Machine Safety Starts with Design
Learn how machine builders can design equipment with programmable safety to help streamline end users’ service procedures.
By Ian Murgatroyd, Product Marketing Manager, Rockwell Automation
New safety technologies, such as Safety programmable logic controllers (PLC), allow manufacturers to streamline their machine access strategies and improve their machine productivity by tailoring the operation of safety systems to the required task. This allows employees to more quickly diagnose, perform minor service on and restore machinery to production, which can yield major improvement to machine uptime.
Many contemporary safety designs use Safety PLCs to assist with streamlining service procedures. Like standard controllers and networks, Safety PLC systems are devices that can be programmed to function differently depending on the task the employee is trying to perform.
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A variety of techniques can be deployed to streamline service procedures and allow machine access for minor servicing functions. Some examples include:
• Improved diagnostics: Safety PLCs are solid-state systems that incorporate modern plant-floor networks. Diagnostic information, such as I/O channel faults in an E-Stop string, can be displayed for an operator on standard machine operator interface panels. This allows plant operators and support personnel to rapidly diagnose safety system related issues.
• Exclusive control: Safety PLCs can be designed to help minimise the risk during machine-servicing tasks by allowing exclusive control of the jog function.
While in jog mode, the Safety PLC can manage which guard doors can be opened for the maintenance task, limiting exposure to moving equipment from outside of the operator’s line of site, and initiate controlled and monitored motion when the appropriate jog controls are permitted.
The safety system may also monitor the jog speed to make sure the robot maintains a safe operating speed. To set up tooling the Safety PLC may remove power from robots, but leave power on to other tooling, potentially allowing the operator to enter the cell without the use of lockout/tagout (LOTO) (based on the risk assessment). To load parts, the Safety PLC can permit the operator limited access to the cell without stopping production, as long as the operator remains within a monitored zone.
• Monitored power control: Operators require multiple modes of operation to perform their job functions in a robot weld cell application. Using a Safety PLC, the system can be placed into a teach mode in which power will remain to the robot(s) as long as the operator maintains his grip on an enable pendant. In addition, the Safety PLC can help prevent hazardous motion by removing the hazardous energy that isn’t required for the maintenance task to be performed.
• Safe speed monitoring: With safe-speed monitoring functionality, manufacturers can reduce and monitor the speed of their applications so an operator can safely perform maintenance work without completely stopping the machine.
• Safe torque off: A safe-off feature disables the output of the servo drive, preventing the servo motor from generating the torque required to move the index table, while maintaining control power and allowing for a rapid restart. Using this system, an operator can do maintenance with power remaining to the servo, helping to reduce system restart time and wear on the motor and drive.
• Zone control: A machine or process is designed so that part of it, or a ‘zone’, can be shut down to allow operator access to the hazardous section for machine servicing purposes while other zones of the machine or process continue to operate. This improves machine productivity by allowing operators to access machines for repetitive tasks without completely stopping production.
A variety of techniques can be deployed to streamline service procedures and allow machine access for minor servicing functions |
In all of these examples, the machinery uptime is improved by helping the employee diagnose, perform service and quickly restore the machinery to production. Stated another way, productivity is improved by optimising equipment mean time to restore (MTTR). In financial terms, an MTTR improvement of 15 minutes per shift on a machine that generates EURO10,000 of revenue per shift can generate productivity improvements of EURO150,000 or more per year (in this case, two eight-hour shifts and 250 work days per year).
The key is risk assessment
The key to helping streamline service procedures is to perform a machinery risk assessment. Risk assessments provide manufacturers a process to:
1. Identify specific hazards on a machine.
2. Quantify the risk these hazards present to employees working around the machine.
3. Evaluate various risk mitigation practices that can reduce the risk to a tolerable level.
The risk assessment process should encompass all tasks and limits associated with interaction to a machine, including those performed by operations, maintenance, quality assurance, sanitation and others. In addition, the risk assessment process is part of the specification of the appropriate safety circuit required for the initial risk rating determined by the risk assessment team.
A variety of risk assessment standards are available to manufacturers and end- users looking to set up a risk assessment programme.
Next steps
A system design to streamline service procedures is most effective during the machine design process, when the basic machine operation and all mechanical, electrical and other elements of the equipment are being determined. Consider updating machinery specifications to request that machine builders take streamlining of service procedures into consideration when developing equipment proposals.
For applications in which equipment is already installed or being installed, the risk assessment process and resulting mitigation requirements can be completed to provide a workplace with reduced risks and with safety systems designed to help optimise productivity.
The concepts associated with designing machines for flexible minor servicing are well established and have been deployed in numerous safety applications. The time is now for manufacturers and OEMs to begin exploring how their design and safety practices can be updated to improve employee safety while improving equipment productivity.
For more information please visit: Rockwell Automation OEM Solutions www.rockwellautomation.com/go/tjoem, Allen-Bradley Safety PLCs www.rockwellautomation.com/go/tjsafetyplc
For more information, please e-mail us at: info_at@ra.rockwell.com with ref: Safety & Design
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