Control system having signal tracking window filters

A control system having a source of control signals and a multi-window electrical signal tracking filter. The signal tracking filter includes an operational amplifier operating in the non-inverting mode as a high impedance load, with a plurality of input stages cascaded at the input to the operational amplifier, each input stage having both a frequency determining filter portion and an amplitude determining threshold detecting portion. Each input stage defines a "window" of operation, such that the portion of a control signal or the like inputted to the filter which falls within the "window" will be filtered or attenuated thereby, while the portion of the control signal which falls outside of the "window" will pass unaffected through the filter. Cascading of the input stages allows one to customize the portions of the signal to be filtered for any particular application.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a control system housing an electrical 
filter circuit to selectively filter out undesirable portions of a signal, 
such as noise, ripple, oscillations or vibrations occurring at frequencies 
within the bandwidth that the control system is expected to operate, while 
not affecting the large amplitude control signals present in the system. 
Such a filter can be especially important in a control system such as used 
in a motorized wheelchair controller, wherein any tremor or spasticity 
occurring in an operator's hand or arm might cause erratic and potentially 
dangerous operation of the chair's drive motors. Controllers which respond 
to these tremor inputs may cause the chair to vibrate and move in 
undesirable ways. 
2. Description of the Prior Art 
Numerous attempts have been made to "soften" the operation of motorized 
wheelchair controllers in the past, including U.S. Pat. Nos. 4,415,049, 
issued Nov. 15, 1983, to John A. Wereb; 4,059,786, issued Nov. 22, 1977, 
to Michael Floyd Jones, et al.; and 3,814,199, issued Jun. 4, 1974, to 
William M. Jones. However, none has approached the problem in the same 
manner as the present invention. Prior attempts to filter or delay the 
undesirable low frequency tremor signals have compromised the response of 
the chair motors to inputs to the controller such that stopping or 
maneuvering the chair next to curbs, stairs, platforms, or vehicular or 
pedestrian traffic become unsafe. 
In the art of electrical filters per se, adaptive filters such as those 
shown in U.S. Pat. Nos. 4,749,951, issued Jun. 7, 1988, to Yasushi Tanaka; 
4,198,612, issued Apr. 15, 1980, to Roger R. A. Morton; and 3,889,108, 
issued Jun. 10, 1975, to Ben H. Cantrell represent attempts to filter an 
undesirable portion of the same signal, but none provides either the 
specific circuitry of the subject invention or the versatility of being 
able to cascade multiple stages of input filter circuitry to customize the 
control system for specific frequencies and/or amplitudes of the signals 
to be filtered as is possible with the present multi-window filter 
invention. 
Other noise reducing systems and signal stripping circuits of the prior art 
have been noted, as shown by U.S. Pat. Nos. 4,322,641, issued Mar. 30, 
1982, to Thomas N. Packard; 4,302,738, issued Nov. 24, 1981, to Richard C. 
Cabot et al.; and 3,889,136, issued Jun. 10, 1975, to William L. Mohan, et 
al., but none provides the flexibility and versatility of the present 
multi-window filter design in elimination of unwanted, low amplitude 
signal portions while passing the desired large amplitude control signals 
virtually unaffected. 
SUMMARY OF THE INVENTION 
This invention discloses a control system or a circuit including a 
plurality of window filters and threshold devices electrically connected 
so that control signals or the like passing through the circuit will be 
filtered to reduce or eliminate undesirable signals which otherwise would 
cause noise, ripple, oscillation or vibration in the control system. 
Filtering of such undesirable signals is effected without adversely 
affecting those control signals passing through the control system circuit 
and retaining the desired amplitude. The undesirable signals are filtered 
at frequencies and amplitudes specifically chosen for each individual 
application. 
