A HYDRAULIC CYLINDER WITH A BUILT-IN ELECTRIC PISTON POSITION INDICATOR

Abstract: A hydraulic cylinder (1) comprising a cylinder housing (3) and a piston rod (7) with a through opening (7a) is provided with a built-in inductive piston position indicator (20) extending through the opening (7a). The housing of this indicator is secured to one end cover (5) of the hydraulic cylinder, and the core of said indicator is secured to the piston rod (7) by way of a rod. When the piston (6) moves a distance out of its bottom position the core of the piston position indicator (2) moves a corresponding distance which causes an electric signal to be transmitted by the piston position indicator (20), which signal corresponds to the moved distance of the piston (6).

Description: Title: A Hydraulic Cylinder With a Built-in Electric Piston Position Indicator. Technical Field The invention relates to a hydraulic cylinder with a built- in electric piston position indicator comprising two mu¬ tually movable members and transmitting an electric signal corresponding to the mutual position of said members. Background Art Several types of hydraulic cylinders of this type are known which comprise built-in electric piston position indicators transmitting a signal corresponding to the position of the piston of the cylinder. One of these cylin¬ ders comprises a housing including two coaxially extending pipes with an inductance coil therebetween. The piston of the hydraulic cylinder operates thereby as a core whereby the inductance coil transmits electric signals correspond¬ ing to the position of the core. It is, however, a condi¬ tion that the inner pipe in the housing of the hydraulic cylinder is made of a non-electrically conductive material. In order to stand up to the normal working pressures of 2-3-00 bar within the hydraulic cylinders and pressure peaks of up to 6-800 bar it is necessary to manufacture the inner pipe with a large thickness which increases the dimensions of the cylinder drastically. Furthermore as the inductance coil and the outer pipe increase the dimen¬ sions of the cylinder this cylinder is of a dispropor ion¬ ately large diameter compared to the actual diameter of the piston and consequently to the actual output of the cylinder. In addition these cylinders are very sensitive to electric noise signals from the outside which may cause an incorrect feedback from the piston position indicator. At worst the latter may cause breakdown of the equipment having the hydraulic cylinder built therein. Description of the Invention The object of the invention is to provide a hydraulic cylinder of the above type which can stand up to the high working pressures usually applying to hydraulic cylinders without involving larger outer dimensions than correspond¬ ing cylinders not including built-in position indicators, and which furthermore is reliable in use and inexpensive to manufacture. The hydraulic cylinder according to the invention is char- acterised in that the electric piston position indicator extends through a longitudinal opening in the piston rod and piston of the hydraulic cylinder, and that one movable member of the piston position indicator is connected to the housing of the hydraulic cylinder and the other movable member is connected to the piston rod of the hydraulic cylinder or vice versa. In this manner a hydraulic cylinder is provided which comprises a built-in piston position indicator, and where neither the cylinder nor the piston position indicator is affected by the high working pres- sures usually applying to hydraulic cylinders. At the same time the hydraulic cylinder is neither of larger outer dimensions nor manufactured by other and stronger materials than a corresponding cylinder not including a built-in position indicator. The electric signal transmit- ted by the position indicator is furthermore not disturbed by electric noise signals from the outside. The electric position indicator transmits constantly an electric signal corresponding to the actual position of the piston of the hydraulic cylinder within the cylinder housing. When the piston of the cylinder moves a short distance out of one of its extreme positions in the cylin¬ der housing the movable member of the piston position indicator - said member being connected to the piston rod - move a corresponding distance from a reference point and the piston position indicator transmits an electric signal of a magnitude corresponding to the moved distance of the piston. A simple signal processing of the signal transmitted by the position indicator in a conventional signal processing equipment, such as a PLC-control or an industrial microcomputer, allows an easy determination of the position of the piston inside the cylinder. The signal processing equipment can simultaneously transmit a feedback to other control means, e.g. for controlling the hydraulic oil. The microcomputer can thus be preprogrammed to ensure that the hydraulic cylinder performs a detailed movement sequence determined in advance. According to the invention the electric piston position indicator may be an inductive position indicator, where the two mutually movable members of the indicator are preferably formed by a core shaped of an electrically conductive material and a tubular housing, an induction coil extending in the entire length of said tubular hous¬ ing, whereby the core is allowed to reciprocate freely in said induction coil. Such position indicators are particu¬ larly reliable and accurate, and the electric signal of the position indicator is particularly simply further processed in the signal processing or adjusting equipment. Furthermore according to the invention the housing of the position indicator may be connected to the housing of the hydraulic cylinder, preferably by way of a pressure and liquidtight connection in the end cover of the hydraulic cylinder, and the core of the position indicator may be connected to the piston rod of the hydraulic