DE20308810U1 - Piston-cylinder unit, especially for a motor vehicle braking system or hydraulic coupling, has a permanent magnet mounted on a support surface on the piston with a securing element to secure it axially - Google Patents

Abstract

The piston-cylinder unit (2), especially for a hydraulically operated coupling and braking system in a motor vehicle has a cylinder (4), a piston (6) that moves axially within it and a sensor system (19) for sensing piston position. The sensor system has a permanent magnet (20) signaler attached to a support surface (23) on the piston and a separately arranged securing element (28) for supporting the magnet.

Description

Reg. No. 15753 -4-

Piston-cylinder unit with a sensor system
5

Description

The invention relates to a piston-cylinder unit according to the preamble of claim 1.

Such a piston-cylinder unit is known, for example, from DE 199 15 832 A1 and is used, for example, to operate a hydraulic clutch or brake device. The piston-cylinder unit, which in this example represents a master cylinder, is activated via the brake or clutch pedal, which is connected to the piston rod of the cylinder. The hydraulic pressure generated in the master cylinder is transmitted via a line system filled with hydraulic fluid to a slave cylinder, which causes a working piston to move there and can thus, for example, actuate a clutch release. The piston-cylinder unit mentioned also has a sensor system which is used to detect the position of the piston within the cylinder. For this purpose, an annular groove open to the inner wall of the cylinder is formed on the outer circumference of the piston, on which an annular permanent magnet is arranged, which can be axially displaced with the piston along the inner wall of the cylinder. A receiving part is attached to the outer wall of the cylinder housing, in which two Hall switches are arranged, the axial distance of which corresponds to a starting and an end position of the piston and which respond to the magnetic field of the permanent magnet on the piston and generate a

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Trigger a switching process or an electrical signal. To pass the signal on to control or monitoring structures, the receiving part has an electrical connection contact part.

The publication in question does not provide any detailed information on how the permanent magnet is attached to the piston. However, based on the single figure, it can be assumed that the piston is manufactured in a first step and that in a subsequent step the ring magnet is pushed from the side of the piston facing away from the pressure chamber into the ring groove of the piston. The disadvantage of this is that with this assembly of piston and permanent magnet, even with the greatest care, it cannot be ruled out that the permanent magnet will subsequently become detached from its mounting position on the piston as a result of sudden, shock-like loads on the piston, and the position of the piston may therefore be incorrectly detected. In extreme cases, the permanent magnet can burst, which can block the piston and thus lead to failure of the hydraulic system.

Based on the above-mentioned prior art, the invention is based on the object of proposing a piston-cylinder unit with a sensor system, in which the permanent magnet is arranged in particular in an axially secured manner on a piston and at the same time can be mounted on it in a simple and cost-effective manner.

The above-mentioned object is achieved by the features specified in the characterizing part of claim 1. Advantageous further developments and embodiments of the invention are specified in the subclaims.

According to the invention, the permanent magnet is arranged in such a way that the permanent magnet rests with one surface on a contact surface formed on the piston and is supported with the other end face on a securing element arranged separately on the piston. This ensures secure axial

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The permanent magnet can be fixed on both sides. A wide variety of axial locking elements are known and can also be easily mounted on the piston without significant costs.

In a further development of the invention, claim 2 provides for the securing element to be designed as a self-locking securing ring, which has at least one locking tongue that is supported on a peripheral surface of the piston. When assembling the permanent magnet, it can be axially fixed by simply placing a self-locking securing ring on, for example, an axial extension of the piston that extends over the permanent magnet. The self-locking reliably prevents the securing ring and thus the magnet from being accidentally released in the opposite direction to the assembly direction.

According to claim 3, a locking effect of the securing element can be realized alternatively or in addition to self-locking if a recess is formed on the piston into which the locking tongue engages and latches.

In a further advantageous embodiment of the invention according to claim 4, at least one spring tongue is provided on the securing element, against which the permanent magnet rests resiliently and is therefore always in contact on the opposite side with a contact surface on the piston, which at the same time represents a reference surface for the sensor system. However, the spring tongues are also effective in another way. In the case of a pulse-like load on the piston in the axial direction, in particular from the outside on the free end of the piston carrying the permanent magnet, the permanent magnet, which is usually made of a brittle material, can be protected from damage by the force introduced onto the piston not being passed directly to the permanent magnet, but first via the spring tongue and then passed on to the magnet. Due to inertia, the

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Permanent magnet initially remains stationary, whereby it performs a slight relative movement on its guide in the direction of the spring tongues. On the spring tongue
The impact energy is briefly absorbed by elastic deformation and then released with a delay, which causes the magnet to weaken the
input pulse.

