EP0618373B1

EP0618373B1 - Position indicator

Info

Publication number: EP0618373B1
Application number: EP19940200818
Authority: EP
Inventors: Ymker Lefert
Original assignee: Hydraudyne Cylinders BV
Current assignee: Hydraudyne Cylinders BV
Priority date: 1993-03-30
Filing date: 1994-03-28
Publication date: 1999-09-01
Anticipated expiration: 2014-03-28
Also published as: EP0618373A1; NL9300565A; DE69420299D1; DE69420299T2

Description

The invention relates to a device for determining the position of a metal piston rod in relation to a chosen reference point.
Diverse solutions already exist for determining the position of a movable piston rod. There are for instance simple position detectors built externally onto the piston rod, mechanical switches built into the cylinder as well as acoustic and magnetic signal processing systems built into the cylinder. All these systems require components to be built into the cylinder or components to be fixed to the piston rod, or a combination thereof. All these solutions have limitations, such for instance in the areas of sensitivity to malfunction, wear, limitation of the stroke length, vulnerability, maintenance and ease of replacement.
The invention relates to an apparatus for determining the position of a piston rod relative to a reference point, in which the circumference surface of the piston rod comprises a substantially tangentially directed profiling with a pattern repeating itself in the axial direction, comprising a magnetic member which has been fixed relative to the reference point for generating a magnetic field which is affected by the profiling, at least two sensors fixed relative to the reference point for sensing the strength of the magnetic field, and a processing circuit for processing the signals generated by the sensors, in which the sensors have a mutual distance in the axial direction, which equals an integer multiple of the pitch of the profiling plus or minus a quarter of the pitch.
Such an apparatus is known from EP-A-0115008.
In this prior art apparatus the means for generating a magnetic field comprise coils, located concentrically relative to the piston rod. Consequently these coils generate a magnetic field, which extends substantially axially relative to the piston rod. Axial displacement of the piston rod will lead to a change in the position of the profiling. However, this change will have a minimal effect on the actual part of the magnetic field. Further, this change is only measurable in a limited extend due to the also axial position of the measuring coils. Expressed differently, the coupling between the generating coil and the measuring coil is only affected in a limited way by the positioning of the piston rod. Consequently the sensibility and the resolution of this known apparatus are limited.
The aim of the invention is to provide such a position sensing apparatus, in which a greater sensitivity and a greater resolution are possible.
This aim is reached in

that the magnetic member is arranged to generate a magnetic field extending substantially tangentially or radially;
that the sensors each comprise a magnetoresistive element being integrated into a bridge circuit;
that the profiling is chamfered; and
that the profiling is covered with a continuous layer in one piece of ceramic material which is smooth on its outer surface and which is adjacent to and fitting onto the profiling of the piston rod and of which the magnetic properties are different from those of the piston rod.

