Hysteresis In Digital Control Systems

Created: 5/15/2020Last Updated: 07/06/2023


Hysteresis refers to the nonlinear behavior of a system where the output response does not directly correspond to the same input values but depends on the history of input changes, including both the direction and magnitude of the input.

This nonlinear behavior is often represented by a hysteresis loop, which describes the relationship between the input and output of the system. In the context of digital control in electronics, hysteresis can be modeled using a simple two-state machine with thresholds for state transitions, such as Vth_high and Vth_low. The hysteresis behavior is often visualized through a loop (often called hysteresis loop)that represents the relationship between the system's input and output states.  For example, when the input voltage goes above Vth_high, the system transitions to state 1, and when the input voltage falls below Vth_low, the system transitions to state 0.

The plot above shows the complete hysteresis loop in a digital control system, capturing both the increasing and decreasing input voltage:

Key Features:

Practical Applications in Digital Systems

A graph comparing a simple comparator (output A) and Schmitt comparator (Ouput B) from input (U). The dashlines in U are the tresholds. Image source(wikipedia: https://en.wikipedia.org/wiki/Schmitt_trigger#/media/File:Smitt_hysteresis_graph.svg)


Hysteresis provides robustness and stability to systems, ensuring that changes in input variables do not immediately trigger undesired or erratic responses. By introducing thresholds and memory of previous states, hysteresis allows for more reliable operation and noise immunity in various applications.

For further reading and more in-depth information on hysteresis, you can refer to the provided link: "Hysteresis" [Link: https://en.wikipedia.org/wiki/Hysteresis]