Specifically, a multi-window electrical filter is provided which includes 
multiple input stages which are cascaded; that is, electrically connected 
in series, to the non-inverting input of an operational amplifier, each 
stage being designed to filter out an undesired signal having a specific 
frequency and amplitude without affecting the remaining portion of the 
signal. Each input stage includes (a) a filter which removes frequencies 
which have been identified as unwanted and permits frequencies to pass 
which have been identified as desired frequencies, and (b) a threshold 
device which determines the amplitude of the signal to be filtered. Such 
amplitude and frequency characteristics identify a "window" or region for 
each stage within which the filter is effective on the control signal or 
the like. Any portion of the signal outside the "window" will be 
unaffected by the filter associated with the amplitude of the respective 
stage. The window tracks or moves with signals having large amplitude 
changes while filtering signals whose amplitudes are within the thresholds 
of the window boundaries. The present invention embodies a multi-window 
electrical filter wherein the filter tracks or moves with the average 
amplitude of the time varying signal and filtering takes place within the 
thresholds of the window. The cascading of the multiple input stages 
allows the multi-window filter to be customized to remove undesired 
portions of a signal as needed. Also, cascading permits tapering the 
amount of attenuation applied to a signal as a function of the amplitude 
of its variation. For example, large amplitudes can be attenuated less 
than small amplitudes, the larger the amplitude of variation from the 
average amplitude of the signal, the less the attenuation present. 
In a particular embodiment, the present invention discloses a multi-window 
electrical filter for use in a motorized wheelchair control circuit. The 
filter includes an operational amplifier having inverting and 
non-inverting inputs and a single output. A plurality of cascaded or 
series connected window filter stages are connected to the non-inverting 
input of the amplifier. One of the input stages comprises a low frequency, 
low pass filter having a threshold device shunting the filter, and another 
of the stages comprises a low pass filter with a higher corner frequency 
than the previous stage and a threshold device shunting same but at a 
higher voltage level. The plurality of input stages is arranged to filter 
an input signal of any unwanted noise, ripple, oscillation and vibration 
as might be caused by tremor or spasticity in the hand or arm of the 
operator of a controller for the motorized wheelchair control circuit, 
with the threshold device sequentially by-passing each filter when the 
signal amplitude level is equal to or greater than a level set by the 
threshold switches, respectively, thus allowing only the average of large 
amplitude control signals to be sensed by the control circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In order to aid in the understanding of the operation of the multi-window 
filter of this invention, reference is first made to the prior art 
single-window filter of FIG. 1. Operational amplifier A is used to provide 
a high impedance load to the first order low pass filter formed by 
resistor R and capacitor C. Amplifier A is utilized in the non-inverting 
or follower configuration. The filter is of the form 
##EQU1## 
where t equals the filter time constant, which is equal to the product R 
C, and S equals the Laplace operator used in this form of the filter 
transfer function. The equation relating input voltage (E in) and output 
voltage (E out) is 
##EQU2## 
for small amplitude signals. The window threshold signal level is 
determined by the forward junction voltage drops of diodes D1 and D2. The 
junction voltage drop for silicon diodes is approximately 0.7 volts. Zener 
diodes or variable reverse biased diodes may be used for applications 
where voltage thresholds greater or less than 0.7 volts are needed. The 
reverse parallel combination of diodes D1, D2 will conduct for voltage 
drops greater than .+-.0.7 volts. These voltages will appear unfiltered at 
the positive input of amplifier A, while voltages within this window 
(.+-.0.7 v.) will be filtered, attenuated or suppressed. 
In its broadest aspect, the present invention relates to a multi-window 
electrical filter as depicted in the control system shown in FIG. 2A. Such 
circuit will selectively filter out unwanted portions of a signal, such as 
noise, ripple, oscillations and vibrations. As shown in FIG. 2A, a signal 
source CSS provides an input control signal to a cascaded chain of filter 
circuits F.sub.1, F.sub.2 . . . F.sub.N which supplies a filtered output 
control signal to the non-inverting input of operation amplifier OP AMP, 
which has its output connected to its inverting input and to a utilization 
device C such as a motor controller U. Each filter stage has a voltage 
threshold switching device TH.sub.1, TH.sub.2 . . . TH.sub.N connected in 
parallel or bypassing relation to its respective filter F.sub.1, F.sub.2 . 
. . F.sub.N. Each succeeding threshold device responds to a succeedingly 
larger threshold voltage. Preferably, the voltage threshold switching 
devices are bi-directional. 
As shown in FIG. 2B, a plurality of series connected input stages is 
provided including a first input stage I1 and at least one subsequent 
input stage I2, each input stage having an input and an output. The input 
of input stage I1 is connectable to the control signal to be processed (E 
in). One of the subsequent input stages is a last input stage. The output 
of such last input stage is electrically connected to the non-inverting 
input of operational amplifier AI. Each input of each subsequent input 
stage is electrically connected to the output of a preceding input stage. 
