Safety is the number one responsibility of every manufacturer. The most challenging applications to ensure safety are those where personnel are required to work with potentially hazardous operations such as machining, welding, movement of a robot arm, etc. The best safety device is the one that offers maximum safety with minimum impact on machine operations at the lowest possible cost. Figure 1 provides a flow chart that will help in selecting appropriate safety device technologies for typical manufacturing applications. The chart organizes safety devices into, input – devices that capture information from the machine to determine if it can run, logic – devices that make safety-related decisions, and output – devices that interact with equipment in order to ensure the safety of personnel. This chart is designed to provide general guidelines; however, all device selections should be confirmed by qualified safety experts.
Input devices
The flow chart leads the reader through a series of questions in order to select the right safety equipment for primary guarding applications to shut equipment down. On many machines, removing power to the motor or actuator will not immediately stop the dangerous motion. Typical examples are, high inertia rotating machines, fast rotating machines and machines where high pressure needs to be released from pneumatic valves. Starting in the upper left of the input section, the flow chart asks: “Can the equipment or process be stopped immediately by an electrical signal?” If the answer is no, then the application calls for the use of movable guards with a guard locking interlock switch that closes a machine guard until power is isolated all the while keeping the guard door locked until the equipment is safe.
If the equipment or process can be stopped immediately, the next question asked: "Is there a need to prevent the operator from accessing the equipment until it has stopped?" This can prevent scrap or waste, if an area cannot be accessed until the process has stopped. The following question asks whether or not there is a need for frequent access into the hazardous area. If the answer is no, then the most economical solution is the use of movable guards interlocked with the power supply sot that power is removed from the actuators whenever the guard door is open. In this case, there is no need for the more stringent guard locking switches previously described. If frequent access is required, the use of presence sensing devices are recommended as they automatically sense the presence of an operator and do not require manual intervention.
The next question is: “Is the highest level of safety required?” If the answer is no, it is possible to improve access to the work area while still protecting employees. If there are no airborne particulates in the environment, safety laser scanners can be used to control access to the work cell by emitting infrared light beams that use a reflective principle to detect personnel, however, they are very sensitive and may be false tripped in certain environments.
Pressure sensitive safety mats provide another alternative for guarding the entrance to the work cell that are particularly useful in the presence of airborne particulates. A matrix of interconnected mats is laid around the entry area and an operator’s footstep will cause the mat control unit to send a stop signal to the equipment. Pressure sensitive mats can be used both in the entry and inside the safe area. Trim is used to hold the mat in place, protect wiring and provide a smooth ramped surface to prevent tripping over the mat.
If the highest level of safety is required, then light curtains can be used. When any of the light curtain’s beams are blocked, the light curtain control circuit sends a stop signal to the guarded machine. These devices are very versatile and can guard large areas, sometimes as large as 20 meters. On the other hand, perimeter access light curtains have a wider beam spacing and can guard areas up to 70 meters wide.
If the potential hazard requires the highest level of safety, however, protection of the operator’s arms, hands and fingers is not required, then perimeter guarding is recommended.
Another question, addressed in the upper right of the flow chart, is whether multiple emergency stops (e-stops) are required alongside the equipment. The need for multiple e-stops usually calls for trip cords. Trip cords, sometimes called rope or wire pulls, are typically cords of braided plastic-coated wire, installed across or around the points of hazard generated by rotating machinery, conveyor motion, etc. which, when pulled or cut (made slack) will cause the attached switch to generate an emergency stop signal removing power to the equipment.
The lower left area of the input section addresses the question of whether additional operator protection are needed during setup, programming or servicing of equipment. If the answer is yes, the solution may be enabling a grip switch that is used to prevent unexpected machine operation when workers perform maintenance or other tasks. The switch has three discernable positions and can be used to switch between normal operating mode and maintenance mode.
Logic devices
After input devices have been selected, the next step is to select safety device monitoring and control devices as shown in the logic section of the flow chart. If three devices or less need to be monitored then a safety monitoring relay that accepts inputs from a variety of safety devices such as e-stops, interlocks, safety mats and light curtains is a good fit. If ten or less devices need to be monitored, then look at a stand-alone safety programmable controller which can be quickly programmed to satisfy the complex safety control needs of small and mid-sized machines and is easily reconfigured to solve evolving machine set-up needs. If an EtherCAT network is being used, then an integrated safety controller with EtherCAT will provide a single communications system for control and safety information. Finally, if DeviceNET or stand-alone operation is required for up to 1024 I/O, then consider a safety network controller that provides safety logic operations, safety I/O control and a DeviceNet Safety protocol.
Output devices
In selecting output devices, the first question to consider is whether switching off voltage or current outside of the range of the logic device is required? If the answer is no and direct safe torque off (STO) shutdown of a servo amplifier or drive is not required then the equipment can be directly connected to the logic device. The STO function safely removes power to the motor within the servo amplifier. If direct STO shutdown is required, the right output device may be a compact frequency inverter that provides harmonized motor and machine control. If it is necessary to switch off voltage or current, less than 10 amps, outside the range of the logic device then a force guided relay with a sufficient amp rating is called for. When current is greater than 10 amps, a power contactor with mirror contacts can meet the challenge.
Conclusion
The recommendations provided this in this article should only be considered general recommendations for primary safeguarding applications. Before making a final decisions, consult with a recognized safety expert. Safety experts can provide detailed risk level identification services including risk reduction recommendations in accordance with recognized standards to bring machines or process lines into compliance with applicable regulatory requirements and specific ANSI, RIA, NFPA, NEC, CSA, EN, IEC, and ISO standards. They will engineer the required safeguarding system, design control circuitry and a guarding strategy appropriate to the identified risk level.
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