To achieve high-speed, high-precision position control for semiconductor product machines and industrial robots, it is well known that PID control is widely applied (Kojima, T., 2004).Many PID control methods have been proposed for such a system so far.In general, proportional position control and proportional plus integral velocity control or integral plus proportional velocity control (P,PI/I-P), which is a type of proportional plus integral plus deferential control (PID), is applied in many industrial applications in Japan.However, the parameters of the control of P,PI/I-P must be changed to maintain a good motion performance when the characteristic of the control target change.As a method to compensate for a change of the control target, a disturbance observer is proposed (Ohishi, K.et al., 1999).Using this method, a load disturbance force is estimated based on a model set beforehand and adds an estimated disturbance force to a control input.However, it might cause the deterioration of the control performance when a control target model is different from real control target greatly.Furthermore, it is difficult to get a correct model because a lot of nonlinear elements such as the friction may change in control target in time and environment (Canudas-de-Wit,C.et al., 1995;Lischinsky, P. et al., 1999; Futami, S. et al., 1990; Otsuka, J. &Masuda, T., 1998; Iwasaki, M. et al., 2000; Tsuruta, K. et al., 2000, 2003).In addition, based on a control target model estimated in real time, the method to change the several control parameters is proposed to compensate it for a change of the control target (Kuwon, T. et al., 2006), but might cause the deterioration of the control response under the influence of an estimate error.Therefore, adaptive control (Suzuki, T., 2001; Sato, K. et al., 2005, 2006, 2007, 2008) and sliding mode control (Nonami, K.&Den, H., 1994; Utkin, V, I., 1977; Harashima, F.&Hashimoto,H., 1986; Fujimoto, T. et al., 2006)are given to solve these problems.However, that is hard to understand these methods to the engineer who got used to PID control; have a problem. In this chapter, we propose a new P,PI/I-P control method that includes a nonlinear compensator, that it is easy to understand for a PID control designer.The control objective is to get a high-speed and high-precision positioning response regardless of the case of control references or load characteristics change.The algorithm of the nonlinear compensator is based on sliding mode control with chattering compensation.Theeffectiveness of the proposed control method is evaluated with three kinds of single-axis slide system experimentally.The first experiment system is two slider tables comprised of an AC servo motor, a coupling and a ball-screw,the second one is a slide table using an AC linear motor and the third one is a slide table using synchronous piezoelectric device driver(Egashira,Y. et al., 2002; Kosaka, K. et al., 2006). By the first experiments, it is evaluated using single-axis slide system comprised of full closed feedback via point-to-point control response and tracking control response when load characteristics of the control target change. By the second experiments, it is evaluated using a linear motor driven slider system via tracking control atlow-velocity, and the resolution of this system is 10nm.By the third experiments, it is evaluated a stepping motion and tracking motion using a synchronous piezoelectric device driver. Then, we derive control algorithm with nonlinear compensator and describe each experimental results.
2. Control method
In this section, we describe a P,PI/I-P+FF control method and propose the P,PI/I-P+FF control method with nonlinear compensator. The control objective is to design a control input to track a given position reference.
2.1. Conventional P,PI/I-P+FF control method
In general, P,PI/I-P control method is applied in many industrial applications. To achieve high-speed positioning response, velocity feed-forward compensation (FF) is usually applied. The FF compensation is effective in compensating it for a response delay of the positioning.As for P,PI/I-Pmethod, positioning and velocity control are comprised of cascade control.When we do not make positioning, we can control velocity by inputting a direct velocity reference.Fig. 1 shows a block diagram of P,PI/I-P+FF control method. In this figure, a signal
The control input
2.2. Proposed control method
In this paper, we will design a PID+FF controller with a nonlinear compensator for high accuracy and a fast response with small overshoot.Fig. 2 shows a block diagram of the P,PI/I-P+FF control method with proposed compensator. The algorithm of the nonlinear friction compensator will be given.
The dynamic equation of positioning table can be modelled as follows
Now, we can define the new signal as
Taking the time derivative of both sides for (8) and multiplying
We define the augmented signal as
Then, (9) can be rewritten as
Now, we give the control input
The control input
To achieve a negative, the following inequality must be satisfied
Then, if we design
3. Experimental results
In this section, we evaluated positioning responses and tracking responses by three kinds of single-axis slide system experimentally.The first experimental system is two slider tables that consists of an AC servo motor, a coupling and a ball-screw, and the second one is a slide table using an AC linear motor and the third one is a slide table using synchronous piezoelectric device driver.
