Vehicle suspension damper with relative velocity sensor having controlled flux path

A relative velocity sensor in a vehicle suspension damper comprises an annular, radially polarized magnet at the top of the shock cylinder adjacent a sensor winding on the inside of the dust tube. A closed magnetic circuit is defined from the magnet through an annular magnetic member inside the magnet to the piston rod and back through the dust tube across the sensor winding to the magnet. Flux below the magnet is similarly confined through the cylinder and does not affect the sensor winding. The air gaps at the winding and the piston rod do not change with axial movement; and the flux linkage variation with axial movement is thus linear across the entire coil. A voltage is generated across the coil which is a strong and accurate signal of relative movement between the sprung and unsprung masses of the vehicle.

BACKGROUND OF THE INVENTION 
This invention relates to vehicle suspension dampers for use in controlled 
damping vehicle suspension systems. Such systems incorporate shock or 
strut devices capable of varying their damping characteristics in response 
to an electronic control signal. This signal is typically generated in an 
electronic control in response to one or more suspension related input 
signals. In a closed loop suspension control, an important input signal 
indicates the velocity of movement between the vehicle sprung mass, or 
body, and the vehicle unsprung mass, or wheel assembly. 
The prior art includes publications describing systems in which the vehicle 
suspension at a wheel includes a suspension relative position sensor such 
as an LVDT. The position signal from such a sensor may be differentiated 
to provide a relative velocity signal; however, the process requires 
additional electronic circuitry with difficult design requirements. 
The prior art also includes relative velocity sensors incorporated in 
suspension components such as dampers. For example, the patent to Herberg 
et al 5,009,450 discloses such a sensor incorporated in a vehicle shock 
absorber of the type having a cylinder attached to one of the sprung and 
unsprung masses and a piston in the cylinder attached through a rod 
extending out of the cylinder to the other of the sprung and unsprung 
masses. The rod further carries a dust tube which extends over a 
substantial portion of the cylinder. An axially polarized annular magnet 
is attached to but magnetically spaced from the top of the cylinder and is 
further magnetically spaced from the piston rod; and a sensor winding is 
distributed axially along the inside of the dust tube, which is made of a 
non-magnetic material. Vertical motion between the sprung and unsprung 
masses causes similar axial motion between the dust tube and cylinder and 
moves the magnet axially along the sensor winding. 
However, the Herberg et al sensor provides a low output through the middle 
of its range and loses its linearity near the ends of its range. The 
variation of flux linkage with position is very flat and actually reverses 
direction near one end. In addition, with its uncontrolled flux paths, it 
is highly susceptible to the presence of nearby magnetic objects, such as 
other suspension members, which may distort the flux paths. Thus, the 
output voltage is low and is not a dependably accurate indication of the 
relative velocity of the sprung and unsprung masses of the vehicle. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide a relative velocity sensor in 
a vehicle suspension damper of the type in which a magnet is moved axially 
across a sensor winding, the sensor exhibiting a linear variation of flux 
linkage with magnet movement across substantially the entire travel 
thereof for an accurate relative velocity output voltage signal. This is 
accomplished by confining most of the flux from the magnet in a structural 
arrangement providing a closed magnetic flux path with no significant flux 
variation as the magnet and sensor winding move axially relative to each 
other. Thus, the variation in the number of turns enclosed by the flux 
path, which is linear, produces a linear flux linkage variation. 
In particular, the dust tube on which the sensor winding is mounted is made 
of a magnetic material, the magnet is radially polarized, and magnetic 
means are provided to form, with the dust tube, a closed magnetic flux 
path for the magnet with substantially constant air gaps. The magnet may 
be annular; and the closed magnetic flux path may include the piston rod 
and an annular magnetic member inside the magnet. The annular magnetic 
member forms an air gap with the piston rod which does not vary with 
relative axial movement; and the magnet forms an air gap with the dust 
tube which likewise does not vary with relative axial movement. Thus, 
relative axial movement varies only the number of turns enclosed by the 
flux path; and this provides a linear flux linkage variation for an 
accurate relative velocity signal.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, a vehicle damper comprises a shock absorber 10. Shock 
absorber 10 comprises a cylinder 12 having a non-magnetic upper rod guide 
14 and is closed at the lower end 16 to define a cylindrical cavity 18. 
Cylindrical cavity 18 is divided into upper and lower chambers by a piston 
19 which is sealingly disposed for axial movement therein. The axial 
movement of piston 19 pumps fluid between the chambers in cylindrical 
cavity 18 and a reservoir, not shown, with orifices and valves providing 
damping in the manner normal for shock absorbers. Since the sensor of this 
invention would normally be used with dampers having variable damping, one 
or more of the valves or orifices may be controllable in response to a 
control signal. The precise structure and operation of the damper itself, 
including damping control, is unimportant to the disclosure of this 
invention; and the prior art shows many known constructions which may be 
modified according to this disclosure. 
