What is Pressure Sensor Error and How to Reduce Error
To achieve high accuracy of the pressure sensor, the error must be reasonable, so the error calibration for the pressure sensor is the key point. Pressure sensors mainly have offset error, sensitivity error, linear error and hysteresis error. This paper will introduce the mechanism of these four kinds of errors and the calibration method to reduce the error.
1. Offset error
Since the vertical offset of the pressure sensor over the entire pressure range remains constant, the diffusion in pressure sensors and laser adjustment correction will produce offset errors.
2. Sensitivity error
The magnitude of the error produced 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 reason for this error is the change in the diffusion process.
3. Linear error
This is a factor which has less impact on the initial error of the pressure sensor. The error is due to the physical nonlinearity of the silicon, but for pressure sensors with amplifiers, the cause should also include the nonlinearity of the amplifier. The linear error curve can be a concave curve or a convex curve.
4. Hysteresis error
In most cases, the hysteresis error of the pressure sensor can be completely ignored, because the silicon wafer has a high mechanical stiffness, and generally only needs to consider the lag error in the case of a large pressure change.
These four errors of the pressure sensor are unavoidable. We can only choose high-accuracy production equipment, use high technology to reduce these errors. In addition, certain error calibrations can performed at the factory to minimize the error to meet customer needs.
1. One point calibration
This calibration method compensates for offset errors by eliminating drift at the zero of the transfer function, which is often called auto-zero. Offset calibration is typically performed at zero pressure, especially in differential pressure sensors, since the differential pressure is typically zero under nominal conditions. For pure sensors, offset calibration is more difficult, because it either requires a pressure reading system to measure its nominal pressure at ambient atmospheric pressure, or requires a pressure controller that can obtain the desired pressure.
2. Select calibration pressure
The selection of the calibration pressure determines the pressure range in which the best accuracy can obtain. The calibration point must be selected according to the target pressure range. The pressure range can not be consistent with the working range. Calibration is usually performed in a mathematical model using a single point calibration method. In order to convert the output voltage to a pressure value, since the actual sensitivity is often unknown, we usually use typical sensitivity for single point calibration in mathematical models. Compared with the one-point calibration method, its requirements are more strict and the implementation cost is higher. By contrast, this method can significantly improve the accuracy of the system, because the method not only calibrates the offset, but also calibrates the sensitivity of the pressure sensor. Therefore, the actual sensitivity value can used in the error calculation instead of the typical value.
As mentioned earlier, the linearity error has a consistent form, and the error curve conforms to the curve of the quadratic equation with a predictable size and shape. This is especially true for pressure sensors that do not use an amplifier because the nonlinearity of the pressure sensor essentially based on mechanical factors (which caused by the pressure of the silicon diaphragm). The description of the linear error characteristic can obtained by calculating the average linear error of a typical example and determining the parameters of the polynomial function (a × 2+bx+c ).The models obtained after determining a, b and c are valid for the same type of pressure sensor. This method can effectively compensate for linearity errors without the third calibration point. This error calibration method requires only two-point calibration to improve the pressure sensor with low-cost to a high-performance device (error is less than 0.05% of full scale).
Of course, the user should select the most suitable calibration method according to the accuracy requirements in the actual application and consider the system cost. With a variety of integration and calibration techniques to choose from, users can choose the appropriate method for different design requirements.
- The unavoidable error in the pressure sensor
1. Offset error
Since the vertical offset of the pressure sensor over the entire pressure range remains constant, the diffusion in pressure sensors and laser adjustment correction will produce offset errors.
2. Sensitivity error
The magnitude of the error produced 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 reason for this error is the change in the diffusion process.
3. Linear error
This is a factor which has less impact on the initial error of the pressure sensor. The error is due to the physical nonlinearity of the silicon, but for pressure sensors with amplifiers, the cause should also include the nonlinearity of the amplifier. The linear error curve can be a concave curve or a convex curve.
4. Hysteresis error
In most cases, the hysteresis error of the pressure sensor can be completely ignored, because the silicon wafer has a high mechanical stiffness, and generally only needs to consider the lag error in the case of a large pressure change.
These four errors of the pressure sensor are unavoidable. We can only choose high-accuracy production equipment, use high technology to reduce these errors. In addition, certain error calibrations can performed at the factory to minimize the error to meet customer needs.
- Error calibration method for the pressure sensor
1. One point calibration
This calibration method compensates for offset errors by eliminating drift at the zero of the transfer function, which is often called auto-zero. Offset calibration is typically performed at zero pressure, especially in differential pressure sensors, since the differential pressure is typically zero under nominal conditions. For pure sensors, offset calibration is more difficult, because it either requires a pressure reading system to measure its nominal pressure at ambient atmospheric pressure, or requires a pressure controller that can obtain the desired pressure.
2. Select calibration pressure
The selection of the calibration pressure determines the pressure range in which the best accuracy can obtain. The calibration point must be selected according to the target pressure range. The pressure range can not be consistent with the working range. Calibration is usually performed in a mathematical model using a single point calibration method. In order to convert the output voltage to a pressure value, since the actual sensitivity is often unknown, we usually use typical sensitivity for single point calibration in mathematical models. Compared with the one-point calibration method, its requirements are more strict and the implementation cost is higher. By contrast, this method can significantly improve the accuracy of the system, because the method not only calibrates the offset, but also calibrates the sensitivity of the pressure sensor. Therefore, the actual sensitivity value can used in the error calculation instead of the typical value.
As mentioned earlier, the linearity error has a consistent form, and the error curve conforms to the curve of the quadratic equation with a predictable size and shape. This is especially true for pressure sensors that do not use an amplifier because the nonlinearity of the pressure sensor essentially based on mechanical factors (which caused by the pressure of the silicon diaphragm). The description of the linear error characteristic can obtained by calculating the average linear error of a typical example and determining the parameters of the polynomial function (a × 2+bx+c ).The models obtained after determining a, b and c are valid for the same type of pressure sensor. This method can effectively compensate for linearity errors without the third calibration point. This error calibration method requires only two-point calibration to improve the pressure sensor with low-cost to a high-performance device (error is less than 0.05% of full scale).
Of course, the user should select the most suitable calibration method according to the accuracy requirements in the actual application and consider the system cost. With a variety of integration and calibration techniques to choose from, users can choose the appropriate method for different design requirements.