Laser Scanners for Work Cell Safety

Designing a safeguarding solution for work cells requires striking a balance between floor space, ease of installation, productivity, safety, and initial and ongoing maintenance costs. When determining the proper method of machine safeguarding, however, it is important that other occasional operations like setup, cleaning, maintenance and adjustments are also taken into account.

08/01/2009


Designing a safeguarding solution for work cells requires striking a balance between floor space, ease of installation, productivity, safety, and initial and ongoing maintenance costs. When determining the proper method of machine safeguarding, however, it is important that other occasional operations like setup, cleaning, maintenance and adjustments are also taken into account.

European laws and directives require the equipment manufacturer to provide the necessary safety equipment to be delivered with the machine. However, in the U.S., the ultimate responsibility for safeguarding falls on the end user company to provide its employees an environment free of hazards that are likely to cause harm or injury. When employees do not have to worry about getting hurt on the job, they ultimately are more productive.

Because there are many ways to safeguard a machine, a risk assessment should be performed first. This will identify the potential for injury at a machine, or the process of interacting with the equipment. Performing a risk assessment is particularly important, considering that complete isolation between a hazard and an operator is often not practical, especially in instances where increased productivity is used to save money and/or increase revenue.

The older and more common techniques of using hard guarding require special procedures when entering a hazardous area. Although hard guarding provides a good visual indication of the location of the hazard, many processes require frequent interaction at or near the hazardous motion and, therefore, do not make hard guarding practical.

How safety laser scanners work

Safety laser scanners operate on the principle of time-of-flight measurement. The scanner transmits an infrared laser beam on a single plane to form a detection area. When an object is detected, the light travels back to the laser scanner. The scanner uses the time between sending and receiving the laser beam to calculate the distance to the object.

There are two user-defined zone types: a safety zone (also called the protective zone or the protective field) and a warning zone (also known as the warning field). If someone enters the warning zone, the laser scanner provides an output that can be integrated with an audible alarm or a visual indicator. If someone violates the safety zone, a signal is generated to initiate a safety stop of the hazardous motion of the machine.

Safety laser scanners do not restrict access, but they do safely detect access when implemented with a control reliable safety circuit. Since the laser in almost all safety laser scanners is not visible to the naked eye, it is practical to provide a visual indication of the safeguarded area with tape and utilize the warning field to indicate by an audible or visual output when someone is nearing the hazard. Both these steps will help to minimize false trips by other workers or visitors who are not familiar with the location of the safety zone.

A laser scanner offers several advantages. First, because it is a non-contact method, it minimizes interference with the machine operation. Second, it typically mounts at the foot of the machine and out of the way of the equipment action, enabling full access to the work area.

Laser scanners are easy to configure, making it simple to make changes to work cell design. Additional benefits include diagnostics to reduce troubleshooting efforts; fewer wires running back to the controllers to reduce wiring problems; no mechanical cycling of components, thereby providing lower maintenance costs than electromechanical safety devices; it is harder to defeat or bypass a scanner, thus providing an ongoing safe working environment even around those highly repetitive activities where safety is sometimes forgotten.

Safe work cell design

Two main objectives of safeguarding a work cell are either to prevent access during dangerous machine action or to stop the dangerous machine motion when the area is accessed. As mentioned earlier, access prevention does not always meet a company's needs when frequent human interaction is necessary for operations like loading or unloading parts in a production operation.

The initiation of a stop, in an open-access design, is commonly done with a pair of vertically mounted safety light curtains, enabling a shorter safety distance and thus requiring fewer operator steps to get to the load/unload station. This action places the machine in a safe state where additional safety functions may be necessary to prevent unintentional restart of the hazardous motion. Traditionally, this is done by requiring the safety device to be manually reset.

The location of the reset button must be outside of the hazardous area and not reachable from inside the hazardous area. In addition, from the location of the reset button, the operator must have an unobstructed view of the entire dangerous area and make sure it is clear before the safety device is reset.

Some argue that an operator should not be relied upon to pay constant attention to important safety functions, like the frequent reset of the safety equipment after loading or unloading. That is where a safety laser scanner can be used to prevent the unintentional restart of the hazardous motion and avoid a permanent reset. So even if the reset button for the safety light curtain is pressed when someone else is in the hazardous area, the dangerous motion will not start.

If floor space is available, the safety laser scanner could be used for both the initiation of a stop and for the unintentional reset of the machine. Unlike a hard guarded entryway or a single pair of vertical light curtains requiring reset, the laser scanners are continually monitoring the safety at and around the work cell. Plus, the need to remember to press a reset button after each operation is eliminated, leading to improved productivity.

Safety laser scanners have become a permanent fixture in almost every factory automation facility. Their ease of configuration, flexible warning and safety zone layouts, multiple zones and reliability make them ideal for a wide range of applications—from mobile vehicles to robot welding cells.



Author Information

Brad Raven is a product manager for Safety Systems with SICK Inc., a global manufacturer of sensors, safety systems, and automatic identification products with North American headquarters in Minneapolis. A white paper entitled, “Guidelines for Safe Machinery” is available for free download from their Website.

Author Information

Brad Raven is a product manager for Safety Systems with SICK Inc., a global manufacturer of sensors, safety systems, and automatic identification products with North American headquarters in Minneapolis. A white paper entitled, “Guidelines for Safe Machinery” is available for free download from their Website.




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