Piston 19 is attached to the lower end of a reciprocating piston rod 20, 
which extends upward through an opening in rod guide 14 and is sealed 
thereto by a standard sliding seal arrangement, shown symbolically in the 
Figures, which retains the fluid in cylindrical chamber 18. Rod 20 extends 
upward in the Figure out of cylinder 12 and ends in a standard fitting 22 
for attachment to the sprung mass or body of a motor vehicle at one corner 
thereof. A similar fitting 24 attached to lower end 16 of cylinder 12 
provides attachment to a member of the unsprung mass or wheel assembly of 
the vehicle such as a control arm thereof. A dust tube 26 comprises a 
radial disk portion 28 attached to rod 20 at the end thereof adjacent 
fitting 22 and a cylindrical portion 30 projecting downward from disk 
portion 28 around a substantial length of cylinder 12. Dust tube 26 is 
normally provided to prevent dirt from entering and harming the seals in 
cylinder 12 around rod 20. Relative movement of the sprung and unsprung 
masses o the vehicle produces relative axial movement between cylinder 12, 
which is attached to and moves with the unsprung mass, and the assembly of 
rod 20, dust tube 26 and the piston, which is attached to and moves with 
the sprung mass of the vehicle. Cylinder 12, piston rod 20, and dust tube 
26 are all made of a magnetic material such as steel. 
A sensor winding 32 is wound on a non-magnetic bobbin 34 which is press fit 
into the inside of the cylindrical portion 30 of dust tube 26, extending 
over a substantial axial length thereof: for example, 110 millimeters. 
Sensor winding 32 is shown in FIG. 2 as comprising multiple layers of 
wound Wire. An embodiment which has been built and tested on a vehicle 
comprises four layers of standard varnish coated 33 gauge wire, the wire 
wound in even layers in a linear manner over the full length of the 
winding; however, the precise number and length of layers, gauge of wire, 
etc. will depend on the particular application. The ends of sensor winding 
32 are connected to terminals in a connector, not shown, for communication 
of the voltage generated in sensor winding 32 as an output signal. No 
special winding arrangements or extra coils are required for accurate 
sensor output or end of coil sensing. 
An annular magnet 36 is attached to the top of cylinder 12. It is polarized 
radially, so that one of its poles (north) is on the outer circumference 
and the lines of flux are directed radially outward across a small gap 38, 
through bobbin 34 and sensor winding 32 to the cylindrical portion 30 of 
dust tube 26. Magnet 36 in the unit built and tested is made of 
Magnequench MQ1 (R) material, although other magnetic materials may be 
used if desired. An annular magnetic flux member 40, which may be made of 
steel, is provided adjacent the other (south) magnetic pole at the inner 
annular surface of magnet 36; and member 40 provides a flux path for the 
flux lines from the other pole of magnet 36 to piston rod 20 across a 
small gap 42. The closed magnetic circuit may be traced from magnet 36, 
through flux member 40, across air gap 42, up piston rod 20, out disk 28 
of dust tube 26, down cylindrical portion 30 of dust tube 26 and across 
the air gap comprising sensor winding 32, bobbin 34 and gap 38 to magnet 
36. Although this magnetic circuit includes the two air gaps described 
above, these air gaps will remain essentially constant in length as 
cylinder 12 moves axially relative to piston rod 20 and dust tube 26 and 
will thus not materially affect the flux through sensor winding 32 with 
such movement. 
Some additional magnetic flux from magnet 36 will tend to follow a path 
downward through the cylindrical outer wall of cylinder 12, the upper end 
of which ends adjacent magnet 36. However, this flux will also be confined 
to a well defined flux path within cylinder 12 that is invariable with the 
axial movement of cylinder 12 relative to piston rod 20 and dust tube 26, 
so that it will not affect the flux path through sensor winding 32. As 
long as magnet 36 is not totally shorted through cylinder 12, the MQ1 
magnet will produce sufficient flux for both flux paths from a small 
magnet. A small air gap may be provided axially between magnet 36 and the 
outer wall of cylinder 12 to prevent such magnetic shorting if the 
magnetic circuit through cylinder 12 has insufficient additional air gap 
length. However, too great an air gap may degrade sensor output and 
linearity through insufficient control of the flux below the magnet. The 
air gap required, if any, is a matter for ordinary design skill in a 
particular case. 
The flux through sensor winding 32 will not vary significantly with 
relative axial movement between cylinder 12 and dust tube 26. Therefore, 
the only variable with such movement will be the number of turns of the 
sensor winding enclosed by the flux path, between the magnet and the top 
of the sensor winding. Since the sensor winding is wound in a linear 
manner, the variation of turns with axial movement of the magnet 36, and 
thus the flux linkage, will vary in a linear manner. A voltage will thus 
be generated in sensor winding 32 proportional to the rate of change of 
flux linkage, which will accurately indicate the relative velocity of the 
sprung and unsprung masses of the vehicle. Tests of the unit described 
above have produced a 1.8 volt output at 1 meter/second relative velocity. 
Cylindrical portion 30 of dust tube 26 is preferably provided with one or 
more axial slits 50, as seen in FIG. 1. Each of slits 50 may be about 1 mm 
in width and should preferably extend in length at least to the axail ends 
of sensor winding 32. Slits 50 interrupt eddy currents induced by the 
moving magnet in dust tube 26 and thereby reduce phase lag in the output 
voltage at high frequencies. A greater number of slits will provide a 
greater reduction in eddy currents; but a very large number of slits may 
adversely affect the sensor output or the protective function of the dust 
tube. The embodiment tested included 8 slits evenly spaced around the 
outer surface of cylindrical portion 30 of dust tube 26; but the optimum 
number may be different and may vary with the requirements of particular 
applications. Slits 50 may be filled with a non-magnetic material if there 
is any chance of dust, stones, etc. getting through slits 50 and damaging 
sensor winding 32 or getting further past sensor winding 32 and bobbin 34 
into the interior of the damper.