cylinder, preferably by means of a rod screwed into the outer secur¬ ing means fixed on the end of the piston rod of the hy¬ draulic cylinder. In this manner a particularly simple securing of the position indicator is obtained as it is particularly easy and quick to replace said indicator, e.g. at service check. As a result the conduits always extending out of one end of the housing of the position indicator can in a simple manner extend outwards directly through the end cover of the cylinder, said housing of the position indicator being secured in the end cover of the hydraulic cylinder. Furthermore according to the invention the housing of the position indicator may extend into the end covers of the hydraulic cylinder, whereby the movement of the core is limited to an area in the housing of the position indicator spaced from the ends of said position indicator. In this manner the core can only move inside the most exact measur¬ ing area of the position indicator, said core never moving in the measuring areas at the ends of the position Indicat- or, which as far as inductive position indicators are con¬ cerned are the most inaccurate or "unlinear" areas. In addition according to the Invention the opening In the piston of the hydraulic cylinder may be pressure and II- quidtightly closed towards the interior of the cylinder, preferably by means of gaskets beteween the piston and the housing of the position indicator. In this manner the area of the position indicator - in which the piston rod thereof extends through the housing thereof - Is not af¬ fected by the pressure and the oil of the hydraulic cylin- der. Moreover according to the invention the cylinder may be provided with one or more electric position indicators, whereby each indicator indicates a predetermined position of the piston, and the electric signal transmitted by these position indicators may co-operate with the signal of the pistion position indicator In achieving a calibra¬ tion or zeroing of the signal of the piston position indi¬ cator each time the piston of the hydraulic cylinder Is in one of these predetermined pistion positions. 'As a result a particularly accurate measuring of the piston position is obtained as the signal of the piston position indicator is calibrated each time the piston of the hy¬ draulic cylinder is at one of the predetermined piston positions . Finally according to the invention the electric position indicators may be inductive sensors provided in openings in the end covers of the cylinder, said openings being pressure and liquidtightly closed towards the interior of the cylinder. In this manner the cylinder is particularly simple to manufacture, and the signals transmitted by the inductive sensors are particularly suited for co-operating with signals transmitted by other electric position indi¬ cators . Brief Description of the Drawing The invention will be described below with reference to the accompanying drawing, in which Fig. 1 is an axial sectional view of an embodiment of a hydraulic cylinder according to the invention with a built- in inductive piston position indicator, where inductive sensors are situated in openings in each end cover of the cylinder , Fig. 2 illustrates the inductance piston position indicator of Fig. 1, partly in section, and Fig. 3 illustrates on a larger scale a portion of a second embodiment of a hydraulic cylinder according to the inven¬ tion. Description of the Preferred Embodiments of the Invention The hydraulic cylinder 1 of Fig. 1 comprises a cylinder housing 3, an end cover 4, 5 secured in each end of the cylinder housing 3 preferably by means of gluing or weld¬ ing, and a piston 6 present in the cylinder housing 3 and mounted on a piston rod 7 extending through a through opening in one end cover 4. The cylinder 1 has a stroke A and Is illustrated with the piston in the bottom position. The end cover 5 of the cylinder 1 and the end of the piston rod 7 opposite the piston are provided, preferably by way of welding, with outer securing means 8 and 9 for the outward securing of the cylinder housing 3 and the piston rod 7. Furthermore, the hydraulic cylinder 1 is provided with in- and outlet gates 10, 11 for hydraulic oil, whereby the reciprocating movement of the piston 6 through the cylinder 1 can be controlled. The piston 6 is furthermore provided with one or more circumferential recesses 12 receiving rings preferably of rubber for the sealing tow¬ ards the inner surface of the cylinder housing 3. The piston rod 7 extends through one 4 of the end covers of the cylinder 1, and said end cover ' 4 is also provided with one or more circum erential recesses 13 receiving the rubber ring, whereby the interior of the cylinder is pressure and liquidtightly closed. The piston rod 7 is preferably a steel pipe through which a piston position indicator 20 extends, cf. Figs. 1 and 2. The piston position indicator of an advantageous embodi¬ ment according to the invention is an inductive position indicator 20 comprising an outer housing 21 in the form of a steel pipe. The inductive position indicator com¬ prises furthermore a rod 28 extending from one end of the housing 21. The outer end of this rod 28 is provided with threads 29 and the inner end thereof Is provided with a core 22 of an electrically conductive material. An in¬ ductance coil 23 extends upwards along the inner surface of the housing and in the entire length of the housing 21. Electric signals of volumes corresponding to the dist¬ ance B or C of the core 22 from predetermined reference positions are transmitted by the position indicator 20 through conduits 27 in response to the position of the core 22 in said position indicator. The inductive position indicator 20 is preferably of the type described in Danish Patent Application No. 335/85. By the preferred embodiment of the hydraulic cylinder 1 according to the invention the housing of the piston posi¬ tion indicator 20 is secured by means of a Seeger ring 25 in the end cover 5 