This paper will examine the wheel loader as a system with two parallel energy conversion systems that show a complex interaction with each other and with the power source. Using a systematic design approach, several principle design solutions for hybridization can be found. Furthermore, the human operator with his/her control actions needs to be considered as part of the total system. This paper will therefore also connect to results from ongoing and previous research into operator workload and operability. http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-51999 http://dx.doi.org/10.1115/IMECE2009-10458

1. Paper 07
Hybrid Power Systems
for Construction Machinery:
Aspects of System Design and Operability
of Wheel Loaders
Reno Filla
VOLVO CONSTRUCTION EQUIPMENT AB, ESKILSTUNA, SWEDEN
Abstract
This paper will examine the wheel loader as a system with two parallel energy conver-
sion systems that show a complex interaction with each other and with the power
source. Using a systematic design approach, several principle design solutions for hy-
bridization can be found.
Furthermore, the human operator with his/her control actions needs to be considered
as part of the total system. This paper will therefore also connect to results from ongo-
ing and previous research into operator workload and operability.
Keywords: hybrid systems, operator workload, adaptive automation

2. It's not what you are underneath but what you do, that defines you.
(from the film “Batman Begins”)
This paper has been published as:
Filla, R. (2009) “Hybrid Power Systems for Construction Machinery: Aspects of
System Design and Operability of Wheel Loaders”. Proceedings of ASME
IMECE 2009, Vol. 13, pp 611-620.
http://dx.doi.org/10.1115/IMECE2009-10458

3. Hybrid Power Systems… 3
1 Introduction
Working machines in construction, mining, agriculture, and forestry are not only be-
coming ever more sophisticated in terms of digital control, but are also complex in ar-
chitecture even in their conventional form. Many of these machines consist of at least
two working systems that are used simultaneously and the human operator is essential
to the performance of the machine in its working place.
This paper will focus on wheel loaders, giving first an overview of the technical sys-
tem and then describing possibilities for hybridization with some examples of how as-
pects of operator workload are linked to system design. Methods to assess operator
workload will also be reviewed.
2 Conventional Wheel Loaders
Wheel loaders are good examples of complex working machines. Drive train and hy-
draulics are both equally powerful and compete for the limited engine torque. Figure 1
shows how in the case of bucket loading the primary power from the diesel engine is
essentially split up between hydraulics and drive train to create lift/tilt movements of
the bucket and deliver traction to the wheels.
Auxiliaries
Bucket
Wheels
Lifting +
Breaking/Tilting
Travelling/
Penetration
Drive train
Hydraulics
Σ Engine
Linkage
External (+/-)
Gravel
pile
Figure 1. Simplified power transfer scheme of a conventional wheel loader during
bucket loading
In addition to auxiliary systems such as cooling systems, there may also be external
systems connected via PTO’s (power take-outs).
3 Working Cycles
Wheel loaders are versatile machines and each working place is unique, yet common
features can nonetheless be found.
The short loading cycle shown in Figure 2, sometimes also dubbed V-cycle or Y-
cycle for its characteristic driving pattern, is highly representative of the majority of
applications. Typical for this cycle is bucket loading of granular material (e.g. gravel)

4. 4 Paper 07
on an adjacent load receiver (e.g. a dump truck) within a time frame of 25-35 seconds,
depending on working place setup and how aggressively the operator uses the machine.
The interaction between hydraulics and drive train is one reason for choosing the short
loading cycle as a kind of standard test cycle when productivity, fuel consumption and
operability are to be assessed. A detailed description with identification of all phases
can be found in [1] and [2].
Figure 2. Short loading cycle
Load & carry cycles, sometimes also called long loading cycles, are commonly used,
too. They resemble short cycles, but involve two longer transport phases of up to 400 m
in forward gear (Figure 3).
Figure 3. Load & carry cycle

