Magnets, sensors create paths for automated guided vehicles

Application Update: An automated guided vehicle (AGV) can be built with as little as two components: a magnetic guide sensor and a dual-channel motor controller.

12/19/2013


Magnetic guide sensor will detect the center position of an adhesive magnetic tape up to 60 mm away. Courtesy: RoboteqAn automated guided vehicle (AGV) can be built with as little as two components: a magnetic guide sensor and a dual-channel motor controller. The AGV will follow a track made of an adhesive magnetic tape affixed on the floor. The magnetic sensor will measure how far from the center of the tape it is and provide the information to the motor controller, which will then adjust the steering so the vehicle remains at the center of the track.

Magnetic markers positioned on the left and right side of the track give the AGV location information that will be used to make stop and fork left/right decisions.

Magnetic track guiding

Magnetic tapes are one of several line-following techniques. The other two main techniques are induction wire guide and optical. A table compares each of these techniques.

Automated guided vehicles table: Tracking comparison

When comparing AGV tracking techniques, magnetic tape is the easiest to lay and modify while providing excellent durability and reliability, compared to induction wire and optical. Courtesy: Roboteq

Chassis design

When designing the vehicle, there are four basic ways of providing drive and steering, shown in the diagrams. Some types are easier to build; others have better steering characteristics. Two of these designs are fully symmetrical and may be operated in both directions. The chassis design table lists the characteristics of each design.

Four typical AGV chassis designs are shown. Courtesy: Roboteq

Automated guided vehicles table: Chassis design

Various AGV designs differ in characteristics, as shown. Courtesy: Roboteq

Sensor mount

An AGV uses a magnetic guide sensor from Roboteq. Courtesy: Artisteril SA of Barcellona, Spain.The sensor should be placed as shown in the above diagrams for each chassis design. For the first two chassis types, the sensor must be placed near the front edge of the chassis. On long AGVs, this means that a little steering will cause a wide swing at the front and will make the steering control more difficult.

On the steerable drive wheel design, the sensor can be placed on the chassis. Or it may be made part of the wheel assembly and turn with it.

For best results, place the sensor at 30 mm above the floor and ensure that the height fluctuates within +/-10 mm at most as the AGV moves along the track.

Sensor-motor controller interface

The MGS1600C has several output types. Features and typical uses are shown in the sensor attributes table.

Automated guided vehicles: Sensor attributes table

Sensors used for AGVs vary in attributes. Courtesy: Roboteq

In the MutliPWM mode, the sensor data is output on one wire in a series of variable width pulses, containing the track detect signal, track position, and left and right marker detect signals. This pulse can be connected to any of the Roboteq motor controller’s pulse inputs. Once the pulse input is configured as “Magsensor,” the sensor information is transferred transparently and continuously to the motor controller, from where it can be processed using the MicroBasic scripting language, or accessed by an external computer or PLC via the controller’s serial or USB port.

Electrical wiring

An example AGV wiring diagram is shown. Courtesy: RoboteqThe wiring diagram shows the magnetic guide sensor and motor controller in a typical four-wheel-drive chassis. This diagram is applicable to all Roboteq dual-channel brushed motor controllers.

A figure shows the controller’s connector details. This wiring is compatible with all Roboteq controllers equipped with a 15-pin DSub connector. The sensor and button can be connected to any other pulse and digital inputs. Refer to the product datasheet for the list of available signals and pin outs. The pulse output is on the blue wire of the sensor cable.

Configuration, testing

The sensor and controller must be configured so that they will each function as desired, and communicate with each other.

A sensor and button connect to the AGV motor controller. Courtesy: RoboteqThe sensor is configured by default to output MultiPWM pulse and therefore can be used without further configuration if the track is made of Roboteq-supplied 25 mm magnetic tape. Once powered, the Tape Detect LED will flash at a low rate if no tape is present. When tape is in range, the LED will be on steady, and its color will change when the tape is at the right or left.

For configuration, monitoring, and troubleshooting, connect the sensor to the PC via the USB connector located under the screw plug. Run the Magsensor PC utility to change the tape width if using a 50 mm tape, or use the waveform display view to monitor the shape of the magnetic field.

Magnetic sensor connectors and LED indicators are shown. Courtesy: RoboteqWith no tape present, the PC utility must show a nearly flat line. For best results, always perform a zero calibration when operating the sensor in a new environment.

Moving a tape under the sensor will cause a "bell" curve to appear on the chart. The curve must be on the positive (up) direction. If the curve is going down, change the Tape Polarity setting in the configuration menu. Makers will also cause a bell curve, but the curve should be going down. A screenshot shows the resulting curve when placing a left marker and a centered track.

Motor controller configuration

To receive and recognize data from the sensor, the controller must first be connected to a PC running the Roborun+ PC utility. In the configuration menu, the pulse input that is connected to the sensor must be enabled and configured as “Magsensor.”

This is the captured magnetic field waveform when no tape is present. Courtesy: Roboteq This is the captured magnetic field when the left marker and center track are present. Courtesy: Roboteq

Next, the controller must be configured to operate in mixed mode so that the steering command will apply a different amount of power to the left and right motor for making turns.

Magsensor to controller

When the sensor and the motor controller are connected to each other using the single wire and MutiPWM mode, the sensor data are transferred periodically, and in the background, into the motor controller from which they can be accessed and used.

An additional set of queries is available for reading this information from the motor controller. The queries can be sent either from the motor controller’s serial port, or from within a MicroBasic script running in the motor controller. (See the query set table.) 

Automated guided vehicle table: Query set

Queries can be sent either from the motor controller’s serial port or from within a MicroBasic script running in the motor controller. Courtesy: Roboteq


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