A methodology for designing robots provides for satisfying both safety and performance requirements. Heretofore, most robot-design efforts have been focussed on maximizing performance, with only incidental regard for safety, under the assumption that humans and delicate equipment would be excluded from robot workspaces during robotic operations. The present methodology was developed out of recognition of the need to ensure safety for humans while realizing the potential ability of robots and humans working together to perform a broader spectrum of tasks than either can perform alone.

The methodology is implemented by a formal design procedure in which quantitative evaluations are performed to effect compromises between the inevitably competing demands of performance and safety. The procedure comprises five main steps and a number of sub-steps (see Figure 1).

Figure 1. This Five-Step Procedure implements the methodology for designing safe robots.

The first two steps, which can be simultaneous, are the determination of task requirements and the determination of safety requirements. "Task requirements" as used here denotes such quantitative performance specifications as robot-tip velocities, robot payloads, robot position and force accuracies, and measures of robot dexterity. For a given application and task, minimum acceptable values can be assigned to these specifications to quantify minimum acceptable performance.

"Safety requirements" as used here denotes the set of all robot-performance measures that are related to safety in that they indicate degrees to which robots can harm humans. Such measures include robot static and impact contact force, robot static and impact vise or pinch forces, robot static and impact contact forces, and crushing forces from robot weights. For a given application and task, maximum allowable values can be assigned to these measures to specify safety limits.

Figure 2. This Safety Diagram with superimposed task envelope for a typical robot illustrates the margin available for designing to satisfy both safety and performance requirements. The intersection of the design and safety envelopes represents the ranges of tip-velocity and payload values of acceptable designs.

The next step is the selection of the top-level robot design. More specifically, this means the selection of the kinematics, range of motion, and geometry of a robot to accomplish the task as specified in the first step.

In the following step, one determines measures of the speed and strength of the robot on the basis of the top-level robot design and the safety requirements. This step is divided into the following four substeps:

  1. Develop mathematical models of robot forces, velocities, and energies to relate design parameters to safety and performance variables.
  2. Using the models developed in substep 1, make a safety diagram, which is a plot showing the boundary (denoted the "safety envelope") between safe and unsafe values of two or more safety-related performance measures. Also plot the boundary (denoted the "task envelope") of the task requirements for these performance measures on the safety diagram (see Figure 2).
  3. Select target performance specifications (e.g., a value of tip velocity and a value of payload weight) that lie within the intersection of the safety and task envelopes.
  4. Select the joint torques and velocities needed to achieve the target performance specifications and design the robot joints and actuators accordingly.

The last step is the application of guidelines for safe design. Six guidelines have been formulated through analysis of those safety and task specifications that are mutually independent (or at least nearly so). Each guideline represents a strategy for independently optimizing some aspect of either safety or performance.

The guidelines are the following:

  1. Maximize robot accuracy,
  2. Maximize robot dexterity.
  3. Minimize robot weight.
  4. Eliminate pinch points and maximize potential vise radii.
  5. Maximize robot contact area.
  6. Maximize robot padding thickness.

These guidelines, if followed during the design process, can help ensure a high-performance, safe robot design.

This work was done by Karl T. Ulrich, Timothy D. Tuttle, Joseph P. Donoghue, and William T. Townsend of Barrett Technology, Inc., for Kennedy Space Center.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to

William T. Townsend
Barrett Technology, Inc.
139 Main Street
Kendall Square
Cambridge, MA 02142
(617) 252-9000

Refer to KSC-11849

NASA Tech Briefs Magazine

This article first appeared in the July, 1998 issue of NASA Tech Briefs Magazine.

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