Cylinder-piston unit with device for measuring the position of the piston

Abstract

 A cylinder-piston unit (1) comprising a cylinder (2) containing in its interior a piston rod (3), at least one end of which emerges from said cylinder (2), said piston rod (3) being rigid with a piston (4) sealedly slidable within said cylinder (2), there being associated with said cylinder-piston unit (1) a device for measuring the axial position assumed by said piston (4) within said cylinder (2); said measuring device comprises M (M ≥ 1) movable sensor elements (9) rigid with said piston (4) and N (N ≥ 1) fixed sensor elements (100) positioned external to said cylinder (2).

Description

[0001] This invention relates generally to a device for measuring the position of the piston rod in a hydraulic cylinder.

[0002] In many industrial applications the technical problem arises of knowing the axial position assumed by the piston rod relative to the cylinder. This technical problem has tormented technical personnel particularly of that sector relative to cylinder-piston units with a through piston rod, i.e. those units in which both ends of the piston rod emerge from the cylinder.

[0003] An applicational sector in which the problem of measuring the axial position of the piston rod is particularly felt is that of steering cylinders for operation with hydrostatic guide units, where the position of the piston rod determines the position of the wheels.

[0004] Various solutions have been attempted in this sector, comprising mechanical, electrical and optical measuring devices or systems.

[0005] Of the various systems, one of particular note is an inductive system in which a series of grooves or notches corresponding to a determined binary code is provided on the piston rod. Said grooves are then read by an inductive sensor positioned external to the cylinder. This solution has proved not totally satisfactory, mainly because of problems of cost. Another solution consists of providing on the piston rod a particular surface machining effect forming a sort of knurling which is read by an optical sensor. This solution presents problems of sealing between the cylinder and piston rod, and has proved unsuitable if the cylinder-piston unit is used in unfavourable environmental conditions.

[0006] All the known solutions, besides being deficient, have proved complicated and costly.

[0007] The object of the present invention is to overcome the drawbacks of the known art within the framework of a simple, rational, reliable and low-cost solution.

[0008] The invention attains this object by providing a cylinder-piston unit provided with a device for measuring the axial position which the piston assumes within the cylinder, said piston being connected to the piston rod, at least one end of which emerges from said cylinder.

[0009] According to the invention said measuring device comprises a fixed sensor element positioned outside the cylinder shell and able to sense the position of a movable element rigid with the piston.

[0010] In some cases the piston position has to be determined by more than one sensor to ensure detection of any individual fault in the measuring devices and to possibly enable the system to operate notwithstanding the fault. Preferably said measuring system therefore comprises several fixed sensor elements associated with the cylinder in order to measuring the position of several movable sensor elements rigid with the piston.

[0011] In detail, said movable sensor element is a magnet, whereas said fixed sensor element comprises a wave guide which is fixed onto the outer shell of the cylinder, and is connected to a transducer/sensor.

[0012] Preferably the movable sensor elements are magnets, whereas said fixed sensor elements each comprise a wave guide, which is inserted into a respective axial hole provided in the outer shell of the cylinder, and is connected to a respective transducer/sensor.

[0013] The signals sensed by the transducer/sensors are then fed to one or more control units which execute a series of predetermined operations on the basis of the axial position of the piston.

[0014] According to a preferred embodiment of the invention, said movable sensors are magnets of annular shape inserted into a respective suitable circumferential seat in the piston.

[0015] The solution in which the at least one wave guide is located in at least one axial hole provided in the wall of the cylinder satisfies the precise functional requirement of minimizing the distance between the wave guide and the movable sensor, in order to free the apparatus from disturbances which could be created by extraneous magnetic fields, and also to maximize the magnetic signal picked up by the wave guide.

[0016] The preferential solution of locating at least one wave guide in at least one axial hole in the cylinder wall also satisfies the requirement of protecting the wave guide from damage.

[0017] The said wave guide location also satisfies precise cost requirements in that it avoids the cost of applying an outer protective tube to the cylinder. In this respect it should be noted that the provision of holes in the wall for receiving the wave guide is virtually of zero cost, considering that the cylinders are produced by extrusion.

[0018] The cylinders obtained in this manner are also suitable for use with or without the invention.

[0019] It should be noted that advantageously the measuring device of the invention can be used in combination either with cylinder-piston units with a through piston rod or with cylinder-piston units in which only one end of the piston rod emerges from the cylinder.

[0020] The special characteristics of the invention are defined in the claims.

