Automatic Traction Control for Articulated Off-Road Vehicles
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GENERIC COLORIZED JOURNAL, VOL. XX, NO. XX, XXXX 2017 1 Automatic traction control for articulated off-road vehicles JOHAN MARKDAHL Abstract— Construction equipment is designed to main- model of a Volvo Construction Equipment A40X articulated tain good traction, even when operating in difficult off-road hauler in the multibody physic simulator MSC ADAMS [13]. conditions. To curb wheel slip, the vehicles are equipped with differential locks. A driver may engage/disengage the locks to switch between two distinct operating modes: the closed mode is characterized by greater off-road pass- A. Background ability while the open mode allows better manueverability. However, many drivers lack the education and experience Articulated haulers are heavy equipment, used to transport required to correctly judge the terrain ahead of the vehicle large quantities of loose materials such as sand, gravel, and and therefore engage/disengage the locks in a suboptimal liquids in off-road enviroments. The articulated steering (see fashion. An automatic traction control solution for locking Fig 5) ensures high maneuverability although at the cost and opening the differentials is hence desirable. This paper compares three on/off differential lock control algorithms, of a lower maximum payload. Articulated haulers may, for all derived from the same kinematic vehicle model but each example, be employed at mines to transport ore over grounds relying on the availability of output signals from different not traversable by ordinary vehicles or on construction sites sensors. The validity of the kinematic model and the algo- to transport building material. rithms’ sensitivity to the values assumed by a couple of Traction is an adhesive friction force in tire/road interface unobservable states, the wheel slip angles, is investigated by comparison to a realistic articulated hauler model in the that serves to drive the vehicle forward. A tire may sometimes multibody physics simulator MSC ADAMS. lose its grip and slip rather than roll over the road, e.g. if subject to full throttle, icy conditions or a steep inclination. Index Terms— Traction control, off-road, articulated vehi- cles, heavy equipment, construction equipment, articulated This is referred to as lost traction. Lost traction is undesirable hauler, wheel loader, differential locks, on/off control. since it reduces the vehicle’s traversability and increases tire wear. Means must therefore be taken to curb wheel slip and regain lost traction, preferably at the onset of wheel slip. 1. INTRODUCTION A differential is a driveline component that distributes The design of articulated haulers is optimized for traversing power, i.e. torque and rotational speed, from an input shaft difficult terrain [1, 2]. This allows the vehicles to take the to two output shafts. The differentials on articulated haulers shortest route on a load, haul, dump run; thereby minimizing have two distinct operating modes: open and closed. An fuel consumption and time expenditure. Part of this opti- open differential distributes rotational speed freely and torque mization is automatic traction control (ATC) which eases evenly. A locked differential forces the output shafts to assume the decision making burden of the driver, protects the tires the lowest of the two wheel speeds while torque is distributed from unnecessary wear, and reduces fuel consumption by up freely. A succesion of locks may be engaged to curb the slip to 6% [3]. Note that the pricetag for a single tire is USD of multiple wheels. 5 000 and tires represent 20–25% of a haulers operating costs The differential locks are of the dog-clutch variety: a pair of arXiv:2104.11289v1 [math.OC] 22 Apr 2021 [4]. Manufacturers like Caterpillar, John Deree, Komatsu, and face gears that are locked together pneumatically and pulled Volvo use ATC [5], but the details of their algorithms are apart by a spring. Being locked together, the two shafts unknown to the public except for glimpses gained from ads assume the same speed. The dog-clutch has a range of angular [3] and patents [6, 7, 8, 9, 10, 11, 12]. The ATC problem velocity differences over which it is safe to engage, otherwise is also interesting from a theoretical point of view since the engagement risks damaging the gear teeth. If the angular nature of the actuators (so-called dog clutches) requires on/off speed differences does not satisfy this constraint, then some control with a strong emphasis on the ‘on’ decision. This paper articulated hauler models allow individual wheel brakes to proposes three traction control algorithms based on different reduce the rotational speed of selected shafts. sensor output. The algorithms are validated against a realistic The articulated hauler often serves as an entry point for beginner drivers who later move on to more advanced vehicles This paragraph of the first footnote will contain the date on which you submitted your paper for review. This work was jointly supported such as wheel loaders and excavators as they gain experi- by the Swedish innovation agency VINNOVA and Volvo Construction ence. Inexperienced drivers tend to be overtly reliant on the Equipment. differential locks, turning them on at all times. By contrast, Johan Markdahl is with the Luxembourg Centre for Systems Biomedicine at the University of Luxembourg (e-mail: system logs from Volvo’s articulated haulers show that their [email protected]). ATC system can outperform even skilled drivers. 