How to avoid chattering of relay?

[Sid2286]

Hi,

I have written a code wherein I set the on/off condition,
once the process value reaches the defined on or off state the relay is suppose to turn off or turn on.

However, when the process value is close to the defined state, the relay starts to chatter.

I have taken care of adding diode to avoid freewheeling condition, but i dont know how to avoid hysteresis through program.

please guide.

Sid

[SherpaDoug]

I think you need to ADD hysteresis to stop the relay chattering. Move your setpoint up or down based on the output state. Either that or slow your loop rate so the output is only updated every few seconds.
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[temtronic]

Since you're using a mechanical relay, the 'process' isn't all that critical to timing or setpoint ,so do all that SherpaDoug suggests.
Typical home heating thermostats have a 1*C hysterisis(2*F) built in,sampling once per second( clock update rate).

You might require different hysterisis values for the positive ramp, versus negative, all depends on what you're trying to accomplish.

[ezflyr]

Hi,

As others have stated, you need to add some hysteresis to your control equations to prevent the relay from chattering. The problem is that your 'Process Value' is varying slightly about the Turn On/Turn Off point, and is causing the problem. So, to solve your problem, the ON point should not be the same as your OFF point!

I have a relatively simple terrarium heater control that has 2 degrees F of hysteresis between the On temperature and the Off temperature, so the relay switches at a different point "on the way up", as it does "on the way down". I use code like this to make that happen:

[sahu77]