 |
Description  |
|
|
BACKGROUND
Noise generated by a vibratory feeder bowl consists of two main components:
noise generated by the parts being fed, and noise generated by the
vibratory feeder bowl itself. Part noise is caused by part to part contact
and pan: to bowl contact and usually manifests itself as a "rattle".
Spectrally, this noise is broad band and usually above 300 Hz.
Traditionally, this noise is treated passively by enclosing the vibratory
feeder bowl. Such enclosures are frequently treated with sound absorbing
foam as well as various damping treatments which are effective at higher
frequencies, where part noise dominates.
The second component of vibratory feeder bowl noise is tonal noise caused
by the motion of the vibratory feeder bowl. This noise is primarily
periodic corresponding to the primarily sinusoidal excitation of the bowl.
This periodic or tonal noise manifests itself as acoustic noise and
mechanical vibration. Acoustic noise refers to the noise caused by the
piston like motion of the vibratory feeder bowl. Acoustic noise is readily
identifiable as a low tone or hum. This tone occurs at the primary
operating frequency and its harmonics. Typical primary operating
frequencies are 50, 60, 100, or 120 Hz. Vibratory feeder bowl users and
manufacturers have attempted to attenuate this tonal noise by the use of
enclosures. Although enclosures often redistribute the radiation pattern
of the tonal noise, they typically do little to attenuate it.
Mechanical vibration is caused by the vibratory feeder bowl imparting
vibration to the table on which it is mounted. Vibratory feeder bowls are
usually mounted on soft elastomeric pads which reduce the forces
transmitted to the mounting surface but do not eliminate them. The force
transmitted through a passive mount is related to the ratio of the mass of
the mounted device to the stiffness of the mount. Softer mounts allow less
force to be transmitted to the mounting surface, thereby reducing the
vibration caused by the vibratory feeder bowl. However, sorer mounts allow
larger gross motions of the vibratory feeder bowl to occur when it is
bumped or when parts are added. Such gross motions can cause the output
track of the vibratory feeder bowl to exceed alignment tolerances causing
parts jams and interrupting production. So in considering the stiffness of
a mount, alignment tolerances are traded off against vibration transmitted
by the mount.
Mechanical vibration can cause acoustic radiation. Because of the
relatively large surface area of the table on which vibratory feeder bowls
are usually mounted, small vibrations can cause effective acoustic
radiation. Furthermore, vibration of the table induces vibration in the
floor, which can also radiate acoustic energy. Table vibration often
reduces the capability of the vibratory feeder equipment to feed parts. A
reduction in vibration is desirable from a mechanical as well as acoustic
standpoint.
SUMMARY OF THE INVENTION
The present invention reduces acoustic noise and mechanical vibration
caused by vibratory feeder bowls or similar equipment. The device consists
of an acoustic noise reduction system, a mechanical vibration reduction
system, and a control system. The acoustic reduction system actively
cancels noise generated by the piston like motion of the vibratory feeder
bowl. The mechanical vibration reduction system actively cancels or
prevents the transmission of forces from the vibratory feeder bowl which
causes vibration in the table on which it is mounted. The control system
monitors and adjusts the performance of the acoustic and mechanical
vibration reduction systems.
Accordingly, it is an object of this invention to provide an active noise
cancellation system for attenuating noise from a vibratory feeder bowl.
Another object of this invention is to provide an active noise cancellation
system for canceling a dipole source of noise.
A still further object of this invention is to use inertial actuators in an
active noise attenuation system to reduce vibration transmitted by
vibratory feeder systems.
Yet another object is the use of piezoelectric devices to attenuate
vibration transmitted by a vibratory feeder bowl.
These and other objects will become apparent when reference is made to the
accompanying drawings in which
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1(A) illustrates an ideal representation of a dipole source consisting
of two monopole sources separated by a distance, d, which is small
compared to the wavelength,
FIG. 1(B) illustrates the far field pressure amplitude distribution
approximation resulting from the dipole source of FIG.1(A), where pressure
amplitude is measured at a radii which is large compared to the separation
distance, d of FIG.1(A), as a function of azimuth,
FIG. 2 is a schematic of a microphone array used to sense acoustic noise,
where although not shown, the number of microphones in the array can be
varied from one, in which case the array would be omni-directional, to
many, in which case the array would be highly directional,
FIG. 3 is a schematic view of a vibratory feeder bowl acoustic noise
attenuation system where accelerometers are used for acoustic reduction,
FIG. 4 is a schematic view of an accelerometer based acoustic sensor signal
conditioning circuit,
FIG. 5 is a side view of a vibratory feeder bowl vibration attenuation
system showing the use of isolation mounts and force transducers,
FIG. 6 is a side/schematic view of a first embodiment of a vibratory feeder
bowl noise reduction system, and
FIG. 7 is a side/schematic view of a second embodiment of a vibratory
feeder bowl noise reduction system.