3.1. A table drive system using AC servo motor with a coupling and a ball-screw
The first experiment system is two slider tables comprised of an AC servo motor, a coupling and a ball-screw.
3.1.1. Experimental system
Fig. 3 shows the experimental setup which consists of the following parts. The control system was implemented using a Pentium IV PC with a D/A converter board and a counter board. The control input was calculated by the controller, and its value was translated into a voltage input for the current amplifier through the D/A board. The positions of the positioning table were measured by a position sensor with a resolution of 50 nm. The sensor's signal was provided as a full-closed feedback signal. The sampling period was 0.25 ms. The table with 5kg weightwas mounted on a driving rail. The total inertia of the moving part of the positioning table was approximately 1.128e-4kgm2. The table was supported by a rolling guide through the coupling that was connected with the motor, and the table was driven by an AC servo motor (SGMAS-02ACA21,Yaskawa Electric. Co., Ltd), a ball-screw lead of 20 mm (KR4620A+540L, THK Co., Ltd). The control parameters were set to
Next, the results of positioning responses and tracking responses are shown.
3.1.2. Experimental results
To evaluate our proposed method, we carried out three kinds of experiments.In the first type experiment, it is evaluated that the case of positioning responses when the acceleration/deceleration changes.In the second type of experiment, it is evaluated by the case of positioning responses when the load changes.In the third type of experiment, it is evaluated that the case of tracking responses when the load changes.Fig. 4 shows the experimental table positioning results with the 5 kg-weight in which the positioning reference acceleration/deceleration was changed. The acceleration/deceleration of the first (left side) positioning reference is ±1.0 G, the second is ±1.5 G, the third is ±2.0 G, and the fourth (right side) is ±3.0 G. In this figure, signal ① was the position reference
These results demonstrated the effectiveness of the proposed control method.Generally, it may be said that the acceleration 3.0 G in this experiment is very large because acceleration is used in less than 2.0 G at the ball screw drive table.Fig. 6 shows a torque reference with the conventional control method, and Fig. 7 shows a torque reference (signal ①) and compensated torque
In the second type of experiment wasa positioning response with load changing. Here, we describe the experimental result that verified the robustness of the proposed control method in the case of real-time load inertia change.To change the load inertia in real time, we prepared two sets of positioning tables, each consisting of a single-axis slider, a coupling, a motor, a servo amplifier, and a linear scale, as shown in Fig. 9.The D/A channel of the torque reference (the voltage) to output through the D/A board from thePC and thecounter channel of the table position signal (the pulse) which is entered from the counter board were made to be able to be changed at the same time by the software. Therefore, the weight added or removed, can be imitated, and it is possible to perform the experiment based on the actual mobile status of the production machine. In this experiment, the trapezoid velocity accelerates from zero velocity to 0.4 m/s in 13.5 ms, moving to a max velocity of 0.4 m/s at the constant in 26.75 ms, decelerates to zero velocity in 13.5 ms in Fig. 10 and Fig. 11.The maximum velocity is 0.4m/s by this experiment, but, by the use of the high lead ball screw and the improvement of the frequency response of the counter, can put up the maximum velocity.The present position reference
In the result using the conventional method shown in Fig. 10, a windup and a big overshootoccurred in the positioning. This is similar to the unstable phenomenon that occurs in the response of the velocity loop to the position loop when the stability is affected by the velocity loop-gain is becoming small. On the other hand, in the result using the proposed method shown in Fig. 11, there is no windup or overshoot when the weight is increased. Moreover, the torque reference is smoothly made and no vibration occurs. Therefore, as shown in both figures, the high-speed positioning responses following load changes were confirmed when the proposed control method was used.
