Active tuned magnetic flux rate feedback sensing arrangement

An arrangement using sensing coils for obtaining flux rate of change information in a magnetic circuit. The arrangement can be used for vibration attenuation in a magnetic forcer system. Active (electric powered) circuitry is used to implement closed loop control of flux rate. The control loop is "tuned" for attenuating a narrow range of vibration frequencies. The arrangement can be applied to magnetic forcer/suspension systems in which vibrations due to magnetic, mechanical/magnetic runouts, system mechanical resonances, or external vibration sources are present.

CROSS REFERENCE TO RELATED APPLICATION 
This application is related to commonly assigned U.S. Pat. No. 5,329,416 
entitled Active Broadband Magnetic Flux Rate Feedback Sensing Arrangement 
which issued to the present inventors on Jul. 12, 1994. 
BACKGROUND OF THE INVENTION 
This invention relates to an arrangement for obtaining flux rate of change 
information in a magnetic circuit such as may be used for vibration 
attenuation in a magnetic forcer system. 
Passive arrangements for the purpose described include spring/mass systems, 
mechanical dampers and hybrid visco-elastic devices. Active arrangements 
may be open or closed loop and may include forcer elements or drive 
elements such as piezoceramic elements, pneumatic/hydraulic drives, or 
electromagnetic devices which, when used in conjunction with appropriate 
sensing elements, can be used to actively accomplish the aforenoted 
vibration attenuation. 
In an active open loop configuration, a command matching the vibration to 
be attenuated as a function of time is applied to the forcer element. This 
configuration works well when the vibration dynamics can be modeled 
accurately. Alternatively, an active closed loop arrangement can be 
employed, whereby the vibration is sensed and the sensed information is 
used to adjust a command matching the vibration. 
The present invention is similar to the above described active closed loop 
arrangement which uses "Hall Effect" devices to measure magnetic flux 
within an air gap. These devices are operative so that presence of a 
magnetic field of a proper orientation induces a small voltage in a 
semiconductor device. The Hall Effect arrangement uses flux as the sensed 
parameter for vibration attenuation. The present invention, on the other 
hand, uses flux rate for this purpose. 
Accordingly, it is the object of the present invention to use a closed loop 
or feedback arrangement for sensing flux rate of change in a magnetic 
circuit. In this regard, it is noted that in the present invention the 
closed loop is tuned whereby a narrow range of vibration frequencies are 
attenuated as may be desired. This is in contrast to the invention 
disclosed and claimed in the aforenoted U.S. Pat. No. 5,329,416 wherein 
the closed loop is broadband in that a broad range of vibration 
frequencies are attenuated as likewise may be desired. 
SUMMARY OF THE INVENTION 
This invention contemplates an active tuned magnetic flux rate feedback 
sensing arrangement wherein a magnetic circuit includes a sensor element 
and a forcer or drive element in the form of wound wire coils for 
providing a magnetic flux path. The sensor output is dictated by a command 
to the forcer element and provides a flux rate output. 
A signal is induced in the sensor element due to the flux rate output. The 
signal is processed by conditioning electronics and a filter tuned to a 
narrow range of signal frequencies. The output of the tuned filter is fed 
back to the drive element loop to provide a closed loop configuration.

