## 5 Experiments
### Active Balancing Experiments  
The balancing experimental devices was constructed as shown in Figure 18. A Siemens stepping motor was chosen to drive the machine tool spindle by belt connection and the maximum rotating speed was 4500 rpm. The designed balancing actuator was installed in the hollow rotor of the spindle and the displacement sensor was used to monitor the vibration of the rotor system. The data collection unit comprised a cDAQ Ethernet chassis with two data acquisition modules and a digital output module. The two data acquisition modules were used to measure the vibration signal, rotational speed signal and counterweight disc positioning signals and the digital output module was used to send out control commands. A PC was selected as the control unit to analyze the vibration signal, calculate the initial unbalance and generate control commands. The power supply unit mainly contained two 15V DC power supplies and four DC-DC solid state relays to generate positive and negative alternating voltage signals.  
Restricted by the conditions of the experimental site, the balancing experiments can only be carried out at the speed of 3600 rpm or lower. Meanwhile, the difficulty of active balancing was raised with the increase of balancing speed, so the balancing results at 3600 rpm were emphatically introduced.  
Figure 17: Control program of Step rotation experiments.  
Figure 18: Overall diagram of experimental device: (**a**) Balancing actuator and the rotor system; (**b**) Other components of the balancing system.  
First, the step rotation experiment was repeated when the actuator synchronously rotated with the rotor. The results in Figure 19 show that the two counterweight discs of the balancing actuator can be stepped in the same direction and the opposite direction according to the commands of the control unit and there was no error during the step-by-step rotation of the whole cycle. The step-by-step test process was repeated for 50 cycles, that is, each counterweight disc rotated 1800 times step by step but there was still no error. Therefore, the step error of the actuator was less than 1/1000.  
The change of unbalance vibration during the balancing process is shown in Figure 20, where the blue solid line and red dotted line represent the unbalance vibration amplitude and phase, respectively. The initial unbalance amplitude was about 3.2 \(\upmu\)m and through the active balancing within 20 s, it was reduced to below 0.4 \(\upmu\)m with an 87.5% decrease. Before the balancing process, the initial unbalance phase was stable and about 140 degree. During the balancing process, the unbalance phase was changed a little and the range was 40 degree. At the end of the balancing process, the amplitude of the unbalance vibration was so small that the vibration phase had a large range of fluctuations. Because the fluctuations of phase was mainly determined by the accuracy of the experimental device itself and had little effect on rotor system, it was ignored and the balancing process was finished.  
Figure 19: Step rotation experiments of the actuator: (**a**) In the opposite direction; (**b**) In the same direction.  
Figure 20: Active balancing experiment results of the actuator.