WO1998002685A1

WO1998002685A1 - Self-testing device for pneumatic drives and position regulator equipped with this device

Info

Publication number: WO1998002685A1
Application number: PCT/DE1997/001487
Authority: WO
Inventors: Klaus-Peter Heer
Original assignee: Siemens Aktiengesellschaft
Priority date: 1996-07-16
Filing date: 1997-07-14
Publication date: 1998-01-22
Also published as: DE29612346U1

Abstract

In the disclosed device, the reaction time (ta) of a drive element (2) to a valve actuation is measured and compared to a stored reference value. A test result is derived from the deviation between the two values and conclusions may be drawn from the test result on the state of springs (4, 5, 6) located within the drive housing (1). The invention is used in pneumatic position regulators.

Description

description

Device for self-testing pneumatic actuators and positioner with such a device

The invention relates to a device for self-testing pneumatic drives according to the preamble of claim 1 and a positioner with such a device.

A pneumatic drive is known from published patent application DE 36 37 068 AI. It is provided with a drive housing, a diaphragm plate movably mounted in it, rigidly connected to a drive rod, which can be pressurized with compressed air on one side, a spring arranged on the diaphragm plate against the action of the compressed air and arranged inside the drive housing, a position meider, who detects the position of the diaphragm plate, and with a valve for adjusting the air pressure on the diaphragm plate. A device for the self-test of the pneumatic drive is not described in the cited publication.

The SIPART PS electropneumatic positioner from Siemens AG is used to control the position of a connected actuator, such as a valve or flap position, on pneumatic linear or part-turn actuators. A process controller or control system specifies a setpoint for the positioner via an interface, for example a digital Profibus connection or an analog 4-20 mA input, and the positioner then forces a corresponding corresponding setpoint on the actuator Position. The pressure in one drive chamber or, in the case of double-acting drives, in both drive chambers is changed until the predetermined position of the actuator is reached. For this purpose, the current position is detected with a displacement sensor, for example a conductive plastic potentiometer, and an actual value signal generated with the displacement sensor is fed to a microcontroller together with the setpoint. This compares both signals, forms a control deviation and, taking into account the dynamics of the pneumatic drive, calculates the required switching reactions of downstream pneumatic valves. One valve is in the supply air branch to increase the air pressure in the chamber, another valve in the exhaust air branch and opens when the chamber is to be vented. This positioner has many special functions integrated, for example an alarm message is issued if a control valve is blocked or the air pressure in the supply network is too low. Further information on the SIPART PS positioner can be found, for example, in an information document published by Siemens AG with the order number E80001-V0343-A020.

Single-acting pneumatic drives generally have several springs, which provide the counterforce to the pneumatic drive force required to reset the drive. If a spring breaks or the force of the springs diminishes over time due to corrosion, the drive will continue to work, but it may not be possible to ensure that a control valve operated with it is sealed. A further disturbance, e.g. B. if a spring breaks or the valve friction high, can lead to a total failure with serious consequences, since the drive may no longer be able to close a valve tightly in an emergency. Such failures have to be avoided through routine, preventive maintenance work, which is very complex.

The invention has for its object to provide a device for the self-test of pneumatic drives, which enables early detection of damage with little effort, and to provide a positioner with such a device.

To achieve this object, the new device of the type mentioned at the outset has the features specified in the characterizing part of claim 1 and the new positioner has the features specified in the characterizing part of claim 3. Advantageous embodiments are described in the dependent claims.

The invention has the advantage that, in addition to the position indicator and valve required in any case for a positioner for pneumatic drives, no additional sensors or actuators are required to adjust the air pressure. The new device takes advantage of the fact that if a spring fails or the spring force is reduced due to corrosion, a lower back pressure in the air pressure chamber is required for a balance of forces. If a setpoint jump for a lower back pressure is specified, a valve for supplying or discharging compressed air to the pressure chamber must also be actuated for a shorter time because due to the reduced spring constant, only a smaller change in pressure has to be generated. This reduces the response time of the drive element to valve actuation, ie the new setpoint is reached more quickly.

A direct measurement of the spring force to diagnose the spring condition would be complex since it would require special force sensors in the drive housing. A poor condition of the springs could also be identified by comparing a measured pressure / displacement characteristic with a reference characteristic. However, a pressure transmitter and an evaluation device with an additional analog input would have to be provided for this diagnostic option. In contrast, a force or pressure transmitter is not necessary in the device according to the invention.

The start-up time can advantageously be evaluated for the self-test of pneumatic drives, ie the time which elapses in a previously depressurized chamber after opening the supply valve until a drive movement begins. With this measurement, the start and end pressure in the chamber are low and thus the difference between the air pressure in the supply network and the air pressure in the pressure chamber of the drive housing is maximum, so that fluctuations in the air pressure in the supply network have relatively little effect on the time measurement and this diagnostic method is almost independent of the supply pressure. Since the pressure difference changes only slightly during the time during which a gas stream flows from the supply network into the chamber, and the ratio of network pressure to chamber pressure is greater than the critical pressure ratio required for Air is about 2, the gas flow is also largely constant. During the start-up time, the drive element is still stationary, the pressure and thus the resulting force on the springs increase linearly over time with a constant chamber volume. The start-up time until a movement of the drive element begins is therefore approximately proportional to the spring preload and almost independent of the pressure in the supply lines of the compressed air network.

