Control law design for haptic interfaces to virtual reality
IEEE TRANSACTIONS ON CONTROL SYSTEMS TECHNOLOGY, VOL. 10, NO. 1, JANUARY 2002 3Control Law Design for Haptic Interfaces to VirtualRealityRichard J. Adams and Blake HannafordAbstractThe goal of control law design for haptic displays is toprovide a safe and stable user interface while maximizing the op-erators sense of kinesthetic immersion in a virtual environment.This paper outlines a control design approach which stabilizes ahaptic interface when coupled to a broad class of human opera-tors and virtual environments. Two-port absolute stability criteriaare used to develop explicit control law design bounds for two dif-ferent haptic display implementations: impedance display and ad-mittance display. The strengths and weaknesses of each approachare illustrated through numerical and experimental results for athree degree-of-freedom device. The example highlights the abilityof the proposed design procedure to handle some of the more diffi-cult problems in control law synthesis for haptics, including struc-tural flexibility and noncollocation of sensors and actuators.Index TermsControl system human factors, force feedback,haptic display, stability criteria, virtual reality.I. INTRODUCTIONT HE WORD haptic means of or relating to the sense oftouch. Haptic feedback is a new and relatively unexploredway of conveying information between a human and a com-puter. A video monitor provides visual information, speakersprovide audio information, in an analogous manner, a haptic dis-play conveys kinesthetic information to the operator through thesense of touch. The haptic display generates force feedback cueswhich may represent the resistance of a virtual wall, the rough-ness of a virtual texture, or the weight of a virtual mass. Hapticdevices come in all shapes and sizes, ranging from large indus-trial manipulators to motorized desktop mice.The field of haptics has been led by research applications.Some of the most exciting work is in surgery simulation. Thegoal is to permit student surgeons to safely practice proceduresusing haptic and graphical interfaces which accurately reflectreal surgical conditions , . Another burgeoning area isforce feedback in computer-aided design (CAD). By allowingthe designer to actually touch the objects under design, hapticsmay greatly increase efficiency and creativity. It is not surprisingthat the most rapidly evolving side of haptics is consumer prod-ucts. The first consumer haptic devices were force feedback joy-sticks for computer gaming. With the acceptance of standardapplication programming interfaces (APIs), the number of forcefeedback enabled computer games has skyrocketed, and new in-terfaces, such as force feedback steering wheels, have appeared.Manuscript received September 13, 1999; revised September 20, 2000. Man-uscript received in final form August 7, 2001. Recommended by Associate Ed-itor D. W. Repperger.The authors are with the Department of Electrical Engineering, University ofWashington, Seattle, WA 98195-2500 USA.Publisher Item Identifier S 1063-6536(02)00327-5.Fig. 1. Excalibur and the virtual building block simulation.The advent of the force feedback mouse  and force feedbackenabled graphical user interfaces (GUIs) may eventually bringhaptic technology to the majority of computer users desktops.The virtual building block (VBB) system is an example ofhaptic simulation, developed at the University of WashingtonBioRobotics Laboratory to demonstrate the utility of haptictechnology in CAD and virtual prototyping applications. Thesystem currently uses the Excalibur three-axis force displaybuilt by Haptic Technologies Inc. of Seattle, WA , . Fig. 1illustrates the simulation. The user can select and manipulateindividual blocks or groups of blocks. Haptic feedback rep-resents each objects inertia, prevents objects from impedingon each other, and renders interaction forces when blocks aresnapped together.In one sense, haptic interaction is very different from othermodes of human-computer interface. In haptic display, there isa bidirection flow of information, both from the device to thecomputer and from the computer to the device. The haptic dis-play in not just a force player, rendering preset force effects. Itis a force feedback device which creates forces in response tothe users input. As with any device where feedback is present,the potential for instability exists. Instabilities will at best de-grade the quality of the force feedback information to the humanoperator. In the case of devices with significant inertia and forceoutput, the consequences of instability may be more severe.The nascent field of controls for haptic displays offers inter-esting challenges. As with most control problems, there are two10636536/02$17.00 2002 IEEE4 IEEE TRANSACTIONS ON CONTROL SYSTEMS TECHNOLOGY, VOL. 10, NO. 1, JANUARY 2002sometimes conflicting goals, performance and stability. Goodperformance implies forces and velocities experienced by thehuman operator are nearly identical to those generated by theapplication software. This is the transparency of the hapticinterface . The performance of any haptic display is limitedat one extreme by how well it can simulate free motion and atthe other extreme by how well it can generate rigid constraints.The minimum and maximum achievable impedance of a hapticinterface delineates its impedance range , another usefulmeasure of haptic performance. The inability to render arbi-trarily small or large impedance to the human operator is due,in part, to the onset of instability. The stability of a haptic in-terface is strongly affected by both the application software andthe nature of human-device contact. The application software,which we will also refer to as the virtual environment, deter-mines what forces are rendered through the haptic display to thehuman operator. If the