MPXM2102AS datasheet(513/670 Pages) MOTOROLA

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AN1573

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Motorola Sensor Device Data

www.motorola.com/semiconductors

Transient Systems

Transient systems are systems with changing conditions such as pressures, flow rates, etc. Measurements in transient systems

are the most difficult to accurately obtain. If the measurement system being used to measure the pressure has a faster response

time than the rate of change in the system, then the system can be treated as quasi–steady–state. That is, the measurements will

be about as accurate as those taken in the steady–state system. If the measurement of the system is assumed to be a snap shot

of what is happening in the system, then you want to be able to take the picture faster than the rate of change in the system or

the picture will be blurred. In other words, the measurement results will not be accurate. In a pressure measurement system, there

are two factors that determine the overall measurement response: (1) the response of the transducer element that senses the

pressure, and (2) the response of the interface between the transducer and the pressure system such as the pressure transmitting

fluid and the connecting tube, etc. For Motorola pressure sensors, the second factor usually determines the overall frequency

response of the pressure measurement system. The vast majority of pressure systems that require measurements today are

quasi–steady–state systems where system conditions are changing relatively slowly compared to the response rate of the

measurement system or the change happens instantaneously and then stabilizes.

Two transient system examples include washing machines and ventilation ducts in buildings. In a washing machine, the height

of the water in the tub is measured indirectly by measuring the pressure at the bottom of the tub. As the tub fills the pressure

changes. The rate at which the tub fills and the pressure changes is much slower than the response rate of the measurement

system. In a ventilation duct, the pressure changes as the duct registers are opened and closed, adjusting the air movement within

the building. As more registers are opened and closed, the system pressure changes. The pressure changes are virtually

instantaneous. In this case, pressure changes are essentially incremental and therefore easy to measure accurately except at the

instant of the change. For most industrial and building control applications, the lag in the pressure measurement system is

negligible. As the control or measurement system becomes more precise, the frequency response of the measurement system

must be considered.

Motorola Pressure Sensors

This application note has covered various types of pressures that are measured and how to tap into a system to measure the

desired pressures. How are the actual pressure measurements made? There are many types of pressure measurement systems

ranging from simple liquid tube manometers to bourdon–tube type gages to piezo–electric silicon based transducers. Today, as

electronic control and measurement systems are replacing mechanical systems, silicon–based pressure transducers and sensors

are becoming the sensors of choice. Silicon micromachined sensors offer very high accuracies at very low cost and provide an

interface between the mechanical world and the electrical system. Motorola carries a complete line of silicon based pressure

sensors which feature a wide range of pressures with various levels of integration on a single chip. These levels of integration start

with the basic uncompensated, uncalibrated pressure sensor all the way to the fully integrated, temperature compensated,

calibrated and signal conditioned pressure sensors. The response time of Motorola’s MPX series silicon pressure sensors is

typically 1 millisecond or less. For static or dynamic systems, Motorola’s pressure sensors are an excellent solution for pressure

measurement systems.

Conclusion

Pressures and pressure measurements can be extremely complex and complicated. However, for most systems it is relatively

easy to obtain accurate pressure measurements if the proper techniques are used.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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