Simulation and Evaluation of an Articulated Forklift Truck

82 Pages·2014·4.04 MB·English

by  Emil Johansson

Simulation and Evaluation of an Articulated
Forklift Truck
Emil Johansson
Department of Fluid and Mechatronic Systems

Master Thesis
Department of Management and Engineering
LIU-IEI-TEK-A--14/01885—SE
ii
Simulation and Evaluation of an Articulated
Forklift Truck
Master Thesis in Fluid Power
Department of Management and Engineering
Division of Fluid and Mechatronic Systems
Linköping University
by

Emil Johansson

LIU-IEI-TEK-A--14/01885—SE

Supervisors: Mikael Axin
IEI, Linköping University
Johan Vestman
Toyota Material Handling, Mjölby
Examiner: Liselott Ericson
IEI, Linköping University

Linköping, Juni, 2014
Linköping University Electronic Press

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© Emil Johansson
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Abstract
Today’s demand on forklift trucks performance and efficiency is high. The productivity is important
but also the experience while handling the forklift. The handling has to be simple and genuine to
make the driver feel confident and safe. To achieve high performance steering in articulated trucks, a
hydraulic power system is often used.
Simulation software are a powerful tool in development processes. The program gives the industry a
possibility to develop, analyze and evaluate constructions and models more efficient.
The purpose of this master thesis is to identify and increase the knowledge about the main
challenges in the hydraulic steering system in an articulated forklift. The hydraulic system has been
modelled in the simulation software Hopsan and validated against data from measurements
performed on the forklift. The different challenges have been identified based on tests and the
simulation results. For a deeper understanding of the system a literature study, mainly about the key
components, has been done during the master thesis.
A number of suggestions for improvement have been developed with focus on increasing the
steering performance. The concepts and ideas have been evaluated and tested in the simulation
model.
The project resulted in a validated simulation model of the articulation and a number of suggested
improvements on the hydraulic steering system.
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Acknowledgments

There are many people that I would like to thank. First of all, I would like to thank the Hydraulic
department on Toyota Material Handling for their time and effort. It has been a great time and a lot
of new experiences. A special thanks to my supervisor Johan Vestman on Toyota for his dedication.
Johan can always spare a moment for discussions and he has been a great support. There are many
people who have been helpful on Toyota. To mention a few of them I would like to thank Lena Löök,
Christian Kjellander and Johan Karlsson. Christian and Johan have been a vital support during the
measurements and have always spared time for questions concerning signal and control. Thank you
also Christoffer Zeipel-Stjerna and Jan Malecki for proof reading and feedback on the report.
At Linköping University, I would like to thank my supervisor Mikael Axin for his guidance throw the
thesis. I would also like to thank Peter Nordin and Robert Braun for their persistent support with the
simulation in Hopsan. My thank goes also to my examination Liselott Ericson.
Last but not least I would like to thank my beloved girlfriend, Lovisa, and my family for their support
and everlasting love.
Linköping, May, 2014
Emil Johansson

