perform on real-world setups and we share our control library (written in c++ and based on Linux-RT and EtherCAT).

Also we present novel insight related to the coupled-stability and passivity of impedance controlled series elastic actuators. In particular, we explain existing limitations when we desire a stiffness that is higher than the physical one and we show how to overcome the issue. Also, we discuss the rendering of linear viscoelastic impedance models.

The course will be half day. Slides and lecture notes will be provided to the registered participants. Register here. It follows a detailed list of topics.

Part 1: Compliant Control (from the ’70s to nowadays)

Introduction to basic concepts

  • compliance & backdrivability
  • mechanical impedance & admittance
  • position & force control
  • the role of compliance in force control
  • coupled stability & passivity

Compliant Control of Stiff Joints

  • history
  • impedance control (Hogan)
  • impedance control (Explicit)
  • admittance control
  • parallel force position control
  • passivity results

Compliant Control of Soft Joints

  • history and motivations
  • force control
  • impedance control
  • collocated admittance control
  • passivity results


  • choosing the mechanics
  • choosing the control
  • what is missing
  • future directions

Part 2: Recent Advances

Impedance control of Series Elastic Actuators: passivity constraints

  • Cascade approach based on nested impedance, torque and velocity controllers
    • Case studies: control of series elastic actuators for wearable robots
  • Passivity constraints for the cascade control approach
  • Virtual rendering of different impedance models
    • Null impedance
    • Pure elasticity
    • Linear viscoelastic models (series and parallel spring-damper systems)

Control of SEAs with motors as ideal velocity sources

  • How to achieve coupled stability during high stiffness haptic display
  • Application of ideal velocity-sourced SEAs: MR-compatible haptic force feedback wrist robot

A paradigm shift: Human-Adaptive Control of Soft Joints

  • Passivity, stability and predictability
  • Human-Adaptive Force Control
    • Rationale
    • Theoretical results
    • Experimental resuls
    • Future directions

Materials: Slides And Reference Links

Slides on Compliant Control (A.Calanca) – Compliant Control
Slides on Passivity Constraints (N.L. Tagliamonti) – Passivity Constraint
Slides on Coupled Stability of actuators as ideal velocity sources (F. Sergi) – Coupled Stability
Slides on Huma-Adaptive Control (A.Calanca) – Human Adaptive Control
Slides on The Role of Compliance in Force Control (A.Calanca) – Ontherole

Link at the PhD Thesis of A. Calanca (available till August)
Link at the article on Human-Adaptive Force Control
Sample C++  Code for the Human-Adaptive Force Control Algorithm – AdaptiveControl
See IAS13 proceedings for the article “The Role of Compliance in Force Control”


Andrea Calanca, PhD.
University of Verona

Prof. Paolo Fiorini,
University of Verona

Riccardo Muradore, PhD.
University of Verona

Nevio Luigi Tagliamonte, Ph.D.
Università Campus Bio-Medico di Roma

Fabrizio Sergi, Ph.D.
Rice University – MEMS Dept.