DE202010018042U1 - Sensor with wear detection - Google Patents

Abstract

Sensor (14) having at least one in particular magnetic or inductive sensor unit (22) which generates a sensor signal as a function of the position of a piston (12) which can be moved in and out in a fluid-driven manner in a cylinder (10), and with an evaluation unit (24, 26 in which, based on the sensor signal, the position of the piston (12) can be detected at at least two switching points (S1, S2), characterized by a wear detection unit (28) which is designed to detect from the points in time at which the piston (12) the switching points (S1, S2) happen to derive motion information about the input and / or extension movement of the piston (12) and to determine a degree of wear by comparison with a reference movement information, wherein the sensor (14) further comprises a warning unit (28, 30, 32), by means of which a wear warning can be output on the basis of the degree of wear.

Description

The invention relates to a sensor having at least one sensor unit for the position detection of a piston in a cylinder at two switching points according to the preamble of claim 1.

The position of the piston in a working cylinder can be determined without contact by means of magnetic or inductive sensors which are mounted in guide grooves on the outer surfaces of the housing of the working cylinder. Such a sensor then outputs a switching signal as a function of the piston position.

Frequently, several switching points are desired per stroke of a piston. In most cases, at least the retracted end position and the extended end position of the piston are to be detected. The EP 1 640 618 A1 describes a sensor system which detects more than one piston position by at least two switching points are adjustable. These switching points are monitored by a plurality of interconnected sensor elements or multiple switching thresholds of a single sensor element.

Such cylinders and generally pneumatically or hydraulically operated components in a plant are subject to wear due to their short, frequent cycles especially. Worn components, for example, consume significantly more compressed air due to internal or external leaks or play, so that early detection of leaks can reduce compressed air requirements by up to 30%. The compressed air consumption in a pneumatically operated plant is a dominant operating cost share.

In order to minimize the compressed air requirement and to increase the plant availability, systems are traditionally very often subjected to preventive maintenance. This means replacing the pneumatic components well before the end of their life. This results in unnecessary maintenance and renewal costs, so that the goal is largely missed to minimize the operating costs of the system.

It is known to check the condition of seals electronically. This leakage is detected immediately at the scene, and it can be timely, wear-resistant maintenance. However, this is bought with an additional sensor for leakage detection alone.

In a different approach, in DE 10 2007 051 468 A1 or in the article S. Fritz, C. Stammen, "On the trail of wear - Possibilities for plant diagnosis in pneumatics", Journal O + P, Issue 9/2006, p. 432-441 is selected, the signals of several position and pressure sensors are evaluated together with Ventilansteuersignalen via an external control to calculate a wear indication. Accordingly, this requires several sensors. In addition, the possibility of recognizing and correctly assessing the existing signals of the sensors is greatly limited by the cycle time of the external control.

It is therefore an object of the invention to monitor the state of wear fluid-driven, so hydraulic or pneumatic components with the least possible effort.

This object is achieved by a sensor according to claim 1. The invention is based on the basic idea to monitor the piston position in a cylinder with a single sensor with multiple switching points. Based on a comparison of the actual and the expected movement is then recognized whether the mobility of the piston has changed during retraction and extension due to wear.

As movement information, in the simplest case the time points or their time difference are directly considered, at which the piston passes the switching points. More complex estimates of the piston movement are conceivable, however, also using additional switching points, which allow a more accurate motion tracking.

The invention has the advantage that a single sensor determines the position and the wear information decentralized and provides. This allows needs-based rather than preventive maintenance, in which a component is replaced just before their failure and not far before the end of life. This ensures system availability while reducing maintenance costs. The compressed air saving minimizes operating costs. Due to the decentralized determination of the degree of wear, there is no limitation due to sampling times of an external controller.

