DE20303784U1 - Active vibration compensation apparatus e.g. for atomic force microscope, digitizes sensor signals, supplies to digital data processor for analysis, and controls actuator accordingly - Google Patents

Abstract

The apparatus includes an actuator for applying forces or deflecting a table (11) based on signals from one or more sensors (17). A control circuit (16) receives and processes the sensor signals and drives the actuator(s). A digital data processor is used to process the sensor signals. An interface including at least one A/D converter digitizes the sensor signals for supply to the digital data processor. The processor analyzes sensor signals produced at different times.

Description

** ·· ta

Patent attorneys

European Patent and Trademark Attorneys

Applicant: Halcyonics GmbH
Attorney file: G-HAL 17

Device for active vibration isolation and/or control

Description

The invention relates to a device for active vibration compensation, comprising a base structure and a support table that can be deflected relative to it, further comprising: at least one sensor coupled to the support table for detecting at least one component of an acceleration and/or a force to which the sensor is exposed as a result of a vibration, and for generating corresponding sensor signals,

at least one actuator for the controlled application of force and/or deflection of the support table as a function of sensor signals from at least one sensor, at least one control circuit for receiving and processing sensor signals and for controlling the at least one actuator.

Such a device, known for example as MOD-I from Halcyonics GmbH, Göttingen, Germany, is used for the active vibration isolation or control of a vibration-sensitive device arranged on the recording table, for example a measuring device for high-resolution imaging applications such as AFM (Atomic Force Microscopy), SNOM (Scanning Nearfield Optical Microscopy) and others. Such measuring tape arrangements, which are generally summarized under the term "Scanning Probe Microscopy", are extremely sensitive to vibrations, whereby low-frequency building vibrations in the range of approx. 0.5 to 30 Hz, acoustic vibrations with frequencies > 20 Hz as well as vibrations caused by motor equipment related to the measuring arrangement itself in the

Range of approx. 10 - 500 Hz (e.g. vacuum pumps) are disruptive. Active vibration compensation systems can be designed for vibration isolation, i.e. to protect vibration-sensitive equipment against vibrations of the foundation, and/or for vibration control, i.e. to compensate for vibrations generated by the equipment itself or devices connected to it. The term "vibration compensation" is understood in this application to mean both forms and mixed forms. The basic principle of active vibration compensation is the detection of the vibration to be compensated using suitable sensors and the fastest possible conversion of the sensor signals into control signals for controlling compensatory actuators. Both the sensors and the actuators advantageously use the piezoelectric effect. The control circuit for converting the sensor signals into control signals for the actuators is usually carried out via an analog, electronic control circuit with suitable filter, differentiation, integration and other elements.

In the known device, the analog sensor signals are not only fed to the control circuit. Instead, an analog interface is provided, which contains a multiplexer that combines the signals of individual sensors in sections to form a single signal and makes this available for external tapping at a BNC socket. The multiplex signal can be made visible on an oscilloscope, for example, in order to be able to qualitatively observe the effect of the active vibration compensation. For example, the failure of a sensor or actuator can be determined by comparing the oscilloscope signal when the vibration compensation is switched on and off. However, this measure is not sufficient for further analysis. The output signal of the analog interface is designed in such a way that a trigger pulse is followed by a number of signal intervals of a few milliseconds in length, each with a different offset, corresponding to the number of sensors. This leads to the oscilloscope

·

· ■

·

loscope screen to a step-like signal in which the "unrest" of each individual step represents a qualitative measure of the size of the vibration recorded by the respective sensor. Due to the obvious inaccuracy of this method, it is only suitable for determining a total failure of a sensor or actuator, but not for any further assessments. There is often interest in a detailed evaluation of the current vibration environment of the compensation device. For example, certain areas of a laboratory room are exposed to a higher level of vibration than others due to structural properties. In order to optimally carry out vibration-sensitive experiments, it would therefore be advantageous to be able to determine an area with particularly low vibrations for setting up the vibration compensation device. There are also daily recurring periods of particularly high and particularly low vibration exposure in a laboratory room, for example due to road traffic outside the building, machines running at certain times, public traffic, etc. Determining these periods can also be valuable for optimizing a vibration-sensitive experiment. The sensor signals available with the current state of the art cannot be used for this purpose.

It is therefore an object of the present invention to further develop a generic device in such a way that a user is supported in determining an optimal installation location of the device itself and/or optimal times for carrying out an experiment compensated for vibrations by means of the device.

