Automated guided vehicles

Flexible manufacturing system

To begin to understand AGV it is necessary to understand the fundamentals of flexible manufacturing systems (FMS). FMS is a means by which to manufacture a product. FMS is more of a philosophy rather than a tangible item. FMS is the idea that faster is better and uses machines to produce their products. Rather than using humans to perform repetitive tasks a machine is used to perform that task 24 hours a day. FMS uses computer numerical controlled machines (CNC) to form a work cell. Each cell performs a specific task to assist in the manufacturing of a product. Although FMS is fast and efficient it is not cheap as it requires a lot of expensive machines in order to work. Typically, it costs millions of dollars to introduce an FMS into a factory. Rather than using a complete FMS, most companies use part of an FMS called a flexible manufacturing cell. This is used to produce part of a product by machine and maybe part by other methods. Often time one or more AGV’s are used in FMS to connect work cells together.

AGVs in FMS are used to transport an object from point A to point B. AGVs navigate manufacturing areas with sensors. There are two main sensors AGVs use for navigation, a wired and a wireless sensor.

Wired

The wired sensor is placed on the bottom of the robot and is placed facing the ground. A wire is placed approximately 1 inch below the ground and a slot is cut in the ground to expose the wire. The sensor senses the radio frequency being transmitted from the wire and follows it. If the AGV is put off course the AGV will correct itself and continue to follow the wire.

Target Navigation

The wireless navigation is done by mounting reflective targets on poles or machines. The AGV carry a laser transmitter and receiver on a rotating turret. The laser is sent off then received again the angle and distance are automatically calculated and stored into the AGV’s memory. The AGV has a grid stored in its memory and can find its location by reflecting the laser off the targets. It can then navigate to a destination target through the grid and the constantly updating sensor.

Gyroscopic Navigation

Another form of a wireless AGV is gyroscope navigation. This method involves using a gyroscope to detect the slightest change in the direction of the robot. Magnets are installed in the ground of the work place and the AGV detects these magnets with a sensor. The magnets give the robot an insurance factor that it is on course. The gyro detects the slightest turn and corrects it in order to keep the AGV on its path. The margin of error for the magnet gyro method is ±1 inch.

Steering control

To help and AGV navigate it can use two different steer control systems. The differential speed control is the most common. In this method there are two sets of wheels being driven. Each set is connected to a common drive train. These drive trains are driven at different speeds in order to turn or the same speed to allow the AGV to go forwards and/or backwards. The AGV turns in a similar fashion to a tank. This method of steering is good in the sense that it is easy to maneuver in small spaces. More often than not, this is seen on an AGV that is used to transport and turn in tight spaces or when the AGV is working near machines. This setup for the wheels is not used in towing applications because the AGV would cause the trailer to jackknife when it turned.

The other type of steering used is steered wheel control AGV. This type of steering is similar to a cars steering. It is more precise in following the wire program than the differential speed controlled method. This type of AGV has smoother turning but cannot make sharp turns in tight spots. Steered wheel control AGV can be used in all applications; unlike the differential controlled. Steered wheel control is used for towing and can also at times have an operator control it.

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