Machine safeguarding with optoelectronic sensors

How to choose the most appropriate equipment

STAMPING JOURNAL® FEBRUARY 2006

February 7, 2006

By:

Optoelectronic sensing devices safeguard machine access and prevent injuries related to hazardous machine motion. The ultimate goals are to prevent access to the hazard, eliminate the hazard before access is attained, and prevent the unintended operation of a machine.

When a metal forming machine is being designed, the potential safety risks must be analyzed and minimized. Some risks cannot be eliminated through design, however, so it is necessary to use safety devices. These devices safeguard operators and other individuals from residual hazards like crushing, shearing, cutting, snatching, clamping, trapping, perforating, puncturing, and shock.

In general, when an operator has to use a machine frequently and is exposed to the risk of hazardous motion, safeguarding devices should be used to prevent exposure to the hazard.

Safeguarding devices, when installed properly, prevent access to a hazard or detect the entry of personnel into a hazardous location. When an entry is detected, the safeguarding device, in conjunction with the control system, either prevents the initiation of hazardous motion or initiates an immediate stop of the hazardous motion, thus eliminating the existence of the hazard.

Optoelectronic sensing devices safeguard machine access and prevent injuries related to hazardous machine motion. The ultimate goals are to prevent access to the hazard, eliminate the hazard before access is attained, and prevent the unintended operation of a machine.

Optoelectronic sensors help reduce access time and eliminate the waiting associated with opening doors or hard guards. In general, they are simple to operate, and they help minimize or eliminate repetitive motions. Safety light curtains and safety scanners can provide protection for all individuals in and near the hazardous area, not just the operator.

Figure 1
A light curtain safeguards entry into an automotive manufacturing cell.

Safety Light Curtains

While the basic concept behind the light curtain has not changed since its introduction more than 50 years ago, the technology has kept pace with the industry's ever-changing demands. Special features and improved direction through regulations and national consensus standards help match a safety light curtain to its intended use.

A safety light curtain consists of at least two units: a sender and a receiver. The sender unit contains emitters that send infrared light beams toward the receiver unit. When the emitted beams reach and are registered by the receiver, the light curtain is operational and allows the motion of the machine or robot to occur.
Interruption of any beam in the safety light curtain generates a safety stop signal to the machine control circuit, which in turn should stop any hazardous motion or prevent the machine from initiating a start sequence. Figure 1shows a light curtain safeguarding entry into an automotive manufacturing cell.

Safety light curtains are designed with various types of functionality to fit different applications. Some are solid-state outputs, fixed blanking, floating blanking, external device monitoring (EDM), coded beams, PC-based configuration, self-documentation, and advanced diagnostics.

In some instances, a third box may be required to house a controller unit or to provide basic or advanced functions, solid-state interfacing for multiple-curtain applications, or relay functionality.

Regardless of the features or functions, safety light curtains are applied to safeguard hazardous applications that can be stopped electrically, quickly, and in any stage of the machine cycle.

Safety Laser Scanners

A laser scanner is an optical sensor designed for 2-D scanning of detection areas using infrared laser beams.

Laser scanners operate according to the principle of time-of-flight measurement. They emit very short light pulses, while an electronic timer captures the time it takes the light pulses to travel. When the light encounters an object, the light is reflected and received by the safety laser scanner. The scanner determines distances from the object during the time elapsed from emission to reception.

The scanner can be configured to safeguard areas of any shape, as well as multiple zones. It also can be reconfigured to handle different materials, depending on production needs. A scanner can be mounted horizontally, vertically, or at an angle and out of the way of the machine, helping to minimize the risk of damage and intrusion.

Vision Systems

New systems for press brake safeguarding use vision-based optoelectronic technology. The Safety Category 4 camera sensors help ensure safety while optimizing the metal folding process by providing multiple safeguarding modes that can be changed automatically while bending a complex part. These systems also optimize machine speed so that throughput is maximized without compromising operator safety. In addition, machine stops are minimized to prolong machine life uptime.