5. Hybrid Power Systems… 5
4 Hybrid Wheel Loaders
As mentioned before, wheel loaders are versatile machines used in a variety of applica-
tions, which makes it hard to find a globally optimal setup. This problem is even more
accentuated when it comes to hybridization of these working machines. Customers will
expect great gains in terms of energy efficiency when using the hybrid wheel loaders for
the same variety of applications they used their conventional machines for.
In order to systematically examine hybridization opportunities, it is meaningful to
first consider the parallel working systems hydraulics and the drive train separately.
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Figure 4. Conventional drive train and hydraulics
The left-hand diagram in Figure 4 shows the conventional drive train, where the in-
ternal combustion engine drives a transmission via a torque converter (here simplified
as a clutch) and further on rotates the machine’s four equally large wheels, passing
through differentials along the way. The hydraulics are driven by one or several parallel
hydraulic pumps, directly connected to the engine (Figure 4, right).
In this paper the focus will be on electric hybrids, but also hydraulic hybrids are un-
der development in the industry and academia [3][4][5].
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6. 6 Paper 07
In a parallel hybrid system, another power source is mechanically connected to the
same drive shaft, enabling torque to be added or subtracted. As shown to the left in
Figure 5, in the case of the parallel hybrid-electric drive train the question arises of
whether the electric machine should be placed upstream or downstream of the torque
converter. Upstream placement permits cranking and torque support of the engine,
while a downstream position offers higher recuperation efficiency. A compromise can
be found by using a lock-up torque converter and an additional clutch. However the
design of a parallel hybrid hydraulic system is fairly straight forward (Figure 5, right).
In a series hybrid configuration (Figure 6) there is no purely mechanical connection
between engine, drive train, and hydraulics. The power flows through electric machines
where it is first converted into the electric domain and then back to the mechanical do-
main.
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Figure 6. Series hybrid drive train and hydraulics
A complex or power-split hybrid system can be seen as a combination of parallel and
series hybrid. There is both a mechanical connection between the engine and the system
to be driven and an electrical connection via two electric machines (Figure 7).
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Figure 7. Complex hybrid drive train and hydraulics
The amount of power to be transferred either way can be chosen according to the
situation at hand. This offers the possibility to combine the advantages and to avoid the
disadvantages of the topologies described above.
An abundance of literature deals with various types of complex hybrid drive trains
and power-split, continuously variable (CVT) or infinitely variable (IVT) transmissions.
It is also possible to let the road act as a summation device, as can be seen in Figure 8.

7. Hybrid Power Systems… 7
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Figure 8. Road as summation device
Power-split hydraulics can in principle be seen as conventional hydraulics with elec-
trically driven boost pumps, but might also be designed so that both pumps share the
generation of hydraulic power in a larger time frame.
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conventional
parallel hybrid complex (power-split) series hybrid
conventionalparallelhybrid
complex(power-split)serieshybrid
Hydraulics
Drivetrain
Figure 9. Combination matrix of hybrid topologies
Combining all the variants, we find a matrix of system possibilities (Figure 9), from
conventional systems over parallel and complex hybrids all the way to series hybrid
systems. The topology of the hydraulic system changes with the column, while the drive
train topology varies between the rows. Some combinations are not drawn because they

8. 8 Paper 07
are not logically possible: if e.g. a series hybrid drive train is installed, then naturally
any installed conventional hydraulics are automatically a parallel hybrid because the
electric machine connected to the engine is at the same time also connected to the hy-
draulic pumps. The same reasoning can be applied to all five excluded combinations.
The system sketches shown are to be understood as simple examples. Neither energy
storage nor power electronic converters are included. Also, several principle solutions
can be found for each combination, each with distinctive sub-variants.
For instance, the full series hybrid design shown in the lower right corner of Figure 9
may be the simplest possible solution in terms of design effort, but not necessarily so in
terms of system complexity. It is a relatively naïve design to drive only the hydraulic
pump and the transmission with an electric motor and keep the rest of the working sys-
tem unchanged. Using electric machines usually opens up for radical new possibilities
in system design, as well as system size (downsizing) and system control.
For hydraulics, one natural path of evolution might be towards pump-controlled
“valveless” systems [6][7], possibly with a separate hydraulic circuit for each individual
function (lift, tilt and steering). It is then a question of whether the flow adaptation
should still be made by means of pumps with variable displacement (as shown in all
figures) or if this should be accomplished varying only the motor speed and using fixed
displacement pumps.
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Figure 10. Designs of full series hybrid wheel loaders
In the case of the drive train, electric machines could be integrated directly into the
wheel hubs. Both solutions are shown on the right in Figure 10, in comparison to the
naïve design shown on the left.
In general, all electro-hybrid system solutions give rise to new system properties be-
cause electric machines offer a different torque envelope and a faster response in a
wider speed range compared to combustion engines. Their introduction should therefore
also include an adaptation and redesign of the system to be driven, rather than just a
substitution of the propulsion source. For a hybrid wheel loader, this gives new possi-
bilities to achieve an optimal compromise between productivity, energy efficiency, and
operability.

