IEEE TRANSACTIONS ON MAGNETICS, VOL. 45, NO. 10, OCTOBER 2009
4459
Contactless Electromagnetic Temperature Sensors for Spinning Devices Oriano Bottauscio1 , Mario Chiampi2 , Gabriella Crotti1 , Enzo Ferrara1 , and Fausto Fiorillo1 Istituto Nazionale di Ricerca Metrologica, Torino I-10135, Italy Dipartimento di Ingegneria Elettrica, Politecnico di Torino, Corso Duca degli Abruzzi, 24 Torino I-10129, Italy Contactless electromagnetic temperature sensors are analyzed and compared, under a number of configurations, for applications in spinning devices. Their working principle is based on the variation of electrical or magnetic properties with temperature. By a preliminary study, carried out through finite element modeling, a number of effective solutions have been identified and a few of them have then been selected for testing in the laboratory. Two significant experimental examples are here discussed. Index Terms—Contactless thermal sensors, hard magnetic materials, soft magnetic materials, spinning devices.
I. INTRODUCTION
M
ONITORING of the temperature of spinning devices is of critical importance in many heavy-duty applications, e.g. turbo-molecular pumps, where strict mechanical tolerances require constant control of the rotor temperature. Since contact measurements on fast rotating elements are quite impractical or even impossible, various types of non-contact temperature measurement methods have been devised. The typical approach, based on infrared sensors (see for example [1]), is often unreliable, because the radiating properties of the emitting surfaces can drift in actual devices. Contactless temperature sensors based on electromagnetic phenomena are being proposed and developed in the literature. They generally rely on the dependence of the saturation magnetization of either hard or soft magnets, alone or in combination, on temperature. Contactless magnetic structures based on the use of Fe-Ni alloys as sensing element have been investigated in [2], [3]. These methods result into fairly non-linear response of the device. Mavrudieva, et al. [4] proposed to use a permanent magnet (GdCo Cu ) as a temperature dependent element. The magnet is employed to bias a soft amorphous strip, which therefore exhibits, upon suitable a.c. excitation, a temperature dependent second harmonic component in the secondary voltage. This device appears to cover, however, a somewhat limited range of temperatures. For special biochemical applications, where minute temperature changes are to be sensed, the temperature dependent flux pattern surrounding a thin ferrite film with proper Curie temperature is detected by a magnetoimpedance element [5]. A remote temperature sensing device has also been proposed, whose resonance frequency can provide (non-linear) temperature readout in the range 20 C to 80 C [6]. The strong non-linear behavior of soft amorphous ribbons has been exploited for temperature measurement by harmonic analysis of the signal induced in a secondary coil [7]. Magnetostrictive properties of amorphous ribbons can be equally used by sandwiching them between metal layers having a different thermal expansion coefficient and detecting the Manuscript received March 06, 2009; revised April 21, 2009. Current version published September 18, 2009. Corresponding author: O. Bottauscio (e-mail:
[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2009.2021859
related drift of inductance with temperature [8]. In any case, it is usually difficult to achieve linear response with such devices, but for somewhat restricted temperature ranges, and it appears that few industrial applications have been implemented so far [9]. This work reports about investigation of different schemes for electromagnetic contactless temperature sensors for spinning devices and it shows that, by combining a SmCo permanent magnet and a low Curie temperature Ni-Fe alloy, a sensor with quasi-linear temperature readout suitable to be integrated into these devices can be obtained. Two sensor categories have been investigated. A first one (named here “electric” sensors) exploits the temperature dependence of a conducting element placed on the rotor and its effect on the intensity of the eddy currents induced by a magnet during spinning. In a second type of devices (named here “magnetic” sensors), the basic sensing action is realised by either soft or hard magnets, taking advantage of their temperature dependent flux channelling properties. Making reference to typical working temperatures between room temperature and some 150 C, the magnetic compositions have been chosen accordingly, that is, tuned to the appropriate Curie temperature . Three setups have been considered, where the sensing elements Ni alloy ( C), a Fe Ni B amorare a Fe C), and a Sm Fe ( C) comphous alloy ( pound, respectively. All the envisaged schemes have been first modelled by finite element analysis [10], making reference to a rotor of diameter equal to 200 mm, rotating at 30 000 r.p.m. Laboratory testing has been made in selected cases and two significant examples are discussed in details. II. MODELED “ELECTRIC” SENSORS This class of sensors exploits the temperature dependence of the electrical resistivity in sensing elements. The temperature is detected through the measurement of either the electric current or the magnetic field. Two basic configurations have been here analyzed. In the first one (see scheme in Fig. 1(a)), the sensing element is a short-circuited conductive coil embedded into the rotor. The m, with temcopper coil (resistivity at 20 C equal to 0.0178 C ) is electrically insulated perature coefficient from and in thermal contact with the rotor. A dc magnetic field is generated by a SmCo permanent magnet, mounted on the
0018-9464/$26.00 © 2009 IEEE Authorized licensed use limited to: Istituto Nazionale di Ricerca Metrologica - INRIM. Downloaded on September 29, 2009 at 10:02 from IEEE Xplore. Restrictions apply.