According to claim 5, it is advantageous if the securing element has several spring and locking tongues distributed around the circumference. The static support forces and dynamic impact forces acting on the permanent magnet can thus be evenly distributed over the effective surface of the permanent magnet.

Claim 6 proposes forming centering surfaces facing each other for the mutual radial position assignment of the piston and the permanent magnet. This measure also serves in particular to facilitate the assembly of the piston-cylinder unit.

To protect the permanent magnet even better against impact, claims 6 and 7 provide for the formation of an axially extending sleeve section on the securing element, which surrounds the permanent magnet on the peripheral surface facing away from the piston or encases the piston, and wherein the sleeve section, together with a radial section on which the locking and holding tongues are arranged, forms an essentially closed receiving space for the permanent magnet. Should the permanent magnet nevertheless be damaged by an unforeseen load, the individual fragments remain safely arranged within the receiving space and cannot pass into the hydraulic system. In such a case, fragments of the permanent magnet remain essentially spatially fixed, which also means that the sensor system is only slightly affected in its function. In this way, the permanent magnet is reliably encapsulated for its entire service life.

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With further advantage, the sleeve section rests with its free end on
an axial surface on the piston. This measure is particularly important with regard to
to an unexpected breakage of the permanent magnet, since otherwise a force connection between the securing element and the magnet
can be reduced.

The safety element is advantageously made from a material that is essentially non-magnetically conductive, e.g. stainless steel, as a thin-walled deep-drawn part. By using such a material, the magnetic field emanating from the permanent magnet is influenced as little as possible. This maintains the steep gradients of the magnetic field in particular, which is advantageous for reliable sensing of the magnetic field during very fast piston movements. By designing it as a thin-walled sheet metal part, the safety element can be designed to be extremely space-saving and the permanent magnet can be arranged very close to magnetic field sensors.

The invention is explained in more detail below using an embodiment with reference to the attached figures. In the figures, identical parts are provided with the same reference numbers. They show:

Fig. 1 is a schematic representation of a master cylinder

Piston-cylinder unit with a sensor system;

Fig. 2a-c a representation of a piston with a permanent magnet arranged by means of a securing element;

Fig. 3 shows a representation of a securing element for mounting a Per

permanent magnets on a piston.

For a general understanding of the invention, Figure 1 shows a schematic representation of a piston-cylinder unit 2 designed as a master cylinder of a hydraulic clutch actuation system of a motor vehicle, which essentially consists of a plastic cylinder 4 and an axially parallel

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The piston 6 is connected to a piston rod 8 which is connected to a clutch pedal. The master cylinder 2 is operatively connected via a hydraulic system to a slave cylinder which in turn can actuate a clutch release mechanism by means of a piston rod connected to a piston.

The front side of the piston 6 and the inner wall of the cylinder 4 define a variable pressure chamber 12, which is sealed from the environment with ring lip seals 14a, 14b in relation to the movable piston 6. The cylinder 4 is designed in two parts and has a front housing part 16 and a rear guide tube 17. The housing part 16 comprises a connection channel 18a, which is in flow connection with a slave cylinder via a fluid line (not shown). Another fluid channel 18b communicates with a reservoir (not shown) for the hydraulic fluid. In order to ensure that hydraulic fluid continues to flow into the hydraulic system, a plurality of recesses 11, so-called sniffer grooves, are arranged on the piston in the circumferential direction. A piston shaft sleeve, which is common in some variants, is not provided in this piston 6, so that the plastic piston 6 is in direct sliding contact with the inner cylinder wall and the seals 14a, b.

The master cylinder 2 is equipped with a sensor system 19 for detecting the axial position of the piston 6. For this purpose, the piston has a ring-shaped permanent magnet 20 which is arranged on an extension 21 extending axially from the base body of the piston 6 and having a smaller diameter than the base body. An alloy of the iron-neodymium-boron (FeNdB) material system is particularly suitable as a material for the permanent magnet 20, as it can produce a particularly high magnetic flux density.

