Magnetic stroke sensor for detecting the stroke of a fluid-power cylinder

A method for easy and accurate positioning of a magnetic sensor in a fluid-power cylinder facing a magnetic scale on the lateral surface of a piston rod, wherein the sensor is supported by a hole in the cylinder, a piece of friction-resistant, non-magnetic material attached to the sensor projects from its detection surface towards the piston rod, and the non-magnetic material is made to keep in touch with the lateral surface of the moving piston rod by a pressure-applying bias. The friction-resistant, non-magnetic material is either fitted to the magnetic detection surface, or is formed into a tubular shape and fitted to the outer circumference of the sensor.

BACKGROUND OF THE INVENTION 
This invention concerns a magnetic stroke sensor installed in a fluid-power 
cylinder which detects the stroke position of the piston rod by reading a 
magnetic scale embodied in it. 
Fluid-power cylinders already exist wherein a piston rod is driven back and 
forth by means of the pressure of a fluid supplied to the cylinder, and a 
stroke sensor is fitted in order to detect the position of displacement of 
the rod. This system may take the form of, for example, a magnetic sensor 
consisting of a magnetic resistance element in the cylinder which reads a 
moving magnetic scale embedded at a given pitch on the lateral surface of 
the rod. 
The magnetic sensor is seated in a fixing hole in the cylinder wall, with 
the magnetic detection surface facing the magnetic scale at a very small 
distance away. When the piston rod moves with respect to the cylinder, the 
magnetic flux changes every time the magnetic element forming the scale 
passes in front of the sensor, and the sensor outputs an alternating 
waveform of which one cycle corresponds to one passage of the magnetic 
element. These wave cycles can then be converted to pulses by a 
comparator, of which the displacement of the piston rod can be found by 
counting. 
If however the distance between the magnetic scale and magnetic detection 
surface of the sensor facing it changes, the amplitude of the sensor 
output or its median value will change. In order to obtain a stable 
output, therefore, it is necessary to control the gap between the sensor 
surface and the scale very accurately. 
Normally, when the magnetic sensor is fitted to the cylinder, a fixing hole 
is formed in the cylinder wall by a drill or other tool, and the sensor is 
positioned by bringing a stepped area previously shaped on its rear end 
into contact with a stepped area formed inside the hole. 
The distance between the sensor and the scale is then directly affected by 
the depth of the stepped area introduced by drilling. In comparison to 
controlling the cylinder wall thickness and the dimensions of the magnetic 
sensor, however, it is not so easy to control the depth of the stepped 
area accurately. This leads either to increased installation cost of the 
stroke sensor, or to a decrease in its precision of detection and 
reliability. 
SUMMARY OF THE INVENTION 
One objective of this invention is therefore to provide easy and reliable 
positioning of the magnetic sensor with respect to the magnetic scale. 
Another objective of this invention is to ensure that the displacement of 
the sensor can follow the scale even when the piston rod is buckled due to 
load variations, and that its distance from the scale is always constant. 
Yet another objective of the invention is to permit the sensor to be 
settled accurately and easily in its fixing hole even when the cylinder 
wall is very thick and the depth of the hole is much greater than its 
diameter. 
In this invention, these effects are achieved by fixing a 
friction-resistant, non-magnetic piece of material to the magnetic sensor 
which projects from its detecting surface towards the piston rod, and by 
providing a pressure-applying means which enables the non-magnetic 
material to keep in touch with the lateral wall of the moving piston rod. 
As this piece of material fitted to the sensor is supported by the 
pressure-applying means and slides on the rod, the sensor is held at a 
fixed distance from the magnetic scale depending on the projecting 
dimensions of the non-magnetic material. It is thus not necessary to 
introduce a stepped area in order to position the sensor in its fixing 
hole, and even if the piston rod is buckled, the non-magnetic material 
continues to keep in touch with the moving rod due to the 
pressure-applying means and thereby maintains the distance between the 
sensor and the scale constant. 
When the friction-resistant, non-magnetic material is fitted to the 
magnetic sensor, it can be either fitted to the magnetic surface, or 
formed into a tubular shape and fitted over the circumference of the 
sensor. 
If a tubular piece is used, dimensional control is still easier if the rear 
edge of the tubular piece is brought into contact with a stepped area of 
large diameter formed on the rear end of the sensor. 
If a holder supporting the sensor is arranged at the rear of the sensor in 
the fixing hole, the pressure-applying means then acts on the non-magnetic 
material via the holder, thereby permitting easy assembly and disassembly 
of the magnetic sensor even when the depth of the hole is greater than its 
diameter. 