The warning unit preferably has an adjustable threshold, from which degree of wear the wear warning is to be output. In other words, this threshold indicates which maximum deviation of the movement information from the reference movement information is still acceptable without maintenance or replacement. Since this depends on the application, it is advantageous to be able to set this limit. To be factory preferably given in advance meaningful, later variable values.

The wear warning advantageously comprises a quantitative information about the degree of wear. This quantitative information can be determined from the degree of deviation between motion information and reference motion information. From this, an operator or a system control can decide for themselves whether the current degree of wear requires maintenance or not.

The wear warning can preferably be output via a display, in particular an LED, whereby this display is preferably mounted directly on the sensor. The LED may be dedicated for this purpose wear LED, or it will be issued a special flashing sequence or a text message. Output to a signal output is also conceivable, especially via a protocol such as IO-Link, so that downstream control units continue to process the wear warning.

The wear detection unit is preferably designed to determine a cause for the deviation in the comparison between movement information and reference movement information together with the degree of wear. Such qualitative analysis, output via a display or an output signal, simplifies troubleshooting and maintenance. For example, a slower retraction of the piston to an internal or external leakage or increased friction, a faster extension to a reduced friction or an external leakage or a slowed down extension, especially due to higher friction and subordinated to an internal leakage.

The wear detection unit is preferably designed to record and statistically evaluate deviations between movement information and reference movement information via a multiplicity of entry and exit movements, wherein in particular the warning unit is designed to output the wear warning when a time trend is detected. On the one hand, it is excluded that accidental single events lead to a wear warning. On the other hand, it can be estimated at an early stage by a temporal trend, when it comes to a failure. For example, the individual deviations may still be far too small for a wear threshold to be exceeded, but at the same time the sensor recognizes a stable statistical trend for deterioration. Extrapolation then leads to an estimation of whether and when critical wear will occur.

The sensor is preferably housed in a housing which is designed so that it can be mounted in a groove on the circumference of the cylinder. Thus, the sensor is firmly mounted in the correct adjustment on the cylinder and disturbs the outer geometry only minimally.

The wear detection unit is preferably designed to automatically teach in the reference movement information. For this purpose, for example, the movement information is used shortly after an exchange, ie with a guaranteed fully functional component, automatically as a reference. Incidentally, it is also conceivable to teach several reference movement information in order to take into account, for example, different loads or load changes on the pneumatic axis.

The wear detection unit is preferably designed to determine the times at which the piston passes the switching points, the movement information about the input and / or extension movement of the piston and / or the degree of wear only in a part of the piston movements. The wear of the cylinder is compared to the measurement of the position only very slowly. Therefore, the time measurement does not have to be done at every position determination or piston movement. It is sufficient to carry out the time measurement only in one part, for example every second, tenth or one hundredth overrun. As a result, the wear detection unit or its processor for determining the wear indication can be significantly relieved of computational power.

The invention will be explained in more detail below with regard to further features and advantages by way of example with reference to embodiments and with reference to the accompanying drawings. The illustrations of the drawing show in:

1 a schematic sectional view through a cylinder mounted thereon inventive sensor with retracted piston;

2 a schematic sectional view according to 1 with the piston extended;

3 a block diagram of the components of a sensor according to the invention;

4 an exemplary course of the switching signals of the sensor according to the 1 - 3 at its two switching points;

5 a diagram for explaining an altered by different wear effects Einfahrzeit the piston; and

6 a representation according to 5 for changed extension times of the piston.

1 shows a sectional view of a working cylinder 10 with a piston which is hydraulically or pneumatically movable therein 12 while in a retracted state 2 is a corresponding sectional view in an extended state. The cross-section of the cylinder is generally but not necessarily circular.

In a groove on the peripheral surface of the cylinder 10 is a position sensor according to the invention 14 assembled. The housing of the sensor 14 It is designed so that it is stable and properly adjusted in the outer contour of the cylinder 10 insert. About a connection 16 becomes the sensor 14 connected to a plant control.

