Control and Evaluation of a Humanoid Robot with Rolling Contact Knees

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Seung Hyeon Bang1, Carlos Gonzalez1, Junhyeok Ahn2, Nicholas Paine3and Luis Sentis1

Abstract— In this paper, we introduce the humanoid robot

DRACO 3 by providing a high-level description of its design and

control. This robot features proximal actuation and mechanical

artifacts to provide a high range of hip, knee and ankle motion.

Its versatile design brings interesting problems as it requires

a more elaborate control system to perform its motions. For

this reason, we introduce a whole body controller (WBC) with

support for rolling contact joints and show how it can be easily

integrated into our previously presented open-source Planning

and Control (PnC) framework. We then validate our controller

experimentally on DRACO 3 by showing preliminary results

carrying out two postural tasks. Lastly, we analyze the impact

of the proximal actuation design and show where it stands in

comparison to other adult-size humanoids.

I. INTRODUCTION

Dynamic motions for legged systems require accurate

sensing of the robot’s states and high performance con-

trol. To realize the latter, several important issues must be

considered. On one hand, efﬁcient transmissions with low

friction/stiction and low backlash are desired. In addition,

mechanical designs capable of achieving a wide range of

motion (RoM) are important and non-trivial to achieve. On

the other hand, these types of mechanisms come at the ex-

pense of an increase in complexity of the controllers needed

to exploit the potential of the high-dimensional system.

Humanoids have often employed collocated actuation (mo-

tors directly located at each joint) because of its simplicity

in design [1], [2]. However, the performance of these robots

degrades when using simpliﬁed models for planning because

of model discrepancy caused by the heavy distal mass on

their legs [3]. Due to this, their design has shifted in favor

of proximal actuation (placing heavy motors near the body)

to reduce the limbs’ distal mass for dynamic maneuvers. As

part of the solution to achieve this, a transmission system

that has been explored frequently in legged robots consists of

cable-based drive systems [4]–[6]. This is due to their light

weight as well as effective power transmission capability,

thus helping provide high torque density when incorporated

into the design. Additionally, this kind of transmission is

more efﬁcient because the mechanical losses due to friction

are small and, thus, provides higher torque transparency.

This work was supported by ONR Grant# N000141512507.

1S.H. Bang, C. Gonzalez, and L. Sentis are with the Department of

Aerospace Engineering and Engineering Mechanics, The University of

Texas at Austin, TX 78712, USA bangsh0718@utexas.edu,

carlos.gonzalez@utexas.edu,

lsentis@austin.utexas.edu

2J. Ahn is with the Department of Mechanical Engineer-

ing, The University of Texas at Austin, TX, 78712, USA

junhyeokahn91@gmail.com

3N. Paine is with Apptronik, Inc., 110701 Stonehollow Dr STE 150,

Austin, TX, 78758, USA n.a.paine@gmail.com

1 DOF Neck

3 DOF Shoulder

1 DOF Elbow

2 DOF Wrist

3 DOF Hip

1 DOF Knee

2 DOF Ankle

(a) (b)

Fig. 1. DRACO 3 Humanoid. (a) Wireframe of DRACO 3 showing the

degrees of freedom on the right side of the robot, and their respective axis

of rotation. (b) DRACO 3 standing at full height.

In a similar manner, DRACO 3 (shown in Fig. 1) was

designed bearing a cable-based drive system in mind for

proximal actuation. In addition, a rolling contact joint (RCJ)

mechanism is employed to enhance its RoM. Although

RCJs have been widely used in other ﬁelds such as in

lower extremity exoskeleton design to reduce misalignment

between the human’s and the device’s joint [7], [8], and in

robotic ﬁngers to realize large RoM as well as to decrease

internal friction [9], it has not been adopted on humanoids

due to its mechanical complexity and complicated control

design. By means of a RCJ on the knee, DRACO 3 achieved

proximal actuation and high ranges of motion.

Typical methods to control complex systems such as

legged robots often make use of a WBC, which compute

joint-level commands to achieve desired operational space

tasks. This type of controller is often designed by considering

a model of the robot along with multiple physical and

environmental constraints (e.g., friction). In addition, any

complexities in the joint mechanisms must be taken into

account either within or outside of the WBC module. The

recent work by [10] presents a rich literature review on

different types of WBCs. In this work, we chose to pursue

a WBC, similar to that in [10], [11] for its high ﬂexibility

and ease of intuitive tuning. Previous works [7]–[9], [12]

have studied the control of complex joint mechanisms (e.g.,

(d)

(e)

(f)

(a)

(b)

(c)

Thigh

Shin

Tensioner

IMU

Gripper

Contact Sensor

Camera

(e)

