Safety sensors in automation

Intelligent and economical machine safety

Competitive advantage, higher productivity and flexibility in relation to item quantity – these are key requirements today for the mechanical engineering and processing industries. Dreams of the automated factory running without human agency have long been abandoned. In many fields the challenge today is that of adapting production processes in a flexible way to constantly changing design variants and batch sizes. Human beings and machines then work hand in hand, so to speak. In this field, safety technology that is both flexible and effective is an integral and indispensable component of modern plant design.

In particular, optoelectronic protective devices like safety light curtains, multibeam safety light barriers and safety laser scanners with new intelligent properties have a contribution to make in this area. Such devices ensure that safety standards are maintained at all times, without any hindrance to working procedures.

Safety light curtains with new integrated functions In addition to the straightforward switchoff function when a protective field is intruded on, a new generation of safety light barriers and multibeam safety light barriers offers a whole series of additional functions that hitherto were not possible, or else could only be managed by complex safety modules connected on the outgoing side or else by an additional PLC system for safety management. These new functions include the following:

• Startup/restart interlock

When the machine is switched on, and/or after intrusion on an active protective field, this prevents the machine starting up until such time as the protective field is clear and the start button provided for the purpose has been pressed and then released again.

• Device monitoring function

This carries out dynamic checking of the relays, contactors and valves, together with their positively driven feedback contacts, on the outgoing side of the safety component. If one of the components on the outgoing side of the safety chain breaks down, the hazardous movement will be switched off, and cannot be restarted again until the fault has been rectified.

• Fixed and floating blanking function

Safety light curtains with a blanking function can for example phase out material transport belts or tool components in the protective field of a safety light curtain, while still offering seamless protection. Teach-in procedures make it possible for the system to become familiar with the blanked out areas it controls quickly and reliably. If blanked out parts, which must extend from the transmitter to the receiver and cannot be allowed to cast any “shadow”, are removed from the protective field, the protective device switches itself off.

• Reduced resolution

This function comes in useful if relatively small parts, such as cables or cooling hoses, need to be blanked out of the protective field. If the safety distance is adjusted accordingly, the protective function will be maintained without restriction. The advantage of this is that, by contrast with the blanking function, the monitoring of components introduced to the protective field at their current positions is no longer necessary. These components may enter the protective field, but do not have to be there.

• 1-cycle, 2-cycle or multiple cycle machine controls

Also known as stroke control. In this case the safety light curtain is not just used as a protective device, in addition it contributes to the control cycle of the machines. The exact position of a workpiece can be precisely detected by an additional sensor, and can then form the basis of a cycle release signal, so as to reduce the risk of damage to the tool or the workpiece.

• Muting functions

These can be incorporated in safety light curtains or multibeam safety light curtains, so as to ensure that the flow of materials (e.g. at automated manufacturing cells or packaging stations) continues without interruption, while at the same time maintaining the protective functionality intact. The protective device will also take responsibility for the control of the muting display lamp, and in case of a malfunction in the muting sequence it ensures that the goods passing through the system will be safely removed from the muting zone.

• 4-sensor sequential muting

This is a useful option when units of identical size (like car kits in automotive manufacture) are continually entering or leaving manufacturing cells (see illustration 2). Controlled by four sensors, the override works both for forward and for backward movement. The determining factor for the blanking is the correct sequence of the muting sensors that are triggered at any given time.

• 2-sensor or 4-sensor parallel muting

This is always used in situations where there is limited space available in front of the station the goods are passing through. The determining factor for the blanking is the simultaneous triggering of the muting sensors (see illustrations 3 and 4).

• 3-sensor directional muting

This works in the same way as 2-sensor parallel muting (see illustration 5). But a third sensor first needs to be activated. This third sensor then determines the direction in which the muting function responds.

• Intelligent quick diagnosis

This functions on the basis of seven segmented displays, and can

for example reliably identify faults in the external circuitry that can

be rectified on site. It offers the additional possibility of bringing

up system data and the times when events occurred by way of an

optical interface.

