what is valve hysterisis

What is valve hysterisis

alan webster said:

What is valve hysterisis

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A pain in the rump.

Any hysterisis (also sometimes referred to as deadband) is a difference between a change in command or input to the change in travel or output. It is most pronounced and of most concern when reversing direction.

There are two kinds of hysterisis in a valve. First is mechanical, the result of backlash or clearances in the gearing. When you reverse the direction of travel some motor rotation and time is involved in taking up these clearances, causing a delay in the actual valve disk or stem movement.

The second, which occurs in analog positioners, is actuator signal hysterisis or deadband. To avoid hunting it is necessary to have a tolerance zone where the feeback pot signal differs slightly from the command signal, but it is "close enough" and so the motor operator isn't given a move command. This can be less than 1% or it can be as much as 5%, depending on the quality of the positioner and the quality of the calibration.

Deadband can be the source of a lot of instability and hunting, particularly in PID control systems.

hysteresis is the lagging effect...ie the change in magnetism lags behind the change in the magnetic field. With a valve it would mean it doesnt respond as quickly as expected.

I like Tom's explanation, but here's another way to look at it:

Imagine you have a linear valve with a 1% deadband at 0% and you tell it to go to 50%. The valve moves and gets to 50.1% in 15s (well inside the 1% deadband). Now lets say you tell it to move to 75% and it moves to 74.9% in 7.5s. Now lets say you tell it to move back to 50%, it moves to 49.9%, but takes 8s to do it...

Now my personal definition (how I think about it) of deadband is the range the valve can be around 50% and still be satisfied or considered to be at 50% (ie, 49.5% - 50.5% is considered 50%).

The way I think of hysteresis is the time (or other factor) it takes for the valve to reverse direction; remember we had a linear valve that too 15s to go from 0% to 50% and 7.5s to go from 50% to 75%, but took 8s to go from 75% to 50%... That extra 0.5s is the time it took for the gears to move from one gear face to the other gear face taking up the little bit of slop in the gear train.

Please note that these are not dictionary definitions, but my way of thinking about these terms.

"phenomenon in which the response of a physical system to an external influence depends not only on the present magnitude of that influence but also on the previous history of the system."

"Magnetic hysteresis occurs when a permeable material like soft iron is magnetized by being subjected to an external magnetic field. The induced magnetization tends to lag behind the magnetizing force."

Sorry to bring this back to life. I liked Tom's explanation in general but disagree with some aspects of it. I am not sure I can explain this properly but I will try.

Hysteresis is a lag in response, in many applications (like a valve) a curve can be created to show the response loop. Usually hysteresis causes the lag to be greater when changing direction...ie an electric coil may energize faster than it will de-energize. In other words hysteresis is a reference to the time it takes to respond to a command.

Also for a valve there may be issues with the fluid and its viscosity which can cause this lag. Most valve manufacturers should have data sheets relevant to this.

Take a piece of bubble wrap, that stuff you wrap items for shipment, and press on one of the bubbles. It will indent where you press it but once released it will slowly attempt to return to its normal shape...this is a simple example of physical hysteresis.

The issue I have is associating it with deadband. Deadband is a range of values that create a response. EX: thermostat set at 70 may turn on an air conditioner when the temp reaches 72 and cut off when temp is 68.

You're right, Ron, in stating that deadband is not identical to hysterisis. However, the effect of deadband on control system accuracy is similar to and linked to hysterisis, and I dodn't think it appropriate to discuss one without the other.

Hysteresis and Dead Band

Hysteresis is non-linear response dependent on the history of the control signal such as whether the control signal is increasing or decreasing. Usually the output lags the input at a 0 Hz by a small amount.



Dead band is a range of where the control signal has no effect on the output. Many hydraulic proportional valves have a volt or more of dead band.



Hysteresis and dead band are definitely different. I find dead band to be the biggest pain in the rear. Hysteresis can be compensated for by the PID. One really can't compensate for dead band in a dynamic sense and they are unsuitable for type 1 (integrating) systems.