Large Workspace Haptic Devices - A New Actuation Approach Michael Zinn * Department of Mechanical Engineering University of Wisconsin - Madison Oussama Khatib † Robotics Laboratory Department of Computer Science Stanford University Bernard Roth ‡ Design Division Dept. of Mechanical Engineering Stanford University J. Kenneth Salisbury § Robotics Laboratory Department of Computer Science Stanford University ABSTRACT Large workspace haptic devices have unique requirements, requir- ing increased power capabilities along with increased safety consid- erations. While there are numerous haptic devices available, large workspace systems are hampered by the limitations of current ac- tuation technology. To address this, the Distributed Macro-Mini (DM 2 ) actuation method has been applied to the design of a large workspace haptic device. In this paper, the DM 2 method is de- scribed and we present experimental results which demonstrate its effectiveness. Finally, the control design is presented along with a discussion of the unique challenges associated with its robustness. Keywords: Actuation, haptics, design, macro-mini, compliance. Index Terms: H.5.2 [Information Interfaces and Presentation]: Haptic I/O – Design 1 I NTRODUCTION Large workspace haptic devices have a unique set of requirements. These include similar requirements which bound traditional desktop devices as well as additional power and force requirements which can be very demanding. A number of researchers have de- veloped device performance and requirement guidelines [4, 9, 11]. These, in combination with the additional power and safety requirements for large workspace devices, can be summarized as follows: Large dynamic range force control: To accurately render a virtual object, a haptic device must have the capability to render forces over a large dynamic range, in both frequency and magnitude. This requirement can be partitioned as follows: Torque Vs. Frequency: As shown in [20], as well as else- where, the required force output for many devices, including haptic devices, is inversely proportional to frequency (1/ω ) while power magnitude is inversely proportional to the square of frequency (1/ω 2 ). At low frequencies, large forces are re- quired to react DC or slowing changing forces, such as would be expected when pressing into a virtual object. At high fre- quencies, brief instances of high frequency force content are required to render stiff surfaces (e.g. during contact transi- tions). While these forces are often short in duration and low power, their presence is critical for the accurate rendering of stiff objects. High Bandwidth: While required torque and power magni- tude falls off with increased frequency, small amplitude actu- ator torques must be capable of supporting a high bandwidth system. This is important to prevent excessive distortion of * e-mail: mzinn@wisc.edu † e-mail:ok@robotics.stanford.edu ‡ e-mail:roth@robotics.stanford.edu § e-mail:jks@robotics.stanford.edu the rendered forces. In the case of admittance devices with a given closed-loop bandwidth, ω CL , the actuator torque output must not introduce phase distortion at frequencies below ω CL . Transparency: An important characteristic of a haptic device is the ability to display zero force over a wide frequency range. Introduc- tion of device friction, inertial, or control forces which deviate from this ideal reduce the effectiveness of the device. This requirement can be further broken down by frequency range: High frequency - Low effective inertia: Regardless of the ar- chitecture of the haptic device (i.e. admittance vs impedance), at high frequencies the transparency is dominated by the ef- fective inertia of the device. The effective inertia is, in turn, affected by the mass properties of the mechanism, the re- flected inertia of the actuation, and location within the device workspace. In the case of admittance devices, the effective inertia is dominant above the closed-loop bandwidth of the admittance controller. For good transparency at high frequen- cies, the physical device must possess low effective inertia. Low frequency - Low output impedance: At lower frequen- cies, the transparency of a device is affected by its frictional characteristics and, to a lesser extent, by its mass properties. For impedance devices, it is important to keep the effective friction forces low. Friction sources include actuation gear- train and joint friction. The low frequency output impedance of admittance devices is determined by its controller design and implementation in combination with the physical charac- teristics mentioned above. High power / large force: In addition to those requirements listed above, a large workspace imposes the additional requirements of high force and power. A major purpose of a large workspace device is to allow full arm or body haptic interaction. This type of task involves higher forces than devices which are designed for desktop use. The larger force, in combination with the larger workspace, implies larger work and power output. It is this requirement, in combination with transparency and force control requirements, which make the design of large workspace haptic devices so challenging. Safety: With the increased torque and power capabilities of a large workspace haptic device comes a new requirement of safety. Device safety is dependent on its mechanical, electrical, and software design characteristics. However, the biggest danger present when working in close proximity with a high power device is the potential for large impact loads resulting from the large effective inertia. To insure a minimal level of safety, a large workspace haptic device should be designed to minimize its effective inertia [19, 20]. Numerous kinesthetic haptic devices have been successfully designed, including a number which have had commercial success [12, 8, 5]. Most devices have been developed for desktop use, with only a few applicable to large workspace applications [17, 1, 5, 18]. While there has been some limited 185 Symposium on Haptic Interfaces for Virtual Environments and Teleoperator Systems 2008 13-14 March, Reno, Nevada, USA 978-1-4244-1972-2/08/$25.00 ©2008 IEEE