The Snaptic Paddle: A Modular Haptic Device Christopher E. Wong and Allison M. Okamura Department of Mechanical Engineering The Johns Hopkins University Baltimore, MD 21218, USA E-mail:
[email protected],
[email protected] Abstract The Haptic Paddle was developed previously as an inexpensive haptic device for educational use, and has been applied to course topics such as dynamic systems and introductory controls. However, it has only a single degree of freedom, limiting it to very simple modeling and simulation exercises. Through modifications to the original Haptic Paddle design, a “Snaptic Paddle” has been created that allows two modular one-degree-of-freedom devices to be connected to form a single two-degree-of-freedom device that operates like a traditional joystick. The devices can also be used separately, creating a flexible, economical haptic interface for educational applications. The device successfully provides force feedback interaction, as will be shown in a demonstration at the World Haptics Conference.
1. Introduction The use of haptics in educational settings has been suggested as an aid to the understanding of physical phenomena in a variety of subjects. Research has demonstrated the need for using different modes of interaction to improve student learning, and cognitive psychologists and education researchers note that the perceived environment plays a strong role in learning [3,7]. Thus, it seems promising that haptic laboratories, which are inherently interactive and multi-modal, will improve students’ understanding and intuition about dynamic systems and controls. Because they are intended for use in an educational setting, a major factor in the design of educational haptic devices is cost. Here, we describe a modular 1-degree-of-freedom (DOF) device that operates in 1-DOF independently and operates in 2DOF when joined with another copy of the device.
Figure 1. The Snaptic Paddle. In the 1-DOF configuration (left), a single rod forms the device handle. In the 2-DOF configuration (right), two rods connect the axes of the individual devices and a third rod forms the handle.
This device is dubbed the “Snaptic Paddle,” alluding to its precursor, the Haptic Paddle [5]. There are a number of other examples of haptic devices used in educational applications [1,2,6]. Some of these applications require only one degree of freedom, while others require two or three. Thus, a modular haptic device that provides flexibility in degrees of freedom can be used for a number of different applications. While we focus on a 2-DOF design here, the concept may be extended to 3-DOF.
2. Device Description The Snaptic Paddle (Figure 1) is motivated by the simple and low-cost design of the Haptic Paddle, and thus shares many of the same components. Like the Haptic Paddle, the Snaptic Paddle consists mainly of laser-cut acrylic. Other shared features include the use of Hall effect sensors and a rotating magnet for position measurement and low-cost fishing line for the capstan drive cable. The final cost of the device is under $60 for two prototypes. This cost could be lessened if the device was mass manufactured. The CAD schematics for laser cutting and the detailed part descriptions/vendors are available from the
authors. System resolutions and force limits are the same as those of [5]. As it is modular in nature, the device has two configurations; the independent 1-DOF configuration and the 2-DOF configuration, which is actually an assembly of two paddles. In the 1-DOF configuration, the device resembles the original Haptic Paddle, but is inverted to move the motor and capstan drive to the base. In this configuration, the device provides single-axis force feedback perpendicular to the handle. The handle of the device is an acrylic cylinder 0.5 inches in diameter. It is secured to the device by inserting a 0.25-inch peg on the bottom of the handle into a hole on the joint of the base. In the 2-DOF configuration, the device kinematics are similar to that of a typical 2-DOF joystick. The handles of the two paddles are removed from their 1DOF positions and instead secured so that they are parallel to the ground. Grooves in the joint prevent rotation of the handle away from the ground-parallel position. A set of acrylic pieces (cut thin to allow bending) are glued to the base, which allows the two paddles to be “snapped” together at right angles. A third handle, the only extra part needed for the 2-DOF configuration besides the two paddles themselves, is attached (snapped) perpendicularly to the other two handles. This handle is identical to the other two and permits rotation at the joints with the other handles. This third, upward pointing handle becomes the new user interface point. The handles are designed in such a way that x and y movements of the handle correspond to decoupled rotation of the base motors.
3. System Control and Performance The original Haptic Paddle used a parallel port interface to communicate with a PC on which a haptic simulation was run. However, only one degree of freedom of control is available via the parallel port’s limited number of pins, so we currently use a commercial data acquisition board. An ideal interface would utilize an IEEE 1394 (Firewire) or similar connection and would include onboard circuitry (A/D, D/A and power amplification) and a printed circuit board. MacLean, et al. have used a USB interface [4] in the past for a 1-DOF haptic device. Control and haptic rendering is performed using a personal computer, using typical collision detection and force computation algorithms. A prototype device was made available to the public at a laboratory open house. The individual 1DOF devices were found to have excellent performance, although limited in the possibility for exciting demonstrations because of the single degree
of freedom. For the 2-DOF configuration, most users indicated that the simulation felt good given the system cost, but that there was undesirable backlash in the snapped-together joints. Other users felt that there might be too much flexibility in the device itself. This suggests that other connection mechanisms and materials should be considered.
Acknowledgements The authors thank Mr. Robert Blakely for his assistance with part fabrication and Drs. Brent Gillespie and Hong Tan for conversations leading to the idea for a modular haptic device. This material is based upon work supported by the National Science Foundation under Grant No. 0347464.
References [1] R. B. Gillespie, M. B. Hoffman, and J. Freudenberg, “Haptic Interface for Hands-On Instruction in System Dynamics and Embedded Control,” 11th Symposium on Haptic Interfaces for Virtual Environment and Teleoperator Systems, 2003, pp. 410-415. [2] M. G. Jones, A. Bokinsky, T. Andre, D. Kubasko, A. Negishi, R. Taylor, R. and Superfine, “NanoManipulator applications in education: The impact of haptic experiences on students’ attitudes and concepts,” 10th Symposium on Haptic Interfaces for Virtual Environment and Teleoperator Systems, 2002, pp. 295-298. [3] J. Lave, “Views of the classroom: Implications for math and science learning research," in Towards a scientific practice of science education, J. G. Greeno, F. Reif, H. Shoenfeld, A. diSessa, and E. Stage, Eds. Erblaum, 1990, pp. 251-263. [4] K. E. MacLean, M. J. Shaver and D. K. Pai, “Handheld haptics: a USB media controller with force sensing,” 10th Symposium on Haptic Interfaces for Virtual Environment and Teleoperator Systems, 2002, pp. 311-318. [5] A. M. Okamura, C. Richard, and M. R. Cutkosky, "Feeling is believing: Using a Force-Feedback Joystick to Teach Dynamic Systems," ASEE Journal of Engineering Education, Vol. 91, No. 3, 2002, pp. 345-349. [6] M. Reiner, “Conceptual construction of fields with a tactile interface," Interactive Learning Environments, 1999, Vol. 7, No. 1, pp. 31-55. [7] W.-M. Roth and M. Welzel, “Learning about levers: Towards a real time model of cognition during laboratory activities,” International Journal of Science Education, Vol. 20, No. 1, 1998, pp. 25-44.
