Making safety a priority when employing tool exchange systems

With technological advancements and the rise of a new generation of high dynamic robots, successful integration of production processes has the potential to yield greater efficiency and productivity. Failure to ensure appropriate safety measures within the production process, however, can pose significant risks with costly ramifications.

When discussing “process safety” at the employment of tool exchange systems, there are two factors that must be taken into consideration. The first, and most obvious, relates to physical safety – ensuring the health and well-being of humans working alongside the line, as well as the protection of the actual equipment employed. The second factor addresses the production process itself – ensuring reliability within the process to reach a production end-goal of 100% capacity.

This paper addresses the prerequisites for process safety at the employment of tool exchange systems on robots to safeguard that the utilization of tool changers does not threaten the welfare of the workers on the factory floor or adversely affect the total output of the production line.

What prerequisites will prevent utilization of tool changers from becoming an additional safety hazard?

Consider the ramifications of the wrong command executed in a robot cell within the robot control, whereby the robot in motion receives the order to drop a 200 kg welding gun. In these instances, where high dynamic robots are providing accelerations in excess of 20 times the earth’s gravitational pull in normal operation and exceeding more than that in emergency shutdown situations, there is simply no room for error or accident.

Admittedly, the thought of a free flying welding gun through the production area may be a bit dramatic, but there have been situations where certain safety precautions were not taken, either due to negligence or ignorance of the predetermined process. According to the United Auto Workers, the majority of accidents reported in the United States occur as a result of a worker found “inside the robot’s safeguarded or restricted space during its automatic operation.” Few reports indicate machinery failure or malfunction. In part, this can be attributed to the safety measures inherent within the robot, its tooling and its environment.

Opportunities to help ensure process safety at the employment of tool exchange systems are possible if the following key process factors are understood and implemented.

Work Cell Zones
The first line of defense in preventing accidental tool drops is to create a safe zone within the robot programming to ensure that the tool will only uncouple in areas that are predetermined as “safe” whereby no harm will come to operators or equipment if a tool is dropped. This work cell zone, defined in the robot controller, prohibits a tool from uncoupling if the robot is not within the programmed zone area.

Coupling and Uncoupling
Coupling and uncoupling represents the greatest opportunity to prevent unnecessary harm. Efforts can be made within the robot’s environment, as well as within the tool exchange system itself to ensure safety. Specific coupling and uncoupling considerations include:

Sensors and Pneumatic Interlocks
In addition to work cell zones, sensors and pneumatic interlocks should also be integrated into the tool exchange system. While it is not required to use both, it is good practice. Use of a sensor as part of the uncouple circuit must be made to actuate, so even if the sensor is unplugged, the tool changer will never uncouple.

There is a prominent misconception that embedding the Program Logic Controller (PLC) sensor either as a hardware or software solution is enough of a safety measure to prevent a tool from accidentally dropping outside of the tool station. Although this allows the tool to release only in the designated tool hanging mechanism, this is inadequate because it will not actually prevent a tool drop should the PLC be told to execute the wrong command.

As an alternate approach, the robot side of a tool changer should include a pneumatic activation switch in addition to the PLC sensor, which is only activated when the tool is placed properly in its storage fixture within the work station. This system does not transmit pneumatic pressure to the uncoupling valve until the robot has correctly set the tool into the final release position. While this is actually a feasible solution, practice has shown that the operator can, and sometimes will, override the switch manually to uncouple the tool. This situation, which occurs most often during line set up, debug and tool maintenance, is an unsafe practice.

It is also important to note that a comparable electrical solution is equally suitable to prevent non-programmed release of tools.

Set Up and Programming
According to the U.S. Department of Labor’s Occupational Safety and Health Administration (OSHA), most problems do not occur during normal operating conditions, but rather during programming, program touch-up or refinement, maintenance, repair, testing, set up or adjustment. Air pressure is an important, but often neglected factor that should be taken into consideration during set up. The valve must be under electrical control, meaning that the coupled state is known and tested before applying air pressure during initial set up. It is also important that the valve is commanded to be in the coupled state before applying pressure. Failure to do so can result in an accidental tool drop.

Another factor to consider when using a tool exchange system, regardless of the type of latching mechanism, is to always incorporate the tool changer couple and uncouple sensors in the programming logic. By doing so, this will determine whether the latching mechanisms are engaged or disengaged from the tool. If this is not done, the latching mechanism may not fully disengage, which can cause the tool to be dragged out of the tool storage fixture and accidentally dropped.

Control of Actuation Valves
Controlling the actuation valve to ensure proper coupling and uncoupling is another important safety measure to be taken into consideration. Control of the actuation valve can be accomplished through electrical interlock logic or pneumatic interlock logic. Regardless of the type of logic, it is critical that multiple layers of interlocks are built into the tool exchange system to ensure 100 percent capacity of the production line.

