To understand how a pressure transmitter works, it is enough to concentrate on its two key elements: one is the pressure sensor that captures the pressure of the medium and converts it into an electrical signal. The other is an application-specific integrated microchip. It is needed to convert the generated electrical signal into a normalized output signal.
Pressure sensor
The pressure sensor consists of a thin film-on-steel cell (Figure 1) in which the resistance bridges are applied to the surface of a steel sensor element as a thin film. This thin film is only a few atomic layers thick. When pressure is applied to the pressure sensor, its membrane deforms at predefined points. The resistors are placed exactly at these points and change their value when they are stretched or compressed. There are four strain resistors on the sensor element. Two resistances each form a path.
In the middle a bridge can be formed, at which the voltage can be measured. This bridge is called Wheatstone Bridge. If there is no pressure, all resistors have the same value, so that there is no tension between the left and the right path. When the pressure deforms the membrane, two resistors are compressed and two resistors are stretched (Figure 2 and 3). This increases the electrical resistance in the stretched areas. On the other hand, the pressure in the compressed areas decreases. This changes the state of the resistance bridge and generates a signal. However, the measured signal is not linear and varies depending on the ambient temperature. This is because the temperature has a strong influence on the resistance of the bridge (Figure 4).
Application specific microchip
Intelligent electronics are required to obtain a linear, accurate and temperature-independent measuring signal from the measured signal. The electronics correct and amplifies the measurement signal. For example, a 10-volta signal is converted into a 10-volt signal. The correction values determined are stored in the application-specific microchip (also application-specific integrated circuit, ASIC, figure 5). These values are determined and saved individually for each pressure transmitter. In order to determine the correction values, a precisely defined pressure is applied to the fully mounted pressure transmitter and the signal is measured. The correction values can be calculated for the pressure applied. This process is then repeated at different temperatures. This allows the correction values for temperature compensation to be determined. The correction values determined in this way are then saved in the chip. In this way, a linear and standardized measurement signal can be generated from the raw signal of the measuring cell. And that over the entire pressure and temperature range (Figure 6). This standardized measuring signal can be transmitted to higher-level control systems.
The application-specific microchip (ASIC) houses millions of circuits on an area of about 2.5 x 2.5 millimeters. Solder points establish the contact between the chip and the electronics of the pressure transmitter (Figure 7).
Conclusion
The best measurement results are achieved when the measuring cell and the microchip are precisely coordinated. For this reason, Trafag produces its own measuring cells and has developed its own ASIC. By developing these two key components under one roof, the pressure transmitter works optimally and quality and reliability can be guaranteed.
