Note: Descriptions are shown in the official language in which they were submitted.
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APPARATUS AND METHOD FOR SENSING UNBALANCE
FORCE AND LOCATION THROUGH FREQUENCY MODULATION
SUMMARY OF THE INVENTION
A force sensing system for sensing unbalance
forces at a spin shaft configured for mounting tire and
rim assemblies in a wheel balancing machine comprises
means for sensing the shaft angular position and fox
providing a shaft position output signal together with a
piezo electric crystal mounted in the wheel balancer
adjacent the shaft and exposed to the unbalance fo~ces.
Means is included for exciting the piezo electric crystal
whereby a crystal frequency output signal is provided.
The system also includes means for comparing the shaft
position and frequency output signals and for providing a
comparison output related thereto, the comparison output
being connected to the means for exciting. As a result
the crystal frequency output is stabilized relative to the
shaft position signal. Further, means is provided for
receiving the comparison output signal and for extracting
the tire and rim assembly unbalance position and magnitude
information therefrom.
An unbalance force detection system for sensing
unbalance force location and magnitude in a vehicle tire
and rim assembly mounted on a rotatable spin shat is
disclosed which comprises means for providing a shaft spin
signal indicative of shaft angular position and speed
together with force sensing means mounted to sense the
~orce imposed on the rotatable spin shaft by vehicle tire
and rim assembly unbalance. A orce signal is provided by
the force sensing means having a frequency indicative of
unbalance force magnitude. Means is also provided for
comparing the shaft spin and fQrce signals and for
providing an unbalance signal containing u~balance force
phase and magnitude information at the rotatable spin
shaft. Further, means is p~ovided for processing the
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unbalance signal to provide indication of unbalance force
and spin shaft referenced angular location.
The method o~ sensing unbalance force magnitude
and location in a vehicle tire and rim assembly mounted on
a spin shaft supported in a wheel balancer, and having an
encoder providing a frequency output indicative of the
shaft angular position and speed and a piezo electric
crystal force sensor mounted to sense unbalance force at
the shaft, includes the steps of exciting the pie20
electric crystal to oscillate a~ a predetermined frequency
in the absence of unbalance forces, comparing the encoder
frequency output with the crystal frequency, generating a
balancer output from the encoder frequency and crystal
frequency comparison, whereby unbalance force imposed
crystal oscillations provide balancer output variations,
and processing the balancer output to obtain unbalance
magnitude and angular location information.
The disclosed invention relates to an unbalance
force detection system for a vehicle tire and rim assembly
mounted on a rotatable spin shaft for providing magnitude
and location of unbalance compensation mass to be applied
to the tire and rim assembly and includes means for
providing a shaft spin signal indicative of shaft angular
position and speed, force sensing means mounted to sense
unbalance force imposed on the rotatable spin shaft by the
vehicle tire and rim assembly rotating thereon and
providing a frequency output indicative of the unbalance
force, and means for detecting the unbaIance force
frequency output and for converting it to unbalance force
and location outputs corresponding thereto. Also included
is means for processing the unbalance force and location
outputs to provide indication of unbalance compensation
mass and spin shaft referenced weight application location.
DESCRIPTION OF THE DRAWINGS
Figure 1 is a diagrammatic depiction of a wheel
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balancer of the type which utiliæes the disclosed
invention.
Figure 2 is a block diagram of one embodiment
of the present invention.
Figure 3 is a block diagram of another
embodiment of the present invention.
Figure 4 is a schematic diagram of a preferred
form of the crystal driver of Figure 3.
Figure 5 is a flow chart relating to the
embodiment of Figure 3.
BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS
A typical wheel balancer for vehicle rim and
tire assemblies is disclosed in U.S. Patent 4,285,240
issued to Gold in August~ 1981 and reissued in August,
lS 1985 as Re 31,971 by the U.S. Patent Office~ An
abbreviated description of a wheel balancer of the type
disclosed therein with which the invention disclosed
herein may be used is made with reference to Figure 1 of
the drawings. The wheel balancer comprises a base 11 on
which is mounted a drive motor 1~ having a pulley 13
mounted on a driven shaft extending therefrom, A drive
belt 14 surrounds the pulley 13 as well as a driven pulley
16 which is mounted to rotate with a spin shaft 17. The
pulley 16 and shaft 17 rotate relative to the balancer
base 11. The spin shaft has mounted at one end thereo a
shaft encoder 18 which may be of the optical type as
described in U.S. Reissue Patent 31,971 referenced
herein. The encoder provides a frequency indicative of
the angular speed of shaft 17 together with information
relating to instantaneous shaft angular position. Mounted
between the base 11 of the wheel balancer and the shaft 17
are a pair of piezo electric crystals 19 spaced along the
length of the spin shaft, Structure is provided on the
end of the spin shaft opposite the end on which the
encoder 18 is mounted for mounting a vehicle tire and rim
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assembly 21. The motor 12 drives shaft 17 with the tire
and rim assembly 21 mounted thereon and the rotating
unbalance force in the tire and rim assembly is sensed as
it rotates and passes through the angular positions
occupied by the unbalance force sensoes 19. It should be
noted that the wheel balancer with ~hich the invention
disclosed herein may be used to advantage need not be
driven by a drive motor 12, but may be driven by any other
drive means including manual means.
The characteristic of a piezo electric crystal
transducer is to produce voltage when a force is applied
to it. On the other hand, if the crystal is excited by an
alternating voltage it will oscillate over a wide range of
frequencies depending upon the excitation frequency. When
the crystal is caused to oscillate at a particular
frequency by applying a particular excitation thereto, the
rate of oscillation will change in response to application
of a force change to the crystal. Therefore, if the
crystal 19 is stimulated to oscillate, an unbalance force
C' resulting from spinning a tire and rim assembly which is
spun on a wheel balancer, such as that described in
connection with Figure 1, will change the frequency of
oscillation of the crystal. The frequency change will be
proportional to the change in the amount of orce applied
to the crystal or the unbalance in the tire and rim
assembly.
Since the time rate of change at which the
oscillations o~ an excited piezo electric crystal is
proportional to the time rate of change of force applied
thereto, it can be said that the frequency of oscillations
in the crystal is modulated by the applied force, or in
this case the unbalance force exerted by the rotating tire
and rim assembly mounted on shaft 17 of the wheeI balancer
of Figure 1. The apparatus and method disclosed herein
for detecting unbalance force will be seen to be
comparatively much more immune to noise disturbance
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because there are no high Q requirements for a noise
reduction filter which receives the unbalance information
signal produced by the system of this disclosed invention,
as is the case when using the force sensor to generate an
analog voltage indicative of unbalance force magnitude and
phase.
Figure 2 depicts the shaft encoder 18 providing
a shaft rotational frequency output as well as a shaft
angular position or phase output. The piezo electric
transducer 19 in Figure 2 is excited by the output from a
voltage controlled oscillator (VCO) 22 to oscillate at a
predetermined frequency with no unbalance force applied
thereto. Generally there is a constant preload force F
applied to the crystal. The output frequency from the
lS piezo electric crystal is provided to a divide by N
counter 20 which provides an output of a desired frequency
to a phase detector 23.
The shaft encoder output is conditionad in a
signal conditioner 24 so that it assumes the
aforementioned desired frequency as well as amplitude and
may be used with the divided pie~o electric crystal
output. Thus, the conditioned shaft encoder output from
signal conditioner 24 and the conditioned piezo electric
ceystal output frequency from the divide by N counter 20
are provided to the phase detector 23 which will provide
an output therefrom (as indicated in Figure 2) which is
proportional to the difference in phase between the two
input signals. The signal arising from the difference in
phase between the conditioned piezo electric crystal
frequency and the conditioned shaft encoder frequency is
connected to a low pass loop filter 26 so that the VCO 22
is not driven to try to correct for high frequency noise
signals provided by the broad band sensitivity of the
crystal 19. Typically, the loop filter 26 is a 100 cycle
low pass filter. This filtered output is an analog signal
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which is connected to the input of the VCO 22 and which
contains the unbalance force magnitude and phase or
anqular location information.
