Embedded safety drives and networks: combating ineffective, over-cautious safety procedures
By Jarred Gibbs, Senior Offer Manager, Industry Business, Schneider Electric
Tuesday, 09 August, 2016
Manufacturing, oil and gas, mining, and energy networks are considered some of the most dangerous work environments, both in Australia and the world. In 2015, mining and manufacturing even ranked fourth and fifth on Safe Work Australia’s list of industries with the highest number of workplace fatalities.
Ensuring operator safety is always a company’s top priority and paramount concern. For this reason, safety does not equal productivity. But ever-increasing demands on outputs have forced companies into a difficult position balancing the two considerations. Traditionally, the functions of automated machine safety and control have been kept separate, resulting in inefficient and overly cautious safety operations that are detrimental to productivity.
Luckily, technological advances and the IIoT now enable safety functionality to be embedded within automation components such as variable speed drives (VSDs) and servo drives. The idea that safety is a costly burden has given way to a new perspective in which safety functionality goes beyond safeguarding people and protecting machinery.
Productivity versus safety — the old debate and the new solution
The challenges in this space are obvious. Safety must always trump production.
Since most industrial operations place a high emphasis on safety, the management of safety constraints is tightly controlled. That is to say, the ‘better safe than sorry’ approach often includes some built-in buffer that affects profitability. Since safety risk is seldom directly measured, most industrial plants assume that the risk is greater than it really is and operate in a more conservative manner than necessary.
A simple safety stop, for example, can shut down production for as long as it takes to identify the problem and restart the systems. Given today’s high-throughput production lines, even a relatively short period of downtime can be costly.
In the past, the functions of automated machine safety and control have been kept separate. Conventional wisdom held that the two systems should be physically and functionally isolated from each other. This was so safety would not be compromised by the same faulty condition that led to the control system’s producing a ‘dangerous’ situation in the first place.
Otherwise, some feared that integrating the safety automation and control automation systems would adversely affect the machinery. Operators were protected from hazardous situations by non-automation control electromechanical hardware such as contactors, safety relays and interlock circuits. The extra expense of OEMs building, and customers maintaining, two discrete systems was seen as an unavoidable cost of doing business.
The concept of embedding safety into processes and product flips these challenges on their head. If the safety constraint functions can be examined with more precision, the operation can be driven harder to increase profitability without compromising the true safety levels of the operation.
For this reason VSDs, and servo drives, with safety functionality embedded within automation components are booming in the Australian market. A single drive component capable of standard control functions as well as safety monitoring offers several advantages to OEMs and their end-user customers including improved productivity, reduced costs and Ethernet embedded safety.
These benefits allow for organisations to operate at a pace not possible with separate safety systems. By having safety embedded in technology there is less reason for overly cautious approaches that are guaranteed to slow a company down.
Embedded safety improves productivity
Embedded safety drives integrate safety functions that control the drive output to the electrical motor in response to safety-related events. These functions can either stop the drive without shutting off power to the motor entirely or control the speed of the drive. Embedded safety functionality means a drive does not have to ‘wait’ for a signal that there is an abnormal condition such as overspeed or a blockage. Three of the most common functions are STO (Safe Torque Off), SLS (Safe Limited Speed) and SS1 (Safe Stop 1), all of which allow a company to bypass overcaution in small but significant ways.
STO ensures that there will be no torque on the motor, so the machine is not able to move and harm operators or damage components. The electronic switching time is much faster than with electromechanical components in a conventional solution.
SLS slows down and then holds the motor at a predefined speed. It automatically kicks in again if this speed is exceeded. SLS protects machinery by ensuring that damage is not caused by machines running too fast. It also enables operators to reduce the speed of a machine to a level where they can safely intervene to correct an abnormal condition.
SS1 stops a motor by controlled braking, according to a predefined speed ramp. It checks that the motor has actually stopped (or reached a predefined minimum speed), then activates STO. This function brakes high-speed motors more quickly and safely than conventional electromechanical solutions. At the same time an automatic switchover to STO enhances operator safety.
A simple example is a woodworking machine. As cutting power to a big machine can take several minutes to stop its spindle, there is always the risk that the operator will try to interact with the machine too soon. The drive with embedded safety functionality cuts off the torque on the motor but does not shut off power altogether, enabling faster start-up. Downtime and potentially costly damages are reduced without compromising operator safety. Once operators are in close proximity to the moving parts of the machine, SLS allows operators to inspect, perform basic maintenance or remove blockages without shutting the machine down completely.
