playing it safe - stanford university

Recently, there has beenincreasing interest in theemerging field of human-centered robotics. Thisfield focuses on applica-

tions such as medical robotics andservice robotics, which require closeinteraction between robotic manip-ulation systems and human beings,including direct human-manipulatorcontact. As a result, human-cen-tered robotic systems must considerthe requirements of safety in addi-tion to the traditional metrics ofperformance. To achieve safety wemust employ multiple strategiesinvolving all aspects of manipulatordesign, including the mechanical,electrical, and software architec-tures. Immediate improvement canoften be realized with the use ofelectronic hardware and software safety mechanisms that intel-ligently monitor and control manipulator operations. Addi-tional improvements can be realized in the mechanical design.The elimination of pinch points and sharp edges can eliminatethe potential for laceration or abrasion injuries. However, themost serious hazard present when working in close proximitywith robotic manipulators is the potential for large impactloads, which can result in serious injury or death. To evaluatethe potential for serious injury due to impact we can makeuse of an empirical formula developed by the automotiveindustry to correlate head acceleration to injury severity

known as the head injury criteria(HIC). A simple two-degree-of-freedom mass-spring model can beused to predict head accelerationsthat would occur during an uncon-trolled impact. In combination withthe HIC index, predicted accelera-tions are used to estimate the likeli-hood of serious injury occurringduring an impact between a roboticmanipulator and a human. For thePUMA 560, an impact velocity of 1m/s produces a maximum HICgreater than 500, more thanenough to cause injury (see Figure1). [The HIC index is correlatedwith the maximum abbreviatedinjury scale (MAIS) to provide amapping from the calculated HICvalues to the likelihood of an

occurrence of a specific injury sever-ity level. In Figure 1, HIC values and the corresponding like-lihood of a concussive injury (or greater) are shown.]

As seen in Figure 1, the addition of a compliant coveringcan reduce impact loading by an order of magnitude or more.However, the amount of compliant material required toreduce impact loads to a safe level can be substantial. (For thePUMA robot, the required thickness of a compliant cover ismore than 5 in, assuming an impact velocity of 1 m/s and anallowable maximum HIC index of 100.) Clearly, adding largeamounts of compliant covering is impractical and does notaddress the root cause of high-impact loads; namely, the large

A New Actuation Concept for Human-Friendly Robot Design

BY MICHAEL ZINN, OUSSAMA KHATIB,BERNARD ROTH, AND J. KENNETH SALISBURY

1070-9932/04/$20.00©2004 IEEEIEEE Robotics & Automation Magazine JUNE 200412

Playing It Safe

©19

98 A

RT

VIL

LE, L

LC

effective inertia of most modern robotic arms.Thus, the mechanical characteristics of a roboticsystem are the limiting factor in improving overallsafety. The solution to reducing the effectiveimpedance, and thus improving safety, is to builda lightweight, low-inertia manipulator.

Some types of robotic manipulators—notablythose utilizing compliant actuation, such aspneumatic actuators, or those employing com-pliant drive trains, such as a cable-driven manip-ulators—do not produce the large-impact loadsassociated with high-impedance designs. While acompliant actuator or drive train can enhancesafety characteristics, the performance of suchsystems is limited. The flexible modes of thecompliant system prevents control bandwidthsgreater than about one-third of the fundamentalresonant frequency. In addition, attenuation offlexible mode oscillations excited by disturbancescan be difficult to achieve. This results from thephase delay introduced above the first mode fre-quency. With the resonant frequencies of many cable-drivenmanipulators in the range of 10 Hz or less, high-perfor-mance control of such systems is difficult, if not impossible.

New Actuation ApproachesNew actuation approaches have been developed to over-come the safety and performance limitations of existing sys-tems. Chief among these are the joint torque controlapproach [11] and series elastic actuation [9]. However, forreasons discussed in the following sections, these approachesdo not simultaneously achieve the characteristics necessaryfor both safety and performance. To address these limita-tions and create a unified high-performance and safe robot-ic manipulator, a new actuation approach—referred to asthe distributed macro-mini actuation approach (DM2)—hasbeen proposed [12].

Joint-Torque-Controlled ActuationJoint torque control was developed to eliminatethe deleterious effects of nonlinearities and frictioninherent in the actuator-transmission systems gen-erally found in industrial robots. Initial implemen-tations were successful in substantially reducingjoint friction effects but wide joint actuation band-width was difficult to achieve without actuallyreducing the friction and nonlinearities in theactuator-transmission system [2], [5], [11].

