What is Hysteresis?

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Thread Starter

boredstudent

Hey,

I'm a 3rd year chemical engineering student doing a Process Control subject and our notes are poor. I'm sure we'll be asked about hysteresis, but have no idea what it is. I've searched a lot of places, but the best I've found is a general description that it describes a system with properties that depend on its history.

Can anyone explain what hysteresis is in regard to Process Control?
 
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Daniel Chartier

Hello;

Just a small example in a flow control process:
you can have two different measured flow rates at the same valve opening % depending on whether it is opening or closing (check valve datasheets from different vendors) so for tight flow control you will have to compensate or take it into account.

Hope this helps,
Daniel Chartier
 
Here's a simple example of hysteresis:

Let's say you have a relay with a 24vdc coil. Suppose you hook up a variable power supply to the coil and slowly start raising the voltage from zero. At some point there will be enough voltage to activate the relay, let say around 21 volts. Continue raising the voltage to 24 volts. Now, if you start slowly decreasing the voltage you'll notice that the relay will not drop out at 21 volts but will stay actuated until some lower voltage is reached, lets say 18 volts.

The relay has hysteresis. If there are 20 volts on the coil the relay could be on or it could be off, the state is going to depend on it's history.

Often we build in hysteresis. For example, say you want an alarm to sound when a tank level reaches 80%. It would be annoying for the alarm to sound repeadedly if the level were bouncing between 79.9 and 80.1. Instead, we would activate the alarm at 80% and program the alarm to recover at some lower level, say 78%.

Some will probably point out that this is not exactly correct, but you can also think of hysteresis as the same as deadband.
 
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Robert Scott

Since you are into Chemical Engineering, I will give an example in that field. Suppose you have a pH meter and you expose it to a strong acid. Then you try to measure something around 7.0 again. Depending on the technology involved, it is likely that the measurement will be skewed because of the previous exposure (that's the history). Another example is hummidity sensors. They will often exhibit some hysteresis. A more common example is a position control system with a gearbox. After turning the input shaft clockwise a while, if you then start turning in the other direction, the output will not move until you have rotated the input shaft a certain amount to take up the slack in the gears.

Robert Scott
Real-Time Specialties
Embedded Systems Consulting
 
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 setpoint 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.

Dick Caro
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idtmemberdoug

Hysteresis represented on graph would be like this. If your optimum measurement was a linear line on a graph, hysteresis lines would appear as arcs or bows both above and below your linear line with the same start and end points. So as you tried to repeat a linear measurement your readings could increase above your optimum on the way up and decrease below your optimum on the way down or vice versa.

idtmemberdoug
 
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