DE102014011079A1 - Sensor for determining the position of pistons in pneumatic and hydraulic cylinders - Google Patents

Abstract

Determining the position of pistons in pneumatic and hydraulic cylinders is important for control and monitoring equipment. So far, the position determination is solved unsatisfactory. An external sensor is undesirable. In some implementations, the piston rod must be laboriously modified or a deep hole must be drilled in the rod. In other implementations, the sensors are prone to be inaccurate or expensive to manufacture and therefore expensive. To determine the position of the piston position here, the magnetic circuit of the piston, piston rod, cylinder cover and cylinder jacket is used, which changes its magnetic properties with the piston position. These altered properties can be measured with a coil under the cylinder cover. In an extension of an additional coil on the piston is given, which is connected like the coil on the cylinder cover vibratory. The resonance characteristic of the coil is influenced by the distance of the other coil. This detuning can be measured and converted to the distance. The measurement can also be performed with multiple coil systems at different frequencies to compensate for disturbances.

Description

In pneumatic and hydraulic cylinders (hereinafter referred to as cylinder.) There is a need to be able to evaluate the position of the piston to this information for z. B. to use a scheme or monitoring. Many systems for position detection have been realized. Their properties are so far not satisfactory overall.

State of the art

Systems that realize piston rod position detection with external sensors have the advantage that classical length measurement techniques can be used, e.g. As magnetostriction, ultrasound or glass measuring rods. Last are not absolute measuring, but incremental. This is generally not desirable, since you can usually afford after the commissioning of a machine with cylinders no Initialisierungsfahrten and tend these systems in principle for Verzählen. External measuring systems have the advantage that repairs for cylinders and sensors can be carried out independently of each other. The disadvantage, however, is that often only one unit must be removed in order to get to the other. A major disadvantage is that the overall system becomes more vulnerable in harsh environments. In the manufacture of the machine additional installation work and constructive an additional space needs to be considered.

The next group of implementations implements a device for evaluating certain properties directly on the piston rod. Also, this device is vulnerable in harsh environments and extends the cylinder system, or reduces the usable hub.

In the first system, the attachment detects position on the piston rod using optical markers mounted on the piston rod. The disadvantage is that these markers are difficult to apply to the chrome-plated and polished rods - so that they are durable. Depending on the implementation of the marker code, either only an incremental evaluation is possible or the axial freedom of rotation of the piston rod is no longer a problem. So far, no high resolution is achieved.

In the next system, the attachment will detect the position on the piston rod using magnetic markers. This system is less susceptible to soiling. However, here too, depending on the implementation of the marker code, either only an incremental evaluation is possible or the axial freedom of rotation of the piston rod is no longer present. So far, no high resolution is achieved here.

In the next system, the attachment detects the position of the piston rod from an axial gradient of the magnetic properties of a layer of material underlying the surface of the piston rod. This system is not susceptible to soiling. The disadvantage is that the gradient layer is difficult to produce. Since it is important to measure analogous quantities and the magnetic effect of the layer also changes, a precise position determination can not be carried out permanently.

In the next system, the attachment detects the position of the piston rod by means of a magnetically active material, which is introduced in waves in the axial direction and radially symmetrical under the surface of the piston rod. This system can only measure incrementally and the topcoat thickness limits the achievable spatial resolution. With decreasing magnetic effect can lead to Verzähl.

The next group of realizations uses sensor technology in the cylinder and is therefore usually mechanically robust against external influences.

In the first system, the position of the piston is detected by means of a resistance track, which is mounted axially in the center of the cylinder. For this system, a hole must be made in the piston rod, which is problematic for deep holes in large cylinders. For small cylinders with thin piston rods there is the problem that the remaining wall thickness can become too thin and therefore unstable. Significant disadvantages are also that the sliding contact wears and the sensor is temperature dependent.

The next system detects the position of the piston using a magnetostrictive sensor. These sensors are very accurate, but here too, the piston rod must be drilled large, which the o. G. Disadvantages. For current systems, a large opening must be made in the cylinder as a fit, which means a considerable manufacturing effort. Overall, this sensor system is relatively expensive.

The next system detects the position of the piston with Hall sensors mounted in the cylinder axis in the axial direction. Again, the piston rod must be drilled out what the o. G. Disadvantages. Due to the discretization of the measuring points, the accuracy is not very high and the electronics are expensive.

The next system detects the piston position with a capacitance measurement between the piston rod and a fixed center tube. Here only a relatively small hole must be made. Nevertheless, the above-mentioned disadvantages arise. The system is susceptible to electromagnetic interference. Due to the relatively small evaluable capacity changes, the accuracy is limited.

