Process Improvement To help understand today’s safety market, it will help to
take a quick look at where and how safety networks came into play. Safety, in the not too distant past, was normally a totally separate system from machine control. A stand-alone control system performed safety functions, with its own sensors, con- trollers, and network communications. If a fault occurred, it was the safety system’s function to stop the machine and sound an alarm. Although the control system was perfectly capable of performing the same safety functions, the idea was to provide a redundant system in case the main control system failed. However, having a second control system became expensive and difficult to manage. In most cases, machine controls came from one vendor, while the safety system came from another. Tis added further complexity to engineering, integration, and aſtermarket support. Over time, consolidation of safety func- tions into the machine control systems began to evolve. Today, safety functions are in-
corporated into the machine control systems, using safety networks to bring sensor information to the control system. Safety PLCs are fully capable of performing both control and safety functions, while meeting the safety requirements of ANSI and IEC. Machine safety over a network is achieved with redundant or dual-channel systems that moni- tor for faults and prevent a restart when a fault occurs. And all of this occurs using a single wired channel for communica- tions, an architecture that is recognized and acknowledged in IEC 61508 and other standards. Tat standard states that redundancy within communications protocols is sufficient to meet the same levels of safety as dual-channel, hardwired systems.
SAFETY LEVEL SIL Level 4 SIL Level 3 SIL Level 2 SIL Level 1
IEC-61508 IEC-61508 is the current standard used in many companies
for functional safety of electrical, electronic, and program- mable electronic systems (PES). Tis standard covers PLC and CNC components installed in typical manufacturing systems found in automotive, and it defines Safety Integrity Levels (SIL) as a means of targeting measures adequate to achieve what is known as “Tolerable Risk.” Additionally, the standard provides example measures for abatement of hazards and covers the entire safety lifecycle including risk analysis, safety requirements and allocations, design, procurement, system build, system verification, installation and commission, valida- tion, operations, and maintenance modifications. Te key to this standard is that it defines both quantitative
and qualitative measures for the development and imple- mentation of a network safety system. Quantitative measures for safety are designed to predict the frequency of hardware failures and compare them with some tolerable risk target. If
82 Motorized Vehicle Manufacturing
the target is not satisfied, the design is revised until the target is met. Qualitative measures, on the other hand, are designed to minimize the occurrence of systematic failures (eg. soſt- ware) by applying a variety of defenses and design disciplines appropriate to the severity of the tolerable risk target. All of this has been developed into a Safety Integrity Level (SIL) measurement that is designed to provide clarity to the “risk” of a system. However, the SIL level determination is actually ap- plication specific and really has nothing to do with the safety system—it is the risk assessment which is used to identify the SIL level of the specific application.
Basic Safety Integrity Levels Tere are four basic levels of SIL in the machine safety
specification, and these levels are tied directly to the probabil- ity of a dangerous failure. Te levels are as follows:
PROBABILITY OF DANGEROUS FAILURE (per hour) 1 in 1 billion
1 in 100 million (highest level for most industrial applications) 1 in 10 million 1 in 1 million
Safety Integrity Level 1 (SIL1) is the lowest safety level
and therefore is the easiest to achieve, providing that ISO 9001 practices were applied throughout the design process. Functional Safety Capabilities must also be demonstrated within the design of the system. Functional Safety Capabilities are covered in IEC 61805, and there are two basic assessments that are required: an assessment of management procedures (similar to an ISO 9001 audit) and an assessment of the implementation of the procedures. Adequate competency for Functional Safety Capability includes the following factors: • Technology knowledge • Safety engineering knowledge • Legal/regulatory knowledge • A link between magnitude of consequences and rigor of competence
• A link between SIL and rigor of competence • A link between design novelty and rigor of competence • Relevance of previous experience • Relevance of qualifications • Need for training to be documented Safety Integrity Level 2 (SIL 2) is incrementally higher than
SIL 1, and still requires that ISO 9001 practices be applied. SIL 2 requires more review and testing, and therefore adds ad- ditional cost, however, it is not difficult to achieve. Safety Integrity Level 3 (SIL 3) involves a significantly
higher degree of effort and competence than is the case from SIL 1 to SIL 2. Development costs and time will be signifi-
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