and is pressure and liquid ightly closed therein by means of a gasket 26. The rod 28 of the position indicator 20 is screwed into the securing means 8 by means of the threaded connection 29, said securing means 8 being secured on the outer end of the piston rod 7. When the piston 6 of the cylinder 1 moves a short distance within the cylinder 1, e.g. its total stroke A, the core 22 of the piston position indicator 20 moves a corresponding distance within the housing 21. The piston position indi¬ cator 20 transmits therefore an electric signal of a magni¬ tude corresponding to the moved distance of the piston 6 of the hydraulic cylinder 1, e.g. out of its bottom posi¬ tion. By the advantageous embodiment of a hydraulic cylinder 1 shown in Fig. 1 inductive sensors 15 are provided in- through openings 18 in the end covers 4, 5 ' of the cylinder 1. The piston 6 of the cylinder 1 is furthermore made completely or partially of an electrically conductive material. The openings 18 are pressure and liquidtightly closed towards the interior of the cylinder 1 by means of preferably ceramic plates, e.g. as described In Danish printed accepted specification No. 150.225. Fig. 3 shows how the ceramic plate 17 is situated in the end cover 5. The inductive sensors 15 emit a magnetic field extending through the ceramic plates 17 and into the interior of the cylinder 1. The magnetic field is altered when the piston 6 enters therein and is situated immediately adjac- ent its extreme position at the end cover 4 or 5. The latter causes transmission of an altered electric signal from the sensor 15. As illustrated in Figs. 1 and 3 the conduits 16 and 27 extend from the inductive sensors 15 and the piston posi¬ tion indicator 20, respectively, and out of the end covers 4 and 5 of the cylinder through the openings 18. The con¬ duits 16 and 27 are optionally connected through an inter¬ face or a signal amplifier to a signal processing or proces controlling equipment, e.g. a PLC control, IPC control, a PC or an industrial microcomputer. Each time the piston 6 is at one of its extreme positions at the end covers 4 and 5, the altered electric signal emitted by the Inductive sensors 15 is used to cause a very accurate zeroing or calibration of the signal from the inductance piston posi¬ tion indicator 20. In this manner it is never necessary to measure directly on the cylinder in order to determine the exact position of the piston. It can also be considered an extra security that an unambigous signal independent of the piston position indicator is emitted each time the piston 6 is " at one of its extreme positions. Fig. 3 illustrates an embodiment substantially correspond¬ ing to the embodiment of Fig. 1, apart from the feature that circum erential recesses are shaped inside the piston rod 7 or the piston 6. Gaskets 14 are situated in these recesses and close pressure and liquidtightly towards the interior of the cylinder 1. In this manner the area of the piston position indicator 20 - in which the rod 28 leaves the housing 21 - is not affected by the pressure nor the oil present in the cylinder 1. In Fig. 3 the hous¬ ing 21 of the piston position indicator 20 is furthermore shown In a position where it is secured in the end cover 5 by means of two screws 24. In the embodiment of Fig. 1 the piston position Indicator 20 is furthermore extending a distance into the end cover 5 and through the end cover 4. In this manner the core 22 of the piston position indicator 20 reciprocates only within an area corresponding to the distance A and cannot at all move into the extreme areas of the piston position indicator 20. As a result the best possible measuring accuracy is obtained for the inductive position indicator 20 as it is generally known that the most accurate measur¬ ing length of these position indicators is approx. the center 70% of the measuring length. Outside this length the linearity between the signal transmitted and the dist¬ ance measured can decrease considerably. The hydraulic cylinder 1 according to the invention can in a particularly advantageous manner be incorporated in painting robots, welding robots, other corresponding types of processing or tool machinery, or machinery as such carrying out a preprogrammed movement sequence. Many of these applications imply that the piston 6 of the cylinder constantly moves various distances out of and back to the bottom position. An obvious advantage is therefore obtained by the inductance sensor 15 transmitting altered electric signals, said signals being used for calibrating the meas¬ uring length of the inductive position indicator 20 each time the piston 6 returns to the bottom position. The desired movement sequence can be preprogrammed in the signal processing equipment, such as the microcomputer, which thereby transmits signals to further equipment, such as the hydraulic " equipment, allowing the hydraulic oil to flow to the gate 11 when the piston 6 is in its bottom position, and reversingly hydraulic oil to flow to the gate 10 when the piston 6 has moved an arbitrary pre¬ determined distance corresponding to the electric signal received from the inductive position indicator 20. It should furthermore be emphasized that the hydraulic cylinder 1 according to the invention is neither of larger outer dimensions nor is made of other or stronger mate¬ rials than a conventional hydraulic cylinder comprising neither built-in piston position indicators nor inductive sensors. As a result the cylinder Is simultaneously inex- pensive, quick, and easy to manufacture. Many modifications can, however, be carried out without thereby deviating from the scope of the invention. Other types of electric piston position indicators may for inst¬ ance be used, such as a capacitor including movable elec- trodes , or the electric position Indicators indicating predetermined piston positions can be built into the cylin¬ der or situated on the outside thereof on other locations than at the end covers.