Other functions such as the additional safety sensors (like safety door switches for securing the back areas of machines, for instance, or an optical interface for the parameterisation and management of settings, based on authorised password access) are now absolutely state-of-the-art technology for intelligent safety light curtains and multibeam safety light barriers.

Safety laser scanners with new fields of application

Like a radar, safety laser scanners poll the complete working area, with an angular monitoring range of 190° and a radius of several metres, with the help of continuous infrared impulses. By measuring the time taken and the angle of the returning impulses that are reflected from any bodies they meet in their path, the system is able to register a “map” of the entire area. So the exact position of an object can be located at any time. If the position lies within an area that has been defined when the equipment is configured – known as the protective field – the laser scanner will switch off the safetyrelated transistor outputs (OSSDs).

Two classic areas of application have been successfully taken over by safety laser scanners in recent years, for the most part with protective fields that are horizontally defined.

• Stationary application

This is designed to secure the area in front of a machine with a level protective field customized to actual conditions, possibly supplemented with a warning field outside it. When this is breached, the warning field can give off a warning signal before intrusion on the protective field occurs, with the resulting STOP command that brings any hazardous movements to a halt (see illustration 6).

• Mobile application

This serves for safety measures in connection with automated floor conveyance systems (see illustration 7). Switchable and graduated protective fields, which will be activated in dependence on the current situation, make it possible to adapt the safety area to the direction of travel and speed of the vehicle. Additional monitoring on the basis of the warning fields can also delivery a preliminary warning, which can step down the velocity of the vehicle and so as a rule avoid the sharp braking that would be necessitated by an Emergency Stop.

New safety solutions with laser scanners, using for the most part vertically defined protective fields, have now also become a reality:

• Entry control

This provides control of access to hazardous zones (see illustration 8). Essential features of this application are the extremely rapid response time of 80 ms, which is all it takes for the system to detect unmistakably a person passing through the field, and the continuous polling of the doorframe and floor area situated opposite the laser scanner as a reference contour. If the appropriate feedback is not provided, as may be the case if the laser scanner has been maladjusted, the protective device will switch off the hazardous movement just as it would in response to the entry of a person into the frame.

• Protecting hazardous zones

This offers hand/arm resolution for Category 3 applications (see illustration 9). Hand recognition calls for a 30 mm resolution in the optical protective device at all points of the protective field, arm recognition calls for a resolution of 40 mm. In view of the systemdependent divergent beam segments, these high resolution levels can be provided by safety laser scanners only in a close-up area of up to around 2 metres. For most such applications, however, this is perfectly adequate. As with entry control, when safety laser scanners are used to protect a hazardous zone the system is required to poll a reference contour along the limits of the protective field situate dopposite. This has advantages as compared with traditional safety light curtains when the protective field offers a complex structure, as well as through the option of switching protective fields, as for instance in a situation involving machines with two adjacent goods input areas.

Connection technology and integration

The correct integration of the safety technology with the machine controls in keeping with industrial standards is a prerequisite for the correct performance of the failsafe function. The connection of the safety sensors will vary in dependence on the complexity of the machine or system, either being based on conventional means with multi-channel switching outputs and safety relays, or in more recent years sometimes also involving safety bus systems.

Future outlook

Generally speaking we can distinguish a trend towards decentralised control strategies – that is to say, people are moving away from massive central PLCs and the centralised switch cabinets associated with such systems. Safety technology will here constitute an integral component. The same holds good for industrial communications.

Here we find users tending to prefer Ethernet-based systems like PROFINET or Ethernet/IP. At the lowest field level AS-Interface has now succeeded in establishing itself as a de facto standard for the networking of binary sensors and actuators, seeing that this bus system – supplemented with a specially developed type 4 ASi safety monitor – is also capable of transporting safety-relevant signals unerringly. Suitable gateways can create connections to almost all higher-level communication systems, so serving to protect investment.