In response, joint torque control systemsemploy high-performance actuator and transmis-sion designs with integrated torque sensors toachieve the performance levels desired. Perhapsthe most successful of these has been the newDLR (German Aeropspace Center, DeutschesZentrum fur Luft-und Raumfhart) lightweightarm design (see Figure 2) [3].

JUNE 2004 IEEE Robotics & Automation Magazine 13

Figure 1. HIC as a function of effective inertia and interface stiffness.

Inte

rfac

e S

tiffn

ess

[kN

/m]

Hea

d In

jury

Crit

eria

Inde

x (H

IC)

Arm Effective Inertia [Kg]

25

20

15

10

5

5 10 15 20 25 30 35 40

600

500

400

300

200

100

PlasticsPuma 560

70% Injury

50% Injury

20% Injury

10% Injury

Hard Rubber

Soft Rubber

Figure 2. DLR light-weight robot. (a) DLR II. (b) DLR III.

(a) (b)

Figure 3. SEA topology.

Ia : Arm Link Inertia Nmotor : Actuator Gear Ratio

Imotor : Actuator Rotor Inertia Ks : Base Actuator (SEA) Compliance

τdesired : Desired Torque Command

Where:

PD ControlD(s)

Imotor

Ia

∆coupling

Nmotor

Ks

Ks

τdesired

–+

The implementation of joint torque control allows fornear-zero low-frequency impedance, which gives the DLRarm excellent force control characteristics. However, abovethe control bandwidth, joint torque control is ineffective atreducing the impedance of the manipulator. The open loopcharacteristics of the manipulator and reflected actuator inertiadominate. Thus, the magnitude of impact loads, which aredetermined by the high-frequency impedance of the contact-ing surfaces, are not attenuated.

While the joint torque control has been successful inimproving the force and impedance control of roboticmanipulators, their fundamental open-loop characteristicsmake inherent safety difficult to achieve and thus do not sat-isfy the human-centered robotic requirements of both per-formance and safety.

Series Elastic ActuationRecently, a class of actuators, known as series elastic actuators(SEAs), has been developed to address the problems of high-impedance actuators [9], [10]. The SEA approach seeks tomitigate the limitations of high-impedance actuators, such asconventional gear-head electromagnetic or hydraulic actua-

tors, by placing an elastic element between the output of theactuator and the robotic link. The elastic element limits thehigh-frequency impedance of the actuator to the stiffness ofthe elastic coupling. To limit the low-frequency impedance alinear feedback system is implemented to regulate the outputtorque of the actuator-spring system (see Figure 3).

The main advantage of the SEA topology is that it pro-vides low output impedance across the frequency spectrum.As shown in [9] and [10], the SEA topology reduces the out-put impedance of the SEA actuator in proportion with thestiffness of the elastic coupling. At frequencies below theclosed loop bandwidth of the SEA controller, the outputimpedance is reduced as a function of the control gains.Impedance reduction of 10x–100x is common and is onlylimited by the maximum obtainable bandwidth. At frequen-cies above the closed-loop bandwidth, the output impedancereduces to the stiffness of the elastic coupling. This is in con-trast to other approaches, such as joint torque control dis-cussed in the previous section, which has good low-frequencyimpedance but suffers from large high-frequency impedance.

It is interesting to note the similarities between the SEAand joint torque control approaches. The topology of joint

torque control is identical to that of theSEA approach (shown in Figure 3). Thedifference between the two approacheslies in their differing control approaches,which are driven by their very differentopen-loop characteristics. As describedearlier, series elastic actuation has a com-pliant coupling between the actuatorand driven link, the stiffness value ofwhich is chosen so that the open loopmode of the system is well below theobtainable closed-loop bandwidth of theSEA control. As a result of the low stiff-ness compliance, the open-loop gain isvery low, which allows for the use of asimple, high-gain proportional-deriva-tive (PD) controller. The resulting sys-tem is stable and possess low impedanceover a wide frequency range. In contrast,

the coupling between the actuation and drivenlink is much stiffer for the joint-torque-controlapproach. Implementation of PD control, in thiscase, is difficult and requires that the control gainsbe kept low to maintain stability. As a result, alter-native control schemes have been implemented,including proportional-integral (PI) control [11]and full-state feedback [3].

There are tradeoffs with using the SEA actua-tors. Due to the velocity and torque saturation ofthe SEA actuator, the maximum output torqueabove the open loop mode of the system (SEAopen loop mode is the unforced coupled motionof actuator and manipulator link inertias throughthe compliant coupling) falls off as 1/ω regardless

IEEE Robotics & Automation Magazine JUNE 200414

Figure 4. DM2 actuation approach.

