Machinery safety advances

Ian Murgatroyd of Pilz Automation Technology explains how today’s machinery safety technologies can deliver benefits to machine builders and end users:

Corby, UK—Technological advances over the last 20 years have made a huge difference to the way safety is implemented on machines. Today’s machinery safety technologies are bringing significant 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.

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. However, traditional electromechanical safety relays are limited in their functionality and diagnostics, so many companies are now using relays that perform various 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 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 configurable controllers, 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?

New 3-D vision-based safety technology can replace the light curtain and some of the physical guarding at lower overall cost and with less disruption to production.

Meanwhile, 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.