Conditional switch rate low pass filter
A conditional switched rate filter system having a two stage digital filter to remove noise from the input signal. The first stage of the filter effectively removes the random noise from the input signal higher in frequency (cut-off frequency) than outside the characteristic response of the closed loop system. The second stage of the filter is used to filter out noise that is at same frequency response as the closed loop system.
1. A conditional switched rate low pass filter for reducing noise in a closed-loop control system comprising:
a first low pass digital filter, the input of which is coupled to an output of the control system, wherein the first digital filter removes noise, in the signal from the output of the control system, higher in frequency than the characteristic response of the control system;
a second low pass digital filter, the input of which is coupled to the output of the first digital filter, and the output of which is coupled back into the control system; wherein the second digital filter removes noise which is at essentially the same frequency as the closed-loop system response;
buffer memory for storing an output signal from the first digital filter;
a signal stability monitor module, coupled to the buffer memory, configured to detect changes in the output signal from the first digital filter; and
a computer implementing the first digital filter, the second digital filter, and the signal stability monitor module, coupled to the buffer memory, for controlling the operation of the switched rate low pass filter;
wherein the signal stability monitor determines the response time of the second digital filter based on monitoring of the output signal from the first digital filter, as stored in the buffer memory, and wherein:
if the output signal has changed by more than a predetermined value within a predetermined time period, the cutoff frequency of the second digital filter is increased to a predetermined value; and
if the output signal maxima and minima have been stable within a predetermined range for at least a predetermined period of time, the cutoff frequency of the second digital filter is decreased to a predetermined value.
2. The system of claim 1 , wherein the cutoff frequency of the second filter is changed in one step.
3. The system of claim 1 , wherein the cutoff frequency of the second filter is changed gradually over a period of time.
4. The system of claim 1 , wherein criteria for increasing the frequency response of the second filter are different than criteria for decreasing the frequency response thereof.
5. The system of claim 1 , wherein:
the second low pass digital filter can be set to either a fast response time mode or to a slow response time mode, and is initially set to the slow response time mode; and
if the second low pass digital filter is set to the slow response time mode, then
if the output signal from the first digital filter has changed by more than the predetermined value within the predetermined time period, the cutoff frequency of the second digital filter is increased to the predetermined value, and the signal stability monitor is configured to check for stability of the output signal;
if the second low pass digital filter is set to the fast response time mode, then
if the output signal maxima and minima have been within the predetermined range for at least a predetermined period of time the, cutoff frequency of the second digital filter is decreased to the predetermined value, and the signal stability monitor is configured to check for large changes in the output signal.
6. The system of claim 5 , wherein the cutoff frequency of the second filter is changed in one step.
7. The system of claim 5 , wherein the cutoff frequency of the second filter is changed gradually over a period of time.
8. A method for conditionally switching the cutoff frequency of a digital filter to reduce noise in a closed-loop control system comprising:
filtering an output of the control system via a first low pass digital filter stage;
receiving the output signal from the first low pass filter stage via a second low pass digital filter stage;
monitoring the output signal from the first low pass digital filter stage to determine the stability of the output signal; wherein:
if the output signal has changed by more than a predetermined value within a predetermined time period, the cutoff frequency of the second low pass digital filter stage is increased to a predetermined value; and
if the output signal maxima and minima have been stable within a predetermined range for a predetermined period of time, the cutoff frequency of the second low pass digital filter stage is decreased to a predetermined value.
9. The method of claim 8 , wherein:
the second low pass digital filter stage can be set to either a fast response time mode or to a slow response time mode, and is initially set to the slow response time mode; and
if the second low pass digital filter stage is set to the slow response time mode, then
if the output signal from the first digital filter stage has changed by more than the predetermined value within the predetermined time period, the cutoff frequency of the second digital filter stage is increased to the predetermined value, and the signal stability monitor is configured to check for stability of the output signal;
if the second low pass digital filter stage is set to the fast response time mode, then
if the output signal maxima and minima have been within the predetermined range for at least a predetermined period of time the, cutoff frequency of the second digital filter stage is decreased to the predetermined value, and the signal stability monitor is configured to check for large changes in the output signal.
10. The system of claim 8 , wherein the cutoff frequency of the second filter stage is changed in one step.
11. The system of claim 8 , wherein the cutoff frequency of the second filter stage is changed gradually over a period of time.
12. The system of claim 8 , wherein criteria for increasing the frequency response of the second filter stage are different than criteria for decreasing the frequency response thereof.