Hysteresis In Control System

Hysteresis:-

Hysteresis  is the dependence of the output of a system not only on its current input, but also on its history of past inputs. The dependence arises because the history affects the value of an internal state

Hysteresis in thermostat:-

Hysteresis can be used to filter signals so that the output reacts more slowly than it otherwise would, by taking recent history into account. For example, a thermostat controlling a heater may turn the heater on when the temperature drops below A degrees, but not turn it off until the temperature rises above B degrees (e.g., if one wishes to maintain a temperature of 20 °C, then one might set the thermostat to turn the furnace on when the temperature drops below 18 °C, and turn it off when the temperature exceeds 22 °C). This thermostat has hysteresis. Thus the on/off output of the thermostat to the heater when the temperature is between A and B depends on the history of the temperature. This prevents rapid switching on and off as the temperature drifts around the set point.

Hysteresis In Valves:- 

Hysteresis is a dynamic response to change that causes the path of movement to be different when the response is increasing than when the response is decreasing. This is found commonly in control valves after some time period as the seal around the stem is tightened to decrease fugitive emissions. The tight packing resists valve movement in any direction making its position change less than demanded by the valve positioning mechanism. Eventually additional force is required to overcome the packing resistance and the valve moves closer to the desired position. 

Control loops depending upon control valves (most) see hysteresis as a dead time in their dynamic response and compensate by applying additional reset (integral) action. When the hysteresis becomes too large, the control loop may become unstable and oscillate about the set point more than desired. The "cure" is to rebuild the control valve - a costly maintenance operation. Control loops may use other final control elements such as variable speed drives to avoid hysteresis from control valves, but this is not often used

In process control, the most serious form of hysteresis is encountered in pneumatic control valves. The control valve is the weakest link in the control system because it is the only moving part and due to the presence of friction it is subjected to varying dynamics quite often. The major causes of control valve problems are non linearities such as hysteresis, stiction, backlash, and dead band. Due to these, the pneumatic control valve’s stem movement does not follow the control signal accurately but deviates from it. In other words, the relationship between controller output and controlled variable often changes. All valves have some hysteresis, but excessive valve hysteresis typically occurs when the valve sticks as it tries to open and close. This can happen for a number of reasons including overtightened packing. Therefore, hysteresis is the one of the biggest problems associated with the final control element. This can be roughly defined as the maximum difference obtained in stem positions for the same input up-scale and down-scale.

In process industries, pneumatic control valves are the most commonly used actuators or final control elements. But in practice it is very difficult to do the modelling of any plant or process having a final control element with hysteresis. 

Hysteresis is a nonlinear phenomenon and cannot be expressed by a transfer function. Therefore simulation of such a plant or process is not an easy job. So far very little work has been reported in the literature regarding the effective modelling of a plant or process having hysteresis elements.