Sensors - October 2000 -An Innovative Passive Solid-State Magnetic Sensor

A new magnetic sensor technology is based on the magnetostrictive and the piezoelectric effects.

Yi-Qun Li,
Spinix Corp.
Robert O’Handley, Massachusetts Institute of Technology

The magnetic sensors in most common use today are variable reluctance coils and Hall effect devices. The drawback of variable reluctance sensors is that they generate a signal proportional to the magnetic field’s rate of change. Signal strength therefore decreases with decreasing speed, and below a certain flux change rate, the signal disappears into the noise. The excess output voltage of the coil at high-frequency magnetic fields also causes problems for circuit designers. Hall effect devices generate a very small raw signal because of low field sensitivities (0.5–5.0 mV/100 Oe applied field), and the device performance is strongly temperature dependent. These features mandate signal conditioning, and require that a certain minimum field be available for device operation.

TABLE 1

Cost and Performance Comparison

Characteristics Coil Hall GMR PSSM¹

Cost² Low Moderate⁴ High Low

Size ~cm ~mm ~mm ~mm

Signal sensitivity (3) 0.01 mV/Oe/V 1mV?Oe?V 20 mV/Oe

Temperature Stability Good Poor Acceptable Good

Input Power 0 ~mW ~mw 0

Dynamic Range 10^-1-10³ Oe 1-10³Oe 10^-3-10Oe 10^-3-10³Oe

1 PSSM: passive solid-state magnetic sensor developed by Spinix Corp.
2 Cost depends on the scale of manufacture; the number here is estimated for large-scale manufacturing.
3 The sensitivity of a coil sensor is frequency dependent.
4 This number includes the price of a bias magnet.

So far, most magnetic sensors are based on one phenomenon that has an intimate connection between magnetic and electric effects; the variable reluctance coil is based on the Faraday efffect and the Hall sensor is based on the Hall effect. We have developed a new passive solid-state magnetic sensor (PSSM) technology (see Photo 1) that is based on two phenomena: the magnetostrictive effect and the piezoelectric effect. These sensors consume no electrical power, and easily produce a raw electrical signal with a magnetic field sensitivity >10 mV/Oe. They combine the advantages of Hall sensors’ miniature size and the passive nature of variable reluctance coil devices (see Table 1).

Photo 1. A new passive solid-state magnetic sensor works by combining two phenomena: the magnetostrictive effect and the piezoelectric effect.

These sensors, typically measuring 1 by 5 by 0.5 mm are diced from a wafer of several square inches. They can be mass produced at low cost and, in most applications, offer better performance than do variable reluctance coils, Hall effect sensors, magnetoresistive semiconductors, and the recently introduced giant magnetoresistive metal multilayers. They have been demonstrated successfully for measurement and control of rotational speed, digital flow, and electric current.

A PSSM sensor typically consists of a piece of piezoelectric material and a piece of magnetostrictive material. In response to a magnetic field, the magnetostrictive component imparts a strain on the piezoelectric element that in turn produces an electrical output signal. One important characteristic of the new technology is that the sensitivity and the operational magnetic field can be controlled through material properties and configurations to suit a variety of applications. Figure 1 shows an example of two sensor designs with magnetic field sensitivities of 28 mV/Oe and 5 mV/ Oe in a linear magnetic field operational range of 2.5 Oe and 200 Oe, respectively.


Figure 1. Root-mean-square output signals are shown as a function of an AC magnetic field at 80 Hz for a PSSM sensor with 28 mV/Oe sensitivity in the 0–2.5 Oe range (A), and with 5 mV/Oe sensitivity in a 0–200 Oe range (B).	Figure 2. The stability of the PSSM sensor over temperature is demonstrated by this root-mean-square signal output plotted as a function of temperature measured in an AC magnetic field at 80 Hz.

As compared to a variable reluctance coil sensor, the PSSM sensor detects near-zero speed (0.1 Hz), is smaller, and costs about the same. Com pared to Hall and magnetoresistive devices, the PSSM sensor has better field sensitivity, better temperature stability, and costs less. Figure 2 shows the stability of a sensor output at a 40ºC–160ºC temperature range without any compensation circuit. The sensor produces a sine wave electrical signal in tens of millivolts when a magnet periodically passes by. The detection distance can be >1 in. when a NdFeB magnet 3 mm dia. by 5 mm long is placed in the object of interest. The target’s speed is measured by the frequency of the output signal, which is an inverse of a time period between the passing magnets. A wheel mounted with equally spaced magnets can thus measure rotational velocity. With a built-in electrical circuit, the sensor outputs a standard pulsed square wave signal. Such a sensor with a bias magnet mounted next to it can detect the rotational speed of a ferromagnetic gear at a standoff distance of a few millimeters, depending on the size (pitch) of the gear teeth.

High sensitivity and low power consumption are the two most important criteria for flow measurement applications. Variable reluctance coil sensors are often used because they satisfy these criteria. The PSSM sensor is well suited for this application because it requires no power source and offers better sensitivity in a smaller size than other magnetic sensors. Furthermore, it works at low frequency for better resolution in low flow measurements. An optional microprocessor-based flowmeter converts directly from flow rotor speed to flow rate (see Figure 3).


Figure 3. A calibration curve of rotor speed can be seen as a function of liquid flow in a a flow sensor with 1/3 in. pipe and a four-blade rotor.	Figure 4. A PSSM sensor placed next to an electric wire produces a root-mean-square signal output measured as a function of an AC electric current at 50 Hz.

A sensor with high linearity over a large magnetic field range is ideal for electrical current measurement. Hall and magnetoresistive sensors, because of their high linearity, are popular for this application. However, their characteristics of zero offset voltage and temperature-dependent output make electrical circuit design a complicated matter. The PSSM sensor has been demonstrated in a simple design and small size for electrical current sensors, current switches, and relays. The device does not have zero offset voltage and requires no temperature compensation for general use in electrical current sensors. It can also be designed to maintain linearity in magnetic fields >1000 Oe for meas urement of large currents.

Figure 4 shows a raw rms output signal with a sensitivity of 2 mV/A as measured by a PSSM current sensor placed next to an electrical wire carrying a current of 0–10 A at 50 Hz.

PSSM sensor technology shows great promise as a replacement for other magnetic sensor technology. It will certainly be explored and developed for many applications such as information storage, magnetic recording read heads, magnetic imaging for nondestructive inspection and biomedical procedures, detection of magnetic anomalies, surveillance, and mineral prospecting.

Yi-Qun Li is President, Spinix Corp., 351 Rheem Blvd., Morago, CA 94556, 925-631-7878, fax 925-631-7892, yqli@spinix.com.

Robert O’Handley is Principal Scientist, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139-4307; 617-253-6913, fax 617-253-9657, bobohand@mit.edu.