## 2 The Online Dynamic Balance System Design for Double-Face Correction
### The Design of the Dynamic Balance Device and Driving System  
Based on the principle of vector synthesis, four balance disks with equivalent fixed unbalance vector were applied to design the double-face correcting system for spindle dynamic balance and the experimental equipment was built. The measurement and control system is shown in Figure 2.  
In Figure 2, the correcting vector (\(\overset{\rightarrow}{Q_{1}}\)) of face 1 can be synthesized by the balance disks 1 and 2, and the correcting vector (\(\overset{\rightarrow}{Q_{2}}\)) of face 2 can be synthesized by the balance disks 3 and 4. There is a flexible joint between the experimental device and motorized spindle. During the experiment, the vibration signals of the rotating spindle ends can be acquired by eddy current sensors v1 and v2, the radial positions signal of balance disks 1 to 4 can be acquired separately by the photoelectric sensors s1 to s4, and the radial positions signal on the spindle can be acquired by the photoelectric sensor st1. All the sensors' signals can be collected by a signal acquisition system and analyzed online in the computer. The signal analysis process can be expressed as following.  
First, the unbalance vibration signals are separated from the vibration signals of the spindle.  
Second, when the amplitudes of the unbalance vibration signals exceed the allowable value, the correcting vectors in faces 1 and 2 are calculated according to the control law.  
Third, the expected radial positions of balance disks 1 to 4 are calculated according to the required correcting vectors.  
Fourth, the control signals are outputted from the computer and then the radial positions of balance disks 1 to 4 are adjusted to reach their expected position.  
Figure 2: The double-face online dynamic balance system.  
In order to meet the requirement of online control, a new type of balance disk structure and its driving system were developed as Figure 3 shows.  
In Figure 3, the inside and outside circles are designed to be a dentate structure of the balance disk body. The installing ring with four elastic claws is installed between the spindle and balance disk body, and it is interference fit between the spindle and installing ring. Four nozzle mechanisms are installed separately under the corresponding balance disks. The working principle of the mechanism is illustrated as follows.  
When the spindle system works in the balance condition, the installing ring is driven by the spindle to rotate synchronously, meanwhile the balance disk body is driven to rotate synchronously by a ratchet-pawl mechanism. On the contrary, a series of pulse air flows with high pressure are jetted from the nozzles to adjust the radial positions of the balance disk body. In theory, the phase corresponding to one tooth can be adjusted by one pulse air flow if the pressure and width of the pulse air flow are controlled properly.  
To meet the requirement of online control, the pneumatic driving system is shown in Figure 4.  
As Figure 4 shows, the driving pressure of pulse air flows can be controlled by adjusting the openness of the pressure regulating valve (3). The air pulse signal can be gained by controlling the magnetic valve to be on or off.  
Figure 4: The pneumatic control system. Notes: 1 two-position two-way valve; 2 nozzle; 3 pressure regulating valve; 4 pressure page; 5 air source.  
Figure 3: The structure principle of the balance disk. Notes: 1 Nozzle mechanism; 2 balance disk body; 3 spindle; 4 elastic claw; 5 thread hole; 6 installing ring; 7 unbalance hole.