## 3 Master Control System and Motion Control System
### Motion Control System  
The motion control of the calibration facility refers to the control of the moving velocity of the carriage that bears the hot-wire probe. The motion control system consists of shaft motion controller, servo motor, servo driver, retarder (speed reducer), position feedback encoder (i.e., magnetic grating displacement transducer), motion control computer and control software, etc. The motion control system also involves some components of a mechanical system such as the moving carriage, the guide tracks, the synchronous belt, and the synchronous pulley, etc. Figure 5: Flow chart of the general calibration process. It is the servo motor that starts the motion of the moving carriage. The synchronous belt is designed as a long-closed loop with two ends that are semi-circular in shape, and at each end one synchronous pulley is placed. One of the two pulleys is the driving pulley, and its rotation center is fixed to the output shaft of the servo motor through a retarder (speed reducer). The motor drives the driving synchronous pulley to rotate, the driving pulley drives the synchronous belt to move, and the belt tows the carriage on the guide tracks, achieving a rectilinear motion (see Figures 2 and 3). In order to improve the velocity control precision, a magnetic grating displacement transducer is installed on the side face of one of the guide tracks with the corresponding magnetic read head fixed on the moving carriage. When the carriage moves, the read head has a relative motion to the magnetic grating transducer, measures the real location of the carriage, the distance it has traveled. Then the carriage velocity is calculated and fed back to the motion (velocity) control system, leading to a closed-loop control(see Figure 6). The master control computer sends orders to start the motion control system and transmits the value of set velocity for calibration to the motion control system. The motion control system then sends the set velocity value to the motion controller via LAN bus line. The motion controller calculates the control information, converts the calculated control data into analog quantity through D/A conversion, and then outputs to the servo driver. The servo driver transforms the analog quantity into driving control signal, amplifies it in power, outputs it, and drives the servo motor. The servo motor rotates according to the specified motion law. The initial velocity value of the rectilinear motion of the carriage is converted from the angular velocity specified by the servo motor encoder, while the real distance the carriage has moved is measured by the magnetic grating transducer and is divided by the time period the motion takes to cover the distance, giving the real rectilinear motion velocity. The encoder installed on the rotating shaft of the servo motor measures the angular output of the motor, and the real rotating speed of the servo motor obtained from here is fed back to the motion controller to form an inner-level closed-loop speed control. The coded signal of the magnetic grating transducer produces the real motion velocity of the carriage. The error between the real rectilinear velocity and the set value is fed back to the servo motor encoder and the motion controller to correct the motion law of the encoder, forming an outer-level closed-loop control. Thus, the servo motor encoder and magnetic grating transducer constitute a two-level closed-loop control scheme (see Figure 6). The motion control system is incorporated into the master control system and shares the same control cabinet, same computer, and same screens with the master control system (see Figure 4). The hardware-integrated motion controller with PMAC is the control core of the extremely low speed calibration facility. It has a powerful processing ability and can realize the complex control algorithm.