The pressure sensor has high precision, the request error is reasonable, and the error compensation of the pressure sensor is the key to its application. Pressure sensors mainly include offset errors, sensitivity errors, linear errors and hysteresis errors. This article will introduce the mechanism of these four kinds of errors and the impact on the test results. Together, we will introduce the pressure calibration method and application for the measurement accuracy. Example.

The variety of sensors available on the market today allows planning engineers to select the pressure sensors required for the system. These sensors include both the most basic converters and the more messy high-integration sensors with on-chip circuitry. Because of these differences, it is necessary for the planning engineer to compensate for the measurement errors of the pressure sensor as much as possible. This is the main process to ensure the satisfactory planning and application of the sensor. In some cases, the compensation can also improve the overall function of the sensor in use.

Offset, scale calibration, and temperature compensation can all be done through a thin film resistor network that uses laser correction during the packaging process.

The sensor is typically used in conjunction with a microcontroller, and the embedded software of the microcontroller itself establishes a mathematical model of the sensor. After the microcontroller reads the output voltage and changes it through the analog-to-digital converter, the model can convert the voltage amount into a pressure measurement value.

The simplest mathematical model of the sensor is the transfer function. The model can be optimized throughout the calibration process, and the maturity of the model will be added as the calibration point is added.

From the point of view of metrology, the measurement error has a proper and strict definition: it represents the difference between measurement pressure and practice pressure. Generally speaking, it is not possible to directly obtain the pressure of practice, but it can be estimated by selecting an appropriate pressure specification. The metering personnel generally use instruments that are at least 10 times higher than the equipment to be tested as the measurement specification.

Since the uncalibrated system can only convert the output voltage to pressure using typical sensitivity and offset values, the measured pressure will produce an error as shown in Figure 1.

This uncalibrated initial error consists of the following:

a. Offset error. Since the straight offset remains constant over the full pressure scale, changes in the transducer dispersion and laser conditioning correction will result in offset errors.

b. Sensitivity error, the size of the error is proportional to the pressure. If the sensitivity of the device is higher than the typical value, the sensitivity error will be an increasing function of the pressure. If the sensitivity is lower than the typical value, the sensitivity error will be a decreasing function of the pressure. The factor that occurs is the change in the dispersion process.

c. Linear errors. This is a factor that has less impact on the initial error. The error occurs in the physical nonlinearity of the silicon, but the sensor with the amplifier should also contain the nonlinearity of the amplifier. The linear error curve can be a concave curve or a convex curve.

d. Hysteresis error: In most scenes, the hysteresis error is completely negligible due to the high mechanical stiffness of the silicon wafer. It is generally only necessary to think about lag errors in situations where the pressure changes a lot.

Calibration can eliminate or greatly reduce these errors, while compensatory skills generally require that the parameters of the system practice transfer function be determined, rather than a brief use of typical values. Potentiometers, adjustable resistors, and other hardware can be selected during the compensation process, and the software can perform this error compensation more sensitively.

The one-point calibration method compensates for offset errors by eliminating drift at the zero point of the transfer function. This type of calibration is called active zeroing.

Offset calibration is typically performed at zero pressure, especially in differential sensors where the differential pressure is typically zero under nominal conditions. With respect to pure sensors, offset calibration is more difficult because it either requires a pressure reading system to measure the nominal pressure at ambient atmospheric pressure or a pressure controller that requires the desired pressure.

The zero pressure calibration of the variable sensor is very accurate, since the calibration pressure is severely zero. On the other hand, the accuracy of calibration when the pressure is not zero depends on the function of the pressure controller or measurement system.

Pick calibration pressure

The choice of calibration pressure is very important as it resolves the pressure scale for the best accuracy. In practice, offset errors that are practiced after calibration are minimal at the calibration point and consistently adhere to smaller values. Therefore, it is necessary to select the calibration point according to the scale of the policy pressure, and the scale of the pressure can not be the same as the scale of the operation.

In order to convert the output voltage into a pressure value, since the sensitivity of practice is often unknown, typical sensitivity is generally selected for single point calibration in mathematical models.

The red curve indicates the error curve after the offset calibration (PCAL = 0), and it can be found that the error curve is linearly offset with respect to the black curve indicating the error before calibration.

This kind of calibration method is more stringent than the one-point calibration method, and the completion cost is also higher. However, compared with the one-point calibration method, this method can significantly improve the accuracy of the system. Since the method not only calibrates the offset, but also calibrates the sensitivity of the sensor. Therefore, sensitivity practice values can be used in error accounting, rather than typical values.

The green curve indicates an improvement in accuracy. Here, the calibration is performed at 0 to 500 megabars (full scale). Since the error is close to zero at the calibration point, it is especially important to set these points correctly in order to get the smallest measurement error within the desired pressure scale. In some applications, it is required to maintain a high degree of accuracy across all pressure scales. In these applications, multi-point calibration can be used to get the best results. In the multi-point calibration method, not only the offset and sensitivity errors are considered, but also some linear errors are considered, as shown by the magenta curve. The mathematical model used here is identical to the two-level calibration for each calibration distance (between the two calibration points).

Three-point calibration

As mentioned earlier, linear errors have a common way, and the error curve fits the curve of the quadratic equation with predictable size and shape. This is especially true for sensors that do not use an amplifier, since the nonlinearity of the sensor is essentially based on mechanical factors (which are caused by the film pressure of the silicon wafer).

The depiction of linear error characteristics can be obtained by accounting for a uniform linear error of a typical example, and determining the parameters of the polynomial function (a × 2+bx+c ). It is concluded that the models obtained after a, b and c are valid for the same type of sensor. This approach effectively compensates for linear errors without the need for a third calibration point.

The error compensation method can improve the low cost sensor to high-performance equipment with only two points of calibration (the error is less than 0.05% of the full scale).