EVS25 Modular driveline concept for underground mining loader

This paper is part of an ongoing HybLab project funded by the Multidisciplinary Institute of Digitalization and Energy (MIDE) of Aalto University School of Science and Technology. First results and plans were presented in EVS24 [1]. The focus in this paper is to describe what kind requirements must be taken in to account when designing and dimensioning an electromechanical driveline for an underground mining loader. For case work machine is chosen 18t loader which was presented in EVS24 [1]. Also concept for a modular driveline concept will be presented. Basic idea in the concept is to avoid over dimensioning of electric drives by using electromechanically actuated mechanical links between the drive units. The concept gives possibility to optimize dimensioning of each electric drive in driveline so that the best efficiency is in use in normal mean power drive conditions. When one drive needs peak power then mechanical power can be driven via mechanical links from other drives. Disengageable mechanical links gives also possibility to avoid internal counter-torques in driveline caused by steering geometries and differences in tire diameters. The concept will be presented in the EVS25. The first results and experiences from the electromechanical driveline will be presented in EVS26. Copyright EVS25 .


Introduction
This paper is focused to describe what kind requirements must be taken in to account when designing and dimensioning an electromechanical driveline for the case machine type of mobile work machine. Also concept for a modular driveline concept will be presented.

Basic Data
The Case machine was presented more detailed in the EVS24 [1]. This chapter tells short version of the basics.

Case machine
The case work machine is a frame steered underground mining loader. The loader total weight is 18t with 4t duty load. Maximum speed is 12km/h. In the conventional production version the engine power is 90kW. The engine drives two hydraulic pumps; one for traction and one for implement hydraulics. Driveline is based on hydrostatic transmission with individual axle motors. Also bucket functions and steering are based on hydraulics.

Duty Cycle
Typically underground mining loaders gets mined rock from ends of mine caverns and brings it to main tunnel where the load will be dumped to a dumpertruck or conveyor. Driving distance for loaders is typically 200-400m and slopes in caverns are typically between 0-20%. The duty cycle, chosen for the project, is described in the fig. 1.  Table 1). The last phase ends-up to the same place where the first phase starts again. In real mining conditionings biggest changes between cycles exist in the phase 4 because the rock pile is not homogenous which causes changes in mass of load and time of loading. Other phases are very accurately repeatable. Driving 12% down-hill (empty bucket) Ph3 Driving on flat (empty bucket) Ph4 Loading the bucket Ph5 Driving on flat (full bucket) Ph6 Driving 12% up-hill (full bucket) Ph7 Driving 4% down-hill (full bucket) Ph8 Dumping the bucket Driving speed in duty cycle is limited to 15km/h which is quite generally used speed limit in mines.

Requirements for the driveline in loader type of work machines
Loader type machines have some specific features which have influence for driveline; the loading situation, frame steering geometries, differences in axle loads and differences in tire diameters.

Loading specific requirements
For a loader type work machine the driveline requirements differ quite much compared to other type work machines. Most of the loader specific requirements are related to loading situation. Especially mining loader's typical loading situation is very extreme because rock piles consist of big and heavy stones which are not coming to the bucket easily like homogenous material, e.g. sand, would come. Time frame to take a load to the bucket from a rock pile is about 30-60 seconds and all that time the loader pushes to the rock pile basically in zero speed with full traction force and at same time pumps with the bucket to improve penetration into the rock pile. The bucket pumping causes big changes in axle loads and normally one of the axles in turns is all the time on air ( fig. 2). When both axles has own motor and no mechanical drive axle between, then full traction power is in use only when both axles are on ground. When one of the axles is on air then total traction power is cut to half. That can be avoided if there is possibility to mechanically connect the axle drives on these situations. Figure 2 Penetration to the rock pile to get load to the bucket requires full traction force and same time pumping the bucket with the dump cylinder. The bucket pumping has significant influents for axle loads between the axles.
For traction system, loading situation means that motors must be capable to keep up high torque in zero speed up to 1 minute and after that still to be capable to drive the load up to the main tunnel. For an electric motor this type use is not a problem because electric motors have typically good peak power/torque capability. When designing motor use so that also power/torque areas above nominal values are in use then comprehensive thermal calculations for the motor and for the cooling system are needed to make sure that motor thermal profile stays in acceptable level. Also advanced controls strategy for the electromechanical actuators in driveline and for system protection against overloads is very important especially when expected service hours of underground mining loaders are up to 30000h.
To get full traction power during loading it's essential to have possibility to drive mechanical power between axles. This enables to drive all driveline power to the axle which is on ground and thus avoid situation where traction force is cut to half.

