Safeguarding employees from workplace hazards not only is a moral and legal imperative; it's a sound business decision. According to the National Safety Council, employers spent an average of $34,000 for each disabling injury and $1.15 million for each employee death in the most recent year for which statistics have been released.
Of course, it's not a difficult engineering challenge to design effective safeguarding for virtually any type of machine. The real challenge is to provide guarding that will deliver high levels of safety while not impeding the productivity of the machine and while maximizing cycle time.
When correctly specified and applied, safety light curtains seamlessly integrate with the machine or other equipment and virtually disappear from the perspective of the machine operator. As such, when properly applied, safety light curtains provide a guarding solution that enables an operator to work unimpeded for increased productivity, coupled with transparent and reliable protection from the machine hazards.
Features, a la carte
Safety light curtains are designed to protect personnel working around moving machinery, and are offered in many different models with a variety of features to achieve transparent protection against a wide range of hazardous conditions.
A high percentage of machine-guarding applications that employ safety light curtains simply require that the light curtain send a stop signal whenever an appropriately sized object penetrates the guarded area while the machine is operating. The remaining applications, which are more advanced in nature, require that the light curtain allow certain pre-planned intrusions into its detection zone such as stock that is being fed into a punch press while still protecting the operator from the machine.
Historically speaking, safety light curtains have been offered with a specific set of features such as fixed blanking, floating blanking and muting to cover all of the potential needs a light curtain may be called to serve. This resulted in users purchasing more features than they actually needed for their application. That is changing as the next generation of safety light curtains that is coming to the market allows the programming of such features on an "a la carte" basis. This enables the user to specify exactly what he needs for a particular application, without having to bear the expense of superfluous features that have no value for the application.
Fixed Blanking The fixed blanking feature can be used to disable selected fixed areas in the light curtains sensing field by masking off various beams at fixed locations. This approach is used when stationary objects, such as tooling, fixtures, conveyors and work tables continuously obstruct a specific portion of the sensing field.
Floating Blanking Floating blanking allows objects of a controlled size and shape to pass through light curtain beams without triggering a machine stop. This approach is used to allow specific objects, such as hoses or machine components, to pass through the light curtain without tripping the safety outputs. The disabled beams are allowed to float through the sensing field.
This feature also can be used in conjunction with fixed blanking, permitting the programmed area to fluctuate in size.
Muting Muting, another common application, temporarily bypasses the protective function of a light curtain and is permitted only during the non-hazardous portion of the machine cycle. Muting can deliver significant improvements in productivity. For example, it can enable pallets containing parts to enter and leave a machine without stopping the machine, yet it immediately can stop the machine should an operator move his or her hand or body toward a hazardous area.
How It Works
These typical examples of muting applications (shown above) allow the safe exit of a full pallet from a palletizing machine where it has been loaded and shrink-wrapped by a robotic system. In order to maximize productivity while maintaining operator safety, the light curtain is configured to allow the pallet to exit the work cell without stopping the system.
Two through-beam sensors arranged in an x pattern are used as the muting inputs. The system is bi-directional, so the light curtain is successfully muted when the pallet approaches from the left or right. For example, sensor A is a dark-on sensor and sensor B is a light-on sensor. Add a dark-on sensor C to detect the pallet's direction of travel so that the light curtain only is muted when the pallet approaches from one specific direction. A bypass-allowed function can be added to enable supervisory personnel to momentarily override the machine stop signal when the sequence of processing operations does not occur in the correct order to allow an automatic transition to the muted state.
New enhancements to muting functionality make it possible to program the light curtain not merely to detect the presence or absence of an object, but to perform positive identification of that object. This technique can even profile the shape of the object to ensure that it matches, for example, the shape of the pallet used to feed parts into the machine. Using this technique, mute sensors are not necessary and it would be extremely difficult to "fool" the system into muting when it would not be safe to do so.
Avoiding Cross-Talk
An issue of significance to keep in mind is that when two or more light curtains are located in close proximity to one other, precautions should be taken to avoid having them interfere with each other. One simple precaution is to avoid mounting the light curtains so that their transmitters are oriented in the same direction. A superior orientation for multiple transmitters is back-to-back, which makes it nearly impossible for the transmitters to interfere with one another.
In cases where many light curtains are used in a small area, it may be difficult to orient them in such a manner that they cannot interfere with each other. In this situation, light curtains with optical scan codes can be used. The receivers in such light curtains are sensitive only to a transmitter that is set to the same scan code.
In most manufacturing operations, the two most important considerations are safety and productivity. The challenge is that improving one of these factors often has, or is at least perceived to have, a negative impact on the other.
There are a number of light curtain configuration options available today to optimize safety and productivity in many manufacturing operations.
It is difficult to address every possible situation or to fully explain every configuration option, so it's important for a machine builder or end user to consult his or her light curtain vendor to obtain full details on how to properly specify, integrate and use safety light curtains for truly transparent safeguarding for a specific application.
Sidebar: Light Curtains vs. Alternatives
How do light curtains compare to alternative machine guarding technologies?
Perhaps the most popular alternative is hard guarding with interlock switches. The greatest concern with interlock switches is the potential for an undetected failure that could leave the machine unguarded. This is because all mechanical devices are subject to failures and interlock switches generally are checked only when the switch is cycled, such as during scheduled maintenance.
Light curtains, on the other hand, have no moving parts and continuously perform self-diagnostic routines that ensure that they are operating properly. If their safety functionality should ever stop operating, they will fail safely by sending a stop signal to the guarded machine.
When compared to devices such as safety laser scanners that are designed to detect lower extremities such as feet and legs, light curtains can provide high resolution so that, for example, they can detect an object as small as a finger moving into the hazard zone. Light curtains also may provide faster response time than laser scanners, so they can be mounted closer to the machine hazard, which saves floor space.
Russ Wood is an application engineering manager with Scientific Technologies Inc. of Fremont, Calif. (www.sti.com). He can be reached at (800) 479-3658.