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This Draft Statement was prepared by:
SAFETY PRACTICES FOR AUTOMATED GUIDED VEHICLES (AGVs)
John W. Mroszczyk, PhD, PE, CSP
Northeast Consulting Engineers, Inc.
74 Holten Street
Danvers, MA 01923
Automated Guided Vehicles (AGVs) are vehicles that are equipped with automatic guidance systems and are capable of following prescribed paths. Unlike traditional robots, AGVs are not manipulators, they are driverless vehicles that are programmed to
follow a guidepath. In automated factories and facilities AGV's move pallets and containers. In offices they may be used to deliver and pick up the mail. They are even used to transport patrons around in airports.
AGVs have been around since the 1950's. The technology was first developed by Barret Electronics from Grand Rapids, Michigan. One of the first AGVs was a towing vehicle that pulled a series of trailers between two points. AGV systems did not catch on at that time, however, they were not well received by unions and were never allowed to perform to their full potential. The Europeans expanded AGV technology in the 1970's. The market has now grown to include the United States.
The main benefit to AGVs is that they reduce labor costs. But in material handling facilities there is another benefit. Material handling has always been dangerous. Injuries occur due to driver lack of attention, driver's driving too fast, or personnel not paying attention. Obstacle detection is therefore a key to allowing AGVs to interact with personnel safely while optimizing vehicle speeds.
AGV technology has been moving forward. There have been advances in navigation systems. Until about10 years ago most AGVs followed electromagnetic wires buried in the floor. Then laser guided systems came into the market. These navigation systems allowed the AGV to determine it's position in the plant based on the location of reflectors within the area. The future may be the in-plant equivalent of a global positioning system. Obstacle detection systems, has largely consisted of mechanical bumpers, giant E-stops that stop the AGV if it contacts a person or obstacle. Now, new advanced "virtual" bumpers may eliminate "driver error" injuries and allow AGV's to operate at optimum speeds.
ANSI B56.5 Safety Standard for Guided Industrial Vehicles and Automated Functions of Manned Industrial Vehicles
ANSI B56.5 defines safety requirements for powered, unmanned automatic guided industrial vehicles. The standard requires that the users are responsible for all factors affecting the operation and maintenance. This responsibility includes load stability and marking the travel path on the floor, including turning and maneuvering clearances. There are also requirements for manufacturers. Deviation from the travel path of more than 3 inches for an external reference (e.g. guide wire), or, more than 6 inches for inertial guidance system shall, require an emergency stop. A loss of speed control also requires an emergency stop. Vehicle warning indicators, audible and/or visual, shall operate when the vehicle is in motion. Emergency controls are required which would stop the vehicle if there is a loss of speed control, loss of guidepath reference, or an object is detected in the direction of travel. Accessible emergency stop switches are required on the vehicle itself.
Proximity Laser Scanner and Laser Scanner Interface (PLS/LSI) Technology
A Proximity Laser Scanner (PLS) creates a sensing field with a pulsed light that is reflected off of a rotating mirror so that it is transmitted in a 180 degree pattern. When an object enters the sensing field, the light is reflected back to the PLS. The distance to the object is computed using the time interval between the transmitted pulse and the reflected pulse, and, the angle of the rotating mirror. The sensing field is divided into three areas; safety zone, warning zone, and surveyed zone. The surveyed area is the maximum radius surveyed by the PLS. When the PLS determines that an object is on the safety zone, hazardous motion is stopped. When an object is detected in the warning zone, it initiates a warning or an avoidance maneuver.
A Laser Scanner Interface (LSI) is essentially a computer that interprets information and acts on it. The LSI can interpret data from up to four PLSs. When the PLS and LSI are combined in this way the PLS acts as the "eyes" and the LSI acts as the "brain". The LSI can also tell the PLS to change it's view depending on the location within the plant. For example, if the LSI gets input from the navigation system that the AGV is near a corner, the PLS view can be changed to go around the corner. The PLS also has a self-checking feature. The exit window is continuously checked for signal loss by an internal sensing array.
What this means for AGVs is that mechanical bumpers can be replaced by dynamic "virtual" bumpers. Although mechanical bumpers have been a low cost and accepted method to prevent contact, there are a number of drawbacks. A contact-sensitive mechanical bumper has only two states: on or off. Once the contact has been made, additional time is required to stop the vehicle, the stopping time depends on the speed of the vehicle. This means lower vehicle speeds resulting in lost productivity. Mechanical bumpers are sensitive to vibration, wear, and need regular maintenance. Depending on the size and shape of the bumper, many mechanical bumpers are not totally effective
when a turning AGV contacts an object in the turning path.
There are human factors issues as well. Workers may be uncomfortable with the fact that they have to be "hit" by the AGV before it will stop. Workers may also be injured trying to "beat" the AGV, thinking that they can get passed the vehicle and a fixed object.
A dynamic "virtual" bumper does not have mechanical parts. Speed can be controlled as the vehicle approaches an object and the configuration of the warning and safety zones can be flexible. For example, the closer an object the slower the vehicle, or, a warning can be sounded if an object enters the warning zone. Systems approved for AGV use should be able to detect a black vertical object with a diameter of 70 mm and a height of 400 mm (representing a leg in black trousers) anywhere in the AGV route.
SICK's Laser BumperTM technology uses PLS/LSI technology to provide a non-contact means for object detection. This advanced system permits increased speed and productivity because the size and shape of the stopping/slow-down/warning zones can be adjusted in real time depending on speed and the load. The protective area can be automatically extended on the sides of the vehicle when turning corners. Vehicle speeds can be increased in straight, unobstructed paths for improved productivity. Response times are as low as 60 ms depending on the system.
AGVs equipped with the Laser BumperTM do not have to hit an object or person to stop. Worker apprehension about whether the AGV is actually going to stop, after contact is made is eliminated. Injuries due to workers trying the beat the AGV are eliminated because the AGV will adjust speed, and stop if necessary, as a person moves in and out of the sensing zones.
Safety Guidelines for Employees and Contractors
AGV travels paths should be clearly marked and should be kept clear of material. Employees should be trained not to ride the AGVs. Training should also include instructing personnel to stay clear of an approaching AGV. Weighed cones or other portable obstacles can be placed where workers may be working on or near an AGV travel path. The training program should also include contractors that may come into a plant or warehouse to perform work.
The following recommendations/conclusions for AGV safety should be followed:
Sharke, Paul. "No More Sweat or Tears" Mechanical Engineering, July 2003; 47-49.
American National Standards Institute, Inc. ANSI B56.5: Safety Standard for Guided Industrial Vehicles and Automated Functions of Manned Industrial Vehicles. 2000.
Laser BumperTM PowerPoint Presentation, SICK, Inc. 6900 West 110th Street, Minneapolis, MN (with permission)
Richard Miller, Automated Guided Vehicles and Automated Manufacturing, Society of Manufacturing Engineers, Dearborn, 1987.
National Safety Council. Accident Prevention Manual for Business and Industry. Itasca, Illinois: National Safety Council.