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Torque Sensors and Domain Theory

What are Torque Sensors?

Torque sensors are devices used to measure torque. There are many kinds of torque sensors, each appropriate for different applications. Of interest here is how domain theory explains properties of materials, and how these properties are used in certain torque sensors.

Torque

Torque is rotational force. Twisting the lid of a jam jar, to remove or replace the lid, is an example of applying torque. If the lid refuses to budge, the torque applied is called static torque. Static torque is torque applied to a non-accelerating object. Therefore, using a force to rotate the jar lid at a constant angular speed is also called static torque. When the jar lid first begins to move, it is accelerating from a stationary state. Torque applied to an object with angular acceleration is called dynamic torque.

The amount of torque developed is proportional to the force applied and to how far from the center of the rotation the force is applied. Pushing a door harder applies more torque to the door. Pushing a door near its hinges may not open the door; pushing with the same force on the door farther away from the hinged side may open the door.

Consumers are not usually interested in exactly how many foot-pounds or meter-newtons of torque are required to unscrew a lid, just so long as they can get at the product. However, manufacturers use high-speed processing equipment that screw hundreds of lids per minute onto containers of various products. Other manufacturers use robots to tighten nuts. If too much torque is used, jars break or bolts get stripped. If not enough torque is used, food spills or spoils, or equipment falls apart. Manufacturers can use torque sensors to package or assemble the goods consumers want. Some torque sensors make use of magnetic properties of specialized materials.

Pseudo-magnets

The magnets most people are familiar with are either permanent or temporary. Permanent magnets

produce a magnetic field with a constant force and a fixed direction. Temporary or soft magnets produce a field with fixed direction that changes (weakens) with time. Pseudo-magnets, used in torque sensors and other devices, are materials whose magnetic field can change in both direction and strength.

The domain theory explains magnetic phenomenon by proposing the existence of domains. Domains are small regions within an object that are magnetic. These regions may be from one to hundreds of microns, which is small, but larger than atomic in size. When the polarities of the individual domains are randomized, their fields cancel one another and the object is not magnetic. When the polarities of all domains are parallel and aligned, their fields reinforce one another and the object is magnetic. Pseudo-magnets are made of special alloys that lock up the domains inside crystalline structures.

Magnetoelastic Materials

Torque sensors are more easily understood as an application of domain theory.

Normally when we think of elastic effects, we think of a material like a clock spring or rubber band. Mechanical work is done to stretch or deform the material, conversely the material can do mechanical work as it is allowed to return to its original shape. However, magnetoelastic material shrinks or elongates or otherwise deforms in a changing magnetic field. Changing shape in response to an external magnetic field is called the magnetorestrictive effect. Conversely, when magnetoelastic materials are stretched or deformed, a magnetic field is created around the material. A changing magnetic field around material being deformed is called the Villary Effect.

How Torque Sensors Work

Some torque sensors work by taking advantage of the Villary Effect. The Villary Effect is the phenomenon of changing magnetization of a magnetic material when the material is stretched, compressed, twisted or bent. Changing the shape of the material distorts the shape of the crystals that make up the material. This changes the direction the domains face, which in turn changes the direction and strength of the material’s magnetic field. These materials are called magnetoelastic materials or pseudo-magnets.

The Villary Effect in torque sensors produces usable electrical signals.

Changing magnetic fields can be used to induce current in electrical conductors. Since the components change mechanical energy into electromagnetic energy, this type of torque sensor may be called a torque transducer. A transducer is a device that changes one type of energy into another type of energy. This current can be converted to a number on an LCD display or it can be used to control manufacturing equipment.

Torque sensors convert mechanical energy to electromagnetic energy and are therefore a kind of transducer.

The example described here is the Torqstar Transducer (http://www.sensorland.com/HowPage020.html). One component of the torque transducer is the shaft. The shaft is what connects the sensor to the object to which torque is applied. Torque is applied to the object through the shaft. A magnetoelastic ring is applied tightly to the shaft. Twisting the shaft also twists the magnetoelastic ring, distorting the crystalline structure of the ring. Left undisturbed, the magnetic field runs parallel to the sides of the ring. As the ring twists, the magnetic field changes direction, cutting across the sides of the ring. The changing magnetic field induces current in the induction coil surrounding the magnetoelastic ring. If more torque is applied through the shaft, more current is produced. The ring and the coil are electrically insulated from each other. The outside housing that encases the torque sensor blocks unwanted external magnetic fields.

Magnetorestrictive Effect

The Villary Effect is sometimes inappropriately referred to as the magnetorestrictive effect. The magnetorestrictive effect is the distortion of ferromagnetic crystals when an external magnetic field is applied; it is the converse of the Villary Effect. When a magnetic field is applied to a ferromagnetic material, the material may elongate or shrink. Alternating current passing through a transformer or fluorescent light ballast creates a magnetic field that changes direction 120 times each second. The magnetic materials in the transformer or ballast interact with the changing magnetic field and vibrate, producing a hum. In this case the transducer is converting electromagnetic energy to mechanical energy. Magnetorestrictive transducers were developed to power sonar systems during World War II to defeat Hitler’s “U-boat” submarines.

domain theory and magnetic field problems

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