Internal counter-torques in driveline
Internal counting forces in drivelines occur in machines which has mechanical drive shaft between axles and no central differential gear ( fig. 3). Most of the loaders have that kind of driveline. The case machine is an exception because it has hydrostatic driveline with one motor per axle and no drive shaft between. The internal counting forces have negative influence for efficiency, for performance and for tire wearing.  Steering geometries and tire diameter differences between the axles causes speed differences between the axles. In frame steered machines the problem can be avoided if the pivot point is longitudally in center of machine which compensates speed differences between the axles during turnings. It seems anyway that the pivot point is not always in center in mining loaders, that's also the situation in the case machine ( fig. 4). Tire diameter difference can be quite remarkable between axles when one axle has new tires and one axle has weared tires. Thickness of wearing-surface in mining loader tires are normally about 100-150mm. Other matter which has influence for tire diameters is load difference between the axles which has influence for dynamic rolling diameter. In case machine the front axle load varies 4t depending is the bucket full or empty. At same time rear axle load becomes lighter because of gearing effect; load in the bucket is in front of the front axle. Also uphill has influence for axle loads by increasing front axle load because the machine goes up backwards when load is on. To avoid internal counter forces it's essential to have possibility to disengage the mechanical link between axles during the machine is driving. Also good controllability of speeds and driving forces on both axles is essential to minimize counter force effect in driveline.

Driveline concept
The basis for the driveline design work has been described on chapter 3. Also other type work machines have been studied to see what kind requirements those have. Main focus has been anyway to find solution for loader type use. The concept consist of 3 main parts; high efficiency high speed electric motor, high speed gear-box which can be 1 or 2 speed version and electrically controllable actuator which is used to control gear shifting and mechanical links in the driveline. The main parts will be designed so that parts are scalable to higher power classes and adaptable to different torque/speed levels. Interfaces between the main components remain always same despite different power classes which make the concept modular. Possibility to use existing standard interfaces e.g. SAE interfaces will be studied. The driveline concept is based on series hybrid construction. In the case machine the construction will be; one motor per axle and 2-speed gearboxes on both motors. Between axles is a controlled power link which allows to transfer power between axles ( fig. 5).

Figure 5 Driveline concept for loader's hybrid version
The mechanical link between the axles will be used normally only during loading. This makes possible to drive torque from both motors to the axle which is on ground. When loading is done then the mechanical link will be opened and both axle can be used freely with them own motors and no counter torques will exist in any circumstances. The 2-speed gearbox between motor and differential is needed to make possible to have high torque during loading and to have high speed during hauling. It also gives better possibility to optimize efficiency and motor dimensioning.

Dimensioning basis
Basic construction of the case machine's hybrid version will remain same as the conventional version. Small differences in weight and weight distribution might exist but it's expected that this will have no influence for performance.
Dimensioning basis for the hybrid loader's power line will be the conventional loader's total power and measured power distribution. The hybrid version will have same level of nominal power in main drives like; traction drive and implement hydraulics drive. To achieve same performance as the conventional has then also total power downsizing would be possible. But when keeping the power at same level it will be then possible to compare also how much the performance can be improved in addition to energy savings. Because of energy buffers, batteries and supercaps, the hybrid version will be capable for higher total power but during test drives the power can be limited to same level as the conventional version has. The conventional loader has total power 90kW which is same as the ICE power. The engine is driven with constant speed and the cooling is directly related to ICE-speed which means that cooling and other auxiliaries takes about 20kW power all the time. Implement hydraulics includes also steering cylinder which must be active all the time. Even thought the steering power is not big, it has anyway remarkable influence for energy consumption. Based on measurement it seems that replacing hydraulic steering with electromechanical steering would give about 6% energy savings [2]

Dimensioning basis for the hybrid driveline
During hauling operation the traction power and auxiliary power are used at same time so total power will be shared between them. In the conventional version traction gets about 70kW and rest of the power, about 20kW, goes to auxiliary devices. In the hybrid version continuous traction power will be 80kW and auxiliary power about 10kW.
Because both axles will have own motor then continuous power per motor will be 40kW. To achieve high traction force during loading and high speed during hauling the motor must be capable for high torque at low speeds and wide speed range. Continuous traction power must be limited normally to the motor's nominal power. Preliminary motor curves are presented in the fig. 6.   Overloading can be used also to achieve better acceleration and to give boost when the loader has to go over some obstacles e.g. fallen stones, on mine track. Based on calculations the mean power during the duty cycle is about 40kW. That gives possibility to use same motor in gen-set use as the traction motor is. Diesel engine's speed range in industrial engines is usually only up to 2200rpm but in smaller power diesel series e.g. 40-60kW, speed range is usually wider e.g. up to 2500rpm or even 3000rpm so 40kW electric motor's nominal point 2500rpm fits fine for gen-set use with same power class diesel engine.

