Digital flux meters DFM 1 and DFM 2
A fluxmeter measures the magnetic flux of a magnetized magnet by integration of the voltage induced in the measuring coil by movement of the magnet. The digital flux meters ECKEL DFM 1 and DFM 2 represent a new generation of integrating flux meters. Unlike all other “electronically” (i.e. analogue) integrating flux meters with digital output of the integration voltage, at the DFM the input voltage is digitally sampled and then integrated by software. All over the world only ECKEL produces digitally by software integrating fluxmeters
The DFM contains a microcontroller with on-board software which performs both the device control and the integration and evaluation.
The test input is digitized my means of a 14-bit A/D converter and with a sampling rate of 0.625 ms (1,600 measurements per second). The high sampling rate is also ensuring high accuracy at high speed measurements. The evaluation and update of the display output is also performed at that frequency. The input voltage is totalled at an integration width of 32 bit in the microcontroller. This allows for a high resolution. At the measurement the magnet is moved out of the measuring coil and the voltage induced by that process within the coil is integrated, and finally, the magnetic total flux is determined.
At this measurement the null balance and its long-term stability is crucially important. An error within the null balance is causing a drift of the integration value. Up to now the calibration had to be performed separately for each measuring coil as the ohmic resistance of the coil was affecting the measurement.
The long-term stability was dependent on the components used. Sooner or later a new drift calibration had to be performed on each analogue integrator. The long-term stability is particularly crucial at automatic sorting machine applications installed within assembly lines.
The DFM 1 Flux Meter performs a highly accurate drift-free flux measurement without any calibration and at maximum errors of less than 0.5%. It is optimized for quality assurance applications performed at manufacturers and users of permanent magnets with a minimum magnetic flux of 100 µVs; for example, magnet segments used for manufacturing fractional-horsepower motors or loudspeaker magnets.
The range of measurement is up to 8191 µVs with a resolution of 1 µVs on a 4-digit display. This range of measurement is related to the magnet and it already considers the number of turns. Seven different input ranges from maximum 93.75 mV to 6 V can be selected. Overload and low modulation are displayed to obtain a high signal-to-noise ratio.
The input is protected against voltage surges. Nevertheless, the DFM must not be connected to measuring coils of capacitor discharge pulse magnetization devices; magnetization periods in the range of milliseconds induce very high voltage surges for the measuring coil. In such cases the flux meter must be disconnected from the coil by a relay during the magnetization process.
The DFM 1 can be used both as a lab measurement device and as a device which is used at assembly lines. It includes all features necessary for the use in full automatic quality inspection and rejection systems. »Reset« and »Hold« can be controlled remotely. The result is checked with adjustable upper and lower limits and it triggers one of three relays for controlling the sorting equipment.
The lower and upper limit can be adjusted from 0 to 8000 µVs.
10, 20, 50 and 100 can be selected as the number of turns of the measuring coil. For adaptation to reference magnets, a calibration relative to the internal reference of 0.500 to 2.000 can be adjusted.
All parameters can be set via push button. The parameters can be stored in the EEPROM and are loaded automatically as soon as the unit is switched on. The DFM 1 has an analogue output (1mV / µVs) for a standard graph plotter or an oscilloscope and has a printer connector for a standard printer.
The DFM 1 has been designed in co-operation with Robert Bosch GmbH, where it is used for quality inspection purposes.
In the DFM 1 the relative calibration using calibration pole housings is integrated according to Bosch standard. In a continuous calibration system the ECKEL Robograph 2 is used for the production of calibration pole housings for flux measurement of ready-to-use pole housings.
Further the DFM 1 offers the absolute function. This function displays the absolute peak value between two extreme values. It is used to measure the flux of turning rotors. For this application the rotor must be turned far enough to guarantee a complete polarity change at each coil. The displayed value corresponds to the difference between the largest negative and the largest positive flux during a rotation.
Also the DFM 1 is can calculate the average flux value of all poles for 2 up to 8 pole motors. When turning the motor the poles are detected and counted automatically. After a full turn the average value is shown.
Due to the combination of the microcontroller and an internal voltage reference, the operation of the DFM is completely maintenance-free. Appropriate algorithms allow for an automatic internal calibration and for a fully automatic internal absolute drift compensation by means of continuously tracking the drift parameters, which is impossible for even the best analogue integrators.
The measurement is independent from the resistance of the measuring coil as the differential input resistance totals 10 GOhm.
Any type of measuring coil can be used. Use your existing measurement equipment with the DFM! It is recommended to perform the measurement using coils which are adapted to the magnet. The measurement of small magnets with large Helmholtz coils can cause problems due to strong scattering effects.
The high resolution model DFM 2 has been designed for the measurement of also significantly smaller magnets. The basic equipment is identical with model DFM 1.
Relative calibration as well as absolute or average value measurement is not possible with the DFM 2.
The model DFM 2 features the following ranges of measurement:
- 9999 µVs with a resolution of 1µVs
and with 4 signal input ranges from maximum 750 mV to 6 V
- 999.9 µVs with a resolution of 100 nVs
and with 4 signal input ranges from maximum 75 mV to 600 mV
- 99.99 µVs with a resolution of 10 nVs
and with 4 signal input ranges from maximum 7.5 mV to 60 mV
- 9.999 µVs with a resolution of 1 nVs
and with 4 signal input ranges from maximum 0.75 mV to 6 mV
All ranges of measurements are related to the magnet using a measuring coil with 10, 20, 50 or 100 windings.
The lower and upper limit can be adjusted individually over the whole range of measurement.
With the DFM 2 the maximum error is increasing with higher input amplifications depending on the external interference level. For accurate measurements at nVs levels, as with all flux meters, an electromagnetic low-interference measuring desk is absolutely necessary. The maximum input amplification at a resolution of 1 nVs is 4000!
The DFM 2 utilizes a modified algorithm for the drift compensation. The DFM 2 indicates the external interference level to simplify the positioning. If measurements are required to be highly accurate while using high resolutions, it is recommended that a calibration should be performed by using a reference magnet. It will then be compensated for variations in the result, originating from varying strong external disturbances caused by different measuring desks. At the DFM 2 a new start of the drift evaluation might be required via push button in case of drastic changes of the external interference level at high resolutions.
The application on assembly lines in close proximity to strong sources of interference and while using high resolutions is absolutely not recommended
The use of intelligent software rather than expensive hardware components leads to a smaller size of only 180x185x65 mm and to a more attractive price.
The DFM can be supplied with a German or an English operating manual.
The devices are offered with a two-year guarantee and a 10 year service guarantee.