Switch Hysteresis – Video Transcript

Welcome to the sixth in our SensorView™ video tutorial series: Switch Hysteresis. Several people have asked, “What is switch hysteresis and why do we need it?” So a moment spent on that topic might be valuable.

We begin where we left off, with two switches set up. We’ll go to the main Workspace window and wiring. I’m going to turn off one of the switches for clarity, and then we’ll go to the switch icon in the switch output window and Switch #1 is set up for a half-inch hysteresis. That’s going to be hard to demonstrate by just moving the sensor in and out from the computer screen, so let’s change that to two inches. And I would like the polarity to be on closer.

Now, the logic of that is: let’s assume we have a tank, and the level will drop until it reaches a point where we turn on the switch. It will stay on until we have passed through the hysteresis band. And turn off, in this case, is two inches. So you would need two-inch waves in the tank, if you will, or a two-inch disturbance to your reading for some reason in order for it to bounce on and off.

If you didn’t have that, and you are right around the setpoint and had some small waves, your control relays could shatter, causing your pumps to turn on and off repeatedly or your valves to open and close repeatedly; and it would degrade life as well as performance in the system.

So that’s the way we’ve set it up. Let’s copy that to the sensor, right-clicking on the workspace and dragging it to the sensor icon. We get our safety warning, and now let’s see how that operates in practice. I’m at about 20 inches. I’m going to come in slowly to the computer screen and note the state of the transistor switch over here on the right as I come through the set point.

I pass 12 inches and it’s on and as I come back, it’s going to stay on. I’m at 13 ½ now, and coming further out. There’s 14 inches, it turns off. And as it comes back in, I’m at 13, it hasn’t gone on yet. When I pass through the threshold, it turns on. I come back, it stays on until I have gone to 14 inches and then it turns off.

So that’s the general concept of implementing hysteresis around a set point. It’s to prevent jittering of the relays in the control circuitry, very similar to what happens with a time delay relay about its setpoint.

In our next session, we’re going to show you how to widen out that hysteresis band to provide a pump control window.

If you have any questions, please contact Senix customer service. We’re here to help!