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The amplitudes of the rotation frequency and tool teeth rotation frequency, and those of the hall sensor signals and ac-celebration sensor signals were correlated with the measured cutting force components using a micro tool dynamometer. Correlations between the amplitudes and the measured cut-ting forces using the dynamometer tool were calculated. From the results, the correlation of the tool teeth rotation and current amplitude showed linearity of 92.0% precision for the hall sensor signals, and 98.0% precision for the acceleration sensor signals (Fig. 6 and Fig. 7). Even though the results obtained using the acceleration sensor showed better linearity than those of the current hall sensors, the signals from the latter can be easily affected by chattering, spindle vibration, and external disturbances. From this perspective, the current hall sensor can provide more robust results.
4.3 Bridge leveling
The experiment using dynamometer dynamometer as sensor, using dynamometer cutting force into the deformation resistance strain gage, the deformation reflect changes for the bridge voltage. Through the relationship between the calibration can know the voltage change and the cutting force, so as to measure the cutting force.
In order to facilitate the measurement and research of CNC lathe cutting force, especially in order to meet the needs of the design and use of machine tool, tool and fixture production, generally the total cutting force Fr is decomposed into three mutually perpendicular to the direction of the force, namely Fz, Fy, Fx to study.
The basic principle of the dynamometer is size will be converted into voltage cutting force. Dynamometer dynamometer is a turning dynamometer. Its working principle is: the dynamometer double parallel octagonal ring is elastic element, in the loop on the inner wall is pasted with resistance strain gauge, the resistance strain gauge and a fixed resistor can be connected to component consisting of bridge, measuring three directions respectively. When turning the work piece, cutting force transmitted to the tool, the tool bar is transferred to a dynamometer. Deformation of the strain gauge dynamometer ring to it also deformations, thus the resistance value change.
When the tensile deformation strain, strain sheet thinning, resistance increased (R+ Δ R)；
When the strain slice under compressive deformation, strain coarse, resistance decreased (R- Δ R)；
The resistance change is change bridge voltage across the bridge. But due to the resistance change is very small, so the change of the output voltage of the bridge is also very small, also need after amplification. The bridge output signal access force composed of instrument and the fixed resistance bridge, after the numerical amplification and AD conversion can be directly display the cutting force.
In practice there is an error due to resistor, fixed resistor can not be completely equal, so measured, before leveling the electrical bridge. But the bridge balance conditions for R1R3=R2R4, namely the relative resistance value equal to two of the product. Bridge leveling method many, many have no longer here, this design uses the potentiometer to the leveling, in the measurement of cutting force, plus a potentiometer in the bridge behind, adjusting its resistance to behind the LED display 0 bridge balance. Because the sensor output voltage is very small (0 - 5mv), So the adjustment potentiometer change quickly, cannot accurately leveling, because when the output voltage is negative LED also showed 0, so the voltage of each sensor are respectively measured by multimedia, regulating potentiometer to close, in with the measuring sensor output voltage regulating potentiometer is million, its output is 0, then LED should also show 0, leveling the end of bridge.
4.4 Scale transformation
The analog voltage while measuring force sensor output A/D converter value is converted to digital, but it is not the actual value of the CNC lathe cutting force, cutting force to get real value requires the following two steps: static calibration and scale transformation.