Torq

ue M

agni

tude

Parallel Actuation Distributed ActuationAnd

Elastic Coupling+ Feedback

Joint Actuation(High Frequency)

PID

PID

Frequency

Elbow JointActuator

Shoulder JointActuator

ShoulderBase Actuator

Elbow BaseActuator

Base Actuation(Low Frequency)

Joint Actuation(High Frequency)

LightweightStructure+ Cable

Drivetrain

Base Actuation(Low Frequency)

Figure 5. Torque versus frequency: 1-Hz2 wave.

Nor

mal

ized

Inpu

t Tor

que

Nor

mal

ized

Inpu

t Tor

que

Mag

nitu

de

Time [s] Frequency [Hz]

1.0

0.0

–1.0

0.4

0.2

0.0

(1.2)

Magnitude ~1/ω

0.0 0.5 1.0 1.5 1 10 20 30 40

of the control loop controller bandwidth [10]. This behavioris an open-loop characteristic of the SEA actuator topologyand represents a fundamental physical limitation of the actua-tor. The choice of the elastic coupling stiffness (in relation tothe manipulator and motor-reflected inertia) determines theopen-loop-mode frequency. A stiffer coupling improves thehigh-frequency torque performance but adversely affects thedesirable closed- and open-loop impedance characteristics.

Tasks such as position control and end-effector impedancecontrol are limited to a bandwidth that is significantly belowthe closed-loop bandwidth of the SEA actuator. This is not amajor consideration for manipulation systems which do notrequire fast dynamics, such as walking robots for which theSEAs were originally developed. However, for tasks requiringhigh bandwidth control such as high-speed trajectory trackingor high-frequency disturbance rejection, the limitations of theSEAs are prohibitive. Other approaches have been proposed,such as the use of a nonlinear elastic coupling, whose compli-ance can be changed through coactivat-ed actuators [1]. Unfortunately, thebandwidth limitations affecting the serieselastic actuator, while mitigated some-what by the variable compliance, is still alimiting factor in performance.

New Actuation Approach:Distributed Macro-MiniActuationTo address the limitations of currentactuation technology, we have proposeda new approach that seeks to relocate themajor source of actuation effort from thejoint to the base of the manipulator [12].This can substantially reduce the effec-tive inertia of the overall manipulator byisolating the reflected inertia of the actu-ator while greatly reducing the overallweight of the manipulator. Performanceis maintained with small actuators collo-cated with the joints. Our approach par-titions the torque generation into low- and high-frequencycomponents and distributes these components to the armlocation, where they are most effective. The overall approachis summarized in Figure 4.

The first element of the actuation approach is to dividethe torque generation into separate low- and high-frequen-cy parallel actuators. The effectiveness of this approach canbe seen clearly when one considers that most manipulationtasks involve position or force control, which are dominatedby low-frequency trajectory tracking or dc load torques.High-frequency torques are almost exclusively used for dis-turbance rejection. Even haptic device torque profiles,which might require rapid changes approximating a squarewave input, have a torque magnitude versus frequencycurve that falls off with increasing frequency by 1/ω (seeFigure 5) . This torque versus frequency profile is ideally fit

using a large-output, low-frequency actuator coupled witha high-frequency servomotor.

In order for the DM2 approach to work properly, both thehigh- and low-frequency actuators must have zero or near-zeroimpedance. This is due to the fact that during power transferthe actuator torques will add nondestructively only if theirrespective impedance is zero. In particular, each actuator mustnot have significant impedance within the frequency range ofthe opposing actuator. Only if this condition is true will theDM2 concept work. For the high-frequency actuation, verylow impedance is achieved by using a small, low-inertia torquemotor connected to the manipulator through a low-friction,low-reduction cable transmission. For the low-frequency actua-tion, we achieve low impedance by using a series elastic actua-tor (see the section “Series Elastic Actuation”). Because theDM2 approach does not require that the base actuator be capa-ble of supplying high-frequency torques, the bandwidth limita-tions of SEA actuators do not pose a difficulty.


Figure 6. DM2 actuation and control topology (single DOF).

Ia : Arm Link Inertia Nb : Base Actuator Gear Ratio

Ks : Base Actuator (SEA) Compliance

Ib : Base Actuator Rotor Inertia Nj : Joint Actuator Gear Ratio

Ij : Joint Actuator Rotor Inertia

Where:

Ib

Ia

Ij

∆coupling

Ks

Nj

Nj

Ks

τdesired

– +

Joint HighFrequency Actuator

Base Low Frequency (Series Elastic) Actuator

PID

Figure 7. DM2 actuation and control block diagram represen-tation (single DOF).

