Ultra high sensitive digital electromagnetic balance for measurement of weights in microgram: Design and experimentation

This paper proposes design of a weight measurement system capable of measuring weight in the range of micrograms to milligrams. The configuration of the system resembles with a conventional straw balance. Unlike straw balance the proposed system employs a force overcome technique. It does not employ any spring as that in case of other types of electromagnetic balance and any vibrating element present in some of the micro-weight measurement technique. The measurement system is digitally controlled using a microcontroller with digital display of measurement. Due to the light weight and extremely low friction, the proposed system becomes the highly sensitive, which can respond to weights in microgram level. The design comprises of a thin aluminum wire as a straw which is suspended by a fine wire and bearing arrangement. A micro-weighing pan is attached to one tip of the wire and a small iron piece is connected to other tip of the wire beneath which a small electromagnet is placed. The unknown weight in micro-weighing pan exerts a force at its end which is overcome by the electromagnetic force exerted on the iron piece present at the other end. Thus the force required to overcome the counterforce exerted by the unknown weight becomes a measure of unknown weight. When the force is overcome the pan is lifted and sensed optically. The electromagnetic force is proportional to the electromagnetic coil current which is again proportional to the DAC output, and hence the digital input of the DAC becomes the measure of the unknown weight. The proposed system was designed and practically implemented for the realization.