4460
IEEE TRANSACTIONS ON MAGNETICS, VOL. 45, NO. 10, OCTOBER 2009
Fig. 3. Configuration for the analyzed “magnetic” sensors, with hard magnet sensing element. Fig. 1. Basic configurations for the considered “electric” sensors.
Fig. 4. Magnetic flux distribution for T
Fig. 2. Time waveform of the voltage U induced in the pick-up coil and variC. ation U (magnified scale) occurring upon T
1
T
(b).
mA/ C should be detected, measuring a current ampliof tude of A. For this sensor, similarly to the ones considered in the followings, the rotational direction is irrelevant.
1 = 10
III. MODELED “MAGNETIC” SENSORS stator. At any passage of the coil (rotating direction indicated by the arrow) before the permanent magnet, a pulsed current is induced, whose amplitude depends on the coil resistivity. This current generates a counterfield pulse, which is detected by a mm ) stuck on the stator at 10-turn pick-up coil (section a certain distance from the magnet. The time waveform of the voltage detected at a given temperature is plotted in Fig. 2, to(magnified scale) occurring upon gether with the variation C. This device calls for a a temperature variation better than mV/ C over 100 mV. To resolution enhance this performance, a solution is envisaged where two permanent magnets of opposite polarisation are mounted on the stator at 45 and the pick-up coil is positioned in the middle. It is correspondingly predicted mV/ C. A second scheme, shown in Fig. 1(b), is based on the use of a miniature yoke facing the rotor and a Fe layer 20 m to 50- -m-thick glued on the rotor surface, which provides partial flux closure. The yoke supply coil is the primary circuit of a mutual inductor, whose secondary circuit is made by the Fe-layer itself. The eddy currents here circulating change with temperature chiefly through the thermal resistivity coefficient of iron; the corresponding variation of its magnetic properties is quite irrelevant in this context, being dominated by the air gap reluctance. The equivalent circuit is thus made of the air-gap inductance and a temperature-dependent resistance. To increase the measuring sensitivity, the magnetizing component of the primary current can be partially compensated by capacitors. A current change
This class of sensors uses soft or hard magnets as sensing element, whose magnetic properties depend on temperature. A. Hard Magnet Sensors A capsule obtained by assembling two hard magnets is mounted on the rotor. A magnet with high Curie temperature (HTCM) is internally mounted, and on the external side is coupled with a hard magnet having low Curie temperature (LTCM), as sketched in Fig. 3. The two hard magnets are magnetized along their axes, with the same direction. If the rotor temperature is considerably lower than , both magnets are magnetized and contribute to generate a magnetic flux which is detected by the field probe mounted , on the stator. When the rotor temperature overcomes the LTCM magnet demagnetizes; the magnetic flux reduces and his spatial distribution is modified (see Fig. 4). The field probe on the stator measures a field variation, detecting the overcoming of the Curie temperature of the LTCM magnet. the HTCM When the rotor temperature becomes lower than magnetizes again the LTCM magnet. Then this sensor works like a “switch”, being able to detect if the rotor temperature is lower or higher than a given threshold. In the example here reported, a threshold temperature of C) and 120 has been considered, assuming SmCo ( Sm Fe alloys for HTCM and LTCM magnets, respectively. The time waveforms of the magnetic field radial component
Authorized licensed use limited to: Istituto Nazionale di Ricerca Metrologica - INRIM. Downloaded on September 29, 2009 at 10:02 from IEEE Xplore. Restrictions apply.