τactual

τdesiredPID

1

1

(Ia + Nj2Ij)s2

Ks

NbIbs2

–

––

+

+

+

+

+

1

Nb

1


The second part of the DM2 actuation approach, whichdiffers from previous attempts at coupled actuation—the mostnotable of which is the parallel coupled macro-mini actua-tions approach [8]—is to distribute the low- and high-fre-

quency actuators to locations on the manipulatorwhere their effect on contact impedance is mini-mized while their contribution to control band-width is maximized. This is achieved by locatingthe low-frequency series elastic actuator remotelyfrom the actuated joint. This is particularly advan-tageous as the low-frequency components of mostmanipulation tasks are considerably larger in mag-nitude than the high-frequency components and,consequently, require a relatively large actuator.Locating the large SEA actuator at the base signif-icantly reduces the weight and inertia of themanipulator. The high-frequency actuators arelocated at the manipulator joints and connectedthrough a stiff, low-friction transmission, provid-ing the high-frequency torque components thatthe low-frequency base actuators cannot. Thehigh-frequency torque actuator must be connect-ed to the joint inertia through a connection thatproduces a high primary mode vibration frequen-cy. By locating the actuator at the joint and byusing a low-inertia servomotor, we can achievethis high bandwidth connection with a minimumamount of weight and complexity.

The DM2 approach is analogous to the designof robotic manipulators for use in zero gravity.Under such conditions, gravity induced torques donot exist. Joint actuators provide torques relatedonly to the task, such as trajectory tracking and dis-turbance rejection, both of which are primarily

medium to high frequency in content. We achieve the zero-gravity analogy by compensating for gravity torques and low-frequency torques using the low-frequency actuators located atthe base of the manipulator. With the effects of gravity and

Figure 8. (a) DM2 actuation control structure G(s)base–closed-loop: base actu-ator closed-loop transfer function. G(s)joint: joint actuator transfer func-tion). (b) Equivalent parallel structure.

Base Actuation (ωbw ~ 20 Hz)

Base Actuation

Base Actuation

τdesired τactual

τactual

τdesired

G(s)Base Closed-loop

1 - G(s)Base Closed-loop

G(s)Joint

G(s)Base Closed-loop +

+_

+

++

Joint Actuation(ωbw ~ 200 Hz)

Frequency

Frequency

ωbase

ωs

(a)

(b)

Figure 9. (a) Perfect torque source: base, joint, and combined DM2 actuator torque magnitude versus phase polar plot. (b) Near-perfect torque source: base, joint, and combined DM2 actuator torque magnitude versus frequency.

180°

150°

120°90°

60°

30°

0°

–30°

–60°–90°

–120°

–150°

G(s)Base SEA Actuator

G(s)Joint Actuation

G(s)DM2 Actuation

ωbase

ωs

Frequency

(b)(a)

Tact

Tdes

1.00.6

0.2

ωbase: Series Elastic Base Actuator Closed-Loop Bandwidth

ωs : Series Elastic Actuator Open-Loop Mode

Where:


low-frequency torques compensated, joint torque requirementsbecome similar to those encountered by a zero-gravity roboticmanipulator. However, unlike robotic manipulators designedfor space applications, the DM2 joint actuators do not require alarge gear reducer to achieve the required torque and powerdensities. Thus, the impedance of the DM2 approach is superi-or to that of current space robotic manipulators.

Actuation Control ApproachPerhaps the most challenging aspect of a DM2 implementa-tion is the development of a control approach that leveragesthe characteristics of the parallel actuator structure while deal-ing with the unique control challenges associated with the useof low-impedance actuation.

At the joint level, the DM2 approach is essentially a dual-input, single-output system. The redundant actuators providean additional degree of freedom that can be used in optimiz-ing system performance while minimizing actuation effort.For example, in the case of trajectory tracking, we can useLQR control techniques to obtain an optimum control lawbased on minimizing control effort and tracking error. Thelow- and high-frequency actuation effort partitioning can beaccomplished in a similar manner. However, this type of con-trol structure is specific to a given task—in this case, to trajec-tory tracking—and does not provide a black-box interface tothe actuation similar to the use of a single actuator. In particu-lar, for applications involving a number of different controlmodes, such as free-space motion with contact transitions, orfor applications requiring a low-impedance torque source,

such as haptics or telerobotic master devices, we desire anactuation control scheme that allows the use of the parallelactuation system as a single torque source.

