Workplan-Task1

Task 1: Multi-wheel system control

Major part of the work in Task 1 is to be performed in IHA. Activities at Aalto has concentrated on Thesis Research on a small electric 6-wheeled rover. The first section below describes the activities at Aalto, the rest is about activities in IHA.

Activities at Aalto

Activities at Aalto has concentrated on Thesis Research on 6-wheeled planetary rover demonstration model 'Marsokhod' producing three master's Theses:

2007, Master's Thesis: Development of Mobility System for Marsokhod Rover (Poornima Muralidhar)
2007, Master's Thesis: Study and Implementation of Wheel Walking for a Mars Rover (Zhongliang HU)
2008, Master's Thesis: Mobility and autonomous reconfiguration of Marsokhod (Jan Philipp Hakenberg)

Activities at IHA

The control system for the four-wheeled hydraulic IHA-machine could be called active flow divider. It will be a digi-hydraulic flow divider which allows independent speed control of individual fixed displacement hydraulic hub motors in closed loop hydraulic circuit. The flow divider and its control system will be studied in simulator before implementing the system in to the real machine. The GIM-machine (based on Avant-635) will be modified in such way, that it is possible to add active by-pass flow control to the hydraulic motors. Benefits of this system will be higher steering accuracy (less under steering) which lead to fewer traces to the environment. This will be demonstrated first with simulator and afterwards also in the real machine in its real operational environment.

Expected results

Expected results of this study are: independent speed control of every fixed displacement hydraulic hub-motor in power transmission system. Rotation direction and speed can be controlled independently, also the maximum torque of each wheel can be limited. Better controllability of fixed displacement hydraulic hub motor makes fast response anti slip systems possible in closed loop hydraulic systems.

System description

Hydrostatic power transmission has multiple advantages such as continuously variable transmission ratio, simple reversing of power direction, and inherently high stiffness. It also has some disadvantages; one of these is the inability to control speed and torque of multiple actuators if load circumstances of these actuators vary. This is the case especially if the actuators in closed loop hydraulic circuit are in parallel. In this research the actuator will be hydraulic motor which rotates wheel of a centre articulated mobile machine (IHA-machine). The common solution for this problem is traditional flow divider, which distributes flow equally to all actuators. The problems with flow dividers are fixed division ratio and low efficiency. Efficiency of flow dividers is low due to pressure difference caused by the orifices of the valve. The fixed division ratio is a result of mechanical structure of flow dividers. This study will take advantage of digi-hydraulics and use theses advantages to create active flow divider.

The active flow divider consists of multiple digi-hydraulic bridge connections; described in figure N. Bridge connection allows control system to decide the direction and amount of flow through each hydraulic motor of the system. In hydrostatic power transmissions with traditional flow dividers the direction of motors is defined by the output flow of the pump. Disadvantage of active flow divider and all other flow divider valves, is that the control is based on restriction control which lowers the efficiency of the system. With digi-hydraulics the effect of this could be minimized, by using special set of digivalves.

The research platforms for this study will be first dynamic simulators, then test benches, and finally IHA-machine. Simulators are going to be built on Matlab/Simulink which offers flexible platform for simulation. Earlier experience of using current simulation programs encourages using it.

With simulators the goal is to derive suitable control algorithms, at first for the speed control of single hydraulic motor. After that the control system will be tested and tuned in test bench with real hydraulic motor and valves. When the control algorithm for single motor is ready and tested, the simulator for multiple actuators will be created. And again the simulator is used as a tool to derive suitable control system.

After the controls system for digital flow divider is derived, the STEP 1 of IHA-machine should be already running. The STEP 2 of IHA-machine will consist of implementation of active flow divider in to the machine. After STEP 2 there will be another 2 steps with IHA-machine, but these steps will be discussed more closely in chapter which deals with further development of rolking.

Active by-pass flow control is a case study, where the effects of turning of a centre linked machine will be studied. Turning of the machine affects on the rotation speed of each wheel of a machine. When the machine is on constant radius the speeds of left-side and right-side wheels are the same. If the rate of turning changes the speeds of wheels will differ, this will cause pressure peaks and cavitation in hydraulic circuit and traces in operational environment. This study will look if it is possible avoid these problems by using by-pass control of hydraulic motors. Also the steerability of the centre linked machine will be increased due to easily rotating wheels. At first this study will be a simulation study. After suitable control algorithm is derived and found, the results will be implemented to GIM-machine.

Planned activities and publications

Time of Acticvity

Activity & Responsible persons
Publication(s) & Journal / Conference

2007-2008

Dynamic Hardware In the Loop simulator of GIM-machine 2007-2008 (Mika Hyvönen, Miika Ahopelto, Antti Vuohijoki, Otso Karhu, Jarno Uusisalo, Juha Järä, and Teppo Moilanen)

2007-2008

Control system of GIM-machine tested and realized HIL simulator 2007-2008.( Ahopelto, Järä, Uusisalo, Hyvönen, and Vuohijoki)
Master student Juha Järä has designed control system for steering of the GIM-machine.
Master student Miika Ahopelto has realized and tested the control system of GIM-machine.
Master’s Thesis (Järä)

End of 2008

Mechanical, hydraulical, and electrical assembly of GIM-machine ready by the end of 2008.
Measurements of the startablity of hydraulic motor of IHA-machine will be done by the end of 2008. (Vuohijoki)
Dynamic simulator for digital flow divider by the end of 2008. (Vuohijoki)
Active by-pass flow control of hydraulic motor will be tested on simulator of GIM-machine end of 2008 (Reza Ghabcheloo, Hyvönen)

2007-2009

Mechanical design of IHA-machine step 1. (Vuohijoki, Timo Julkunen)
Active by-pass control will be tested on GIM-machine, summer 2009 (Ghabcheloo, Hyvönen)
Dynamic Hardware In the Loop simulator of IHA-machine step 1, 2009. (Hyvönen, Vuohijoki)
Design of control system of IHA-machine, 2009. (Vuohijoki, Uusisalo, Karhu, and Hyvönen)
IHA-machine step 1, 2009 will be realized
Master’s Thesis (Ahopelto)
Active flow divider report 1: ‘Simulation study on digi-hydraulic speed control of hydraulic motor’(Vuohijoki)
Active by-pass flow control report 1: ‘Simulation study on active by-pass flow control of hydraulic motors in mobile machines’ (Ghabcheloo, Hyvönen)
Active flow divider report 2:’Study on digi-hydraulic speed control of hydraulic motor’ (Vuohijoki)
Active by-pass control report 2:‘Active by-pass flow control of hydraulic motors in mobile machines’ (Ghabcheloo, Hyvönen)

2010

IHA-machine step 2.
Active flow divider report 3: ‘Simulation study on digi-hydraulic flow divider’(Vuohijoki)
Active flow divider report 4:’ Digi-hydraulic flow divider in hydraulic mobile machine’ (Vuohijoki)

2011

IHA-machine step 3.
Doctoral Dissertation (Vuohijoki)