Each respective input stage includes first means F1, F2 electrically 
connected between the input and output of such respective input stage for 
filtering out unwanted frequencies of the signal while permitting desired 
frequencies of the signal to pass through the first means to the output of 
such respective input stage, and second means T1, T2 electrically 
connected to the first means for providing a voltage threshold which 
permits current flow, of the signal having voltages with amplitudes that 
are less than the voltage threshold, through the first means and to the 
output of such respective input stage, and which permits current flow, of 
the signals to be processed having voltages with amplitudes that are equal 
to or greater than the voltage threshold, through the second means and to 
the output of such respective input stage. 
Although FIG. 2B depicts a two-window filter, that is, a multi-window 
electrical filter having two input stages I1 and I2, as shown in FIG. 2A, 
it will be apparent to those skilled in the art that there is no 
theoretical limit to the numbers of windows or input stages which may be 
added to such a circuit. For example, if desired, an additional subsequent 
input stage I3 depicted in FIG. 2C can be provided as described herein. 
In the embodiment of FIG. 2B each first means F1, F2 is a filter. Filter F1 
includes resistor 2 and capacitor 4 and filter F2 includes resistor 5 and 
capacitor 8. Although each such filter is a first order filter, it will be 
obvious to one skilled in the art that other types of filters can be used. 
For example, second order, notch, Butterworth, Chebychev and Bessel are 
examples of other forms of filters F1, F2 which may be used with similar 
results. In the embodiment of FIG. 2, each second means T1, T2 is a 
threshold device. In the embodiment depicted in FIG. 2, threshold device 
T1 includes silicon diodes 10 and 12 and threshold device T2 includes 
silicon diodes 14, 16, 18 and 20. Although each such threshold device 
includes silicon diodes, it will be obvious to one skilled in the art that 
other threshold devices can be used. For example, voltage threshold 
detectors, FET signal switches and reverse biased diodes are examples of 
other forms of threshold devices T1, T2 which may be used with similar 
results. In more complex situations, the threshold devices can be 
controlled to change the various threshold levels. It will be apparent to 
one skilled in the art that the selection of component parameters and 
discrete values or types will depend upon the nature of the signals to be 
processed. For example, as to the threshold devices T1, T2, switching 
thresholds will be determined by the amplitudes of the signals in need of 
filtering. As to the filters F1, F2, filter frequencies will be dependent 
upon the frequency content of the signals to be filtered. 
In the embodiment, the filter F1 is formed by the combination of resistor 2 
and capacitor 4 which are electrically connected together in series to 
form a first order filter the time constant of which is determined by the 
product of the value of the resistance of resistor 2 and the capacitance 
of capacitor 4. The filter F2 is formed by the combination of resistor 6 
and capacitor 8 which are electrically connected together in series to 
form a first order filter the time constant of which is determined by the 
product of the value of the resistance of resistor 6 and the capacitance 
of capacitor 8. As depicted in FIG. 2, filter F2 is electrically connected 
in cascade or series with filter F1 and to the non-inverting (+) input 22 
of an operational amplifier 24. The signal to be processed is in the form 
of a voltage input which is produced by signal source 26 and applied to 
the terminals 28, 30 (E in). Terminal 28 is electrically connected to the 
resistor 2 and diodes 10, 12 through junction 32. Junction 32 constitutes 
the electrical input to, and junction 46 constitutes the electrical input 
of, the input stage I1. Terminal 30 is electrically connected to the 
circuit common or ground 34. The circuit of the filters F1, F2 is 
completed by electrically connecting the output of capacitors 4 and 8 to 
the circuit common or ground 34. 
The operational amplifier 24 is used as a voltage follower which means that 
the output of the amplifier will exhibit the same signal voltage that 
exists at the non-inverting input 22. The output of the amplifier 24 is 
connected by conductor 36 to the inverting input (-) 38 of the amplifier 
24. Direct current voltage connections are made to the amplifier at 40(+v) 
and 42(-v) to provide power to the amplifier. The voltage follower circuit 
provided in this manner is well known in the prior art and is used in the 
embodiment of FIG. 2 because a very high impedance exists at the 
non-inverting input 22 of the amplifier 24 which means that the RC filter 
network will not be loaded by the circuit connected to its output. The 
voltage output of the amplifier 24 is electrically connected to the load 
44 developing a voltage (E out) across the load from the output of the 
amplifier to the circuit common or ground 34. 