The next system detects the position of the piston using eddy current sensors. Again, the sensor is mounted in the axial direction in the cylinder axis. Again, only a relatively small bore is necessary. And here, too, the o. G. Disadvantage. The realizable length is limited. Therefore, this method is z. B. not suitable for very large cylinders.

The next system detects the position of the piston using inductive distance sensors. It is advantageous that these sensors can measure very accurately and can be realized as a screw-in element in the cylinder bottom. The disadvantage is that they can only measure a very small stroke.

The next system detects the position of the piston using ultrasound. Transmitter and receiver can be realized separately or together as a screw-in part in the cylinder bottom. The disadvantage is that the system is relatively sensitive to changes in temperature, viscosity and contamination of the filling medium and mechanical shock from the outside.

The next system detects the position of the piston using microwaves. Again, transmitter and receiver can be realized in a screw on the cylinder bottom. For the waves results in a propagation cone, which is bounded by a cylinder. As a result, depending on the piston position more or less complex fashion characteristics. The detection of the reflections is correspondingly complex and also dependent on the temperature, viscosity and contamination of the filling medium.

The next system detects the piston position by evaluating capacity changes of the entire piston rod / cylinder system. The structure for this is relatively simple. However, extensive shielding measures are necessary. The disadvantage is that piston and piston rod must be permanently isolated from the cylinder electrically. This can not be guaranteed if the filling medium is dirty. Another problem is that the change in capacity is relatively small and therefore prone to failure. There are dominant constant capacities between the lateral surface of the piston and the cylinder and between the inner surface of the guide bush and the piston rod. It is also expected under water or in the rain a change in capacity. Several systems on one machine can then interfere with each other. The system depends on the permittivity of the filling medium and varies greatly with the type, temperature, viscosity and contamination of the filling medium. In particular, even if there are different conditions in the Zylinderzug- and pressure chamber. It is a trouble-prone sliding contact or a fault-prone capacitive / inductive coupling to the piston rod necessary. Particularly problematic is that cylinder and piston rod form a dipole and this is prone to external electromagnetic interference.

The next system detects the position of the piston by counting the volume of fluid that is exchanging and going in and out of the cylinder. The disadvantage is that the system measures only incrementally and is dependent on temperature, viscosity and contamination of the filling medium. If the counting elements are designed as toothed wheels, this also results in mechanical wear with consequent leaks and consequent inaccuracies.

Technical task

A measuring system is to be created which can accurately detect the piston position in a pneumatic or hydraulic cylinder. The system should be absolutely measuring and robust against external influences. There should be no deep hole / deep cavity in the piston and piston rod and no modification to the outer piston rod necessary. It should only result in a small additional axial space. It should be used for very thin and very large cylinder. The measuring method should be independent of the type, temperature, viscosity and contamination of the filling medium. It should not be prone to external mechanical and electromagnetic interference. No special shielding measures should be necessary. There should be no material fatigue. It should be simple and inexpensive to implement.

Solution of the task

To determine the piston position, a magnetic circuit is to be used, which consists of piston, piston rod, cylinder cover and cylinder jacket.

When the piston undergoes a change in distance relative to the cylinder cover, there are changed magnetic properties of the magnetic circuit, which are to be evaluated for distance determination.

For this purpose, an exciter coil may be mounted under the cylinder cover, whose inductance changes with the magnetic circuit.

The magnetic properties can be measured with different excitation frequencies to detect and compensate for disturbances.

The exciter coil can be connected vibrationally. On the piston, an additional coil may be attached, which together with their wiring represents a band filter for the magnetic circuit. This acts on the excitation coil back and detunes their resonance characteristics. This is done depending on the properties of the magnetic circuit. From the detuning the distance can be determined.

Several coil systems with different resonance frequencies can be used to compensate for disturbances.

Realization / embodiments

Furthermore, the invention will be explained with reference to exemplary embodiments.

The attached pictures show an inductive measuring system in a cylinder. No pneumatic / hydraulic connections and seals are shown. These may be typically attached. Show it

1 Arrangement of excitation and piston coil in a pneumatic / hydraulic cylinder

LIST OF REFERENCE NUMBERS

1

cylinder surface

2

cylinder base

3

cylinder cover

4

piston

5

piston rod

6

excitation coil

7

signal generator

8th

decoupling

9

signal analyzer

10

piston coil

11

Resonanzbeschaltung

2 Magnetic field course - static.

3 Magnetic field course - dynamic.

Variant 1: Measurement of the exciter coil

The exciter coil ( 1 , 6 ) is for all variants under the cylinder cover ( 1 , 3 ) appropriate. It encloses the piston rod ( 1 , 5 ).

