Hydraulic Pressure-Flow Rate Control of a Pallet Handling Robot for an Autonomous Freight Delivery Vehicle
Abstract
:1. Introduction
- It is not common practice to use hydraulics in robotic systems. We have taken a new approach by using hydraulics instead of electric drives for our robotic forklift.
- Furthermore, the axes need force density and very modest precision: An electric motor and gears also give additional precision not needed for our tasks. Additionally, the electric motor and gear pump can have a rigid transmission, and this too is a feature not required for FURBOT application: for this reason, hydraulics are not used much in robots because robots are designed for tasks needing accuracy and made stiff to simplify the control (controlling flexible axes is more complex in trajectory and under variable loads).
- Validation of our methodology was achieved by conducting separate tests for speed control, the pressure loop, the speed test and the bi-state regulator. Moreover, repetitive experiments were performed to increase the faith in the robustness of our system.
- The classical approach to regulate speed/force is through pressure reducing valves while the pump turns at full speed; this means it is not possible to set both the maximum force that will be exerted and the speed during the motion. With the help of bi-state regulator, we are able to set both the maximum force exerted and speed during motion.
- A lot of energy is wasted using pressure reducing valves. The technique proposed is not only safer, but also reduces the energy loss in the system.
1.1. Hydraulics on FURBOT
1.2. Synoptic View of Control Architecture
- The pump is always running at full speed and the pressure regulators are used uneconomically. Given the oil compressibility, the rise in pressure is quasi-instantaneous; in other words the pump can start when actuation is needed without wasting energy for the majority of the operation time when in fork mode and waiting for actuation.
- The forklift’s cylinders do not always need 120 bars of pressure to work properly; so the maximum flow rate is not always required.
- There is no safety regarding the speed of the unloaded cylinders as it is not controlled while working at maximum capacity. However, when the pressure is regulated, lamination valves allow regulating the speed of actuation by limiting the flow rate. Additionally, if the cylinders are stuck, the hydraulic pressure rises to 120 bars of pressure. Thus, it is better to set a maximum pressure set point in order to set the maximum force exercised by the cylinders. The pressure set point will give the maximum force and the lamination valve the maximum speed in steady state.
2. Research Objectives
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- They are designed for a simple service and dissipate energy by bypassing a certain amount of oil into the main tank.
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- One can set the pressure manually but cannot control or regulate it over time.
3. Hydraulic Equations and Control Strategy
4. Implementation of the Control Algorithm
4.1. The Control Strategy
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- The speed of the pump: through intelligent servo-controller.
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- The hydraulic pressure: through the installed pressure sensor.
4.2. Algorithm
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- The libcanopen++ API is used to create new can buses handlers and CANOpen “master controllers” which can be linked to a bus. A “master controller” is a CANOpen master node with a specific master object dictionary and which can run several “application controllers” with the use of threads.
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- The software’s “application controllers” are linked to a specific hardware-device on the bus. They work with the use of threads and callbacks from the libcanopen++ stack and have an internal state machine to operate the vehicle safely.
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- A FURBOT class wraps all the buses and controllers. The latter also includes a network handler to broadcast information via UDP (User Datagram Protocol) to all hosts on the local network. All the objects are instantiated and the threads launch with the call to the Furbot() constructor inside the main function.
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- Changing the object dictionary for one of the master nodes on the bus. New user entries were added for the pressure process-values coming from the pressure sensor and we did the right PDO (Process Data Object) mapping.
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- Writing a controller for the pressure sensor (two new files), to get the data and to send it to the local network inside UDP packets. A network subsystem for the pressure sensor was added using pre-existing functions.
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- Creating a class for the algorithm (two new files) to be able to get the behavior explained above. The pressure control mode is a PID control while the flow regulation just consists of setting the speed of the pump.
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- Changing some functions into the furbot class, especially the function starting the pump and running it at full speed by a new function according to the current requirements.
5. Preliminary Work
5.1. The Open Motion Controller and CAN
5.1.1. The OpenMCP
5.1.2. CAN
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- The data frame transports the data.
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- The remote frame asks for the transmission of a data frame.
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- The error frame is broadcast by one node which detects an error.
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- The overload frame is broadcast by a node which detects an overload.
5.2. CANOpen
5.3. Preliminary Work on the Pressure Sensor
6. Modeling and Results
6.1. Simulation Modeling
6.2. Simulation Results
7. Prototype Testing and Results
7.1. Pressure Sensor Test and Results for the Pressure Loop
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- Blue represents pressure.
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- Red represents set speed from the control loop.
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- Green represents actual speed.
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- Yellow represents torque from the motor, measured from the servomotor.
7.2. Final Algorithm Test
8. Conclusions
Author Contributions
Funding
Conflicts of Interest
Abbreviations
FURBOT | Freight Urban Robotic Vehicle |
SHOW | SHared automation Operating models for Worldwide adoption |
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Share and Cite
Masood, K.; Dauptain, X.; Zoppi, M.; Molfino, R. Hydraulic Pressure-Flow Rate Control of a Pallet Handling Robot for an Autonomous Freight Delivery Vehicle. Electronics 2020, 9, 1370. https://doi.org/10.3390/electronics9091370
Masood K, Dauptain X, Zoppi M, Molfino R. Hydraulic Pressure-Flow Rate Control of a Pallet Handling Robot for an Autonomous Freight Delivery Vehicle. Electronics. 2020; 9(9):1370. https://doi.org/10.3390/electronics9091370
Chicago/Turabian StyleMasood, Khayyam, Xavier Dauptain, Matteo Zoppi, and Rezia Molfino. 2020. "Hydraulic Pressure-Flow Rate Control of a Pallet Handling Robot for an Autonomous Freight Delivery Vehicle" Electronics 9, no. 9: 1370. https://doi.org/10.3390/electronics9091370