Near-Perfect Torque SourceAs such, our control approach seeks to exploit the DM2 actu-ation’s unique characteristics to construct a near-perfecttorque source. The characteristics of a perfect torque source,consisting of zero output impedance and infinite controlbandwidth, would enable a manipulator to possess the charac-teristics necessary for both inherent safety and high-perfor-mance tasks. While a perfect torque source is impossible toachieve, a near-perfect torque source, with low outputimpedance relative to the driving load and high bandwidthtorque capability, offers many of the same advantages.

A physical schematic of the control structure along withan equivalent block diagram representation are shown inFigures 6 and 7, respectively. The transfer function of thecontrol structure shown in Figure 7 has unity gain and zerophase over all frequencies (τactual(s)/τdesired(s) = 1).

A simplified representation, shown in Figure 8, demon-strates how the control approach utilizes the low-frequencybase actuator’s low pass filter characteristics to partition thecontrol torques into low- and high-frequency components.

By using the actual measured torque output from the low-frequency base actuators in combination with the desiredtorque, we automatically compensate for the nonideal behaviorof the base actuators. Assuming that the smaller joint actuatorscan produce this torque, the combined torques’ sum is a perfect

Figure 10. Two-axis DM2 prototype.

Elbow Joint Base Actuator(Shoulder Actuator Not Shown for Clarity)

Drive IdlerPulley

InternalDrive Cables

Elbow Joint

Joint ActuatorServo Motor

Base ActuatorDrive Pulley

(Section View)

Shoulder Joint

Base ActuatorServo Motor andHarmonic Drive

AdjustableStiffnessCoupling

LOHETMagnetDeflectionSensor

Single StageCable Drive


realization of the desired torque. The frequency partitioning canbe clearly seen if we rearrange the structure in Figure 8(a) into apure parallel structure, as shown in Figure 8(b). As seen in Fig-ure 8(b), the base actuator’s transfer function falls off above itsclosed-loop bandwidth, ωbase, while the equivalent joint actua-tor’s transfer function approximates a double lead filter, whichadds phase to the combined system above the open-loop modefrequency, ωs, and attenuates the dc and low-frequency compo-nents commanded to the high-frequency actuator.

The combined actuator control structure creates a perfecttorque source in the linear sense, where the torques sum tounity magnitude and zero phase, as seen in Figure 9(a) and(b). Thus, by using the simple control structure describedabove, we can create a unified actuator with the desirablecharacteristics of the low impedance necessary for inherentsafety and the high bandwidth torque control necessary forhigh performance.

Manipulation ControlThe DM2 control structure allows for straightforward imple-mentation of the DM2 approach in multidegree-of-freedommanipulator systems. Assuming that the assumptions of a near-

perfect torque source hold, the DM2 approach is particularlywell suited to control methods, such as operational space con-trol [6], which assume that the control torques are directlyapplied to the joint with little or no unmodeled disturbancesfrom sources such as actuator friction or reflected inertia.

The perfect torque source structure breaks down when theassumptions of the model shown in Figures 6 and 7 are nolonger valid. The main challenge in implementing the controlscheme is in identifying and avoiding the situations where thisideal model breaks down (see [13] for more details).

Promising Results: Safety and PerformanceTo demonstrate the effectiveness of the DM2 approach, wehave designed and built a two-axis prototype robotic arm thatincorporates the important characteristics of the DM2

approach. The overall design approach is shown in Figure 10.Preliminary experimental and simulation results have

demonstrated the effectiveness of the DM2 approach. Thereduction in impact loading by an order of magnitude, as com-pared to conventional joint actuated manipulators, substantiallyimproves the inherent safety of the manipulator. In the case ofa two-axis prototype developed at Stanford (see Figure 10), the

Figure 11. Comparison of impulse load due to impact for various actuation concepts. (a) Normalized impulse vector: impulse dueto collision of manipulator end effector with rigid object. Impulse magnitude changes with angle due to variation of end-effectoreffective inertia as a function of impact direction. (b) Normalized impact impulse versus collision velocity direction for various actu-ation concepts and values of end-point load (Pload). (c) Comparison of normalized impact impulse load for various actuation con-cepts and values of end-point load (Pload). Impulse values are normalized by impact velocity and maximum effective inertia.

