Dario Dallefrate, Safety Product Manager at Control Techniques, gives an overview of the machine safety market and the trend towards increasing safety technologies for original equipment manufacturers.
The Factory Act of 1844 was a landmark in industrial safety for British factory workers. Burgeoning industry and increased market competition had not been conducive to sufficient safety self-regulation by factory magnates. Rising casualties demanded action and the act of 1844, which extended to all textile factories, prohibited the cleaning of machines in motion and required secure fencing around significant moving apparatus.
From 1844 to today, health and safety legislation throughout most industrialised nations around the world rightfully protects workers and machinery is declared to meet associated safety standards and levels, of which Safety Integrity Level (SIL) and Performance Level (PL) are the most highly regarded. Both SIL and PL aim to gauge probability of dangerous failure, normally measured in probability of dangerous failure per hour (FPH).
The International Electrotechnical Commission (IEC) standard 61508 defines the four Safety Integrity Levels – the relative levels of risk reduction provided by a particular system – with SIL 4 as the most dependable and SIL 1 the least. Meanwhile, the International Organization for Standardization (ISO) and its standard 13849 references Performance Levels (PL), significant for their citation within the European Union’s Machinery Directive and their replacement of the previous EN 954 safety level descriptions.
While SIL and PL levels are similar in many ways, using slightly varying algorithms to achieve their calculations, a difference is that SIL is specifically used to classify electric, electronic and programmable safety devices like electric drives, whereas PL has typically been associated with a wider range of technology types, including hydraulics, pneumatics and electric technologies
The machine safety market, currently worth $2.4bn, is reportedly growing at 10% per year. It is largely centred around machinery automation as well as the metals, automotive and food & beverage industries. While safety legislation has increased, the market has also grown as a result of demand for new safety technologies which reduce machine downtime and therefore increase overall productivity.
In basic terms, increased productivity is achieved by safety devices which typically slow or stop a machine when required, for example during maintenance or to clear obstructions, as opposed to a complete power shutdown. Not only does a power shut-down slow the manufacturing process, thus reducing productivity, but it can also require maintenance to create a new production set-up. Furthermore, shutting off the power has the potential to damage the product – not to mention the capacity to damage the machine itself as a result of sudden power down.
A further factor towards matching safety and productivity is speed of configuration. OEMs are increasingly creating more flexible machines with safety features which allow faster set-up and therefore increased productivity. Fast configuration becomes increasingly important if product types in manufacture are regularly changed on a given machine.
Let’s now take a look at the safety technologies which are commonly deployed today, together with their advantages and disadvantages.
Safety relays act as simple on/off switches and therefore perform only a single function in a safety system, for example isolating an actuator which is responsible for moving or controlling a mechanism or part of a system.
Relays are still the most common safety product in the market place. The advantage of relays is that they are individually inexpensive plus they have a strong legacy and are ubiquitous throughout industry, meaning that they are relatively easy to supply and replace. Relays can also be the most cost effective solution; they are well suited for controlling a limited number of safety devices where the use of a safety controller for some applications can be unnecessary to requirements.
The primary drawback of relay-based safety is a result of its wired system. This solution reduces flexibility in upgrade or change of components as a result of greater reliance on wire connections, meaning increased time and difficulty for required changes.
The primary advantages of drive-based solutions – safety controllers integrated within a variable speed drive – are twofold: they reduce the number of components and wiring compared to safety relays and they offer a smaller machine footprint, especially compared to centralised/PLC-based systems. Specifically regarding the latter, drives are commonly located in close proximity to motors – the primary source of danger – so they can react quicker than PLC-based systems. As the reaction time is faster, safety barriers or other sensors can be mounted closer to the dangerous areas, therefore making for a smaller machine overall. Enabling closer safe access to dangerous areas also means potentially less movement required for an operator to and from their machine, which can result in a faster process and therefore increased throughput.
The intelligence now available in drive-based safety systems also means that safety devices such as light curtains can be connected directly to the drive. Previously, multiple safety relays and monitoring devices would have been required, especially for servo/motion-based applications. The advantages of the drive-based system and subsequent reduction in components and wiring are numerous: reduced cost, reduced complexity in machine design and reduced footprint.
In addition to simplified machine design, machine commissioning time can also be saved; drive-based safety functions can be pre-tested instead of relying on physical tests with relays, which would be required on the actual machine. Similarly, drive-based safety also makes the implementation of some machine operations faster and easier as these modes can be software programmed rather than hardwired.
Reduced component count also means lower maintenance requirements. Furthermore, on certain complex safety systems, higher integrity safety level components may be required to compensate for the failure potential of a large number of relays and monitors which constitute a complex safety system. A drive-based system can remove this need, therefore potentially further reducing costs.
Speed and ease of maintenance is also improved using a drive-based safety system thanks to the ability to collate and manage data. If a safety component is operated, for example to impede a machine’s throughput, the system can record this data to enable swift maintenance and potential minimisation of the issue in future.
Centralised controllers, including safety PLCs, share many of the same benefits of drive-based safety systems. Safety controllers however have greater ‘visibility’ over the complete machine and so therefore can be more flexible in their actions and configuration, whereas a drive generally has visibility over a specific area or components of the machine. There’s greater possibility for adding extra safety functionality components with a PLC and as all components within a safety system are likely to share the same programming platform, expansion or reconfiguration of the system could be faster, easier and more cost effective.
Disadvantages can include a larger machine build as the addition of a machine controller brings its own footprint, plus extra cost for the safety controller or PLC. Neither can the PLC generally achieve the close proximity to the motor, where a more localised drive-based system provides advantages in safety and productivity as previously described.
The market trend is moving increasingly towards ‘smart’ devices – drives and controllers – rather than relay-based safety technology. Drive and centralised controller safety devices share a roughly equal market share but drive-based control is growing at a faster rate. Market growth towards high technology safety is also supported by a perception of the association between high quality machines and high level safety standards. This serves as a key differentiator for OEMs who have adopted the safety technology to achieve a sufficiently high SIL or PL level, and a potential market barrier for those that don’t.