ELECTRONIC MEASUREMENTS

Electronic Measurements

Electronic Measurements

Introduction

In this essay on electronic measurement and instrumentation, we concentrate on theoretical principles relevant to all measurements for electrical, thermal and mechanical systems. We follow a system science approach rather than employing the more common method of instrument description. We deal with all the fundamental aspects of measurement, including theory of measurement, systems of units, standards, measurement methods, data acquisition, sampling, multiplexing and aliasing. We also cover more practical aspects of measurement, including transducers, interference, noise, AD and DA conversion and instrument data busses. This essay is targeted at engineers and scientists in both industry and academia.

Analog and digital meters for measuring voltage, current and power

The AD775x family of energy-measuring integrated circuits accepts voltage inputs representing local voltage and current in an electrical power system and converts them to digital using over sampling A/D converters. An on-chip digital processor continuously computes the product of the two signals, which is proportional to instantaneous power. (Birolini, 2007, 54) Input conditioning, filtering, further processing, and other features, which are specific to each type within the AD775x family, provide metering solutions for a variety of power-system applications. (Birolini, 2007, 54)

For example, the most general-purpose member of the family, the AD7750, low-pass filters the computed product, then uses digital-to-frequency conversion to output a complementary pair of pulse trains of frequency proportional to the instantaneous real power—for driving a counter or two-phase stepping motor—plus a higher-frequency output, suitable for calibration and test.Before continuing with a discussion of system applications and the features they give rise to, let us consider recent progress in power measurement in the electrical industry.

Electrical utilities, like many other heavy industries, have greatly increased their familiarity with sophisticated electronics in recent years. For example, the rate of replacement of long-used electromechanical meters by electronic devices is expected to quickly increase as decentralization in some countries and deregulation in others encourages customers to have a greater voice. (Birolini, 2007, 54) Energy producers and consumers alike can benefit significantly from electronic energy meters.

A typical meter will convert signals proportional to instantaneous voltage and current to digital, then compute average and instantaneous real power, reactive power, active energy, etc., and transmit the information serially.

Customer service is improved through remote and automated meter reading and efficient data management. Besides receiving more credible utility bills, consumers benefit from more reliable power distribution. When customer meters are communicating through a network, power outages can be detected, identified, and corrected more quickly. (Birolini, 2007, 54)

If the required ratio of peak power to average power in a system is reduced, the consequent reduction in required generating capacity will reduce environmental disturbance and pollution. The incentives provided by multiple-rate billing will help to greatly reduce peak usage despite population growth. Distribution cleanliness is maintained by monitoring the power-quality pollution (e.g., excessive reactive power, nonlinear loads, dc offsets) imposed by individual consumers. Consumers can benefit from lower electricity bills with the installation of smart card controlled energy meters that lower operational costs of providing service, reading meters, and processing ...