Magnetic element position sensing device, using misaligned electromagnet

Strange, M. G.

English

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NATIONAL AERONAUTICS AND SPACE AD.MINISTRATION 
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November 6, 1970 
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Copy of Patent aited above 5 
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April 28, 1970 M. G. STRANGE 3,509,469 
POSITION SENSING DEVICE! EMPLaOYING MISALIGNED MAGNETIC 
FIELD GENERATING AND DETECTING APPARATUS 
Filed May 17, 1967 
OSCILLATOR 
i REFERENCE POSITION 
W E N T O R  
Maxwell G. Strange 
3,509,469 United States Patent Office ,,,, ,, , ,,,, 
POSITEON SENSINC-~@%CE EMPLOYING MIS- 
ALIIGNED MAGNETIC FIELD GENERATING 
AND DETECTJNG APPARATUS 
Maxwell G. Strange, FuBton, Md., assignor to the United 
States of Amedca as represented by the Admfn!strator 
of the Nabionaal Aeronautics and Space Administration 
FlBed May 17, 1967, Ser. No. 640,453 
Int. Cil. GOln 27/00 
U.S. C1. 328-1 6 Claims 
ABS'P1RACT QF THE DISCLOSURE 
This invention is a position sensing device for sensing 
the position of a magnetic element. The magnetic element 15 
moves between a pair of fixed electromagnets-one a 
bar magnet connected to an oscillator and the other a 
horseshoe magnet connected to a phase detector through 
a limitig amplifier. The oscillator is also connected to the 
phase detector. The bar magnet is misaligned with the 20 
ends of the horseshoe magnet to create a bias voltage on 
the horseshoe magnet's output. When the magnetic ele- 
ment moves between the bar and horseshoe magnets it 
creates a phase shift in the output of the horseshoe mag- 
net. This phase shift rapidly passes through zero at  a well 25 
defined point. This point is detected by a phase detector 
and a pulse-forming network creates an output pulse. 
BACKGROUND OF  THE INVENTION 30 
eration of false outputs. Furthermore, considerable elec- 
tronic circuitry is required. 
Magnet sensing devices are limited due to noise pickup 
and lack of resolution. Also, the use of permanent mag- 
nets, as required by some magnetic devices, is often re- 
stricted in some environments such as on a spacecraft, 
for example. 
Furthermore, simple magnetic sensors have outputs 
proportionat to the rate-of-change of flux; this places a 
definite lower limit on the velocity of motion which can 
generate a useable pulse. 
- It is an object of this invention to provide a new and 
improved position sensing device. 
It is another object of this invention to piovide a new 
and improved position sensing device that is simple and 
reliable. 
It is a further object of this invention to provide a new 
and improved position sensing device that senses very 
slowly moving elements and generates a sharp output when 
said element reaches a predetermined position. 
It  is still another object of this invention to provide ,a 
new and improved magnetic position sensing device which 
requires very little power for its operation. 
It is yet another object of this invention to provide a 
new and improved magnetic sensing device that avoids 
the use of permanent magnets and minimizes radiated 
magnetic interference. 
SUMMARY OF THE INVENTION 
The invention described herein was made by an em- In accordance with a principle of this invention a new 
ployee of the United States and may be manufactured and and improved magnetic position sensing device i s  prcr- 
used by or for the Government for governmental pur- vided. The device comprises a first magnet and a second 
poses withornt the payment of any royalties thereon or 35 magnet. The first magnet is separated from the second 
therefor. magnet and the magnetic axes of the magnets are mis- 
Generally, there are three types of electrical position aligned. The ferrous element t~ be sensed passes between 
sensing devices which do not require ac- the misaligned magnets. The first magnet is connected to ;i 
tuated contacts-optical, capacitive and magnetic. An op- frequency generating source and the second magnet op 
tical position sensing device generally comprises a light 40 crates as a pickup or sensor, The frequency   en era tin^ 
source and a light detecting element. The element whose Source is also connected to a phase defector as is the out- 
position is to be sensed either passes between the light P U ~  from the pickup magnet. As the item or element to be 
source and the light detecting element or has a reflective detected Passes between the two magnets the phase de- 
element mounted on it that reflects light from the light t~ctor's output signal changes in accordance with said pas- 
source to the light sensing element when it reaches a pre- 43 Sage. This change is due t~ a 180' phase shift kiween the 
determined position. Capacitive ensing devices comprise phase detector's inputs, which occurs at a well defined 
a pair of plates with the element to be sensed varying the position the 
capacitance between the plates when it passes between In acco~dance with a still further principle of the inven- 
them. Magnetic sensing devices generally comprise a mag- tion, the output from the phase detector is applied to a 
net or a pair of magnets connected to a signal generating 50 pulse-formint: network, and a pulse is generated when the 
means and a signal detecting means. me element to be element to be sensed reaches the position between the first 
sensed varies the mametic field of the magnet or pair of and ~econd masnets that creates the 180" phase shift, 
magnets to vary the detected signal. It will be appreciated by those skilled in the art and 
