RoboCart: Ultra low-cost AGV Demonstrator
Engineers at Roboteq have developed an ultra-low cost, lightweight AGV demonstrator based on a Utility Cart and inexpensive hub motors. We call it RoboCart.
In the 21st Century, the Automated Guided Vehicle (AGV) is utilized in nearly every industry, such as factory assembly, food delivery, warehouse inventory control, and in general manufacturing. For many industries, the AGV has become an indispensable part of the control process. Unfortunately, the high cost and complexity of many typical AGV’s has precluded their use in many smaller industries that could greatly benefit from their usage.
To address this important issue, the Engineers at Roboteq have developed an ultra-low cost AGV demonstrator. We call it RoboCart. This robot is ideally suited to transporting lightweight materials, for example food, linen or medicine in hospitals.
Robotic Utility Cart following a magnetic tape guide
The primary design considerations for this robot design were driven by observing the high cost of most industrial AGV systems on the market, and the need for smaller companies with less demanding requirements to be able to implement a smaller scale autonomous delivery system.
The design greatly reduces the cost of the mechanical chassis by using a common utility cart.
Available from multitude of vendors, these carts can be assembled in minutes and cost between 100$ and 150$. They are made of sturdy plastic or metal, include two fixed and two caster wheels that can carry loads up to 250kg. They can therefore serve for a multitude of real-life transport and logistics application without any mechanical modification.
Our engineering team at Roboteq built the prototype using Roboteq standard motor controllers and an off the shelf magnetic guide sensor for detection of the magnetic strip on the floor. A detailed look at the underside of the drive system is shown below
RoboCart's drive system.
Robocart steers in a very different fashion than the standard AGV. Most AGV’s use the “Differential Steering” method where the drive wheels are mounted under the center of the body of the robot’s frame, and by driving them independently, the robot can achieve all manners of forward and reverse movements, including sharp turns. Differential drive was not practical to adapt to a utility cart without substantial mechanical modifications.
Typical Mobile Robot Drive mechanism and RoboCart's variant
RoboCart uses an alternate method, referred as Differential Steerable Drive in the diagrams above. It is a variant of the Steerable Drive Motor mechanism, with two motors mounted as a Differential Drive on a rotating plate.
Here, all the electronics including motor controller, batteries, magnetic guide sensor, and both motors are rigidly mounted to a single rotating plate.
The rotating plate is attached to a stationary (relative to the cart) base plate using a turntable bearing. The bearing holds both plated firmly together while allowing for free and smooth rotation movement even at full load capacity. The stationary plate is attached to the utility cart using the cart's mounting holes that were originally used to mount the caster wheels. The cart is used without any modification whatsoever.
Mini Hub Brushless Motors
Recent developments in small but powerful brushless hub motors used in powered skateboards opened the possibility of a highly maneuverable and cost-effective robot solution. While these motors are made for consumer applications, they are full-featured, direct-drive 10 pole motors. They are fitted with Hall sensors and are therefore capable of high torque during stall or low speed. The Hall sensors can also be used to operate in closed loop speed mode and to determine robot’s travelled distance over the track.
These motors are design to withstand the brutal use of skateboarders and therefore can be expected to operate very reliably in AGV use. Robocart uses one of these very inexpensive motors assemblies in an innovative manner to provide both thrust and steering. Using the mixed mode available in all Roboteq’s dual channel motor controllers, the drive plate can be oriented in the desired direction by applying a different speed on the right and left motors.
Motor Controller and Magnetic Guide Sensor
The complete line following functionality is achieved with only two components: The SBL2360 dual channel BLDC motor controller and the MGS1600GY magnetic guide sensor.
The sensor interfaces with the motor controller using only two power supply wires and a single wire that carries the tape position information using a modulated PWM signal. The motor controller’s MicroBasic programming language implements the algorithm that moves the motors based on the sensor’s data. In addition, a WiFi to RS232 adapter is used to communicate wirelessly with the cart. The complete wiring diagram is shown in the figure below.
RoboCart in Operation
The cart operates very simply: The controller is loaded with the very same script listed in our main AGV Applications Blog. Once the magnetic tape is detected, both motors turn on. If the sensor reports that the tape is off center, one of the motors will turn faster while the opposite motor runs slower. This causes the drive assembly to turn in the direction needed to center the tape. This results in flawless line following motion. Here are some photos of the behavior of the robotic utility cart initiating a turn.
|The first image in the set shows the cart tracking a straight section of magnetic tape, with the steering plate following directly behind the magnetic guide sensor.|
|In the second image, we approach a turn. The motor assembly is still facing straight.|
|Finally, in the third shot the steering plate rotates about 45 degrees by pivoting on the turn table bearing by changing the relative speed of both drive wheels. Steering can be very sharp, and even approaching 90 degree turns.|
The robot can autonomously detect markersalong the path of the track for providing additional navigation information. Markers a small piece of magnetic tape of inverse polarity compared to the main track. They are used to identify special locations along the track, such as upcoming fork turn direction, charging location ahead, stop at mark for loading or unloading, and changes in speed.
RoboCart was built and tested within only a few hours of work. The skateboard dual-motor assembly was useable as-is and interfaced directly with the motor controller. Most of the robot’s originality is the mounting of rotating drive assembly using the turntable bearing. The motor controller / magnetic guide sensor performed immediately upon loading the AGV script. The WiFi connection allowed continuous monitoring of the motor’s voltage, current and speed. Basic line following functionality was successfully achieved with minimal work.
The RoboCart design is a demonstrator which can be easily improved into a full-featured professional AGV. Here are some of the possibilities:
Automatic Charging: Using Roboteq’s RoboPads, the cart can be made to autonomously dock with the charging power supply and renew its reserves periodically. Typically, the charging station is in a location with little or no traffic, and off the main pathway for transporting goods. Battery voltage can be measured by a simple script in the motor controller and cause the robot do take a fork to the charger, and resume normal operation after charge.
Live Location Reporting over WiFi: The Hall encoder inside the hub motor change state 60 times per wheel turn. Roboteq’s motor controllers can use these changes to measure travelled distance with 5mm steps increments. Used in combination with fixed location markers, the robot can communicate its location along the track in real time to a server. This capability is essential in fleet management in a multiple AGV installation.
Obstacle avoidance / Safety: A simple contact switch type bumper on the front of the slow-moving cart will provide ample sensing of impact with an obstacle on the track. Such bumper switches are commonly constructed using small micro switches that connect directly to the motor controller. Alternatively, ultra-sound or infrared obstacle sensor can be used in installations where the robot operates at higher speed or/and higher loads. A script can be written to react to any impact conditions. Additionally, a small inexpensive piezo buzzer can be made active during movement to allert people of the robot's presence.
The development of an inexpensive - under $1000, but highly capable AGV Cart for small business and light industry enables nearly limitless applications in both the service industries and small companies. Here at Roboteq, we acknowledge such forward momentum, and strive to be at the forefront of a rapidly growing industry.