MRO Today - Light curtains: It's curtains for you

It's curtains for you
Light curtains and other machine guarding equipment not only address safety concerns, but improve productivity

by John Stroup

Machine safeguarding technology and business has grown substantially in recent years.

Companies purchase machine safeguards for three reasons:
1) To prevent injuries.
2) For legal and financial issues tied to reason No. 1. Companies want to avoid costly civil and criminal penalties that occur after someone gets hurt.
3) Profit margins. Machine safeguarding equipment can actually help your company cut bad costs out of the production process.

More than accident avoidance
Protecting employees from injury is the most important aspect of any safety system. As machine safeguarding technology has developed and matured, though, companies have started to recognize that the benefits of such equipment go beyond accident and injury avoidance.

Safety technology improves machine productivity in two ways.

The first way is protection. While it is true that safety equipment protects people from machines, it is also true that it protects machines from people.

By guarding against improper usage practices, this reduces machine and product damage. Such damage reductions lead to cost reductions associated with repair, scrap, rework, lost production, overtime and additional training.

For a typical manufacturing business, the costs associated with the defect and repair process account for 30 to 40 percent of sales. Reducing or eliminating the opportunity for machine and product damage works to limit the cost of poor quality.

Second, adding “intelligence” (configuration abilities with names like “floating blanking” and “channel select”) to safeguarding equipment lets machine operators adapt to working conditions or save steps in the production cycle.

Cut cycle time, be more productive
Safeguarding systems take many forms, from mechanical safety interlocks, pressure sensitive mats, acoustic systems and light curtains. Adding intelligence to safety light curtains creates a guarding system that reduces cycle time and boosts productivity. Here’s how:

A safety light curtain contains a series of light sources and receivers mounted in pairs. When the light beam between one or more pairs of transmitters and receivers is broken, a stop signal is sent to the guarded machine. By adding intelligence to a light curtain, users can configure the curtain to work only in a variety of time (sequence) and space configurations. This simplifies and speeds machine actions.

One example of timing is called “muting.” This lets an operator approach a machine when it is in a safe position (without power), and a safety system is not necessary. At this point, an operator can place a part into a punch press or palletizer, back away and cycle the machine. When the machine is powered, the light curtain is activated. The machine stops if the light beams are broken.

From this point it becomes a straightforward development to configure a light curtain to incorporate Presence Sensing Device Initiation, or PSDI.

A light curtain can work in coordination with the operation of a machine. An operator can place a workpiece in the machine and back away. The machine waits for a pre-set time and then cycles if the beams are not broken again. The curtain senses when the part is removed and another part is then positioned for processing.

Again, when a certain pre-set time has passed, the machine automatically cycles. If the beam is broken, the machine does not operate. PSDI allows a machine process to be automated and still be safe.

PSDI removes a step in a machine cycle by eliminating the time it takes to activate a two-hand control to start the machine. This can remove as much as one second in a two- or three-second operation.

PSDI can increase production up to 45 percent by eliminating process steps. It also reduces operator fatigue and associated physical problems.

Space considerations
In some cases, it is appropriate to tell the safety light curtain to ignore situations when either certain light beams, or one or two light beams, are interrupted.

This is called channel select and floating blanking. This eliminates the need for a special mechanical guard for the unprotected area, and having to work with that guard. You may need to adjust the safe mounting distance of the light curtain, which is a function of its ability to detect objects of a certain size.

With channel select, you can configure the curtain to ignore a certain fixed section of beams when the machine, or, for instance, a long piece of feed stock, extends beyond the curtain’s plane. You may want to use two sets of light curtains framed around the plane of entry at right angles to minimize the unprotected area.

Floating blanking is the ability to configure a light curtain to ignore instances where one or more beams are broken. Use includes cases where a workpiece “floats” or moves through a light curtain’s detection zone during the machine cycle, such as bending a long pipe.

Considerations for using safety light curtains
Before using any safety system, it is necessary to use a formal process of risk assessment to determine suitability. A misapplied safety light guard can give an operator a false sense of security.

Light curtains prevent injuries from a machine but they don’t prevent someone from passing through the plain of light beams. Only use light curtains on a machine that can stop anywhere in its operating cycle. Do not use a curtain on any machine with inadequate stop controls, or one that has a non-repeatable stopping time.

Also, light curtains don’t prevent objects from “flying” off a machine. Don’t use them to protect against environmental hazards such as gases or hot surfaces.

Use a hard-guard safety device to prevent ejection of hard objects or release of hazardous materials. Since a light curtain depends on sensing a beam of light, it should not be used in smoky or dusty environments.

John Stroup is vice president of marketing at Scientific Technologies Inc., a California-based manufacturer of machine guarding equipment.

This article appeared in the October/November 1999 issue of MRO Today magazine. Copyright, 1999.