Collision ImpulseMagnitude

Collision VelocityDirection

NormalizedImpulse Vector

ly

lx

ly ly

lx lx

ConventionalJoint Actuation

ConventionalCable Drive

SEAApproach

DM2

Approach

Pload = 1 [kg] Pload = 10 [kg]

Pload = 1 [kg] Pload = 10 [kg]

1.0 1.0

(b)

(c)

(a)

Nor

mal

ized

Impu

lse

Load

1.0

0.8

0.6

0.4

0.2

0.0

Conventional Joint Actuation

Conventional Cable Drive

DM2 Approach

SEA Approach

5 × Reduction10 × Reduction


effective joint inertia was reduced by almost a factor of ten. Wecan use the effective inertia, graphically illustrated as a beltedellipsoid [7], to calculate the impulse due to impact at anypoint on the manipulator. To demonstrate the effectiveness ofthe DM2 approach in reducing impact loads, Figure 11 showsthe normalized impact impulse for two cases of end-point load(Pload) for a two-degree-of-freedom planar manipulator. Theimpact impulse reduction increases rapidly with increasingload, as the required increase in actuator torque capabilityaffects the reflected inertia of the conventional and cable-dri-ven manipulators while minimally affecting the reflected iner-tia of both the DM2 and SEA approaches. Because the DM2

joint actuators are lightweight and have very small reflectedinertia, their contribution to the effective inertia is minimal. Asa result, the improved performance that results from the addi-tion of the small joint actuators does not compromise the safe-ty levels of the DM2 as compared to the SEA actuatedmanipulator. While this is just an illustrative example, we seethat in combination with a lightweight structure and compli-ant covering, this new actuation approach can be used todesign a manipulator that reduces impact loads substantially,thus ensuring inherent safety.

In addition to safety, the DM2 approach, with the introduc-tion of the high-frequency joint actuator and implementationof the control approach described in the section “DM2 Actua-tion Control Approach,” has been shown experimentally toimprove manipulator performance. As shown in Figure 12,open-loop end-effector force control with the DM2 approachimproves the speed of response over that of the base-series elas-tic actuator alone. Both approaches have very low steady-stateerror due to their very low output impedance.

Trajectory tracking experiments carried out on the two-axis planar manipulator testbed demonstrate the feasibilityof the DM2 approach. Initial experiments demonstrated aposition control bandwidth of approximately 5 Hz as com-

pared to a 2-Hz bandwidth using the base actuator alone(see Figure 13), reducing the position tracking error bymore than a factor of ten. The higher achievable closed-loop position bandwidth allows the DM2 actuated arm toaccurately follow trajectories at rates that are not possiblewith the base actuator alone.

Using the two-DM2 axis testbed, we performed end-effector position tracking control experiments along a 15-cm linear path at cycle rates of .25 Hz, 1.0 Hz, and 2.0 Hz.The results of the experiments, which contrast the DM2

actuated and base (SEA) actuated performance, are shown inFigure 14. The DM2 actuated testbed showed good trackingcontrol for all three cases, with only a small amount ofamplitude and phase distortion occurring during the 2.0-Hzrate experiment. The same experiment performed using thebase actuators alone produced significant tracking error.During the 1.0-Hz and 2.0-Hz rate experiments, significantphase and amplitude distortion were observed.

Figure 12. Open-loop end-effector force (step) response.

Mea

sure

d F

orce

[N]

25

20

15

10

5

0

InputCommand

Base and JointActuator (DM2)

Base SEAActuator Only

Time [s]

0.00 0.05 0.10 0.15 0.20

Figure 13. Comparison of position tracking performance using base actuation only with combined base and joint actuation (DM2).

Act

ual A

ngle

[rad

]

0.10

0.10

0.05

0.05

0.00

0.00

–0.05

–0.05

–0.10

–0.10

Elbow: Desired Versus Acutal Angle Shoulder: Desired Versus Acutal Angle

Desired Angle [rad]

Act

ual A

ngle

[rad

]

0.10

0.10

0.05

0.05

0.00

0.00

–0.05

–0.05

–0.10

–0.10Desired Angle [rad]

BASE SEAACTUATOR ONLY

2 Hz BandwidthTracking Error ~ .06 rad

BASE SEAACTUATOR ONLY

2 Hz BandwidthTracking Error ~ .04 rad

BASE + JOINT5 Hz Bandwidth

Tracking Error ~ .005 rad

BASE + JOINT5 Hz Bandwidth

Tracking Error ~ .004 rad

(a) (b)


Distributed Macro-Mini ImplementationFinally, a few words should be said aboutthe implementation of a DM2 actuatedrobotic system. The DM2 approach isessentially a tradeoff between safety, per-formance, and design complexity. How-ever, this design trade is not necessarily azero-sum game.

