Conceptual Design of A Universal Bilateral Manual Controller

Walter Conklin and Sabri Tosunoglu

Department of Mechanical Engineering
Florida International University
Miami, Florida 33199 USA
E-mail: conklinw@eng.fiu.edu & tosun@fiu.edu

Abstract - A bilateral controller or a force reflecting manual controller (FRMC) allows an operator to sense forces which are exerted on a remote system. A universal controller can control any remote system as opposed to a master/slave system. Forerunners to this technology were large, bulky and impeded by inertial forces, large frictional forces and backlash. Inertia forces are one of the inherent disadvantages common to serial-structured controllers. This paper describes design concepts for a new 3-DOF FRMC which focuses on miniaturization and simplicity. Relative advantages and disadvantages of serial and parallel-structured controllers are also discussed in an effort to justify these new concepts.

INTRODUCTION

Force reflection in teleoperator systems is quickly becoming a field with commercial potential in industry as well as the private sector. Potentially, a teleoperator could do everything a person would normally do to perform a taskóas though remote objects were actually seen and handled. And if the operator also had a full sense of being at the remote site, this would seem ideal. That condition is called telepresence [1]. Recently, the inclusion of force reflection is referred to as telesensation in the context of a virtual reality environment. Such an environment provides the operator with a 3-D feel of the remote site as well as a feeling of touch and sound feedback.

Industrial applications may be concerned with unstructured environments (such as nuclear, space, marine) and private sector may use it in more challenging diverse applications such as remote training, medical operations and video games. Presently these controllers are more specialized for individual tasks and research. In order for the controllers to become feasible for most applications, their design must be made practical, compact and universal. The concepts developed at FIU will combine a parallel-structured controller with a compact design.

Topics discussed here cover advantages of force feedback and the structural benefits of serial and parallel systems when used with the controller. Major components of a force-reflecting controller system (controller mechanism, actuation, sensing and transmission systems) and possible design alternatives for such controllers are also addressed.

BILATERAL FEEDBACK ADVANTAGES

The involvement of the human operator in teleoperation is necessary, and the human/machine interface and the operator's abilities to interact efficiently with the manipulator systems is a crucial issue [2].

In industry a vast majority of robots work autonomously: once programmed, it does one assigned task over and over. A teleoperator hardly ever repeats the same task. And it is usually seen not in a factory but in a variety of other settings. Most important, a human operator sees, feels, and controls the remote task through the teleoperator [1].

Task completion time is the deciding factor when a decision is made whether bilateral feedback needs to be implemented or not. Several experiments have shown that force feedback provides a large improvement in performance. Also, adding information through one type of feedback (force) could make up for degradation in another type of feedback (visual frame rate). Decreasing the quality of visual feedback had less of an effect on performance when force feedback was used , as shown in Figure 1 [3]. Therefore, whenever visual feedback conditions are extremely poor and cannot be improved, the use of force feedback could be expected to enhance performance.

Figure 1. Interaction of Subtended Visual Angle [3]

SERIAL VS PARALLEL CONTROLLERS

Both the serial structure and the parallel structure have advantages and disadvantages when applied to robotic systems. We briefly review the essential characteristics of each structure below.

Serial Structure Controllers
Serial structures in general have been thought of as superior to parallel structures because of their exceptional workspace [4]. This might hold true for the slave/manipulator but not for the master/controller. As discussed above, a serial structure tends to add weight and inertial forces to a controller and/or manipulator. When used as a controller, these forces will conflict with any bilateral force feedback or true telesensation. As mentioned earlier, telesensation is a new term which implies a combination of all feedbackó3-D visual, force (touch), thermal, audible, etc.óreceived from the remote manipulator to create a virtual environment for the operator.

Telesensation, which also may be thought of as synonymous with telepresence, has been gaining acceptance because of the advent of virtual reality. Force feedback is the component of telesensation covered in this paper.

Effort has been made to balance the inertia effects of serial structures with counter weights and serial/parallel hybrid systems. Even with these efforts the serial controller still experiences its inherent drawbacks. The advantages are greater when the serial structure is used as a manipulator (large workspace, simple joint actuator position control, etc.). When implemented as a manual controller, their size often becomes too large, and their weight too heavy for practical use.

Figure 2. Kraft Serial Controller

Parallel Structure Controllers
A parallel structure usually allows to place all motors, brakes, and accessories at one central locationóthe base. This eliminates the necessity to move around most of the actuators as in the serial case. Hence, the input power is mostly used to support the payload. Also, the errors at each joint do not add up as one moves towards the payload as we have seen in serial structures. This implies a higher precision at the end-effector when parallel structures are used. Similarly, deflections at joints and links do not add up as in the serial case, which again implies better precision at the end-effector.

Figure 3. FRMC Integrated into a
Teleoperation System [6]

It is interesting to mention here that it is possible to have a serial frame with parallel control strategy if the motors are located at the base and each joint is controlled via a series of pulleys and cables. Serial/Parallel hybrids introduce unnecessary mechanical complexity to manual controllers which might be beneficial to manipulators. This mechanical complexity often results in bulky designs which are not acceptable for the design sought in this work. Also, the cables offer low precision due to slippage, and need frequent maintenance and replacement.

Table 1. Functional Characteristics of Typical Industrial Manipulators and Universal Bilateral Manual Controllers

Because this paper is concerned mainly with controller design, the beneficial traits of hybrid systems for manipulators are covered further. Table 1 lists some of the characteristics found with controllers and manipulators [5].

