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bio s yste ms e n ginee r ing 185 (2019) 45 e55 Available online at ScienceDirectjournal homepage: / locate/issn/15375110 Special Issue: Agricultural Machinery Safety Research PaperExperimental characterisation of front axle suspension systems for narrow-track tractorsMichele Mattetti a, Stefano Davoli a, Mirko Maraldi a,*,1,Francesco Paolini b, Stefano Fiorati b, Giovanni Molari aa Department of Agricultural and Food Sciences (DISTAL), University of Bologna, Viale G. Fanin 50, 40127 Bologna,Italyb CNH Italia, viale delle Nazioni 55, Modena, Italya r t i c l e i n f o Article history:Published online 5 February 2019Keywords:Front axle suspension ComfortWhole-body vibration Narrow-track tractorOver the last few years, stricter comfort and stability standards for agricultural vehicles have generated tough challenges for manufacturers. Front axle suspensions can considerably improve tractor comfort. This solution has been typically installed on vehicles over 100 kW but recently, it has also been introduced for narrow-track tractors. These types of tractor require special features in terms of dimensions, manoeuvrability and stability; indeed, the small turning radius and the roll-over performance required present tough constraints. In addition, the lack of space for positioning suspension systems is a concern. A lack of back- ground knowledge has forced companies to search for methods to systematically validate suspension performance. The aim of this work was to design and validate a test-set that can enable the front axle suspensions of these specialised tractors to be fully characterised. The proposed test-set comprises of two different bumps tests and a braking test which are designed to excite each tractor rigid mode selectively. In tests, a series of sensors was installed to measure accelerations at different tractor positions and other dynamic param- eters (tractor speed, pitch rate and roll rate) in real-time. Aggregated data (root mean square and peak-to-peak values) were calculated from the acquired signals which allowed to compare two tractors of the same power class, one equipped with a front axle suspension, and one with a rigid front axle. The collected data could lead to a benchmark definition of the tractor class features, to be compared with newly designed, improved suspension systems. 2019 IAgrE. Published by Elsevier Ltd. All rights reserved.1. IntroductionThe harsh conditions under which tractors operate often leads to poor vibrational comfort, limiting field travellingspeed and productivity (Scarlett, Price, Semple, & Stayner, 2005, pp. 1e249). Vibrational discomfort can also affect the perceived quality of vehicles (Bubb & Estermann, 2000) and cause health problems to drivers (Griffin, 1996). It has been* Corresponding author.E-mail address: maraldi.mirko (M. Maraldi).1 The Cooks distance is a measure that quantifies the changes in the regression model when observation i is removed from the model. /10.1016/j.biosystemseng.2019.01.0131537-5110/ 2019 IAgrE. Published by Elsevier Ltd. All rights reserved.Cooks distance eFront-ballasted tractor Inertial measuring unit Data point indexTotal number of observation for single test Unladen tractor configurationNumber of coefficients in the regression model Peak-to-peakParallel bump test Rear-ballasted tractor Root-mean-squareMean squared error of the regression model Fully-ballasted tractorTractor without front axle suspension Tractor with front axle suspension Low tractor speed m s1Medium tractor speed m s1 High tractor speed m s1Predicted value of observation j various Predicted value of observation j according to a regression model built without data point i variousLongitudinal axis Transverse axis Vertical axisybjybjiX Y ZAB test Asymmetric bump testANOVA Analysis of variance BRK test Brake testD FB IMUjnNBpP2PPB test RB RMSs2 TB TN TSv1 v2 v3Longitudinal acceleration m s2 Lateral acceleration m s2 Vertical acceleration m s2axay azNomenclaturesuggested by several studies that the vibration total value of tractors is often higher than the daily exposure limit estab- lished by European Directive 2002/44/EC. In such cases, the operation of tractors has to be restricted during prolonged working days (Directive 2002/44/EC, 2002; Scarlett, Price, & Stayner, 2007).To improve vibrational comfort, and to comply with the regulations in terms of operator vibration exposure, tractors can be equipped with various suspension systems, such as: seat suspension, cab suspension and front axle suspension. Originally, vibrational comfort was ameliorated by employing suspended seats. However, the benefit of this solution is rather limited, because the drivers mass (i.e. the suspended mass) is much lower than the tractor mass (i.e. the unsus- pended mass) (Hilton & Moran, 1975). Thus, front axle sus- pension systems were introduced onto tractors to mitigate the negative consequences of tractor pitching, primarily consist- ing of power-hop instability, occurring when tractors were operated under high draft loads (Rill, Salg, & Wilks, 1992). Front axle suspension systems also help reduce the vibration total value by attenuating both vertical and longitudinal ac- celerations (Giordano, Facchinetti, & Pessina, 2015). However, results in this respect are not consistent among the different studies appeared in the literature. Indeed, Marsili, Ragni,Santoro, Servadio, and Vassalini (2002) reported mixed ob- servations on the benefits of a front axle suspension: a reduction up to 15% of the vibration total value for road ap- plications, as opposed to an amplification of the vibration total value on dirt track and a large