The primary reason for the introduc-tion of our new actuation approachwere to reduce contact impedance andmaintain task performance levels. If thetask is performed by a manipulator’s endeffector, then high-frequency torque andforce capabilities need only be providedat the end effector. As shown in [7], thedynamics of a redundant manipulator isbounded by the dynamics of the outer-

most degrees of freedom that span the task space. In the caseof a redundant manipulation system, such as a dual manipula-tor–mobile base’s system depicted in Figure 15, the mobilebase degrees of freedom need not employ our new actuationapproach to maintain task performance levels which, due tothe redundancy of the system, are bounded by the outer sixdegrees of freedom. Another possible approach is to designthe wrist such that required task torques are small, as wouldbe the case for a compact wrist design. In this case, the wristactuation could be provided by smaller conventional EMactuators. The large dc and low-frequency torques providedby the base actuators of the DM2 approach would not berequired. The higher impedance of the wrist actuators wouldnot compromise safety because impact loads would be limitedby the inner three degrees of freedom.

SummaryWe have presented a new actuation concept for human-friendly robot design, referred to as DM2. The new conceptof DM2 was demonstrated on a two-degree-of-freedom pro-totype robot arm that we designed and built to validate ourapproach. The new actuation approach substantially reducesthe impact loads associated with uncontrolled manipulatorcollision by relocating the major source of actuation effortfrom the joint to the base of the manipulator. High-frequencytorque capability is maintained with the use of small, low-inertia servomotors collocated at the joints. The servomotors,integrated with a low-reduction, low-friction cable transmis-sion, provide the high-frequency torque required for high-performance tasks while not significantly increasing thecombined impedance of the manipulator-actuator system.The low output impedance and complete frequency coverageof the new actuation approach allow the combined manipula-tor system to approximate a pure torque source. This in turnallows for very good open-loop joint torque control over awide frequency range. Initial experimental and simulationresults validate the DM2 approach.

Figure 14. End-effector position tracking control experimental results.

Figure 15. Implementation of DM2 actuation for multi-DOFmanipulator.

Inner DOFs(DM2 Actuation)

Outer DOFs(Single Actuators)

Distributed Macro-Mini Actuation

Single Actuator

End-Effector Trajectory Tracking: Linear Path (~15 cm Full Scale)

0.25 Hz Scan 1.0 Hz Scan 2.0 Hz Scan

BASEACTUATOR

ONLY

DM2

(BASE/JOINTACTUATOR)

Desired End-Effector X-PositionEnd-Effector X-Position (Base Only Actuation)End-Effector X-Position (Base and Joint Actuation)

X P

ositi

on [m

] 8

4

0

–4

–8

X P

ositi

on [m

] 6

2

–2

–6 X P

ositi

on [m

] 6

2

–2

–6 X P

ositi

on [m

] 6

2

–2

–6X

Pos

ition

[m] 8

4

0

–4

–8 X P

ositi

on [m

] 8

4

0

–4

–8


AcknowledgmentsThe authors would like to thank Gunter Neimeyer, KenWaldron, and Gene Duval for their helpful insights and dis-cussion in preparing this article. The financial support ofNSF grant EIA-9977717 is gratefully acknowledged.

KeywordsHuman-friendly, low impedance, safety actuation, parallelactuation, distributed actuation.

References[1] A. Bicchi, L. Rizzini, and G. Tonietti, “Compliant design for intrinsic

safety: General issues and preliminary design,” in Proc. Int. Conf. Intell.Robots Syst., Maui, HI, 2001, pp. 1864–1869.

[2] G. Hirzinger, A. Albu-Schäffer, M. Hähnle, I. Schaefer, and N. Sporer,“A new generation of torque controlled light-weight robots,” in Proc.Int. Conf. Robotics Automation, Seoul, Korea, 2001, pp. 3356–3363.

[3] G. Hirzinger, N. Sporer, A. Albu-Schaffer, M. Hahnle, and A. Pascucci,“DLR’s torque-controlled light weight robot III—Are we reaching thetechnological limits now?” in Proc. Int. Conf. Robotics Automation, 2002,pp. 1710–1716.

[4] J. Hollerbach, I. Hunter, and J. Ballantyne, A Comparative Analysis ofActuator Technologies for Robotics. Cambridge, MA: MIT Press, pp.299–342, 1991.

[5] R. Holmberg, S. Dickert, and O. Khatib, “A new actuation system forhigh-performance torque-controlled manipulators,” in Proc. 9th CISM-IFToMM Symp. Theory Practice Robots Manipulators, Udine, Italy, Sept.1992, pp. 285–292.

[6] O. Khatib, “A unified approach for motion and force control of robotmanipulators: The operational space formulation,” IEEE J. Robot.Automat., vol. RA-3, pp. 43–53, Feb. 1987.

[7] O. Khatib, “Inertial properties in robotic manipulation: An object-levelframework,” Int. J. Robotics Res., vol. 14, no. 1, pp. 19–36, Feb. 1995.