application calls for an unachievable levelof impedance, for example by creating an excessively rigid vir-tual wall, the haptic display may oscillate violently. The mannerin which the human operator contacts the haptic display alsoaffects the stability of the overall system. With improperly de-signed controls, instability may ensue when the operator graspsthe device very tightly or, conversely, oscillations may occurwhen the device is released.In early applications of haptic display, no distinction wasmade between the virtual environment and the control law forthe haptic device. In effect, the virtual environment was the con-trol system. The stiffness and damping of a virtual wall becameproportional-plus-derivative feedback gains on device position.Some examples are found in . This approach has twomajor drawbacks. First, it is very difficult to ensure that a com-plex dynamic virtual environment translates into a stabilizingcontrol law for a haptic device. The application software mustbe tuned and tested extensively, and even then, stability is not as-sured. Second, since the virtual environment must be tuned fora specific device, the application software must be redesignedif it is to be used with a different haptic display. In other words,a haptic enabled CAD package that works properly with oneforce feedback mouse may induce instabilities when linked toa different mouse which has slightly less mechanical damping.An improved approach to control law design for haptic displayswould separate the problem of device control from the designof application software.The use of an artificial coupling between the haptic displayand the virtual environment was first proposed by Colgateet al.. Zilles and Salisbury  suggested a similar godobjectapproach which couples a haptic device to a virtual environmentthrough a virtual spring-damper. Adams and Hannaford  putthe problem of stable haptic simulation into a two-port frame-work. The two-port approach allows for rigorous stability andperformance analysis for a very general class of haptic displays.Virtual couplings may be designed for devices with structuralflexibility, force sensing, noncollocated sensors and actuators,and measurement delay.This paper builds upon the theoretical work presented in. A review of the two-port framework for analysis anddesign of haptic interfaces precedes the introduction of newdevelopments. Conservatism in previous results is reducedFig. 2. Network model of haptic simulation.using a new technique which restricts the assumed human op-erator impedance to realistic levels. A model for the Excaliburforce display is developed, and numerical data provided for acritical design point. The work of  is extended to permitregulator and virtual coupling design for a much broader classof haptic displays. Two haptic display implementations areexplored, numerically and experimentally, for Excalibur. Thestrong agreement between theoretical and experimental resultshighlights the utility of the proposed two-port framework forhaptic interface analysis and design.II. TWO-PORT FRAMEWORK FORSTABILITY AND CONTROLTwo-port methods are rooted in linear circuit theory, wherethey are used to characterize the effects of different loading con-ditions on two terminal electrical networks. We can consider amechanical analog to this electrical two-port, the haptic inter-face, which is subject to variable loading conditions both at thepoint of interaction with the human operator and at the pointof interaction with the virtual environment. In using this me-chanical analog, we substitute velocities for currents in repre-senting flow and forces for voltages in representing effort. Thetwo-port haptic interface model characterizes the exchange ofenergy between the human operator and the virtual environment.It is useful in studying the stability of the overall system and indescribing the performance of the haptic interface. Fig. 2 showsthe network model of haptic simulation. The human operatoraffects the velocity, , and force, , at the physical point ofcontact with the haptic display. The virtual environment modu-lates the velocity, , and force, , at the point of informationexchange with the haptic interface. The virtual environment isa digital system. The star superscript indicates that a variable isdiscrete, defined only at the time of sampling.In the analysis and design of haptic interfaces, we are in-terested in a particular representation of the two-port system,known as animmittance matrix. A matrix , which maps theinput to output , is an immittance matrix characterization ofthe haptic interface if . There are fourways to form such a matrix. These are the impedance matrix,,the admittance matrix, , the hybrid matrix, , and the alter-nate hybrid matrix, . Any one of these forms can be formedas a algebraic combination of the elements of any of the others,provide that the matrices exist. By definition, if any one of thesematrices is positive real, then all of the others will be as well.The converse is also true. More details on two-port representa-tion can be found in , .We will say that the overall haptic simulation isstableif, forgiven human operator and virtual environment impedance, theADAMS AND HANNAFORD: CONTROL LAW DESIGN FOR HAPTIC INTERFACES TO VIRTUAL REALITY 5resulting characteristic equation has no roots in the right half-plane and only simple roots on the imaginary axis. Unfortu-nately, we do not havea priori knowledge of the level of humanor virtual environment impedance. We would like the hapticsimulation to remain stable under any foreseeable variation. Auseful notion for handling this issue of robustness is absolutestability. A linear two-port is said to be absolutely stable if thereexists no set of passive terminating one-port impedances forwhich the system is unstable. Thus, if the haptic interface can bemade absolutely stable, the haptic simulation