This object is achieved in conjunction with the features of the preamble of claim 1 in that a digital data processing device is provided for the digital processing of sensor signals and at least one interface comprising at least one A/D converter for the digitization of sensor signals and for the transmission of the digitized sensor signals to the digital data processing device.

*· Y Y

device and that the digital data processing device is programmed in such a way that analyses of sensor signals generated at different times and, on the basis of the analysis results, a comparative, vibration-related assessment of the vibration environments at the different times at which the sensor signals are generated can be carried out automatically.

This special design of the vibration compensation device makes it possible to carry out a method which comprises the following steps:

Generating at least one first analog sensor signal at a first time, digitizing the at least one first analog sensor signal by means of at least one A/D converter included in an interface, transmitting the at least one first digitized sensor signal to the digital data processing device, vibration analysis of the at least one first digitized sensor signal, buffering the at least one first digitized sensor signal and/or the corresponding first analysis result, generating at least one second analog sensor signal at a second time, digitizing the at least one second analog sensor signal by means of at least one AD converter included in an interface, transmitting the at least one second digitized sensor signal to the digital data processing device, vibration analysis of the at least one second digitized sensor signal, buffering the at least one second digitized sensor signal and/or the corresponding second analysis result, comparing the buffered analysis results and outputting a value to display a best analysis result determined according to pre-programmed criteria. This method can be used, for example, to determine the best time to carry out an experiment. The recording of the sensor signals with the active vibration compensation switched off essentially corresponds to a measurement of the

Vibration environment at the installation location of the vibration compensation device at the current measurement time. A vibration analysis of the digitized sensor signals enables a quantitative assessment of this vibration environment. It is of course possible to record the vibration environment of the compensation device at more than two different times. In principle, there are no restrictions here. The design of the vibration compensation device according to the invention also makes it possible to record the optimal installation location of the device. This is preferably done by setting up the device at different locations between the different recording times of the vibration environment. The comparison of the measurements at different times thus corresponds to a comparison of measurements at different locations. The output value thus serves as an indication of the optimal installation location.

In a particularly preferred embodiment, the digital data processing device is designed as an external calculation unit that can be connected via the interface. This ensures the greatest possible flexibility and at the same time minimizes the space requirements of the basic structure. The device according to the invention can be easily connected to the interface, which is preferably designed as a standard interface, in particular as a USB interface, to a laboratory computer that is usually available anyway for carrying out the intended experiments.

Although a particularly cost-effective embodiment provides for the interface to be designed for wired data communication with the external calculation unit, an advantageous further development may provide for the interface to be designed for wireless data communication with the external calculation unit. Infrared or radio links are particularly suitable for data communication.

In an alternative embodiment, the digital data processing device can be integrated into the basic structure as an internal unit. This enables the analysis and evaluation of the vibration environment without the need for an additional, appropriately configured computer and enables the vibration compensation device to operate completely as a so-called "stand-alone" unit. The data processing device can be designed, for example, as a microprocessor, digital signal processor or the like.

In contrast to the use of sensor signals to generate control signals for the actuators, the speed of data acquisition and processing is not a relevant factor when analyzing the vibration environment. Time losses due to digitization, transmission, digital calculation, etc. of the sensor signals can be accepted here. In the context of vibration compensation, however, the time factor is one of the key factors, so analog control circuits are currently used. In principle, however, it can also be provided that the digital data processing unit to which the digitized vibration data is transmitted can also be used as a control circuit. An embodiment is conceivable in which the vibration environment analysis and the generation of the control signals are carried out by an external laboratory computer. However, an alternative embodiment is also possible in which these tasks are carried out by an internal, possibly hardware-optimized calculation unit.

A disadvantage of the prior art, in which the analog signals of several sensors were multiplexed into a single analog signal, is the lack of simultaneity in the generation of the jointly displayed measurement data. In a particularly advantageous embodiment of the present invention, it is provided that the interface is able to bundle simultaneously generated, AD converted signals of several sensors into a communication packet and transmit this to the digital data processing device. This requires

of course requires the signals to be buffered at least temporarily, although this is within the scope of expert knowledge for interfaces with A/D converters. In this way, it is possible to analyze the data recorded simultaneously by different sensors together or separately, but in any case to achieve a reliable analysis of the entire vibration environment at a specific point in time. It is advantageous for the interface to have a number of A/D converters corresponding to the number of sensors. If these are operated in parallel, the data from all sensors in the vibration compensation device can be recorded together. Of course, other variants are also possible in which only a subset of the simultaneously generated sensor signals is digitized and bundled into a communication package.