Dimensioning basis for the implement hydraulics
Implement hydraulics in a mining loader type machine is active relatively short periods. Mean power is also quite low even thought power peaks can be quite high. In the conventional version implement hydraulics are needed also for steering which means that pump must be running all the time. For the hybrid version is considered to implement electromechanical steering so implement hydraulics pump needs to be running only during loading and dumping operations. Even thought the 40kW traction motor is a bit too powerful for pump drives in implement hydraulics' circuit, its anyway attractive option for that use too. Short term load-profile gives possibility to use simple air cooling for the motor which reduces available power but still fulfills power requirements what is needed to drive the implement hydraulics pump. It's also beneficial if the loader has only one type motor for all main functions because that reduces the number of different components.

Dimensioning basis for the auxiliary power
Auxiliary power will be reduced about 10kW by using electrically controlled electromechanical actuators in the cooling system. Also improved total efficiency and about 50% down-sized ICE will reduce cooling needs which gives possibility to use down-sized cooling devices. Also the fact that cooling devices can be used from high voltage side gives possibility to improve efficiency because currents can be kept much lower then. Better efficiency can be achieved also in low voltage side when 24V will be converted from high voltage side with DC-DC converter which has better efficiency than traditional ICE driven alternators.

Expected performance
Best way to describe loaders performance is to define how many tons per kilowatt-hour [t/kWh] or how many tons per hour [t/h] the loader is capable to process from rock pile to dumping place. Improved efficiency and better system controls leaves more power for traction. Two speed gearbox makes possible to achieve higher speeds. Also the conventional loader would have potential to achieve higher speed with two speed gearbox instead one speed which it originally has. Based on calculations [1] and simulations [3] it seems that a higher speed actually improves efficiency. When cycle time is shorter then energy consumers which uses energy all the time has less time to consume energy so amount of energy used per cycle is then smaller. In the speed range 0-30kmh is no significant increase in traction resistances, like tire resistance, so even if the top speed would be doubled the traction resistances remains at about same level. In the table 3 is shown how much only speed has influence for energy efficiency during one cycle. Assumption in this comparison is that gear ratio in each selected top speed case is optimal for the traction motors. In the first two columns are presented speeds which are possible for the conventional case machine and in the next columns are presented speeds which are possible for the hybrid version.  The cycle time shows that effective way to improve performance is to use higher speed. Other way to improve performance would be to take bigger loads but that would mean then also bigger machine and probably also bigger tunnels so this option is out of scope. In the tunnels where loaders haul rock, speed is usually limited for safety reasons. Quite often used speed limit is 15km/h. In main tunnels the limit can  be higher so it's desired that the loaders top speed is anyway at least about 30km/h. Increasing speeds in small tunnels would need improved driving controllability and also improved driving comfort. The case machine begins to be quite difficult to drive over 10km/h speed especially on bumpy roads because of primitive steering and uncomfortable cabin where driver and user interface moves back and forth. Up-hill is the most time consuming phase in the duty cycle. Only way to increase speed on up-hill is to get more power for traction. When total power remains about same, then only way to get more traction power, is to improve efficiency in whole driveline. Also loading and dumping has some potential to increase performance. Improving these phases needs anyway also improvement for bucket usage. For example if penetration to the rock pile can be improved then time and power needs can be reduced.

Conclusions
To get full traction power during loading it's essential to have possibility to drive mechanical power between axles. This enables to drive all driveline power to the axle which is on ground and thus avoid situation where traction force is cut to half.
To avoid internal counter forces it's essential to have possibility to disengage the mechanical link between axles during the machine is driving. Also good controllability of speeds and driving forces on both axles is essential to minimize counter force effect in driveline. Increasing driving speeds reduces energy consumption during work cycle. Higher driving speed requires improvements for drivability and driving comfort.