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Wiegand Effect Sensors The Wiegand effect, which refers to the generation of an electrical pulse in a coil wrapped around or located near a Wiegand wire subjected to a changing magnetic field, can be used in a variety of sensing applications. David J. Dlugos, HID Corp.
The
Wiegand Effect In other words, the patented cold-working process that produces the Wiegand wire permanently locks in the ability to exhibit Barkhausen jump discontinuities in the material. To achieve magnetic switching, the wire is put in the presence of alternating longitudinal magnetic fields. The resultant hysteresis loop contains large discontinuous jumps known as Barkhausen discontinuities that occur due to shell and core polarity switching.
The magnetic switching action of the Wiegand wire induces a voltage across the pickup coil lasting ~10 µs (see Figure 1). It is important to understand that the induced voltage amplitude is not totally dependent on excitation field strength and orientation. It is actually the alternating positive and negative magnetic fields of equal saturating strength that are used to magnetize and trigger the Wiegand wire. These alternating magnetic fields are typically produced by magnets affixed to rotating or moving equipment, by a stationary read head and moving Wiegand wires, or by an AC-generated field. Many applications are feasible because the Wiegand effect is operational at temperatures from -80ºC to 260ºC. The functional temperature range of each sensor is typically a factor of the limitations of various component subparts of the individual sensor, not the Wiegand wire itself.
There are two modes of magnetic excitation of the Wiegand effect, symmetric switching and asymmetric switching. In symmetric switching (see Figure 2), alternating positive and negative magnetic fields of equal strength are used to magnetize and trigger the Wiegand wire. First, a saturating magnetic field of one polarity orients the core and shell polarities in the same direction (A). Due to the movement of the magnets, this magnetic field is replaced by an opposite field of equal strength. As the strength of this opposing field increases, the Wiegand wire core switches polarity (B), and produces a large voltage pulse. As the magnetic field increases further, the shell of the Wiegand wire switches polarity (C), producing a much smaller pulse of the same polarity. The pulse is often not visible on an oscilloscope when compared to the large pulse produced by the core. This opposite magnetic field then fully saturates the Wiegand wire (D). At this point, the magnetic field changes back to its original polarity, causing the core to again switch polarity (E), producing a large voltage pulse in the sensing coil. Then, as the magnetic field strength increases, shell switching occurs and produces a much smaller pulse of the same polarity. This pulse is often not visible on the oscilloscope when compared to the large pulse produced by the core. In the asymmetrical switching mode, the Wiegand wire is magnetized and triggered by magnetic fields of opposite polarity but unequal strength (see Figure 3). A
In most Wiegand effect applications symmetrical switching is recommended because it is the easiest to produce with most permanent magnets. Using
the Wiegand Effect Magnetic Actuated Sensor. One of the most common applications of a Wiegand effect sensor is as a rotational counting pulser. The sensor consists of a short piece of Wiegand wire inside a pickup
 . A wide variety of magnets can be used, from
simple bar types to multipole ring magnets. These sensors
will produce a positive or negative voltage pulse
depending on which magnetic pole (north or south)
triggers the sensor.
Read Head and Wiegand Wire.
Another way of making a rotary pulser is to use a
stationary magnetic read head and to embed the Wiegand
wires in a drum. The wires would then rotate through the
fixed magnetic field (see Figure 5). The output of this
type of sensor would be ~2 V, 10 µs pulse width, into a
24,000
lies in its ability to indicate direction of rotation. In one direction, it will produce all positive pulses; in the other direction, all negative pulses. The Wiegand read head can also be used as a linear pulser (see Figure 6). The Wiegand wires could be arranged in a code pattern to give a reference value to the object being passed by the read head.
AC-Generated Field. Instead of permanent magnets, electromagnetic fields can be used to trigger the Wiegand wire. This operation can be as simple as winding a coil around a Wiegand sensor (see Figure 7) and applying an alternating current or as elaborate as making an electromagnet to trigger the sensor. Current Applications. Wiegand effect sensors are used in water, gas, and electric meters for electronic indexing. They also have many automotive applications such as antilock braking, speed sensing, and position indicators. They have been used in anemometers and other wind speed applications, machine controls, shaft speed sensing, and numerous rotational counting applications. Dave Dlugos is Manager-Marketing Services, HID Corp., 333 State St., North Haven, CT 06473; 800-243-2563 or 203-287-9000, fax 203-407-5990, ddlugos@prox.com or www.hidcorp.com. Or contact Eric Widlitz, Sales Engineer, HID Corp., 800-243-2563, ewidlitz@prox.com
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fter 40 years of
research, John R. Wiegand discovered a way to cause the
magnetic fields of specially processed, small-diameter
ferromagnetic wire to suddenly reverse, generating a
sharp uniform voltage pulse. Sensors based on the
proprietary, patented Wiegand effect require only a few
simple components to produce Wiegand pulses. These
sensors consist of a short length of Wiegand wire, a
sensing coil, and alternating fields, generally derived
from small permanent magnets.
Figure 1.
The amplitude of a typical Wiegand pulse
will vary with the style of sensor, but
the pulse width will generally remain the
same.
Figure 2.
Symmetric switching of Wiegand wire
occurs when alternating positive and
negative magnetic fields of equal
strength are used to magnetize and
trigger the wire.
Figure 3.
Asymmetric switching of Wiegand takes
place when the wire is magnetized and
triggered by magnetic fields of opposite
polarity but unequal strength.
Figure 4.
Permanent magnets can be used in three
ways to activate Wiegand sensors.
Figure 5.
A rotary sensor can be made using a
stationary magnetic read head and
embedding the Wiegand wires in a drum.
The wires rotate through the fixed
magnetic field.
Figure 6.
In a linear Wiegand sensor, the Wiegand
wire passes by the read head and produces
positive pulses in one direction and
negative pulses in the opposite
direction.
Figure 7.
Alternating current fields can be used to
trigger the Wiegand wire. In this
example, a coil wound around a Wiegand
sensor is powered by the AC field.
