High electric voltages are required for many applications. However, they always involve a potential danger. Static discharges and high field strengths, for instance, can easily destroy electronic components. If a discharge is accompanied by a spark, flammable vapors may even explode. In view of these hazards, shielding and grounding are mandatory in many areas. On the other hand, the advantages of electrostatic charging are frequently exploited by state-of-the-art technology, e.g. laser copiers, electrostatic application of coatings, or flue gas scrubbing. In such cases it is important to know the actual amount of electric potential. In areas such as these, an electric field meter is the appropriate instrument. These handy devices are also equipped with miniature drives. As a result, field strength, and hence the applied voltage, can be determined on a non-contact basis without any distortion of measurements.
High voltages can be produced in many different ways. There is the "natural" charge separation, which occurs when rubbing surfaces against each other, when transferring liquids with a pump, or when exposing an object to radiation. Another method is the artificial generation of high voltage potentials for technical purposes. In all cases it is important to determine the strength of the electric fields. Electrostatic charges are very difficult to measure without actually influencing them. With this in mind, Kleinwächter GmbH, based in Hausen, Germany, developed a compact electric field meter for precision measurement.
Designed along the lines of an electrostatic generator, the meter is designed specifically for non-contact measurement of electric field strength. A rotary shielding vane temporarily prevents the penetration of the measuring instrument by the field. In this way, the field hits the measuring electrode at regular intervals; electrostatic induction thus generates an alternating voltage. What is actually measured is the alternating current proportional to it. Applying this principle, it is possible to measure electric charges and fields without removing energy from them. Since field strength E is measured in volts per meter at a known distance (d), the instrument uses the figure to calculate the voltage (U=E*d). If the phase angle of the voltage generated by electrostatic induction is compared with the position of the impeller, the polarity of the voltage and the direction of the field can also be determined.
The user-friendly instrument, which comes in an antistatic plastic housing, can cover a wide span of voltages in five measuring ranges. The ranges are measuring distances of 1, 2, 5, 10, and 20 cm. For example, at a distance of 1 cm it is possible to measure a voltage of 0 to 10 kV, while
at a distance of 20 cm one can measure 0 to 200 kV. For use in inaccessible locations the display can also be frozen and read afterwards. The digital display shows the selected measuring distance at the top and the measured charge at the bottom. The high zero-point stability of the measuring principle makes it possible to dispense with the otherwise customary procedure of zero-point balancing. The instrument is only 122 x 70 x 26 mm in size and weighs approx. 130 g. An extended version of the instrument provides additionally an analog voltage output, ±1V. As a result, measurements can also be subjected to further processing on a PC, with the aid of the A/D converter UAC 110. Power is supplied by a 9 V rectangular battery. To allow longer operating times with the limited power supply of the battery, all the components of the field meter must be optimized for minimal power consumption. This particularly applies to the drive motor for the impeller. To drive the shielding vane in the new electric field meter, Kleinwächter soon found a suitable motor in the extensive product range of FAULHABER.
Small, light, efficient
The selected Flat DC-Micromotor is scarcely 6 mm in height, not including the shafts, which saves space for integration into the head of the measuring instrument. The net diameter is only 15 mm. The DC motor features precious metal commutation and is ideal for use in devices operated by rechargeable batteries. Depending on the power supply of the device, motors can be used with an operating voltage of 3 V, 6 V, or 12 V. The level of efficiency is 67% and that extends the operating time per battery charge. Idling speed is around 12,000 rpm. With its plastic housing the motor weighs only 4.3 g. Depending on the voltage version the power output is between 0.15 W and 0.22 W. As a Flat DC-Micromotor, the speed of the motor can easily be controlled via the supply voltage. Another advantage of these miniature motors is their low starting voltage and easy start-up even after a lengthy period of non-use. Consequently, it is also possible to safely operate measuring instruments that are not used on a regular basis. If the standard version does not optimally match the particular application, the drive can be customized accordingly. In addition to the mounting plate, the armature shaft can be adapted to suit special requirements. The shaft then projects at both ends of the motor; through it the impeller charge essential for measurement is supplied via the motor from below.
Nowadays, state-of-the-art DC miniature drives are suited for a very wide field of applications. They range from measuring and control equipment and simple actuators to precision instruments. Sturdy, long-life design, simple control via operating voltage, and good electromagnetic compatibility enable use in sensitive measuring instruments or medical equipment.