HYSTERSIS LOOP AND ITS SIGNIFICANCE

The word ‘Hysteresis’ comes from original Greek word ‘hustérēsis’ meaning shortcoming or deficiency. It was coined in 1881 by Sir James Alfred Ewing to describe behaviour of magnetic materials where magnetic response lags behind the change in magnetic field.

The word now has broader scope to describe behavior of a system output where the output fails to return along the exact path it followed during the initial application of input. It is a phenomenon where output does not immediately respond to a change in input.

A simple example of a mechanical system is a sluggish door hinge. Force required to open the door might be different from that needed to close it. This is caused by internal friction, structural changes within materials, or mechanical play in components.

A graph can be plotted with input variable on X-axis against the resulting output variable on the Y-axis. When the input is cycled—increased to a maximum and then decreased back to its starting point—the two paths do not overlap, but form a closed loop. Shape and size of this loop characterize specific hysteretic behaviour of system.

In almost every physical system that exhibits a closed loop, area enclosed by loop represents energy lost during the cycle. The lost energy is primarily dissipated as heat due to internal friction, whether molecular friction in a magnetic core or friction in a mechanical joint.

This energy loss is among factors affecting the efficiency of devices like electric motors and transformers. Conversely, same hysteretic behaviour provides a mechanism for system memory, where the material or device retains a state even after external force is removed.

Hysteresis phenomena categories

1. Magnetic Materials: When dipoles of a ferromagnetic material are exposed to a magnetic field, they align with the field (get magnetized). Upon removing the field, some magnetization remains, creating a magnetic B-H hysteresis loop.
2. Mechanical Systems: Rubber bands, alloy springs, door hinges etc.  exhibit hysteresis due to internal friction or structural changes, where the force required to stretch or release differs depending on prior deformation.
3. Electronics: Devices like Schmitt triggers, memory devices and thermostats use hysteresis to prevent rapid switching by introducing a controlled lag between input and output. 
4. Other Fields: Hysteresis is also observed in biology, economics, and environmental systems, where past conditions influence current behaviour.

1. Hysteresis in magnetic materials

In magnetic materials, magnetic field density (or magnetic force) H and magnetic flux intensity B bear a relation along a hysteresis loop.  An applied external magnetic field forces its dipoles to get aligned along the field direction. When the field is gradually removed, magnetization does not return to zero, but a certain residual magnetization is retained.

Initially, the B-H curve starts from (0,0) and goes along the dotted line up to saturation. It does not return to this point (0,0) on removal of field, but follows a different return path as in the curve above. At zero field, some flux is still retained. It takes a force (coercion) in the negative direction to bring the flux to zero. The B-H curve thereafter follows the loop (Hysteresis loop) as the force is reintroduced and removed in cycles. It takes some energy to bring the material from negative saturation level to upper saturation. This energy is not fully recovered on removal of field, and energy covered by area of loop is lost in every cycle. This is the hysteresis loss.

Materials with a wide hysteresis loop (high coercivity and retentivity) are classified as hard magnetic materials. They are used for permanent magnets and data storage because they resist demagnetization.  On the other hand, soft magnetic materials with narrow loop are easily magnetized and demagnetized.  These are suitable for magnetic cores for inductors and transformers where rapid field reversal is possible with minimal energy.

2. Mechanical Hysteresis

In mechanical systems like spring, hysteresis is observed when a material is subjected to a cycle of stress (input) and strain (output), and the deformation path during loading differs from recovery path during unloading. This is called elastic hysteresis and is a property of materials like rubber and polymers. For example, when rubber is stretched and then released, it returns to original length with a delay in its elastic response.

This mechanical lag is due to internal friction and molecular rearrangement within the material, which dissipates some of the mechanical energy as heat. This energy dissipation is a design feature in applications like shock absorbers and vibration dampers, where hysteretic behaviour is used to absorb kinetic energy.  

3. Hysteresis in Electronics

In electronics is switching system, output depends not only on its current input but also on its past states, creating a memory effect that stabilizes circuit behaviour. An idealized hysteresis curve for such circuits takes a rectangular shape. A ‘dead band’ is introduced between the crossover points to prevent output chatter, or to keep output at stable value.

In control circuits, memory units and process controls, it is important to ensure positive switching at defined discreet values of inputs, between which output level must remain constant. In the curve above, output levels are defined at level 1 and 0. When input starts dropping from upper threshold, output remains stable at 1. When it reaches below lower threshold, output switches to 0. When input starts rising, there is no change in output till input reaches upper threshold. At upper threshold of input, the output changes to 1.  For any change beyond these input levels, output does not change. This memory is fundamental to data storage and is sometimes intentionally introduced into control systems to prevent rapid, unnecessary switching between states.

4. Mechanical Sensor Hysteresis

Measurement devices, such as pressure sensors, temperature sensors, and load cells, also exhibit hysteresis, which affects their accuracy and repeatability. Sensor hysteresis is the maximum difference in the output signals for a given difference in physical input, when that value is approached from opposite directions. For instance, a pressure sensor may indicate a slightly higher reading when the pressure is falling from higher level, than when it is rising.

This lag in sensor response is often due to minor mechanical favtors like friction in linkages, or elastic properties of the sensing element itself. This error is taken into account during calibration to ensure that sensor provides a consistent reading regardless of the input history. For high-precision control systems, understanding this non-linearity is important for maintaining system stability and accuracy.

5. Hysteresis in Metrology

Hysteresis affects the accuracy and reliability of measurement systems in metrology and inspection. Understanding this factor is crucial for quality and precision of measurement.

In metrology, hysteresis occurs when response of an instrument to a changing input is not the same when the input is increasing, as it is when the input is decreasing. For example, a pressure gauge may read 5.2 psi when pressure is rising, but when the same pressure is reached on the reducing journey of scale, it might read 5.3 psi at the same absolute pressure.

Hysteresis is common in various measurement applications like:

• Dimensional: Coordinate measuring machines (CMM) show hysteresis due to mechanical friction or backlash in measurement mechanism.
• Force: This arises due to non-linear behavior of sensor element.
• Pressure: Pressure sensors, particularly those using diaphragms or Bourdon tube tend to show hysteresis.