By Dave Bird and Henry Menke
Metalforming Industry Manager & Robotics Systems Product Manager, Balluff, Inc

Robots have been used to transfer metal in stamping operations for many years, but what safety issues should be addressed when humans need to share space in the robotic work cell?

Robots have been used to transfer metal in stamping operations for many years, but what safety issues should be addressed when humans need to share space in the robotic work cell? What techniques can be applied to ensure personnel safety while maintaining the productivity of the robot?

U.S. robot safety standard ANSI/RIA R15.06-1999 provides guidelines for the safe interlocking of robot position to the presence of a human operator. This process is known as Dynamic Envelope Restriction. Section 10.2.3 of the standard permits the use of Dynamic Limiting Devices to reliably monitor the position of the robot and allow it to continue working while it is safely out of the area occupied by the operator. The use of dynamic limiting devices, also known as DLDs or Zone Limits, can significantly increase the hourly productivity of a robot without compromising the safety of personnel working with the robot. The challenge for robot integrators and control system designers has been to devise a way to economically and reliably implement zone limits while meeting the strict ANSI/RIA requirements for Control Reliability under section 4.5.4.

Since the late 1990s Balluff, a global manufacturer of sensors, precision mechanical limit switches, and linear position transducers, 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 DLD implementation. Kits are available for most Fanuc robots, as well as several popular robot models from Motoman and Nachi. 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.

At the heart of the Balluff systems is the rugged and compact Multiple Safety Limit Switch. Anywhere from 2-8 plunger-operated limit switches are arranged vertically in a single connectorized housing. The rigid plungers actuate the positive-drive switch elements for forced contact opening, even if contacts become welded. To implement "diverse complementary redundant" operation per ANSI/RIA requirements, each zone is monitored by two redundant limit switches.

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.

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.

Balluff Zone Limit Systems are intended to be monitored by standard dual-channel safety relay modules available from manufacturers such as Pilz or STi. When properly connected to these devices, the overall system can achieve a Category 4 safety level.

Why Robot Zone Limit Systems Are Being Installed in Metal Forming Operations

• Increased throughput of the robot (eliminates robot standby waiting for personnel to clear the cell. The robot keeps working in other areas of the cell).
  
• Easy implementation of multiple zones on Axis 1, 2, and/or 3.
  
• Crash avoidance between adjacent robots.
  
• Tooling protection (verification that the robot cleared before a die is closed).
  
• The zone limit system does not create an additional obstacle in the cell that the robot must maneuver around (such as, for example, robot-sensing light curtains).
  
• Balluff Zone Limit Systems are compatible with a wide range of robot brands and models commonly found in metal forming operations. These systems can be installed on new robots or can be retrofitted to many legacy robots.
  
• Implementation of dynamic envelope restriction per ANSI/RIA R15.06-1999 and addressing the requirement for Category 4 safety and control reliability when used with appropriate safety relays.

System Overview 

• A heavy-duty mechanical cam switch assembly mounts to the stationary robot base. The mechanical cam switch assembly is a double-changeover switching system made up of six limit switches actuated with solid stainless steel plungers with high-speed ball bearing rollers. The positive-break, force-guided contacts are rated for 5 million operations.
  
• A circular cam drum that attaches to the robot's rotating flange.
  
• Hard chromed, long-life circular cams mount on the cam drum to define location and angular size of zones.
  
• Balluff Zone Limit Systems implement "Dynamic Restricted Envelope" per ANSI/RIA R15.06-1999 and address the requirement for Category 4 control reliability.

System Function

1. Each zone is monitored by a pair of redundant safety switches, designed to VDE 0113 specification, 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.

2. 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. Examples of single point system faults that can be detected include:

• Missing cam
  
• One cam overlapping another
  
• Open/short circuit
  
• Stuck switch plunger
  
• Switch disconnected
  
• Welded contact
  
• Broken contact
  
• Bad electrical connection.

3. The Zone Limit System provides a control-reliable means to interlock safety light curtains and safety mats 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.

4. When the robot enters the operator zone, safety light curtains and safety mats 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 until the operator reenters the area.

New Trends In Zone Limit Systems

R & D marches on. The need for robotic zone limit safety systems continues to grow. For example, manufacturers of RTUs (Robotic Transfer Units) are incorporating linear cams into their zone limit systems. Likewise, development of zoning systems for high-speed gantry robots using linear cams and noncontact 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 80 meters in length. There is also growing demand for noncontact, pulsed inductive solid-state sensors for rotary zone limiting in abrasive dust applications where mechanical switches can't be used.

Human and robot doing work in the same cell? Absolutely. With dependability, safety, and productivity.

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