Hydraulic Cylinder With Piston and With a Magnetic Device for Piston Position Determination

The present invention relates to a hydraulic cylinder with a piston slidable in the inner of the cylinder and with magnetic position indication comprising at least one magnetically sensitive component mounted in the cylinder wall.

It is known to sense the position of a hydraulic piston by means of a magnetic sensor. In the known devices, the permanent magnet is usually mounted in the slidable piston and an inductive sensor is placed at the location in the cylinder wall where a position indication is desired. The known sensors are, however, less reliable in hard frost.

The object of the present invention is to provide a hydraulic cylinder with a reliable, exact, and yet inexpensive position indication which can use a magnetic sensor of the Hall-element type in order to preserve the reliability at low temperatures, as it must be considered that hydraulic cylinders on working machines such as bulldozers and tractors sometimes must work in hard frost.

According to the invention, the cylinder is characterised in that the magnetically sensitive component is embedded in a magnetically essentially non-conducting material and forms part of a first closed magnetic circuit comprising at least one permanent magnet, and that the slidable piston in a predetermined position forms a second closed magnetically better conducting circuit including said permanent magnet(s), but excluding the magnetically sensitive component. When the piston is placed in the position in which an indication is wanted, the steel material of the piston forms part of the second closed magnetic circuit and almost all the magnetic flux passes through the piston. In this situation the first closed circuit with the sensor in the magnetically bad conducting material carries only little flux. The magnetically sensitive component is thereby brought into a first, non-magnetised condition when the piston opposes the position indicator. When the piston is placed elsewhere in the cylinder, the entire magnetic flux will tend to follow the first magnetic circuit so that a magnetic south pole is formed on one side of the magnetically sensitive component and a magnetic north pole on the other side. As a result the component is placed in a magnetic field and is brought into another, magnetised condition which can be registered in electric circuits connected to the sensor circuit.

The invention is explained more detailed in the following with reference to the accompanying drawing, in which

FIG. 1 shows a hydraulic cylinder with several position determination devices according to the invention,

FIG. 2 is an example of the structure of a position determination device according to the invention,

FIG. 3 shows the structure of FIG. 2 with an indication of the magnetic circuit when the piston is displaced from the position determination device, and

FIG. 4 shows same structure as FIG. 2 and the magnetic circuit when the piston opposes the position determination device.

FIG. 1 shows a hydraulic cylinder with several position determination devices according to the invention. These are preferably placed in the bottom members of the cylinder, but may also be placed in the side wall.

FIG. 2 is a sectional view through a cylinder wall 11 which is preferably the end wall of a cylinder. The pressure of a hydraulic liquid such as oil 13 reciprocates the piston 12. A cylindric bore is provided in the cylinder wall, the position determination device being placed in a tight fit in said bore. It must be emphasized that this type of hydraulic cylinders must be able to handle considerable pressures. The position determination device comprises a magnetic material 14 which at one end forms a circular plate filling the cross section of the bore, said material abutting via a central stem 14a an electronic sensor circuit 15 surrounded by non-magnetic material 20, e.g. an epoxy resin. A magnetic post 16 abuts the opposite side of the electronic sensor circuit 15 which is thereby placed between the two magnetic materials 14, 16. A permanent magnet 17 shaped like a disc with a hole in the middle is mounted around the magnetic post 16. The hole fits the magnetic post. The outer circumference of the disc forms one magnetic pole and the central hole form the other magnetic pole. The electronic sensor circuit is connected to electronic control circuits via electric conductors 18, said central circuits not being explained more detailed in the present connection and besides they may be of a known type.

The permanent magnet is covered by a non-magnetic material 19 towards the inner of the cylinder.

FIG. 3 illustrates the piston displaced from the position indication device, whereby only the hydraulic liquid e.g. oil 13 being present at the inner surface of the cylinder wall in front of the position indication device. A first magnetic circuit is shown by a dot-and-dash line and comprises the permanent magnet 17, a portion of the magnetic post 16, the sensor circuit 15, the magnetic material 14, and the circuit is closed through the cylinder wall 11 which must be of a magnetic material, preferably steel. In this situation the sensor circuit is thus actuated by the magnetic field and the magnetic circuit is dimensioned so that the main part of the magnetic resistance, i.e. the magnetically hardly conducting material is located at the sensor circuit which then is placed between a magnetic north pole N and a magnetic south pole S which can be registered from the outside through the electric conductors 18.

FIG. 4 illustrates the piston at its dead centre where it fits tightly to the end wall 11 of the cylinder. In this piston position an alternative magnetic circuit will comprise the permanent magnet 17, part of the magnetic post 16, the piston 12 preferably being of steel, and the cylinder wall 11 also of steel. The magnetic circuit surrounds the non-magnetic material 19, e.g. brass. As appears from the FIG., the brass member or ring 19 is preferably of a larger outer diameter than the rest of the position determination device in order to achieve a tight fit around the position determination device, especially in order to tighten the hydraulic cylinder, which may be exposed to high pressures. The alternative magnetic circuit shown here must be dimensioned with a low magnetic resistance in other words with a high permeability so that the magnetic field strength across the sensor circuit 15 in the first circuit becomes so low that the sensor circuit changes state which can be registered from the outside through electric conductors 18.

The magnetic sensor described here is temperature independent in a wide range of temperatures when using an appropriate sensor circuit and furthermore a great security against leakages is obtained as the device shown can be made very tight. At a suitable dimensioning, a good accuracy is obtained and the device is substantially less expensive than the position determination devices used today. A Hall-effect transducer typically working in a range of temperature from -40.degree. to +150.degree. C. can be used as sensor circuit.

The term of material "steel" used in this application refers to steel alloys magnetizable and preferably soft magnetic steel alloys.