Light curtains improve machine safety and productivity
Fig. 1. Safety light curtains come in a variety of sizes so they are adaptable to nearly any machine guarding application.; Fig. 2. The exact channel select feature of a flexible light curtain allows an operator to feed stock to a machine while protecting the balance of the work area. The use of flexible light curtains eliminates any "shadows" in the safety field, which can be a common problem with conventional single segment light curtains.; Fig. 3. "Floating blanking" allows a safety light curtain to work
By Russ Wood -- Plant Engineering, 2/1/2000
Safety light curtains use infrared light to detect the presence of an object or person. Used properly, safety light curtains improve a machine operator's work environment. In addition to safety, these devices aid in reducing operator motion, leading to improved machine productivity and operator ergonomics (Fig. 1).
As a noncontact control technology, light curtains deliver a versatile means of providing machine safety. A properly applied safety light curtain increases productivity by eliminating the extra motions associated with opening mechanical guard doors, resulting in reduced operator stress.
Proper use of a safety light curtain is achieved by first considering all aspects of an application. Before installing light curtains, engineers need to evaluate the machine's operation, as well as the curtain's features, response, and diagnostic capabilities.
Machine and safety monitoring
When using safety light curtains, engineers must monitor both the operations of the machine and the light curtain. To do this, the light curtain monitors a machine primary control element (MPCE). The MPCE, typically an electrically actuated device (other types include hydraulic and pneumatic), is the last element in a time sequence of circuits used to start or stop a machine's operation.
The light curtain monitors the action of the MPCE to make certain that it operates within a reasonable period of time (typically 300 milliseconds). If the MPCE does not perform as required, the light curtain enters an interlocked state. In this case, it requires that the problem be corrected prior to resetting the curtain. For safe design considerations, two MPCEs are used in a safety circuit to provide redundant machine control.
Modern safety light curtains are control-reliable devices that monitor themselves for correct operation. Should a user also wish to have his machine control system monitor the safety light curtain, a machine test signal (MTS) is used. An MTS allows a machine controller to simulate an interruption of the safety light curtain and monitor the response of the light curtain's output relays.
Applications
Some machines and operations present the user with difficult guarding situations. In these situations, either it is necessary for a portion of the machine to permanently block the sensing field of the light curtain, or a piece of equipment may need to move through the sensing field.
Because safety light curtains use a nonmechanical technology, relying on beams of light and sophisticated electronics, they can be easily adapted to specific situations. Several types of programming allow the curtains to adapt to these situations.
Exact channel select
When a fixed part of the machine or workpiece permanently blocks part of the safety light curtain sensing field, the device can be programmed to ignore that particular area. This approach is called "exact channel select" (Fig. 2). In this case, the safety light curtain is programmed to ignore a set of beams in a fixed location, simplifying what would be a difficult task for a mechanical guard.
Floating blanking
A similar concept applies to a beam blocked by a workpiece that extends through the sensing field of a safety light curtain and also moves up and down, breaking the sensing field in multiple locations. It is possible to program the safety light curtain to ignore the blocking of one or two beams. This capability is called "floating blanking" (Fig. 3) Here, the safety light curtain is programmed to not send a stop signal to the guarded machine when one or two beams are broken, to accommodate a moving work piece. It does, however, send a stop signal if three or more beams are broken.
How close a safety light curtain can be mounted to a point of operation hazard is determined (in part) by the size of the object it can detect. Use of either exact channel select or floating blanking reduces the light curtain's sensitivity. To compensate for this situation, it is necessary to adjust the safe mounting distance of the light curtain.
Muting
Another typical application is "muting." This term refers to a situation in which it is necessary for the machine operator to enter the guarded area of the machine to manually feed and remove a workpiece.
Upon a worker's entry, the light curtain controller suspends sending a stop signal when the machine is in the nonhazardous portion of its cycle.
Consider the installation
When installing a safety light curtain, it is necessary to consider the appropriate protected height and overall dimensions of the unit for the application. Light curtain mounting is critical; the need to make adjustments during installation and the use of different types of mounting brackets become part of the light curtain selection process. In addition, consider the need for remote functions when the operator is required to be distant from the machine.
In some cases, a light curtain must monitor more than one point of access. In this case, a segmented safety light curtain design allows a device to monitor two, three, or four points of entry around a machine.
Additional considerations
It may be necessary for an engineer to include indicator lights and diagnostic capabilities to inform an operator about the light curtain's status. This approach allows operators to determine if it is operating correctly and speeds troubleshooting.
Safety light curtains use infrared light to detect the presence of an object or person. Since this light cannot be seen, an indicator light to signal the alignment status of each individual beam is a desirable feature. A second set of lights can provide diagnostic information and identify operating conditions.
Reducing machine downtime affects productivity. In addition to diagnostics, consider how a safety light curtain should best be serviced and repaired. It is possible to use safety light curtains that have transmitters and receivers that can be changed out quickly. If a problem occurs in the light curtain that cannot be immediately resolved, it may be necessary to enlist the use of the manufacturer's 24-hr repair service.
-- Edited by Cheryl M. Firestone, Senior Editor, 630-320-7136, cfirestone@cahners.com
More info
The author is available to answer technical questions regarding this article. Mr. Wood can be contacted at 888-510-4357 ext. 5453 or by e-mail at russ_wood@sti.com .
