Development of a Rotary Transformer
and Its Application to SRC Connectors
by DongZhi Jin * , Fumihiko Abe * and Hajime Mochizuki *2

Spiral-type SRC connectors, which are a key element in air-bag systems, present

ABSTRACT problems with respect to the noise produced by the spiral cable and the adjustmentof the center point that is required. Non-contact SRC connectors using a rotary transformer, on theother hand, have the advantages of completely noiseless operation and infinite rotating life, butsuffer from a variety of severe restraints on their practical use. Furukawa Electric has been working
on the development of a non-contact SRC connector, and has solved a major problem in applying
rotary transformers--that of obtaining the detonating current needed to deploy an air bag with adelay of 1 msec or less, even when the core-to-core gap is 1 mm or more. The principle of the noncontact SRC is introduced, and its structural features and energy transmission characteristics aredescribed.

. Center positioning assurance: Because steering has to1.

INTRODUCTION

A variety of safety systems are installed in modern automobiles. The air bag system, known as a major safety system, has experienced almost 10 years of practical use
since its introduction and nowadays almost all types ofvehicles, from compact car to high-class car, are equippedwith this system.
Furukawa Electric has been dealing with steering rollconnector (SRC), a key component of air bag system, formany years ahead of other companies making spiral SRCs
that the company has developed a large share holder inthe domestic market as well as abroad. Spiral SRC consists of a flexible flat cable (FFC) of high reliability thatprovides electrical connections between the steering column (stator) and steering wheel (rotator) sides, thereby
featuring a very simple structure. Although the spiral SRC
is currently employed in many cars for its simple structure,it also presents some technical problems as mentioned
below owing to the FFC that is based on a wiring connection principle.
. Noise: Requirements for lower noise levels of SRC arebecoming even more severe as the quietness in cars
has so far been improved. The FFC in spiral SRC makes
noises due to sliding and dancing during car cruising.
Most of current issues in quality assurance of SRC areconnected with this noise, and FFC is related with manyof them.
* Sensor Development Sec., Facilities Development Center, Plant & Facilities Div.
* 2 Electric Appliance Div., Automotive Products Gr.

be rotatable right and left equally, FFC has to be positioned at its center point in the SRC assembly line. This
makes not only assembly very troublesome but also
center positioning after every in-line test unavoidable,
thus causing problems of cost as well as of quality.
Furukawa has been developing non-contact SRCs
based on a rotary transformer principle, doing efforts toobtain a fundamental solution to the problems mentioned
above.
Non-contact SRCs, in which the electric energy for detonation of air bag is provided through a rotary transformer,
is advantageously capable of limitless rotation and is entirely noiseless. On the other hand, the development imposes such demanding requirements as instantaneoustransmission of a large amount of electric power, and thus
a number of problems have to be solved to realize practical use of this technology.
We have opened up a prospect for practical use of rotary transformer having a very simple structure which enables, with a core gap of as large as the millimeter order,transmission of an electric power of several watts with adelay time of less than 1 ms.
Primary coilDC-AC
converter
Car battery(12V)

Switch under control
of collision sensor

Secondary coilAir-bagdetonating
deviceRotary transformer

Figure 1

Principle of electric energy transmission for noncontact SRC

13

Rotator
Secondarycoil
Gap

RotatorCore

SteeringcolumnPrimary
coil
StatorFigure 2

StatorBasic structure of rotary transformer

The principle of operation of the non-contact SRC will
be presented in this paper together with its structural features as well as its characteristics in electric power transmission.

2.

BASIC PRINCIPLE

Figure 1 shows the principle of electric power transmission from the battery to the air bag detonating device. Whenthe switch is activated by a signal from the collision sensor, the DC-AC converter converts DC power from battery into AC power which is applied to the primary coil ofthe rotary transformer. The electric energy transmitted tothe secondary coil is supplied to the air bag detonating
device and heats it directly to trigger detonation. This principle is so called "direct transmission" and compares favorably with "indirect transmission" in terms of simple structure, smaller mounting space, and high reliability; in thelatter principle, the electric energy is temporarily stored ina storage device on the secondary side of the transformer
to effect discharge and detonation as occasion demands.
The key component of the non-contact SRC is rotarytransformer. Figure 2 shows the basic structure of a transformer in which a ring shaped magnet core with the primary coil is fixed onto the stator, while another ring magnet core with the secondary coil is fixed onto the steeringcolumn via the rotator. A gap is provided between the primary and secondary coils making them entirely non-contact with each other. A soft magnetic material is used forthese cores to make the magnetic resistance appearing
in the magnetic circuit of the two coils small enough, sothat an AC electric energy impressed on the primary coilis transmitted to the secondary coil instantaneously.