The objectives and advantages of the present invention will become further 
apparent from the attached drawings and the embodiments described below.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIGS. 1 and 8, a magnetic sensor 4 is supported in sensor fixing hole 3 
of cylinder 2 facing the magnetic scale formed on the lateral face of 
piston rod 1. Sensor fixing hole 3 is a throughhole of unique diameter 
formed in cylinder wall by means of drill cutting. 
In order to maintain the specified distance between the magnetic detection 
surface 5 of the leading edge of sensor 4 and piston rod 1, a tubular 
piece 6 is inserted free to slide in fixing hole 3 such that it fits over 
the circumference of sensor 4. Piece 6 is constructed of a 
friction-resistant, non-magnetic material such as Teflon or brass, and its 
leading edge slides on piston rod 1. 
Another part 7 of different and larger diameter is formed on the rear edge 
of sensor 4. The rear edge of tubular piece 6 is brought into contact with 
this part 7, and its length is such that its leading edge projects towards 
piston rod 1 by a specified distance from magnetic detection surface 5. 
When the leading edge of piece 6 is sliding on piston rod 1, therefore, 
the distance between magnetic detection surface 5 and the magnetic scale 
on piston rod 1 is maintained constant. 
A tubular holder 8 is arranged at the rear of magnetic sensor 4. Holder 8 
fits inside fixing hole 3 such that it is free to slide on its inner 
surface, and its leading edge is fixed by pressing over the circumference 
of the rear edge of tubular piece 6 so as to cover the outer surface of 
part 7 of sensor 4. 
In addition, as shown in FIG. 2, a pin 10 is inserted from the outer 
surface of holder 8 into a vertical groove 9 in part 7, thereby preventing 
the rotation of sensor 4 with respect to holder 8. 
The upper edge of holder 8 projects toward the outer surface of cylinder 2, 
with a stepped area 11 of larger diameter than hole 3 on the projecting 
edge. This stepped area 11 prevents holder 8 from falling through the hole 
3 together with sensor 4 and piece 6 when cylinder 2 is disassembled. 
A cover 12 is fixed to the outer surface of cylinder 2 such that it 
encloses the upper edge of holder 8. 
A spring 13 is inserted between holder 8 and cover 12 as a 
pressure-applying means which allows piece 6 to keep in touch with piston 
rod 1. By means of the recoil force of this spring, holder 8 presses piece 
6 towards piston rod 1 via magnetic sensor 4. 
Even if piston rod 1 should become buckled, therefore, the leading edge of 
piece 6 always keeps in touch with the rod 1, and the distance between 
magnetic detection surface 5 and the magnetic scale is maintained 
constant. 
By making pin 14 which projects to the upper edge coincide with hole 15 in 
cover 12, holder 8 restricts rotation. When sensor 4 is assembled, it is 
fixed to holder 8 and inserted in fixing hole 3, the sensor being set 
accurately in the correct orientation by this pin 14 and by said pin 10 
which is located between sensor 4 and holder 8. 
Further, by fixing sensor 4 to the leading edge of holder 8, it can be 
settled easily and accurately in the specified position in fixing hole 3 
even when the hole is deep and of small diameter. 
Although the above are preferred embodiments of the invention, they are not 
exhaustive, and various other design modifications are possible within the 
scope of the claims. 
For example in order to maintain a constant distance between the surface 5 
and piston rod 1, instead of the tubular piece 6, a cap 16 of 
friction-resistant, non-magnetic material can be fitted to the leading 
edge of sensor 4 as shown in FIG. 3, or a plate 17 of similar material can 
be bonded to the detection surface 5 of sensor 4 as shown in FIG. 4. 
Alternatively when the lateral wall of cylinder 2 is thin or the sensor 
hole 3 is shallow, instead of using holder 8, a ring of elastic material 
18 can be inserted between sensor 4 and cover 12 to keep the leading edge 
of piece 6 in touch with the circumference of piston rod 1, as shown in 
FIG. 5 and FIG. 6. In this case, the leading edge of piece 6 is shaped to 
the circumference of piston rod 1 as shown in FIG. 6, and piece 6 is 
bonded to sensor 4 so as to position both and prevent rotation. 
Further instead of inserting a stepped area in piece 6, its upper edge can 
be brought into contact with the part 7 of different diameter in sensor 4 
as shown in FIG. 7, part 7 being made to slide in fixing hole 3. In this 
way, the invention can be embodied in a simplified form.