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Sensing Movable Receiving Coils by Detection
of AC Current Changes on the Primary Side of a
Multi-Coil System
Article in Procedia Engineering · January 2017
DOI: 10.1016/j.proeng.2016.11.323
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Procedia Engineering 168 (2016) 991 – 994
30th Eurosensors Conference, EUROSENSORS 2016
Sensing Movable Receiving Coils by Detection of AC Current
Changes on the Primary Side of a Multi-Coil System
B. Kallela,b*, O. Kanouna, H. Trabelsib, M. Roesc
a
Chair for Measurement and Sensor Technology, Technische Universität Chemnitz, Reichenhainer Straße 70, 09126 Chemnitz, Germany
Computer, Electronic & Smart Engineering systems design, National Engineering School of Sfax, Road of Soukra km 3, 3038 Sfax, Tunisia
c
Electromechanics and Power Electronics, Eindhoven University of Technology, Postbus 513, 5600 MB Eindhoven, The Netherlands
b
Abstract
Position detection is very important for inductive power transfer systems with movable receivers, in order to switch off inactive
coils and to reach a high efficiency. The solution presented in this paper is based on the measurement of the current amplitudes in
sending coils, which show a significant difference between no targets, metallic targets and receiving coils. A galvanically separated
circuit is designed to implement the new method by transforming the current of the sending coils to a DC voltage and then
comparing it to the detection threshold. Finite element simulations were carried out to predict the value of the detection threshold.
Both simulation and experimental results show that the circuit fulfils the detection requirements with high precision and high
detection height. The proposed method represents a smart, efficient and low cost detection technique for inductive systems with a
movable receiver, independently on position and speed.
©
Published
by Elsevier
Ltd. This
© 2016
2016The
TheAuthors.
Authors.
Published
by Elsevier
Ltd.is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference.
Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference
Keywords: Inductive power transmission; Multi-coil system; Movable receiver; Position detection
1. Introduction
Inductive power transmission IPT is used for different power range applications including biomedical implants [1],
battery recharging of electrical vehicles [2] as well as powering autonomous sensors [3] and portable electronic
* Corresponding author. Tel.: +49-371-531-34857; fax: +49-371-531-834857.
E-mail address: [email protected]
1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference
doi:10.1016/j.proeng.2016.11.323
992
B. Kallel et al. / Procedia Engineering 168 (2016) 991 – 994
devices [4]. In case of movable receiver, the two-coil inductive system is no more suitable for power transmission and
it is proposed to use multi-coil inductive system.
Multi-coil inductive link is one of the most promising methods of energy transfer because it provides a safe,
convenient and flexible power transfer technology [5 - 7]. However, powering all sending coils simultaneously
decreases system efficiency due to the high loss of magnetic field. To overcome this problem a receiver detection
system is required and only the coils that are underneath the receiver should be powered. The design of such a system
presents a very challenging task for which only few methods were proposed in literature. In [8], authors have proposed
a load-detection method to reduce the power consumption of an inductive system by analyzing the transmitting coil
voltage and supply current space using a microcontroller with a built-in ADC. Their aim is to turn off the sending coils
when no valid receiving devices is placed on the transmitting platform. The low detection height of this method
presents its major limitation. In [9] a detection method was proposed using additional coils located in every transmitter
coil. Sensing principle is based on the detection of ferrite material in the receiver that increases the inductivity of the
additional coils and thus reduces the resonance frequency. However, this method shows some drawbacks due to the
necessity to use of receivers covered by ferrite material and the low detection height. In [10], a detection method was
proposed, which is based on the measurement of the impedance change of sending coils. Changes of impedance
amplitudes and phases can be detected by measuring the voltage and the current of every sending coils. For that, the
AC voltages are measured and at the same time, the load type can be estimated. However, this solution requires four
extra estimation circuits attached to each sending coil and other electronic components that increase the system
complexity and cost.
In this paper, we present an effective solution to reduce the power consumption of a multi-coil inductive system in
case of no load and metallic targets present on the transmitter platform. This is by powering only the coils underneath
the valid receiver which is the receiving coil. The proposed solution overcomes the use of any additional sensors or
coils. It is realized by introducing only simple circuits and uses the sending coils themselves as detectors.
