The present invention concerns a device for positioning according to the preamble of claim 1.

Previously known devices using magnetic sensing for determining the piston position inside a piston cylinder aggregate, where magnetic bodies on the piston rod are being used as position indicators, have been based on the principle of adding agnetizble material to grooves in the piston rod, said material subsequently being magnetized to become permanently magnetic. This principle is however costly in use as there is no possibility of using standard components for different desired indicator positions, and as the magnetizing of the magnetizble material necessitates a stage of operation in a specially designed device, which must be highly adapted to the geometrical conditions, such as the diameter of the piston rod, indicator positions etc. Besides, a piston rod is obtained having a envelope surface that has to be finished in order to obtain a sufficient smotheness for preventing leackage from the cylinder from occurring at the sealing bet¬ ween the cylinder end wall and the piston rod. When there are considerable temperature changes, there is always a risk for surface cracks due to the coefficient of heat expansion not being the same for the material of the piston rod and the magnets. Cracks in the surface increases the wear and con¬ siderably reduces the working life.

Due to the relatively low magnetic fields of the previously known devices, relatively sensitive detectors are necessary for the detection, such detectors unfortunately also being relatively susceptible of noise signals. Further, it is necessary for the detector to be positioned in great proximity to the piston rod, which may lead to problems in situations where great amounts of impurities are present in the surroundings.

It is an aim of the present invention to provide a solution to the problems entailed with the previously know devices, a solution which to a high extent allows the use of cheap standard components and at the same time guarantees a safe detection of the piston position by means of sensors which may be comprised of commercially available sensors, being simple, contact-free and unsensitive to noise. It is further an aim of the present invention to provide a device where safe sealing between the piston rod, being intended for indication, and the cylinder end wall may easily be accomplished.

These aims are achieved in a device according to the features of the characterizing part of claim 1. Hereby commercially available standard magnets may be used directly without further treatment. The special arrangement of the magnets leads to a clearly detectable magnetic flux, admitting the use of cheap standard type sensors, which are unsensitive to noise. Finally, satisfactory sealing between the piston rod and the cylinder end wall is easily obtained.

A very advantageous embodiment of the invention is obtained by the feature in claim 2, whereby it is possible to apply the device in extremely difficult surroundings as e.g. in aluminium smelting plants or in soda boilers, where the piston cylinder aggregate is subject to considerable heat and/or high humidity. By this solution, where the device is applied on the opposite side with respect to the operational piston rod, simple adjustability and possibility of maintainance or replacement is also achieved. This embodiment is also parti- cularly useful when the piston cylinder aggregate is to be provided with a through piston rod, particularly for compensa¬ tion of masses, such as tools, being applied to the working piston rod, and for adaption of the working areas of the piston.

By the features of claim 3, it is achieved that the magnets may be applied on a central rod, which substantially

facilitates mounting of the magnets into one unit, which may be applied within the covering. When the piston rod containing the magnets also comprises the operational piston rod, said central rod may be force transmitting.

Particularly advantageous embodiments of the invention are achieved by the features in claims 4 to 6. By the distance plates having a diameter which corresponds to the inner dia¬ meter of the covering, and which is greater than the diameter of the magnets, it is achieved, on the one hand a more even distribution of the magnetic flux and thus better sensing con¬ ditions for the sensor when using magnetizable plates, and on the other hand that only the distance plates have to be adjusted to the respective covering, while the diameters of the magnets may differ therefrom, thus making it possible to use magnets of standard sizes within widely differing piston rod dimension. By the thickness of the plates being adjustable for optimizing the magnetic field when using magnetizable plates and/or for adjustment of the axial positions of the magnets, a large extent of freedom of using magnets of standard sizes is also achieved, at the same time as very simple adjustment of the entire magnet unit to the present situation is possible.

By the features of claims 7 to 9, very advantageous adjustment possibilities are achieved for adjusting the reversing positions and the like to the present application.

By the modification according to claim 10, it is achieved that the invention is also applicable in piston cylinder aggregates of a conventional type. It may e.g be used in rodless cylinders.

Further aims and advantages with the present invention are explained in the following detailed description.

The invention will now be described in reater detail in the

light of an embodiment and with the aid of the annexed drawings, whereon:

Fig. 1 shows, partly in a central axial section, a part of a positioning device according to the present invention,

Fig. 2 shows a diagrammatic coupling diagram for the device according to Fig. 1, and

Fig. 3 shows diagrammatically the magnets with indicated flux lines and, underneath in the form of a diagram, the signal from the sensor to the control device.