As may be seen from the foregoing description,
Figure 2 shows a phase locked loop circuit which allows
the unbalance signal phase information to be maintained
and gives rise to an inherentl~ stable oscillator The
phase locked loop controls the oscillation frequency of
the crystal 19, keeping it locked to the reference signal,
the conditioned shaft encoder output from signal
conditioner 24. This allows the unbalance force signal to
be referenced to the known phase o~ the encoder in order
to calculate the unbalance correction weight angular
location. The voltage controlled oscillator 22 utilizes
the output signal from the loop filter 26 to adjust the
oscillation frequency of the piezo electric crystal to
keep it in phase wi~h the reference signal from signal
conditioner 24. The unbalance force signal output from
the loop filter 26 may be connected to an analog to
digital converter 27 and analyzed with a computer or
microprocessor 28 to derive the correction weight
magnitude and phase or angular location relative to the
encoder output. The correction or compensation weight
magnitude and angular location are then communicated to an
operator by means such as the display 29 shown in Figure 2.
An alternative embodiment of the disclosed
invention may be seen with reference to Figure 3~ A`micro
controller 31 such as a SA~80C53S, manuactured by Siemens
Components, Inc., of Orange, California, is connected to
receive input from the shaft encoder 18. The
micro-controller is also connected to receive input from a
bu~fer 32 connected between the oscillating crystal 19 and
the micro controller. A crystal driver 33 is set to
provide a known frequency output when there is no
unbalance load being measured. The piezoelectric crystal
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may be preloaded with a force F. A change in unbalance
force F applied to the force sensing pie20electric crystal
transducer 13 will cause a change in frequency output from
the transducer. The micro controller receives the new
frequency from crystal transducer 19 through buffer 32
and converts it to force and location of the changed
unbalance. Wheel parameter entry structure 34 is provided
to allow the unbalance force and location to be converted
to accommodate the specific rim and tire assembly 21
mounted on the wheel balancer,
A preferred embodiment of the crystal driver 33
is shown in Figure 4. An oscillating circuit is connected
as shown having a pair of inverters 36 and 37 having
feedback resistors Rl and R2. The inverters may be 74LS04
Hex inverters manufactured by Motorola, Texas Instruments
and others. The buffer may be the same type of inverter.
A capacitor C is shown connected between the output of
inverter 37 and the input of inverter 36. This capacitor
is selected to tune the combination of the crystal driver
~3 and the crystal transducer 19 of Figure 4 to some
appropriate frequency such as 40~Hz. From the foregoing
it may be seen that the frequency output from the force
transducer 19 is allowed to change and the new frequency
together with information from encoder 18 is converted by
2-~ the micro controller to an unbalance force and location.
Turning to Figure 5, a flow chart of the manner
in which the left crystal transducer output together with
the encoder output is processed i~ shown. It should be
noted that the right crystal of the usual pair of
transducers in a wheel balancer is treated in the same
way. AS seen by ~A~ in Figure 5 and the box stating rall
operations within ~A~ are performed for the right crystalW
the process is the same and the resulting signals are used
as input to resolve forces into plane forces. The~efore,
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the processing relating to the left transducer only will
be explained, it being understood that the process for the
right transducer is the same.
The left transducer output is connected to an
input port of the micro controller 31 as stated
hereinbefore. ~he micro controller senses the oscillation
period of the frequency output from the transducer. The
sensed frequency period is compared with the last
frequency period to detect any change. The change is
converted to a force change by calculation.
The encoder output is also input to an input
port of micro controller 31. Shaft position is sensed.
The position is compared with a last sensed encoder
position and shaft speed and anqular location are
calculated. The shaft speed is used to scale the
converted force change. The scaled force and shaft
location are input to that section of the controller which
resolves the forces into forces in the selected wheel
assembly planes. Thereafter the unbalance compensation
weight mounts and rim weight application locations are
indicated to an operator, as by a display. It may be seen
that in this embodiment the crystal transducer is allowed
to put out the frequency in accordance wi~h the unbalance
force applied and that the system operates on that
frequency to e~tract the force magnitude.
Although the best mode contemplated for
carryinq out the present invention has been herein shown
and described it will be apparent that modification and
variation may be made without departing from what is
regarded to be tha subject matter of the invention.
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