The coil profile can be chosen so that there is hardly any space consumption in the axial direction.

Piston ( 1 , 4 ), Piston rod ( 1 , 5 ), Cylinder cover ( 1 , 3 ) and cylinder jacket ( 1 , 1 ) form a magnetic circuit of ferromagnetic materials. It is completely closed, so that there is little interaction with external fields and ferromagnetic objects. The magnetic circuit is variable in length. Since the materials are magnetically lossy, there is a correspondingly variable inductance for the exciter coil. This can be measured electrically.

The filling medium is not ferromagnetic. Since almost all relative non-ferromagnetic materials have a relative permeability, the system is therefore independent of the type, temperature, viscosity and contamination of the filling medium. This is a great advantage over capacitive measuring methods in which the filling medium exhibits strongly varying relative permittivities in these properties.

However, with strong external magnetic fields or very close and large ferromagnetic objects, a significant disturbance may result.

The excitation coil can be measured with different frequencies. At higher frequencies, there are fewer interactions with external objects (Figure 2 + 3). If several measurements are made, the ratio of the results for each piston position is known. If differences occur here, this information can be used to compensate for influences of external objects. In part, this also effects of external fields can be compensated.

Variant 2: Use of a piston coil

The piston coil ( 1 , 10 ) is applied in the magnetic circuit directly on the piston surface. It is intended to represent a band filter for the magnetic circuit. For this purpose, an external wiring ( 1 , 11 ) be necessary with a capacitor. But it may also be sufficient to wind the coil so that the internal capacity is sufficient. In this case, the coil only has to be short-circuited.

About the magnetic circuit of this band filter acts on the excitation coil back. If this is wired so that it is able to oscillate, its resonance frequency is detuned depending on the distance of the piston coil. It changes both the vibration amplitude and the frequency.

The distance of the piston coil determines the magnetic coupling, which also determines the resonance frequencies for both coils. If these frequencies are close to each other, there is a strong interaction and thus a detuning of the exciter resonant circuit. This effect is very strong, so that external disturbances are correspondingly less significant.

As signal generator ( 1 , 7 ) can z. As a Colpitts or Royer oscillator can be used, which automatically adjusts to the resonant frequency. The signal analysis ( 1 , 9 ) is then limited to converting the frequency detuning into an output signal.

Variant 3: Use of multiple coil systems

Several exciter coil oscillator systems with different resonant frequencies can be used. The coils can have several taps for this purpose or separate coils can be used (if necessary with additional circuitry) that form resonant circuits with suitable resonant frequencies. As already described in variant 1, measurements at different frequencies can then be used for the further suppression of disturbing influences.

Realization / Features

The system realizes an absolute measurement.

It is an in-cylinder measuring system that is robust against external influences.

There is no deep bore / cavity in piston and piston rod and no modification of the outer piston rod necessary.

There is only very little additional axial space required.

It can also be used for very thin and very large cylinders.

The measuring method is independent of the type, temperature, viscosity and contamination of the filling medium.

The system is not prone to external mechanical or electromagnetic interference.

There are no shielding measures necessary

There are no relevant material aging effects to be expected.

The system is simple.

The system can be realized extremely cheaply.

Claims (7)

Use of a closed magnetic circuit in the piston, characterized in that it is formed from piston, piston rod, cylinder cover and cylinder jacket. Change in the magnetic properties of a magnetic circuit according to claim 1, characterized in that they change with the change in position of the piston. Use of an exciting coil, characterized in that this coil is attached to the cylinder cover and can detect the variable magnetic properties according to claim 2. Oscillatable wiring of the exciter coil according to claim 3 and additional coil on the piston, which has a local band filter circuit, characterized in that the piston coil in response to the magnetic properties according to claim 2 and thus in response to the piston distance to the cylinder cover on the exciter coil according to claim 3 and so back Resonance characteristic of the coil and its wiring detuned. Vibratory wiring of the exciter coil according to claim 4, characterized in that a Colpitts or Royer oscillator is used and this automatically adjusts to the resonant frequency. Measuring systems according to claim 1-5, characterized in that they are used for different resonant frequencies and for the coils have multiple taps or separate coils are used (if necessary, with additional circuitry), forming the resonant circuits with matching resonant frequencies. Use of different measuring frequencies on the measuring systems according to claims 1-5 or 6, characterized in that by the additionally obtained measuring information, disturbances of external magnetic fields and field distorting objects can be compensated.

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