1The HapticMaster, a new high-performance haptic interfaceVan der Linde R.Q., Lammertse P., Frederiksen E., Ruiter B.

FCS Control Systems, The Netherlands

Richard.van.der.linde@fcs-cs.com

AbstractThe article addresses the technical principles of a newhigh-performance haptic device, called HapticMaster,and gives an overview of its haptic performance. TheHapticMaster utilizes the admittance control paradigm,which facilitates high stiffness, large forces, and a highforce sensitivity. On top of that the HapticMaster has alarge workspace, and a huge haptic resolution. Typicalapplications for the HapticMaster are therefore found invirtual reality, haptics research, and rehabilitation.

1. Introduction

More than a dozen haptic interfaces are commerciallyavailable today. Lately, a new high-performance hapticdevice has been developed by FCS Control Systems(Fig. 1). It is called the HapticMaster, and it is nowcommercially available. In Japan it is called theForceMaster out of respect for Prof. Iwata, who built adevice with a similar name earlier.

Fig. 1 The HapticMaster, a new haptic device built byFCS Control Systems and now commercially available.

Haptic devices come in two distinct classes: impedancecontrolled devices, and admittance controlled devices.The HapticMaster belongs to the latter class. Bothclasses will be briefly discussed.

Impedance control was first introduced by Hogan(1985). The essential control paradigm is this: the usermoves the haptic device, and the device will react with aforce if a virtual object is met. So, viewed from thehaptic device, the paradigm is: displacement in, andforce out. The user will inevitably feel the mass andfriction of the actual device, but these can be made verysmall by careful mechanical design. Impedancecontrolled devices are by nature lightly built and highlybackdrivable. They are typically cable driven by high-performance DC motors. A prime example of theimpedance control paradigm is Sensable's well-knownseries of Phantom devices.

Admittance control is the inverse of impedancecontrol, hence the name. In admittance control theparadigm is this: the user exerts a force on the hapticdevice, and the device will react with the properdisplacement. So, viewed from the haptic device, theparadigm is: force in, and displacement out. Admittancecontrol allows considerable freedom in the mechanicaldesign of the device, because backlash and tip inertiacan be eliminated. As a result, the mechanism can bequite robust, capable of displaying high stiffnesses andhigh forces. Admittance control has been used forcontrol sticks in the flight simulator industry for manyyears. A recent example of a generic haptics deviceusing the admittance control paradigm is the FCSHapticMaster.

Impedance control and admittance control are dualnot only in their cause-and-effect structure, but also intheir performance. The impedance control device istypically lightweight, backlash free, and renders lowmass [Adams & Hannaford, 2001]. Consequently,performance is lacking in the region of higher forces,high mass and high stiffness. Adding complex endeffectors is also a problem. Admittance control deviceson the other hand are capable of rendering very highstiffnesses and minimal friction, giving a very free feelto the motion. They are very suitable for largerworkspaces, and also for master-slave applications andfor carrying complex end effectors with many degrees offreedom. Also, they intrinsically register forcesencountered, and are therefore very suitable for hapticsand neurological research. However, they are often notcapable of rendering very low mass.

2The differences between impedance and admittancecontrolled haptic devices have an effect on their typicalareas application. These will therefore be discussed atthe end of this article, preceded by the principles ofoperation and the haptic performance of theHapticMaster.

2. Principles of operationThe principles of operation of the HapticMaster arediscussed in three sections. First the control algorithm ispresented, next the hardware and the software involvedare discussed.

2.1 The control algorithm

The HapticMaster measures the force exerted by theuser, preferably measured close to the human hand witha sensitive force sensor. An internal model thencalculates the Position, Velocity, and Acceleration(PVA), which a (virtual) object touched in space wouldget as a result of this force. The PVA-vector iscommanded to the robot, which then makes themovement by means of a conventional control law. Thegeneral control algorithm is illustrated in Fig. 2. Theinternal model will typically contain a certain mass, toavoid commanding infinite accelerations. The innerservo loop will cancel the real mass and friction of themechanical device.

human PVA

force force

sensor

position

(virtual)model

+

-

HapticMASTER

controller robot arm PVA

sensors

Fig. 2 The general control scheme of the HapticMastercomprises an outer control loop, and an in inner servoloop. A (virtual) model converts the force sensor signal toa Position/Velocity/Acceleration setpoint vector. Theinner servo loop controls the robot to the PVA setpointvalues.

2.2 The hardware

The hardware comprises two main functionalcomponents (Fig. 1): the robot arm, and the control box.Logically the first serves as the actual force display,whereas the second houses the amplifiers, and the hapticserver.

2.2.1 The robot armThe mechanism of the robot arm is built for zerobacklash, which yields some friction in the joints.However, the friction is completely eliminated by thecontrol loop, up to the accuracy of the force sensor. The

result is a near backlash-free and smooth movingbehavior at the end effector.