BOTTAUSCIO et al.: CONTACTLESS ELECTROMAGNETIC TEMPERATURE SENSORS
4461
Fig. 5. Computed time waveform of the signals detected by the field probe mounted on the stator. Fig. 7. Temperature dependence of the magnetisation curves of a Ni B alloy. Fe
Fig. 6. Basic configurations for the considered “magnetic” sensors, with soft magnet sensing element.
measured by the field probe for in Fig. 5.
or
Fig. 8. Magnetic field lines in proximity to the amorphous ribbon for a temperature of 25 C (a) and 122 C (b).
are shown
B. Soft Magnet Sensors Two basic configurations have been here analyzed. In the first configuration (see Fig. 6(a)) the magnetic sensing element is a package of amorphous ribbons stuck on the rotating drum. It provides a flux closure for the magnetic field generated by is a yoke mounted on the stator. When Curie temperature approached, the flux closure sharply fails, and this is detected through a secondary coil wound on the static yoke. In the configuration here analysed, the amorphous ribbons (thickness 23 m) are made of Fe Ni B alloy, with C. The magnetisation curves of this alloy, produced and characterized at the Istituto Nazionale di Ricerca Metrologica (INRIM), are shown in Fig. 7 for three different temperatures. The modification of the magnetic properties of the amorphous ribbons with temperature determines different spatial distributions of the magnetic field lines, as shown by the finite element simulations reported in Fig. 8. This behavior causes a sudden modification of the circuit reluctance in proximity to the Curie temperature, giving rise to an increase of the magnetizing current in the primary coil. Thus, the sensor works like a switch-off device. In the second considered configuration (Fig. 6(b)) a magnetic cylindrical capsule is obtained by assembling a low soft magnet (sensing element) with a high hard magnet. The capsule is mounted on the rotating drum. In the present configuration a SmCo hard magnet is considered, while the sensing elalloy ( C), produced ement is made of a Fe Ni and characterized at INRIM (Fig. 9). A field probe, stuck on
Fig. 9. Temperature dependence of Fe
Ni
alloy magnetisation curve.
the stator frame, detects the field amplitude at every turn by the passage of the capsule. With increasing rotor temperature, flux channelling by the soft cylinder becomes increasingly less effective and the field pattern emerging from the composite capsule fans out. The flux intercepted by the field probe is correspondingly affected and it turns out that such variation relates in a quasi-linear fashion with the rotor temperature. Finite element simulations show that a monotonic or non-monotonic behavior of the field probe signal with temperature is found, depending on the magnetic capsule height. This is evident in the diagrams reported in Fig. 10. IV. DISCUSSION AND EXPERIMENTAL VALIDATION The “electric” sensors considered in Section II show linear behavior, but exhibit low sensitivity to temperature variations. It
Authorized licensed use limited to: Istituto Nazionale di Ricerca Metrologica - INRIM. Downloaded on September 29, 2009 at 10:02 from IEEE Xplore. Restrictions apply.
4462
IEEE TRANSACTIONS ON MAGNETICS, VOL. 45, NO. 10, OCTOBER 2009
Fig. 12. Computed temperature response of the “magnetic” sensor of Fig. 6(b) (h = 4 mm) and its comparison with the experiments. Fig. 10. Temperature dependence of the magnetic flux density at the field probe position (distance d) for three different Fe-Ni cylinder heights (h). B is the residual induction of the permanent magnet.
simulated results have been experimentally verified in the labomm), confirming the predicted ratory static prototype ( quasi-linear behavior. V. CONCLUSION In this work we compare different methods for detecting the temperature of spinning devices through non-contact sensors, exploiting the temperature dependence of their electric/magnetic properties. In particular, in the here considered “magnetic” sensors, the use of tunable Curie temperature soft/hard magnetic materials is envisaged. Some of the investigated methods, which provide either linear response or switching behavior with temperature, appear ready for industrial applications.