All three of the foregoing position sensing devices have others that the invention is inherently simple and reliable. 
found widespread use. However, their operation has not 55 It merely involves the use of a pair of magnets in corn- 
proven to be entirely satisfactory in many respects. Opti- bination with a frequency generating source and a phase 
cal position sensing devices require light sources that con- dctsctor. The magnets are misaligned to create a bias sig- 
sume a relatively large amount of power and are subject nal wh-n the element to be sensed is not between the ma@ 
to deterioration and failure. Further, optical sensing de- nets. \%'hen the element to be sensed passes between the 
vices require complete shielding against ambient light. 60 magnets the output of the phase detector varies drre to 
Moreover, a high degree of resolution is difficult to ob- a 180" phase shift. This phase reversal point is well-de- 
tain in a compact device. fined as a function of elenlent position and is not de- 
Capacitive sensing devices are subject to misalignment, pendent upon its velocity; hence, the positlorn indication 
as well as failure due to dirt or other matter varying the 65 is sharp and accurate no matter how slowly the element 
capacitance between the plates and resulting in the gen- is moving. 
"7 
3,509,469 
4 4 
BRlEP DISCUSSION OF THE DRAWINGS and, the network may be used to introduce a cofitiolkci 
The foregoiag objects and many of the attentive advan- 
tages of t h ~ s  invention will become more readily appreci- 
ated as the same bccomes better understood by reference 
to the following detailed description when taken in con- 
junction with the accompanying drawings wherein: 
FIG. 1 is a paltially schematic and partially block 
functional diagram of a preferred embodiment of the in- 
degree of hysteresis. 
The sensed or refcrence position is insensitive to the 
speed of the movement of the element. That is, element 
29 can be rapidly moved between the magnets or it can 
be slowly moved. When it reaches its predetermined posi- 
tion the output from the limiting amplifier passes through 
an abrupt phase reversal and the pulse forming network 
generates an outout oulse to indicate the vosition has 
vention; and been reached. ~ e n c e :  in addition to being- a basically 
FIG. 2 is a graph illustrating the output of the sensor lo simple position sensing device, the invention also has the 
as thc element passes between the magnets. advantage of sensing slowly moving elements. 
DESCRIPTION O F  THE PREFERRED While the foregoing has described the preferred em- 
EMBODIMENT bodiment of the invention, it will be appreciated by those 
1 j skilled in the art and others that this is merely by way of 
FIG. 1 is a partially schematic and partially block example. The invention as illustrated in FIG. 1 uses a 
functionaf diagram of 3 preferred embodiment of the in- bar magnet and a horseshoe magnet. However, other mag- 
vention and comprises a bar magnet 1 P  having a coil 13, net configurations can also be used. Generally, the inven- 
a horseshoe magnet 15 having a coil 17, an oscillator 19, tion requires the use of magnets having misaligned axcs. 
a limiting amplifier 21, a phase detector 23, and a pulse 20 The misalignment of the axes creates a bias output voltage 
forming network 25. for the quiescent condition. This bias level prevents ex- 
The oscillator 19 bas two outputs; one output is con- traneous signals from creating erroneous outputs. That 
nected to the coil 13 of the bar magnet 11, and the other is, if the magnet center linss were aligned, as provided by 
output is connected to one input of the phase detector 23. prior art magnet sensing devices, the output from the coil 
T h e  coil 17 of the horseshoe magnet 15 is connected to 25 17 would be zero in the absence of element 28 and the 
the input of the linliting amplifier 21, and the output of output from the phase detector would be indeterminate 
the limiting amplifier is connected lo the second input of and very susceptible to noise or spurious signals. The 
the phase detector 23. The output of the phase detector is simple expedient of biasing the output, as hereinabove 
connected lo the input of the pulse forming network 25 described, insures a definite signal and phase condition 
and the output of the pulse forming network is connected 30 at all times. Hence, the invention provides a high degree 
to an output terrninal 27. of noise immunity and simplifies the electronic circuitry 
The bar magnet 11 has one of its poles spaced from the necessary to sense the element's position. That is, if 
poles of the horseshoe magfiet 15 to create a passageway. spurious or extraneous magnetic sources created false out- 
However, the magnetic center line of the bar magnet put signals an electronic system for eliminating these false 
along its longitudinal axis is displaced from the magnetic 35 signals would be required. The invention achieves the 
center line of the horseshoe magnet between its legs. That same result by simply misaligning the magnetic elements 
is, as illustrated in FIG. 1, the center line of the bar mag- to create a bias level. Moreover, the misalignment does 
net is slightly below the center line of the horseshoe not require that the centerlines be parallel as illustrated in 
magnet. FIG. 1; any suitable configuration can be used. 
Also illuslrated in FIG. 1 is an element to  be sensed 29 40 Further, while the invention illustrated in FIG. 1 
preferably formed of a magnetic material. The element utilizes an oscillator to create an AC signal, it will be ap- 