variability in field operations (ranging from no attenuation up to a 36% of reduction of the vibration total value), instead. Giordano et al. (2015) also observed a reduction up to 7% in the vibration total value for the tractor alone, and a lower reduction when the tractor operated with an implement connected through the three- point hitch with respect to the case of tractor with no imple- ments connected. Scarlett et al. (2005) reported that, despite the largest accelerations occur along the vertical direction, the most influential term in the calculation of vibration total value (which is a weighted composition of the accelerations along the longitudinal, transversal and vertical directions) is the longitudinal acceleration for trailer transportation and the lateral acceleration for implement operations. In turn, accel- erations along these two axes are influenced by tractor pitch and roll (Molari et al., 2011). The extent of such influence de- pends on some characteristic dimensions of the tractor, such as seat height position, wheelbase and axle track width (Gomez-Gil, Gomez-Gil, & Martin-de-Leon, 2014; Mattetti et al., 2012). For instance, the reduced wheelbase and axle track width that is typical of narrow-track tractors, leads to an in- crease in the pitch and roll rates and ultimately to an increase in the seat lateral and longitudinal accelerations.The variability in the results appearing in the literature can be explained by the fact that front axle suspension behaviour has been evaluated under real operating conditions or with the tractor travelling over the standardised ISO test track (ISO 5008, 2002), where drivers can bring inconsistencies and data dispersion over several test repetitions due to differences in their driving style. Moreover, the analyses were limited to comparing seat accelerations or of the vibration total value of the tractors with activated and deactivated suspension. However, tractors are operated on highly variable ground conditions, carrying out a variety of tasks, utilising a variety of tools and with a wide range of operating parameters (e.g. travelling speed, ballast configuration). This leads to suspen- sion systems receiving significantly different input excitations and having a range of natural frequencies.As a result, few insights into how front axle suspensions effectively work have so far been reported. In order to properly analyse the performance of front axle suspensions, selective tests to individually excite each vehicle rigid mode are required. This can be successfully achieved using 4-post testing (Mattetti, Molari, & Vertua, 2015), but, due to the high cost of 4-posts test rigs, such solutions are not feasible for small-sized companies or research centres.The aim of this work was to design and validate a test-set able to fully characterise a front axle suspension. The basis on which the test-set design is grounded is the selective excitation of each tractor rigid mode; each mode was excited using road bumps placed at specific locations depending on the values of wheelbase and axle track width of the vehicle under study (Gillespie, 1992). The methodology was then applied to the case of narrow-track tractors where front axle suspensions have been recently introduced (Uberti, Gadola, Chindamo, Romano, & Galli, 2015).49bio s yste ms e n ginee r ing 185 (2019) 45 e55 2. Materials and methodsA methodology was applied to compare two narrow-track tractors: one without front axle suspension (labelled TN), and another equipped with a front axle suspension (labelled TS). Tractors brands and models are not indicted to avoid commercial interest but both two tractors were similar in terms of specifications (Table 1). The suspension of tractor TS was composed of two hydraulic units connected to a longi- tudinal trailing arm and a Panhard rod; each hydraulic unit consisted of a hydraulic cylinder plus an accumulator. Both tractors were set up in order to have the most similar config- urations. In particular, the front and rear axle tracks were set in order to be as similar as possible for both tractors.The following tests were carried out on paved surface for each tractor:Table 1 e Specifications of the tractors tested.TSTNFront axle suspensionyesnoUnladen tractor mass kg29743020Unladen tractor mass4041at front axle %Front wheels inflation280/70 R18 120280/70 R18 120pressure kPaRear wheels inflation420/70 R28 120420/70 R28 120pressure kPaWheelbase mm21802180Axle track: front/rear mm1177/12851220/1280Seat base height from8882the cabin floor mm Parallel bump test (PB test): two bumps located at the same longitudinal position, so that the tyres of each axle hit the bumps simultaneously (Fig. 1a). This test was designed for pitch excitation. Asymmetric bump test (AB test): two bumps located at different longitudinal positions, so that the right-side tyres hit the bumps first, followed by the left-side tyres (Fig. 1b). This test was designed for roll excitation. Brake tests (BRK test): straight-line braking at maximum driver effort from maximum tractor speed (around 40 km h1) to stop; this test was designed for evaluating the suspension anti-dive performance.All the tests were carried out with the same driver (driver mass - 80 kg) to limit as much as possible the influence of the driving style among the different test repetitions.Both tractors were equipped with a data logger (HBM Somat eDAQ, Darmstadt, Germany), in order to monitor all the information related to driver inputs, wheels response, front axle suspension excitation, front axle suspension response, tractor frame state and seat excitation. In particular, the following sensors were installed using silicone adhesive (Fig. 