ACOUSTIC NOISE REDUCTION SYSTEM
Because the pressure field resulting from the motion of a vibratory feeder
bowl is similar to the field generated by a dipole source, the vibratory
feeder bowl is well modeled as an oscillating source. One technique of
spatially matching a dipole source is to place one or more additional
dipole sources near the original source. Placed in the same orientation,
and close to the original source, the pressure field generated by these
additional sources will be spatially similar to the original source and
can be used to effectively cancel the field generated by the original
source.
Displacement of an acoustic actuator is measured in units of volume. The
volume displacement of a given acoustic actuator may be visualized as the
volume swept out by a surface of given area vibrating at a given
amplitude. The acoustic actuator must produce at least the same volume
displacement as the vibratory feeder bowl at the controlled frequencies.
For example, if the acoustic actuator is a vibrating plane, and the
effective area of the acoustic actuator is one tenth the area of the
vibratory feeder bowl, the acoustic actuator must be capable of generating
at least ten times the displacement of the vibratory feeder bowl.
The acoustic noise reduction system is intended to reduce the acoustic
noise created by the piston like motion of the vibratory feeder bowl. The
acoustic noise reduction system consists of an acoustic actuator and
acoustic sensor.
ACOUSTIC ACTUATOR
The acoustic actuator must be spatially similar to, and be capable of
producing the same volume displacement as the offending source. The piston
like motion of the vibratory feeder bowl is best modeled as an acoustic
dipole source. The acoustic actuator should also be well modeled as a
dipole source. The acoustic portion of this invention is generalized to
the use of oscillating sources to cancel noise from the offending dipole
source.
A dipole source is academically defined as two monopole sources oscillating
180 degrees out of phase at a given frequency, and separated by a given
distance, which is small compared to the wavelength of sound at the
excitation frequency. Such a source is illustrated in FIG. 1(A). At
distances large compared to the source separation distances, the pressure
field approximation exhibits a unique amplitude pattern. The pressure
field is symmetric about the axis connecting the two sources, and anti
symmetric about the plane which separates the two sources and is
orthogonal to the axis connecting the sources. The pattern is illustrated
by a pressure amplitude distribution diagram in FIG. 1B and in
Fundamentals of Acoustics, Kinsler et al, 1982, John Wiley & Sons. The
diagram depicts the variation of pressure amplitude as a function of
azimuth from the source. The pressure field amplitude is zero in the plane
separating the two sources and is at a maximum on the axis connecting the
sources. The phase of the pressure field is anti symmetric about the plane
separating the sources. All points in the pressure field on either side of
this plane are in phase. Points on separate sides of this plane are 180
degrees out of phase.
One acoustic actuator which may be described as a oscillating source is an
unenclosed loudspeaker. An unenclosed loudspeaker is well modeled as a
oscillating source because the diaphragm or cone of the loudspeaker moves
in a piston like fashion. A loudspeaker used to cancel acoustic noise from
a vibratory feeder bowl differs in application from typical uses of
loudspeakers. In most loudspeaker applications, it is desirable to prevent
the pressure radiating from the back of the loudspeaker from destructively
interfering with the pressure radiated from the front of the loudspeaker,
thus increasing the radiative efficiency of the loudspeaker. This is
accomplished by placing loudspeakers in cabinets, which vary in
complexity, in order to increase the radiative efficiency over a frequency
band of interest. However, the primary goal in using a loudspeaker to
cancel noise from a dipole source is to ensure that the loudspeaker is
spatially similar to the vibratory feeder bowl. Speaker cabinets could be
used to increase the radiative efficiency of the loudspeaker in this
application, provided the enclosed speaker retains the characteristics of
a dipole.
ACOUSTIC SENSOR
The acoustic sensor must provide a signal to the controller which is
indicative of the far field acoustic energy radiated. The degree to which
the acoustic sensor is representative of the far field energy radiated is
largely a function of how spatially similar the acoustic actuator is to
those sources to which the acoustic sensor is sensitive. If the acoustic
sensor is sensitive to sources which the acoustic actuators are not
spatially similar to, the system may not attenuate overall acoustic
radiation.