will remain stableas long as the human operator and virtual environment are pas-sive.Llewellyns stability criteria provide both necessary and suf-ficient conditions for absolute stability of linear two-ports ,has no poles in the right half-plane, only simplepoles on the imaginary axis,(1)(2)Together, the two inequalities imply . The satis-faction of the absolute stability criteria for any one of the immit-tance matrix forms ( , , , or ) is necessary and sufficientfor them to hold for the other three . By demonstrating thatthe haptic interface two-port satisfies these criteria, the stabilityof the system is assured for any level of passive human operatorand virtual environment impedance.Treating energetic interaction between the human arm anda mechanical device as passive appears to be a reasonable as-sumption. In experimental studies, Hogan found that despiteneural feedback within the arm and a high degree of adapt-ability in the neuromuscular system, the impedance exerted bya human is passive . Requiring that the virtual environmentact as a passive operator can be challenging. It is intuitive thatthe simulation of physically motivated effects (masses, springs,dampers) should obey conservation laws of physics, and thusbe passive. However, formulating numerical integration routineswhich achieve strict adherence to these laws can be difficult.Brown  showed that explicit discrete-time passive integra-tion of the equations of motion is impossible. Fortunately, expe-rience has demonstrated that absolutely stable haptic interfacesare very robust when coupled to virtual environments which arealmost passive , .For some haptic interfaces, representing the human operatoras an arbitrary passive impedance can be overly conservative.Arbitrary passivity allows for the extreme cases of zero humanimpedance (no contact) and a perfectly rigid (infinitely stiff)human grasp. If a haptic display has very little mechanicaldamping, it becomes difficult to achieve any level of perfor-mance while maintaining absolute stability. In this case wemay want to assume some minimum level of impedance, ,which will be provided by the human operators contact withthe device. This strategy should always be coupled with someform of dead-mans switch since the system may well beunstable if the operator breaks contact with the haptic device.Fig. 3. Human operator impedance model.If the haptic display implementation includes force sensingat the human-device interface, the stability analysis may bedominated by the unrealistic scenario of infinitely high humanstiffness. This conservatism can be abated by postulating amaximum level of impedance for the human operator.Fig. 3 shows how the analysis model can be modified to includeminimum and maximum levels of human impedance. Wecharacterize the human operator in terms of three impedances,, , and . is an arbitrary passive impedance function,, and . Notice that whenis zero (short circuit) the resulting human impedance is. When is infinite (open circuit), the resulting humanimpedance is .The stability conditions in this paper are contingent uponthe linearity of the system and the fidelity of linear model. Itis possible for nonlinear systems to be treated using a similarpassivity-based approach, but this problem is not addressed inthe present work. A fairly direct approach to nonlinear prob-lems would be to first apply some form of feedback lineariza-tion before performing the linear design discussed here. Struc-tured singular value analysis techniques might be used to ana-lyze the effect of uncertainty in the linear model due to param-eter variations and unmodeled dynamics. Unfortunately, theseapproaches are not currently viable as a synthesis scheme forvirtual couplings.III. EXCALIBUR MODELExcalibur is a three degree-of-freedom Cartesian manipu-lator, designed to act as a haptic interface to virtual or remoteenvironments. Brushless motors provide control forces througha steel cable transmission along three mutually orthogonaltranslational axes. The user grasps a handle mounted on the endeffector as shown in Fig. 1. The device is capable of renderingpeak forces of up to 200 N and continuous forces of up to100 N in each axis over the workspace of 300300 200mm . The dynamics of the manipulator are a function of thelocation of the handle within the workspace. All three axesof motion ( , , ) must be considered. For brevity, we willrestrict our attention to a single, worst-case design point forthe Excalibur. This is the location at which the device exhibitsa structural resonance at the lowest frequency. A successfuldesign for this condition yields a control law which satisfiesour stability criteria for all other locations in all three axes. Thisworst-case point represents-axis motion when the handle isin the neighborhood of mm, mm,mm, measured from the lower-left-bottom position as seen inFig. 1. Details on the Excalibur model can be found in .6 IEEE TRANSACTIONS ON CONTROL SYSTEMS TECHNOLOGY, VOL. 10, NO. 1, JANUARY 2002The internal dynamics of the system can be represented instandard second-order form(3)where , , and are the mass, damping, and stiffness ma-trices, respectively. is the control distribution matrix. Thevector represents the internal state of the system.is the inputvector(4)is the force applied at the handle by the human operator inthe -direction. is the force generated by the actuator alongthe -axis. The outputs of interest are velocity at the handle,, velocity of the actuator, , and force measured by a straingauge built into the handle,(5)(6)The final element to consider is an analog filter, , integratedinto the load cell by the manufacturer for noise reduction andanti-aliasing, The output of this filter is the measured force at thehandle, . This force measurement and the actuators are non-collocated. The equations governing the dynamics of the systemcan be written in Laplace form(7)(8)Numerical values, derived from a lumped mass model, for allrelevant parameters are provided in the Appendix. The equa-tions of motion can alternatively be written in terms of indi-vidual transfer functions- - - - - - - - - - - - - - - (9)(10)Note that two subscripts are attached to each transfer function.The first one matches the subscript of the corresponding outputvariable. The second subscript matches that of the input vari-able.IV. HAPTIC INTERFACEDESIGNThe haptic interface is more than just the mechanical device.It encompasses everything that comes between the human op-erator and the virtual environment. To better understand the sta-bility properties of the system, we separate the haptic interfaceinto a cascade combination of two subnetworks. These are theFig. 