Fig 1 thus shows a piston cylinder aggregate 1 in a central axial section. In the cylinder, which comprises a cylinder end plate 2 and a cylinder tube 3, an operational piston is axial- ly moveably arranged (not shown) . A piston rod 5 extends from the piston and passes through the cylinder end wall 2 via conventional sealing elements. Piston rod 5 comprises a covering 6 of a non magnetic material as stainless austenitic steel, an aluminium alloy, a reinforced plastic or any other suitable material. Plate shaped magnets 8 are contained within the covering 6, said magnets being arranged with their magnetic axis essentially coaxial to the axis of the piston rod 5. Every second magnet 8 is turned in such a way that the poles of two adjacent magnets of the same kind are place oppo¬ site to each other. The axial pole sequence of two adjacent magnets will thus be: south-north-north-south or north-south- south-north. The magnets 8, being of a standard type, are comprising an external diameter which is somewhat smaller than the inner diameter of the covering. Between adjacent magnets 8 a distance plate 9 of a magnetizable material is inserted. The distance plates 9 correspond to the inner diameter of the covering so that a suitable fit is obtained between the plates and the covering. By the plates having a somewhat larger diameter than the magnets, more distinct outgoing positions for the magnetic flux are achieved and thus better sensing

conditions. The thickness of the distance plates 9 may also be choosen in such a way that the magnetic flux is opimized, which may easily be tested by one skilled in the art. In com¬ bination with, or as an alternative thereto, the thickness of the distance plates 9 may be adapted for achieving desired sensing positions for a sensor 10 which is placed adjacent the piston rod 5, said sensor via a cable 11 communicating with a central control unit. Optimizing the magnetic flux may be very essential, e.g. when the sensor for some reason may not be applied in proximity to the piston rod, or when it is desired to use a thick covering. To facilitate mounting of the magnet/plate unit, a central rod 7 is situated within the piston rod 5. If the piston rod 5 is comprising the opera¬ tional piston rod, the covering 6 and/or the central rod 7 may comprise force transmitting elements. In the example shown in Fig. 1, the piston rod 5, however, is only comprising a posi¬ tioning piston rod, which opposes the actual operational piston rod. The piston rod 5 may also serve to adjust the working area of the piston within the cylinder.

By using a positioning piston rod 5, which opposes an opera¬ tional piston rod 5, a high degree of safety is achieved when using the aggregate in surroundings which are difficult with respect to heat, impurities and humidity. Service, replacement and adjustment is also simple to undertake in this applica¬ tion.

Fig. 2 shows diagrammatically a coupling diagram for the positioning device according to Fig. 1, with the sensor 10 being placed adjacent the piston rod 5. A programmable logic computer (PLC) 13 is used as the control unit, said computer via the cable 11 receiving inpulses from the sensor 10. Possibly a relativly simple counter may also be used as an alternative to such a computer. After having received a determined number of pulses from the sensor 10, the control unit 13 transmits signals to the valve 1 , to set this element: for axial movement to the right or to the left, in the Figure,

or for cylinder stop. 15 indicates a pressure fluid source.

On top of Fig. 3, four magnets, being positioned adjacent to each other in a magnetic unit are diagrammatically shown with the poles indicated as well as flux lines 16a and 16b, and flux directions by means of arrows 17a and 17b. The lines 18 and 19 indicate the levels of operation on and off respective¬ ly of the sensor (here the applicant's own magnetic piston sensor 322-20, -21). The different levels are explained by a certain sensor hystereses.

At the bottom Fig. 3 shows the signal in volts from the sensor as a function of the lenght of the movement when the sensor is moved in parallel to the rod. The signal from the sensor is thus a square wave.

The embodiment of the invention according to the description is only to be consider as an example and the invention is only limited to what is stated in the following patent claims. The sensor may thus, as was indicated earlier, be of any suitable kind if earlier mentioned conditions with respect to among others unsensitivity to noise are fulfilled. The magnetic piston sensor described, which is of a magneto resistive type, may of course also be replaced by other sensors of this kind. Sensors of the Hall-element type may also come in question as well as e.g. reed sensors. The sensor of the latter kind hand¬ les processes at least up to 50 Hz, while the other sensors mentioned handle still faster switching processes.

In its most simple embodiment the described positioning device is adapted to what may be defined as rough positioning, where the position normally is allowed to vary within ± 1 mm. More accurate positioning may be achieved if more than one sensor are applied axially displaced adjacent the piston rod (c.f. indication in e.g. sliding calipers) . A typical distance between the poles of the magnets is 4 - 15 mm, also other distances may, however, come into question. Tne shortest

distances are now fully realistic in view of the stable permanent magnets now being available on the market.

The invention also provides good possibilites of adjusting the indicator positions axially in different ways. This may be accomplished by the sensor being axially displaceably mounted adjacent the piston rod on a guide, on threaded pins, or in any other way. The magnet unit may also be displaceable within the covering by means of threaded distance fittings, axially expanding plates or in any other suibable manner. In applica¬ tions with considerable piston movements, the piston rod may contain groups of magnets along only determined portions of the axial extention of the piston rod, whereas rod or tube shaped distance elements (12 in Fig. 1) may be mounted between the groups of magnets, defining e.g. reversing positions or stop positions.

It is also possible to use distance plates 9 of a non magnetizable material as plastic, stainless austenitic steel, an aluminium alloy or any other suitable material. What is stated above with respect to the plates dimensions is also applicable with respect to non magnetizable plates (except the optimation of the magnetic field) .

Finally, it is possible to apply the invention also in an indication rod of a kind previously known per se, being attached in parallel with the piston or the piston rod 5. In this case the indication rod contains magnets 8, distance plates 9 and possibly a rod 7 and distance means in accordance with what has been described above in connection with the piston rod 5.