The workspace of the HapticMaster is depicted inFig. 3. The kinematic chain from the bottom up yields:base rotation, arm up/down, arm in/out, illustrated inFig. 4. This makes 3 degrees of freedom at the endeffector, which spans a volumetric workspace. TheHapticMaster is designed for exchangeable endeffectors, to match an appropriate end effector to the endapplication. For instance an end effector with 3additional rotations can be mounted at the end plate ofthe robot arm.

Fig. 3 The end effector plate workspace of theHapticMaster spans a 3-dimensional space with avolume of approximately 80 liters. The end plate of therobot arm allows the mounting of different end effectors.

= actuator

Fig. 4 The actuator arrangement and the kinematics ofthe HapticMaster

2.2.2 The haptic serverThe haptic renderer and the robot control loop both runon a dedicated industrial PC with the VxWorks real-time operating system. These two loops run at a fixedupdate rate of 2500 Hz. Because this frequency isapproximately ten times higher than the maximal human

3discrepancy value [Burdea, 1996], it is assumed to behigh enough to guarantee a haptic quality for a smoothand realistic experience. Using the principle of a hapticserver also unloads the host PC.

2.3 The software

The software comprises a programming interface, tocreate haptic worlds, and a real-time operating system,to render the haptic world. The real-time applicationincorporates issues like safety guards, communicationprotocols, the control loop, collision detection of virtualobjects, etc. Only the collision detection algorithm willbe briefly discussed.

2.3.1 The programming interfaceThe HapticMaster is programmed by means of aHapticAPI, which is a C++ programming interface thatenables the user to control the HapticMaster and createvirtual haptic worlds. The HapticAPI is used to make anEthernet connection to the HapticMaster to control theinternal state machine and to define or modify the virtualhaptic world. Haptic effects can be created, like dampersand springs, and spatial geometrical primitives can bedefined, like spheres, cones and cubes. Also, themeasured force, position, and velocity can be read. Anexample of a virtual haptic world, created by means ofthe FCS HapticAPI, and visualized by means ofOpenGL is given in Fig. 5.

Fig. 5 An example of a virtual haptic world, created withthe HapticAPI and visualized with OpenGL. Geometricalprimitives like spheres and cubes, and force elementslike springs and dampers can be placed into a virtualworld.

2.3.2 The collision detection algorithm

When the end effector collides with a virtual object, anappropriate force and displacement must be presented tothe user. The relationship between these two is given bythe object properties (e.g. stiffness, damping, friction,etc.). With a penalty-based method the appropriaterelation between force and displacement is calculated bythe real-time operating system.

3. Haptic performance

A plethora of performance indicators can be given forhaptic interfaces [Hayward and Astley, 1996]. Forquantification of the haptic performance of theHapticMaster the following indicators were chosen:workspace, position resolution, stiffness, nominal/maximal force, minimal tip inertia, maximum velocity,maximum deceleration, force sensitivity, frequencyresponse, haptic resolution, force depth, and inertialratio. The first eight indicators will be given in a simplespecs table, whereas the latter four will be separatelydiscussed. All indicators are given with respect to theend effector.

3.1 Specs table

The haptic specifications of the HapticMaster are givenin Table 1.

Table 1, specs of the HapticMaster

Workspace 8010-3 [m3]

Position resolution 410-6-1210-6 [m]

Stiffness 10103-50103 [N/m]

Nominal/max force 100/250 [N]

Minimal tip inertia 2 [kg]

Maximum velocity 1.0 [m/s]

Maximum deceleration 50 [m/s2]

Force sensitivity 0.01 [N]depending on the degree of freedom and the position

3.2 Frequency response

The transfer function HN () of a haptic device at the endeffector can be quantified by dividing the commandedacceleration (aC) with the measured acceleration (aM)

)()()(

C

MN a

aH =

4Where N is the degree of freedom measured.If we assume the force sensor to be infinitely fast, thenthe force at the end effector is proportional to the endeffector acceleration. Fig. 6 gives the magnitude plot ofthe defined transfer function for the up/down movementfor frequencies up to 25 Hz. It can be seen that thefrequency response is fairly straight till 10 Hz, with aslight amplitude incline from 10 to 25 Hz.

1 2 3 4 5 6 7 8 9 10 20 30-20

-15

-10

-5

0

5

10

15

20

frequency [Hz]

gain

[dB]

vertical movement

Fig. 6 The frequency response of the vertical movementof the HapticMaster. The gain is obtained by division ofthe measured end effector acceleration and thecommanded acceleration.