4. Haptic interface two-port.Fig. 5. Impedance display implementation.haptic display two-port and the virtual coupling two-port, asshown in Fig. 4.A. Haptic Display ImplementationsThe haptic display includes the physical structure of the ma-nipulator, as well as actuators, sensors, analog filters, amplifiers,digital-to-analog/analog-to-digital conversion, digital filtering,and control software. There are two ports by which the hapticdisplay can be accessed. At one end there is a physical port, thehandle, at which the human operator exchanges energy with thedisplay. At the other end is an information port, characterizedby the discrete variables, velocity,, and force, . Unlike thephysical port, a specific causality must be associated with the in-formation port. There are two possibilities. The haptic displaycan measure motion and display force or it can measure forceand display motion , . The former case is animpedancedisplay, is an output and is an input. The latter case is anadmittance display, is an input and is an output.Within the individual classes of impedance and admittancetype haptic displays, a number of implementations are possible.Two of the most common will be described here. A third im-plementation, impedance display with force compensation, isaddressed in . It is straightforward to follow the examplesbelow to perform design and analysis for other implementations.1) Impedance Display:This is by far the most common im-plementation of a haptic display. Either optical encoders or po-tentiometers provide a measure of device position,, at thepoint of actuation. This signal is sampled with periodto createthe digital signal, . The device velocity is estimated, in ourcase using a simple first difference approximation, to generatethe output variable, . The digital force command, , is passedthrough a zero-order hold to provide the control input to the ac-tuators. Fig. 5 shows the impedance display implementation.The two-port equations for the haptic display can be derivedADAMS AND HANNAFORD: CONTROL LAW DESIGN FOR HAPTIC INTERFACES TO VIRTUAL REALITY 7from Fig. 5 and the haptic device two-port admittance form, (9).Including the integration of device velocity and the zero-orderhold at the input, we have- - - - - - - - - - - - - - - - - - - - - - - - - - - - -(11)The Laplace transform of the digital signal is(12)A sampling period of will be used throughoutthis paper. If we can assume that is very small for, then no significant aliasing will take place whenthe signal is sampled. This is a reasonable assumption forour system thanks to the low pass properties of the lower twotransfer functions in (11). We can therefore say(13)This assumption limits analysis to frequencies up to the Nyquistrate. Neglecting higher frequency effects is valid, provided thesystem provides sufficient roll-off. Applying (13) to (11) givesus- - - - - - - - - - - - - - - - - - - - - - - - - - - - -- - - - - - - - - - - - - - (14)The final step is to include the first difference velocity approx-imation, giving us the equations for the impedance type hapticdisplay (in admittance matrix form)- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -- - - - - - - - - - - - - - (15)This is an effective implementation. Equivalent strategies havebeen used by numerous researchers , , , . One dis-advantage of this approach is that no compensation is made forFig. 6. Admittance display implementation.the open-loop impedance of the device. In other words, whenmoving about in free motion, the human operator will alwaysfeel the full inertia and friction of the manipulator. For light-weight, highly backdrivable devices, this is acceptable. Whendevice inertia and friction are significant, an alternative imple-mentation is desirable.2) Admittance Display:The presence of high levels of in-ertia and friction, common in industrial robots, may make animpedance display implementation impractical. An alternativeis to configure the device as an admittance display. This im-plementation has been used in a number of applications whereback-drivability is a concern , . Fig. 6 shows a blockdiagram of an admittance display. We can use (10) and (14) asstarting points for the derivation of the haptic display two-portequations. By combining the force transfer functions in (10)with the zero-order hold, we get(16)Assuming sufficient analog filtering is present to preventaliasing, the result of sampling the measured force is(17)Using (10) and (14) and closing a position control loop(18)we find the admittance display two-port equations (in alternatehybrid form) as shown in (19) at the bottom of the next page. Theposition regulator determines the maximum impedance whichcan be rendered by the admittance type haptic display. In orderto simulate rigid virtual objects, should be designed tohave the highest gain possible, without violating stability con-straints.B. Virtual Coupling ImplementationsThe second half of the haptic interface, as shown in Fig. 4is the virtual coupling network, an artificial link between thehaptic display and the virtual environment. The virtual couplingnetwork