Data Acquisition

Analog and digital signals 17Figure 2.6An analog AC signal in the frequency domainAnalysis in the frequency domain allows for easier detection and extraction of the wantedsignal by filtering out unwanted noise components having frequencies much higherthan the desired signal. The digital signal processing (DSP) required to convert the timemeasuredsignal into frequency information and possibly perform analysis on thefrequency spectrum, can be achieved with software or with special DSP hardware.2.2 Sensors and transducersA transducer is a device that converts one form of energy or physical quantity intoanother, in accordance with some defined relationship. Where a transducer is the sensingelement that responds directly to the physical quantity to be measured and forms part ofan instrumentation or control system, then the transducer is often referred to as a sensor.In data acquisition systems, transducers sense physical phenomena and provide electricalsignals that the system can accept. For example, thermocouples, resistive temperaturedetectors (RTDs), thermistors, and IC sensors convert temperature into an analogvoltage signal, while flow transducers produce digital pulse trains whose frequencydepends on the speed of flow.Two defined categories of transducer exist:• Active transducers convert non-electrical energy into an electrical outputsignal. They do not require external excitation to operate. Thermocouples arean example of an active transducer.• Passive transducers change an electrical network value, such as resistance,inductance or capacitance, according to changes in the physical quantity beingmeasured. Strain gauges (resistive change to stress) and LVDTs (inductancechange to displacement) are two examples of this. To be able to detect suchchanges, passive devices require external excitation.2.3 Transducer characteristicsTransducers are classified according to the physical quantity they measure (e.g.temperature, force etc).