On a press brake, the vision system is mounted on the machine and travels with the ram. It monitors the hazardous area with a camera image and constantly evaluates the safety fields. These fields can be flexibly programmed based on the specific folding operation.

Figure 2
A vision-based system that is connected to the machine controls offers safeguarding during the fast downward movement of the press.

Installation and maintenance of a camera-based press brake safety system are relatively simple because of its built-in self-diagnostic tools. In essence, the camera-based system is able to report any need for adjustment, minimizing troubleshooting time and maximizing machine uptime. The alignment of the device takes less than five minutes, for example, because the camera system is able to tell the installer which direction to move the camera to achieve perfect alignment of the components. Software and hardware features help simplify configuration and alignment.

Figure 2shows an example of a vision-based system for press brake safeguarding. It offers safeguarding during the fast downward movement of the press, and it is connected to the machine controls. If the safety volume is infringed during the dangerous movement of the press, the device sends a signal to stop the machine.

Selecting the Appropriate Sensor

Several criteria need to be considered in selecting optoelectronic safeguarding equipment for a machine application. National consensus standards help companies that manufacture, integrate, or use machines define the tasks and hazards associated with their machines. The standards also help users perform risk estimation, determine a corresponding risk reduction strategy, and understand the safeguarding and safety circuit performance requirements for their application.

The fundamental question in implementing safety with optoelectronic sensors is which device to use for which machine. The following are guidelines for selecting the appropriate sensor for the machine and application at hand.

Identify and Quantify Machine Risk. To determine the suitability of a safety device for a machine and application, the machine risks must be identified and assessed. The following points need to be considered:

  • The dimensions of the safety zone that require safeguarding
  • The different points of access and any other hazards related to the machine and its use
  • The risk of machine initiation after workers pass through the safeguarding device and are in the hazardous area undetected

Depending on the machine, standards and technical reports outline the requirements for performing a risk assessment. The process of risk assessment must be completed initially during the design stage before installation, at final installation, during configuration, and each time the system configuration changes.
A number of methodologies for risk assessment may be consulted:

  • ANSI B11.TR3-2000.Risk Assess-ment and Risk Reduction — A Guide to Estimate, Evaluate and Reduce Risks Associated With Machine Tools is a technical report (not a standard) that outlines the process of performing risk assessment for the machine tools industry.

  • ANSI/RIA R15.06-1999.Indus-trial Robots and Robot Systems — Safety Requirements outlines safeguarding requirements associated with robot and robot system applications. In addition to risk assessment, this standard also outlines other machine safeguarding implementation requirements.

  • ISO 14121 1999 (formerly EN 1050) Safety of Machinery — Principles of Risk Assessmentis an international standard that outlines general risk assessment requirements.

In general, the first two steps of the risk assessment process are:

  1. Assume no safeguards are installed, and identify the tasks and associated hazards of the machine, robot, or robot system.

  2. Select the safeguards based on risk reduction and safeguard selection criteria.

Once these steps have been completed, safeguard performance and circuit performance must be defined. When suitable, engineering controls such as safety light curtains can be used to safeguard personnel. Circuit performance of the safeguarding system also should comply with applicable sections of applicable standards.

Circuit performance requirements include control reliability, single channel with monitoring, single channel, or simple. It is prudent to consult all applicable standards for the machine being evaluated when performing a risk assessment or implementing any type of machine safeguarding strategy.

Define the Safeguarding Method. Primary functions of safeguarding devices include causing the hazard to cease before access is attained and preventing the start of a machine when safety requirements are not met.

Point-of-operation safeguardsare designed to detect a finger or hand entering or existing in the safeguarded space. These safeguards generally are used for applications in which work is performed on the material or workpiece in close proximity to personnel.

Perimeter safeguardsare designed to detect a torso or body entering a safeguarded space or area. In perimeter safeguarding, the safety functions of the system must use a manual reset placed outside the protected area. This forces the operator to return to a safe area before reinitiating the robot or machine.

