The pressure sensor has high precision and requires reasonable error. The error compensation of the pressure sensor is the key to its application. Pressure sensors mainly include offset error, sensitivity error, linearity error and hysteresis error. This article will introduce the mechanism of these four errors and their influence on the test results. At the same time, it will introduce pressure calibration methods and application examples to improve measurement accuracy.
There are various types of sensors on the market, which allows design engineers to select the pressure sensors required by the system. These sensors include not only the most basic transducers, but also more complex highly integrated sensors with on-chip circuits. Because of these differences, design engineers must compensate as much as possible for the measurement error of the pressure sensor, which is an important step to ensure that the sensor meets the design and application requirements. In some cases, compensation can also improve the overall performance of the sensor in the application.
The simplest mathematical model of the sensor is the transfer function. The model can be optimized during the entire calibration process, and the maturity of the model will increase as the calibration points increase.
From the perspective of metrology, measurement error has a fairly strict definition: it characterizes the difference between the measured pressure and the actual pressure. Usually, the actual pressure cannot be obtained directly, but it can be estimated by adopting an appropriate pressure standard. Measurement personnel usually use instruments whose accuracy is at least 10 times higher than that of the equipment under test as measurement standards.
Since the uncalibrated system can only use typical sensitivity and offset values to convert the output voltage into pressure, the error of the measured pressure. This uncalibrated initial error consists of the following parts:
Offset error
Since the vertical offset remains constant throughout the pressure range, changes in transducer diffusion and laser adjustment correction will produce offset errors.
Sensitivity error
The size 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 pressure. If the sensitivity is lower than the typical value, the sensitivity error will be a decreasing function of pressure. The cause of this error is the change in the diffusion process.
Linearity error
This is a factor that has a small effect on the initial error. The cause of the error lies in the physical nonlinearity of the silicon chip, but for sensors with amplifiers, the nonlinearity of the amplifier should also be included. The linear error curve can be a concave curve or a convex curve.
Hysteresis error
In most cases, the hysteresis error is completely negligible because of the high mechanical stiffness of the silicon wafer. Generally, only the hysteresis error needs to be considered when the pressure changes greatly.
Calibration can eliminate or greatly reduce these errors, and compensation techniques usually require determining the parameters of the actual transfer function of the system, rather than simply using typical values. Potentiometers, adjustable resistors, and other hardware can all be used in the compensation process, and software can more flexibly realize this error compensation.
Calibration method of pressure sensor:
One-point calibration
This calibration method can compensate the offset error by eliminating the drift at the zero point of the transfer function. This type of calibration method is usually called automatic zero. Offset calibration is usually performed at zero pressure, especially in differential sensors, because the differential pressure is usually zero under nominal conditions.
Select calibration pressure
The selection of the calibration pressure determines the pressure range for obtaining the best accuracy. The calibration point must be selected according to the target pressure range, and the pressure range may not be consistent with the working range. The sensitivity calibration is usually carried out by a single-point calibration method in the mathematical model.
Three-point calibration
Linear error usually has a consistent form. It can be obtained by calculating the average linear error of a typical example and determining the parameters of the polynomial function (a×2+bx+c). The model obtained after determining a, b, and c is valid for the same type of sensor. This method can effectively compensate the linear error without the third calibration point.