3.3 Haptic resolution

Most large-workspace haptic devices have less positionaccuracy at the end effector than small-workspacedevices. This is a logical consequence, because forarticulated devices rotational measurements of the jointsmust be more accurate to obtain the same positionaccuracy at the tip. To correct for this size influence, theworkspace is divided by the third power of the positionresolution, which yields the haptic resolution (HR):

3 resolutionpositionworkspaceHR =

The haptic resolution gives an approximation of thenumber of volumetric units (voxels) that the device canrender in space. For the HapticMaster the hapticresolution thus equals 1.251015 [voxels], which is thelargest haptic resolution of all commercially availablehaptic devices today. For comparison, this isapproximately 3000 times higher than the hapticresolution of a Phantom desktop.

3.4 Force depth

Most powerful haptic devices have less force accuracy atthe end effector than weak devices. This seems to belogical, because for impedance controlled devices morepower implies more mass and more joint friction. Since

impedance controlled haptic devices without forcefeedback are not able to render forces below the backdrive friction, this is the smallest increment for forcerendering. By dividing the nominal force by the backdrive friction, the number of force increments isobtained, which we will call the force depth (FD):

3resolutionpositionforcenominalFD

=

For the HapticMaster the force depth yields 10.000increments. This is approximately 200 times higher thanmost impedance controlled devices.

3.5 Inertial ratio

There is a physical relationship between the tip inertiaand robot mass. Simply comparing the tip inertia of abig and strong device with the tip inertia of a small andweak device, gives an incomplete picture of hapticperformance. To correct for this, the minimal tip inertiais divided by the maximum stiffness of a haptic device,which yields the inertial ratio (IR):

stiffnessinertiatipIR

max min

=

The inertial ratio is a measure for the haptic dynamicrange, because it gives the ratio between freely movingobjects and constrained objects. For the HapticMasterthe inertial ratio yields 0.0410-3 [s-2]. For comparison,this is similar to a Phantom Premium 1.0.

4. Typical applications

In the previous section performance parameters weregiven for the HapticMaster. By comparing theHapticMaster to other commercially available hapticdevices, it can be concluded that the HapticMasterexcels on: force, force depth, stiffness, positionresolution, and haptic resolution. Based on thesecharacteristics the HapticMaster highly qualifies for thefollowing typical applications.

Virtual realityVirtual reality applications especially benefit from theHapticMasters performance when stiff or heavy objectsmust be rendered. Also, the rendering of curved stiffobjects (e.g. car skins) requires a high haptic resolution.The HapticMaster therefore typically qualifies for virtualdesign and assembly tasks.

Haptics researchHaptics research is performed in many different areas,from finger movement to arm movement in acombination with auditory, visual, kinesthetic or tactilesensing. Because the HapticMaster measures real force

5with the force sensor in the end effector, additional man-machine interaction measurements can be accuratelyperformed.

RehabilitationRehabilitation and neurological research requires theforce of the HapticMaster because the human limbs (e.g.the arm) must be carried and moved around [Harwin,1999]. Given the workspace, and ability to measureforce, the HapticMaster is highly suitable to perform thisfunction.

5. Conclusions

A new high-performance haptic device, called theHapticMaster, is discussed in this article. TheHapticMaster excels on the haptic performanceindicators force, force depth, stiffness, positionresolution, and haptic resolution. These key advantagesfacilitate typical applications like virtual assembly,haptics research and rehabilitation.

6. Future developments

In the near future different end effectors will bedeveloped, facilitating different applications. A gimbalend effector will soon be commercially available. It willhave three measured passive degrees of freedom, andone degree of freedom for an active gripping function.Also, the HapticAPI will be functionally extended tosupport software interfaces of third parties andincorporate issues like triangularization, dynamicenvironments, etc.

References

Adams R.J., Hannaford B., Control law design forhaptic interfaces to virtual reality. IEEE Trans. onControl Systems Technology (2001), June.

Burdea G.C., Force and Touch feedback for virtualreality, 1996 John Wiley & sons inc. ISBN: 0-471-02141-5.

Harwin W.S., 'Robots with a gentle touch: advances inassistive robotics and prosthetics'. Proc. Int.Biomechatronics Workshop. pp. 95-99, April 19-211999, Enschede, The Netherlands.

Hayward V., Astley O.R., Performance measures forhaptic interfaces. In: Robotic Research: the 7th Int.Symp. G.G. Hirzinger (eds.), 1996 Springer Verlag,p.p.: 1995-207.

Hogan N., Impedance Control : An Approach toManipulation: Part I-Theory, Part II-Implemen-

tation, Part III-Applications. Journal of DynamicSystems, Measurement and Control. (1985).

The HapticMaster, a new high-performance haptic interfaceAbstractFig. 2 The general control scheme of the HapticMaster comprises an outer control loop, and an in inner servo loop. A (virtual) model converts the force sensor signal to a Position/Velocity/Acceleration setpoint vector. The inner servo loop controls the rFig. 6 The frequency response of the vertical movement of the HapticMaster. The gain is obtained by division of the measured end effector acceleration and the commanded acceleration.References