Two sensor types, induction coils and magnetic field sensors, are commonly used to measure the magnetic field or flux density.

Induction Coils.

The most commonly used type of sensor on MFL tools is an induction coil. Induction coils incorporate several turns of fine wire

A changing magnetic field induces a voltage across the wire. Coils can have different orientations, as shown on on the right of the above figure; each orientation measures only one component of the magnetic field. Coils can take on many shapes and sizes and have many subtleties and strategies that enhance the performance in one way or another.

The output voltage V across the ends of the coil moving through a magnetic field is time rate of change of magnetic flux density B across the coil. Coils have an aperture or area equal to A. (Note the direction of A is defined by its normal vector.) So, the output voltage of a coil can be written as:

V = d(B·A) / dt

Coils are sensitive only to the flux that is perpendicular to their apertures. (To obtain the perpendicular component, the dot product (·) is used between B and A). The flux density is constant in a region with no imperfections. Therefore, no voltage will be induced when no defect is present. When an imperfection causes flux to leak into the air, a voltage is induced because the flux density is changing.

The output voltage of an induction coil is directly proportional to tools speed, S, in the axial direction (dz/dt). So, the output voltage can be written as

V µ d(B·A) / dz

where dz is a change on distance along the pipe axis during the measurement. Therefore, the output voltage of an induction coil is directly proportional to tool speed, and the tool speed must be known to calculate the actual leakage field level.. So, accounting for velocity effects with coil sensors is difficult.

Other Coil Configurations. Coil sensors come in many shapes and sizes and have many subtleties and strategies that enhance performance in one way or another.

One of the most common strategies in coil design is the use of differential coils to reduce noise. One coil is placed near the pipe surface where a flux signal can be detected; this signal includes a certain amount of noise. A second coil is placed away from the pipe surface, where only noise is detected. By subtracting the output of the second coil (noise) from the output of the first coil (signal plus noise), only the flux leakage signal should remain. This subtraction can be easily performed by winding one coil clockwise and the other coil counterclockwise and wiring the coils in series. The noise signals never cancel out completely, but significant noise reductions can be made.

In coil construction, it is difficult to make two coils have the same output characteristics. But MFL inspection systems have many sensors (10 to 30 in conventional tools, and nearly 100 in some advanced tools). One method to fabricate duplicate coils is using printed circuit card technology as shown in (Item c above). Then each coil will have the same geometry and number of turns. The design of MFL sensors can benefit from other technologies. For example, variations of magnetic tape recording heads, (Item d above), can be used to measure the magnetic fields.

Other Coil Design Considerations. One other design consideration related to coils is the choice of a core material. The core of a sensor is the region inside of the wire coils. In some MFL sensors, a solid core is used to change the sensitivity of the sensor to flux leakage fields. Solid cores are generally more sensitive to small leakage fields, but as a result, they are also more sensitive to noise variations.

Many new MFL inspection systems use a magnetic field sensor. Unlike a coil sensor, a magnetic field sensor measures directly the magnetic field. The most common type is a Hall-effect sensor, which directly converts the magnetic field level to an output voltage. Since field and flux density are related by a constant in air, the output voltage of a Hall-element is directly proportional to the flux density.

The figure below shows a Hall-effect sensor. Typical Hall-effect sensors are relatively small, thin devices that are nominally 0.25 inches square and 0.03 inches thick, although devices exist that are twice as big and five times smaller. Hall-effect sensors, like coils, measure only one component of a magnetic field.

Developments in other industries have help the implementation of hall effect sensors. The most significant improvements are in the area of magnetic field sensors, with antilock braking system development improving Hall-effect sensor technology and reducing the price. In the past, Hall-effect sensors were fragile, needed additional circuitry for the supply current and amplification, were temperature sensitive, and had large variation in offset and gain. The Table below shows the development of Hall effect sensor to overcome these problems.

Problem	Solution	Company/ Model	Year	Cost, $
Fragile leads	Kapton film leads	FW Bell	1991	15.00
Additional circuitry for current supply and amplification	Integrated into circuits and hall plate on a chip. These 3-pin devices need power and ground and provide an output proportional to flux.	Allegro 3501, 3503	1993	10.00
Temperature Drift	Integrated temperature compensation into circuit	Microswitch SS495, and Allgero 3507	1996	6.00
Offset and gain variation	Programmable Hall sensors to adjust sensitivity, offset, gain and temperature compensation. One additional pin (4th) for programming.	Melexis	1998	6.00 (est)

Hall-effect sensors require power to operate, on the order of tenth of a watt per sensor. This power drain is important for long distance inspection requirements. Hall-effect sensors are also temperature sensitive, with a drift in output voltage on the order of a tenth of a percent per degree Celsius (-0.06 percent/degree F). Operational temperatures for commercial sensors range from -76 F to 185 F. During a typical MFL inspection, though, the change in temperature is likely to be too small to significantly impact measurements. The bandwidth of these sensors exceeds 10 KHz. Therefore, the output is not a function of velocity for normal inspection speeds.

Like differential coils, differential field sensors can be used to reduce the effects of noise. In addition, other types of field sensors can directly measure the magnetic field. These devices are usually referred to as magnetometers and operate on a variety of principles. At least one in-line inspection system uses a proprietary magnetometer to measure magnetic field levels.

Sensors between the magnet pole pieces measure the flux leakage field. The purpose of sensor systems is to convert the flux leakage field into a signal that can be stored and analyzed. The sensor system consists of the sensors themselves, the mounting system used to support the sensors, wear plates between the sensors and the pipe, and cabling between the sensors.

The GRI MFL test bed vehicle has 48 sensors heads, 6 on each magnet bar. In each sensor head has four axial hall element sensors. This spacing is similar to commercial high resolution systems. To minimize noise, some amplification of the signal takes place very near the sensor. An example of a integrated Hall element is the SS495 Series, solid-state, ratiometric, linear sensors manufactured by Honeywell Micro Switch. These sensors operate on supply voltages ranging from 4.5 volts to 10.5 volts. Outputs are ratiometric, and are set by the supply voltage. The sensors measure a minimum of +/- 600 Gauss, and they include an amplifier integrated into the circuit.