The filtering software which use the software to identify useful signals and interference signals can filter out the interference signal. Following we will list some common methods of software filtering method and briefly introduce the advantages and disadvantages.

## 1. Limiting filter method (also known as procedure determine filtering method)

A. method: it could identify the maximum allowable deviation value of the two samples and set the maximum value as A according to experience, each time it detects a new value it will judge: If the difference between the current value and the previous value <=A, then the current value is valid. If the difference between the current value and the previous value >A, then the current value is invalid, discard the current value, use the last value instead of the current value

B. advantages: can effectively overcome pulse interference due to causal factors.

C .disadvantages: unable to suppress that kind of periodic disturbance, smoothness is poor.

## 2. Median filtering method

A. method: take the samples for n times in a row (n get the odd), arrange the sample values by the size, and set the intermediate values as the valid values.

B. advantages: can effectively overcome wave interference due to causal factors, and the filtering effect of the measured parameters about the slow changes of temperature, liquid level are good.

C. disadvantages: for rapidly changing parameters such as flow, speed, it would not be appropriate.

## 3. The arithmetic Average filtering method

A. method: continuous access to n sample values for arithmetic operations. When the value of n is large: the signal smoothness is high, but with lower sensitivity; when the value of n is small: the signal smoothness is lower, but the sensitivity is higher. Select N value: general flow, N=12, pressure: N=4

B. advantages: apply to filter the signal with random interference, this signal is characterized by an average, the signal in the vicinity of a range of values fluctuate.

C. disadvantages: it is not suitable for the measuring speed slower or requiring faster real-time control, and it is a little wasting RAM.

## 4. Average recursive filter method (also known as the moving average filter method)

A. method: take the continuous n sample values as a queue, the length of the queue is fixed as n, put each sample new data into the end of the line and throw away the original data (basis on first in, first out), do mean queue operations arithmetic for the n data, you can get new filter results. N value select: flow, N=12, pressure: N=4; liquid, N=4~12 temperature, N=1~4

B. advantages: it has a good interference for periodic inhibition, the smoothness is high, and it is suitable for high-frequency oscillation system.

C disadvantages: the sensitivity is low, it is poor interface in occasional impulse inhibition and difficult to eliminate the sampling deviation of pulse interference, it is not suitable for more severe pulse interference, it wastes more RAM.

## 5. The median average filter method (also known as the average pulse interference filter method)

A. method: it is equal to “median filter method” + “arithmetic average filter”. Take N continuous data, get rid of a minimum and a maximum value, and then calculate the arithmetic mean of N-2 data. N value select: 3~14

B. advantages: blend the advantages of two filtering methods, it can eliminate impulse interference caused by sampling bias for the occasional pulse interference.

C. disadvantages: measured speed is slow and it is wasting RAM the same as arithmetic average filter.

## 6. Amplitude limiting the average filter method

A. method: it is the equal to “limiting filter method” + “average recursive filter method”, it will do the amplitude limiting for the new sample data each time, and send the new data to the queues to do the average recursive filter process.

B. advantages: blend the advantages of two filtering methods, it can eliminate impulse interference caused by deviation of sample values for the occasional pulse interference.

C. disadvantages: wasting RAM.

## 7. First order lag filter method

A. method: set a=0~1, this filter result = (1-a) * this sample value +a* the last filter results.

B. advantages: periodic interference with good inhibitory effect, suitable for occasions with higher frequency.

C. disadvantages: phase delay, sensitivity low degree of latency depends on a size cannot eliminate filter more frequently than the sampling frequency one-second interference signals.

## 8.The weighted average recursive filter method

A. method: it is the improvement of average recursive filter method which adds the different right for the data at different time. Usually, the closer the moment of data, the bigger the right value it gets. The bigger the right value is given, sensitivity is higher, but the signal smoothness is lower.

B. advantages: for a large time delay relatively short sampling period and the time constant of the object system.

C. disadvantages: it could not rapid respond the current severity when the delay time constant is small, the sample time is long and the signal change is slow. The filter effect is worse.

## 9.Eliminate shake filter method

A. method: setting a filter counter which can compare the sample value with the current valid values: If the sample value = current valid value, the counter is cleared to zero if the sampled value <> current judging effectively counter is >= limit n (overflow) when the counter overflows, then this value replaces the value currently in effect, and clear counter.

B. advantages: better for slow changes of measured parameters of the filter effect can be avoided near the threshold controller on/off beat or numeric on the monitor repeatedly.

C. disadvantages: for rapidly changing parameters because it was difficult to counter that sampling for the value of the overflow happened to be jamming, the interference will be deemed valid values into the system.

## 10. Amplitude limiting consumer shake filter method

A. method: it is equal to ” amplitude limiting filter method” + ” eliminate shake filter method ” amplitude limiting first and eliminating shake after

B. advantages: inherit “limiting” and “Xiao shook” the advantages of improved “eliminating mechanically dithered filter method” certain deficiencies, avoiding interference into the system.

C. disadvantages: it is not suitable for quick change parameters.

## 11. IIR digital filter

A. method: determine the signal bandwidth, filtered. Y(n) = a1*Y(n-1) + a2*Y(n-2) + . + ak*Y(n-k) + b0*X(n) + b1*X(n-1) + b2*X(n-2) + . + bk*X(n-k).

B. advantage: high pass, low pass, band pass, band stop arbitrarily. Simple design (using MATLAB)

C. disadvantage: the computing capacity is very large.

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