# Spindle Bearing Preload Control Using NI MyRIO
## 3 Construction of the Research Stand  
From a mechanical point of view, the spindle in a machine tool should be considered as a beam supported by two supports. The optimal spacing of bearing supports wasthe subject of literature research as early as the 1980s [14; 15]. Various researchers have determined both the shaft diameter and the tip overhang so that the spindle operates outside the area of natural vibration frequencies. This type of solution is relatively easy to achieve for milling spindles where the natural vibration frequencies are above 1 kHz. The characteristics of grinding spindles are completely different. They are characterized by low spindle stiffness, and additionally, a grinding wheel is mounted on their front end, i.e., an element of significant mass and dimensions, which additionally change during its operation. This additional mass, the greater it is, the more it lowers the natural vibration frequencies of the spindle system. Then there is a probability that the spindle system will operate near these frequencies, which leads to resonance. Then even a small forcing force can cause resonance vibrations of the entire spindle system. During his own research, the author of the publication developed a research stand with a model spindle system. Its detailed description can be found in the publication [16]. The main element of this station is a spindle with active support. It is shown in Figure 1. There is a spindle system equipped with piezoelectric elements, which were used to preload the bearings in the model spindle. The piezoelectric elements were three piezo actuators type PSt 150/40/10 from Piezomechanik GmbH (Munich, Germany) with a built-in strain gauge system. They were positioned along the spindle axis and spaced every 120\({}^{\circ}\). Their correct setting was possible thanks to the use of a screw pre-adjustment system separately for each piezo actuator. The structure of this system is shown in Figure 2.  
Each of the piezo actuators was calibrated and its hysteresis was determined. During assembly, using adjustment screws, the piezo actuators were set in such a way that each of them pressed against the outer bearing race with the same force. This prevented the bearing from skewing. The relative movement of the bearing races was also controlled by means of three displacement sensors positioned parallel to the spindle axis. They were placed in the same support as the piezo actuators, but offset relative to them by a rotation angle of 30\({}^{\circ}\). These were MDKa-F1 displacement sensors from VIS (Kombinat Przemyshu Nazrzedziowego \"VIS\" Fabryka Wyrobor Precyzyinych, Warsaw, Poland) with an accuracy of 0.4 \(\upmu\)m. During preliminary tests, they were connected to a QuantumX MX 840 measuring system from HBM (Hottinger Bruel & Kjaer, Darmstadt, Germany), and the active support was controlled using a type 9264 measuring card from National Instruments (Austin, TX, USA). This card supplied a voltage signal to the piezo actuator amplifier, which allowed changing the extension value of these elements. The entire application was developed in LabVIEW. The author then demonstrated that the proposed control algorithm allows for a statistically significant reduction in the amplitude of vibrations of the front end of the  
Figure 1: Tested spindle system: 1—equivalent mass, 2—front support, 3—active preload module, 4—rear support. spindle, which is subjected to external random excitations. The entire research is presented in the publication [17]. Unfortunately, this solution had many hardware limitations, such as the fact that the forcing signal was generated from a different computer than the one on which the measurement was recorded. This caused problems with precise synchronization of the forcing signal with the measurement signal. The QuantumX measurement system recorded the measurement, which was saved in the LabVIEW application to a table. Then, based on this datum, the controller determined the conditions for changing the piezo actuator extension.