One example which causes far field performance deterioration is the effect
of noise from adjacent, uncontrolled vibratory feeder bowls. In this
example loudspeakers serve as the acoustic actuators and are placed around
a controlled or primary vibratory feeder bowl. A microphone serves as the
acoustic sensor and is placed above the primary vibratory feeder bowl. An
uncontrolled or secondary vibratory feeder bowl, is close enough so that
the microphone is sensitive to its noise. When the active acoustic
reduction system is not operating, the noise measured by the microphone is
partly due to the primary vibratory feeder bowl, and partly due to the
secondary vibratory feeder bowl. However, the secondary vibratory feeder
bowl is not sufficiently close to be considered spatially similar to the
loudspeakers. When the acoustic reduction system operates, the
loudspeakers are actuated so that the signal from the microphone is driven
to zero. The acoustic result can be described as a summation of two
signals: one which cancels the contribution of noise from the primary
vibratory feeder bowl, and one which cancels the contribution of noise
from the secondary vibratory feeder bowl. Because the loudspeakers are not
spatially similar to the secondary vibratory feeder bowl, cancellation of
its noise occurs locally, near the microphone only. At locations in the
far field, the component of the loudspeaker signal which cancels noise
from the secondary vibratory feeder bowl may interfere constructively with
the noise from the secondary vibratory feeder bowl, increasing radiative
efficiency. In general, the loudspeakers may be considered an additional
source, which is roughly equal in strength to the secondary vibratory
feeder bowl as measured at the location of the primary vibratory feeder
bowl, with the control system not operating. So, noise from other sources
which are measured by the acoustic sensor is in effect "echoed" by the
acoustic reduction system and deteriorates far field performance.
The acoustic sensor can be designed to avoid far field performance
deterioration due to additional sources which are spatially dissimilar to
the acoustic actuator. The goal is to decrease sensitivity of the acoustic
sensor to additional sources in comparison to the sensitivity of the
sensor to the primary source. If the acoustic radiation resulting from the
primary vibratory feeder bowl is considered "signal," and the acoustic
radiation resulting from additional sources is considered "noise," then
the goal can be restated as the desire to increase the
signal-to-noise-ratio of the acoustic sensor.
One design, as in FIG. 2, which increases the acoustic sensor
signal-to-noise-ratio takes advantage of the known acoustic
characteristics of a oscillating source. Signals from a plurality of
microphones 10 placed in a physical array may be conditioned and combined
12 so that sensitivity is increased in the direction of the primary
vibratory feeder bowl 11, but decreased in other directions, such as those
of secondary sources. The array could be used to measure intensity and
oriented such that the axis of sensitivity coincides with the axis of
maximum intensity for a oscillating source parallel to the vibration of
the vibratory feeding system "echoes" noise from secondary sources.
Another design which increases the acoustic sensor signal-to-noise-ratio is
shown in FIG. 3, accelerometers are used to estimate acoustic radiation
from the physical displacement of the surface of a given dipole source.
Accelerometers 20, 23 are placed on the vibratory feeder bowl 21, and on
cones or diaphragms of one or more loudspeakers 22. The signals from the
loudspeaker accelerometers 20 and vibratory feeder bowl accelerometer 23
are weighed proportionally to the volume displacement of the device to
which they are attached. The signals are then summed, conditioned as at 24
and used as the acoustic sensor. The resulting signal is proportional to
the net volume displacement and therefore representative of the net
acoustic energy radiated by the sum of the loudspeakers and vibratory
feeder bowl. This signal is minimized by the controller via the acoustic
actuators when the system is in operation.
FIG. 4 illustrates the signal conditioning portion of this process. Here,
one accelerometer 30 is placed on each of two loudspeakers 22, and an
accelerometer 31 is placed on the vibratory feeder bowl. Each
accelerometer 30, 31 is assumed to have the same sensitivity. The signal
32 from the vibratory feeder bowl accelerometer 31 is conditioned at 33 by
multiplication by a gain factor equal to the ratio of vibratory feeder
bowl area (Avfb) to total speaker area (Asp). The signals 34, 35 from the
loudspeaker accelerometers 34, 35 are averaged at 36 and summed at 37 with
the conditioned vibratory feeder bowl accelerometer 31 signal. The result
is representative of the volume displacement produced by the vibratory
feeder bowl and loudspeakers.