is an additional control element, designed such thatthe haptic interface is absolutely stable. In principal, it can bea general two-port function. In practice, by limiting the choice8 IEEE TRANSACTIONS ON CONTROL SYSTEMS TECHNOLOGY, VOL. 10, NO. 1, JANUARY 2002(a) (b)(c) (d)Fig. 7. Virtual coupling implementations.of these networks to specific topologies, explicit design criteriafor the coupling function can be found .For impedance displays, we will consider a two-port with asingle shunt impedance(20)This impedance display virtual coupling induces a limit on themaximum impedance which can be rendered. In effect, it actsa spring-damper placed between the virtual environment andthe haptic display. It never allows the virtual environment todrive the system unstable by generating an excessively rigidconstraint. For performance, we would like the magnitude ofto be as large as possible, while maintaining the abso-lute stability of the haptic interface.For admittance displays, we will limit the virtual couplingtwo-port to consist of a single series impedance(21)While the virtual environment may simulate an infinitesimallysmall mass, there is a limit to what the haptic display canstably render. The admittance display virtual coupling acts as afrequency-dependent damper, providing the required level ofimpedance to stabilize the system. For performance, we wouldlike to be as small as possible, permitting unconstrainedfree motion, while still meeting the requirements for absolutestability. Further details on the motivation for these virtualcouplings are found in .The actual implementation of the virtual coupling networksis dictated by the type of haptic display and virtual environmentused in a simulation. At one end, the virtual coupling two-portmust match the causality of the haptic display. If an impedancedisplay is used, the coupling must accept velocities,, and gen-erate forces, . The inverse is true if the coupling is connectedto an admittance display. On the other end the virtual couplingtwo-port must match the causality of the virtual environment.It is possible for a virtual environment to act as an impedance,, or as an admittance, , . The fourpossible virtual coupling implementations are shown in Fig. 7.C. Design for Absolute Stability1) Impedance DisplayBasic:When we combine theimpedance display implementation with an appropriate virtual- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -- - - - - - - - - - - - - - (19)ADAMS AND HANNAFORD: CONTROL LAW DESIGN FOR HAPTIC INTERFACES TO VIRTUAL REALITY 9coupling network, Fig. 7(a) or (b), we get the admittance matrixfor the combined haptic interface- - - - - - - - - - - - - - - - - - - - - -(22)Note that (22) is invariant to the choice of virtual environmentcausality and thus so are the analysis and design results whichfollow . The difference lies only in the implementation ofthe virtual coupling. By applying (1) and (2) to (22), we get theconditions for absolute stability of the haptic interface(23)(24)By definition, is the transfer function from force appliedat the handle to velocity at the handle when the force of actuationis zero. Since the unpowered mechanical device is inherentlypassive, this open loop admittance function will always be posi-tive real and thus (23) is satisfied without further consideration.The (24) can be rewritten to get an explicit expression whichseparates the unknown quantities (virtual coupling impedance)from known quantities (haptic display two-port parameters)(25)This is the virtual coupling design equation. The right side isa real valued function which can be plotted against frequencyfor . To achieve an absolutely stable haptic in-terface, we must choose the virtual coupling such that the realpart of its admittance function exceeds the lower bound formedby this plot. To maximize performance, we want to maximizevirtual coupling impedance. The best performing, absolutelystable solution is achieved by selecting the spring constant,,and damping, , which minimize the difference between theleft and right hand side of (25) under the constraint that the in-equality is satisfied. These values can be found by performing arapid two-dimensional numerical search.2) Admittance Display:Taking the cascade combinationof the admittance display two-port (19) and a single seriesimpedance virtual coupling gives us the alternate hybrid formof the haptic interface equations- - - - - - - - - - - - - - - - - - - - - -(26)The absolute stability equations for this case are(27)(28)There are two control design steps for the admittance displaycase: First, the position regulator, , is chosen such thatis a positive real function.In other words, the handle of the regulated device must behavepassively when the virtual environment velocity is zero. It turnsout that we already have developed a way of designing such aregulator. The haptic interface equation for the impedance dis-play, (22), can be rewritten in alternate hybrid form (we are onlyconcerned with the upper left term)- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -(29)Notice that if we make a substitution,, the upperleft term becomes . Thus, finding whichsatisfies (25) defines a regulator which forces the upper leftterm in (29) to be positive real. Once the impedance displayvirtual coupling is found, the position regulator follows:(30)which satisfies (27). Second, the admittance display virtual coupling function,, is chosen to satisfy (28).This entails plotting the right side of (28) and choosing andsuch that the graph of the left side exceeds that curve for allfrequencies, . For best performance in this case,we want to minimize the impedance of the virtual coupling. Theparameters are selected to minimize the difference between theleft and right sides of (28) under the constrain that the inequalityis satisfied.3) Impact of Human Model on Stability Condition:For Ex-calibur, we