(a)

(b) (c)

(d)

(c)

(g)

(a)

(f)

(b)

IMU

Gripper

Contact Sensor

Camera

(a)

(b) (c) (d)

(e)

(f)

qdistal

qknee

Fig. 2. Mechatronic Components of DRACO 3. (a) Hip actuation: Lateral view of thigh, showing cable routing (- -) and the location of the terminations

(×) of the stainless steel cables. (b) Cable terminations of the Hip pitch joint. Soldered termination (left) and crimped termination (right). (c) Mechatronic

components in DRACO 3: the camera, the IMU, the grippers, and the contact sensors are Carnegie Robotics MultiSense S7, VectorNav VN-100, SAKE

Robotics EZGripper, Apptronik load cell, respectively. (d) Rolling contact joints. Snapshots of leg conﬁgurations in sequence when qknee is 0◦, 90◦, and

180◦.(e) Knee joint: Lateral view of knee, showing the location of its cable terminations (×). (f) Cable terminations of the Knee joint.

rolling contact joints) in an isolated manner, by developing a

high ﬁdelity characterization for the modelled and controlled

joint. The presence of such complex joint mechanisms in

high-dimensional robotic systems is emerging and, similarly,

control architectures and WBCs that perform effectively

using these complex mechanisms. Some WBCs that use non-

traditional collocated actuators include the use of parallel

linear actuators, which can be solved through a transmission

mapping [2], [13]. Others have taken into account in the

WBC formulation both series and parallel mechanisms by

deﬁning their optimization problem in actuation space [14].

However, to the best of our knowledge, this is the ﬁrst WBC

that takes into account a RCJ mechanism and is successfully

deployed on a high-dimensional legged robot.

The main contributions of this paper are: 1) a high-

level presentation of the new humanoid DRACO 3 built

by Apptronik, where we highlight key design modiﬁcations

that we have found beneﬁcial to increase the durability and

performance of the robot, 2) an extension to our previously

presented WBC framework to take into account RCJs by

means of internal constraint inclusions, 3) an analysis on

the centroidal properties of the DRACO 3 showing how it

compares to other known and relatively large humanoids,

and 4) a set of preliminary results showing the humanoid

performing disturbance rejection and a squatting and swaying

task to validate the compliance of our design propositions

as well as the integration of our extended WBC to control

DRACO 3.

II. SYSTEM OVERVIEW

In this section, we introduce the overall actuation mecha-

nisms in DRACO 3, placing greater emphasis on the hip and

knee mechanisms, which make up the most complex parts

of the robot.

A. Robot Design

DRACO 3 stands 1.35 mtall, weighs 39 kg, and has a

range of motion (RoM) similar to that of a human in its

actuated degrees of freedom (DoF): neck pitch, 6-DoF arms,

and 6-DoF legs. The joint conﬁguration shown in Fig. 1 (a) is

used by the low-level controller. DRACO 3’s proprioceptive

and exteroceptive sensors are illustrated in Fig. 2(c). Its

lower-body constitutes the most complex part of the robot as

its actuation principle exploits proximal actuation and seeks

to achieve a high range of motion.

In general, the robot leverages both off-the-shelf and App-

tronik’s motors, all of which communicate through EtherCAT

by means of Apptronik’s motor and sensor boards. The

usage of multiple encoders equips the robot with accurate

sensing capabilities, even in non-collocated actuated joints,

thus resulting in accurate control authority over all joints.

B. Hip Pitch Design with Cable-Pulley Actuation

The lateral view of the hip is shown in Fig. 2(a). This

joint is actuated by means of a rolling contact, with the

colored curves delineating the routing of the tensioned cables

inside. This design has taken into account several factors:

the cable material selection, the termination design, and

the sensor placement. Typical materials for cable-driven

actuators include stainless steel, Vectran, and Kevlar, among

others [5], where it is often desirable to have high stiffness,

low creep, and low D-to-d ratio (excluding environmental

resistance speciﬁcations, e.g., water resistance). Since the

bending radii in our design are fairly big in comparison

to the required cable diameter, the D-to-d ratio is relaxed,

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ControlandEvaluationofaHumanoidRobotwithRollingContactKneesSeungHyeonBang1,CarlosGonzalez1,JunhyeokAhn2,NicholasPaine3andLuisSentis1Abstract—Inthispaper,weintroducethehumanoidrobotDRACO3byprovidingahigh-leveldescriptionofitsdesignandcontrol.Thisrobotfeaturesproximalactuationandmechanicalartifactstop...

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Control and Evaluation of a Humanoid Robot with Rolling Contact Knees

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