The application of magnetoresistive sensors allows a greater freedom in the determination of the direction of the magnetic field, so that is can be directed such, that the sensors are subject to a greater change in the strength of the magnetic field, when the position of the piston rod is changed. The magnetic field can be directed tangentially or radially relative to the piston rod. In both cases the magnetic field at the circumference of the piston rod will be strongly affected by the position of the profiling. Further, such a direction of the magnetic field allows a greater freedom in the position of the magnet for generating the magnetic field.
According to a preferred embodiment, the profiling comprises ridges and grooves and the ridges and the grooves have the same width.
The invention will be further elucidated with reference to the accompanying drawings, wherein:
figure 1 shows a perspective, partially cut away view of a device according to the present invention; and
figure 2 shows a schematic representation of a sensor co-acting with an enveloped, profiled piston rod.
Figure 1 shows a metal piston rod 1 provided with a longitudinal surface profiling 2 of tangentially and successively arranged grooves and which is enveloped with an electrically non-conducting cover layer 3. A cylinder 4 with a sensor 6 fixed in the cylinder head 5 serves to determine the position of piston rod 1. Sensor 6 is situated between the piston rod 1 and means 7 for correctly positioning a magnetic field. The figure further shows means 8 for adapting and processing the changes in the magnetic field converted by sensor 6 into electrical signals.
As appears from figure 2, the sensor 6 comprises essentially two bridge circuits 9, 10 respectively, each being composed of two magnetoresistive elements 11, 12, 13, 14 respectively and two normal resistors 15, 16, 17, 18 respectively. Both bridge circuits have been positioned such, that the distance between the centre of both bridge circuits is equal to an integer multiple of the pitch of the profiling to plus or minus a quarter of the pitch. In the present case the distance is 1¾ times the pitch. As consequence of this feature it is ensured, that always one of the sensors is in a transient part of the substantially sinusoidal magnetic field, so that the resolution is as large as possible.
Further, it is noted, that in the preferred embodiment depicted in figure 2, the edges of the profiling are chamfered. This causes an aproximate sinusoidal effect on the magnetic field generated by the magnet by the profiling, which is an advantage in the processing of the signals thus generated.
Both bridge circuits 9, 10 respectively are supplied by a generator 19. The generator 19 generates a sinusoidal signal with a frequency of several tenths of kHz. According to a first preferred embodiment the phase of both signals to be supplied to the bridge circuits 9, 10 is equal. According to another preferred embodiment, there is a phase difference of 90° between the signals to be supplied to both bridge circuits. The signals coming from the bridge circuits 9, 10 respectively, are fed an amplifier circuit 21, 22 respectively, being integrated in the sensor 6. Further, they are supplied to a processing circuit 8 through a common cable 23. In this respect it is noted, that the sine generators 19, 20 can be present in the sensor 6, but that they can also be incorporated into the processing circuit 8. In the embodiment shown, the magnetoresistive elements 11-14, together with the resistors 15-18 and the amplifier circuits 21, 22 are incorporated into the sensor 6. It is however, also possible to provide the magnetoresistive elements 11-14 in the sensor 6. It is also possible, departing from the depicted situation, to integrate the processing circuit 24-35 into the sensor.
The processing circuit 8 comprises a circuit 24 for generating a signal representing an angle ϕ. The output signal is supplied to a sine signal generator 25, the cosine signal generator 26 respectively. The output signal of the circuit 25 is a sine function of the an angle ϕ representing signal, and the output signal of the cosine circuit 26 is a signal representing the cosine of the angle ϕ.
The output signal of the circuit 25 is supplied to a comparator circuit 27, together with the output signal of the circuit 21, and the output signal of the signal 26 is together with the output of the circuit 22 supplied to a comparator circuit 28. It is noted, that the output signal of the circuit 21 varies cosine shaped as a function of the replacement of the piston rod, whereas the output signal of the circuit 22 varies as a sine shaped signal of said relocation.
The output signals of both comparator circuits 27, 28 are added, taking the signs of said signals into account in an adding circuit 29 and are supplied to control circuit 30. It is remarked, that both output signals of the circuit 27, 28 respectively, can be expressed as, A . cos ( x p . 2 π) sin ω t . sinϕ or respectively A . sin ( x p . 2 π) sin ω t . cos ϕ The difference of both signals is only equal to zero, when: x p . 2 π = ϕ The circuit uses the property by having the control circuit 30 generating a control signal which is supplied to the angle ϕ generator 24 through an intergrating circuit 31, in which the control signal is determined such by the control loop, that the above mentioned condition is satisfied.
Thus, the output signal of the angle ϕ generator is an indication for the position of the piston rod 1. This output signal is in analoge format available on the output signal 32, and is also in digital format available on an output signal 34 through an A/D converter 33. Further, the circuit comprises an output 35, for the control circuit 30, on which an output signal in the format of a speed signal is available.
The output signal of the angle ϕ generator in binary format is converted into incremental counting pulses after an adjustable resolution coding circuit 32 by means of pulse shapers 33.
In a first embodiment these pulses are available on output circuit 34. In a second embodiment these pulses are added in a bi-directional binary counter 36, and the counting value is available in binary format on the output circuit 34.
In a third embodiment the binary counting value is converted into a Gray-format, after which this information is available on the output circuit 34.
In a last embodiment the already mentioned second embodiment is supplied with a D/A converter 38, after the position information in analoge format is available on the output 39.

Claims

Apparatus for determining the position of a piston rod (1) relative to a reference point, in which the circumference surface of the piston rod (1) comprises a substantially tangentially directed profiling (2) with a pattern repeating itself in the axial direction, comprising a magnetic member (7) which has been fixed relative to the reference point for generating a magnetic field which is affected by the profiling (2), at least two sensors (9,10) fixed relative to the reference point for sensing the strength of the magnetic field, and a processing circuit (8) for processing the signals generated by the sensors (9,10), in which the sensors (9,10) have a mutual distance in the axial direction, which equals an integer multiple of the pitch of the profiling (2) plus or minus a quarter of the pitch, characterized in

that the magnetic member (7) is arranged to generate a magnetic field extending substantially tangentially or radially;

that the sensors (9,10) each comprise a magnetoresistive element (11-18) being integrated into a bridge circuit (9,10):

that the profiling (2) is chamfered; and

that the profiling (2) is covered with a continuous layer (3) in one piece of ceramic material which is smooth on its outer surface and which is adjacent to and fitting onto the profiling (2) of the piston rod (1) and of which the magnetic properties are different from those of the piston rod (1).
Apparatus according to claim 1, characterized in that the profiling (2) comprises ridges and grooves and that the ridges and grooves have the same width.
Apparatus according to claim 1 or 2, characterized in that the bridge circuits (9,10) are each connected with an AC voltage source (19,20), and that the phases of the signals generated by each of the AC voltage sources (19,20) differs pi/2.
Apparatus according to claim 3, characterized in that the processing circuit (8) has been arranged for for comparing the output signals of each of the bridge circuits (9,10) with a cosine, respectively a sine of the signal representing an angle which is a function of the comparison.
Apparatus according to claim 4, characterized in that the processing circuit (8) has been arranged for generating a digital incremental signal.
Apparatus according to claim 4 or 5, characterized in that the processing circuit (8) has been arranged for generating a digital and an analog signal, each representing the position of the piston rod (1).