2000-01-0132 Integrated Motor Drive Unit A Mechatronics ... - Delphi

SAE TECHNICAL
PAPER SERIES 2000-01-0132
Integrated Motor Drive Unit
A Mechatronics Packaging Concept for
Automotive Electronics
Jeff Burns and Suresh Chengalva
Delphi Delco Electronics Systems
SAE 2000 World Congress
Detroit, Michigan
March 6-9, 2000
400 Commonwealth Drive, Warrendale, PA 15096-0001 U.S.A. Tel: (724) 776-4841 Fax: (724) 776-5760

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2000-01-0132
Integrated Motor Drive Unit
A Mechatronics Packaging Concept for Automotive Electronics
Copyright © 2000 Society of Automotive Engineers, Inc.
ABSTRACT
This study presents a unique design for combining an
electric motor and its drive circuitry into one integrated
package.
Electric motors used in systems such as electric power
steering applications commonly consist of a separate
electric motor and drive controller. There are several disadvantages
to this approach. First, the mounting of two
separate items into the system takes extra room and
increases assembly complexity. There are several disadvantages
associated with the wires that interconnect the
two units. The long wires introduce additional electrical
resistance. Also, they can act as antennas that both
receive and radiate electromagnetic interference.
Mounting a miniaturized motor drive unit directly to the
side of the motor achieves some of the advantages of a
fully integrated approach but only to a limited extent.
Wires still exit the motor and must be attached to the
drive unit. Angular position sensors must be mounted to
the motor separate from the drive unit. This results in
extra electrical connections that add cost and are potential
points of failure.
The design presented in this paper places the power
drive section of the controller directly on the end of the
motor. The digital control board is so placed that sensors
for measuring the angular position of the motor can be
placed directly on the PCB board. This results in a small
package with a minimum number of connections. The
viability of this design, from the standpoint of heat transfer,
is demonstrated using finite element analysis. Varying
parameters in the finite element model identifi the
most critical design parameters.
The insights into the nature of mechatronic design gained
through this design study are discussed.
PROBLEM DEFINITION
Jeff Burns and Suresh Chengalva
Delphi Delco Electronics Systems
Electric Power Steering (Figure 1) systems use an electric
motor to provide steering assistance. The space
available in the automobile for this motor, and it’s associated
drive circuitry, is minimal.
Figure 1. Electic Power Steering System
Traditionally the power module and electronic controller
for an electric motor are in a box, or boxes, separate from
the motor. There are several disadvantages to this. First,
multiple boxes take extra room and increases assembly
complexity. There are also several disadvantages associated
with the wires that interconnect the units. The long
wires introduce unwanted electrical resistance, and can
act as antennas that both receive and radiate electromagnetic
interference.
To solve these problems it is desirable to integrate the
Motor, Power Module, and Electronic Controller into one
package. Mounting a miniaturized motor drive unit
directly to the side of the motor (Figure 2) is a good step
in this direction. Unfortunately, wires still exit the motor
and must be attached to the drive unit. The angular position
sensors are mounted separately from the drive unit.
This results in extra electrical interconnections that are
costly and prone to failure.

Figure 2. Early Integrated Motor
The goal of this design study is to integrate an electric
motor and its associated electronics to the maximum
extent possible.
DESIGN DESCRIPTION
This design approach, illustrated in figures 3 and 4,
mounts semiconductor switches, normally MOSFET's, on
a power distribution bus that is mounted between the
motor and the motor drive unit. Also, motor position sensors
rest directly on the system control board.
1
2
3
4
5
6
Figure 3. Cutaway View
7
8
9
14
13
12
11
10
2
5
11
8
10
Figure 4. Exploded View
3
The semiconductor switches (2) are attached directly to a
power distribution bus (11). This power distribution bus is
made of a highly thermally and electrically conductive
material such as copper. This power distribution bus is
attached to the bottom of the case (5) using an electrically
isolating, but thermally conductive, double sided
tape. A second electrical isolation pad is placed between
the distribution bus (11) and the motor endcap(8). This
pad must be highly thermally conductive. Heat dissipated
in the semiconductor switches (2) is conducted through
the power distribution bus (11) to both the motor end cap
(8) and the case (5). From these two parts the heat conducts
through various parts of the integrated motor structure
until it is dissipted into the surrounding air.
Inside the case (5) an interconnect board (4) is used to
connect various electrical parts (6) and the power distribution
bus (11) together to form electrical circuits. The
semiconductor switches (2) are connected to the interconnect
board (4) with wire bonds through windows in
the case (5).
Above the interconnect board (4) a system control board
(13) is mounted. This control board (13) contains various
electronic devices that together control the motor system.
Also on the control board are sensors that together with
the encoder wheel (14) monitor the angular position of
the motor.
6
1
14
13
4