The sensor 14 monitors two switching points S1 and S2, thus generating a switching signal when the piston 12 located in the appropriate positions. Shown in 1 the retracted end position at switching point S1 and in 2 the extended end position at switching point S2. The switching points S1, S2 are, however, as by arrows 18 indicated, arbitrarily adjustable, and there are also additional switching points possible.

The movement of the piston 12 gets through with an arrow 20a -B indicated pressure change in the respective cylinder chamber causes. Due to wear, the pressure difference prevailing in the two cylinder chambers caused by the pressure build-up changes, which drives the movement. A distinction is made between internal leakage and external leakage. Internal leakage concerns leaks between the two cylinder chambers, while external leakage occurs both as a leak on the piston rod and in the pressure-conveying system. Internal and external leaks influence the movement behavior of the piston 12 , Another influence is the friction force of the piston 12 in the cylinder 10 with a friction coefficient r, which counteracts the actual direction of movement. Wear is primarily noticeable here through increased friction. However, the reverse is also conceivable when lubricant escapes and thus at least temporarily makes the movement too smooth.

The sensor 14 according to the invention not only has the function of detecting the position at the switching points S1 and S2, but also detects the degree of wear. 3 shows the structure of the sensor 14 in a block diagram. Two sensor elements 22a -B provide a sensor signal depending on the position of the piston 12 , The sensor elements 22a For example, -b work magnetically or inductively. Such sensor elements 22a -B are known per se and are therefore not explained in detail.

The sensor signals become an evaluation unit 24 fed. There is in a position detection unit 26 Based on a threshold assessment, determined whether the piston 12 is in a position corresponding to the switching points S1, S2 and generates a respective switching signal. The switching points S1, S2 are in the position detection unit 26 teachable. It is not necessary for the sensor 14 as shown, a sensor element 22a -B has each switching point. Instead, only a common sensor signal with several switching thresholds can be evaluated in order to detect the switching points S1, S2.

A wear detection unit 28 evaluates the times at which the switching points S1 and S2 are reached, in a below using the 4 - 6 explained way. If a degree of wear is detected which requires maintenance or replacement, a corresponding signal is sent via an LED 30 displayed or via an output 32 and the connection 16 output. The exit 32 In various embodiments, it either serves merely to output switching signals, or it supports a communication protocol such as IO-Link. With such an IO-Link connection, conversely, the teaching-in or the configuration of the sensor is the same 14 possible. Alternatively, operating elements such as teach buttons are provided for this purpose.

4 shows the switching signals for the two switching points S1 and S2. According to the position of the switching points S1, S2 in the 1 and 2 near the end positions of the piston 12 the switching signals run alternately. The distance between the falling edge at the switching point S1 and the rising edge at the switching point S2 is a measure of the extension time t _{out of} the piston 12 Accordingly, the distance between the falling edge at the switching point S2 to the rising edge at the switching point S1 t a measure of the running-in period _a of the piston 12 , The sensor 14 determined in this way, in addition to the position indication at the switching points S1, S2 and the times that are required for driving over the switching points S1, S2 in the entry and exit movement.

This allows the sensor 14 determine the degree of wear, for increasing wear is recognized by a change in the running-in period t _a and the extension time t _of the piston 12 noticeable. For this purpose, a comparison is made with reference times for undisturbed function. Such reference times can be learned automatically, for example, shortly after maintenance or replacement, ie when the operation is undisturbed. Depending on the size and Direction of a deviation between the measured times and the reference times, the wear detection unit 28 determine a statement about the degree of wear and possible causes of failure. The threshold for issuing a warning or maintenance message is freely adjustable or programmable by the user.

Beyond a simple comparison with a reference, even more complex evaluations are conceivable. For example, break-in period be t and _an extension time t _over a larger number of working cycles recorded and jointly evaluated statistically. Possible drift effects via temperature dependencies of the sensor elements 22a -B can be compensated via an additional temperature sensor or be included in the statistical evaluation.