DETAILED DESCRIPTION OF THE INVENTION 
With reference first to FIG. 1, the orientation of a flux rate sensing coil 
in relation to a drive or forcer coil in a magnetic circuit is 
illustrated. Thus, a stator is designated by the numeral 2 and a rotor is 
designated by the numeral 4. Stator 2 carries a drive coil 6 having legs 8 
and 10 and carries a sensor coil 12 having legs 14 and 16. The output of 
sensor coil 12 at legs 14 and 16 is equal to the number of sensor coil 
turns times the rate of change of magnetic flux. 
Legs 14 and 16 of sensor coil 12 are disposed close to an air gap 18 
between stator 2 and rotor 4. With this arrangement, magnetic flux induced 
in stator 2 moves from the stator to rotor 4 and then back to the stator 
with minimal pick-up of stray leakage magnetic flux fields in the magnetic 
circuit. 
With reference to FIG. 2, sensor coil legs 14 and 16 are connected to 
conditioning electronics 20. Conditioning electronics 20 provides a flux 
rate signal which is applied to a tuned filter 22 which passes a narrow 
range of signal frequencies. 
The signal from tuned filter 22, is fed back as an input to a current drive 
device 24 via a summing device 26. An external source 25 provides a 
command signal I.sub.c which is applied to a summing device 28 and is 
summed thereby with the output from current drive device 24. 
Summing device 28 sums the signal from current drive device 24 with command 
signal I.sub.c and provides a summed signal which is applied to a current 
forward loop compensator 30 and therefrom to summing device 26. Summing 
device 26 sums the signal from compensator 30 with the signal from tuned 
filter 22 and provides a summed signal which is applied to current drive 
device 24. Current drive device 24 provides a signal which is applied as 
current feedback to summing device 28 and is applied to drive coil 6 of 
stator 2 for energizing the drive coil. 
With the arrangement described, a signal is induced in sensor coil 12 due 
to the rate of change of magnetic flux and is used in a tuned filter 
closed loop configuration. This signal is processed by conditioning 
electronics 20 and tuned filter 22. The processed signal is then fed back 
to a drive command loop including summing device 28, compensator 30 and 
summing means 26. 
The arrangement including conditioning electronics 20 shown generally in 
FIG. 2 is shown more specifically in FIG. 3 and includes overload 
protection devices 32 and 34, load resistors 36 and 38, a differential 
amplifier 40 and a noise filter 42. 
Overload protection device 32 is connected across leg 14 of sensor coil 12 
and overload protection device 34 is connected across leg 16 of the sensor 
coil. Overload protection devices 32 and 34 may be diodes. Load resistor 
36 is connected across overload protection device 32 and load resistor 38 
is connected across overload protection device 34. The signal from sensor 
coil 12 is applied through overload protection devices 32 and 34 and load 
resistors 36 and 38 to a differential amplifier 40 and therefrom through 
high frequency noise filter 42 which provides a filtered signal. The 
filtered signal is applied to tuned filter 22 as shown in FIG. 2. This 
arrangement is useful for preventing extraneous noise from interfering 
with the operation of the closed loop configuration heretofore described. 
Differential amplifier 40 rejects common mode voltages that may be present 
due to IR drops, or other non-linear effects. Load resistors 36 and 38 are 
selected in conjunction with overload protection devices 32 and 34, 
respectively, to limit peak loads. Since the induced voltage is 
proportional to the rate Of change of flux or the rate of change of 
current in drive coil 6, for high D.sub.i /D.sub.t systems, overvoltage 
protection as described is mandatory. 
FIG. 4 shows a typical frequency response plot of tuned filter 22. The gain 
or authority is low at low frequencies (where the sensor coil output is 
low) and is particularly high for a narrow band of frequencies of higher 
frequencies. This arrangement results in good vibration attenuation at a 
particular frequency as shown in FIG. 5. At lower frequencies, the current 
loop (FIG. 2) has authority. 
It will now be appreciated that flux rate feedback using sensing coils as 
in the present invention provides superior vibration attenuation compared 
to prior art Hall Effect devices at frequencies greater than zero. In this 
regard, it will be noted that the voltage obtained from a Hall Effect 
device is typically small and must be amplified with a high gain device. 
This makes such an arrangement susceptible to noise which is obviated by 
the present invention. In further contrast to the prior art Hall Effect 
devices, said devices are typically limited to a temperature range less 
than ninety to one hundred and twenty degrees Celsius. Further, the Hall 
Effect devices require both temperature correction and calibration for 
non-linear effects and a precision current source is required. The present 
invention obviates these requirements while providing a flux rate sensor 
which is relatively simple and does not require calibration. 
With the above description of the invention in mind, reference is made to 
the claims appended hereto for a definition of the scope of the invention.