Embedded safety drives reduce costs
Another benefit of embedded safety drives is that organisations are investing in one component that handles both control and safety, rather than two devices. Although variable speed drives or servo drives with embedded safety are likely to cost more than drives solely used for control or for safety, the overall hardware cost savings can be as high as 30%.
Reducing the number of circuit breakers or contactors needed to achieve a safe stop is another way this cuts costs. The STO functionality on the drive enables the machine to be put into a safe mode automatically without the need for a breaker or contactor downstream. Conventional approaches require two circuit breakers or contactors in series (or a breaker and a monitoring device), so reliability is dependent on several components working in conjunction. Embedded safety drives achieve the same level of safety integrity inherently.
Cabling, which can account for 15% of the installation costs, is another saving consideration. A single embedded safety drive eliminates the cabling cost associated with connecting numerous devices. This can be a significant amount and is likely only to increase in line with material costs.
Most importantly, a safety system with fewer devices is likely to be more robust as generally, a simpler system is a safer system. Put simply, fewer devices and less cabling reduces the potential points for failure. With every additional device that is needed to construct the safety system, the overall performance and reliability are lessened. This is true regardless of how highly rated the individual components are.
Replacing hardware with embedded software, which is certified for specific conditions, increases the overall reliability index of the system. When a system segregates safety and control functions, companies must ensure control and safety are well coordinated. With embedded safety on the other hand, the function and the behaviour of the control are certified to be linked to this safety function — precluding a source of failure and allowing a company to move at a quicker pace without fear of danger.
Safety embedded over Ethernet
Modern industrial processes in Australia are employing ever larger, faster and more complex machinery. While more productive, today’s machines present more challenging safety conditions and call for more sophisticated safety control, especially as connectivity reaches new heights with the Industrial Internet of Things (IIoT).
Luckily, technological advancements and the IIoT enable safety devices to be networked as a cohesive system. Networking simplifies the integration of automation and safety control, offering all the operational and cost-saving advantages of a less complex system, as well as additional benefits.
A number of safety protocols are already used widely for networking discrete machine safety components. They all sit on top of industrial Ethernet variants that are used for networking of a broad range of industrial automation components. Technological advances have overcome the disadvantages of Ethernet communication in previous years, and today its low cost and high speed have established it as the most widely installed network technology. The compatibility of both the safety-based and control-based protocols means that integration to a single network is quick and straightforward.
Integration allows safety and control systems to ‘talk’ with each other and share data such as diagnostics, system status, alarms, events and other critical information. Safety over Ethernet simplifies safety system communications, reducing the need for multiple hardware components and their associated expense.
For example, hardwiring communications for a 5-axis machine would require terminal adapters at each drive. Ethernet-networked embedded safety drives would be fully operational as soon as a single standard RJ45 cable is connected — as opposed to a hardwired system, which would require six different wires to achieve the same level of functionality.
The commissioning process is also simplified with Ethernet-networked embedded safety drives. Safety parameters can be specified using standard function blocks in a programmable logic controller (PLC), which can then be connected via Ethernet to multiple drives. This streamlines an often complex architecture, as a single safety PLC can be used to control multiple parts of the production line.
Overall, this makes for a simpler system and improved productivity due to better synchronisation of modules. An example would be a machine where motors are virtually linked, like a packaging machine (in the case of servo drives) or load-sharing applications (in the case of VSDs), in which the wear and tear of the mechanical link among motors could be avoided. Embedded safety over Ethernet also benefits OEMs and their end-user customers during the certification process, saving time by fast-tracking processes.
Throwing overcaution to the wind
Balancing safety risk, production value, energy cost and material cost in a simultaneous manner is equivalent to solving a multiple objective optimisation problem. This process is challenging and time-consuming, but within reach of operators if they have access to the right tools and advice from reliable experts.
Machine process companies today must respond to pressure to improve their bottom line through increased productivity without sacrificing operator safety. Now is not the era for ineffectual safety procedures that hold companies back from achieving their production potential. While it is certain downtime will remain the nemesis of productivity, safety no longer has to be.
Embedded safety drives represent a new, forward-looking alternative to the conventional approach that allows safety to remain a company’s top priority while enhancing the efficiencies that drive success in competitive markets.
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