Redundant Control Method for Automated Guided Vehicles
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Redundant Control Method for Automated Guided Vehicles Scott Jagolinzer, Sabri Tosunoglu Department of Mechanical and Materials Engineering Florida International University 10555 W Flagler St Miami, Florida 33174 sjago001@fiu.edu, tosun@fiu.edu ABSTRACT ruleset that autonomous vehicles on a road are able to use. The fact The purpose of this research is to explore the current state of that the applications can have the vehicles operating either indoors automated guided vehicles (AGVs), sensors available for the or outdoors or even moving between the two creates tracking vehicles to be equipped with, control systems for the vehicles to run challenges for the system. on, and wireless technology to connect the whole system together. Controllers that operate autonomous vehicles must either be With a technological push towards increasing automation and contained fully on the vehicle, as with self-driving cars, or can be maximizing the possible throughput of systems, automated centrally located and connect wireless to a plurality of vehicles. technology needs to improve for trackless and wireless systems While trackless vehicles that connect to a wireless control system such as vehicles that can be used to move loads in a vast array of still need to have on-board computing capability to process the applications. signals they get from the central controller, they are usually tethered The goal of this research is to develop and propose improvements to the signal and rely on having a stable connection. There must be in both vehicle and control system design that allows for improved ways to improve the balance of computing capability between safety and efficiency. Right now, the main issues are autonomous systems that rely on a central control system that allow maneuverability of vehicles and control systems being adaptive for more forgiving operation in situations where signal interference enough to deal with connection issues between systems. While is possible. prolonged connection issues will result in a stoppage of operation of any system that relies on wireless communication, intermittent 2. AUTOMATED GUIDED VEHICLES issues can also cause systems to have an emergency stop. I have Typical AGVs tend to be of a simplistic design and control due to looked into ways to offload tasks from the central system and allow their autonomous nature. Keeping the vehicle and controller simple the vehicles themselves to have more computational privileges such limits the chance that there is something that can go wrong for these that they can operate in a semi-independent manner. potentially huge machines that are moving around without much The result is a proposed system that remedies or limits negative human oversight. In general, AGVs consist of platforms that have effects that currently cause issues with trackless vehicles and one or two drive wheels with the rest being free-moving castor control systems working with remote systems that communicate via wheels to carry weight and provide balance. wireless means. 2.1 Steering Keywords Steering is one of the most important factors in the AGV because the method of steering influences the mobility and control of the Automated Guided Vehicle, Hybrid Controller, Redundancy, platform. There are three majors steering schemes used on AGVs. Sensors, Trackless Vehicle. The most basic steering control is from a differential drive wheel 1. INTRODUCTION configuration. This is when there are two drive wheels that are Automated guided vehicles began simply as a vehicle following a parallel to one another and the turning process is created when there radio signal emitted from a wire on the ground. This has advanced is a difference in wheel speeds. This is similar to how a tank steers. to the point where we are very close to passenger vehicles being It benefits from its design simplicity but is unable to perform more able to drive themselves on a widespread level. Vehicles are used complex movements. in a wide variety of applications from personal transportation, to Another steering method is a three-wheel steer drive configuration. carrying payloads, to entertainment purposes. Industrial There are two castor wheels and the one powered drive wheel that applications for autonomous trackless vehicles are tough because can also spin to control direction. This is the type of steering that is the vehicles have to be able to operate in a very dynamic and hectic available on most forklifts. This method is more maneuverable than environment that doesn’t have the benefit of infrastructure and 30th Florida Conference on Recent Advances in Robotics, May 11-12, 2017, Florida Atlantic University, Boca Raton, Florida differential steering and can perform smooth and accurate motions the required mechanics for the vehicle such as power source but still isn’t the most maneuverable option. (batteries or generator), the motors that control wheel orientation, The third option is a two steer-drive wheel configuration. This most of the onboard sensors, and the computer system. configuration has two drive wheels that can be rotated and two castor wheels. This steering configuration allows from the vehicle to move in any direction and turn on the spot. Similar to the 2 steer- drive wheel configuration is the Quad wheel configuration. This combines two fully rotatable drive wheels with two castor wheels to give the best range of motion and highest maneuverability [37]. A possible improvement that will be explored is having four drive motors that can all be rotated independently. This has the advantage over the two steer-drive configurations in that all four wheels are driven, allowing for double the available power and can also allow for power efficiency and reliability by disengaging a pair of wheels to drop the power consumption by half or allow for continued Figure 2. Underside of platform showing a possible wheel operation if a motor for a wheel breaks or a sensor malfunctions. configuration with respect to the center of the vehicle This fault-tolerance system will allow for increased efficiency and reliability. To make control of the wheels as simple as possible, they are laid out along a circular pattern with respect to the center of the vehicle. 2.2 Vehicle Design By having the wheels laid out in a circular pattern about the center A more robust control system for an AGV requires a more robust of the vehicle, it creates an easy reference not only between the vehicle. The design of this vehicle allows for size scalability and central point of the vehicle, where the position will be taken from, maneuverability for a wide array of uses. The vehicle has a very but also an easy reference between wheels so that rotational simple design consisting of 3 main parts. maneuvers have the most evenly distributed output requirements. This layout allows for the required wheel velocities and orientations to be easily calculated getting eight outputs (velocity and orientation of each wheel) from three desired inputs (liner velocity, direction, and vehicle orientation). Knowing only the length and width of the wheelbase, the location of all four wheels can be resolved for the system to properly calculate the required motor outputs to the wheels. Since each wheel is independently controller that mean each wheel has two motors, one each for linear and rotational velocity outputs. Each wheel system is connected to the main platform via a cuff that is designed to be able to carry the required loadings for the vehicles desired application. The rotational motor is mounted within the platform and interfaces with the wheel system via a gear. The tricky part comes with the mounting location of the drive motor. There Figure 1. Wheel mounting bracket showing rotational gear are two options, on the platform or directly to the wheel mounting interface and space for linear motor mounting bracket. Mounting from the platform would require a driveshaft that can not only operate at an angle, but be able to compensate for First is the wheel and mounting bracket. This consists of the wheel, the rotational movements with the vehicle without affecting wheel the bracket that the wheel is mounted to, and the drive motor for velocity. Mounting from the wheel bracket gives an easy, direct the wheel. Since the wheels are meant to spin a full 360 degrees the connection to the wheel but creates an issue with wiring back to the bracket must be able to easily rotate with respect to the main platform to be able to receive power and a signal. Mounting to the platform while also being able to stably support the loading of the wheel bracket was determined to be preferable since slide contacts vehicle. To account for both these needs, the cuff of the bracket has can be used for the transfer of power and required data. a wide plate on the top and bottom that help distribute the loading for each wheel assembly while also limiting the frictional forces Figure 3 shows how given a desired linear (red arrow) and that need to be overcome for rotation of the cuff to be performed. rotational (blue arrow) velocity for the vehicle as a whole, it can be Figure 1 shows an earlier design of the wheel and cuff. As can be broken down to the wheel components which combine for the total seen midway up the bracket, there is a gear built into it. This is for output for each wheel (green arrow). Notice how the blue arrows the second motor to interface with to control the wheel orientation. for each wheel are always tangent to the circle that the wheels all In figure 1 the placement of the drive motor is shown on the outside lie on. The resultant motion shown is of the vehicle driving in a of the bracket lined up with the axis of the wheel.