Ian Murgatroyd of Pilz Automation Technology explains how today's machinery safety technologies are bringing benefits to machine builders and end users.
Technological advances over the last twenty years have made a huge difference to the way safety is implemented on machines. Today's machinery safety technologies are bringing significant additional benefits to machine builders and end users alike.
Before considering the benefits, however, it is worth analysing what it is that machine builders and end users are seeking from machinery safety. Obviously they want a safe machine and one that complies with current safety standards and regulations, but they also want ease of set-up and reliability. Machine builders and system integrators are keen to have diagnostics that can help them when they are commissioning the machine, and end users want diagnostics so they can identify and rectify faults as fast as possible. Although it is an extreme example, a modern car plant produces one car every 48 seconds, or 75 per hour, so you can see why the potential cost of lost production is massive.
Fortunately modern safety technologies can help to achieve these goals, whether we are talking about an automotive production line or a machine on a smaller scale. Starting at the lower end, simple machines still tend to use one or two safety relays and guard interlocks, emergency stop switches, safety light curtains, and so on. Having said that, traditional electromechanical safety relays are limited in their functionality and diagnostics, so many companies are now using Pilz PNOZsigma relays that are capable of performing one of several different functions. This makes circuit design, ordering and stockholding easier for machine builders, plus the LEDs aid fault-finding during commissioning and operation.
As the complexity of a machine increases, the complexity of the safety-related control system also increases. Once you need more than a few safety relays, the benefits of a configurable controller - such as the Pilz PNOZmulti - come into their own. Configuring the logic with drag-and-drop software is much easier than using relay logic, and the chance of wiring errors being made is greatly reduced. For series machines, the configuration can just be copied to all of the machines, and diagnostics are also far superior as well. Another thing to be aware of with medium-complexity machines is that they are more likely to require changes. With safety relays, this could require significant amounts of rewiring but, with PNOZmulti, all you have to do is connect the new safety sensors/actuators and make the necessary changes to the configuration.
Once you have a project the size of an automotive production line, you really need a programmable safety system (sometimes called a safety PLC) to handle the I/O and the logic, and probably a safety fieldbus for communications - such as SafetyBUS p or the forthcoming SafetyNET p.
Having considered the monitoring and control of the safety-related functions, we can now look at how safety sensor technologies can help machine builders and end users. In most cases safety sensors can be used across the board, on machines of all sizes and complexities. For example, whatever the machine, tongue-actuated guard switches or interlocks are still appropriate for physical guards that are opened on an occasional basis, while guards that are opened more frequently will benefit from non-contact magnetic switches. And if you need higher integrity, there are coded non-contact switches available.
Light curtains are very popular, but you have to remember that they are not suitable for something like a CNC machine tool where parts might be ejected at high speed should the tool or workpiece break. Light curtains are also limited to the extent that they can only guard a flat plane. Mirrors can be used to 'bend' the plane around a corner, but setting up the light curtain and integrating it with physical guarding is time-consuming and, consequently, expensive.
Light curtains can also be the source of problems if the machine is modified. Consider the example of a machine that is upgraded to run faster in order to increase throughput. If it runs faster, the time it takes to stop will most likely also be longer. This might mean that the safety light curtain has to be moved further away, but this will also require the physical guarding to be modified. And what if the light curtain has to be moved so far back that it encroaches on the walkway between the machines? Pilz recently had a customer who found himself in a similar situation and he was considering replacing the light curtain and some of the physical guarding with a Pilz SafetyEYE 3D vision-based safety system, as this would be cheaper and easier overall, and could be done with less disruption to production.
SafetyEYE is the latest safety technology to have come to the market and is unlike any other safety product currently available. The nearest thing you could compare it to is a safety light curtain or a laser area scanner, but both of these just monitor a flat plane and, after someone has broken the beam, there is no way of telling which side of the beam they are unless you use another safety sensor. SafetyEYE, on the other hand, monitors a three-dimensional volume - which is configured in software - and the system therefore knows when the hazardous zone (or the outer warning zone) is being breached. But what you have to bear in mind with both light curtains and scanners is the hidden costs associated with the physical guarding that you still need, plus the substantial cost of setting-up and maintaining the system. Engineers at Pilz in Germany have estimated that a SafetyEYE system can be 70 per cent cheaper to install and maintain.
Something else that SafetyEYE can do is perform standard logic functions. For example, if SafetyEYE is being used to safeguard a robot that is placing product in a collection bin, the system can also be used to monitor the level in the bin and signal when it needs emptying. In this type of application you effectively get the bin monitoring hardware 'for free', and you just need to configure the program using the software.
Indeed, there is a definite market demand for integrated safety and standard control functions. Pilz launched its 'Safety & Control' initiative three years ago, taking the view that safety controllers are becoming so sophisticated that in many cases they could absorb the standard control functions currently performed today by separate PLCs. By adopting this approach, machine builders, system integrators and end users can benefit from lower costs, reduced development time, closer integration of safety and standard control, and improved machine availability due to the standard control being handled by higher-integrity hardware and software.
Elsewhere in the industry, however, suppliers are starting to integrate a limited amount of safety functions within standard automation products. The most obvious way this is being done is with inverter and servo drives that include a 'safe torque off' function, which avoids the need to use contactors to cut the power to the drive in response to a safety-related 'stop' signal. In most cases this is to EN 954-1 Safety Category 3, but some suppliers offer a Category 4 stop and others can deliver more sophisticated safety functions such as monitoring speed or direction, or enabling the torque on the motor to be maintained.
The desire to integrate safety and standard controls all goes back to where we started: people want ease of set-up and improved uptime - which is partly achieved through the better diagnostics available via a more closely integrated control system.