The advantage of using accelerometers to estimate acoustic pressure or
energy is that their sensitivity to secondary sources is negligible. The
disadvantage of this technique is that gain errors in the signal
conditioning result in an incorrect estimate of acoustic pressure or
energy and deteriorate acoustic performance.
The speaker accelerometers 20 and vibratory feeder bowl accelerometers 21
of FIG. 3 may be replaced with microphones. Typically, these microphones
would be placed within ten centimeters of the loudspeaker cones 22 and
bowl portion of the vibratory feeder bowl 21 and would be used to estimate
the position of the loudspeaker cone 22 and bowl portion of the vibratory
feeder bowl 21, respectively. The signals from the microphones would be
conditioned as in the discussion above, which refers to FIG. 4.
MECHANICAL VIBRATION REDUCTION SYSTEM
The mechanical vibration reduction system is intended to reduce the
vibration induced in the support structure by the vibratory feeder bowl.
The mechanical vibration reduction system consists of a mechanical
actuator and mechanical sensor.
MECHANICAL ACTUATOR
The mechanical actuator may actuate to prevent vibration in the table on
which the vibratory feeder bowl is mounted in two ways: force
cancellation, and vibration isolation. Both techniques are well developed.
The force cancellation actuator must exert forces that are equal in
magnitude and spatially similar to the forces caused by the vibratory
feeder bowl. If the table is stiff at the controlled frequencies, spatial
similarity may be achieved by placing the force actuator so that it exerts
canceling forces at the center of action of the offending forces. If the
axis of interest is vertical, the center of action may be near the
centroid of the vibratory feeder bowl mounting points. In this case, a
plurality of force actuators would be placed symmetrically about this
centroid, or alternatively, a single force actuator would be placed at
this centroid. If, however, the table is flexible at the controlled
frequencies, one actuator should be placed beneath each mounting point to
effectively cancel vertical forces. In such a case, it may be necessary to
apply independent signals to each force actuator.
Vibration isolation, as in FIG. 5, is achieved by inserting active mounts
41, 42 between the vibratory feeder bowl 40 and the support frame 43. The
active mounts are controlled to be extremely compliant at the bowl
excitation frequencies. As a result, vibration is not transmitted to the
table.
Vibratory feeder bowls induce table vibration in many axes. Because it
impractical to cancel vibration in all axes, only the most offensive axes
should be controlled. Vibration caused by vibratory feeder bowls tends to
be primarily vertical. Also, vertical table motions radiate acoustic
energy most effectively. So, if one must decide on a single axis to cancel
vibration, the vertical axis is the natural choice.
MECHANICAL SENSOR
Placement of the mechanical sensor depends upon the type of sensor used,
and the degree to which the mechanical actuators are spatially collocated
with the offending source. If the mechanical actuators apply forces or
compliance which spatially collocated with the offending forces or
vibration, the mechanical sensor may be placed virtually anywhere
uncontrolled motion can be measured in the axis of interest. However, if
the forces are not spatially collocated with the offending sources, the
sensor should be placed so as to be sensitive primarily to vibration in
the axis of interest. The preferred location of the mechanical sensors is
one that is sensitive to the vibration along the axis of interest.
An accelerometer is one example of a mechanical sensor. Used in the
vertical axis, the accelerometer is mounted to the top or bottom surface
of the table. Used in a horizontal axis, an accelerometer would be placed
on the side of the frame corresponding to the direction of interest. Used
in a rotational axis, two accelerometers are placed at locations off of
the axis of rotation and the difference of the signals is used as the
mechanical sensor signal.
If active isolation mounts are used as mechanical actuators, force
transducers 44 may be used as mechanical sensors, as shown in FIG. 5. In
this application the transducers are inserted between the isolation mounts
41, 42 and the support frame top 45. When force transducers are used as
mechanical sensors, the isolation mounts actuate so that force is driven
to zero at controlled frequencies. As a result a corresponding reduction
in table vibration occurs.
CONTROL SYSTEM
The function of the control system is to provide signals to the mechanical
and acoustic actuators so that the mechanical and acoustic sensor signals
are driven to zero. The control system monitors signals from the sensors,
and applies an output signal to the actuators which, after dynamically
altered by the filters, amplifiers, actuators and the medium between the
actuator and sensor, causes a reduction in the sensor signals. Often,
sensors are sensitive to inputs to more than one actuator. If such is the
case, the system is said to interact between channels. If, for example,
the acoustic sensor is sensitive to signals sent to the mechanical
actuator, the controller would need to account for this in driving the
signal from the acoustic sensor toward zero. This process is described in
detail in U.S. Pat. No. 5,091,953, entitled "Repetitive Phenomena
Cancellation Arrangement with Multiple Sensors and Actuators" by Steven A.