29 moves in the direction of the arrow through the pas- preciated that this can be a rather unsophisticated, simple 
sageway between the bar and horseshoe magnets. device. That is, its frequency can be relatively unstable 
FIG. 2 is a graph illustrating the output of the sensing since only phase differences are being sensed. 
coil on the horseshoe magnet as the element 29 moves 45 In some environments it may be advantageous to 
between the magnets. As illustrated in FIG. 2, there is a eliminate the limiting amplifier to provide an even sim- 
bias voltage for the quiescent condition. That is, the pler, less expensive device than that illustrated in FIG. 1. 
oscillator 89 applies an AC signal to the winding 13  of Moreover, it may be desirable to eliminate the pulse 
the bar magnet 11; it applies to the same signal to one forming network and directly use the output from the 
input of the phase detector 23. When the element 29 is 50 phase detector. It is also possible to place all magnetic 
not located in the passageway (illustrated as a solid line parts on the same side of the element. Hence, the inven- 
position in FIG. I),  the limiting amplifier detects a signal tion may be practiced otherwise than is specifically de- 
and applies it to the phase detector. The signal from the scribed herein. 
oscillator and from the limiting amplifier are, for ex- It will also be appreciated by those skilled in the art 
ample, in phase whenever the element 29 is well removed 55 and others that the invention is useful in various environ- 
from the gap. ments. Specifically, the element 29 could be mounted on 
As the element 29 moves between the magnets, the the rotor of a brushless DC motor; that is, a motor 
output from the coil 17 starts to rise as illustrated in FIG. wherein a means is used to sense the position of the rotor. 
2. After reaching a maximum, it starts to drop. When the This sensed position is then used lo control the energiza- 
element 29 reaches a predetermined position (illustrated 60 tion of windings located on the motor's stator. Or, the 
as the dotted line position in FIG. I ) ,  the output from invention could be used in a spacecraft with electronic 
the sensing coil 17 passes through zero. That is, the ele- measuring devices. That is, the invention could be used 
rnent reaches a null position in the passageway. There- to sense the position of a directional measuring or scan- 
after, the output increases in amplitude, but with reversed ning device to give meaning to the parameter being meas- 
phase, as the element 29 continues to move upward 65 ured. Further, the invention could be used as a timing 
through the gap, Finally, as element 29 moves away from device by having a pendulum attached to the element. 
the gap area, another phase reversal occurs and the out- Each time the element moves between the magnets a 
put of the coil 17 returns to the bias level. pulse is generated. This pulse could be a timing or clock 
The limiting amplifier 21 removes all amplitude varia- pulse for use in controlling other electronic circuitry. 
lions from the oatput of coil 17, preserving only the axis 70 Therefore, it will be appreciated that the invention is suit- 
crossings shown in FIG. 2 as sharp phase reversals. The able for use in numerous envi~onments. 
phase reversal ~esults in an abrupt voltage shift at the What is claimed is: 
phase detector output, Pulse forming network 25 deter- 1. A magnetic position sensing device comprising: 
mines the length and shape of the output pulse and blocks (a) first magnetic means having a coil for generating 
the norillaliy extraneous positive-slope phase reversal. 75 a magnetic field; 
5 6 
(b) second magnetic means having a coil for detecting 3. Apparatus is claimed in claim I wherein said signal 
a magnetic field and generating an output signal in generating means is an oscillaior. 
response thereto, said first magnetic means separated 4. Apparatus is claimed in claini 3 wherein said first 
from said second magnetic means wit11 the magnetic magnetic means is a bar magnet and said second mag- 
centers of the magnetic means misaligned; neiic means is a horseshoe magnet. 
(c) signal generating means having first and second 5.  Apparatus is clai~xed in claim 4 including 21 pulse 
outputs for generating an electronic signal; forming network coilnected to the output or said phase 
(d) phase detecting means having first and second detecting means. 
inputs for detecting the phase difference between 6. Apparatus is claimed in claim 5 iillcluding a limiting 10 amplifier connected between the coii of said horseshoe 
said output signal of the second magnetic means and 
magllet and said detector. 
said electronic signal and generating an output in 
response thereto; References Cited 
(e) the first output of said signal generating means 
connected to the coil of said first magnetic means l 5  UNITED STATES PATENTS 
and the second output of said generating 3,058,059 10/1962 Bockemuehl ---------- 324-41 
means connected to the first input of said phase 3,395,341 711968 Malaquin ----------_ 323-70 
detecting means; and 
(f)  the coil of said second magnetic means connected JOHN S .  I-IEYMAN, Primary Examiner 
to the second input of said phase detecting means. 20 S. D. MILLER, Assistant Examincr 
2. Apparatus is claimed in claim 1 wherein said first 
magnetic means is a bar magnet and said second magnetic U.S. C1. X.R. 
means is a horseshoe magnet. 328-5, 133; 307-309, 232; 324---41 


Position sensing device employing misaligned magnetic field generating and detecting apparatus Patent

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