The device for the self-test can be implemented both with a hardware circuit and with software in a digital, microprocessor-controlled positioner. The implementation with a positioner has the advantage that no additional circuit parts are required, since time measuring devices are usually present as so-called timers in microprocessors. When commissioning a positioner on a pneumatic actuator for the first time, a reaction time can be measured and saved as a reference value for a later self-test. This has the advantage that a positioner automatically adapts to different actuators, which may have different start-up times, without an operator having to enter these start-up times.

Embodiments and advantages of the invention are explained in more detail below with reference to the drawings, in which an embodiment of the invention is shown. FIG. 1 shows a block diagram of a drive with positioner and FIG. 2 shows a time diagram to explain the start-up time.

A pneumatic drive has a drive housing 1 in which a drive element, here a diaphragm plate 2, is movably mounted. The drive element 2 is connected to a push rod 3, which is used to actuate, for example, a control valve not shown in FIG. 1. On the top of the diaphragm plate 2 there are three springs 4, 5 and 6 within the drive housing 1, which exert a compressive force Fo on the diaphragm plate 2. The underside of the diaphragm plate 2 forms, together with the lower part of the drive housing 1, a pressure chamber 7, which can be aerated via a feed line 8 and vented via a discharge line 15. Depending on the pressure prevailing in the pressure chamber 7, a force Fu acts on the underside of the diaphragm plate 2. For static conditions, the drive element is in equilibrium of forces, ie the force Fu on the underside and the force Fo on the top of the diaphragm plate 2 are the same size, if you disregard the force acting through the push rod 3. To detect the current position of the push rod 3, a position indicator 9 is provided, which is coupled to the push rod 3 via a position tap and supplies an electrically evaluable position signal 10 to a positioner 11. To actuate the pneumatic drive, the positioner 11 opens or closes a piezo valve 12, which on one side has a line 13 of a compressed air supply network and the other side of which is connected to the feed line 8. If necessary, a further valve 14 can be provided for quick ventilation, which opens or closes the discharge line 15 to the surroundings. A display 16 is connected to the positioner 11 and serves to output current position values and error messages in the event of an error. In addition, test results can be displayed to a higher-level process control system via a bus interface, which is not shown in FIG. 1.

The measurement of a start-up time ta is now to be illustrated on the basis of FIG. In the case of a previously vented pressure chamber 7 (FIG. 1), ie when the piezo valve 14 was open, the piezo valve 14 is closed and at the time tO the piezo valve 12 is opened, so that a gas stream begins to flow into the pressure chamber 7. According to curve 17 in FIG. 2, the pressure in the pressure chamber 7 and thus the force Fu acting on the underside of the membrane plate 2 increases essentially linearly. Since the pneumatic drive is pressed into its end position due to the prestressing of the springs 4, 5 and 6, it remains there until the moment tl there is a balance of forces between the forces Fo on the upper side and the force Fu on the lower side of the diaphragm plate 2. With further supply of compressed air, the diaphragm plate 2 begins to move away from its end position, since the force Fu on the underside is greater than the preload Fv of the springs 4, 5 and 6. This beginning movement is detected by the position indicator 9 and the positioner 11 , which stops an internally running time measurement at the start of a movement. The time between opening the valve 12 at the time tO and the onset of movement of the push rod 3 at the time t1 is evaluated as the starting time ta. When the positioner 11 is started up for the first time, the measured value of the start-up time ta is stored in a memory within the positioner 11. Start-up times measured during later operation of the pneumatic actuator are compared with this stored reference value and monitored for impermissible deviations. If the start-up time has decreased considerably compared to the reference value, this is an indication of corroded or broken springs in the drive.

Instead of the start-up time ta, the positioner 11 can similarly monitor the positional speed of the pneumatic drive and compare it with a speed / position characteristic curve stored as a reference. If the effect of the springs 4, 5 and 6 declines over time due to wear, then a predetermined change in position is achieved after a shorter opening time of the valve 12 or the valve 14, i. that is, the positioning speed increases.

Claims

claims

1. Device for the self-test of pneumatic drives with a drive housing (1), a drive element movably mounted therein, connected to an actuating means, in particular a push rod (3) for a control valve, drive element, in particular a diaphragm plate (2) or a piston, at least on one side An adjustable air pressure can be applied, with at least one spring (4, 5, 6) acting on the drive element (2) against the action of the air pressure and arranged within the drive housing (1), with a position indicator (9) which directly or indirectly controls the Position of the drive element (2) detected, and with at least one valve (12, 14) for adjusting the air pressure on the drive element (2) by supplying or discharging compressed air, characterized by:

a time measuring device (11) with which the reaction time (ta) of the drive element to a valve actuation can be measured,

- an evaluation device (11) with which a measured reaction time (ta) can be compared with a reference value for determining a deviation and a test result can be derived from the deviation, and - a device for displaying the test result.

2. Device according to claim 1, characterized in that the measured response time (ta) is the time which elapses until the drive element (2) after a supply line from Compressed air begins to move away from an end position that it assumes when the drive is depressurized.

3. Positioner with a device according to claim 1 or 2, characterized in that the device for self-test in the positioner (11) of the pneumatic actuator is integrated.

4. Positioner according to claim 3, characterized in that a measured response time (ta) can be stored as a reference value for a later self-test in a commissioning mode of the positioner (11).