Ref: C0600
Draft simulator to reproduce field work on the road
Michele Mattetti and Giovanni Molari, Distal Unibo, viale Fanin 50, Bologna Italy.
Matthew Walker, CNH Italia, viale delle Nazioni 55, Modena Italy.
Abstract
The increase of the tractor reliability can be reached only through the design of the tractor in
function of the real loads applied in different components during the machine usage. In recent years the introduction of CAE software in the design and development phase has greatly contributed to this advancement in reliability, however the final validation of the machine is
only completed after experimental tests. These tests - aimed at reproducing the vehicle load
cycles and validate design model - require a large amount of time and represent a significant
cost to the manufacturer. In any case the field test of the tractor in real world conditions is
difficult due to the problems connected with the weather, the seasonality of the operations
and the variability of the test conditions. As a consequence the tests performed on the field
are not repeatable and fully reliable. In the past different procedures have been introduced to
reproduce the loads acquired during field test on proving grounds or on 4 post-test rigs. Despite the use of both methods manufactures were only able to accurately reproduce vertical
loads and accelerations, these methods were not able to reproduce the longitudinal loads
frequently introduced on the tractors during field operations. It is possible to introduce longitudinal loads during proving ground tests by connecting a load cart to the tractor, but the
commercially available load carts are not designed to generate impulsive loads transmitted to
the tractor due to the implements interaction with the soil. Moreover commercial load carts
are connected to the hitch instead of the three point hitch commonly used to connect an implement to the tractor. The aim of the paper is a feasibility study of the design of a load cart
able to reproduce longitudinal impulsive loads transmitted by a three point hitch to the tractor
to perform durability test on tractors. Measurements using different implements on different
soil types were performed to define the loads transmitted by the implement to the tractor
through the three point hitch. Then a dynamic model to simulate the connection between
tractor-implement was defined to design the load cart. The load cart is constituted by a trailer
connected to the three point hitch. The wheels of the trailer are braked in order to reproduce
the draft acquired during field operations. The use of this load cart on proving ground will
allow to reproduce the loads measured on the tractor during the whole life independently by
the weather conditions.
Keywords: Tractor, durability, three-point hitch, testing.
1
Introduction
The increasing competitiveness between different manufactures of agricultural tractors has
stimulated the reduction of the prototype developing time and the increase of the reliability
such as in the automotive sector (Hughes, Jones, & Burrows, 2005). One of the most costly
activities in tractor development is durability approval, that is the application of a load sequence to a specific component or the whole vehicle, equivalent of the real one applied during the machine usage on field (Oelmann, 2002). Despite in the recent years the CAE tools
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are widespread in vehicle companies, the final approval of vehicles has been carrying out
through tests due to the difficulties to model some vehicle components (Ensor, Cook, &
Birtles, 2005) that limit the models’ accuracy and as a consequence a wrong predicted components lifetime. The development time has been reducing through accelerated tests, able to
apply to the structure a damage equivalent to that applied to the vehicle during the work on
field in a reduced time (Dodson & Schwab, 2006). A test could be accelerated through: the
increase of the load amplitudes; the increase of the load frequency; the removal of the not
damaging cycles, but an accelerated test is valid only if it is able to reproduce the similar load
spectras, damage and failures caused by the real vehicle usage (Lee, Pan, Hathaway, &
Barkley, 2005). Field tests would be preferable for the ability to reproduce the real use of the
machine though not repeatable and controllable, but the main problem of these tests is the
dependency by the weather conditions not easy to combine with the manufacturer schedules. For these reasons the manufacturers have tried to reduce the field tests replacing with
alternative methods such as the proving grounds (Mattetti, Molari, & Sedoni, 2012) or the
four post test rigs (Anthonis, Vaes, Engelen, Ramon, & Swevers, 2007; Mattetti, Molari, Vertua, & Guarnieri, 2013; Vaes, Engelen, Anthonis, Swevers, & Sas, 2007). Both methods are
able to reproduce the test in any weather conditions but are able to faithfully reproduce only
the vertical dynamic and not the horizontal one. The horizontal loads induced by the draft
created by the soil-implement interaction (Mattetti, 2012) are considerable, and the lack to
reproduce these loads may cause a not complete evaluation of the tractor durability and as a
consequence a not respect of the reliability standards required by the manufacturers. This
problem could be overcome driving the tractor on the proving grounds with a load unit, but
the used load units are usually designed to perform drawbar tests such as the draw bar power test included in the OECD Code 2 (OECD, 2012) or the Nebraska test (NTTL, 1998).