In order to optimize the database on which the analysis of the vibration environment is based for each individual case, a particularly advantageous embodiment of the invention provides that the digital data processing device has an input interface via which parameters can be entered that relate to the digitization and/or transmission of sensor signals. This can, for example, be information on sampling frequency, trigger parameters, the length of the time interval to be recorded, the arrangement of individual signals within the communication bundle, etc.

Further details of the invention emerge from the following detailed description and the accompanying drawings, in which preferred embodiments of the invention are illustrated by way of example.

In the drawing, the only

Figure 1: a diagram for the symbolic description of the device according to the invention using a preferred method.

A known part of a vibration compensation device is symbolically summarized under the reference number 10. The purpose of the vibration compensation device 10 is to isolate a vibration-sensitive apparatus (not shown) such as a measuring setup mounted on a support table 11 from vibrations of the foundation 12 or to compensate for vibrations generated by the apparatus itself. The spring 13 between the support table 11 and the foundation 12 serves as a decoupling in the high frequency range. The effect of a suitably dimensioned spring 13 as a mechanical low-pass filter is known to those skilled in the art. Only the low-frequency vibrations therefore require active compensation. For this purpose, an accelerometer 14 is provided which is firmly connected to the support table 11. The accelerometer 14 essentially consists of an inertial mass 141 which acts on a piezoelectric plate 142. Depending on the alignment of the inertial mass 141 and the piezoelectric plate 142, different components of a vibration of the recording table 141 can be detected. Due to the mechanical stresses generated by the inertial mass 141 in the piezoelectric plate 142, electrical sensor signals are generated at the output of the accelerometer due to the piezoelectric effect, which are related to the size of the measured vibration component. These are pre-amplified by a preamplifier 15 and fed to the analog control circuit 16 (ctre). There they are converted into control signals to control the actuator 17. There are different concepts for its operation, two of which are implemented in the present embodiment. In a first concept, the actuator 17 works according to the principle of the plunger coil. For this purpose, a permanent magnet 171 is attached to the recording

table 11. Coils 172 controlled by control signals from the control circuit are immersed in the magnetic field of the permanent magnet 171, the magnetic fields of which interact with that of the permanent magnet 171 and cause a deflection of the receiving table 11, which is suitable for vibration compensation. In a second basic principle of vibration compensation, piezoelectric actuators 173 are used, which can compensate for vibrations through their change in length, which occurs when suitable voltages are applied.

This basic principle of a vibration compensation device is known from the prior art. The innovation according to the invention lies, among other things, in the fact that an interface 20 is additionally provided, which comprises at least one analog-digital converter (A/D) 21, but preferably one for each sensor 14. This converts the pre-amplified sensor signals into a digital character string 30. Essential digitization parameters, such as the sampling frequency, the length of the interval to be digitized, the resulting number of digital characters, the digitization depth, etc. can preferably be entered via a user interface (not shown). Of course, it is also possible to set these in an interface-specific memory. For the sake of clarity, only one sensor 10 is shown in Figure 1. To explain the further method steps, four digitized character strings 30 are shown, each of which was generated in a comparable manner. They symbolize the digitized signals of other sensors. The number of sensors used has no fundamental restriction and can also be varied by the user in special embodiments.

In a next process step, the digital character strings are buffered in the interface in a suitable manner, which is symbolically represented by the reference number 22. The purpose of buffering is to achieve a suitable bundling of digitized sensor signals from several

Sensors to form a communication packet 23. Various options are available to the person skilled in the art for bundling into a communication packet 23. Figure 1 shows a bundling method in which short data blocks, for example individual characters, of different sensor signals are interleaved with one another intermittently. This enables a quasi-continuous transmission of measured values recorded at the same time. It is of course also possible to connect larger data blocks serially or in parallel and in this way to bundle suitable communication packets. Since there is no real-time requirement for the intended analysis of a vibration environment, the person skilled in the art is subject to hardly any restrictions when designing the bundling mechanism. It is advantageous if, as shown in Figure 1, a header 24 is added to the communication packet 23, preferably preceded by it, which contains important communication and coding parameters as well as parameters characterising the signal itself. These include, for example, the digitization parameters already mentioned above, the type of interleaving and the length of the data blocks and the like. The bundled communication packet 23 is then forwarded to suitable transmission means 25 for transmission. Figure 1 shows a symbolic antenna, which indicates wireless communication between the interface and an external data processing unit. However, a wired connection is usually implemented for reasons of lower complexity. Of course, the transmission of the communication packet does not have to be carried out to an external data processing unit. Rather, transmission to an internal unit in the vibration compensation device is also possible.