3.

TASKS TO BE ADDRESSSED IN THE DEVELOPMENT OF ROTARY TRANSFORMER

It is anticipated that instantaneous transmission of electric energy by means of rotary transformer to the detonating device of automobile air bag system is subject to thefollowing restraints.
(1) Squib impedance: The DC resistance of the detonating device is around 2Ω, a quite small value.
(2) Detonation delay time: The allowable delay time for14

air bag to detonate after a collision detection is about 1
ms.
(3) Gap distance between magnet cores: Gap distance
between magnet cores should desirably be in the orderof mm, taking the assembling accuracy and vibration
during car cruising into consideration.
(4) Transformer diameter: Structurally, the transformer diameter has to be larger than that of steering column.
These constraints can be reduced to the requirements
for rotary transformers as summarized below.
(1) Efficient power transmission over a large gap distance:
The higher the coupling coefficient between magnet
cores, the more efficient energy transmission becomes.
Thus, a rotary transformer that maintains high coupling
coefficient irrespective of larger gap distances between
cores is desirable.
(2) Impedance matching: As is well known, the impedance of a coil is in proportion to the square of the coildiameter. Thus, in the present case where a high efficiency transmission at high frequency under a constraintof large coil diameter has to be achieved, it is very important to realize good impedance matching between
the large coil impedance and the small squib impedance.
(3) Frequency characteristics of transmission: Directswitching of battery DC power source into pulsed AC
power is advisable to make the transmission systemeconomical and simple structured. Accordingly, broadening the frequency bandwidth of transmission characteristics is desirable as much as possible to improvetransmission of high frequency components containedin the pulsed AC power, or in other words, to raise thetransmission efficiency.
(4) Stability: High reliability is required for the air bag system because it is a key safety system. Thus, stabilization technologies are indispensable for such parameters
as transformer coupling coefficient, coil impedance, andthe like during car cruising.

4.

IMPROVEMENT OF COUPLING COEFFICIENT BETWEEN COILSThe basic idea for improvement of transmission efficiency,
which means high coupling coefficient, consists in how
much amount of the AC magnetic flux due to the AC current impressed on the primary coil is made to interlink
with the secondary coil.
Ordinary transformers use a magnetic material of highpermeability to form a magnetic circuit including coils. Whenthe gap shown in Figure 2 is in the order of µm, The AC
flux concentrates on the core area because the magnetic
resistance of the interlinking magnetic circuit is smallenough in comparison to that of the leakage flux circuit,so that the coupling coefficient between coils becomes
sufficiently large. However, this configuration presents a
problem of large fluctuation in coupling coefficients due toeven smaller gap variations.
In car assembly line, as mentioned before, it is neces-

Furukawa Review, No. 18. 1999

Rotator

RotatorInterlinking
magnetic path

Secondary
coil
SteeringcolumnPrimary
coil
StatorStatorFigure 3

Dispersed distributionof magnetic flux at thegap
StatorRotary transformer prototype

sary to provide a large gap allowance. When the gap distances are of the mm order in particular, the ratio of thetotal magnetic circuit length to the gap distance (filled withair) becomes smaller than the relative permeability of thecore, making the effective relative permeability of the wholemagnetic circuit entirely dependent on the gap distance.
That is, the effective relative permeability of the wholemagnetic circuit is determined entirely by the gap distancehowever high the relative permeability of the core material
may be.
In the development of a new rotary transformer, we haveselected a magnetic material having a comparatively low
value in relative permeability placing much importance on
the mechanical strength of material. As a result, following
advantages were obtained.
(1) Impedance matching with detonating resistance: Since
the coil inductance is proportional to the effective relative permeability of the magnetic circuit comprising coils,
the use of a low permeability material reduces the coilinductance, thus improving the impedance matching of
the circuitry.
(2) Performance stability: Due to the large gap distance,parameter fluctuations that influence transformer performance can be suppressed regardless of gap distance fluctuations caused by vibration during vehicle
cruising. This results in higher reliability.
Below are described fundamental technologies that wehave developed in order to improve the coupling coefficient between the coils.
4.1 Core Configuration
In designing a rotary transformer, it is normal practice toplace the end faces of core and coil on the same plane as
Metallic shielding
layer with slit

