## 8 Calibration of Instrumentation for Aerodynamic Testing
### Multichannel Aerodynamic Pressure-Sensing Instrument: General Description  
_Aerodynamic testing_ of fast-moving objects, such as aircraft and road or railway vehicles, is a necessary step in their development [65; 66]. Some large stationary objects, such as civil engineering and architectural structures (bridges, buildings etc. ), are also subjected to aerodynamic tests in order to estimate the influence of wind on their stability [67]. The tests can be performed either in a _wind tunnel_ or in open space. A wind tunnel is a facility that generates a controlled air flow around the object under test, so as to produce aerodynamic effects that approximate those occurring in a real environment. Some wind tunnels are big enough to hold full-sized vehicles or other large objects, while many of them can only be used for testing of scaled models. Wind tunnels provide a well-controlled environment in terms of air flow quality, and can offer a wide flow velocity range, which would be difficult to achieve in open space. _Multichannel pressure-measuring instruments_ are essential for performing aerodynamic tests. In general, a multitude of small openings (pressure taps) are made on the aerodynamic surfaces of the tested object, and the taps are connected via pneumatic tubing to the instrument that measures the corresponding pressure values and sends the measurement signals to an acquisition system. Such instruments are usually called pressure scanners, because their early implementations had only one pressure sensor and an electromechanical valve for multiplexing of pressure channels, so that the channels were scanned, i.e., measured, one by one in a rapid succession. Contemporary pressure scanners are typically based on miniature MEMS pressure sensors, and use a separate sensor for each pressure channel. In terms of sensor signal processing, most instrument implementations perform only analog multiplexing and conditioning (typically amplification). In that case, the acquisition system receives the multiplexed analog signal, digitizes it, and stores the measured values. More demanding operations, such as sensor linearization, are performed in the digital domain on the stored measurement data. Pressure scanners of the latest generation digitize the measurement signals, perform digital signal processing internally, and send measurement data to an external computing equipment via a high-speed digital interface. Increasing requirements, especially regarding the miniaturization of the instrument (the highest possible number of pressure measurement channels per unit volume, as well as the instrument's overall dimensions), scanning speed (the number of samples per channel per second), and low measurement uncertainty, are difficult to meet at the same time, relying only on existing concepts and solutions. This fact has motivated researchers at the Department for Microelectronic Technologies (Institute of Chemistry, Technology and Metallurgy, University of Belgrade) to develop a novel pressure scanner concept, based on MEMS multisensor chips [68], simultaneous measurement at all pressure channels, and advanced digital signal processing. A block diagram of the novel pressure scanner designed according to this concept is shown in Figure 9. Two exchangeable multisensor modules contain several MEMS multisensor chips. Each chip has an array of piezoresistive MEMS pressure-sensing elements, as well as one or more resistive temperature-sensing elements; all of these are monolithically integrated on the same silicon die. Signals from the temperature-sensing elements are digitized by circuitry built into the multisensor modules; meanwhile, the conditioning and analog-to-digital conversion of signals from the pressure-sensing elements are performed by the analog front-end module. The latter consists of multichannel simultaneous-sampling 24-bit analog-to-digital converters, sensor excitation sources, and accompanying circuitry.