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Principles of Assistive Robotic Manipulators

Working Definition of Assistive Robotic Arms

A robotic manipulator is a set of rigid links connected by actuated joints. The most common joints are revolute joints, allowing adjacent links to turn around them. The number of joints in a robotic manipulator usually determines the number of degrees of freedom (DOF) of the manipulator, that is, the number of independent variables that needs to be specified to locate all parts of the manipulator (Craig 2005). The higher the number of DOF, the higher the flexibility in positioning the end-effector of the manipulator (i.e., the device attached to the manipulator wrist; examples in this chapter are a spoon, a cup bearing a forearm, or a gripper). The set of all positions and orientations that the end-effector can attain is called the workspace of a given manipulator. Many robotic manipulators have been designed for industrial purposes. These applications typically require high speed and high accuracy. Many industrial robots are also required to handle heavy payloads. Because of these high speeds, high accelerations, and heavy payload capabilities, you can find most industrial robots behind a fence because it is not considered safe for humans to be close to them. This would be a problem for our application: We want the robot to be near a human to assist that person. Therefore, we need to have another group of robotic manipulators, the so-called assistive robotic manipulators (ARMs). ARMs are designed to interact with humans in a safe manner. This will of course give the robotic manipulator some restriction with respect to speed and force and require some special sensors or other measures to guarantee the safety of interactions with humans.

Another important difference between (most) industrial robots and (most) ARMs is the way they are controlled. Most industrial robots are set up at a fixed position and a given (structured) environment. Within this environment, the robot can perform its task. The robot, again in general, is following the instructions coming from a program, following the same sequences over and over again. For several ARMs, the location in space is changing continuously (e.g., mounted on a wheelchair), and the environment is, in general, unstructured. A wheelchair-mounted robot can be used one time as a feeder, making repetitive movements from the plate to the mouth, and the same robot is used to pick up a glass for drinking. Neither will the plate nor the glass ever be at the same spot. The shape of the glass might also change. Usually, the human is in the control loop, meaning that the user has control over at least some of the manipulator’s

DOF (e.g., the user is able to move the manipulator end-effector in the Cartesian space with the help of a joystick that allows for up/down and left/right movements, although the actual joint angles are controlled by the robot to hold the end-effector orientation fixed).

Assistive robotic manipulators can be seen as augmentative and alternative manipulation tools for people with upper limb physical impairments. Their goal is to provide these persons an alternative way of manipulating objects in their environment. They differ from the robotic prosthesis described in Chapter 4 because they’re not attached to the person’s body. The fact that joints should be actuated by motors is critical in the definition of a robotic manipulator.

This chapter addresses robotic manipulators with at least 6 DOF. This number of DOF is the minimum required to be able to position and orient arbitrarily an object in three-dimensional (3-D) space. When describing the movement of the robot end-effector, we usually do it in reference to an inertial frame located at the base of the robot with the x and y axes on the plane where the robot sits and with the z axis pointing up. Rotations about the x, y, and z axes are usually termed roll, pitch, and yaw movements, respectively. Six DOF are required for an ARM to be able to perform relatively complex tasks. Imagine that you have a full glass of water on a table. You want a robot to pick up the glass and bring it to your mouth without spilling so you can start drinking. Table 3.1 lists the robot joint actions necessary to accomplish this task. This typical application would not be possible for any robot with less than 6 DOF.

Although 6 DOF are enough to position and orient the cup, some movements require more DOF. Suppose you have a robot holding a glass of water at the same position and orientation in space but with different joint

TABLE 3.1 Required DOF for Drinking

Action

Robot Joint Actions

Required DOF

1

Robot holding a glass

Starting point

2

Robot lifts glass

z-axis movement up and simultaneously movement of gripper down (pitch) to keep glass horizontal

z + pitch

3

Move glass to user

x-axis and y-axis movement toward the user and simultaneous rotation of the gripper on the horizontal plane (yaw) to keep the same gripper orientation

x, y + yaw

4

Start drinking

z-axis movement and roll of gripper

z + roll

configurations (see Figure 3.1). to go from the “elbow-up” to the “elbow- down” configuration, while keeping the glass of water in exactly the same position, an extra DOF is necessary. In fact, a human arm has 7 DOF (Figure 3.2), which enables us to move our elbow up and down while keeping our wrist in the same position. It can be shown that, except for some singular points in the robotic manipulator workspace, a given position and orientation of the end-effector of a 6-DOF manipulator can be achieved at least by two joint configurations. An extra DOF allows moving from one configuration to another while keeping the end-effector orientation. With a 7-DOF manipulator, there are an infinite number of possible joint angles that achieve the same position and orientation of the end-effector.

Industrial robots mostly have dedicated end-effectors for dedicated tasks, often with an end-effector exchange option. Because rehabilitation robots have to perform many different tasks and be able to pick up and hold many different objects with different shapes, dedicated end-effectors are not an option. We are looking more toward a “universal” gripper. This gripper may not be the ideal solution for every task; but it must do the job.

The human hand is effective for multipurpose tasks, so it makes sense to give it a closer look. Most of us use all five of our fingers unconsciously, but imagine what you can do with only three fingers or even with only two fingers. You will soon find out that most of the tasks can be done by two fingers only. Many current ARMs have grippers as end-effectors that emulate the functioning of the human hand. Opening and closing a gripper is not considered as an extra DOF (Poole 1989).

In summary, our definition of an ARM is as follows: any robotic arm that is not body bound with a minimum of 6 DOF and a gripper, which is safe to interact with humans, especially humans with special needs or a disability, and can assist with several activities under the control of the user.

Joint configurations

Figure 3.1 Joint configurations: “elbow” up and down.

The 7 DOF of the human arm

Figure 3.2 The 7 DOF of the human arm.

 
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