Design and Control of a Multiple Degree of Freedom Haptic Interface

The role of robotics in society is no longer restricted to assembly and manufacturing. Robots are fmding their way into -a wide spectrum of tasks that directly link human and machine. Haptic interfaces are robots that are physically coupled to humans and produce desired tactile stimulation. These devices are integrated into active joystick control for aircraft, bilateral teleoperation, robotic assisted surgery and human rehabilitation and training. This paper discusses the design and control of a new haptic interface at the Georgia Institute of Technology. HURBIRT (Human Robot Bilateral Research Tool) is a two degree of freedom planar manipulator used to investigate control issue in teleoperation and humanmachine interaction. Nomenclature dij ijth component of inertial tensor ~i gravitational component of i DOF 'ti force of i DOF qi generalized displacement of i DOF mi mass of ph link Ii length of ith link lei distance to COM of ith link x Tip position of manipulator Xo Desired tip position of manipulator F Human applied force vector M Desired tip inertial tensor B Desired tip damping tensor K Desired tip stiffness tensor Introduction Since the fIrst robots designed in the late 1940's (Goertz, 1952), the integration of human and robot has been a popular research interest. The initial applications focused on assisting humans in remote handle hazardous materials. Ironically, this is still an active research topic. Today, many diverse applications can benefit from the improved integration of human and robot that result from research on haptic interfaces. The role of robots in human rehabilitation and training was fIrst investigated by Book, et.al. (1979). A manipulator can produce constrained paths to guide the human along desired trajectories. The advantage of such systems is the ability to actively control and record resistance. A mUltiple degree of freedom system can facilitate a wide range of exercises. For rehabilitation, measurement of human resistance can assist in the isolation and quantification of injuries as well as gage progress. The first prototypes were mechanically and electronically crude. The evolution of computers and mechanical motion systems has renewed interest in this field. One critical issue currently under investigation is the modeling of human resistance. Hollerbach and Kazerooni (1992) discuss the modeling of humans coupled to robots. Horowitz, et.a! ' (1993) considers a control algorithm that uses self tuning control to optimize the work done by the human. This paper describes the design and control of a new robot at the Georgia Institute of Technology used for investigating a variety of tasks involving humans coupled to robots. The design takes advantage of the well-documented characteristics of closed kinematic chains. A discussion of current and future work is included. Robotic Transformer There are many control issues concerning haptic interfaces that are currently under investigation. The issues span a range of applications that include unilateral and bilateral teleoperation, time delays in teleoperation, exercise physiology, and the coupled stability of human and manipulator. During the design process ofHURBIRT, illustrated in Figure 1, an attempt was made to build a robot which could address many of these topics. As an exercise machine, the robot may need to facilitate a workspace optimized for arm andlor leg motion. The mechanical assembly of the links can be easily reconfigured to produce a variety of workspaces. Many teleoperation algorithms are initially tested on a simple single DOF testbed. Figure 2 illustrates how HURBIRT can be configured as a single 2 DOF master arm or two single DOF manipulators. When satisfactory performance is obtained, the system is reconfigured as a 2-DOF master robot. A project that is currently using HURBIRT consists of teleoperation using long reach manipulators. HURBIRT acts as an active master arm electronically connected using serial communication to RALF(Robot Arm Long and Flexible), an elastic long reach manipulator. Issues under investigation include bandwidth limitations due to control and transmission delays, the role of different cues in training and performance, and the performance of different teleoperation schemes. A second advantage to the current kinematic design is the use of a closed kinematic chain. One benefit of the closed chain is the ability to remotely drive one of the degrees of freedom. By constraining the lengths of the closed chain to form parallelogram, the kinematics and dynamics are simplified (Spong and Vidyasagar, 1989). This kinematic design constraint eliminates the Corio lis terms in the dynamic equations of motion, Equation (1). '1: d q + d cos (q + q ) q 1 11 1 12 1 2 2 d cj 2 sin(q +q ) + <I> cos(q ) 12 1 1 2 1 1 '1: = d q + d cos (q + q ) q + 2 22 2 12 1 2 1 (1) d cj 2 sin (q + q ) + <I> cos (q ) 12 2 1 2 2 2 All of the coefficients in the equations of motion are constant. The term dl2 has special significance in the design and control ofthe robot. This coefficient is dependent upon the mass properties of the links.