Whatever the data processing unit to which the communication packet is sent or the actual transmission path is designed, the data 23 are received by suitable receiving means 25 ^of the data processing unit 40. Here, the data 22 are first decoded according to a decoding scheme corresponding to the coding scheme.

J IJL · · · _t *· · ? I *♦

encoded and restored as representations 50 of the individual sensor signals. According to the invention, the data processing unit 40 is set up so that the individual representations 50 of the sensor signals are each subjected to a vibration analysis. Which vibration parameters are determined in detail depends on the intended decision-making process and can be adapted by the expert. For example, it is possible to determine a power spectrum of the vibrations. Figure 1 shows the analysis result generally as Ei(t=ti), which is intended to indicate that this is the result of the analysis on which the sensor data recorded at time ti was based.

The entire procedure described above is carried out again at a time t ₂ and provides the analysis result value E ₂ (t=t2). In this way, N different analysis results can be determined from sensor data recorded at N different times.

Depending on the specific task, a comparison of the analysis results Ei to E _n is carried out using pre-programmed criteria and a result value W is output as an indication of the "best" vibration environment. The term "best" vibration environment is to be interpreted in relation to the analysis and comparison criteria used. The output value W does not have to be a single value. Rather, an ordered list can also function as the output value, for example.

According to the invention, the method outlined above runs automatically in its essential points, which allows the user to identify an optimal vibration environment for his needs in a particularly simple manner. If the vibration compensation device is set up at different locations between the recording of the individual sensor measurements, ie between the times ti, t ₂ ,.., t _N , the method described, which is made possible by the inventive design, is suitable.

•&igr;«&iacgr; · · · &Iacgr;

design of the device is made possible in order to identify an optimal installation location for the device. It is of course also possible to change the configuration of the measuring apparatus or other parameters between the times ti, t2,.., tN, so that the term "vibration environment" can basically be interpreted in a wide range of ways.

Of course, the embodiment of the device according to the invention and the method made possible thereby described with reference to the embodiment of Figure 1 merely represents a particularly advantageous embodiment for the purpose of illustration. In particular with regard to the number and specific type of generation of sensor signals, signal coding and transmission as well as the concrete analysis and decision-making, the expert has a wide range of design options.

Claims (10)

1. Device for active vibration compensation, comprising a base structure and a support table 11 which can be deflected relative to the base structure, further comprising: - at least one sensor ( 14 ) coupled to the receiving table ( 11 ) for detecting at least one component of an acceleration and/or a force to which the sensor ( 14 ) is exposed as a result of a vibration, and for generating corresponding sensor signals, - at least one actuator ( 17 ) for the controlled application of force and/or deflection of the receiving table ( 11 ) depending on sensor signals from at least one sensor ( 17 ), - at least one control circuit ( 16 ) for receiving and processing sensor signals and for controlling the at least one actuator ( 17 ), characterized ,
that a digital data processing device is further provided for the digital processing of sensor signals and at least one interface comprising at least one A/D converter for digitizing sensor signals and for transmitting the digitized sensor signals to the digital data processing device and that the digital data processing device is programmatically set up in such a way that analyses of sensor signals generated at different times and, on the basis of the analysis results, a comparative, vibration-related assessment of the vibration environments at the different times of generation of the sensor signals can be carried out automatically. 2. Device according to claim 1, characterized in that the installation location of the device can be changed between the different times of generation of the sensor signals, so that the comparative evaluation of the vibration environments corresponds to a vibration-technical comparison of the different installation locations. 3. Device according to one of the preceding claims, characterized in that the digital data processing device is designed as an external calculation unit which can be connected via the interface. 4. Device according to claim 3, characterized in that the interface is designed for wired data communication with the external calculation unit. 5. Device according to claim 3, characterized in that the interface is designed for wireless data communication with the external calculation unit. 6. Device according to one of claims 1 or 2, characterized in that the digital data processing device is integrated as an internal unit in the basic structure. 7. Device according to one of the preceding claims, characterized in that the digital data processing unit can additionally be used as a control circuit. 8. Device according to one of the preceding claims, characterized in that the interface is able to bundle simultaneously generated, A/D-converted signals from several sensors into a communication packet and to transmit this to the digital data processing device. 9. Device according to one of the preceding claims, characterized in that the interface has a number of A/D converters corresponding to the number of sensors. 10. Device according to, characterized in that the digital data processing device has an input interface via which parameters can be entered which relate to the digitization and/or the transmission of sensor signals.