Claims: Claims : 1. A hydraulic cylinder (1) with a built-in electric piston position indicator (20) comprising two mutually movable members (21, 22) and transmitting an electric signal corresponding to the mutual position of said mem¬ bers, c h a r a c t e r i s e d in that the electric piston position indicator (20) extends through a longitu¬ dinal opening (7a.) in the piston rod (7) and piston (6) of the hydraulic cylinder (1) , and that one movable member (21, 22) of the piston position indicator is connected to the housing (3) of the hydraulic cylinder and the other movable member (22, 21) is connected to the piston rod (7) of the hydraulic cylinder or vice versa. 2. A hydraulic cylinder as claimed in claim 1, c h a r a c t e r i s e d in that the electric piston position indicator (20) is an inductive position indicator, where the two movable members (21, 22) of the indicator are preferably formed by a core (22) shaped of an electric¬ ally conductive material and a tubular housing (21) , an induction coil (23) extending in the entire length of said tubular housing, whereby the core (22) is allowed to reciprocate freely in said induction coil. 3. A hydraulic cylinder as claimed in claim 1 or 2, c h a r a c t e r i s e d in that the housing (21) of the position indicator (20) is connected to the housing (3) of the hydraulic cylinder (1) , preferably by way of a pressure and liquidtight connection (26) in the end cover (5) of the hydraulic cylinder (1) , and that the core (22) of the position indicator is connected to the piston rod (7) of the hydraulic cylinder (1) , preferably by means of a rod (28) screwed into the outer securing means (8) fixed on the end of the piston rod (7) of the hydraulic cylinder (1) ■ 4. A hydraulic cylinder as claimed in claim 2 or 3 , c h a r a c t e r i s e d in that the housing (21) of the position indicator (20) extends into the end covers (4, 5) of the hydraulic cylinder (1) , whereby the movement of the core (22) is limited to an area in the housing (21) of the position Indicator (20) spaced from the ends of said position indicator. 5. A hydraulic cylinder as claimed in one or more of the claims 1-4, c h a r a c t e r i s e d in that the opening (7a.) of the pistion (6) of the hydraulic cylinder (1) is pressure and liquidtightly closed towards the inte¬ rior of the cylinder (1) , preferably by means of gaskets (14) between the piston (6) and the housing (21) of the position indicator (20) . 6. A hydraulic cylinder as claimed In one or more of the preceding claims , c h a r a c t e r i s e d in that the cylinder (1) furthermore is provided with one or more electric position indicators (15) , whereby each indicator indicates a predetermined position of the piston, and that the electric signal transmitted by these position Indicators (15) co-operates with the signal of the pistion position indicator (20) in achieving a calibration or zeroing of the signal of the piston position indicator each time the piston (6) of the hydraulic cylinder (1) is I one of these predetermined pistion positions. 7. A hydraulic cylinder as claimed in claim 6 and com¬ prising a piston (6) containing completely or partially an electrically conductive material, c h a r a c t e r¬ i s e d in that the electric position indicators are inductive sensors (15) provided in through openings (18) in the end covers (4, 5) of the cylinder (1) , said openings (18) being pressure and liquidtightly closed towards the interior of the cylinder (1) , preferably by means of ceram¬ ic plates (17) .

Inventors: HVILSTED NIELS, PEDERSEN KAJ, HVILSTED, NIELS, PEDERSEN, KAJ
Applicants: HVILSTED NIELS, PEDERSEN KAJ, HVILSTED, NIELS, PEDERSEN, KAJ
Application year: 1988
Main classification: F15B15
IPC codes: F15B15, G01B7