Hysteresis Error

Definition

Hysteresis error is a phenomenon in instrumentation and control systems where the output of a device depends not only on the current input but also on its previous states. It represents the maximum difference between the output readings when the same input value is measured once while increasing the input (upscale) and once while decreasing the input (downscale).

Main Content

1. The Hysteresis Loop

• Hysteresis occurs due to internal friction, elastic deformation, or magnetic retentivity within the mechanical or electrical components of a sensor.
• Because the system does not return to its exact original state during the reverse cycle, a "loop" is formed when plotting input versus output.

2. Causes of Hysteresis

• Mechanical friction: Moving parts in a sensor may stick or experience resistance, leading to different positions for the same applied force.
• Magnetic properties: In magnetic sensors or relays, the material may retain some magnetism even after the current is removed, affecting future measurements.

3. Quantification of Error

• It is usually expressed as a percentage of the full-scale output (FSO) of the instrument.
• Mathematically, it is defined as the difference between the upscale reading and the downscale reading at any specific input point, divided by the full scale.

Output
  ^          /--- (Upscale)
  |         /  _--
  |        /  /
  |       |  /
  |       |_/ --- (Downscale)
  +----------------------> Input
        Hysteresis Loop

Working / Process

1. Calibration Upscale

• The sensor is subjected to a slowly increasing input signal starting from zero.
• Readings are recorded at specific intervals to establish a baseline performance curve.

2. Calibration Downscale

• Once the maximum input is reached, the input is slowly decreased back to zero.
• Readings are recorded at the same intervals used during the upscale phase to identify deviations.

3. Error Calculation

• The difference between the upscale reading (Y1) and the downscale reading (Y2) is calculated at every test point.
• The maximum of these differences is identified as the "Hysteresis Error" and compared against the full-scale range of the device.

Advantages / Applications

• Understanding hysteresis allows engineers to compensate for errors in control systems through software algorithms or mechanical design.
• It is vital in the design of smart materials like shape-memory alloys used in actuators.
• Necessary for precision calibration of pressure transducers, strain gauges, and magnetic storage devices.

Summary

Hysteresis error is the discrepancy in sensor output readings caused by the direction of input change, reflecting the system's "memory" of its previous state. By identifying this error, manufacturers can improve instrument accuracy and reliability in high-precision measurement applications.

Important terms to remember: Calibration, Full-Scale Output (FSO), Upscale, Downscale, and Hysteresis Loop.