The threshold devices serve as switches which direct signal current to flow 
either through a filter or around it in a bypass manner as described 
hereafter. In the embodiment of FIG. 2B, the threshold device T1 includes 
diodes 10 and 12 which are electrically connected in reverse parallel by 
which is meant that the diodes are connected in parallel, and the cathode 
of one diode is connected to the anode of the other diode. Such a parallel 
diode combination is connected across the resistor 2 at junctions 32 and 
46. The threshold device T2 includes a first network including diodes 14 
and 16 which are connected in series and a second network including diodes 
18 and 20 which are also connected in series. The two networks are 
connected in reverse parallel, a cathode of one of the diodes of one 
network being connected to an anode of one of the diodes of the other 
network. The two networks of diodes which form the threshold device T2 are 
connected across the resistor 6 at junctions 48 and 50. Junction 48 
constitutes the electrical input to, and junction 50 constitutes the 
electrical output of, the input stage I2. 
In the embodiment of FIG. 2B, silicon diodes are selected so that in the 
forward conduction direction, the diodes 10, 12 will not permit the flow 
of current until the voltage across the diodes exceeds 0.7 volts. For 
opposite polarities of voltage, such diodes will block current flow. When 
two silicon diodes are connected in series such as diodes 14 and 16, such 
diodes will not permit the forward conduction of current until the voltage 
across the series combination is 1.4 volts. Such diodes also serve to 
block current flow for opposite polarities of voltage. In this manner, the 
diodes 10 to 20 control the threshold voltage at which signal current will 
flow through a filter F1, F2 or bypass a filter entirely as explained in 
greater detail herein. 
In the operation of the embodiment of FIG. 2, if the change in input signal 
voltage at terminals 28, 30 is below .+-.0.7 volts, all of the signal 
current will flow through filter F1 and filter F2. In this manner, signals 
below .+-.0.7 volts will be filtered by the two filters (F1, F2) in 
cascade or series and thereby be subjected to the maximum filter effect. 
If the input signal voltage change at terminals 28, 30 is between .+-.0.7 
and .+-.1.4 volts, and produces an equivalent voltage change across 
resistor 2, signal current will bypass filter F1 by flowing through diodes 
10, 12 and into the second stage filter F2. In essence, the diodes of 
threshold device T2 block current flow therethrough so that current will 
flow through filter F2. In this manner, only the second filter stage 
provided by filter f2 is functional when the input voltage change is in 
the range of .+-.0.7 to .+-.1.4 volts. Finally, if the input signal 
voltage change at terminals 28, 30 is above .+-.1.4 volts, and produces an 
equivalent voltage change across resistor 6, the signal current will flow 
directly to the follower or non-inverting input 22 of the operational 
amplifier 24; that is, the signal current will bypass filters F1 and F2, 
flowing directly through threshold devices T1 and T2 to the input 22. 
The time constant or bandwidth of filters F1 and F2 can be the same or 
different. For example, the time constant of filter F1 can be chosen to be 
larger than that of filter F2 so as to enable the signal that is within 
the smaller threshold levels to be attenuated or filtered more than 
signals whose amplitudes cause them to pass through the larger threshold 
window. 
FIG. 3 represents the shape of the voltage and time relationship 
representing a desired input voltage, i.e., an ideal time varying waveform 
signal independent of any connection, hardware or processing. FIG. 4 shows 
what the actual existing waveform looks like with noise, ripple or other 
time varying voltage content. This view represents the actual input 
voltage, E in, which contains the desired signal, with undesired ripple or 
oscillation present, which the circuit of FIG. 2 will process. By passing 
the actual signal through the circuit of FIG. 2, the filtered output 
voltage, E out, as depicted in FIG. 5 is obtained. 
One possible use for the multi-window filter of FIGS. 2A, B and C is in 
conjunction with a joystick controller for a motorized wheelchair. As 
often happens, the wheelchair patient may suffer from tremor or 
uncontrollable vibration or spasticity in his hand or arm, making smooth 
operation of the joystick controller and the chair very difficult. 