These load units are conceived to be attached to a tractor trailer hitch to apply a constant
load for a reduced time. However the loads generated from the soil on a three point hitch are
random (Hayhoe, Lapen, McLaughlin, & Curnoe, 2002) and the durability test requires to be
performed for at least 10 hours per day to reduce the testing period. Finally a part of the
loads are transmitted to the tractor through the three point hitch with the influence of the
loading control. For these reason the load units used to measure the tractive performances
can not be used to perform accelerated tests but it is necessary to design a specific load unit
able to reproduce the same draft produced by the implements. This goal could be reached
using a feedback loop control of the load applied by the load unit (Mattetti, Molari, & Walker,
2013). Between the different techniques, the Simulation in the Loop (SIL) is one of the most
frequent used (Castiglione, Stecklein, Senseney, & Stark, 2003). This technique permits to
test a control system in a simulated environment instead of the real hardware. In this way the
performance of the whole system is evaluated before the construction, allowing to reduce the
costs and the developing time and to obtain excellent performances (Steiber et al., 2003;
Winkler & Gühmann, 2006).
The goal of the paper is the design of a new load unit to perform durability test on agricultural
tractor with SIL techniques and the following design constraints:
 connection to the three point hitch;
 allow the three point hitch movement and the load control activation during the braking load application;
 permit a good steerability to permit a driving on proving grounds;
 capability of the braking system to apply a braking force necessary;
 possibility to work until 10 hours.
2
Material and methods
To design a load cart able to perform accelerated tests on tractors with a maximum power of
80 kW, some measurements using different implements on different soils were performed on
a tractor with a weight of about 4200 kg. The implements were connected to the tractor
through a three-point hitch coupler (ASAE, 2003) on which 3 load pins were located to
measure the draft for each testing condition. Three different implements (plough, ripper, and
cultivator) were tested on two different fields. The measured signals were sampled at a fre-
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quency of 500 Hz and filtered with a low pass filter with a cutting frequency of 250 Hz to delete the high frequency noise.
The load unit has been designed started from a commercial trailer modified in function of the
design constraints, in particular one axle pig trailer has been used due to the adherent mass
similar to the total mass of the trailer. The connection between the tractor and the trailed has
been realised with a ball joint fixed on a frame connected to the three point hitch to allow a
good steerability of the trailer and the possibility to move the three point hitch while braking.
The wheels are connected through a transmission to a friction brake with a rated of 67 kW
and a maximum braking torque of 3500 Nm (Eaton, 2014). Finally are presented the pneumatic system to actuate the brake and the cooling system with glycol-water mixture. The
scheme of the system is reported on Fig 2:
FIG 2: The trailer and a layout of the braking sistem.
The pressure into the brake cylinder is defined by a proportional directional control valve controlled in voltage and that vent directly to the atmosphere in case the draft is higher than the
reference value. The replication of the draft is guaranteed by a feedback control loop that
control the valve to minimise the error between the required draft and the really applied
measured from the load pins at the hitch points of the three point hitch (Fig 3):
Ref draft
-
+
Error
PID
Voltage
4
2
5
Pressure
Torque
Draft
3
1
Fig 3: layout of the control system
Following a multidomain system model has been designed in SimulinkTM
(http://www.mathworks.com/). The system is composed by a multibody model of the tractor
and of the three point hitch, a hydraulic model of the actuation of the load control, a pneumatic model of the brake drive and a powertrain model of the brake-wheel system of the trailer.
From the described model the capability of the chosen component to reproduce the draft and
has been evaluated and the optimum control able to reproduce the working condition has
been defined (Macia & Thaler, 2005).
3
Results and Discussion
The trend of the draft measured on the U-frame hooks during a field test has been compared
with that obtained from the model in Fig 4.
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40
Simulated
Measured
35
30
Draft [kN]
25
20
15
10
5
0
0
20
40
60
80
100
120
140
Time [s]
Fig 4: comparison between the draft measure on the three point hitch and that obtained from the model.
The two signals are comparable, the model reproduce oscillation similar with respect to that
obtained on field. The chosen hardware is sufficiently fast for tracking the measured signal.
The peaks of the signals obtained from the model are average lower with respect to that
measured with a difference lower than 15%.
4
Conclusions
Performing tests in the field is more and more difficult due to the high cost, the time constraints due to weather and season. Despite the numerous solutions adopted to reproduce
on test benches or specific tracks the service loads, solutions able to faithfully reproduce the
tractor dynamics on the field are not known. In this paper a load unit to reproduce the draft
created by the soil has been designed and modelled using a multi-domain system model.
The model permits an evaluation of the designed system. The system is able to reproduce
sufficiently the draft measured on field and to reproduce the movements of the three point
hitch. A better tuning of the controller is necessary to improve the reproducibility of the
measured draft.
The project will be developed with the building of the loading cart and the controller identified
to actuate the real system.
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