ABSTR ACT
A critical aspect of electric motor fabrication
is the assembly process of the stator winding
electromagnetic circuit. A common winding
configuration consists of copper magnet wire that
is die-formed into hundreds of complex hairpin
geometric structures, which are then inserted
into a laminate core to form the stator winding.
For performance and efficiency, extremely tight
tolerances are required for the wire-form geometries
to avoid improper automated insertion. Quality-
control measurements are increasingly critical
for consistently producing wire-forms that meet
dimensional specifications. However, current
technology capable of accurately measuring these
complex 3D shapes requires far more time than
it takes to produce individual wire-forms, limiting
quality control to only a small number of audits. To
push toward in-process verification in keeping with
the wire-form production rate, significant advances
in data-acquisition strategy and analysis techniques
are necessary. This paper summarizes the noncontact
structured light sensor array system and analysis
methodology developed to allow rapid assessment
of the full 3D wire-form shape. When used in
conjunction with a robust calibration technique, it
becomes feasible to build an in-process inspection
system that can be implemented in production.
KEYWORDS: quality inspection, machine vision,
3D reconstruction, structured light, electric motors
1. Introduction
In recent years, manufacturing has seen an increased adoption
of automation, largely due to advancements in robotics,
modern sensor technology, and data analytical techniques
such as artificial intelligence (AI). These efforts have resulted
in a more data-driven manufacturing environment that aims
to boost plant productivity and improve plant efficiency
while reducing overall costs. Consequently, many traditional
quality-control inspection methods have been replaced by
automatic, nondestructive evaluation (NDE) techniques. One
of the most prolific within manufacturing is the use of machine
vision (Shirmohammadi and Ferrero 2014), which has experi-
enced significant advancements. Sensors coupled to computers
capable of running advanced analytical tools can automatically
extract and analyze useful information from digital representa-
tions of manufactured components.
Machine vision is becoming ubiquitous within industrial
applications for automating real-time inspection for discon-
tinuities (Wang et al. 2017) and providing proactive alerts for
process feedback control (Lee and Kim 2020), whether for
electronic devices (Flack and Hannaford 2005), automotive
products (Wagner and Agapiou 2024), or other components.
Regardless of the application, automated precision machine
vision systems combined with advanced analytical methods
aim to eliminate the need for manual visual inspection on the
production floor. For this reason, there has been an ongoing
effort for industries to acquire tools capable of meeting the
challenges associated with fully automated inspection.
Though there are numerous applications of automated
machine vision within the automotive industry, the global shift
toward electric vehicle (EV) production has placed increased
demand on the precise inspection of battery and electric
motor–related components. Electric motor manufacturing in
particular requires high-fidelity machine vision solutions to
automate quality control along the production line due to its
complex architecture, which becomes increasingly difficult
to assess as it advances toward final assembly. NDE tech-
niques applied to electric motors present unique challenges
that demand rigorous inspection of subassembly components
throughout the fabrication process to ensure they meet the
performance and efficiency demands necessary for vehicle
propulsion.
This paper outlines some of the challenges associated with
implementing fully automated nondestructive quality-control
systems that can perform evaluations at the same pace as the
RAPID IN-PROCESS 3D SHAPE INSPECTION OF
MAGNET WIRE HAIRPINS DURING ELECTRIC
MOTOR ASSEMBLY
SEAN R. WAGNER*
ME
|
TECHPAPER
*Materials &Manufacturing Systems Research Lab, General Motors
Research &Development, 30470 Harley Earl Blvd., Warren MI 48092, USA
(ORCID: 0000-0003-3540-1501) sean.wagner@gm.com
Materials Evaluation 84 (1): 34–44
https://doi.org/10.32548/2026.me-04552
©2026 American Society for Nondestructive Testing
34
M AT E R I A L S E V A L U AT I O N • J A N U A R Y 2 0 2 6

electric motor fabrication process—specifically, the assembly of
the stator winding, one of the most complex aspects of manu-
facturing. Current inspection technologies and their limitations
are discussed, along with a newly developed combined non-
contact measurement and analytical technique that has shown
the ability to overcome these barriers.
Automotive Electric Motor Assembly
Electric motors consist of both a rotor and a stator subassem-
bly that, when coupled together, serve as the primary source
of propulsion for EVs. The specific product application can
have significant implications for the general architecture of the
motor design options typically applied to achieve an optimal
solution. Though a variety of electric motor design architec-
tures could be discussed at length, for simplicity, this paper
focuses primarily on a common architecture used in the auto-
motive industry for hybrid and fully electric vehicles.
Figure 1a shows a breakout illustration of a distributed bar-
wound permanent magnet electric motor. Both the rotor and
stator consist of stacks of electrically isolated steel laminations,
with individual laminations typically 0.3 mm or less in auto-
motive applications. Lamination geometries are formed by a
die-punching process, which creates the slots in the rotor stack for
permanent magnet insertion and in the stator stack for insulated
copper wire insertion. The dimensional tolerances associated
with the slots that are formed in the laminations, as well as the
inner diameter of the stator lamination and the outer diameter
of the rotor lamination, are extremely tight—typically 0.1 mm
along the profile of the slot perimeter (Wagner et al. 2024).
The number of slots for the rotor and stator cores is
dictated by operational requirements for the electromagnetic
interaction responsible for producing the rotational motion
needed for EV propulsion. Laminations are interlinked to form
the core stacks, which can vary in overall length depending on
design requirements such as torque, power, operating speed,
and efficiency. It is important to note that the tolerance spec-
ified for individual laminations must be maintained through
the entire length of the core to ensure no damage occurs
during insertion processes. In addition, the air gap between
the rotor and stator stacks is minimized for efficiency. The
rotor assembly is completed once the permanent magnets,
shaft, and bearings are inserted into the core. For the stator
assembly, after the insulated copper wire material is inserted
into the core, a welding operation is performed to generate the
electromagnetic winding coil circuit. Figure 1b shows an illus-
tration of the final distributed bar-wound permanent magnet
electric motor installed into a vehicle drive unit.
Both the rotor and stator assemblies can be complex
depending on application and performance requirements.
However, there is generally a higher risk of defect introduction
during the stator assembly process, largely due to the wire
insertion and welding steps. Automotive electric motor designs
such as the one shown in Figure 1 use stator windings of heavy-
gauge, rectangular-shaped (bar) magnet wires. The magnet
wire consists of copper coated with a thin electrical insulation
layer—typically a polymer enamel film such as polyimide,
or in some cases polyetheretherketone (PEEK). This insula-
tion protects the stator winding from short-circuit conditions,
allowing the electromagnetic circuit to operate as intended and
provide propulsion to the vehicle.
The winding geometry is often complex to optimize
packing efficiency and thermal management of the coil
during operation. To complicate matters further, the insu-
lation material must not be damaged during insertion into
the steel laminate core otherwise, it can cause complete
motor failure. For this reason, tight tolerances over the entire
three-dimensional (3D) surface profile of these complex
copper windings are necessary to prevent improper insertion
into the stator core, which can result in damaged assemblies
that must be scrapped, and to maximize electromagnetic
Bar-wound wire
Laminated steel rotor core sections
Laminated steel stator core
Magnets for
installation
Bearing support
assembly
Rotor hub Steel plate
Figure 1. (a) Breakout illustration of a distributed bar-wound permanent magnet electric motor assembly with critical components
labeled (b) illustration of the electric motor assembly installed into a drive-unit configuration for vehicle propulsion.
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