CAD,CAM AND ROBOTICS
104
There are two aspects of the safety issue in robotics. The first deals with the justification of robots. Historically, one of the fundamental reasons for using robots in industrial applications is to remove human operators from potentially hazardous work environments. The hazards in the workplace include heat, noise, fumes, and other discomforts, physical dangers (potential injuries or even loss of limbs), radiation, toxic atmospheres, and other health hazards. The problem of removing or reducing these hazards from the workplace has provided one of the important justifications for industrial robots in applications such as welding, forging, spray painting, and die casting. Since the Occupational Safety and Health Act (OSHA) was enacted in 1971, worker safety has become a significant factor in promoting the substitution of robots for human labor in these kinds of dangerous jobs. The second aspect of the safety issue involves the potential hazards to humans posed by the robot itself. The use of robots presents a new set of possible dangers to the human worker fro which precautions must be taken. It is this second aspect of the safety issue which we will address in this chapter. In considering the potential hazards that are encountered the use of the robot are robot, it seems reasonable to question when during the use of the robot are humans in contact or close proximity to it. There are three occasions when humans are close enough to the machine to be exposed to danger There are: During programming of the robot During operation of the robot cell when humans work in the cell During maintenance of the robot The types of risks encountered during these times include physical injury from collision between the humans and the robot, electrical shock, objects (parts or tools) dropped from the robot gripper, and loose power cables or hydraulic lines on the floor. Some of these risks can be reduced with straight forward safety measures such as proper grounding of electrical cables to prevent shock, and raised floor platforms to cover power cables and hydraulic lines. In other cases, operator safety is improved by requiring certain common sense procedures to be followed. For example, when the robot is being programmed, the speed of the arm should be set at a low level during teaching and testing of the program. Another example would be when the maintenance personnel are servicing the robot. During maintenance, the power to the machine should be turned off under normal circumstances. More extensive safety measures must be taken to guard against hazards that arise during robot operation. Many of these safety measures must be designed into the work cell, wither as part of the work cell, either as part of the work place design or as part of the work cell control system. Workplace design consideration for safety Certain safety features can e designed into the robot work cell. These include physical barriers to limit intrusion into the cell, emergency stop buttons to hall the cell operation, and laying out the equipment in the cell for maximum safety. The most common approach is to construct a physical barrier around the periphery of the robot work cell. The periphery of the robot must be defined to be out side the farthest reach of the robot in all directions with end effectors attached to the wrist. The work cell would also include any equipment in the cell which operates with the robot. The barrier should not be designed only for the programmed work cycle envelope because a malfunction of the controller may cause the robot to follow a trajectory different from its normal program. The barrier has the effect of preventing human intruders from entering the vicinity of the robot while it is operating. The barrier often consists of a fence with a gate fro access to the work cell. The gate is equipped with an interlock device so that the work cycle is interrupted when the gate is open. A positive restart procedure is designed into the cell for resumption of the cycle, rather than using the gate closure for restarting. Other possible physical barriers include safety rails and chains, although these are not as effective as a full fence. Another approach that has been used in industry as a physical barrier is a steel post in the floor at the limits of the programmed motion cycle, so that a cut of control robot arm crashes into the post
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