Daniel Roth - Robot Programming: A Practical Guide to Behavior-Based Robotics (Tab Robotics)

A Practical Guide to Behavior-Based Robotics

Joseph L. Jones

Robotic Simulator by Daniel Roth

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To Sue, Kate, and Emily

I got my first taste of robot programming in the early 1980s when I joined the research staff at MITs Artificial Intelligence Laboratory. My group was trying to solve a classic challenge in robotics called pick-and-placemake a robot pick up an object at one spot and put it down somewhere else. Given an object and a destination all the robot has to do is to figure out the actual arm and gripper motions needed to move the object to the goalthe sort of thing any two-year-old can do. Four of us worked on the problem for about five years.

Other group members worked on the parts of the program that would generate the large-scale motions of the robot arm, motions to move the arm from one region of the workspace to another. My job was to write the software that would enable our robot arm (see Figure P.1) to work out how to move the last few inches toward an object and grasp the object. The solution to the overall problem has many constraints: the robot has to grasp the object at a viable spot; the robot must avoid bumping into anything as it moves about; the robot must avoid violating what are called kinematic constraints.

Figure P.1 This Puma model 560 with custom-built gripper was one of the manipulator robots used in the Handey project. In the foreground the robot picks up a motor that will be added to an assembly (contained in the white box) at the back right. The Handey program generates all the joint motion commands needed to move the motor from the point where it is picked up, avoiding all the obstacles, and insert it into the assembly. (Photograph courtesy of Prof. Toms Lozano-Prez of the MIT Artificial Intelligence Laboratory.)

Yet another part of our work was to write code that would figure out how to reposition the object in the robots gripper if the initial grasp conflicted with obstacles or the robots kinematic limits at the putdown point. The tricky bit was that our software was supposed to be completely generalthe code had to work for any robot, in any environment, transporting any part.

In order to accomplish all these things we had to first build a world model. A world model tells the robot the geometric shape of every object in the robots workspace and where every object is located in relation to the robot. And in the same meticulous way that we modeled the environment, we also had to model the robot and to program the equations that described the robots kinematicshow the robots joints relate to each other and in which ways and how far each joint is able to move.

Our task was excruciating. Any small error in the world model could cause the robot to collide with an object when the robot tried to execute the motions it had planned. Any little mistake in the equations that describe the robot meant the robot might fail to reach the designated pickup-object or whack something along the way. If one of us accidentally bumped some object in the robots workspace, thus creating a mismatch between the real world and the robots world model, the robot would most likely strike that object. And, when at last the robot came up with a successful plan for moving an object from one place to another, the robots motions invariably looked awkward and unnatural.