The concept of providing machinery safety with a light curtain is ostensibly simple. You erect a light grid around the machinery and any object passing through the grid will result in the machinery being immediately shut down, quickly enough to eliminate any risk of injury. There are, however, many crucial aspects to be considered if the light curtain is provide the level of safety assurance required.
A light curtain is built around a transmitting element and a receiving element, which are mounted opposite to one another. Pulses of infra-red light emitted by the LEDs in the transmitting element are picked up by corresponding photo-diodes in the a receiving element, with the two elements electrically synchronised. If any one or more of these abeams' are broken -- say by an operator passing through the grid -- then the system outputs a machine shut-down command, causing any potentially hazardous motion of the machine to be safely stopped.
When looking to build a safety system around a light curtain, the first thing to consider is whether optical protection is even suitable for use at all. For it to be so, the machine control must be able to be electrically influenced using the light curtain's semiconductor output, and it must be possible to immediately terminate or exit the hazardous state in each operating phase.
Further, there must be no danger of injury due to heat, radiation or from materials or components being flung out of the machine. If there is such danger, then either the optical system is not suitable, or the danger must be completely excluded by applying additional safety measures. Finally, ambient conditions must not have a negative impact on the effectiveness of the contactless, semiconductor-based protective equipment.
The light curtain option
If it is determined that an optical protection system is suitable for use, then a first consideration in actually implementing a safety light curtain is its area of coverage. In particular, it must not be possible to pass around, pass below or go behind the protective field. If this cannot be guaranteed, then additional protective devices must be used.
System resolution is also part of this equation. The resolution of the safety light curtain is defined as the minimum size of object that can be reliably and safely detected at any position in the protective field.
At one extreme, an application might require finger protection resolution, whilst at the other it might be permissible to keep the machine running until the whole body of an operator completely passes through into the danger zone. Resolution of the system is a function of the number of LEDs and corresponding photo-diodes in any given length of transmitting/receiving element.
The time taken to shut the machinery down must also be considered. The light curtain must be positioned such that any machine movement or motion that could potentially be hazardous is safely stopped before there is any potential for injury.
European standard EN999 defines a formula for calculating the minimum safety clearance between the light curtain and the hazardous location. This formula is:
S = (K x T) + C
here: S is the minimum clearance in millimetres, measured from the hazardous area to the protective field; K is a parameter in millimetres per second derived from the data associated with the velocity of approach of the body; T is the run-on time of the machinery (the time that elapses before the hazardous motion stops); and C is an additional clearance in millimetres which relates to how far a body could potentially penetrate the hazardous field before the protective device or equipment is triggered. The values of K and C depend upon the protective function required (such as hand/finger protection or more general access security), the system's resolution and the approach direction.
When securing hazardous areas using horizontally mounted light curtains, the height of the protective field may be a maximum of 1000mm. Further, if the height is greater than 300mm (or 200mm where children are present) then it is considered that it will be possible to crawl below the protective field, and this must be taken into account when evaluating risk of injury. The lowest permissible mounting height depends on the resolution of the light curtain, such that it can be guaranteed that a human leg or foot joint can be reliably detected.
While the criteria for their implementation are stringent, light curtains can also provide high levels of flexibility in setting up a safety system. Different blanking modes, for example, allow fixed objects permanently positioned in the light grid to be ignored unless they are removed, or allow specific moving objects (such as parts on a conveyor) to exit through the grid without triggering a shut down.
Gary Provis is Product Manager, Industrial PCs and Safety Systems with Siemens Automation and Drives