Method and apparatus for digitally processing and filtering signals in industrial control applications

• US 5,339,335 A
• Filed: 12/15/1992
• Issued: 08/16/1994
• Est. Priority Date: 12/15/1992
• Status: Expired due to Fees

• Current Assignee: Elsag International BV (ABB Limited)
• Original Assignee: Elsag International BV (ABB Limited)
• Inventors: Molnar, Richard J.
• Primary Examiner(s): Chin, Stephen
• Assistant Examiner(s): Le, Amanda T.
• Application Number:
  US07/990,516
• Time in Patent Office: 609 Days
• Field of Search: 375/103, 375/60, 375/99, 375/58, 364/551.01, 364/552, 364/571.01, 364/572, 364/574, 328/149, 328/167, 328/162
• US Class Current: 375/350
• CPC Class Codes: H03H 17/02 Frequency selective networks digital computers for complex mathematical operations G06F17/10

Abstract

A device which is a combination of a method and apparatus and a filter, such as but not limited to, a conventional lag filter for digitally processing and filtering signals used in industrial control applications. The method and apparatus provides a factor which is used by the filter to process the digital signal at the input to the device. The method determines if the change in the input signal is either due to noise or a valid signal change. Information which gives a past analysis of the trend in direction of the signal at the input to the device is used in the method. The trend information includes a factor representative of the past history of the trend. The method uses the most recent trend indicator to determine to a reasonable degree of confidence if the signal currently at the input to the device is or is not characteristic of noise. If the signal at the input is not believed to be characteristic of noise, then the method adjusts the trend indicator by a tracking credit. The tracking credit is, in effect, a further degree of confidence that change in the input signal is not characteristic of noise. The method minimizes the tradeoff between valid signal response time and noise rejection associated with the prior art filters.

Claims

1. In a device having an input and an output, said device operating in repetitively occurring predetermined intervals of time, a method for processing a digital signal at said input during that one of said predetermined intervals of time which is presently occurring to thereby provide a filtered digital signal at said output, said device including means for storing said filtered digital signal at said output and a trend indicator, said method comprising the steps of:

a) calculating the difference in amplitude between said digital signal at said input during said presently occurring one of said predetermined intervals of time and said filtered digital signal at said output stored in said means for storing during a previously occurring one of said predetermined intervals of time;

b) determining whether said trend indicator stored in said means for storing during said previously occurring one of said predetermined intervals of time permits a tracking credit;

c) calculating the permitted tracking credit as a function of an assumed noiseband of said digital signal at said input during said presently occurring one of said predetermined intervals of time and said difference in amplitude;

d) comparing said difference in amplitude;

i. to said trend indicator; and

ii. to a predetermined amount based on a function of said assumed noiseband;

e) changing said trend indicator;

i. to zero either when said difference and said trend indicator disagree or when the absolute value of said difference is less then said predetermined amount based on a function of said assumed noiseband;

or

ii. by adding to said trend indicator the permitted tracking credit and a predetermined amount related to the polarity of said difference;

f) calculating a filter factor based on a function of said trend indicator changed during said presently occurring one of said predetermined intervals of time;

g) using said filter factor in said device to thereby provide said filtered digital signal at said output; and

h) replacing said filtered digital signal at said output and said trend indicator stored in said means for storing during said previously occurring one of said predetermined intervals of time with said filtered digital signal and said trend indicator changed during said presently occurring one of said predetermined intervals of time in said means for storing.

2. In a device having an input and an output, said device operating in repetitively occurring predetermined intervals of time, a method for processing a digital signal at said input during that one of said predetermined intervals of time which is presently occurring to thereby provide a filtered digital signal at said output, said digital signal at said input having an assumed noiseband, said device including means for storing said filtered digital signal at said output, a first trend indicator and a second trend indicator based on said assumed noiseband, said method comprising the steps of:

a) calculating the difference in amplitude between said digital signal at said input during said presently occurring one of said predetermined intervals of time and said filtered digital signal at said output stored in said means for storing during a previously occurring one of said predetermined intervals of time;

b) determining whether said first trend indicator stored in said means for storing during said previously occurring one of said predetermined intervals of time permits a tracking credit;

c) calculating the permitted tracking credit as a function of said assumed noiseband of said digital signal at said input during said presently occurring one of said predetermined intervals of time and said difference in digital signal amplitude;

d) comparing said difference in said digital signal amplitude;

i. to said first trend indicator;

ii. to a first predetermined amount based on a function of said assumed noiseband; and

iii. to zero;

e) changing both said first and said second trend indicators to zero either;

i. when said difference and said first trend indicator disagree;

or

ii. when said difference is equal to zero;

or

iii. when the absolute value of said difference is less then said first predetermined amount based on a function of said assumed noiseband and said difference disagrees with said second trend indicator;

or

adding to said first trend indicator the permitted tracking credit and a first predetermined amount related to the polarity of said difference, when the absolute value of said difference is not less than said first predetermined amount based on a function of said assumed noiseband;

or

changing said second trend indicator by adding thereto a second predetermined amount related to the polarity of said difference, if said difference agrees with said second trend indicator, and

then, if the absolute value of said second trend indicator is greater then a second predetermined amount based on a function of said assumed noiseband, changing said second trend indicator to zero and adding to said first trend indicator the permitted tracking credit and said first predetermined amount related to the polarity of said difference;

f) calculating a filter factor based on a function of said first trend indicator changed during said presently occurring one of said predetermined intervals of time;

g) using said filter factor in said device to thereby provide said filtered digital signal at said output; and

h) replacing said filtered digital signal at said output and said first and second trend indicators stored in said means for storing during said previously occurring one of said predetermined intervals of time with said filtered digital signal and said first and second trend indicator changed during said presently occurring one of said predetermined intervals of time in said means for storing.