To protect against corrosion, the surface of the permanent magnet is coated with nickel. The permanent magnet 20 is supported on a

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The permanent magnet 20 is secured against loss axially on both sides by a contact surface 23 formed on the base body and a thin-walled securing element 28 made of sheet metal with a material thickness of approximately 0.2...0.3 mm, which is arranged separately on the axial extension 21. This holds the permanent magnet 20 in its position, particularly when forces are applied from the outside to the piston end shown.

The exact arrangement of the permanent magnet 20 on the piston 6 and the design of the securing element 28 are shown in Figures 2a-c and 3.

The sensor system 19 further comprises a sensor housing 22 mounted on the outside of the cylinder 4, in which two magnetic field-sensitive Hall sensors or reed contacts 24 and 26 are contained. Alternatively, continuously operating capacitive or inductive displacement sensors can also be used as sensors. The axial positioning of the permanent magnet 20 on the piston 6 is in principle freely selectable, but in the position shown the axial distance of the magnet 20 to the pressure chamber 12 is minimized, which results in a design advantage in the positioning of the sensors 24, 26 and the sensor housing 22, which can be designed in a space-saving manner on the front housing part 16 and thus directly axially in the area of the pressure chamber 12.

The axial distance between the sensors 24, 26 corresponds to a starting and an end position of the piston 6. When the piston 6 moves axially along the inner wall of the cylinder 4, the sensor 24 or 26 located axially at the height of the permanent magnet 20 is detected by the magnetic field of the permanent magnet 20 and then carries out a switching movement and/or generates an electrical signal which is forwarded to an evaluation unit for further processing.

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Figures 2a-c show an end region facing the pressure chamber 12 of a cylindrical piston 6 injection-molded from a plastic, preferably from a thermosetting plastic material or alternatively a thermoplastic material, with an axial extension 21, the cylinder surface 30 of which serves as a centering surface for the ring-shaped permanent magnet 20 and which rests against a centering surface 32 formed on the latter. The securing element 28 for fixing the position of the permanent magnet 20 is designed as a self-locking securing ring 28 with a central recess 34 receiving the axial extension 21 of the piston 6, as shown in Fig. 3. To create a locking effect, the locking ring 28 has a plurality of locking tongues 38 distributed in the circumferential direction and directed radially inward on a radial section 36, the free ends 40 of which define an inner diameter DS in the unassembled initial state that is slightly smaller than the diameter DA of the axial extension 21. When the locking ring 28 is assembled, the locking tongues 38 spread slightly on the outer surface 30 of the axial extension 21 against the assembly direction and remain in this pre-tensioned position in the assembled state. To reinforce the locking effect, it is possible, as shown in Fig. 2c, to form a circumferential groove 44 on the axial extension 21, in which the locking tongues 38 engage in a locking manner.

The securing element 28 also has spring tongues 46 in the circumferential direction, which are evenly distributed between the locking tongues 38. The spring tongues 46 rest against the permanent magnet 20 under a pre-tension and thus always hold it in contact with the axial surface 23, which represents a reference surface for the sensor system 19. The spring tongue 46 is shown in Figure 2a in an unloaded starting position and in Figure 2b in a pre-tensioned installation position. At the same time, this arrangement also allows pulse-like changes in the movement of the piston 6 to be better absorbed without exposing the permanent magnet 20 to the risk of destruction. In the event of a force pulse acting via the clutch and the slave cylinder, the permanent magnet 20 can remain in space for a short time. As a result of the movement of the piston, a slight relative movement of the permanent magnet 20 occurs in-

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within the axial movement range of the spring tongues 46 on the axial extension 21. The diameter DF formed by the spring tongue ends 48 is slightly larger than that of the axial extension DA, whereby the spring tongues 46 arranged adjacent to the locking tongues 38 through slots 50 can be freely pivoted relative to the outer surface 30 of the axial extension 21.