9. Hybrid Power Systems… 9
5 Operability
While there are usable definitions for productivity (for a wheel loader: material loaded
per time) and energy efficiency (preferably: fuel consumed per loaded unit, rather than
per time unit), a generally agreed definition of the human operator’s difficulty in work-
ing with the machine is still to be found. For vehicles, the concept of drivability has
been the main focus for quite some time. Recently, however, the operability of working
machines has also become a major research subject (the word operability acknowledg-
ing the fact that while the drive train is the only major power-consuming system in a
vehicle, working machines like wheel loaders have at least two working systems that
are used simultaneously).
In [8], a definition is offered that also works well for working machines: “Operability
is the ease with which a system operator can perform the assigned mission with a sys-
tem when that system is functioning as designed”. The limitation to states where the
system is functioning as designed effectively excludes somewhat related yet separate
properties such as robustness and reliability.
The challenge in designing a wheel loader, from a manufacturer’s point of view, is to
find an appropriate, robust, and maintainable balance between productivity, fuel effi-
ciency, and operability over the complete area of use. At Volvo, the term Harmonic
wheel loader has been coined, describing a machine possessing a high degree of ma-
chine harmony which makes it intuitively controllable and able to perform the work
task in a straightforward manner without much conscious thought or strategy. The lower
the degree of machine harmony, the more effort the operator has to put in in order to
perform the work task (Figure 11) and thus the higher the workload.
Primary harmony
(matched and appropriately specified hardware)
Operator
effort
Secondary harmony
(added control systems)
M
achine
harm
ony
Figure 11. Machine harmony and operator effort
In general, the assumption made in this research is that the operator’s impression of
the working machine’s operability stems from the amount of workload the operator is
subjected to. We exclude physical workload and therefore also operator comfort with
aspects like exposure to vibration, ergonomics etc, and concentrate on the mental work-

10. 10 Paper 07
load of the operator, which is affected by the operator’s control efforts and thus by the
following requirements (the fourth item probably being the most prominent):
• fast response
• high precision of machine and bucket positioning
• several operator controls to be actuated simultaneously in order to achieve
a specific effect
• several machine functions to be synchronized.
In a short loading cycle as shown in Figure 2, as well as in a load & carry cycle ac-
cording to Figure 3, there are three phases in which the operator is challenged to a
higher degree:
• Bucket filling (phase #1)
• Reversing (phase #4)
• Bucket emptying (phase #6 in Figure 2, #9 in Figure 3)
Each of these phases will be examined in more detail with regard to operability and
operator workload in the following sections.
6 Operability Aspects in the Bucket Filling Phase
Extending the schematics of the technical system (Figure 1), Figure 12 depicts how the
human operator interacts with the wheel loader during bucket filling.
Auxiliaries
Bucket
Wheels
Lifting +
Breaking/Tilting
Travelling/
Penetration
Drive train
Hydraulics
Σ Engine
Linkage
External (+/-)
Gravel
pile
OperatorECU
ECU
ECU
ECU
Figure 12. Simplified power transfer and control scheme of a wheel loader during
bucket loading
In order to fill the bucket, the operator needs to control three motions simultane-
ously: a forward motion that also exerts a force (traction), an upward motion (lift) and a
rotating motion to fit in as much material as possible (tilt). This is similar to how a sim-
ple manual shovel is used. However, in contrast to a manual shovel, the operator of a
wheel loader can only observe, and cannot directly control these three motions. Instead,
he or she has to use different subsystems of the machine in order to accomplish the task.
The gas pedal controls engine speed, while it is lift and tilt lever control valves in the
hydraulics system that ultimately control movements of the linkage’s lift and tilt cylin-
ders, respectively.