assume human arm impedance to be bounded byN/(m/s), corresponding to a stiff-ness of 1000 N/m and a damping of 300 N/(m/s). These valuesare consistent with previous work  as well as with exper-imental measurements taken by the authors. We do not limitthe minimum impedance imparted by the human, .While the linear human impedance bound may be simplisticfor frequencies below 5 Hz, it is assumed the human operatordoes not deliberately attempt to destabilize the system. In fact,one may argue the haptic display should not attempt to sta-bilize unstable motion deliberately induced by the operator.The limit on maximum human operator impedance may be in-cluded in the stability analysis by simply replacingin all of the preceding equations.10 IEEE TRANSACTIONS ON CONTROL SYSTEMS TECHNOLOGY, VOL. 10, NO. 1, JANUARY 2002Fig. 8. Impedance display virtual coupling design.V. NUMERICAL AND EXPERIMENTAL RESULTSControl laws have been designed for Excalibur for thetwo haptic display implementations described above. Theworst-case configuration, detailed in the Appendix, is the focusof the numerical design. The resulting regulators and virtualcouplings have been implemented in software and tested onExcalibur as part of the VBB system. The VBB environmenthas admittance causality, so virtual couplings are implementedas shown in Fig. 7(b) or (d).A. Impedance DisplayWe use (25) to find a virtual coupling of the form (20) whichmakes the haptic interface absolutely stable. The lower boundformed by the right side of (25) is shown as a dashed line inFig. 8. The line disappears at some frequencies when the boundbecomes negative. When the haptic interface equations are mod-ified to include the human operator impedance model, the re-sulting design bound is shown as a thin solid line in Fig. 8. Atfrequencies above 70 Hz, there is little difference between thesetwo curves, and there is no difference in the resulting controldesign. Using the design bound the best performing, absolutelystabilizing virtual coupling parameters, and , are found.The left side of (25) with the resulting values,N/m and N/(m/s), is plotted on Fig. 8 as a bold line.B. Admittance DisplayThe first step in control law design for the admittancedisplay implementation was accomplished when we foundthe impedance display virtual coupling parameters. Following(30), the position regulator is(31)The second step is to use (28) to find a virtual coupling whichprovides absolute stability. Fig. 9 shows the lower bound onthe real part of virtual coupling impedance. The dashed line isthe bound calculated without a human impedance model. Thethin solid line shows the bound modified to include a limit onFig. 9. Admittance display virtual coupling design.maximum human impedance. Here we see that the virtual cou-pling design is dramatically affected by the introduction of thehuman model. Conservatism induced by allowing unreasonablelevels of human interaction drives the required virtual couplingimpedance to excessive levels. The virtual coupling designedusing the human model is represented by the bold line in Fig. 9.The corresponding virtual coupling parameters which definein (21) are N/(m/s) andkg. We should note that the position regulator in (31) repre-sents the stiffest allowable controller. These gains may be re-duced, which in turn allows reduced impedance in the virtualcoupling. The tradeoff between regulator gain and virtual cou-pling impedance is explored further, later in the paper.C. Experimental ResultsThe control laws for the different haptic display implemen-tations have been tested on the VBB system. This simulationfalls under the class of admittance type virtual environments.The virtual world consists of a cursor and up to 50 buildingblocks. By default, the virtual coupling connects the haptic dis-play to the cursor. When the cursor is moved inside a blockand a mouse button clicked, the device grabs that block, con-necting the virtual coupling to it. When a selected block col-lides with another block or blocks in the virtual environment, itsposition is constrained to lie on the surface of the obstruction.The blocks may also be vertically mated together by aligningtheir knobs and overcoming inter-block friction. The equationsof motion are integrated using an Euler velocity approximationand a trapezoidal position estimate. As noted by Brown and Col-gate , since an explicit integration routine is used, the vir-tual environment does not strictly satisfy discrete-time passivity.The implication here is that we cannot simulate an infinitesi-mally small mass while maintaining a stable numerical integra-tion of the equations of motion. The experimentally determinedminimum value for cursor and block masses is 0.25 kg. Underthis condition, both of the control designs described above pro-vide a stable haptic simulation. We use the term stable here toimply that there are no detectable undamped or divergent os-cillations under any combination of virtual environment stateADAMS AND HANNAFORD: CONTROL LAW DESIGN FOR HAPTIC INTERFACES TO VIRTUAL REALITY 11TABLE ITHEORETICAL VERSUSEXPERIMENTAL STABILITY BOUNDARY GAINSand human operator grasp. The important virtual environmentstates are: free motion, unilateral blockblock collision, and bi-laterally constrained block. Possible human operator grasp con-ditions are: hands-off, relaxed operation, and tight grip with armfully extended. The first condition corresponds to zero humanimpedance, the last to maximum grasp impedance.The control parameters were tuned to find the values whichmake the system marginally stable. Table I shows these experi-mentally derived stability boundary gains along with their the-oretical counterparts. For the impedance display virtual cou-pling, damping was held constant and stiffness increased untilinstability was first detected. This occurred when the value wasaugmented by 50% in the virtual