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Contents
1 Introduction ..................................................................................................................................... 1
1.1 Background .............................................................................................................................. 1
1.2 System Description .................................................................................................................. 2
1.2.1 Mechanics ........................................................................................................................ 2
1.2.2 Hydraulics ........................................................................................................................ 2
1.3 Method .................................................................................................................................... 3
1.4 Objectives ................................................................................................................................ 4
1.5 Delimitations ........................................................................................................................... 4
1.6 Outline ..................................................................................................................................... 4
2 Theory .............................................................................................................................................. 6
2.1 Valve and Cylinder ................................................................................................................... 6
2.2 Ideal Constant Flow Valve ....................................................................................................... 7
2.3 Flow Force ............................................................................................................................... 8
2.4 Constant Flow Valve .............................................................................................................. 10
2.5 Accumulator .......................................................................................................................... 10
2.6 Stick-slip phenomenon .......................................................................................................... 12
3 Models ........................................................................................................................................... 13
3.1 Simulation Model .................................................................................................................. 13
3.1.1 Simulated Mechanic Model ........................................................................................... 15
3.1.2 Simulink and Hopsan ..................................................................................................... 16
3.2 Mechanical Model ................................................................................................................. 16
4 Measurements .............................................................................................................................. 19
4.1 Equipment ............................................................................................................................. 19
4.2 Results ................................................................................................................................... 20
4.2.1 Test 1 ............................................................................................................................. 20
4.2.2 Test 7 ............................................................................................................................. 22
4.2.3 Pump speed ................................................................................................................... 24
5 Validation of Model ....................................................................................................................... 25
5.1 Valve and signals ................................................................................................................... 25
5.1.1 Validation Model for the Valve ..................................................................................... 25
5.1.2 Dynamic Validation of the Valve ................................................................................... 27
5.1.3 Leakage in the Valve ...................................................................................................... 27
5.2 Flow Forces ............................................................................................................................ 28
5.3 Mechanics .............................................................................................................................. 29
6 Simulation Result ........................................................................................................................... 31
6.1 Free Maneuvering ................................................................................................................. 31
6.2 Wire Guidance ....................................................................................................................... 33
6.3 Analysis of Results ................................................................................................................. 35
7 Opportunities for Improvement .................................................................................................... 36
7.1 Introduction ........................................................................................................................... 36
7.1.1 Robust Wire Guidance ................................................................................................... 36
7.1.2 Interference ................................................................................................................... 37
7.2 Analysis .................................................................................................................................. 38
7.2.1 Cylinder Placement ........................................................................................................ 38
7.2.2 Valve Characteristics ..................................................................................................... 39
7.2.3 Double Valves ................................................................................................................ 41
7.2.4 Valve and Accumulator ................................................................................................. 46
7.2.5 Interference ................................................................................................................... 48
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7.3 Suggestions for Improvement ............................................................................................... 50
7.3.1 Cylinder Placement ........................................................................................................ 50
7.3.2 Valve Characteristic ....................................................................................................... 50
7.3.3 Double Valves ................................................................................................................ 50
7.3.4 Valve Size ....................................................................................................................... 50
8 Discussion ...................................................................................................................................... 51
8.1 Simulation Model .................................................................................................................. 51
8.2 Suggestions for Improvement ............................................................................................... 51
9 Conclusions .................................................................................................................................... 53
10 Future Work .................................................................................................................................. 54
Bibliography ........................................................................................................................................... 55
Appendix ................................................................................................................................................ 56

