Implementation of Auto-Rotation Function
in a Narrow-Aisle Fork Lift
September 22, 2014
1
Introduction
The following document describes the project activities supervised during the project work occasions and
the intermediate goals that must be fulfilled. Each goal have to be approved and signed by a supervisor
before the project can continue. A completely signed document serves as an acknowledgment for
the student group that the goals have been fulfilled.
Project Group:
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TMMS04
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Project Task
2014 HT-1
Project Description
The aim of this project is to develop an auto-rotation algorithm for a narrow aisle forklift. The forks on
the forklift can both rotate and traverse simultaneously, see figure 1. The goal is to complete one such
auto-rotation within 5 seconds. The algorithm shall use a closed-loop controller, and the auto-rotation must
be able to run autonomously without user interaction.
Figure 1: The aim is to perform a complete auto-rotation of the forks within 5 seconds.
One approach to solve this problem could be to iteratively test the implementation on the algorithm directly
on the forklift in the laboratory, see figure 2. Normally, however, the access to physical prototypes in industry
is limited, due to cost and space limitations. Many people in a company might also want to use the same
prototype. A solution to this is to instead develop the algorithm against a simulated model of the system,
and only use the physical prototype for validating the model and the final testing, as shown in figure 3. This
method will be used in this project. The control algorithm shall be built in Simulink, and the forklift shall be
modeled in the Hopsan simulation environment. For control and data acquisition, the dSpace environment
will be used.
Figure 2: Product development based on testing with physical prototypes can be expensive and time consuming.
Figure 3: With the help of simulation, the need of physical prototypes can be reduced, saving time and
money.
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TMMS04
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Project Task
2014 HT-1
System Description
The fork truck used in the course is a BTtruck, of the type VFLEX AC, with rotating forks. All the fork
motions are controlled by hydraulic actuators, see figure 4. The forks can be moved in three directions (3
DOF) - vertically, horizontally and rotationally.
Figure 4: The movement of the forks are controlled by hydraulic actuators.
3.1
Auto-Rotation
Auto-rotation means that the operator can shift the fork directions (see figure 1), by one push-button. A
typical situation is that the operator has picked up stock goods from the right side of the aisle, which later
on shall be delivered on the left side. This auto-rotation must be possible to perform during transportation
in a store room with narrow aisles. Therefore, the fork tips must remain inside the aisle space during
the whole auto-rotation. In the control system that shall be built in this project work the auto-rotation
shall be able to start from any fork positions, not only from the end positions.
3.2
Parameters
• Supply System
– Pump displacement Dp = 18.9 cm3/rev
– Max pump speed np,max = 3600 rpm
– Overall pump efficiency (np =1450 rpm) ηtp = 0.85
– Volumetric efficiency ηvp = 0.95
– Max system pressure pmax = 180 bar
• Traverse motion
– Displacement of hydraulic motor Dm = 100 cm3/rev
– Max motor speed nm,max = 1.25 rps
– Gear wheel circumference Ogw = 272 mm
– Traverse motion length Lt = 1.3 m
– Traverse carriage weight m0 = 170 kg
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TMMS04
Project Task
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• Rotation
– Piston diameter D = 50 mm
– Piston rod diameter d = 25 mm
– Piston attack radius rc = 56 mm
– Piston stroke (for φ = 180°) s = 177 mm
– Rotation radius for the fork center of mass rg = 250 mm
– Rotation radius for the load center of mass rL = 780 mm
– Weight of rotating plate and forks mg = 185 kg
– Load weight mL = ? kg (will be given)
4
Constructing the Model
The main focus in the beginning of the project is to construct the architecture of the simulation model in
Hopsan. The complete Hopsan model consists of a hydraulic part and a mechanical part.
4.1
Hydraulic Part
The hydraulic part of the Hopsan model has to be constructed from scratch. This is accomplished by using
the hydraulic library and the signal library in Hopsan. The exception is the electric motor component found
in the mechanical rotation library. When modeling advanced dynamic systems, it is important to consider
the level of complexness needed. The goal should always be to achieve a model that is as simple as possible,
but still captures the important behaviors of the system. Since the hydraulic circuit of the forklift is quite
complex, it is essential to find ways of simplifying the model. As a aid to the group, a schematic of a
simplified circuit is given that should serve as target for the Hopsan model, see figure 5
A hydraulic pump (1) with fixed displacement is driven with constant speed. The pump supplies oil
flow to two valve controlled actuators, one hydraulic motor (2) that controls the linear motion of the forks
and two hydraulic cylinders (3) that control the rotational motion of the forks. Both actuation systems
are controlled by 4/3 directional valves, (4) and (5), that only serves to control the direction of the flow to
the actuators. The amount of flow to the actuators are controlled by two constant flow valves (6) and (7).
One check valve (8) makes sure that negative flow is avoided and one pressure relief valve (10) protects the
system from too high pressures. Note that the constant flow valves are mounted in series which would give
a priority to the linear motion over the rotational. The process of modeling the hydraulic part is preferably
done in steps as suggested below.
