Modeling and Analysis for Thermal Control of Spindles for Reconfigurable Machines

Control of thermally induced bearing loads remains an important but unsolved problem for precision, high-speed, metal cutting, machining spindles. Spindle dynamic performance, as well as spindle life, depends on bearing loads. Because these loads can change drastically with a change in process conditions, poor spindle dynamic performance, and dramatically reduced bearing life can result. The purpose of this paper is to evaluate the feasibility of controlling bearing loads by controlling the heat generated by a thermal actuator placed around the housing of the spindle. A mathematical model of the open loop response of a laboratory prototype spindle is developed and validated. The model is then used to evaluate the closed loop performance. The results show that closed loop control of the bearing load is achievable in steady state and under bandwidth limited transient conditions. The proposed system has reasonable command authority when additional heat is required, however, it is possible for the system to lose control when the heater is required to “provide” negative heat. This situation can be mitigated by proper choice of initial preload. As expected, measurement noise limits the control gain but is not a limiting factor. More open loop tests are suggested to validate the model under a broader set of conditions. In addition, closed loop validation is suggested. However, based on results obtained it appears bearing load control is achievable by controlling the thermal field around the spindle.