Slit
B1

B31

Secondary coilshown in Figure 2. This structure, however, permits a major portion of fluxes distributed in the gap between thecoils to turn into leakage flux degrading the coil coupling
conditions.
We have developed an enclosed type core shown in
Figure 3 intending to reduce the flux leakage; in which
both coils are enclosed in the secondary core so that the
gap between coils and that between cores lie on different
levels with each other. In this configuration, the gap between cores is located apart from the coil end face by adistance equivalent to the coil length, making the fluxes
distributed over the core gap less liable to turn into leakage flux. Moreover, since the distributed fluxes themselves
eventually interlink with the secondary coil, the configuration can greatly suppress the adverse effect of gap fluctuation to influence the coil coupling coefficient.
4.2 Shielding Layer
The well-known shielding effect of metal conductors onhigh frequency fluxes has been utilized in this development to improve the coupling conditions between coils.
The theory will be explained below with the help of illustration in Figure 4.
The shielding layer, located as shown in Figure 3, isprovided with an insulating slit as shown in Figure 4 so as
not to make the layer an electric coil. Fluxes are generated around the primary coil by the current impressed on
the coil, comprising two sets of fluxes: the one whichinterlinks with the secondary coil, denoted as B1 and theother which does not, called leakage flux and is denoted
as B2. B2 induces eddy currents in the conductor of theshielding layer as it crosses the layer. While the AC fluxes
generated by the eddy current is directed in the reverse
direction to B2, some of them lie in the direction of B1.
That is, equivalently, whereas B2 is reduced by the eddy
current, B1 is increased. This results in an increased ratioof (interlinkage flux / leakage flux), and improves the coupling conditions between coils. In other words, the metalshielding layer functions as a kind of magnetic resistanceagainst AC magnetic fields, causing shielding effects only
on leakage fluxes with a degree of effectiveness that depends on its assembling position. The greater the shielding effect, the better the coupling conditions between coils.
Theoretically, the shielding effect improves the thicker
the shielding layer thickness, the smaller the material's
electric conductivity, and the higher the transmission frequency. The use of high frequency is indispensable withthe present rotary transformer to reduce the delay time for

Sinusoidal signal
oscillator

0.01Ω Precision resistor

0.01Ω Precision resistor

V1

Shielding layer

Load resistor
simulating thedetonating device(2Ω)

Bipolar power source(High frequency
current amplifier)

B32

Schematic view ofshielding layer

Figure 4

Primary coil

Schematic view of shielding layer and principleof operation

V2

B2

Rotary transformer

Figure 5

Block diagram of transmission efficiency
measuring system

15

120

50%

100

40%
G=0
G=0.5mm
G=1.0mm
G=1.5mm
G=2.0mm
G=2.5mm
G=3.0mm

30%
20%
10%
0%
0

5

10 15 20 25 30 35 40 45
Transmission frequency (Sinusoidal, kHz)

50

Transmitted power (W)Transmission efficiency

60%

80
G=0mm
G=0.5mm
G=1.0mm
G=1.5mm
G=2.0mm

60
40
20
0
0

5

10

15

20

25

30

35

40

45

50

Transmission frequency (Pulsed, kHz)

Figure 6

Frequency response of transmission efficiency(G: gap distance)

0.01Ω
Precision
resistor

V2

Car battery
Pulse
generator
Current
amplifierD

Rotary transformer

G
S

Figure 7

High power
MOS FET

Load resistor modelingthe detonating device(2Ω)

Block diagram of transmission power measuringsystemdetonation, and this is totally compatible with the shielding technology we have developed.

5.