Controllers which respond to these tremor inputs may cause the chair to 
vibrate or move in undesirable ways. The multi-window filter provides a 
means for filtering the low amplitude signals generated by joystick 
movement due to tremor while not affecting the larger amplitude control 
signals. For this particular application, the filter F1 might be a first 
low pass filter and the filter F2 might be a second low pass filter, 
respectively, the second low pass filter having a higher frequency than 
that of the first low pass filter. 
Referring to FIGS. 6 and 9, operation of the multi-window filter circuit of 
FIGS. 2A, B or C will now be described as it would apply to a motorized 
wheelchair control circuit. Since a joystick controller has four normal 
quadrants of operation, forward, backward, left, and right, four of the 
circuits of FIGS. 2A, B or C could be employed, one for each quadrant of 
operation. Since most control signals are bipolar, however, it is possible 
to use two of such circuits, as depicted in FIG. 6, instead of four. One 
would be used for fore and aft operation and one would be used for left 
and right operation. 
FIG. 9 shows the powered wheelchair 210 wherein a patient 211 sits in the 
wheelchair and operates a joystick 212 (element 118 in FIG. 6) to initiate 
direction and speed of movement. If the patient has spasticity or erratic 
hand shaking 204, these tremor-like hand movements will command the 
wheelchair 210 to move in a like or erratic manner. If the joystick 
electrical output signal is filtered with a window filter 204 before 
passing it on to the power controller 214, the effect of spasticity of 
hand tremor can be eliminated or greatly attenuated. However, if it is 
necessary for the patient to stop or make a directional change quickly, 
the corresponding large amplitude corrective input command from the 
patient's hand movement will be passed from the joystick 212 to the power 
controller 214 virtually unattenuated. Such response is required in 
emergency operation. In contrast, with conventional filtering intended to 
attenuate spastic inputs, response to emergency input signals would be 
attenuated, as well as the spasticity effects. 
In the circuit of FIG. 6, the motorized wheelchair control circuit 100 
produces command signals 102 to a first drive motor 104 which is 
mechanically connected to a first wheelchair wheel 106 and command signals 
108 to a second drive motor 110 which is mechanically connected to a 
second wheelchair wheel 112 in response to output signals 114 and 116 
produced by at least one joystick controller 118. Multiple window filter 
means 120 is electronically connected between the joystick controller 118, 
on the one hand, and the drive motors 104, 110, on the other hand, for 
selectively filtering out unwanted portions of the output signals 114, 
116, such as noise, ripple, oscillations and vibrations. Multiple window 
filter 120 includes a first multi-window electrical filter 122 and a 
second multi-window electrical filter 124, each of which includes the 
circuitry, and operates in the manner of FIG. 2. 
Joystick 118 is a prior art-type joystick which comprises two transducers 
such as, for example, electrical potentiometers, inductive transducers, 
and the like. The two transducers typically are connected to a joystick 
lever which can be displaced forward or backward while in a centered 
position relative to any lateral disposition, and can be displaced to the 
right or to the left relative to such centered position. Movement of the 
lever in a forward direction relative to neutral produces a positive 
voltage, and movement of the lever in a rearward direction relative to 
neutral produces a negative voltage. Such voltage is in the form of output 
signals 114, 116 which will be equal. Such output signals are summed by a 
summer 126, the resultant signal 128 representing a command for forward or 
reverse movement of motors 104, 110, each motor moving in the same 
direction and with the same speed, to cause forward or backward movement 
of wheels 106, 112, in a known manner, depending upon whether the joystick 
lever is moved forward or backward. Movement of the lever to the right or 
left of its centered position while the lever is in the forward or 
backward position produces a positive voltage or a negative voltage, 
respectively. Such voltage in the form of output signals 114, 116 will be 
unequal and will be directed to summer 130 which will produce a difference 
signal identified as resultant signal 132. Resultant signal 132 will 
represent a command for forward or reverse movement of the motors 104, 
110, each motor moving in the same direction but with different speeds to 
cause rightward or leftward movement of the wheelchair, in a known manner, 
depending upon whether the joystick lever is moved to the right or to the 
left, respectively. 