[0021] The constructional and functional characteristics of the invention will be more apparent from the ensuing description of a preferred embodiment thereof having a single movable sensor and a single fixed sensor, given by way of non-limiting example and illustrated in the accompanying drawings.

[0022] Figure 1 is an axial section through the invention.

[0023] Figure 2 is the section II-II of Figure 1.

[0024] Figure 3 is a view from III of Figure 1.

[0025] Figure 4 is a partial view of Figure 1.

[0026] Said figures show the cylinder-piston unit 1, comprising a cylinder 2 containing a through piston rod 3.

[0027] In detail., the ends of the piston rod 3 emerge from two end caps 20 and 21 of the cylinder 2, suitable seal gaskets 200 being interposed therebetween. To the centre of the piston rod 3 there is fixed a piston 4, which can slide sealedly within the cylinder 2. The piston 4 divides the interior volume of the cylinder into two chambers 40 and 41, each of which is supplied with fluid via a conduit indicated by 400 and 410 respectively.

[0028] With reference to Figure 4, the piston 4 presents in its lateral surface three annular seats indicated by the reference numerals 5, 6, 7 respectively. The central annular seat 6 houses a seal gasket 60, the annular seat 5 houses a guide shoe 50, and the annular seat 7 houses a magnetic sensor element 9 of annular shape. The sensor element 9 is a magnet forming part of the device for measuring the position of the piston with which the invention is provided. In addition to the sensor element 9, said device comprises a fixed second sensor element 10, consisting of a wave guide 100 inserted into an axial hole 201 provided in the wall of the cylinder 2.

[0029] The wave guide 100 is connected to a transducer/sensor 102 connected in its turn to a control unit, not shown. The purpose of said transducer/sensor 102 is both to generate current pulses and to sense voltage pulses.

[0030] The axial position of the piston 4 is measured by virtue of the interaction of two magnetic fields, one of which is generated by the sensor element 9 and the other by a current pulse which is induced in the wave guide 100 and flows through it. The interaction of said magnetic fields produces in the wave guide 100 a mechanical torsion which propagates along the wave guide and is sensed by the transducer/sensor 102 fixed to one end of the wave guide 100.

[0031] By measuring the time which passes between the inducement of the current pulse in the wave guide 100, also known as the interrogation pulse, and the reception by the transducer/sensor 102 of the signal generated by the interaction of the two magnetic fields, the axial position of the piston 4 can be determined.

[0032] The transducer/sensor 102 feeds said pulse to the control unit, which executes a series of predetermined operations based on the axial position assumed by the piston 4 within the cylinder.

[0033] A typical applicational variant, given by way of non-limiting example, comprises 3 fixed sensors and 2 movable sensor elements.

[0034] Said variant is not described in detail, being totally evident to an expert of the art.

Claims

1. A cylinder-piston unit (1) comprising a cylinder (2) containing in its interior a piston rod (3), at least one end of which emerges from said cylinder (2), said piston rod (3) being rigid with a piston (4) sealedly slidable within said cylinder (2), there being associated with said cylinder-piston unit (1) a device for measuring the axial position assumed by said piston (4) within said cylinder (2), characterised by comprising at least one movable sensor element (9) rigid with said piston (4) and at least one fixed sensor element (100) positioned in at least one respective axial hole (201) provided in the cylinder wall.

2. A unit as claimed in claim 1, characterised in that said at least one movable sensor is a magnetic sensor.

3. A unit as claimed in claim 2, characterised in that said magnetic sensor is a magnet of annular shape.

4. A unit as claimed in claim 3, characterised in that said magnet of annular shape is housed in a corresponding matching annular seat (7) provided in the lateral surface of said piston (4).

5. A unit as claimed in claim 1, characterised in that said at least one fixed sensor (100) is connected to at least one control unit which controls the execution of determined operations on the basis of the axial position which the piston (4), and hence the piston rod (3), assume within said cylinder (2).

6. A unit as claimed in claim 1, characterised in that each of said fixed sensor elements (100) comprises a wave guide (100) connected to a respective transducer/sensor (102).

7. A unit as claimed in claim 6, characterised in that said at least one wave guide (100) is inserted in a tube fixed to the outside of the cylinder (2).

8. A unit as claimed in claim 6, characterised in that said wave guides (100) are connected to said control units via said transducer/sensors (102).

9. A unit as claimed in claim 6, characterised in that said transducer/sensors (102) each comprise at least one electrical pulse generator and at least one mechanical pulse detector.

Drawing