EP0306507B1

EP0306507B1 - A hydraulic cylinder with a built-in electric piston position indicator

Info

Publication number: EP0306507B1
Application number: EP19880902110
Authority: EP
Inventors: Niels Hvilsted; Kaj Pedersen
Original assignee: Individual
Current assignee: Individual
Priority date: 1987-02-20
Filing date: 1988-02-11
Publication date: 1991-07-17
Also published as: FI884853A0; EP0306507A1; FI884853A; DK88687A; DK88687D0; JPH01502298A; DK165562C; WO1988006243A1; DK165562B; BR8805634A; AU1393088A

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

The invention relates to a hydraulic cylinder with a built-in inductive piston position indicator transmitting an electric signal corresponding to the actual piston position, said piston position indicator being received in a longitudinal opening extending in the piston and piston rod of the hydraulic cylinder and comprising a core made of an electrically conductive material and an induction coil, in which the core is allowed to reciprocate freely, said core is connected to the piston rod of the hydraulic cylinder, and said induction coil is connected to the housing of the hydraulic cylinder or vice versa.
From the electric position indicator there will constantly be transmitted an electric signal corresponding to the present position of the piston of the hydraulic cylinder in the housing of the cylinder. When the piston of the cylinder is moving a distance out from one of its bottom positions in the housing of the cylinder, that one of the core or the induction coil of the piston position indicator being connected to the piston rod will move a corresponding distance out from a reference point, and the piston position indicator will transmit an electric signal of a magnitude corresponding to the distance that the piston has moved. By simple control of the signal transmitted from the position indicator in normal signal controlling equipment, e.g. a PLC control or an industrial microcomputer the position of the piston in the cylinder can thereby easily be determined. At the same time feedbacks to other control members, as e.g. for the control of the hydraulic oil, may be emitted. Thus the microcomputer can be preprogrammed so that the hydraulic cylinder carries out an exactly fixed movement sequence.
From EP-A- 95.267 is known a hydraulic cylinder with a built-in inductive position indicator of the type mentioned in the introduction, where the induction coil is wrapped uncovered around a glassfibre reinforced tube of epoxy resin, in which tube the core in form of a steel rod is able to reciprocate during the piston movement of the hydraulic cylinder. Both the induction coil, the glass-reinforced tube of epoxy resin and the steel rod are thus always bathed in hydraulic oil and are thereby completely unprotected against the metal particles always being present in the hydraulic oil. The metal particles will thereby easily be able to become fixed between the steel rod and the tube of epoxy resin and destroy their mutual sliding surface. Further the metal particles will be able to become fixed on the induction coil itself or between this and the opening in the piston rod and thereby being able to short-circuit the induction coil with the consequense of a breakdown of the inductive piston position indicator. As neither the glass reinformed tube of epoxy resin nor the steel rod are supplied with any kind of draining canals the enclosed amount of the hydraulic oil, which is always present in the tube of the piston position indicator in front of the steel rod, cannot quickly enough flow out and in between the inside of the tube and the cylinder room outside the tube. If this inductive piston position indication is to work under the high working pressures of 2-300 bar usually applying to hydraulic cylinders and especially pressure peaks of up to 6-800 bar and piston speeds of up to 15 m per second there is consequently every probability that it quickly breaks down. The by the piston movement of the piston rod in the cylinder uncovered piece of the induction coil in the cylinder is also very much exposed to outside coming noise signals which may cause incorrect feedbacks from the piston position indicator. This hydraulic cylinder with built-in inductive piston position indicator is for that reason only suited to work with especially pure hydraulic oil and under conditions with very low working pressures for the hydraulic oil and very low piston speeds.
The object of the invention is to provide a hydraulic cylinder with built-in inductive piston position indicator of the type mentioned in the introduction which can stand up to the high working pressures usually applying to hydraulic cylinders without involving larger outer dimensions than corresponding cylinders not including built-in position indicators, and which cylinder is furthermore reliable in use and inexpensive to manufacture.
The hydraulic cylinder according to the invention is characterised in that the induction coil is secured inside and extends over the entire length of a tubular housing which liquid tightly surrounds the core. In this manner it is achieved that a hydraulic cylinder with a built-in inductive piston position indicator is provided, which is working most reliable under all conditions and especially under the high working pressures from the hydraulic oil usually applying to hydraulic cylinders. This is especially due to the fact that the mutual movement between the core and the induction coil of the piston position indicator is neither affected by the hydraulic oil itself nor by the impurities present impurities, e.g. the always present torn off metal particles, as the core and the induction coil of the piston postion indicator are protected against coming into contact with the hydraulic oil by means of the tubular housing of the position indicator. The by the piston position indicator transmitted signal will not be affected by outside coming electric noise signals either, as the by the movement of the piston in the cylinder uncovered and no longer by the piston rod protected piece of the induction coil will always be encapsulated in the tubular housing of the piston position indicator.
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 liquitight 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 outer securing means fixed on the end of the piston rod of the hydraulic 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 measuring area of the position indicator, said core never moving in the measuring areas at the ends of the position indicator, which as far as inductive position indicators are concerned 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 liquid-tightly closed towards the interior of the cylinder, preferably by means of gaskets between 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 affected by the pressure and the oil of the hydraulic cylinder.
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 calibration or zeroing of the signal of the piston position indicator 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 hydraulic cylinder is at one of the predetermined piston positions.
Finally according to the invention the electric position indicators may be inductive sensors provided in through-openings in the end covers of the cylinder, said openings being pressure and liquid-tightly 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 indicators.