4.1.1
Valve controlled cylinder
Start by modeling the directional valve connected to two cylinder with a damping orifice. Use the hydraulic
library in Hopsan and supply the direction valve with a constant pressure source and a tank component. Use
lossless connectors to connect the orifice in parallel to the cylinder. Connect each piston to a translational
mass component with a zero force source. Make sure the pistons can move from -1 m to 1 m. The
direction valve is not a proportional valve and should only be able to hold two positions. To give it such
characteristics it is necessary to change the input signal to the valve. Use a function block from the signal
library and connect it to the signal port of the valve.
Use a sign component followed by a gain, and connect them to the valve to test the configuration. Adjust
the gain to scale the signal to the maximum spool position of the valve (0.01), according to:
0.01
if x>0
y=
(1)
−0.01 if x<0
Set the supply pressure to 100 bar and the orifice flow coefficient to 1e-11. In the piston components, set
the leakage coefficient, viscous friction and dead volumes to zero. Control the valve with a ramp signal from
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Project Task
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(a) Complete circuit
(b) Simplified circuit
Figure 5: Hydraulic schematics of the forklift
-10 to 10 to test your system. Run the simulation and plot the cylinder position and the pressures. Also
try the animation mode. Is the system behaving according to your expectations?
4.1.2
Valve controlled motor
Use the same methodology as for the valve controlled cylinder. Use a "C-type variable displacement machine"
component.
4.1.3
Constant flow valve
The constant flow valves consist of an adjustable orifice and a pressure regulating valve, see figure 6. The
pressure drop over the orifice is kept constant by the valve, and any excess flow is emitted through port 3.
The flow rate between port 1 and 2 can thus be adjusted by changing the opening area of the valve. Build a
model of a constant flow valve in Hopsan and make sure it works correctly. Then add a second flow control
valve, and connect them to the actuators. Note that port 3 in the first one is to be connected to the inlet
port of the second one. This means that the linear actuator runs on the excess flow from the hydraulic
motor.
1
2
3
Figure 6: Schematics of a three-port constant flow valve
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TMMS04
4.1.4
Project Task
2014 HT-1
Supply system
Build the supply system, connect it to the rest of the system (i.e. to the inlet port of the first constant flow
valve) and verify that everything works as intended. There shall be a pressure relief valve that protects the
system for too high pressure levels.
4.2
Mechanical Part
The mechanical part of the forklift is already given (figure 7) and is included in the special Hopsan version
found on the homepage. The component is modeled as two separate motions; one traversing and one
Figure 7: The forks component in Hopsan
rotational. For simplicity and performance, the motions are assumed to be decoupled. A bilinear transform
solver, similar to the trapezoid rule, is used to solve the differential equations. The physical parameters of
the model must, however, be adjusted. See the following list for the important parameters to adjust:
Rotational motion
Radius to the pistons
Moment of inertia
Dry friction
Viscous friction
Minumum angle
Maximum angle
Traversing motion
Radius of the pinion
Linear inertia (mass)
Viscous friction
Minumum position
Maximum position
To change the starting position of the forklift, adjust the a parameter between -0.6 and 0.6 and the x
parameter between -1.5708 and 1.5708.
4.3
Complete model
Assemble the complete forklift model by using copy and paste between your model files. For both motions,
it is the same signal coming to the direction valve as to the control valve. The control valve should also
have a dead-band modeled. Make sure that all model parameters have reasonable values. You can find
a list of some parameter values in the project description. If there are other unknown parameters, try to
make a sophisticated estimation. Simulate a rotating motion by using a step signal source. Use a constant
signal source for the translational motion and set it to 0. Are the forks rotating? Do the same test with
the translational motion and then run them both together. If the model is behaving the way you would
expect, it is time to validate the model against the real forklift. This is done by performing measurement
tests on the real forklift and then comparing them with the results gained from the simulation model. It is
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TMMS04
Project Task
2014 HT-1
important that the group has planned the experiments and listed what needs to be measured before coming
to the forklift (see laboratory exercise 3).
Intermediate Goal 1: Complete Model and Validation Plan
Supervisor
5
Date
Validation
Validation is an iterative process where measurement data is continuously compared with simulation results.
If the simulation model is shown to be inaccurate, the parameters must be adjusted and the simulation
model run again. This is process is repeated until the model resembles the experimental data sufficiently.
For easier comparison between measurements and simulation, it is desirable to convert the Hopsan data to
Matlab format. This can be done by exporting data from a plot window in Hopsan to a comma-separated
values file (.csv) or to a Matlab script file (.m). Use the plot command to compare graphs from simulation
and experiments.
Intermediate Goal 2: Validated Model
Supervisor
6
Date
Co-simulation Hopsan - Simulink
Follow the steps in "Exporting Hopsan to Simulink" tutorial for detailed instructions on how to set up the
Hopsan - Simulink co-simulation. Once the co-simulation is set up, run the simulation from Simulink.
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Development of Control Algorithm
Use your previous knowledge in control theory to develop the control algorithm in Simulink for an autorotation applied to your forklift model in Hopsan. When an auto-rotation can be performed under 5 seconds
in simulation, the group is ready to try the algorithm on the real forklift (Laboration 4).
Intermediate Goal 3: Simulated Auto-rotation
Supervisor
Date
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