5 _shows and their possible causes is a diagram illustrating the changes in the break-in At. A retraction delay is an indicator of increased friction, internal or external leakage. The friction is opposite to the desired movement. Both an internal and an external leakage means that the pressure difference can not be built up to the expected extent. Will the amount of At _too large, the sensor outputs 14 a maintenance request. Even with a strongly accelerated retraction thus takes place a maintenance request. The cause is to be found in a reduced friction. This indicates lubricant leakage and is in any case a significant deviation from normal operation.

6 shows one of 5 corresponding diagram for the extension time .DELTA.t _off . With a reduced extension speed, the cause of the error is mainly to be found in an increased friction and inferior in a larger internal leakage. An increased extension speed is due to increased external leakage or reduced friction. The external leakage preferably occurs at the piston rod, which draws impurities from the environment into the seal with its working movement. Leakage at this point helps vent the rod-side cylinder chamber and thereby accelerate the extension movement.

All current information, such as entry and exit time, size and direction of the deviation, degree of wear, cause of wear, can be displayed 30 displayed or the output 32 be queried, in particular via IO-Link. The switching signals can also be transmitted time-stamped via IO-Link in order to carry out the evaluation in the system control with further signals, such as the valve control.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• EP 1640618 A1 [0003]
• DE 102007051468 A1 [0007]

Cited non-patent literature

• S. Fritz, C. Stammen, "On the trail of wear - Possibilities for plant diagnostics in pneumatics", Journal O + P, Issue 9/2006, pp. 432-441 [0007]

Claims (9)

Sensor ( 14 ) with at least one in particular magnetic or inductive sensor unit ( 22 ), which generates a sensor signal as a function of the position of a cylinder ( 10 ) fluid-driven retractable and retractable piston ( 12 ) and with an evaluation unit ( 24 . 26 ), in which, based on the sensor signal, the position of the piston ( 12 ) at at least two switching points (S1, S2) can be detected, characterized by a wear detection unit ( 28 ), which is adapted from the times when the piston ( 12 ) the switching points (S1, S2) passes, a movement information about the input and / or extension movement of the piston ( 12 ) and to determine a degree of wear by comparison with a reference movement information, the sensor ( 14 ), a warning unit ( 28 . 30 . 32 ), by means of which a wear warning can be output on the basis of the degree of wear. Sensor ( 14 ) according to claim 1, wherein the warning unit ( 28 ) has an adjustable threshold, from which degree of wear the wear warning is issued. Sensor ( 14 ) according to claim 1 or 2, wherein the wear warning comprises quantitative information on the degree of wear. Sensor ( 14 ) according to one of the preceding claims, wherein the wear warning is displayed ( 30 ), in particular an LED, or via a signal output ( 32 ) is dispensable. Sensor ( 14 ) according to one of the preceding claims, wherein the wear detection unit ( 28 ) is designed to determine a cause for the deviation in the comparison between movement information and reference movement information at the same time as the degree of wear Sensor ( 14 ) according to one of the preceding claims, wherein the wear detection unit ( 28 ) is designed to record deviations between movement information and reference movement information via a multiplicity of entry and exit movements and to evaluate them statistically, wherein in particular the warning unit ( 28 ) is adapted to output the wear warning when detecting a time trend. Sensor ( 14 ) according to one of the preceding claims, which is housed in a housing which is designed so that it is in a groove on the circumference of the cylinder ( 12 ) is mountable. Sensor ( 14 ) according to one of the preceding claims, wherein the wear detection unit ( 28 ) is adapted to teach the reference movement information automatically. Sensor ( 14 ) according to one of the preceding claims, wherein the wear detection unit ( 28 ) is adapted to the times at which the piston ( 12 ) passes the switching points (S1, S2), the movement information about the input and / or extension movement of the piston ( 12 ) and / or to determine the degree of wear only in part of the piston movements.

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