environment/human operatorcombination of bilateral constraint/hands-off. The experimentaladmittance display virtual coupling was found by reducing themass while holding constant the theoretical damping and po-sition regulator gains. Instability occurred in the combinationof free-motion/maximum grasp when the mass was reduced by15%.A number of factors may account for the difference betweentheoretical and experimental results. The theoretical controllaws are based on a linear model of Excalibur. Any discrepancybetween this model and the true systems behavior will leadto design error. In addition to any error in the linear model,nonlinearities are not accounted for in the design. Certainly,effects such as motor cogging torque and transmission cableslack contribute to the difference. Finally, while our experi-ments attempt to consider worst-case combinations of virtualenvironment state/human grasp impedance, they by no meanscomprise an exhaustive search of all allowable port impedances.If the tests missed the true worst-case scenario, the experi-mental results may be optimistic. Without an exhaustive searchof all possible terminating impedance combinations, it willalways be possible the worst-case scenario was missed. Whilethe experimental gains achieved better performance than thetheoretical values, they put the system on the edge of oscillationand did not provide the user with an ideal kinesthetic response.During practical use of the VBB system, the theoretical gainswere normally used to provide a robustness margin againstoscillations.D. Performance ComparisonQuantifying the performance of a haptic interface can be dif-ficult, since our ultimate goal has to do with human percep-tion. We want the interface to act as a transparent window intothe virtual environment. When the virtual environment simu-lates free motion, such as free cursor motion in the VBB sim-ulation, the human operator should feel zero (or very small)force at the handle. Ideally, the operator would not even re-alize that the handle was there. When the virtual environmentsimulates zero velocity, such as when a selected building blockis stuck between other objects, the handle should be immo-bile. One way of quantifying haptic interface performance isthe notion of impedance range . The impedance range is de-lineated by the minimum and maximum impedance which thehaptic interface canstablyrender to the human operator. If thehaptic interface is absolutely stable, then it is straightforward tocalculate these bounds on realizable impedance. The minimumimpedance is simply the input impedance at the human operatorport with the virtual environment port short-circuited(32)The maximum impedance is found with an open-circuit virtualenvironment(33)The magnitude of the impedance range bounds for the twohaptic display implementations considered in this paper areshown in Fig. 10(a). The thin lines represent the impedancedisplay and the bold lines show the admittance display bounds.The frequency responses are calculated using the worst-casedesign model and the theoretical control laws.The lower bound represents the free-motion response of thehaptic interface. For the impedance display, this is simply theopen-loop response of the system. Below 10 Hz, this responseis dominated by rigid body damping [20 N/(m/s)] and the totalinertia (3.9 kg). The lower bound for the admittance display im-plementation is driven by the virtual coupling impedance. Atfrequencies below 10 Hz, the response is dominated by the cou-pling mass of 5.0 kg. In this case, the operator feels greaterinertia than in the impedance implementation, but with zerodamping. When the virtual environment simulates free motion,the handle literally floats around the workspace like a friction-less mass.The upper bound on impedance range represents the handleimpedance when the virtual environment simulates an infinitelyrigid constraint. Below 10 Hz, the haptic interface acts as agrounded spring, . For an impedance displayimplementation, the effective spring constant, 20 000 N/m, isthe series combination of virtual coupling stiffness and mechan-ical stiffness between the handle and the actuators. The effectivespring constant for the admittance display is the series combina-tion of the position regulators stiffness (gain on displacement)and the device mechanical stiffness. Its value is also 20 000 N/m.Since we chose the admittance display position regulator gainsto be the same as the impedance display virtual coupling gains,these two cases have identical upper bounds.We have described in some detail two different realizationsof a haptic interface. Both are implemented on the same hard-ware, both are absolutely stable, and both can be connected to animpedance or admittance type virtual environment. The obvious12 IEEE TRANSACTIONS ON CONTROL SYSTEMS TECHNOLOGY, VOL. 10, NO. 1, JANUARY 2002(a)(b)Fig. 10. Impedance range for different haptic display implementations (thinlineimpedance display, bold lineadmittance display).question is: Which is best? In general, the answer is device de-pendent. The impedance display is the simplest implementation(no force sensor is required), but is also the least adaptable. It isdifficult to improve the free-motion response of the display be-yond the backdrivability of the open-loop device. For a devicewhich is lightweight and with low-friction, this may be the bestimplementation. The admittance display implementation is tai-lorable, and is likely the best implementation for a device whichhas high levels of nonlinear friction and high gear ratios.The ability to tailor the performance of the admittance dis-play is highlighted in Fig. 10(b). By reducing the inner loop po-sition gain to 10 000 N/m, the virtual coupling impedance pa-rameters can be reduced to N/(m/s) andkg while still providing absolute stability. The new impedancerange bounds show the corresponding improvement