Tretter which is herein incorporated by reference in its entirety.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Enclosed System
FIG. 6 depicts the first embodiment of the vibratory feeder bowl noise
reduction system enclosed within a passive enclosure 60. Here a vibratory
feeder bowl 50 is mounted to a table 51. Two loudspeakers 52,53 are
mounted to the support structure 54 such that the cones or diaphragms of
the loudspeakers are parallel to the plane of the base of the bowl portion
of the vibratory feeder bowl 50. The loudspeakers 52, 53 are positioned
vertically above the table 51 to be approximately the same height as the
bowl portion 55 of the vibratory feeder bowl 50. The loudspeakers 52, 53
are sized so that the loudspeakers are capable of producing the same
volume displacement as the vibratory feeder bowl at the frequency of
vibratory feeder bowl oscillation (typically 50, 60, 100 or 120 Hz).
The acoustic sensor 56 is depicted as a microphone placed above the
vibratory feeder system in FIG. 6. The microphone may be placed anywhere
the pressure field of the feeder system is measurable. Ideally, the
microphone 56 is placed above or below the vibratory feeder bowl 50 since
the sound field of an oscillating source is largest along the central axis
parallel to the oscillating motion.
The mechanical vibration reduction system is depicted as an inertial
actuator 57 and an accelerometer 58 in FIG. 6. Actuator 57, which produces
a periodic force on the magnet causing it to move periodically. This force
is also exerted on the underside of the mounting surface 51 as a reaction
force. The inertial actuator must be capable of exerting the same periodic
force on the mounting surface 50 as the vibratory feeder bowl exerts on
the mounting surface 51.
Placement of the actuator 57 depends on the stiffness of the table 51. If
the table top 51 is stiff at the controlled frequencies, and bending of
the table top 51 is negligible at this frequency, the inertial actuator 57
may be placed centrally beneath the vibratory feeder bowl 50, as shown in
FIG. 6. However, if the table 51 is flexible at controlled frequencies,
one inertial actuator 57 should be placed beneath each mounting point of
the vibratory feeder bowl 50.
The mechanical vibration sensor is depicted in FIG. 6 as an accelerometer
58 mounted next to the inertial actuator 57. The stiffness of the table 51
at controlled frequencies must also be considered in placing the
accelerometer 58. If the table 51 is stiff, and moves rigidly at
controlled frequencies, the accelerometer 58 may be placed anywhere on the
top or bottom surface of the table 51. If the table 51 is flexible at
controlled frequencies, the accelerometer 58 should be placed close to the
inertial actuators 57.
The control system is depicted as a box 59 containing electronics in FIG.
6. Here, the control system 59 receives signals from microphone 56 and
accelerometer 58. It provides signals through an amplifier 59, to the
loudspeakers 52,53 and inertial actuator 57 such that the signals from the
sensors 56, 58 are driven to zero at the vibratory feeder bowl operating
frequency and perhaps harmonics of the vibratory feeder bowl operating
frequency. This is accomplished through what is known as destructive
interference. In the case of the acoustic system, the control system 59
sends a periodic signal to the loudspeakers 52,53 such that they produce
sound at the microphone 56 which is the opposite of the sound produced by
the vibratory feeder bowl 50, the inertial reduction system, and other
sources. The sound produced by the loudspeakers 52, 53 at the microphones
is equal in amplitude and phase shifted by 180 degrees, as compared to the
sound radiated by the vibratory feeder bowl 50, vibration reduction
system, and other sources. As a result, the pressure or sound at the
microphone 56 is driven to zero at those frequencies controlled. Also,
because the loudspeakers 52, 53 exhibit the quality of being spatially
similar to the vibratory feeder bowl 50, the energy radiated by the entire
system is reduced. The control system used which accomplishes this is
described in U.S. Pat. No. 5,091,953.
Care must be taken to ensure the contribution of sound from sources other
than the controlled vibratory feeder bowl 50 is small compared to the
sound produced by the bowl when the cancellation system is operating. If
other sources are significant, a reduction of sound pressure at the
microphones 56 may not correspond to a significant far field noise
reduction.
