Fault tolerance becomes even more critical as the number of sensors increases because each one represents a possible point of failure. If a camera stops sending valid video to the human detection algorithm, somebody could wander into a robot arm operating at full speed and be at danger for potentially serious injury. The rule of thumb must dictate the graceful stopping of the robot when sensors are compromised in any way. A carpenter would not keep hammering nails if somebody pulled a bag over his head. During the design phase, keep in mind that not all sensors are rated for use in human safety applications, so use caution when selecting them.
A power outage is a special type of failure because a robot’s intelligent awareness capabilities disappear when the electricity is terminated. Redundant backup batteries are often too costly and bulky for most applications. Passive resistance is an option to slow the descent of mechanical arms being held up by motors and gears. Strategies are required to accommodate the possibility of heavy objects held in the hand of a robot that loses power. Latching mechanisms are a commonly used method to avoid dropping a heavy chunk of metal on somebody’s foot.
Designers have the responsibility to prevent the user from entering in control parameters that could cause dangerous robotic behaviors. For example, fast arm speeds and rapid changes in direction could exceed the gripping force of a robot’s hand and result in parts being thrown. A user should be prevented from setting force detection thresholds too high, so the arm will not bump into somebody harshly.