An Area-Guarding Puzzle

May 31, 2006
With the introduction of new machine-guarding technologies, design engineers are faced with the challenge of choosing the right technology to meet both production and safety needs.

The introduction of laser scanners over the last few years adds another exciting technology that can be used on the manufacturing floor for combined safety and productivity. The puzzle a design engineer faces is when to use one in place of other established area protection technologies such as safety mats and light curtains.

Area-guarding design starts with the same process that is applied to safeguard any machine: the safety review. If physical guarding is needed to lower risk but methods such as mechanical barriers, interlocked fences, siding gates and pull-back restraints are too restrictive for operations then area-guarding technologies will be necessary. To maintain safe operation, since there is no barrier from the hazard, the layout and size of the safety zone must be determined so that the machine can be safely stopped. This "safe minimum distance" takes into account how far the person can walk during the machine's stopping time.

How to Calculate the Safe Minimum Distance

The basis for the following safe minimum distance calculation is ANSI standard B11.19-2003. The ANSI formula consists of: Ds = K (Ts + Tc + Tr + Tspm) + Dpf.

Ds = The minimum safe distance.
K = The maximum speed at which an individual can approach the hazard, inches per second.
Ts = The total time that it takes, in seconds, for the hazardous motion to stop.
Tc = The response time, in seconds, of the machine control circuit to activate the machine's brake.
Tr = The response time, in seconds, of the safety mat system.
Tspm = The additional stopping time, in seconds, allowed by the stopping performance monitor before it detects stop time deterioration.
Dpf = The added distance, in inches, due to the depth penetration.

(Note that the European standard EN999 has three formulas that could be applied. They include floor, raised-surface and combined-technology versions.)

Caution: What Can't be Automated

Before we jump too far into the technologies, it is important to further clarify where these "physically borderless" technologies are inadequate. First of all, physical barriers are best when access to the protected area is very infrequent. In such a case, hard guards coupled with safety switches are a feasible choice.

Also, some environments are too severe to allow personnel close to the operations. This would include areas that have dangerous debris being ejected, gases, heat and even radiation. In such cases, a light beam or safety mat will not protect against the machine or its variant environment, specifically in regard to flying shrapnel and toxic, carcinogenic or flammable materials.

Requirements for Applications

In general, if the application requires repetitious activity that, in itself, can cause operator fatigue and unsafe "work-arounds" such as system bypasses, an area guard could be the right solution. Regardless of the technology selected, several important principles should be followed:

* First, the safe distance must be correctly calculated and used in the design. If any modifications are made after the installation, the original safety measures should be re-evaluated. The technology should be audited and maintained at predefined intervals with written procedures. You also may consider stating these in the operator's job description. Also, be aware that the stopping time could be extended due to machine wear, especially in the case of mechanical brakes.

* Concerning the area protected, there should be no other points of entry that allow someone to be endangered. Since the protected area is invisible when using photo-electrics, a visible outline should be depicted. It could be either a painted line, a rail or a chain. Another measure is to post a warning sign as well.

* Lastly, the safety-reset switch should be placed outside the zone and the operator must be able to see the complete area before starting the apparatus. For added safety, you may want to consider using a keyed switch to restrict this potentially dangerous activity further.

If your application fits the requirements of the last two sections, you can now evaluate which electronic area-guarding technology will work best for you. In each case, we generally discuss the technology, provide installation details, state distinct benefits and suggest industrial fits.

Safety Mats

Since the 1950s, automatic door mats have used in numerous commercial and industrial applications around the world. Surprisingly, the first safety mats were not introduced until 30 years later for industrial safety. They now are a very practical solution for many applications.

General Technology

The safety mat is a simple, normally open switch. When a specified minimum weight (66 pounds by ANSI B11.19-2003) is applied to the safety mat, the "switch" closes. This sends a signal to the safety mat controller, which sends a stop signal to the guarded machine. If you have any malfunctions such as broken wires or shorted plates, the machine will not operate.

When the safety mat is not exposed to sufficient actuating force, the signals are unimpaired. The output relays in the controller are energized, permitting the guarded machine to run. The mat itself typically is rated for more than a million actuations.

If you have a large area, you may want to interconnect multiple mats with joining trim. Mats are available in a wide variety of standard sizes. Custom shapes can frequently be made to order.

Installation Details

Although mats are simplistic devices, they do require proper installation procedures. First, the surface on which the safety mat(s) will be placed should be flat, smooth and free of debris. After the mat is in place, you should use care in routing the mat cables to prevent damaging the insulation or cutting/shorting the internal wires. Lastly, you should ensure perimeter trim is used to support and tie multiple mats together.

Distinct Benefits

Safety mats have several key benefits that should be highlighted in their evaluation. For starters, they will work in many areas and are an alternative to light curtains in areas that may limit optical sensing such as mists, dust and smoke. Also, they generally are resistant to cutting oils, salts, welding splashes, shavings, grinding particles, water, mineral acids, organic acids, alcohols, aldehydes, caustics and petroleum products.

Chemical resistance is dependent on factors such as time, temperature and concentration. Confirm mat chemical resistance with the manufacturer prior to purchase.

