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Robot retrofits: Axis 1 safety

Robot retrofits: Axis 1 safety
As more manufacturers embrace robotics with new systems or look to redeploy their existing robots in other areas of the plant, the importance of safety zoning increases. Bryan Hall, Dave Bird and Henry Menke of Balluff discuss safety retrofits.

Robots have been used by the automotive industry for many years, and in this ever changing environment an individual robot may fulfil several roles over its lifespan. As a consequence there is often a need to retrofit additional robot hardware features during any refurbishment that may take place in order for the robot to meet the next phase of its life cycle. One such addition to so-called legacy robots is an Axis 1 safety zoning system. The system would be interlocked with either mechanical guards and/or photoelectric light curtains to aid protection of the operator when for instance entering the working area of the robot cell.

Since the late 1990s Balluff has been developing a broad family of pre-engineered and kitted Zone Limit Systems. Each kit is specifically tailored to a particular make and model of robot for a quick, no-guesswork solution to the problem of safety zoning implementation. These Zone Limit kits can be applied to Axis 1, Axis 2, and/or Axis 3 for the ultimate in flexible and comprehensive zone limiting. The kits can be fitted to both new and refurbished robots.

At the heart of the Balluff system is the rugged and compact Multiple Safety Position Switch with either safety rated rigid mechanical limit switch plungers and contact blocks or non-contact pulsed inductive inserts monitored by a safety PLC or safety switching relay. Anywhere from two to six limit switches or inductive sensors are arranged vertically in a single housing wired to a multi pole connector. The switches are operated by a complementary set of cams - for example, 90 degrees and 270 degrees. One switch, working in combination with the first cam, is always on, while the other switch, tracking the second cam, is set for off so that the switches operate in a diverse configuration. The cams come as a set of segments so that many combinations of complementary angles can be configured in 15-degree increments.

System implementation
One set of cams is required per zone. Typically, up to three zones can be implemented on Axis 1, while Axes 2 and 3, if implemented, are normally each set up for a single zone. The cams can be positioned anywhere in a 360 deg circle around the robot, allowing not only the size of the zone but also the zone location to be quickly set up with minimal delay.

Each zone is monitored by a pair of redundant safety switches, tripped by a pair of complementary cams. The cams cover 360 deg of the robot's range of motion. Restricted zones are configurable from 15-345 deg in 15 deg increments. When the limit switches are monitored by a dual-channel safety relay, control reliability is assured. A single fault point in any part of the system will not compromise the safety function. 

The Zone Limit System provides a control-reliable means to interlock safety light curtains, mechanical guarding with the angular position of the robot. When the robot is clear of the operator area, the safety relay temporarily mutes the output of personnel-sensing devices, allowing workers to access part of the robot cell without stopping the robot.

When the robot enters the operator zone, the safety light curtains are instantly re-enabled. If an operator is present, the robot is stopped. If no operator is present, the robot is free to continue working in the restricted zone unless the operator re-enters the area.

The need for robotic zone limit safety systems continues to grow. For example, development of zoning systems for high-speed gantry robots using linear cams and non-contact pulsed inductive solid-state sensors is allowing vastly increased productivity from these robots. Balluff has already successfully integrated up to eight zone systems for robot gantries 80m in length. There is also growing demand for the non-contact, pulsed inductive solid-state sensors for rotary zone limiting in abrasive dust applications where mechanical switches can't be used.
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