Area safeguardsfunction similarly to perimeter safeguards, but with the added function of sensing the presence of personnel inside the defined hazardous area. Area safeguards use a nonvertical (angular or horizontal) approach to detect personnel entering or within the hazardous area.

The ultimate goals for each configuration are the same: to prevent access to the hazard, cause the hazard to cease to exist before access is attained, and prevent the unintended operation of a machine or robot.

Proper operation of a safeguard should not require a specific conscious action by plant personnel. Also noteworthy is that often one type of safeguarding is not suitable or sufficient for every machine or robotic application. For example, safety light curtains should not be used alone without fencing in applications that require containment of parts or tooling.

Calculate the Safety Distance. After the safeguarding method has been determined, the minimum safety distance requirement must be considered.

The theory behind minimum safety distance is to allow sufficient time for a hazard to cease before personnel are exposed to any danger. Components of the minimum distance requirement are based on the average speed a
person would travel per second, the overall response time of the system, and how far a person penetrates the area before he is detected (depth of penetration).

The minimum safety distance is described in Section 10.4.3 of ANSI/RIA R15.06-1999, as well as in the informative Appendix B of the standard. Derivatives of this formula also are presented in OSHA Regulations in Title 29 of the Code of Federal Regulations Part 1910 Subpart O (1910.212 to 1910.217).

Based on the ANSI/RIA R15.06-1999 definition, the minimum safety distance formula is:

Ds= [ K x ( Ts+ Tc+ Tr) ] + Dpf
where:

Ds= Minimum safety distance

K = Speed constant: 1.6 m/s (63 in./s) minimum

Ts= Worst-case stopping time of the machine/equipment (ms)

Tc= Worst-case stopping time of the control system (ms)

Tr = Response time of the safeguarding device including its interface (ms)

Dpf= Depth penetration factor (in.)

It is important to note that the value for Dpf will change depending on the configuration of the application (vertical for perimeter or point-of-operation configurations; horizontal or angular for area configurations) and depending on the resolution of the safeguarding device. Values for Dpf will range from about 1 in. for high-resolution safety light curtains in a vertical orientation to 48 in. for horizontal configurations. Appendix B of ANSI/RIA R15.06-1999 can provide additional information.

Safeguard devices should be mounted so that their protective field is located farther from the hazard than the calculated minimum safety distance. It also is worth noting whether a margin for error should be considered with the calculated minimum safety distance requirement to account for other factors such as reduced machine or robot braking capability over time.

Validate the System and Determine Residual Risks. After determining the minimum safety distance and the safeguarding configuration, it is necessary to validate the choices to confirm that the safeguard accomplishes what is intended. Personnel should not be able to reach over, under, or around the safeguard to bypass the safeguarding functionality.

Users also must determine what residual risks may still exist. If the risks are deemed to be "not tolerable," the safeguarding method chosen should be re-evaluated, or supplementary safeguards may be necessary. In some cases, these additional safeguards may be used in conjunction with the safety light curtain to accomplish safety goals.

Israel E. Alguindigue, Ph.D., is a market manager for SICK Inc., 6900 W. 110th St., Minneapolis, MN 55438, 952-941-6780, fax 952-941-9287, israel.alguindigue@sick.com, www.sick.com.

Sources

American National Standards Institute, 1819 L St. N.W., 6th floor, Washington, DC 20036, 202-293-8020, fax 202-293-9287, info@ansi.org, www.ansi.org

International Organization for Standardization, 1, rue de Varemb, Case postale 56 CH-1211 Geneva 20, Switzerland, 41-22-749-01-11, fax 41-22-733-34-30, www.iso.org

U.S. Department of Labor, Occupational Safety and Health Administration, 200 Constitution Ave., Washington, DC 20210, 800-321-6742, www.osha.gov



Israel E. Alguindigue

Ph.D.

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