Once properly installed, mats require no maintenance other than the occasional sweeping and test procedure. They also can be used in conjunction with other safety equipment guarding a machine or work cell, such as opto-electric devices.

Industrial Fits

The resulting potential applications for safety mats include welding robots, assembly machinery, material handling, palletizers, packaging machinery, presses, robotic work cells, food processing, wood processing and automated assembly equipment.

Light Curtains

Light curtains have been with us since the 1950s. They employ advanced methods utilizing harmless infrared beams to protect an area. They also are called light screens, optical guards and presence-sensing devices,

To protect an area, light curtains are laid horizontally so that they will detect an object anywhere in the safety area. They also can be used in perimeter and point-of-operation applications.

General Technology

Light curtains utilize photoelectric transmitters to project an array of synchronized, parallel infrared light beams to a receiver unit. When an object blocks one or more beams, the light curtain controller sends a stop signal to the guarded machine.

Most vendors offer a range of lengths, resolutions and environmental-hardening options.

Installation Details

Utilizing a manufacturer's mounting kit, light curtain installation is relatively straightforward. In regard to configuration, one unique option you may want to consider is muting.

Muting allows the complete bypass or disabling of the light curtain's sensing field to the protected machine during the non-hazardous portion of a machine cycle. Muting will allow entry and exit to and from the protected area whenever the hazard is not present, such as when used with conveyors in material-handling applications.

Distinct Benefits

When compared to the other technologies, benefits gained by light curtains include the fact they can be used across wide areas (a 20-meter range is not uncommon). If you are already using them for perimeter and point-of-hazard installations, then you can lower your training and spares requirement by using them for area protection as well.

Light curtains also can be designed for use in explosive environments if you buy the environmentally hardened versions. They also have the highest resolution capability.

Industrial Fits

The resulting potential applications for light curtains include robotic and transfer areas, assembly machinery, material-handling, packaging machinery, punches, presses, robototic work cells, large weaving machines, food processing, automated assembly equipment, palletizing areas, power presses, thermo-forming equipment, stone stacking and wood processing.

Laser Scanners

Among the safety sensor products, the highest market growth rate is forecasted for laser scanners. This is a relatively new type of product that is expected to provide a more cost-effective method of area guarding, particularly in "machine cell" factory floor applications.

Laser scanners offer optimized detection of the lower extremities of personnel entering a hazardous area from the perimeter. It is not intended to be used for the detection of hands and fingers.

General Technology

Safety laser scanners function by using a "time-of-flight" measurement to determine distance. First, the scanner emits a light pulse. The light then hits the first object in its path and is reflected back to the scanner. Then the scanner compares the distance against the known size of its safety zone. If the scanner senses an intrusion into the safety zone, it sends a stop signal to the guarded machinery. This sequence is repeated as the optical assembly rotates, scanning the guarded area over and over.

Installation Details

The laser scanner uses a non-contact monitoring of a freely programmable area. Since separate reflectors are not required, installation is simplified due to the sender and receiver being located in one housing.

A scanner is typically mounted at the floor level for fixed applications. For an automatic guided vehicle (AGV), scanners can be mounted on both the front and back for both vehicular movement directions.

Distinct Features

One unique benefit of scanners is that, in addition to a safety zone, a warning zone can be programmed into the unit for annunciation of safety zone encroachment. This complements the irregular shapes (circles, rectangles, polygons and self-learned) that can be configured.

Many manufacturers also have used scanners as replacement of bumpers on AGVs, which are gaining popularity on the manufacturing floor. Along with this, its small size makes it easily adapted to a wide range of applications. If you also consider its advanced diagnostics and interface to communication buses, you will understand why its popularity is increasing at such a high rate.

Industrial Fits

The resulting potential applications for safety scanners include automatic guided vehicles, assembly and production lines, automatic die cart changes for inline presses, back-step protection, cranes, crossings in production plants, forklift trucks, packaging machines, presses, service robots, transfer shuttles, tube benders, automated weld cells lines, work stations and loading stations.

Comparison of Area-Guarding Technologies

Now, having covered the background, installation, benefits and industrial fits of each technology, which one fits your application? In an attempt to answer this question, Table 1 lists "side-by-side" ratings across 15 categories.

The key elements of electronic area safety protection devices were concisely reviewed after starting with the minimum requirements. The devices included are safety mats, light curtains and laser scanners. For each of these technologies, we investigated their background, installation requirements, key benefits and industrial applicability.

To further aid selection, a final comparison table that listed 15 key properties and ranked the technologies "side-by-side" was developed.

Of the results, only two or three of the listed properties will potentially dictate which technique to use. If they are key criteria, then that technology must be selected. If not, preferential ratings of each criteria and then summation should hopefully clarify your decision process.

Generally speaking, mats were found to be best for their simplicity and ruggedness. Curtains are noted for their support of large or high-resolution areas. Scanners were best-suited for stationary, irregular shaped areas or mobile applications that require warning, selectable and configurable zones.

If you have thoroughly reviewed your area application requirements, this table should help rectify your area-guarding puzzle.

Be safe out there!

Joe Lazzara is president and CEO and Russ Wood is application engineering manager for Scientific Technologies Inc., a leading supplier of machine safeguarding products in the United States headquartered in Fremont, Calif.. For more information, visit or call (800) 479-3658.

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