We mentioned that ARM end users would like to be as independent as possible. This means that they will utilize the ARM on a daily basis to perform several tasks that rely on a perfect working ARM. Moreover, many people with ARMs are known to be alone or to have a working spouse. They are on their own for a large part of the day. This means that the ARM has to be reliable. But, just like any other piece of equipment, it could malfunction. Most problems are cable-related problems, such as a damaged wire. For these end users, it is extremely important that a broken ARM can be repaired/serviced as soon as possible. This requires not only a special design in terms of serviceability (e.g., a modular design, ease of mounting/demounting of the ARM and its components) but also a dealer network with professional service engineers.
Safety and Norms
Safety is a key issue with assistive robotic arms. ARMs are usually capable of relatively fast movements and of exerting relatively large forces. How do we prevent the robot from hurting someone or damaging something? This concern is especially important given that most of the end users are almost defenseless.
Consider a practical example: To feed himself, a user puts a fork in the ARM’s gripper and picks up some food with this fork. Now, he can bring the fork to his mouth. How do we prevent the user from hurting himself with the fork? We cannot say “don’t get near your face” because that would not make sense. How is this dilemma solved? The first approach is to instruct the user on how to perform the activity with the ARM, which is not necessarily in the same way the person would be doing the activity with a natural arm. For example, end users can be told not to use sharp objects like knives, forks, needles, and so on with the ARM. If they want to eat, a spoon can be used. Also, end users can be taught not to enter the spoon into the mouth. The spoon could be placed just before the mouth and then the end user would move his neck or body to eat from the spoon. This would solve some of the safety issues.
However, safety measures should be implemented in the ARM. The robot should be equipped with sensors (e.g., force sensors, cameras, tactile sensors) to provide automatic feedback on the robot operation. For example, the force exerted by the robot end-effector can be measured and kept within a safety limit. This countermeasure solves some safety issues when the robot is applying force to an object. But, the robot should also be able to handle some external forces. Suppose someone is leaning on the robot. Forces on a joint can then easily reach 100 N. In this case, either the controller can give a warning and stop trying to keep the robot in this position or a countermeasure can be taken from the mechanical construction of the robot, applying slip clutches to the joints. Slip clutches will prevent too much force from being applied to a joint. When the force to a joint is higher than the maximum allowed value, the clutch will slip.
The 2014 standard “Robots and Robotic Devices—Safety Requirements for Personal Care Robots,” (International Organization for Standardization [ISO] 13482) provides more information on safety considerations for ARMs (please refer to Chapter 1).