# Research on Temperature Compensation Method in Crankshaft Online Measurement System
## 2 Temperature Compensation Method for Crankshaft Online Measurement
### Temperature Compesaiton Method  
Crankshafts are measured in workshop where temperature changes will not only affect the measurement accuracy of the displacement sensor itself but also cause the thermal expansion and contraction of the workpiece, and will bring great errors to measurement results. The comparison method is used for shafts parts in measurement. The radial value of main journal to be measured is calculated by using the radius value of calibration part and the displacement difference of LVDT probe. The indication value of calibration part is measured in a normal temperature environment. In the workshop environment, the sensor will be affected by temperature, which will cause measurement errors. At the same time, the measurement data of crankshaft and the calibration part will also be influenced by temperature because of the temperature characteristics of their material. The flow chart of the temperature compensation for the crankshaft measurement system is shown in Figure 2.  
Figure 1: Temperature compensation block diagram. As shown in Figure 3, multiple LVDT sensors are arranged to measure the main journal of the crankshaft. The specific implementation steps of temperature compensation during measurement are as follows:  
1. Measurement of main journal under ideal conditions  
Before measurement, the calibration part needs to be calibrated to obtain the displacement zero point. The structure of the calibration part is shown in Figure 4, where the LVDT probe is directly facing the center of the main journal, and \(R_{0}\) is the main journal calibration radius. Figure 3: LVDT sensors arrangement of crankshaft. Figure 2: Flow chart of the temperature compensation for crankshaft measurement. Take calibration of the main journal as an example, set the original distance between LVDT probe and the center of journal in undetected state as \(C\), set the displacement of LVDT probe when calibrating the main journal is \(d_{0}\), then there is  
\[R_{0}=d_{0}+C \tag{1}\]  
When measuring shaft part, suppose the actual displacement of LVDT probe is \(d\), then the actual radial distance of the journal at this moment is  
\[R=d+C \tag{2}\]  
Reorganize Formulas (1) and (2), we get  
\[R=\Delta d+R_{0} \tag{3}\]  
2. The deviation value of LVDT probe \(\Delta X\) supposed in Section 3  
3. Temperature compensation of calibration part and test part  
The changes in radial quantity of calibration part and measured crankshaft have a linear relationship with the temperature. The measurement compensation is performed under three different temperature environments: the calibration laboratory temperature, the ambient temperature when the equipment is calibrated and the actual measurement temperature. The correction coefficient is obtained, which effectively reduces the influence of temperature changes on the test results. Three temperatures are involved in the online temperature compensation of the crankshaft main journal. Suppose the temperature of the calibration laboratory is 20 \({}^{\circ}\)C [19], the ambient temperature when the equipment is calibrated is \(T\), the temperature of the crankshaft is \(T_{1}\), the thermal expansion coefficient of the crankshaft is \(\alpha_{1}\), and the thermal expansion coefficient of the calibration part is \(\alpha_{2}\). During the measurement process, it is necessary to recalibrate the zero position of the probe at regular intervals. At this time, the calibration part is at a non-calibration temperature (20 \({}^{\circ}\)C), and radial expansion or contraction occurs. So the compensation value is:  
\[\Delta C=(T-20)\times\alpha_{2}\times L \tag{4}\]  
where \(T\) is ambient temperature, \(\alpha_{2}\) is the thermal expansion coefficient of the calibration part, \(L\) represents original length. The main journal of crankshaft is still at a non-standard temperature during the measurement.