Vibration of the mounting surface is reduced by the vibration reduction
system using the same principle of destructive interference. In this case,
the control system 59 sends a periodic signal to the inertial actuator 57
such that it produces a vibration at the accelerometer 58 which is the
opposite of the vibration produced by the vibratory feeder bowl 50 and
acoustic reduction system. The vibration produced by the inertial actuator
57 at the accelerometer 58 is equal in amplitude and phase shifted by 180
degrees, as compared to the vibration produced by the vibratory feeder
bowl 50, acoustic control system, and other sources. As a result,
vibration as measured by the accelerometer 58 is driven to zero at the
controlled frequency. Also, because the inertial mounts 57 are spatially
similar to the vibratory feeder bowl 50 from the standpoint of table 51
vibration, the overall vibration of the table is reduced. Physically, the
inertial actuators 57 apply a periodic vertical force (at the controlled
frequencies) to the table 51 which is equal and opposite the sum of the
vertical component of forces (at the controlled frequencies) applied by
the vibratory feeder bowl 50 and the floor. Because the table 51 vibration
is virtually eliminated, it no longer acoustically radiates noise. In many
cases, the mechanical vibration reduction system is necessary for
acceptable acoustic reduction. Such is the case in applications where
acoustic radiation due to vibration of the table 51 and floor is
significant compared to acoustic radiation due to the piston like motion
of the vibratory feeder bowl 50.
Additional axes of control may be applied when additional equipment is
mounted to the table 51. The requirement for additional axes would stem
from the severity of vibration in those axes. For example, if a linear
pans track causes severe vibration in the vertical, rotational, and
horizontal directions, additional channels of control could be added to
cancel vertical force, horizontal force, and moments exerted by the parts
track on the table. Although controlling force and vibration in additional
axes significantly reduces mechanical vibration, additional acoustic
reductions may not be as significant as those achieved by reducing
vertical vibration. This is because table vibration in the horizontal and
rotational directions does not radiate acoustic energy as efficiently as
table vibration in the vertical direction.
Unenclosed System
A second embodiment of the device is shown in FIG. 7. In this case, no
enclosure has been placed around the vibratory feeder bowl 80. Once again,
two loudspeakers 81,82 are mounted horizontally so that their cones or
diaphragms are parallel to the plane of the bowl portion of the vibratory
feeder bowl 80. They are vertically positioned to be at approximately the
same level as the bowl portion of the vibratory feeder bowl 80.
The acoustic sensor in this embodiment takes the form of multiple
accelerometers 83, 84, 85. One accelerometer 83, 84 is mounted to each
loudspeaker 81, 82 cone, and to the bowl portion of the vibratory feeder
bowl 85. The accelerometers 82, 83, 85 measure acceleration in the
vertical direction. The signals from each loudspeaker 81, 82 accelerometer
are averaged by the signal conditioning box 86, creating a resultant
loudspeaker acceleration signal. The signal from the accelerometer 85
mounted on the vibratory feeder bowl 80 is multiplied by a weighting
factor and summed with the averaged signal from the loudspeaker
accelerometers 83, 84, as shown in FIG. 4. The weighting factor is
nominally the ratio of the total cone or diaphragm area of the
loudspeakers 81, 82 to the cross sectional area of the bowl portion of the
vibratory feeder bowl 80. This weighted sum of the signals then input to
the controller amplifier 87, taking the place of the microphone signal in
the previous embodiment. The purpose of using a weighted summation of
acceleration signals is to decrease the sensitivity of the system to
additional acoustic sources, such as other vibratory feeder bowls, thereby
enhancing far field performance.
A similar concept is employed in the mechanical vibration reduction system
of this second embodiment. Active isolation mounts 88 are used as
mechanical actuators and force transducers 89 are used as mechanical
sensors. The mounts 88 and transducers 89 are inserted in series between
the mounting points of the vibratory feeder bowl 80 and the table 90. In
this case, the active isolation mounts 88 reduce the force between the
vibratory feeder bowl 80 as measured by the force transducers 89. Although
vibration due to the operation of the vibratory feeder bowl 80 is
eliminated, vibration caused by other sources such as other equipment or
vibration transmitted from the floor is not necessarily reduced. The floor
induced table vibration is reduced.
The control system 87 operates in the same manner as in the first
embodiment. It sends the necessary signal to each actuator 81, 82, 88 so
that the signals from the acoustic and mechanical sensors 83, 84, 85, 89
are driven to zero at the controlled frequency.
Having described the invention and the preferred embodiments attention is
directed to the claims.
* * * * *
|
|
 |
|
 |
Description |
|