STRUCTURAL CONCEPT

The overall benefit of the universal parallel force-reflecting manual controller (FRMC), as the word universal implies, is that it can be used with any manipulator as long as there exists a control software interface. The FRMC is used to position the manipulatorís end-effector at any desired position and orientation. The controllerís workspace is not as critical because only the manipulatorís end-effector position/orientation is being controlled. The FRMC simply feeds the control software with translational or rotational directional signals which the control software then uses to drive the respective actuators located at the manipulator. The design takes the characteristic that parallel mechanisms are more compact and have smaller working volumes and takes advantage of it. Figure 3 illustrates the major components of a universal FRMC system [6].

The proposed 3-DOF FRMC will be capable to position the end-effector of any remote manipulator. Then, with a simple one-key input, the operational mode will switch to that of orientation. The 3 degrees of freedom will then correspond to the three orientation parameters (e.g., a set of Euler angles) of the end-effector. As the operator wishes, the control mode may be switched back and forth between the position and orientation modes. Hence, the design emulates a 6-DOF controller by combining a 3-DOF controller and special control software.

CONTROLLER HARDWARE

Here we address the three major components of the FRMC design. They are the motors, encoders and gear heads. We note that these components are the most critical ones to affect the performance of the FRMC. By comparison, we list the kinematic structure of FRMC as the most critical issue, followed by the selection of these critical components.

Actuation: Servo Motors
The servo motors used for the controller will be the most difficult to acquire. The small size neededóless than 2 inches in diameter and less than 2 inches in lengthówill be difficult to find. Because the human hand is able to sense forces from 0.016 to 4.5 lbf, the desired minimum/maximum reflecting forces of the manual controller need to be selected accordingly to fully utilize the human capabilities. In the design of the 3-DOF controller, the continuous maximum reflecting forces of 4 lbf and the peak reflecting forces of 6 to 8 lbf at the joystick will be used as design goals.

Sensing: Encoders
The encoder signals to the control software at what radial location each of the controller/manipulator joints is positioned. For the manipulator the encoder is used to its full potential because the joints usually rotate to their full limits. For the 3-DOF controller, the rotational limits will most likely be in a range between 0 and 90 degrees. In order to keep the controller size small, the positional sensors will be high precision potentiometers. The potentiometers will give the resolution needed and the desired size.

Figure 4. 3-DOF Gimbal Design Concept [5]

Transmission: Gears
Because of the low forces being transmitted and the revolutions constrained from 0 to 90 degrees, the goal is set to implement a direct-drive configuration. Eliminating gear sets from the design will also reduce the friction in the system and any backlash effects inherent in gears. The direct-drive system will allow the controller to be fully backdrivable. This means that the motor may be driven from the input shaft or the output shaft without being restricted. As an example, worm gears can be driven from the input shaft but not from the output shaft because of its gear configuration.

FRMC DESIGN ALTERNATIVES

Gimbal Design
As the kinematic sketch in Figure 4 illustrates, the gimbal design uses a parallel structure. The arcing gimbals, which are powered by motors placed on the base, produce a large workspace in the X- and Y-axis directions.

The third joint can be implemented as a linear displacement of the joystick in and out of the gimbal structure. However, because such a selection will increase the size of the overall system, it is not considered to be an attractive design option. Instead, the third joint is visualized as a revolute joint rotating about the main axis of the joystick as discussed later.

This design enables better encoder resolution. When a large displacement is produced along the X and Y axes, a relatively small displacement is produced at the base servo motors and encoders. This high resolution enables the user to control the manipulator with higher precision. Also, due to the relatively simple kinematic structure, the input-output equations will be simple and possibly have a closed-form solution.

Modified Gimbal Design

Figure 5. 3-DOF Modified Gimbal Concept

This design is a more compact version of the gimbal design presented above. The X- and Y-axis servo motors are located in the same area but the arcing gimbals are replaced with one straight shaft and one straight shaft with arcing center as shown in Figure 5. The third degree of freedom can be located in either of two positions that is covered in the next section. The compact design of this concept reduces friction considerably and simplifies the mechanical structure.

Third-Degree-of-Freedom
The third-degree-of-freedom can either be positioned inside the hand gripper, as shown in Figure 6, or made to rotate the base about an axis perpendicular to the base where the X and Y axes intersect.

If a servo motor with relatively tight tolerances which can fit inside the gripper is available, the first option will be favorable. With the motor inside the gripper, the weight of the controller will be kept at a minimum and the design will remain compact without affecting the other axes.

If the servo motor is positioned to rotate the base, the structure will not be as compact as in the above case. The advantages of such a design are easy accessibility of components and the available gearing options. The third motor will rotate the entire base with the X and Y axes attached to it. The extra weight and size will require that the servo motor be geared to the base. In this case, it is expected that a simple spur gear configuration will suffice.

Figure 6. Modified Gimbal with
Rotational Gripper

CONCLUSIONS

Serial and parallel structures were discussed as two alternate kinematic structures to develop force-reflecting manual controllers (FRMC). It is shown that parallel structures have more desirable characteristics to build very compact FRMCs. After briefly addressing the desirable characteristics for the major FRMC components, namely the actuators, encoders and transmission systems, we introduced two conceptual FRMC design. These design alternatives offer compact FRMC systems which can readily be integrated into a teleoperation or a telesensation system.

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