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Filter on Digital Inputs

Created by: MotyS at: 8/29/2011 9:46 PM (5 Replies)

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8/29/2011 9:46 PM	Rate (0)
MotyS Posts: 3 Rating: (0)	S7-1200 offers multiple inputs on their Digital Inputs, both in the CPUs and in the I/O units. Can anyone explain in simple words why do we need so many optional filters? Thanks, MotyS
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8/29/2011 11:07 PM	Rate (0)
0_Einstein Diamond Expert Joined: 7/7/2010 Last visit: 10/4/2024 Posts: 15264 Rating: (2422)	To help with noisy or bouncy input signals like pushbuttons. Think of it as a debouncer. Think of it as a built-in delay timer without any PLC logic. At least those are the ways I think about those filter options.
	science guy
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8/30/2011 5:32 AM	Rate (0)
MotyS Posts: 3 Rating: (0)	Thank you very much for your answer, but: Yes, I understand the need to debounce (filter) the input and I have been using this for devices that bounce for seneral msec using the larger filter values. However, the Siemens filters can be set to any of the following:0.2, 0.4, 0.8, 1.6, 3.2, 6.4, and 12.8 ms and I am wonderingwhich application will require a filter of 0.2 msec and will fail at 0.4msec? It must be something that changes faster than every 0.4msec (2,500 Hz) and that bounces for up to 0.2msec every time it changes. Do you happen to know examples of devices that when connected to the Digital Input will require the following: A 0.2msec filter? A 0.4msec filter? A 0.8msec filter? A 1.6msec filter? A 3.2msec filter? It can not be a button ??????
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8/30/2011 2:16 PM	Rate (0)
watr_fallr Member Joined: 7/22/2011 Last visit: 5/15/2019 Posts: 3 Rating: (0)	These are also filters for noisy environments. So, a 0.2mS filter would require a signal that is in the on or off statelonger than 0.2mS before transitioning to this state. It also will filter any noise that is less than 0.2mS. So, the noisier the environment the longer of a delay filter can be used. The default is 6.4mS.However, some customers / applications may desire to get the process signal as fast as possible so they can lower the filter time to suit the speed of the application and the amount of noise in the environment.

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8/30/2011 2:26 PM	Rate (0)
MotyS Posts: 3 Rating: (0)	Thank youwatr_fallr. But, can you give me one example of a device whose input to the controller is so fast that it will require such a narrow debouncing filter of 0.2msec or even 0.4msec? MotyS
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8/30/2011 3:32 PM	Rate (0)
0_Einstein Diamond Expert Joined: 7/7/2010 Last visit: 10/4/2024 Posts: 15264 Rating: (2422)	MotyS Thank youwatr_fallr. But, can you give me one example of a device whose input to the controller is so fast that it will require such a narrow debouncing filter of 0.2msec or even 0.4msec? MotyS I can imagine several situations where I want fast precise control with fast low-hysteresis position sensors and not delay unnecessarily for the input state change to be recognized. I can also imagine a saw mill with the majority of the time sawing and minimal time positioning and using the longer filters to let things settle out a bit before resorting to a debouncing TON. Bouncy logs rolling into position. Hope that helps.
	science guy
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