The Importance of Verification and Validation in Machine Safety
In this final part of our machine safety series, we’re focusing on the verification and validation stages of the machine safety lifecycle.
During a machine safety risk assessment (SRA), hazards and potential risks are evaluated and ways of mitigating those risks are determined. This information is used to formulate a safety requirements specification (SRS) that helps a designer understand how to build required safety measures into the system. But, how do you know if the machine was designed correctly to match the requirements of the SRA? How do you know if it was built to the proper specifications? Two major stages during the machine safety lifecycle that are often overlooked are verification and validation.
Verification
Verification determines if the design of the machine meets the requirements of the SRA and considers all details in the SRS. This step ensures that what is being built is safe and will not require a costly retrofit in the future.
In the verification process, you examine all elements of the SRS—safety functions, descriptions of the diagnostic covers, trigger events, response of the system, safe state of the system, and fault exclusions—to determine if the required performance level identified from the SRA was achieved and/or exceeded.
A schematic of machine components and a bill of material (BOM) is also needed to understand the design. Certain layouts of components can have hidden faults that drastically reduce the reliability of the safety system.
There are four key functions of the performance level that are being verified in this process:
Reliability of the components – the mean time between failing dangerously (MTTFd) and is the value used to calculate the performance level. Certified components are tested by a nationally recognized testing laboratory (NRTL) to determine the number of cycles the component can last. If you are using non-certified components, it might be difficult to determine their reliability, which adds more complexity to design and cost.
Structure – the architecture of the safety related functions of the control system. There are five different structures for control system design – B,1,2,3, or 4. With each category, the level of safety improves. A Category B system is very basic, like turning on a light switch. A Category 4 system is fully redundant and would be able to respond safely and efficiently during a fault event. For example, if a door interlock fails, it would be considered a Category 4 system.
Diagnostic coverage – the ability of the system to detect the state of a safety system including a fault. Are you able to detect when a component has failed?
Design – the accountability to identify common causes of failure and precautionary measures taken in the design phase. This ensures good engineering practices have been followed.
During this process, it’s also critical to consider the number of operations and time between cycles. For example, if you install a safety gate, how many times during a specific time frame will it be used? It’s important to design the system to match the number of operations it requires. If cycle time is low, it could result in greater mission times. Consider asking yourself, “how long is the safety function guaranteed to be safe?” Instead of cycling safety components, you can cycle non-safety components to extend the mission time.
Ideally, the verification process is done at a 60% design review, 75%, and at the final design. Using this schedule allows time to address issues that arise and evaluate any changes needed before installation. This will keep costs from inflating.
To complete the verification process, you need a certified safety engineer who has knowledge of industry standards ISO 13849-1-2. Verifications can be completed manually or using a tool like SISTEMA, a software program for the evaluation of safety-related machine controls. Similar to a risk assessment, you must have detailed documentation of the verification to prove the design meets the requirements.
Validation
After you have verified the machine was designed correctly, you need to determine if it was built correctly. This includes revisiting the SRS, checking all of the wires and programming, and actuating all safety functions to validate that they’re performing as required. In addition, this stage introduces faults into the system to see if the system responds as designed.
Validation is ideally completed during the factory acceptance testing (FAT). It’s easier to fix a machine while it’s still at the builder’s site, as opposed to when it’s on the facility floor. Due to long component lead times, staying on target with the verification and validation reviews is especially important.
The validation stage is the last part of the process before the machine is installed. Once installed, it’s more costly to make changes.
During a FAT, a good designer will include not only a FAT checklist, but also a safety validation checklist.
A safety validation checklist could include the following questions:
- Is the safety controller programmed correctly?
- Did you choose the right input cards and the right configuration for those cards?
- Are the safety functions wired according to the validated drawings?
- Did you swap out components due to longer lead times? If so, did you recalculate how those components could affect the system?
- Are the drives and relay configurations correct?
- Is the machine reaction time less than previously calculated?
- Do the safety functions react as intended?
- Do the safety functions continue to react as intended when a fault is introduced?
Return on Investment for Verification and Validation
The verification and validation stages are crucial steps in the machine safety lifecycle. Proper planning allows for a smooth, cost-effective installation and startup. Catching mistakes during the design phase saves time and money, as well as reduces complexity.
Gain peace of mind that your machines are built with safety in mind and perform as intended. Our certified safety engineers have the industry knowledge to guide you through the entire machine safety lifecycle. Contact us today.