11. Hybrid Power Systems… 11
One difficulty is that no operator control directly affects only one single motion. The
gas pedal controls engine speed, which together with the selected transmission gear
affects the machine’s longitudinal motion. But engine speed also relates to the speed of
the hydraulic pumps, which in turn affects the lift and tilt cylinder speed. The linkage
between the hydraulic cylinders and the bucket acts as a non-linear planar transmission
and due to its design a lift movement will also change the bucket’s tilt angle and a tilt
movement affects the bucket edge’s height above the ground. Furthermore, when simul-
taneously using lift and tilt function, a reduction in the lift cylinder speed can be ex-
pected as the displacement of the hydraulic pump is limited and the tilt function is pri-
oritized over lift due to its lower pressure demand. The more the tilt lever is actuated,
the higher the tilt cylinder speed, while the speed of the lift cylinder decreases.
Finally, there is also a strong interaction between traction and lift/tilt forces: penetrat-
ing the gravel pile with the bucket requires tractive force, which is transferred through
the wheels to the ground (Figure 13). In accordance with Newton’s Third Law of Mo-
tion, the Law of Reciprocal Actions, the gravel pile exerts an equal and opposite force
upon the loader bucket. A typical sequence for actually filling the bucket is then to
break material by tilting backwards a little, lifting, and penetrating even further.
Figure 13 shows how these two efforts work against each other: in order to achieve a
lifting force, the cylinders have to create a counter-clockwise moment around the load-
ing unit’s main bearing in the front frame. At the same time, the reaction force from the
traction effort creates a clockwise moment that counteracts the lifting effort.
Figure 13. Force balance during bucket filling (simplified)
The ground and the gravel pile thus connect drive train and hydraulics, forcing them
to interact with each other, and to experience each other’s effort as an external load.
Since both working systems are already mechanically connected via the engine, an in-
teresting situation arises in which engine torque is transferred to the wheels to accom-

12. 12 Paper 07
plish traction, but at the same time counteracts that part of the engine torque that has
been transferred through hydraulics to accomplish lifting and tilting – in turn requiring
even more torque to be transferred.
The momentary torque or power distribution to drive train and hydraulics is specific
for the working task at hand and is controlled by the operator, who ultimately balances
the complete system and actively adapts to both the machine, the task at hand, and the
working place.
The traditional way of decreasing the severity of these phenomena has been to design
the wheel loader with a high load margin by utilizing the internal combustion engine at
a high speed (often near governed speed), backed up by a steep torque rise, and to em-
ploy weak torque converters that require a high rate of slip in order to transfer torque.
This creates a certain decoupling effect in the drive train, but it is unfortunately prohibi-
tively costly in terms of fuel efficiency.
In modern working machines the main idea is to utilize the engine at as low speeds
as possible by employing a stiffer torque converter and hydraulic pumps with larger
displacement. Also, major components and subsystems are controlled by electronic con-
trol units (ECUs), all connected within a network, as shown schematically in Figure 12.
This makes it possible to give the operator support in controlling the wheel loader. In a
hybrid working machine, this opportunity is even greater. Depending on the chosen
topology, several of the difficulties discussed above can be avoided.
While the gravel pile will still connect drive train and hydraulics downstream via the
bucket, in a series hybrid these systems would not be connected upstream via the en-
gine. This enables the operator to control only one motion at a time via only one con-
trol: the gas pedal will only control the machine’s longitudinal motion and the hydraulic
levers can control flow independently, all independently of each other and of the current
engine speed.
The responsiveness of electric machines also makes it possible to quickly ramp
torque up or down such that tractive force and lift/tilt force can be actively controlled.
This can be used in essentially all hybrid variants (Figure 9).
Furthermore, the cylinder speeds can be controlled in such a manner that the non-
linearity of the linkage is compensated for and thus the speed of bucket lift and tilt is
proportional to the angle of the tilt and lift lever.
However, it is important to observe that not all limitations that arise because of con-
ventional wheel loader design are experienced as negative by the operator. During their
work with a pump-controlled hydraulic system with a separate hydraulic circuit each for
the lift and tilt function, the authors of [6] and [7] noticed that professional operators
use the aforementioned prioritization of the tilt function over the lift function to their
advantage.
Instead of operating the lift and tilt lever separately, the operators pull the lift lever to
a certain level, hold it there and then use the tilt lever to control the speed of both tilt
and lift cylinder. By using this procedure during bucket filling, the operator has only to
actively handle two controls (tilt lever and gas pedal) instead of three (because the lift

