# Temperature Fluctuation Attenuation of Circulating Cooling Water Using Dynamic Thermal Filtering
## 1 Introduction  
In the fields of precision manufacturing, more than 40-70% of machining errors result from thermal deformation of machine tools and work pieces [1], while circulating water cooling is a key technology against thermal pollution. Circulating cooling water (CCW) is usually used for temperature maintenance of motors, drivers, and other power devices. Therefore, the requirement for CCW with high temperature stability and quick response of temperature control is found in the fields of lithography, optical machining, and so on [2; 3; 4; 5]. Lithography, for instance, has a requirement for overlay accuracy of up to 1.4 nm, while the technical node of the advanced stepping and scanning lithography machine is below 45 nm. However, the peak power of the thermal pollution produced by motors, drivers, laser sources, and various electrical circuits reaches up to several kilowatts. Thus, CCW with a temperature stability of \(\pm 0.01\) K is used as a direct cooling medium to maintain the temperature stability of the objective lens within \(\pm 0.01\) K, and also as a temperature reference medium to produce an \"air shower\" for temperature stability of up to 2.3 mK/5 min, to maintain the air refractive index for the ultra-precision laser interferometry [3; 5]. Besides, it is preferred that the temperature of CCW is able to vary quickly along with the pre-set power curve of motors, rather than conventional motor cooling with CCW of constant temperature. In the applications of the electrical spindle cooling of optical machining [6; 7; 8], a CCW temperature control response time of tens of seconds of is also demanded, to achieve \"servo\" thermal managementaccording to the variation of the spindle rotating speed. For example, in 2015, [PERSON] used two CCW machines to provide two streams of CCW with different temperatures, then mixed the two streams to produce the CCW for spindle thermal control. As a result, the temperature of mixed CCW can be regulated quickly by modifying the mixing percentage [6]. Therefore, precision manufacturing is marching forward to an \"atom era\"; accordingly, CCW temperature control with high stability and good dynamic performance becomes a challenging task to be solved. Conventionally, in order to improve the temperature stability of CCW, thermal inertia of CCW is enlarged by introducing a water tank into the temperature control system of CCW. As a result, a quick temperature control response can hardly be achieved due to the enlarged distributive delay which comes from the introduced thermal inertia [9; 10; 11; 12; 13], i.e., the dynamic performance is sacrificed for higher temperature stability. [PERSON] et al. presented temperature fluctuation attenuators based on the principle of introducing additional thermal capacity medium. In 2001, [PERSON] used water, which is sealed in one chamber of a heat exchanger, as thermal capacity medium. Then, CCW flows through another chamber of the heat exchanger, thus the temperature fluctuation of CCW is reduced by the large thermal capacity of the sealed water [14]. In 2003, [PERSON] used stainless-steel balls as thermal capacity material, with CCW flowing through the balls, thus the temperature fluctuation of CCW is reduced by the large thermal capacity of the stainless-steel balls [15]. Materials with large thermal capacity, including stainless-steel balls and water, are introduced as the medium, and the temperature fluctuation of water is smoothed by heat exchanging with the medium. This method provides a new approach to the high attenuation ratio of water temperature fluctuation. However, the attenuators only function around a fixed temperature point. Once the set value of the CCW temperature is changed, the attenuator is functionally invalid, and becomes a thermal interference source, which means that the method is only applicable for water cooling applications that demand CCW of constant temperature.