2 GENERIC COLORIZED JOURNAL, VOL. XX, NO. XX, XXXX 2017 B. Problem statement this paper appear in the author’s master’s thesis [16]. A draft The key question for ATC is when to lock the differentials. version of this paper also appears in the PhD thesis [4]. This comes down to comparing the revolutions per minute (RPM) of driveline shafts that are coupled through open 2. PRELIMINARIES differentials. A difference in RPM indicates slip, unless the vehicle is turning or braking. The most interesting automatic A. Articulated hauler model in ADAMS control challange is to discern differences in RPM due to The results of this study are validated by simulations of an turning from those due to wheel slip. In this paper we use articulated hauler model in MSC ADAMS which is owned by the hauler geometry to develop a kinematic model that lets Volvo Construction Equipment (VCE). Part of the presentation us answer this question. Moreover, we explore the advantages is therefore based on VCE articulated haulers (note that that may be gained from utilizing information obtained from the author decleares no conflict of interests). In particular, e.g. a ground speed sensor ( a ground speed radar or a GPS assumptions about the vehicle geometry, the sensor network receiver) and individual wheel tachometers (angular speed and actuators that are available on all units are based on the sensors). These sensors are not standard, so this paper also VCE ATC system [3]. However, the key ideas of our control addresses questions that are of interest to the manufacturers. algorithms generalize to other geometries and sensor networks. The VCE sensor network is displayed in Fig. 1. The fol- C. Literature review and contribution lowing five sensors are availible on all hauler units: a steering The literature on traction control and various related topics angle sensor; four tachometers: the transfer case (dropbox) such as antilock breaking systems, electronic stability control, in/out sensors and the bogie axle in/out sensors. The following and anti-slip regulation mainly concerns automobiles, see five actuators are available: three transversal differential locks, e.g. the survey [14], and the survey [15] on traction control one for each wheel axle (not in Fig. 1); one longitudinal for electric vehicles. Traction control for articulated off-road differential lock which locks together the tractor and trailer vehicles is largely unexplored in the academic literature, unit axles; and one bogie lock which engages the 6 × 6 drive although there are some exceptions [4]. Rather, the state-of- (the vehicle usually drives on 6 × 4 for better fuel economy). the-art exists as inhouse software solutions that are unavailable The geometry of the MSC ADAMS vehicle is the same as to the public. To gain an idea of what ATC algorithms are the A40X Volvo CE articulated hauler (X denotes the genera- used by manufacturers we may turn to commercials, product tion, which varies from D to G). Some relevant specifications: specifications, white papers, grey literature, and patents. Vehicle mass is 28 500 kg. • Some algorithms presented in patents are based on braking Max load is 39 000 kg. • the slipping wheel, including [6, 8]. This actuation is not Front wheel axle to steering joint, l = 1:278 m. Front • 1 without limitations, including additional wear on the brakes. It boogie wheel axle to steering join, l2 = 3:265 m. requires sensors that can detect which in a pair of wheels that The axle track, i.e. the distance between a pair of wheels, • is slipping, i.e. individual wheel tachometers. Moreover, the is 2c1 = 2:636 m [17]. heat generated by braking can only be sustained by the vehicle A picture of the ADAMS model powertrain corresponding to from a limited time [12]. The majority of relevant patents use the VCE A40X hauler is displayed in Fig. 2. differential locks for actuation [7, 9, 12]. The algorithm [9] is unique in that it uses detection of oscillations in the driveline shafts to detect the presence or absence of wheel slip. Most algorithms make use of a steering angle sensor [6, 8, 9, 10, 12] or a related output signal [7], but unlike our paper they do not use the steering angle derivative. For motor graders an additional angle sensor for the front wheel pair is needed [11, 12]. The algorithm [7] uses data from presure sensor in the steering hydraulics to decide when to lock or unlock the differentials.