Pneumatic Safety Switch
A pneumatic safety switch is an additional measure that can be employed. In this scenario, the pneumatic line to the uncoupled valve passes through a robot side mechanical valve that is only activated when the robot arm is in the designated end position at the tool station.

Handling High Current
When discussing the safety of tool exchange systems, another factor that must be taken into consideration is the voltage level of the current flowing through the system. In welding applications, there can be as many as several hundred amperes with voltages as high as 600 volts depending on the type of welding system utilized. If the tool changer is not adequately equipped to handle such current and degrades over time, a malfunction and subsequent tool failure may occur. Therefore, appropriate measures must be employed.

The materials used in the tool changer system must be durable enough to safely handle high current flow even under adverse conditions. Materials like FR4, a fiberglass epoxy laminate, provide a suitable dielectric constant to prevent leakage through the tool changer housing, eliminating such problems as sensor shortages, motors burning and even electrocution. In addition to durable materials, the insulation must also be able to withstand the welding current and prevent current transferring back from the welding gun.

Additionally, it is important that electrical operators on the floor are not exposed to hazards. In compliance with European Standards, when the tool changer is uncoupled the robot contacts must be “touch safe.” This is especially important when changing from a welding gun to a gripper – high-KVA to non-KVA. The robot side must be covered to shield the exposed high-KVA contacts.

On occasion, in addition to the actual current carrying contacts, so called “mass contacts” are utilized that are capable of handling the total current capacity of all electrical contacts together. For example, with 3x200 amperes at 600 volts or respectively 120 KVA per contact, the mass contact must be able to handle a minimum capacity of 360 KVA.

Capacity for High Dynamic Robots
As previously mentioned, newer robots that are designed to help reduce cycle time and generate greater productivity on the line have entered the market. Robots like the ABB IRB 6600, IRB 7600 and the Kuka KRC series can be characterized as “high dynamic,” since they are capable of generating high dynamic forces, particularly at an emergency stop. As a result, it is important that all tool exchange systems meet the stringent demands of these robots, both with direct mounting and more robust locking mechanisms.

These high dynamic robots are capable of generating moments and torques with tooling payloads in excess of half a ton, with accelerations equaling as much as 20 times the earth’s gravity – especially at emergency stops where tooling payloads may easily reach or exceed 10.0 KNm at the base. Therefore, tool changing systems and locking mechanisms must be able to safely handle such load conditions and be able to provide more than the commonly projected 5 million change cycles without compromising accuracy or performance.

Deceleration/Emergency Stops
The proper tool changer for these high dynamic robot models should be engineered for the “worst case” robot capability and not the actual loads generated within a specific tool.

According to ABB, in the extreme condition of full velocity, full reach, full payload and the worst case emergency stop, the IRB 6600 can generate a maximum of 10.0 KNm and the IRB 7600 can generate 14.0 KNm at the tool mounting surface. Similarly, the Kuka KRC series is capable of generating 9 to 16 G’s at emergency stop depending on the specific model at the tool mounting surface. These forces must be well below a tool changer’s failure point, providing inherent strength greater than the moments or torques generated by the robot even under the most adverse conditions.

In an effort to reduce weight, older tool changers have traditionally been designed to mount to the robot via aluminum adapter plates, typically with six M8 or M10 screws. These mounting methods, which can not withstand the forces generated by newer robots, can become a dangerous failure point. Any tool changer that uses an aluminum adapter plate mounting between the robot flange and the tool changer may not provide the full moment and torque transfer given by the manufacturer’s robot mounting pattern. The same holds true for the replicate robot mounting pattern on the tool side of the robot. Such applications constitute unsafe practice and may present significant liability issues. One possible solution to this problem is to replace the aluminum adapter plate with steel. However, the additional weight and increased inertia on the robot or tool mounting screws reduces the available payload capacity for that robot.

It is an essential safety requirement that the tooling equipment be capable of withstanding the maximum forces that can be generated by the specific robot rather than just absorbing initial tooling concepts. This will protect the line builder and end-user through the life of the line from inadvertent overloads caused by process changes or tooling modifications, especially with new lines, which generally see more changes of this type.

It is also important to note that all of these considerations not only apply to just the tool changer, but also to the tooling that is mounted to the tool changer.

Perceptions regarding the utilization of tool changer systems on robots have shifted from not only providing the utmost flexibility on the line, but also providing additional value by increasing throughput on lines that do or do not currently employ tool change systems. While it is clear that technological innovations within tool changers have brought greater efficiency and flexibility to automation, it does not imply that the basic safety issues can be ignored. To safeguard the protection of operators working on the line and the production output of the line itself requires redundancy – a series of safety measures with inherent checks and balances built into the tool change system. Work cell zones, sensors, interlocks, actuation valves, proper materials and sufficient mounting plates should all be considered and employed to minimize the risk posed to workers and productivity.