Keeping Your AGV On Track

If walking around the ProMat 2007 expo floor taught me anything, says C.G Masi of Control Engineering US, it's that the big trend in warehouse automation is automated guided vehicles (AGVs).

These little puppies (while 'little' may not be apropos many of these AGVs, 'puppy' is getting to be more appropriate as the devices' intelligence grows) turned up in nearly every booth I visited.

There are two problems AGV designers must solve—beyond the obvious one of giving them enough muscle to carry the loads required. First, they have to be able to navigate from point A, where they accept their load, to point B, where they drop it off. Second, they have to make the trip without running into or over anything significant, such as employees, walls, stacks of product, or other AGVs.

In general, AGV designers seem to be dealing with these problems separately. That is, they design a navigation system that simply navigates. Avoiding the supervisor's toes is a separate problem usually handled by a separate collision-avoidance system, such as the ultrasonic proximity sensors mounted around all sides of the AGVs demonstrated at the Egemin Automation booth. I could write a book on either system, but want to briefly explain how combining multiple navigation technologies makes AGV navigation systems more robust.

Three AGV navigation technologies are in general use:

- Wire guided systems are the oldest, having been in service since the 1940s. Typically, the installer attaches some physical guide, such as a magnetic tape, to the floor. The AGV carries a sensor that finds the guide structure and follows it around.

- Laser guided systems have an infrared laser mounted in a turret that rotates around its vertical axis on top of the AGV, and targets strategically located in the space the AGV roams around in. As the laser beam reflects from each target, a photodetector picks up the echo returning to the turret. The time at which the system sees a target correlates with the line-of-sight direction to that target. A computer triangulates lines of sight to multiple targets, computing the AGV's position and orientation in the horizontal plane.

- Inertially guided systems use the fact that the distance moved is the second integral of acceleration. System designers use accelerometers mounted anywhere in the AGV to monitor linear and rotational accelerations in the horizontal plane. An onboard computer does the double integrals numerically to determine the AGV's current position and orientation. This system is particularly useful in outdoor applications like the one Allan Quimby described at the Transbotics booth, where laser and wire guided systems are harder to maintain.

The problem, of course, is that any of these systems can get lost. If a wire-guided system loses contact with its guide, it has no way to find it again. Laser guidance works only as long as the laser can see at least some of its targets. Inertial systems work only until inevitable measurement uncertainties and numerical-integration errors accumulate beyond tolerable levels.

Some AGV manufacturers are starting to deploy more than one guidance method on each AGV. This strategy allows the AGV to crosscheck its position, catching, and in most cases correcting, guidance errors before they become problematical.

For example, a wire-guided AGV that also has a laser guidance system can find its way back to the nearest point on its guide track should it get lost. A laser guided AGV has no problem zipping along an isle between cases stacked higher than its turret.

As time goes on, expect to see additional AGV guidance devices, all with some failure mechanisms. Deploying more than one guidance system will keep automated warehouse facilities running smoothly.