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List of Figures
Figure 1: Illustration over change of aisle during wire guidance ............................................................................ 1
Figure 2: The hydraulic schematic ........................................................................................................................... 3
Figure 3: A 4/3 directional valve and a hydraulic cylinder. ..................................................................................... 6
Figure 4: A pressure compensating valve together with a restrictor, thus a constant flow valve. ......................... 7
Figure 5: Characteristics of a constant flow valve [2]. ............................................................................................ 8
Figure 6: Vena Contracta in a sharp edged restrictor and static pressure graph [2]. ............................................. 9
Figure 7: Flow force acting on a valve as a function of the opening distance x [2]. ................................................ 9
Figure 8: Static flow forces acting on a spool [5]. ................................................................................................. 10
Figure 9: Accumulator in hydraulic system. .......................................................................................................... 11
Figure 10: Simulation model in the Hopsan interface. Signals are marked with dotted blue lines and the blue
component is signal related. ................................................................................................................................. 13
Figure 11: Pump Logic block in the Hopsan model. Input to the Pump Logic block is the accumulator pressure
and output is the pump signal............................................................................................................................... 14
Figure 12: Plot over the functionality of Pump Logic block. Pump start to charge the accumulator at 130 bars
and stop the refill at 230 bars. .............................................................................................................................. 15
Figure 13: The mechanical approximation for the simulation model in Hopsan................................................... 15
Figure 14: Simulink interface with the compiled Hopsan model (blue box). Input values are specified in Matlab
and the outputs are delivered as a matrix. ........................................................................................................... 16
Figure 15: A sketch over the articulation geometry showing the cylinder position with the radius for cylinder
motion ( ) and the fixed length ( ). ....................................................................................................................... 17
Figure 16: Motion model for the right hand side cylinder of the articulation. ...................................................... 17
Figure 17: Left picture: The sensors are mounted on test point connections on the hydraulic block. Right picture:
Hydac HMG 3000, used for collecting pressure data. ........................................................................................... 19
Figure 18: Left picture: A tachometer, Testo 470, used for pump motor speed measurements. Right picture:
CAN-logger, Vector GL 1000 mounted in the forklift for logging CAN signals. ..................................................... 20
Figure 19: Pivot angle and cylinder pressure from test 1 ...................................................................................... 21
Figure 20: Pivot angle, accumulator and pump pressure from test 1. .................................................................. 21
Figure 21: The steering signal, PWM, and the resulting pressure, test 1. ............................................................. 22
Figure 22: Pivot angle and cylinder pressure from test 7. The test was performed during wire guidance. .......... 22
Figure 23: Pivot angle, accumulator and pump pressure from test 7. Pump pressure is increased during second 9
to second 12 because of raising the forks. ............................................................................................................ 23
Figure 24: The steering signal, PWM, and the resulting pressure from test 7. Observe the different time scale. 23
Figure 25: Signal input to simulation model. ........................................................................................................ 25
Figure 26: Validation model for the directional valve in simulation program Hopsan. ........................................ 26
Figure 27: Graf from validation model for the directional valve. Flow as a result from a specific current. Blue line
is data from the supplier and the red line is the results from simulation. ............................................................. 26
Figure 28: The blue solid line is the control signal to the valve for a small correction of pivot angle during wire
guidance. The dotted lines are the position change of the spool in the directional valve for Low pass filter and
Rate limiter ............................................................................................................................................................ 27
Figure 29: Pressure in piston 1 and 2 during wire guidance, test 6. ...................................................................... 28
Figure 30: The flow forces effect on the pressure level in the cylinders. ............................................................... 29
Figure 31: Position feedback for the mechanical model. ...................................................................................... 30
Figure 32: Accumulator pressure during free maneuvering compared with data from Test 4. ............................ 31
Figure 33: The right hand cylinder position during free maneuvering. ................................................................. 32
Figure 34: Pressure from the right hand piston during free maneuvering. ........................................................... 32
Figure 35: Pressure from the left hand piston during free maneuvering. ............................................................. 33
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Figure 36: Accumulator pressure during wire guidance compared to measurements from Test 6. ..................... 33
Figure 37: The right hand cylinder length during wire guidance........................................................................... 34
Figure 38: Pressure in the right piston during wire guidance. ............................................................................... 34
Figure 39: The left cylinder pressure during wire guidance................................................................................... 35
Figure 40: Showing the tractor antennas’ position on the forklift [1]. .................................................................. 36
Figure 41: Pivot angle and cylinder pressure from Test 7, wire guidance without load. Zigzag tendencies can be
observed during the wire guidance. ...................................................................................................................... 37
Figure 42: The flow change as a result of the broadening of the cylinder placement. ......................................... 39
Figure 43: Representative valve characteristic from supplier data. ...................................................................... 40
Figure 44: The desirable valve characteristic with estimated working regions. ................................................... 41
Figure 45: Simulation model with double valves and one pressure compensator. ............................................... 42
Figure 46: Two different strategies of controlling double valves to the articulation. ........................................... 43
Figure 47: Simulation results with ramped input signal. ....................................................................................... 43
Figure 48: The delivered flow from valve 1 and 2 during Test 4, free maneuvering. ............................................ 44
Figure 49: A comparison of the simulated and measured piston length during Test 4. ........................................ 44
Figure 50: The delivered flow from valve 1 and 2 during Test 6, wire guidance. .................................................. 45
Figure 51: The comparison of piston length during Test 6. ................................................................................... 45
Figure 52: Measured pivot angle and valve current during maximum steering speed, Test 11. .......................... 46
Figure 53: Graf over the utilization of the directional valve. ................................................................................. 47
Figure 54: Description of the force acting on the lifting cylinder. ......................................................................... 48
Figure 55: Pump, accumulator and piston pressure during wire guidance. Some instability in the piston pressure
can be seen during the raised pump pressure. ...................................................................................................... 49

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