German Research Offers Insights for Sensing Edges
© 2005 Door & Access Systems
Publish Date: Fall 2005
Author: Heiko Hutter
German Research Offers Insights for Sensing Edges
By Heiko Hutter
Electrical Engineer, Bircher Reglomat, Switzerland
Between 1990 and 2000, nearly 25,000 people were injured in automatic gate-related incidents, according to the U.S. Consumer Product Safety Commission (CPSC). Of these 25,000, 9,000 were children under 15 years old. Each year, more than 2,000 people, including 800 children, are treated in emergency rooms for gate-related injuries to the head, neck, arm, or hand.
In 2000, the CPSC and Underwriters Laboratories (UL) responded to the dangers by developing a tougher safety standard, requiring automatic gates to have at least two mechanisms to prevent entrapment. Sensing devices such as electric eyes or edge sensors reverse the door or gate if it encounters an obstruction when opening or closing.
Safety Research in Germany
One way to increase safety at doors and gates is to limit the forces that contact the body. A helpful study by the BGIA, the institute for accident insurance and prevention in Germany, tested the maximum forces that can be applied to human body parts.
Within the study, female and male subjects, aged between 18 and 60 years, were exposed to various forces on different parts of the body. The personal sensation and grade of pain were analyzed and verified by medical examinations.
The study provides a basis to define the maximum forces on body parts without causing serious injuries. The results show that dynamic forces can be much higher although briefer than static forces that apply for a longer period.
The Bottom Line
Results: The study recommends that dynamic forces be no higher than a maximum of 88 lbs. However, within 0.75 seconds, the force must be reduced to less than 33 lbs. This is typically done by reversing the movement of the door or gate. After 5 seconds, the force must be less than 5.5 lbs.
These values do not guarantee injury prevention, but they provide a general guideline for producers of garage doors, gates, and safety components. The values do not, however, provide enough safety for older people, small children, or the disabled. In such applications, contact-free safety systems are highly recommended.
Edges: Different Types, Different Performance
Limiting the force to these recommended values is typically achieved with safety edges. A safety edge is typically made of a rubber profile and a switching unit integrated in the edge. When the rubber profile is compressed, the switching unit triggers the door controller to stop the door movement.
Different profile types perform differently and allow different maximum forces. The height, material, and shape of the profile affect performance and the “overrun distance” of the door or gate. This is the distance between the position of the stop signal and the actual stop position.
This distance depends on the speed of the door/gate, the performance of the motor, the reaction time of the electronics, and the mechanical design. The profile type affects the force applied, and that force must be less than the specified values. When the overrun distance is longer, the profile must be taller.
Three Edge Technologies
Currently, the most common safety edge technologies for industrial doors and gates are:
1. Air pressure wave profiles and switches
2. Electrical edges (switching element inside rubber profile)
3. Optical safety edges
Air pressure wave edges offer several advantages, including excellent sensitivity, a sturdy design, and low cost. The air wave presses on a diaphragm in an electrical switch, thus signaling the door controller. Testing, however, is a bit more involved than self testing, “fail-safe” electric edges.
Electrical edges are typically triggered by a defined resistance. The controller continuously measures the resistance across the safety edge. When the resistance reaches a certain level, the controller stops the door/gate movement. Advantages include the system’s self-testing ability, and only one side of the edge must be connected. This technology can also be used for longer safety edges.
Optical safety edges are relatively new. A light signal is sent through the edge, received on the other side, and is analyzed by a special controller. A certain variance in the light signal sends a stop signal to the controller. This system is easy to install; you simply plug the sender/receiver into the rubber profile. The length of the edge may be limited (typically to 25 ft.), connections are needed on both sides of the edge, and power is required by both the sender and receiver.
Edges require physical contact. In some cases, contact is not acceptable, and higher safety standards are required.
In such cases, touchless or contact-free technologies are available. The garage door industry is typically familiar with these systems, such as infrared-based sensors/beams that create light barriers in front of doors or gates. When the beam is broken, the door/gate is stopped or reversed.
Before choosing the appropriate technology, conduct a risk analysis to determine the level of safety required. Ask key questions: What kinds of hazards are possible? What is the probability of injury or damage? Will the door/gate be mounted in a public area?
In public areas, such doors/gates require a higher level of safety. People in these areas have not received any special instruction and probably do not even recognize the risks.
Once the appropriate technology has been selected, trained technicians must install the system correctly and perform regular maintenance. In all cases, the suppliers of safety systems are available to assist you.
Heiko Hutter holds university degrees in electrical engineering and economics. He is a business unit manager at Bircher Reglomat of Switzerland, the parent company of Bircher America, a supplier of air wave switches, electrical and pneumatic sensing edges, industrial and pedestrian door sensors, and similar products.