Servo - June 2009

where the AGV has to travel a long distance before employing more accurate positioning at the end points.

Steering Systems

The only type of steering that most of us ever thought of was what we use in our cars. Called Ackermann steering, this type of system uses two front wheels that turn at the same time and at approximately the same angle. The car’s body and the fixed rear wheels then follow the curved path quite accurately.

Ackermann steering has advantages for robots and AGVs because of its accuracy in following a curved path. Its disadvantage is the large turn radius required. AGVs that tow carts or other objects use this type of steering to prevent jack-knifing of the carts.

A second type of steering is called ‘differential steering’ or ‘tank-type steering’ as tanks and bulldozers use this with two side treads running at different speeds to turn. Differential steering has gained favor with robot experimenters as it is easy to drive two side wheels at different speeds and have a small robot turn at any angle, or even spin on its axis. Of course, with only two wheels one or two free-swiveling casters are needed at the front or back (or at both ends) to keep the robot or AGV from tipping over.

Figure 10 shows the DX- 40 AGV from Savant Automation with differential steering. Smaller AGVs that need to make tight turns or even rotate on their axis use this type of steering.

FIGURE 9.

FIGURE 10.

AGV Traffic Control

As you can imagine, it would not be very productive if a warehouse full of roving AGVs just ran about without any concern for other AGVs or humans in their paths. A flashing light or even a constant beeping is not enough to warn of an imminent collision. The AGV must be able to detect a human, another AGV, or an obstacle and safely stop. Ultrasonic, IR, and focused laser or optical beam sensors can detect objects ahead and a bumper sensor can be used as a last fail-safe resort.

When it comes to AGV vs. AGV, the best solution is to have each of them operating in areas separate from the others. This can be done in larger systems with a master computer that knows the location of each vehicle and has bi-directional communications with all the AGVs. The ‘zone control’ method uses an RF transceiver that emits a signal in a specific area and each AGV in that area receives and

re-transmits that signal back to the
master transceiver. The first AGV to
enter the area is given an ‘all clear’
signal to enter or cross, whereas
all others are given a ‘stop’ signal.
If the zone control goes down for
some reason, each AGV has its own
proximity sensors to prevent crashes.

The Evolution of the OmniMate

I have long admired the HelpMate that I mentioned at the beginning of this article. It was first created by Engelberger’s earlier company, Transitions Research Corporation. TRC later marketed the LabMate robotic platform “trucks” which were essentially a base from the HelpMate. These differentially-driven bases have been used in small AGV applications and are equipped with a laser range finder to detect obstacles and accomplish basic navigation.