FEATURE MACHINE SAFETY
A QUESTION OF STANDARDS Sticking to the speed limit
Would you ever feel safe driving a car without a speedometer? Of course not, and - just as with motor vehicles - machinery can have safe operating speeds that should be adhered to, says Dr Martin Kidman, safety s pecialist at SICK UK
requirements and makes recommendations for the design, development, integration and validation of adjustable-speed electrical Power Drive Systems (PDS). These are the systems that are suitable for use in safety-related applications in terms of their functional safety. The Performance Level of the speed
A
speed monitoring system can be essential for safe control of a
machine and to protect the operator. So how do we know what the permissible safe limits of operation should be? To answer these questions, we can turn to the relevant legislation. However, the plethora of directives and standards, combined with the very specific terminology used, can make it difficult. The Machinery Directive, like any other
directive, is long and complex, with many clauses and sub-clauses. It would be very difficult for someone to design a machine that complies with every clause. So the specifications, to cover each aspect of the design and manufacture of machines, are helped by agreeing standards. For the time being, the EU’s ‘harmonised’ standards give purchasers the confidence that a machine meets the requisite safety levels.
STANDARDS A list of standards for the Machinery Directive can be found in the freely available Official Journal of the EU, and each standard is one of three types. A-Type Standard relates to basic safety containing basic terminology, principles of design and general aspects that can be applied to all machinery.
B-Type Standard addresses a safety aspect, or protective device, that can be used in a wide range of machinery. C-Type Standards contain all safety
requirements for a specific machine or type of machine. If this standard exists, it
24 DECEMBER/JANUARY 2017 | ELECTRICAL ENGINEERING
has priority over the A-type or B-type. The relevant directives and standards
are usually added to the machine builder’s declaration of conformity, which has to accompany any machinery.
SPEED MONITORING Speed monitoring is covered by B and/or C-Type standards, depending on the machine in question. There are various machine speed parameters that could influence the risk of harm on a machine against identified limits. If the machine exceeds those limits then suitable measures are initiated. Thus, the monitoring of the speed becomes a safety function. Of course, there are many different types of machine; if a C- Standard exists for your machine, then you may find the speeds that have been agreed, though not always.
PERFORMANCE LEVEL/ SAFETY INTEGRITY LEVEL IEC 61800-5-2 defines the safety functions for drives. It specifies the
monitoring circuit may be defined in the C-Standard. If no C-Standard exists, ISO 13849 or IEC 62061 can be used. These both outline a method for determining the required safety performance level for a safety function. Each method can be used to determine the required performance level (PL), or the required Safety Integrity Level (SIL), respectively. Once the required safety PL has been
determined, the speed monitoring control system can be designed. The two ways that speed monitoring can be implemented are either externally, when the safety is implemented outside the drive and the speed controller requires feedback or, internally, where the safety feedback is inside the drive as an integral function. Either way, it is common to use rotary encoders and, as a general rule, when used with appropriate and relevant associated speed monitoring equipment, a single encoder can give you PLc/SIL1 and two encoders can achieve up to PLe/SIL3. There are also encoders designed specifically for safety, such as SICK’s DFS60S Pro (a safety-rated Sin/Cos encoder). If such a Sin/Cos encoder was used with a compatible safety controller, such as the SICK Flexi Soft, then this would provide you with a SRP/CS that achieves PLd/SIL2.
SICK UK
www.sick.com/gb/en
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