13. Hybrid Power Systems… 13
lever has been locked in a fixed position). This behavior has been confirmed in the au-
thor’s own interview studies [1].
During measurements on a research prototype with the pump-controlled hydraulic
system described above, it was observed that the fuel consumption in short loading cy-
cles was unusually high during the bucket filling phase. It could be shown that the op-
erators tried, but were not able, to use the new system just like to old one. With the
speeds of the lift and tilt cylinders now independent of each other, they could no longer
be controlled by just one lever. No record exists as to whether the operators themselves
commented on this and complained about the new functionality (in the author’s own
studies on a similar machine one did), but it is clear that the prototype’s operability was
reduced and thus the workload of the operator increased. To solve this, a similar behav-
ior was implemented by software, artificially decreasing the flow demand for the lift
function when the tilt lever was activated (Figure 14).
0
20
40
60
80
100
0
20
40
60
80
100
0
20
40
60
80
100
0
20
40
60
80
100
0
20
40
60
80
100
Lift speed
Lift
command Tilt command
Figure 14. Reduction of lift speed due to prioritization
Learning from the example above, the question arises of how many changes one can
make in the design of the operator interface and the behavior of a hybrid wheel loader
without professional operators experiencing this in a non-positive way – and then, how
long it would take for the operators to adapt.
7 Operability Aspects in the Reversing Phase
In this phase of a short loading cycle the operator changes the direction of the machine
from reverse to forward and aims at the load receiver, while at the same time continuing
to raise the bucket using the lift function. As examined in [9], there is again a conflict
between the working systems:
In order to reverse the machine, the operator lowers the engine speed (otherwise gear
shifting will be jerky and the transmission couplings might wear out prematurely). Less

14. 14 Paper 07
torque is available at lower speeds. Engine response is also worse at lower speeds,
mainly due to the inertia of the turbo-charger and smoke limiter settings.
When the operator shifts from reverse to forward the loader is still rolling backwards.
This forces an abrupt change in the rotational direction of the torque converter’s turbine
wheel, greatly increasing the slip, which leads to a sudden increase in torque demand
from the already weakened engine.
Continuing to lift the bucket while reversing requires high oil flow, which is propor-
tional to the pump’s displacement and shaft speed (and thus engine speed). In order to
satisfy the flow requirement at lower speed, pump displacement is usually at maximum
in this phase, which together with a high hydraulic pressure due to a full bucket and the
loading unit’s geometry leads to a large demand for driving torque.
Thus, both drive train and hydraulics apply high load to the engine, which at lower
speed has less torque available and a longer response time. In a conventional wheel
loader it is the operator’s task to control the machine correctly. In order to lower fuel
consumption, operators of conventional wheel loaders with torque converters are usu-
ally advised to release the gas pedal and use the brakes in order to lower machine speed
almost to stand-still before shifting into forward. This greatly reduces torque converter
slip and thus saves fuel.
In a wheel loader with a hybrid drive train, it would actually be better, both operabil-
ity- and energy efficiency-wise, to go back to the traditional non-optimal method of
reversing and just command a change of direction at a certain point in time and then let
the machine automatically initiate regenerative braking by using the electric machine in
generator mode, followed by a smooth acceleration forward. Requiring the operator to
use the brake pedal leads to a higher workload and ideally requires the hybrid working
machine to incorporate brake blending, i.e. use the electric machine for regenerative
braking in combination with the mechanical brakes.
Related to this, the author witnessed another example of how productivity, energy
efficiency (in the form of fuel consumption) and operability (in the form of operator
workload) are intertwined. In a research prototype machine a system had been imple-
mented that automatically lowered engine speed and applied the brakes when the opera-
tor commanded a change of direction the traditional way by just putting the transmis-
sion gear lever from reverse into forward. The initial expectation was to save fuel,
according to a chain of reasoning as shown in Figure 15.
Machine automatically
uses brakes when
beginning to reverse
Reduced slip
over torque converter
Reduced
fuel consumption
Figure 15. Expected effect of the automatic reversing feature
Comparing fuel consumed per time unit with and without the automatic reversing
feature, it showed that the new feature actually led to higher fuel consumption. The rea-
son for this was the operator using the automatic reversing feature to increase produc-