The resultant signal 128 is fed to the multi-window electrical filter 122 
and the resultant signal 132 is fed to the multi-window electrical filter 
124. Resultant signals 128 and 132 are filtered by the multi-window 
electrical filters 122 and 124, respectively, which function in the same 
manner as discussed above regarding the circuit of FIG. 2. The 
multi-window electrical filters 122 and 124 produce command signals 134 
and 136, respectively, which are fed to a two channel power controller 138 
which applies modulated power through command signals 102 and 108 to 
motors 104 and 110, respectively. The two channel power controller 138 
will include the source of power such as a battery (not shown) to energize 
the control circuit 100 and motors 104 and 110 in a known manner. 
In operation, when the joystick lever is moved forward or backward, 
relative to its neutral position, the drive motors 104, 110 will rotate 
together with the same speed in the forward or backward direction, 
respectively, thereby moving the wheelchair in such direction. If in 
combination with such forward or backward level displacement, right or 
left level movement is also introduced, the wheelchair will move right or 
left, respectively, while moving forward or backward, as a result of 
having one drive motor rotating faster than the other. 
Considering a normal voltage command range of wheelchair operation of 0-5 
volts from rest to full motor speed, the effects of patient tremor may be 
assumed to be in the range of 0-1.4 volts. In considering FIG. 4, the 
ripple in the curve may be caused by such tremor. Utilizing the circuit of 
FIG. 2, a change in the tremor signal of 0-1.4 volts may be filtered out 
with the two-window electrical filter depicted, leaving voltage changes 
outside of this range unfiltered. 
By using a two-window electrical filter according to this invention, small 
amplitude signals that are less than .+-.0.7 volts from the average of the 
desired signal are attenuated more than larger amplitude signals that have 
amplitudes between .+-.0.7 to .+-.1.4 volts. If a corner frequency that is 
0.1 of the ripple frequency is used, very high attenuation results within 
the windows as shown in FIG. 5. Small amounts of rounding of the signal 
are evident at points where signal changes are very abrupt. The window 
filter tracks the average amplitude of the time varying signal and filters 
within a window that moves with this average amplitude. 
The first window filter I1 formed by filter F1 and threshold device T1 is 
set by selecting the proper resistor 2 and capacitor 4 to filter out low 
frequency, low threshold (0.+-.0.7 volt) tremors such as might occur when 
the patient is simply resting his hand on the controller in preparation 
for inputting a signal to cause the chair to move or hanging onto the 
control in trying to move at a constant speed. Referring to FIGS. 2 and 6, 
a tremor signal of less than .+-.0.7 volts would appear at 128, 132, and 
the combination of resistor 2, capacitor 4 and the silicon diodes 10 and 
12 would filter the signal and block its passage to the positive input of 
amplifier 24. If, upon applying pressure to the joystick controller, the 
signal voltage amplitude change should increase above .+-.0.7 volts but 
less than .+-.1.4 volts, the second window filter formed by resistor 6, 
capacitor 8, and diodes 14 to 20 may be adjusted to react at a higher 
frequency by choosing appropriate values for resistor 6 and capacitor 8, 
while the dual silicon diodes 14, 16, 18, 20 will act to filter the 
voltage input change from .+-.0.7 to .+-.1.4 volts. Thus, only signal 
changes from the controller greater than .+-.1.4 volts from the steady 
state value will reach the input 22 of amplifier 24 and the E out 
terminals from which they are then sent in an unfiltered condition to the 
two channel power controller 138 and motors 104, 110 as described herein. 
The most significant use of window filters, according to the invention, is 
in applications where a servosystem is expected to operate responsively 
and in a stable fashion over a specific bandwidth or frequency response 
range wherein there is noise, vibration or other unwanted signal content 
in the system whose frequencies are within the expected bandwidth of the 
servosystem. Sources of these unwanted signals, among others, are: 60 Hz 
line noise, vibration due to spring and mass resonance with low damping, 
slip stick oscillation in movement wheel hop in vehicles moving over 
nonuniform roads, wheel runout or out of round conditions causing a once 
per revolution force variation and vibrations from engine or motor 
imbalance conditions. Electrical transducers that measure force (load 
cells), pressure, accelerations (accelerometers), velocity (tachometers) 
and position (potentiometers, LVDTs and encoders) all respond to physical 
motion, both desired and undesired. The unwanted signals are often dealt 
with by conventional filtering with the accompanying penalty of 
restricting the bandwidth, response and performance of the servosystems. 