The radial section 36 of the retaining ring 28 is followed by an axially extending sleeve section 52 which envelops the permanent magnet 20. The sleeve section 52 is supported with its free end 54 on the axial surface 23 arranged on the piston 6, just like the permanent magnet 20, and together with the radial section 36 forms a substantially closed receiving space 56 for the permanent magnet 20. When the permanent magnet 20 is assembled, any clearance that may be present is compensated for by the elastic spring tongues. So that the magnetic field of the permanent magnet 20 can penetrate the receiving space 56 to the outside, the securing element 28 is made of stainless steel or generally of a material that is essentially non-magnetically conductive. The retaining ring 28 can be manufactured particularly cost-effectively using punching and deep-drawing techniques and mounted on the piston 6 using an automated manufacturing process.

It should not go unmentioned that, alternatively, instead of the ring magnet 20, a disk-shaped or rod-shaped permanent magnet 20 can also be arranged on or within a cylindrical recess of the piston, wherein the securing element 28 can also be structurally adapted in a simple manner.

Reg. -Number 1 5753 -13- »·
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&bull;■ 2 Piston-cylinder unit 4 cylinder 5 6 Pistons 8th Piston rod 10 Ball joint 11 Sniffernut 12 Printing room 10 14a ,b Ring lip seal 16 front housing part 17 rear guide tube 18a , 18b Connection channel 19 Sensor system 15 20 Permanent magnets 21 Axial process 22 Sensor housing 23 Contact surface 24, 26 Sensors (24, 26) 20 28 Securing element 29 Evaluation unit 30, 32 Centering surface 34 Central recess 36 Radial section 25 38 Locking tongue 40 Locking tongue end 44 Groove 46 Spring tongue 48 Spring tongue end 30 50 slot 52 Sleeve section 56 Recording room 58 End area

Reg. No. 15753

DS, DA, DF diameter

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Claims (11)

1. Piston-cylinder unit ( 2 ), in particular for a hydraulically operated clutch or brake system on a motor vehicle, with a cylinder ( 4 ), a piston ( 6 ) axially movable therein and a sensor system ( 19 ) for sensing the position of the piston ( 6 ) in the cylinder ( 2 ), which comprises a permanent magnet ( 20 ) arranged on the piston ( 6 ) and at least one stationary magnetic field-sensitive sensor ( 24 , 26 ), characterized in that it is supported axially on both sides by a bearing surface ( 23 ) and by a securing element ( 28 ) arranged separately on the piston ( 6 ). 2. Piston-cylinder unit according to claim 1, characterized in that the securing element ( 28 ) is designed as a self-locking securing ring with at least one locking tongue ( 38 ) which is supported on a peripheral surface ( 30 ) of the piston ( 6 ). 3. Piston-cylinder unit according to claim 2, characterized in that the locking tongue ( 38 ) is locked in a recess ( 44 ) formed on the piston ( 6 ). 4. Piston-cylinder unit according to one of the preceding claims, characterized in that the securing element ( 28 ) has at least one spring tongue ( 46 ) for contact with the permanent magnet ( 20 ). 5. Piston-cylinder unit according to one of the preceding claims, characterized in that the securing element ( 28 ) has a plurality of spring and locking tongues ( 38 ; 46 ) distributed around the circumference. 6. Piston-cylinder unit according to one of the preceding claims, characterized in that for the mutual radial position assignment of the piston ( 6 ) and the permanent magnet ( 20 ) have centering surfaces ( 30 , 32 ) facing each other. 7. Piston-cylinder unit according to one of the preceding claims, characterized in that the locking tongues ( 38 ) and the spring tongues ( 46 ) are arranged on a radially extending portion ( 36 ) of the securing element ( 28 ). 8. Piston-cylinder unit according to one of the preceding claims, characterized in that the securing element ( 28 ) has an axially extending sleeve section ( 52 ) which surrounds the permanent magnet ( 20 ), and wherein the sleeve section ( 52 ) together with the radial section ( 36 ) forms a substantially closed receiving space ( 56 ) for the permanent magnet ( 20 ). 9. Piston-cylinder unit according to one of the preceding claims, characterized in that the sleeve section ( 52 ) is supported with its free end ( 54 ) on an axial surface ( 23 ) on the piston ( 6 ). 10. Piston-cylinder unit according to one of the preceding claims, characterized in that the securing element ( 28 ) is formed from a substantially magnetically non-conductive material. 11. Piston-cylinder unit according to one of the preceding claims, characterized in that the securing element ( 28 ) is designed as a thin-walled deep-drawn part.