in free-mo-tion response (1.7 kg) and degradation in rigid-constraint per-formance (8000 N/m).VI. CONCLUSIONTwo-port absolute stability criteria have been developed forthe design of stabilizing haptic interface control laws for a widerange of human operator and virtual environment impedance.The approach has been demonstrated in the design of regulatorsand virtual couplings for two different haptic display implemen-tations: impedance display and admittance display. Numericaland experimental results for the Excalibur device have validatedthe efficacy of the theoretical methods.APPENDIXThe worst-case design model for Excalibur is defined in termsof the matrices, , and . This model charac-terizes the local behavior of the device along the-axis at itspoint of maximum flexibility. See equations (34)(39) shown atthe top of the next page. The analog filter on load cell output is(40)REFERENCES C. V. Edmondet al., ENT endoscopic surgical training simulator,Proc. Medicine Meets Virtual Reality, pp. 518528, 1997. R. Baumann and R. Clavel, Haptic interface for virtual reality basedminimally invasive surgery simulation, inProc. IEEE Int. Conf. Robot.Automat., 1998, pp. 381386. C. J. Hasser, User performance in a GUI pointing task with a low-costforce-feedback computer mouse, inProc. ASME Int. Mechanical Eng.Congr. Exhibition, Anaheim, CA, 1998. R. J. Adams, M. R. Moreyra, and B. Hannaford, Excalibur, A three-axisforce display, inProc. ASME Symp. 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Brown, Passive implementation of multibody simulations forhaptic display, Doctoral dissertation, Mech. Eng., Northwestern Univ.,Evanston, IL, Dec. 1998. J. M. Brown and J. E. Colgate, Passive implementation of multibodysimulations for haptic display, inProc. ASME Int. Mech. Eng. Congr.Exhibition, Dallas, TX, 1997, pp. 8592.ADAMS AND HANNAFORD: CONTROL LAW DESIGN FOR HAPTIC INTERFACES TO VIRTUAL REALITY 13M =0:9271 0 0 0 00 0:3875 0 0 00 0 2:4489 0:2799 00 0 0:2799 0:3343 00 0 0 0 0:1600(34)D =8:5600 2:7895 1:1447 0:0487 0:00952:7895 12:2139 7:4320 0:0529 0:05511:1447 7:4320 68:2359 48:6630 47:41500:0487 0:0529 48:6630 49:2727 47:36530:0095 0:0551 47:4150 47:3653 48:2606(35)K =10 0:0236 0:0280 0:0045 0:0003 0:00010:0280 0:1044 0:0764 0:0000 0:00000:0045 0:0764 5:3954 5:3238 5:32350:0003 0:0000 5:3238 5:3270 5:32350:0001 0:0000 5:3235 5:3235 5:3236(36)G =0 10 00 00 01 0(37)C =0 0 0 0 11 0 0 0 0(38)C =10 [ 0 0 5:3236 5:3236 5:3236] : (39) R. J. Adams, M. R. Moreyra, and B. Hannaford, Stability and perfor-mance of haptic displays: Theory and experiments, inProc. ASME Int.Mech. Eng. Congr. Exhibition, Anaheim, CA, 1998, pp. 227234. T. Yoshikawa, Y. Yokokohji, T. Matsumoto, and X.-Z. Zheng, Displayof feel for the manipulation of dynamic virtual objects,Trans. ASME,J. Dyn. Syst., Measurement, Contr., vol. 117, no. 4, pp. 554558, 1995. R. J. Adams, Stable haptic interaction with virtual environments,Doctoral dissertation, Elect. Eng., Univ. Washington, Seattle, WA, Aug.1999. T. H. Massie and J. K. Salisbury, The phantom haptic interface: A de-vice for probing virtual objects, inProc. ASME Int. Mech. Eng. Congr.Exhibition, Chicago, IL, 1994, pp. 295302. Y. Yokokohji, R. L. Hollis, and T. Kanade, What you see is what you canfeelDevelopment of a visual/haptic interface to virtual environment,in Proc. IEEE Virtual Reality Annu. Int. Symp., Los Alimitos, CA, 1996,pp. 4653. C. L. Clover, G. R. Luecke, J. J. Troy, and W. A. McNeely, Dynamicsimulations of virtual mechanisms with haptic feedback using industrialrobotics equipment, inProc. IEEE Int. Conf. Robot. Automat., Albu-querque, NM, 1997, pp. 32053210. F. A. Mussa-Ivaldi, N. Hogan, and E. Bizzi, Neural, mechanical, andgeometric factors subserving arm posture in humans,J. Neurosci., vol.5, no. 10, pp. 27322743, 1985.Richard J. Adams is a Major in the United StatesAir Force. He received the B.S. degree from the U.S.Air Force Academy, Colorado Springs, CO, in 1989as a Distinguished Graduate. In 1990, he received theM.S. degree in aeronautics and astronautics from theUniversity of Washington, Seattle. He received thePh.D. degree in electrical engineering from the Uni-versity of Washington in 1999.He worked from 1991 to 1993 in Wright Labo-ratory, Dayton, OH on advanced flight controls forfighter aircraft. From 1994 to 1996, he served as anExchange Scientist with the C.E.R.T. research center in Toulouse, France.Major Adams received the Air Force Scientific Achievement Award for hiswork on F-16 high angle-of-attack control in 1994.Blake Hannaford (S82M84SM01) received theB.S. degree in engineering and applied science fromYale University, New Haven, CT, in 1977 and theM.S. and Ph.D. degrees in electrical engineering fromthe University of California, Berkeley, in 1982 and1985, respectively.Before graduate study, he held engineeringpositions in digital hardware and software design,office automation, and medical image processing.At Berkeley, he pursued thesis research in multipletarget tracking in medical images and the control oftime-optimal voluntary human movement. From 1986 to 1989, he worked onthe remote control of robot manipulators in the ManMachine Systems Groupin the Automated Systems Section of the NASA Jet Propulsion Laboratory,Caltech. He supervised that group from 1988 to 1989. Since September 1989,he has been at the University of Washington, Seattle, where he has beenProfessor of Electrical Engineering since 1997, and served as Associate Chairfor Education from 1999 to 2001. His currently active interests include hapticdisplays on the Internet, and surgical biomechanics. He is the founding editorof Haptics-e, The Electronic Journal of Haptics Research(www.haptics-e.org).Dr. Hannaford was awarded the National Science Foundations PresidentialYoung Investigator Award and the Early Career Achievement Award from theIEEE Engineering in Medicine and Biology Society.