The multi-window filters of this invention are sensitive to signal 
amplitude in cascaded filter applications, the larger the signal amplitude 
is, the less it is filtered. Filtering takes place inside of a window or 
threshold that moves with changing primary signal levels as explained 
above. 
In the simplest application, multi-window or amplitude tracking filters can 
be placed in a signal path where filtering is desired. For example, in 
FIG. 7 above, a signal, which could originate in a tachometer 200 where, 
in addition to a d.c. voltage 249 that is proportional to shaft speed 201, 
a noise signal or ripple 249 is present due to voltage commutation and 
rides on top of the d.c. voltage produced in response to the speed of the 
armature. Also, such a signal as in FIG. 8 could be present in an 
electrical circuit that produced the response shown in FIG. 7, but in 
addition, 60 Hz powerline a.c. is coupled through stray coupling and makes 
its presence known by modulating the primary desired voltage as shown in 
FIG. 7. The result again, is a composite, modulated signal as depicted in 
FIG. 7. 
If a conventional filter were used, distortion of the primary signal or 
voltage would result in signal variation in the frequency range of the 
filter. If a multi-window filter were used, filtering would occur only 
inside of the window or established threshold, allowing large signal 
response to take place without filtering. 
There are virtually hundreds of specific applications in which these 
filters could be used. The following describes a few specific examples of 
applications. 
FIG. 7 shows a block diagram of how a multi-type window filter 203 could be 
used with a d.c. tachometer 200 to remove unwanted amplitude modulating 
ripple voltage from the speed signal on line 202. 
FIG. 8 shows a block diagram of how a multi-window filter 204 could be used 
to remove 60 Hz line noise 206 from a circuit where it is present through 
stray inductive or capacitive coupling. 
FIG. 10 shows an application in which vibrations due to structural 
resonances, unbalance in the wheel 220, unbalance in the vehicle engine or 
other force variations are present in a servocontrol system. The system is 
an active vehicle suspension system. Vertical motion of the wheel relative 
to frame 221 is controlled by an actuator or hydraulic piston 222. 
Hydraulic oil flow to and pressure across the actuator are modulated by a 
servovalve 223 that is operated by an electrical signal 250 from a 
controller 224. The controller produces this output signal in response to 
comparing preselected control conditions to the vertical movement of the 
wheel 222 relative to the frame 221 of the vehicle. Vertical acceleration 
is measured by an accelerometer 225, vertical displacement is measured by 
a position transducer 226 and vertical velocity is measured by a velocity 
transducer 227. These three signals are connected to the controller 224 
through separate filters 228A, 228P and 228V, respectively. Because of 
vibration present in the vehicle and road system, and the structure has 
resonant spring-mass components, both vibrations and resonances must be 
filtered for control purposes. The window tracking filters 228A, 228P and 
228V will remove these effects without compromising the bandwidth of the 
primary servosystems, even though vibration and resonant frequencies are 
inside of the servosystem bandwidth. The result of application of these 
filters is that wide bandwidths are possible for controlling the 
servosystem. 
Other applications where system resonances interfere with servosystem 
responses are positioning gimballed rocket engines for steering rockets, 
controlling airfoil surfaces on high performance aircraft, robotic 
positioners and numerically controlled (NC) machines. 
FIG. 11, an ABS or anti-lock braking system 240, is shown schematically. A 
wheel speed transducer or pickup 241 measures the rotational speed of the 
wheel 242. The wheel speed signal 243 is connected to the brake controller 
245 and the pressure control system through filter 244. The controller 245 
modulates the hydraulic brake pressure to operate the brake caliper 247 on 
disk rotor 248 in a manner that does not allow the wheel 242 to lock up, 
and thus develop the greatest vehicle stopping performance. Without the 
multi-window filter 244 of the present invention, vibration present in the 
vehicle, resonances in the vehicle, and force variations at the tire and 
road interface would disrupt the smooth and responsive performance of the 
ABS 240. The multi-window filters can also be used in traction control 
systems for vehicles. In these systems, both the vehicle acceleration and 
deceleration or braking, are controlled. Similar resonance and vibration 
conditions apply to both ABS and traction systems. 
The embodiments which have been described herein are but some of the 
several which utilize this invention and are set forth here by way of 
illustration but not of limitation. It is apparent that many other 
embodiments which will be readily apparent to those skilled in the art may 
be made without departing materially from the spirit and scope of this 
invention.