15. Hybrid Power Systems… 15
tivity, which also increases fuel consumption per time unit. Figure 16 shows the actual
course of events.
Machine automatically
uses brakes when
beginning to reverse
Reduced
operator work load
Reduced slip
over torque converter
Increased
productivity
Operator compensates
for lower work load
and drives harder
Increased
fuel consumption
Figure 16. Actual effect of the automatic reversing feature
Besides showing how productivity, energy efficiency, and operability are connected
in a working machine, the example above also proves how clearly inappropriate it is to
use fuel consumption measured per time unit in such comparative testing. Fuel effi-
ciency expressed in fuel consumed per loaded mass unit of material would have been a
much better choice.
8 Operability Aspects in the Bucket Emptying Phase
Having driven the machine from the gravel pile to the load receiver and having raised
the full bucket to dumping height at the same time, the operator now needs to coordi-
nate four controls in order to successfully empty the bucket‘s contents:
• Tilt lever in order to dump the material
• Lift lever, gas pedal and brake pedal in order to safely guide the bucket
over the edge of the load receiver and to evenly distribute the material
In this phase, the aforementioned difficulty of no operator control affecting just one
motion is again apparent. The connection between desired increase in hydraulic flow
and thus commanded increase in engine speed now leads to a possibly unwanted in-
crease in tractive force, propelling the machine forward at a faster rate. With the wheel
loader already standing in front of the load receiver and only small forward movements
required, this increases the operator’s workload since the machine may risk driving into
the load receiver, which can only be prevented by also applying the brake pedal.
In a series of measurements on a research prototype with a stiffer torque converter
(i.e. one that can create the same torque at a lower slip rate than that of a baseline ma-
chine) the author again witnessed the connection between productivity, energy effi-
ciency, and operability. Originally it was expected to see a decrease in fuel consumption
due to the stiffer torque converter in the research prototype machine theoretically lead-
ing to less slip and thus lower drive train losses (Figure 17).

16. 16 Paper 07
Stiffer torque converter,
yet same traction demand
Reduced slip
over torque converter
Increased
fuel efficiency
Figure 17. Expected effect of a stiffer torque converter
Comparing fuel efficiency, the operator when using the research prototype machine
with the stiffer torque converter was shown to require more time per loading cycle and
thus consumed more fuel in total and per loaded unit of material (but not per time unit,
as the additional consumption per time unit was close to the average and thus led to
approximately the same mean fuel consumption figure). It was concluded that the main
reason for the longer cycle time was difficulties in bucket filling and bucket emptying
due to the stiffness of the torque converter. Figure 18 shows the course of events.
Stiffer torque converter,
yet same traction demand
Higher traction force
propels machine faster
for same pedal angle
However, same pump
speed and thus same
engine speed demand
Increased cycle
time and reduced
productivity
Operator compensates
by applying brakes
during bucket emptying
Reduced
fuel efficiency
Figure 18. Actual effect of a very stiff torque converter
Especially during bucket emptying the operator found it difficult to handle the higher
tractive force at lower slip rates and thus lower engine speeds if compared for the same
propeller shaft speed. In order to increase hydraulic oil flow, operators apply the gas
pedal to increase pump speed, which due to a conventional wheel loader’s system de-
sign also increases converter speed and thus increases tractive force. To avoid colliding
with the load receiver, the operator had to apply the brake pedal, which resulted in both
increased workload and increased drive train losses (because the torque converter essen-
tially acted as a retarder brake).
In most hybrid wheel loader designs (Figure 9) the above described course of events
can be avoided, since the gas pedal would be used to control the machine’s traveling
speed rather than the internal combustion engine’s rotational speed.
9 Assessing Operability
In summary it can be concluded that operability issues must be factored in when design-
ing working machines, and more so when designing hybrids.
Virtual Prototyping has been generally adopted in product development in order to
minimize the traditional reliance on testing of physical prototypes. The complex archi-
tecture of working machines with tightly coupled, non-linear subsystems in different
engineering domains make simulation of the complete system’s dynamic behavior diffi-