Fig. 1 (a) depicts the studied multi-coil systems. Sending coils are similar. Every coil has a diameter of 3 cm and 4
turns. The receiving coil has a dimeter of 6 cm and 8 turns. The vertical distance separating the two sides equals to 5
cm. This is to perform a high detection height.
b
a
Fig. 1. (a) Studied multi-coil inductive system (b) sending coil S1 current for different load types
2. Implemented detection method
The proposed solution consists of the measurement of the peak of the AC current of every sending coils, converts
it into DC voltage value and then compare it to a threshold limit which is dependent, in general, on the supply voltage,
the sending coil parameters as well as on the receivers.
In order to predict the current value of the sending coil, finite element simulations were carried out. Results
presented in Fig. 1 (b) demonstrate a significant difference between no loads (without receiver), metallic targets
(conductive material) and receiving coils (valid receiver). They show that the current amplitude decreases only in the
case of the presence of a receiving coil in the proximity. Based on the aforementioned results, a detection threshold
can be chosen between 0.61A and 0.73A.
B. Kallel et al. / Procedia Engineering 168 (2016) 991 – 994
The system converts, first, the AC current circulating in the sending coil to a DC voltage and then compares it to a
reference voltage. The reference voltage is chosen within the gap between the voltage thresholds corresponding to the
case of a valid and invalid load. For a threshold current of 0.66 A (see Fig. 1 (b)), a detection voltage threshold “Vref”
of 2.4 V was chosen.
Fig. 2. Detection circuit
Fig. 2 depicts the designed detection circuit. Via a toroidal transformer, the current in the sending coil is transferred
to a galvanically separated circuit. The measured current in the secondary side after the transformer is converted to a
voltage “Vcoil” by a simple peak detector circuit and then compared to the detection voltage threshold “Vref”. A timer
and flip-flop circuits were added to periodically test for valid loading conditions when the transmitting coil is switched
off. The time interval for detection can be adjusted by the user by modifying the value of resistances R3 and R4 and
the capacitor C2.
Fig. 3. Laboratory set-up for the sensing circuit
Fig. 3 shows the laboratory set-up for the sensing circuit. A ferrite ring was used for the toroidal transformer, two
integrated circuits NE555 and one LM311P are used for the timer, flip-flop and comparator respectively. In order to
test the circuit experimentally, an AC voltage generator was used.
3. Results and discussions
Two current values corresponding to the sending coil current are programmed as an input to the designed detection
circuit. After transformer and AC/DC conversion, two voltage values are obtained: 1.5 V which indicates the presence
993
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B. Kallel et al. / Procedia Engineering 168 (2016) 991 – 994
of valid load and 3.4 V which indicates either no load or invalid load (see Fig. 4, black signal). A short time interval
was chosen for the evaluation of the circuit to clearly identify its two operation modes. The timer and flip-flop are
also programmed to test every 1.8 ms for valid loading conditions when the transmitting coil is switched off. This
value could be changed by the user and depending on the velocity of the receiver. The green lines of fig. 4 represent
the output voltage for the inverter’s driver. It is clear that if Vcoil > Vref so the output voltage Vout = 0 V and if Vcoil <
Vref so the output voltage Vout = 1 V. The choice of the value of Vout is only to check the good operation of the circuit.
It could be changed to another value chosen by the user. The main advantages of the designed circuit that it requires
no communication link between the sending and receiving sides and its high detection height which equals to 5 cm.
Fig. 4. Simulation results of the implemented detection circuit
4. Conclusion
In this paper, a detection circuit for movable receiver of a multi-coil inductive system is designed. It consists of the
measurement of AC current peak of every sending coils, converts it into DC voltage value and then compare it to a
threshold limit. The coil current threshold is predicted by finite element simulation. Both simulation and experimental
results show that the designed circuit fulfils the detection requirements. It presents an efficient, low cost detection
technique which is also suitable for high distances. Additionally, it doesn’t require any communication link between
the sending and receiving sides.
References
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