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 invention.

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 welding, 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 towards 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 circumferential recesses 13 receiving the rubber ring, whereby the interior of the cylinder is pressure and liquid-tightly 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 embodiment 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 comprises 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 inductance 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 distance 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 position indicator 20 is secured by means of a Seeger ring 25 in the end cover 5 and is pressure and liquidtightly 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 indicator 20 transmits therefore an electric signal of a magnitude corresponding to the moved distance of the piston 6 of the hydraulic cylinder 1, e.g. out of its bottom position.
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 liquid-tightly 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 adjacent 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 position indicator 20, respectively, and out of the end covers 4 and 5 of the cylinder through the openings 18. The conduits 16 and 27 are optionally connected through an interface 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 position 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 corresponding to the embodiment of Fig. 1, apart from the feature that circumferential recesses are shaped inside the piston rod 7 or the piston 6. Gaskets 14 are situated in these recesses and close pressure and liquid-tightly 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 housing 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 measuring 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 distance 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 measuring 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 predetermined 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 materials than a conventional hydraulic cylinder comprising neither built-in piston position indicators nor inductive sensors. As a result the cylinder is simultaneously inexpensive, quick, and easy to manufacture.
Many modifications can, however, be carried out without thereby deviating from the scope of the applications. For instance may the inductive position indicator be connected to the housing of the hydraulic cylinder, and the housing of the piston position indicator may be connected to the piston of the hydraulic cylinder. The electric position indicators to show predetermined piston positions may for instances be built into the cylinder or situated outside thereof on other locations than at the end covers.

Claims

1. A hydraulic cylinder (1) with a built-in inductive piston position indicator (20) transmitting an electric signal corresponding to the actual piston position, said piston position indicator (20) being received in a longitudinal opening (7a) extending in the piston (6) and piston rod (7) of the hydraulic cylinder (1) and comprising a core (22) made of an electrically conductive material and an induction coil (23), in which the core (22) is allowed to reciprocate freely, said core (22) is connected to the piston rod (7) of the hydraulic cylinder and said induction coil (23) is connected to the housing (3) of the hydraulic cylinder or vice versa, characterised in that the induction coil (23) is secured inside and extends over the entire length of a tubular housing (21) which liquid-tightly surrounds the core (22).

2. A hydraulic cylinder as claimed in claim 1, characterisedin 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 liquid-tight connection (26) in the end cover (5) of the hydraulic cylinder (1) and that the core (22) of the position indicator (20) is connected to the piston rod (7) of the hydraulic cylinder (1), preferably by means of a rod (28) screwed by way of a threaded connection (29) into outer securing means (8) fixed on the end of the piston rod (7) of the hydraulic cylinder (1).

3. A hydraulic cylinder as claimed in claim 1 or 2, characterised in that the housing (21) of the position indicator (2) 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.

4. A hydraulic cylinder as claimed in one or more of the claims 1-3, characterised in that the opening (7a) of the piston (6) of the hydraulic cylinder (1) is pressure and liquid-tightly closed towards the interior of the cylinder (1), preferably by means of gaskets (14) between the piston (6) and the housing (21) of the position indicator (20).

5. A hydraulic cylinder as claimed in one or more of the preceding claims, characterised 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 piston 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 in one of these predetermined piston positions.

6. A hydraulic cylinder as claim in claim 5 and comprising a piston (6) containing completely or partially an electrically conductive material, characterised 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 liquid-tightly closed towards the interior of the cylinder (1), preferably by means of ceramic plates (17).