[8] J.B. Morrel, “Parallel coupled micro-macro actuators,” Ph.D. disserta-tion, Massachusetts Institute of Technology, Cambridge, MA, 1996.

[9] G. Pratt and M. Williamson, “Series elastic actuators,” in Proc.IEEE/RSJ Int. Conf. Intell. Robots Syst., vol. 1, Pittsburgh, PA, 1995, pp. 399–406.

[10] D. Robinson, “Design and analysis of series elasticity in closed-loopactuator force control,” Ph.D. dissertation, Massachusetts Institute ofTechnology, Cambridge, MA, June 2000.

[11] D. Vischer and O. Khatib, “Design and development of high-perfor-mance torque-controlled joints,” IEEE Trans. Robot. Automat., vol. 11,pp. 537–544, Aug. 1995.

[12] M. Zinn, O. Khatib, B. Roth, and J.K. Salisbury, “A new actuationapproach for human friendly robot design,” in Experimental RoboticsVIII, Springer Tracts in Advanced Robotics, B. Siciliano and P. Dario, Eds.Berlin: Spinger-Verlag, 2002.

[13] M. Zinn, O. Khatib, B. Roth, and J.K. Salisbury, “Actuation methodsfor human-centered robotics and associated control challenges,” in Con-trol Problems in Robotics, A. Bicchi, H. Christensen, and D. Prattichizzo,Eds. Berlin, Germany: Springer-Verlag, 2003.

Michael Zinn received his M.S. and B.S. degree fromM.I.T. in 1988 and 1987, respectively, and is currently com-pleting his Ph.D. in mechanical engineering at StanfordUniversity. He has extensive experience in mechanical andcontrols design with more than 15 years experience design-ing complex electro-mechanical systems. His currentresearch interests include human-centered robotics andmedical device robotics.

Oussama Khatib received his Ph.D. in 1980 from Sup’Aero,Toulouse, France. His current research is in human-centeredrobotics, human-friendly robot design, dynamic simulations,and haptic interactions. His exploration in this research rangesfrom the autonomous ability of a robot to cooperate with ahuman to the haptic interaction of a user with an animatedcharacter or a surgical instrument. Prof. Khatib is the presi-dent of the International Foundation of Robotics Research,IFRR, and co-editor of the Springer Tracts in AdvancedRobotics. He is a “Distinguished Lecturer” of the IEEE and arecipient of the JARA Award.

Bernard Roth has been a faculty member of Stanford Uni-versity’s Mechanical Engineering Department since 1962.He teaches courses in design, robotics, kinematics, and the24 societal and personal aspects of technology. He is anactive researcher with over 150 publications in the areas ofrobotics, kinematics, and design. He and his students havebeen pioneers in the design and construction of roboticdevice and in the development of a rational theory for robotdesign and control. He has served as president of the Inter-national Federation for the Theory of Machines and Mecha-nisms and as chairman of the Design Engineering Divisionof the American Society of Mechanical Engineers. He is adirector of the International Federation for RoboticsResearch. Dr. Roth is the recipient of many awards for hisresearch and teaching, including The Joseph F. EnglebergerAward for Robotics, the Machine Design Award, theMelville Medal, five Best Paper Awards from the AmericanSociety of Mechanical Engineers, and two Japanese Societyfor the Promotion of Science Awards.

J. Kenneth Salisbury received his Ph.D. from Stanford Uni-versity in mechanical engineering in 1982. At MIT from 1982-1999, he served as principal research scientist in mechanicalengineering and as a member of the Artificial Intelligence Lab-oratory. Some of the projects with which he has been associat-ed include the Stanford-JPL Robot Hand, the JPL ForceReflecting Hand Controller, the MIT-WAM arm, and theBlack Falcon Surgical Robot. His work with haptic interfacetechnology led to the founding of SensAble Technologies Inc.,producers of the PHANTOM haptic interface and 3DFreeForm software. In 1997, he joined the staff at Intuitive Sur-gical, in Mountain View, California, where his efforts havefocused on the development of telerobotic systems for theoperating room. In 1999 he joined the faculty at Stanford inthe Departments of Computer Science and Surgery, where hisresearch focuses on human centered robotics, cooperative hap-tics, and surgical simulation. He currently serves on theNational Science Foundation’s Advisory Council for Roboticsand Human Augmentation as scientific advisor to Intuitive Sur-gical, Inc. and as technical advisor to Robotic Ventures, Inc.

Address for Correspondence: Michael Zinn, DesignDivision, Department of Mechanical Engineering, StanfordUniversity, Stanford, CA. E-mail: [email protected].

playing it safe - stanford university

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