2):a) 4 single-axis accelerometers (Model:3711E1110G, PCB Piezotronics Inc., Depew, NY, USA) to measure the vertical accelerations at the four wheel-hubs (Fig. 2b);b) 1 tri-axial accelerometer (TE 4630-02, TE Connectivity, Schaffhausen, Switzerland) placed beneath the engine radiator to measure the acceleration of the front-axle support along three orthogonal axes (Fig. 2c);c) 1 inertial measuring unit (IMU; HBM EGPS200, HBM, Darmstadt, Germany) to measure pitch rate, roll rate and travelling speed of the tractor frame (Fig. 2d);Fig. 1 e The three tests performed: a) parallel bumps test (PB test); b) asymmetric bumps test (AB test); c) brake test (BRK test).Fig. 2 e a) Sensors location on the tractor: single axis accelerometers at wheel hubs (circles and subfigure b); front axle support triaxial accelerometer (yellow hexagon and subfigure c); IMU (green hexagon and subfigure d; and seat base triaxial accelerometer (violet hexagon and subfigure e).d) 1 tri-axial accelerometer (TE 4630-02, TE Connectivity, Schaffhausen, Switzerland) installed next to the seat rail to measure the accelerations at the seat base along three orthogonal axes (Fig. 2e);e) 1 load cell (LP-50Kb, Kyowa Electronic Instruments Co., Ltd., Tokyo, Japan) to measure the load placed on the brake pedal by the driver.According to the standards (ISO 2631-1, 1997b; ISO 5008, 2002), the vibration total value should be calculated measuring the accelerations along three orthogonal axes (longitudinal, transverse and vertical axes indicated with X, Y and Z, respectively) at the contact points between the driver and the tractor, i.e. mainly the seat surface. However, such accelerations were not measured in this study, because they are not appropriate to highlight the effect of a front axle sus- pension. Indeed, the value of the accelerations at the seat is also affected by the features of the seat installed on the tractor, which may be different from case to case, according to customer preference.2.1. Parallel and asymmetric bumps tests: tests specifications, signal conditioning and data post-processingFor the PB and the AB tests, two commercial speed bumps made of steel and having a trapezoidal shape with a height of 90 mm and a length of 1200 mm were used. To vary excitation, tests were performed at 3 different speeds. Different ballast configurations were used to simulate the different ballasts the tractor may be equipped with (Table 2). Each test condition was repeated at least three times. As a consequence of the different ballast configurations, the mass at the front axle ranged from 34% to 45% of the total mass for the two tractors. Signals were acquired at a sampling frequency of 200 Hz and then filtered before being analysed. A low-pass Butter- worth filter of 10th order with a cut-off frequency of 14 Hz(MATLAB R2016b, Mathworks, Inc., MA, USA) was used.The next step was the selection of the event window. For each test, a number of relevant signals was identified among all the acquired signals, namely: for the PB test the front axle support vertical acceleration, the seat base verticalTable 2 e Parallel bump test and asymmetric bump test overview.Ballast masskgTest speedTractors testedNB e unladen tractor0v1 e lowTSe with front axle suspensionFB e front ballast170v2 e mediumTNe without front axle suspensionRB e rear ballast450v3 e highTB e full ballast (F R)620acceleration and the pitch angle; and for the AB test the front axle support lateral acceleration, the seat base lateral accel- eration and the roll angle. The values of pitch and roll angle were obtained by numerical integration of the pitch rate and roll rate signals, respectively; the algorithm used for the nu- merical integration of the signal was the trapezoidal rule (MATLAB, Mathworks, Inc., MA, USA). The event onset was set when any of the relevant signals changed its amplitude significantly (indicating a perturbation in the signal related to the passage of the tractor on the bumps), while the event end was set when the last of the signals was considered to return to the amplitude values observed prior to the tractor hitting the bumps. Thus, the procedure could identify an event win- dow for each test.Within the event window, the vertical accelerations az at four different locations were analysed: front left wheel-hub; front right wheel-hub; rear left wheel-hub; and rear right wheel-hub (Figs. 2 and 3). The analysis allowed the identifi- cation of sections of the event window that corresponded to each tractor wheel travelling over the bumps; indeed, the major perturbation in any of the aforementioned signals occurred when the corresponding axle ran over the bumps, and for each signal the negative peaks of acceleration corre- spond to the ascent/descent of/from the bumps (see the in- dicator functions in Figs. 2 and 3).To evaluate the effects of ballast, tractor speed, and the influence of the front-axle suspension on the performance of the tractors, root-mean-square (RMS) and peak-to-peak (P2P) values of the relevant signals were calculated; all the analysis and calculations were performed on the portions of signals inside the event window. These two metrics were chosen because they correlate well with subjective ratings of ride comfort in these tests (Previati, Gobbi, & Mastinu, 2016); in particular, the RMS value was considered for vertical, longi- tudinal and lateral accelerations, whereas the P2P value was considered for pitch and roll rates and angles. In order to eliminate the effects of inaccuracies in tractor manoeuvring, outliers were identified and removed from the analysis of the aggregated data. This was done using, for each tractor and for each ballast configuration grou