17. Hybrid Power Systems… 17
cult. But in early design stages such as conceptual design, simulation is the only viable
option.
In order to capture the full scope of the interaction between the machine (the techni-
cal system), its environment (working place, gravel pile), and its operator, all three must
be modeled at an appropriate level of detail if the simulation is to give valid results.
Usually, the technical systems are modeled at a very intricate level of detail, while its
environment is modeled crudely (for instance by various types of rolling resistance),
and the operator model is virtually non-existent, following a predefined trajectory with
control inputs at given points, and thus capturing none of the essential phenomena.
Advances in simulation-related research, among those the author’s own work
[10][11] and the work of others inspired by it [12][13] as well as the many advances in
research on autonomous bucket loading [14][15][16], have resulted in a growing body
of knowledge on the operator’s behavior and how to approximate his or her control ac-
tions in a simulation. Using for example a weighed, piece-wise analysis of control effort
in the different cycle phases might be one way of quantifying the (simulated) operator’s
workload and measuring the (simulated) machine’s operability. However, some work
still remains to be done in this regard.
Meanwhile, research in the field of Human-Computer Interaction has resulted in the
development of Cognitive Architectures that can be used for modeling cognitive aspects
of human beings while interacting with technical systems [17]. However, for tasks like
operating a machine within a simulated environment the operator model needs to have a
spatial awareness in order to orient itself in space, detect objects to interact with etc.
One promising approach to address this has been reported in [17].
It can also be argued somewhat philosophically that the closer a simulation comes to
reality, the more complex it necessarily has to be until it is finally as complex and diffi-
cult to understand as reality itself, also introducing challenges to its validity in terms of
repeatability. That means that there will be some limit to the extent of how operability
can be simulated using operator models.
The other approach then would then be to still simulate the technical system yet use a
human operator to control it in so-called human-in-the-loop simulations. Examples can
be found in [19][20][21]. The question then is how to quantify a (simulated) machine’s
operability or the operator’s workload leading to an impression of operability.
In the examples given earlier, a physical prototype already exists, yet the same ques-
tion of quantification of operator workload still needs to be answered. Fortunately,
much research has been done on this topic, initially in aeronautics but later also in on-
road vehicles and other applications.
In the beginning, workload was quantified using self-report measures like the modi-
fied Cooper-Harper scale or the NASA Task Load Index. These methods rely on the
operator trying to be objective, i.e. not consciously trying to skew results. However, due
to the fact that such self-report assessments are often performed afterwards, the time
delay itself leads to a certain amount of unreliability. Also if for example a wheel
loader’s operability in the reversing phase is to be rated, the operator may find it hard to

18. 18 Paper 07
concentrate on this and not let his/her impression of the machine’s total behavior in the
complete loading cycle influence the rating.
It has therefore been found better to assess workload by measuring psychophysi-
ological parameters, predominantly heart rate variability and finger temperature. A
comprehensive overview of the subject and references for further study can be found in
[22][23][24][25].
In the research published in [26], a real-time flight simulator is used with a human
pilot in the loop and the results from these simulator flights are compared to real flights.
We will conduct similar measurements, yet not focused on the operator’s training effect
but on using the operator’s workload in order to assess the tested machine’s operability.
This does not need to be limited to the development of advanced working machines like
hybrids, but can also play an important role in rating selected features of conventional
machines.
10 Workload-adaptive Machines
An earlier chapter discussed how the introduction of hybrid power systems can lead to
machines that are easier to use due to a different system design.
But there are also a great many opportunities in new human-machine interfaces,
made possible by hybrid systems and extensive computer control. The question was
earlier raised of how many changes one can make in the design of the operator interface
and the behavior of a hybrid wheel loader without professional operators experiencing
this in a non-positive way – and then, how long it would take for the operators to adapt.
This issue can be made less prominent by adapting machine behavior and operator
interface to the individual operator’s needs and preferences. This could be as relatively
easy to do as redefining the resolution and mapping of various operator controls to suit
the individual operator, either online or offline.
Augmented Cognition in order to raise the operator’s situation awareness is another
solution, for example by giving advanced visual, haptic or acoustic feedback [27][28].
Instead of requiring the operator to adapt, one could also move the adaption task to
the technical system and use adaptive automation to give the operator help in certain
situations [29]. Possibilities to do this are especially high in hybrid machines whose
systems have greater degrees of freedom than is the case for conventional machines.
11 Conclusions
Working machines like wheel loaders are complex in architecture and behavior, espe-
cially since the operator plays an important role in controlling the total system. This
paper has shown several examples where the operator’s influence has led to unexpected
results. It has been discussed that in future measurements, assessment of operator work-

19. Hybrid Power Systems… 19
load needs to be included in order to fully understand the course of events. Methods for
assessing workload in the context of this research have also been discussed.
Hybrid systems give the opportunity to not only vastly improve energy efficiency
and possibly productivity of the working machine, but also increase operability. Trying
to optimize a machine for such different properties as power, productivity, fuel effi-
ciency, operability, initial purchase cost, and total cost of ownership is difficult due to
conflicting demands. Examples have been shown on how hybrid systems can help to
find a better compromise.
Acknowledgments
The financial support of Volvo Construction Equipment and Energimyndigheten, the
Swedish Energy Agency, is hereby gratefully acknowledged.
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