The motor winding terminations (3) pass through the
motor end cap (8), the case (5), and the system control
board (4). The power distribution bus (11) has tabs that
are bent upward and connect to the motor winding terminations
(3). This connects the motor to the drive unit.
Parts that dissipate a large amount of heat (12) may be
mounted to the inside of the case.
Some of the parts of the motor include the outer sleeve
(10), end cap (8), bearing (9), and motor core (7).
The drive unit is enclosed with a cover (1).
FINITE ELEMENT ANALYSIS
In this design both the semiconductor switches and the
motor dissipate heat into the same structure. An obvious
concern is that the two parts will heat each other. To
address this question a Finite Element Analysis has been
performed.
Dielectric
FET Frame
Solder
Axis of Symmetry
FET
Figure 5. Schematic of EPS System Level Model (2D)
Table 1. Factors Considered
Motor End Cap
Motor Case
Heat Sink
Housing
Winding/Core
Load on End Cap Motor Case Convection He at Sink
Motor/FET Thicknes Material Coefficient (Block)
(%) (mm) (W/m2-C)
100 6 Aluminum 0 With
60 12 Steel 7 Without
20 20
A 2D cross-section model is schematically presented in
figure 5. The maximum temperature reached by the FET
is the primary response variable. Because this 2D-model
will overestimate the actual die temperatures, the model
is primarily useful for assessing the relative effects of various
design options. A full factorial experiment with 60
distinct cases was completed using the factors listed in
table 1. The object labeled "heat sink" in figure 5 represents
a gearbox. Because the thermal sinking abilities of
the gearbox were not known it was included as a constant
temperature sink in some models. For the rest of
the models it was assumed be an isothermal boundary.
3
The motors power consumption and duty cycle are specified
according to table 2.
Table 2. Power Input
Case 1 Case 2 Case 3
% Max Power 100 60 20
On Time (s) 7 90 Infinity
Off Time (s) 180 1000 N/A
Power in FET (W) 38.1 13.7 1.5
Power in Motor (W) 210 75.6 8.4
N
o
r
m
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i
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e
d
T
e
m
p
e
r
a
t
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r
1.200
1.000
0.800
0.600
0.400
With Sink
0
With Sink
7
Temperature vs LC
With Sink
20
Heat Sink Condition
Convection Coefficient
Figure 6. 2D Analyses Results
No Sink
7
No Sink
20
The results of all 60 cases were analyzed using graphs
such as shown in figure 6. From these graphs it is evident
that the percent of power applied is the most important
variable affecting the FET temperature. The 100% load
case always produces the highest temperatures. Furthermore,
at 100% load all the other variables have little
effect on the temperature. This occurs because during
the short high-power pulse the energy dissipated in the
FET does not have time to dissipate into the motor. This
burst of energy is then falowed by a long cool down time
that allows the energy to be dissipated even through a
less than optimal structure. The variables in the model
also have little effect on the 20% load case because the
energy applied is small enough to easily be dissipated.
The presence of the heat sink has a large effect in the
60% power load case. This occurs because the power off
portion of the cycle is not long enough to dissipate the
energy through convection alone.
It is also evident that for the 100% load only a small part
of the structure, immediately around the FET, warms up
during the power on cycle. With this in mind, a 3D model
of the area immediately around the FET was constructed.
The geometry for this model is pictured in figure 7. This is
a 1/8’Th symmetry model of a square cutout around the
FET. Two versions of the model were created. One model
has a one mm thick copper layer to which the FET is
mounted. In the other model the copper layer is two mm
thick. Heat is applied to the top surface of the silicon chip.
Because of the short duration power pulse, it is not necessary
to consider any heat output from the model. The
heat rise at the model extremities is small and consistent
Load:
20 %
60 %
100 %

with the 2D model results. The FET's used in this application
can not exceed 175°C. A 3D model is necessary to
assess if this condition can be met.
2
0.18
0.18
Dielectric
Mg
Figure 7. 3D Model Dimensions
The temperature of the silicon in the one mm thick copper
model reached 174°C. When the copper layer is
increased to two mm thick the temperature is reduced by
seven degrees. Obviously the goal of keeping the temperature
of the silicon below 175°C can be achieved with
this design concept.
THE NATURE OF MECHATRONIC DESIGN
Cu
Al
The design of most complex systems is achieved by
breaking the various parts into pieces that can be
designed independently. The design of each sub part
may be completed by an individual working on each part
in series, or by several teams that complete the parts in
parallel. When the design of all the sub parts is completed
they are combined together to make a complete
system.
Highly integrated designs, such as the one presented
here, make dividing up the design into sub parts very difficult.
Each and every part becomes highly dependent on
the others. Members of the design team may no longer
work independently, and everyone working on the project
must have a strong understanding of the entire system.
The challenges to integration extend beyond the design
process. Electric motors and their controllers are often
produced in separate manufacturing facilities. Sometimes,
different corporations may even produce the two
parts. Because of the high integration achieved in
designs such as this one it becomes necessary to combine
the manufacturing operations. Often the manufactur-
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5.842
Solder
0.63
0.1
20
2
4.318
1-2
4
ing processes are not compatible with each other. For
example, motors require heavy metal working operations
that are inherently dirty, but electronics requires a very
clean environment.
Highly integrated designs can be extremely sensitive to
changes in requirements. For example, suppose the
torque output requirement for this motor was increased
by 50 percent. This would require an increase in current
through the motor. This would necessitate doubling the
number of silicon devices to handle the current. Because
there is no more room for these devices the whole design
concept would require re-evaluation. A less integrated
design would likely be more easily adapted to such
changes.
Highly integrated designs have many potential advantages,
but they must be weighed against the added complexities
during design and manufacturing.
CONCLUSION
This design achieves a tight integration between an electric
motor and its controller. This results in a very small
system package. Problems associated with interconnecting
wires are minimized. The viability of the novel thermal
management approach is shown through finite element
analysis.
CONTACT
Jeffrey H. Burns
Advanced Project Engineer
Advanced Engineering
Delco Electonics Systems
World Headquarters
M/C D28
One Corporate Center
P.O. Box 9005
Kokomo, Indiana
46904-9005 USA
Tel: 765.451.3279
Fax: 765.451.3600
Email: Jeff.H.Burns@delphiauto.com
Suresh K. Chengalva
Project Engineer
Advanced Engineering
Delco Electonics Systems
World Headquarters
M/C D28
One Corporate Center
P.O. Box 9005
Kokomo, Indiana
46904-9005 USA
Tel: 765.451.3278
Fax: 765.451.3600
Email: Suresh.K.Chengalva@delphiauto.com