Fig. 11. Temperature response of the “magnetic” sensor of Fig. 6(a). The heating-cooling curve is experimentally verified and compared with simulated results.
is reasonable to believe that temperature variations lower than 10 C are not easily detectable in industrial applications. The “magnetic” sensor based on a hard magnet sensor shows interesting properties, but the preparation of the low Curie temperature magnet can be critical. The most interesting solutions are the soft magnet based “magnetic” sensors. They have been experimentally verified by working out specific prototypes. The experiments have been carried out under static conditions, by emulating a turbo-molecular pump rotor with an aluminium cylinder internally heated by a resistor. The rotor temperature has been directly measured by a PT100 sensor, located close to the prototype of “magnetic” sensor. The response of the sensor of Fig. 6(a) is shown in Fig. 11, where the voltage measured on the secondary coil is plotted against the rotor temperature. It is confirmed that this sensor works like a switch-off device. The repeatability of the measured results has been verified by a series of heating-cooling processes. The response of the sensor of Fig. 6(b) is shown in Fig. 12. The diagram reports the dependence of the probe signal on temmm, and a perature, computed for a soft cylinder height capsule-probe distance ranging between 1 mm and 3 mm. The
ACKNOWLEDGMENT This work was supported by Varian S.p.A. (Italy), which is gratefully acknowledged. REFERENCES [1] J. D. Vincent, Fundamental of Infrared Detection Operation and Testing. Santa Barbara, CA: Wiley, 1990. [2] D. Mavrudieva, J. Y. Voyant, A. Kedous-Lebouc, and J. P. Yonnet, “Magnetic structures for contactless temperature sensor,” Sens. Actuators, A, vol. 142, pp. 464–467, 2008. [3] D. Mavrudieva, J. Y. Voyant, A. Kedous-Lebouc, and J. P. Yonnet, “Magnetic structures for contactless temperature sensor,” Sens. Lett., vol. 5, no. 1, pp. 319–322, 2007. [4] D. Mavrudieva, J. Y. Voyant, J. Delamare, G. Poulin, N. M. Dempsey, and R. Grechishkin, “Contactless harmonic detection of magnetic temperature sensor,” Sens. Lett., vol. 5, no. 1, pp. 315–318, 2007. [5] H. Osada, S. Chiba, H. Oka, H. Hatafuku, N. Tayama, and K. Seki, “Non-contact magnetic temperature sensor for biochemical applications,” J. Magn. Magn. Mat., vol. 272–276, pp. e1761–e1762, 2004. [6] R. R. Fletcher and N. A. Gershenfeld, “Remotely interrogated temperature sensors based on magnetic materials,” IEEE Trans. Magn., vol. 36, no. 4, pp. 2794–2795, Jul. 2000. [7] K. G. Ong and C. A. Grimes, “Magnetically soft higher order harmonic stress and temperature sensors,” IEEE Trans. Magn., vol. 39, no. 5, pp. 3414–3416, Oct. 2003. [8] P. M. Zelis, F. Sanchez, and M. Vasquez, “Magnetostrictive bimagnetic trilayer ribbons for temperature sensing,” J. Appl. Phys., vol. 101, p. 034507, 2007. [9] A. Conrad, “Device for Contactless Measurements of Rotor Temperature,” U.S. Patent 0127551 A1, Jun. 7, 2007. [10] O. Bottauscio, M. Chiampi, and A. Manzin, “Different finite element approaches for electromechanical dynamics,” IEEE Trans. Magn., vol. 40, no. 2, pp. 541–544, Feb. 2004.
Authorized licensed use limited to: Istituto Nazionale di Ricerca Metrologica - INRIM. Downloaded on September 29, 2009 at 10:02 from IEEE Xplore. Restrictions apply.