EVALUATION OF BASIC PERFORMANCEOF PROTOTYPES

We fabricated prototypes of rotary transformer having enclosed cores along with shielding layers and evaluatedtheir basic performance.
5.1

Frequency Characteristics of Transmission
Efficiency
The electric power necessary for the detonation of air bag
system is several watts. Considering such system parameters as the instantaneous current available from the carbattery, transmission attenuation between the battery to
the primary coil of rotary transformer, and the conversion
efficiency of DC-AC converter, it is estimated that a transmission efficiency of about 20% is practically sufficient for
the rotary transformer.
Figure 5 shows the basic configuration of the systemused to measure transmission efficiencies. Sinusoidal signals of constant voltage having a frequency specified at
the oscillator are power amplified by the bipolar amplifierand subsequently are impressed onto the primary coil ofa rotary transformer. By measuring the voltages appearing on the two precision resistors (0.01Ω), the currents
flowing into the primary coil and the load resistor modeling the detonating device are calculated respectively. Trans-

16

Figure 8Frequency response of transmitted power(G: gap distance)

mission efficiency η is defined as follows.

η = V2 × I2 / V1 × I1
where, V1 and I1 are the impressed voltage and current on the primary coil, V2 and I2 are the voltage andcurrent on the load resistor modeling the detonating
device respectively.
The frequency characteristics of transmission efficiency
for the prototype rotary transformer are shown in Figure 6.
The following became clear from measurements.
(1) Below a transmission frequency of 15 kHz, transmission efficiency improves with higher frequencies, indicating that the shielding effect of the shielding layer increases as the frequency rises.
(2) Above 15 kHz, the frequency response somewhat flattens. At high frequencies, while the coupling between
coils improves, the matching condition between coil impedance and load deteriorates. These effects counteract each other resulting in a fairly wide frequency bandwidth, which is quite favorable for practical applications.
(3) The transmission efficiency declines as the gap distance increases. However, the efficiency decline decreases at larger gap distances. When the gap distanceis in the mm order, its small variations are seen to cause
little effects on transmission efficiency fluctuations.
(4) At transmission frequencies above 20 kHz, a transmission efficiency of more than 25% can be ensured
with a gap distance of as large as 2 mm, demonstrating the practicality of the direct transmission method.
(5) The delay time can easily be reduced to 1 ms or lessdue to the use of high frequency transmission, since
the transmission delay corresponds to a half cycle timeof transmission frequency.
5.2 Evaluation of Power Transmission Characteristics
The electric power instantaneously transmitted via rotarytransformer from the car battery to the detonating device
is influenced by the conversion efficiency of the DC-AC
converter as well as by the matching between battery output impedance and transformer input impedance. The available transmitted power was confirmed, simulating actual
vehicle assembly, by using a measuring system shown in
Figure 7; where a single high power MOSFET was used
to constitute a DC-AC converter capable of switching the

Furukawa Review, No. 18. 1999

battery DC source into the pulsed source of high frequency.
The transmitted power was evaluated based on the product of effective values of voltage and current, where theeffective current to flow into the resistor modeling the detonating device was calculated by measuring the voltage on
the precision resistor (0.01Ω) on the secondary side. The
frequency response of the transmitted power thus evaluated is shown in Figure 8, clarifying the following points.
(1) The electric power necessary for detonation can be
amply transmitted with a gap distance of as large as 2
mm.
(2) The transmittable power declines as the gap distanceincreases. However, the rate of power decline decreases
at larger gap distances in absolute value. When gap
distance is in the mm order, similarly with the transmission efficiency mentioned before, its small variations
cause transmitted power fluctuations that are negligibly small.
(3) The transmittable power demonstrated this time, nearly
50 W at a gap distance of 2 mm or more, can be improved further by optimizing the impedance matching
between the coil and the output side of battery.
(4) The bandwidth of frequency response is sufficiently
wide and flat, and is capable of reducing fluctuations in
the transmitted power even with varied spectrums intransmission signals.
(5) Peaks of frequency characteristics are seen to haveshifted to lower frequencies. This is probably due to thefact that the pulsed power source contains plenty ofhigh frequency components.
Furthermore, the transmission delay time was actually
measured to be less than 50 µs at a transmission frequency of 10 kHz, a value that surely cause no problem ofpractical importance.

6.

CONCLUSIONS

We have described the present situation of developmentfor non-contact SRC based on a rotary transformer principle, which is capable of directly transmitting detonationenergy into automobile air bag systems.
While the new non-contact SRC is much more advantageous than conventional SRCs for its noise-less as wellas limit-less rotation, we have encountered many challenging tasks in putting the SRC into practical use. We
believe some of the restrictions on practical applications
have successfully been removed by the present development.
We intend to realize practical applications of this SRC
in the near future by developing peripheral technologies
that remain incomplete this time.
Manuscript received on July 21, 1998.

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