In the third type of experiment we evaluated the tracking control characteristic when the trapezoid velocity was constant at 13 mm/s or 6.5 mm/s using the single-axis rolling guide slider, as described in section3.1.1for a 2-cycle period. There is no weight on the table at 1st period (0-3.6s), and there is 5 kg weight on the table at 2nd period (3.6-7.2s). The result with the 1st period when driving with the conventional control method is shown in Fig. 12 (left side), and the result with the 2nd period is shown in Fig. 12 (right side). Also, the result with the 1st period when driving with the proposed method is shown in Fig. 13 (left side), and the result with the 2nd period is shown in Fig. 13 (right side). In these figures,
Also, there was an error having to do with a ripple under constant velocity. The comparison results of tracking errors are shown in Table 1. It is obvious that the proposed method remains robust under controllability with or without the weight in the case of low-changing load conditions.
3.2. A table drive system using AC linear motor
Next, we evaluated a tracking response in the low speed using a table drive system driven a linear motor, and the resolution of this system is 10nm.After having investigated friction characteristics of this system because it was easy to receive a bad influence of the friction at the low-velocitymovement, we inspected the effect of the proposed method.
3.2.1. Experimental system
Fig. 14 shows the photograph of single axis slider and the experimental system shown in Fig.15. It consists of the following: (i) a one-axis stage mechanism consisting of an AC linear coreless motor which has no cogging force, (ii) a rolling guide mechanism, (iii) a position-sensor (1pulse=10nm),(iv) two current amplifiers, and (v) a personal computer with the controller,a D/A board and a counter board.In a practical application, high precision positioning at a low velocity is required, but in general, it is well known that the conventional control methods can not accomplish such a requirement. Moreover, the tracking error becomes large at the end of a stroke because of the effect of a friction force.
In the previous researches, a friction force can be regarded as a static function of velocity in spite of its complicated phenomenon. Therefore, the servo characteristics of this experimental system were investigated.Experiments have shown that there is a deflection or relative movement in the pre-sliding region, indicating that the relationship between the deflection and the input force resembles a non-linear spring with a hysteretic behavior. In this experiment, general PID control is used.Thus, the present study focuses on the nonlinear behavior at the end of a stroke during changes in velocity as shown in Fig. 16 (left side). In this figure, the signals of ①,④ and ⑦are velocity references, the signals of ②,⑤ and⑧ are velocity responses, the signals of ③,⑥ and ⑨ are output forces with constant acceleration-deceleration profiles of 10mm/s, 5.0mm/s, and 2.5mm/s, respectively. The forces in the actual experiment are calculated values and not the values actually measured. It seems that the trackingerror of velocity are almost zero. From this figure, it is seen that the output forces are different during constant velocity and the force of 2.5mm/s is the largest in all cases. The moving force generally needs a big one where velocity is large. The reason is influence of viscous friction. When the velocities are decreasing, output forces have not decreased and when the velocities are increasing, output forces have not increased.
Further,when the output forces are set to zero, the spring- like behavior occurs at the end of a stroke, as shown in Fig.16 (right side). In this figure, the signals of ①,② and ③ are the displacement,the command velocities which are 10mm/s, 5.0mm/s, and 2.5mm/s, respectively. At values of low command velocities, the spring-like behaviors produce large displacement. The displacement, which exceeds 15μm can negatively influence precision point to point control. The frequency of vibration was observed to be 40Hz. The spring-like characteristic behavior is thought to be due to the elastic deformation between balls and rails in the ball guide-way. Thus, friction is a natural phenomenon that is quite hard to model description by on-line identification, and is not yet completely understood.Particularly, it is known to have a bad influence in a trackingresponse at the low-velocity movement. Next, in this table drive system with such a nonlinear characteristic, we evaluate the effectiveness of the proposed compensation method.
Fig.17 shows a block diagram of the proposed control method, which consists of a PID controller (
The PID controller is tuned using the normal procedure, where a signal
3.2.2. Experimental results
To show the effectiveness of the proposed method, experiments were carried out. Digital implementation was assumed in experimental setup. The sampling time of experiments was 0.25ms. Parameters of PID controller was chosen as
It is obvious that the tracking errors of the case with compensation are reduced by more than 2/3 compared to the case of without compensation at the end of a stroke. Table 2 shows the tracking errors at the end of stroke. The errors are greatly reduced by our proposed compensation method.Thus, the proposed method is judged to have better performance accuracy. Fig.21 shows the compensate force inputs
3.3. A table drive system using synchronous piezoelectric device driver
For the future applications of an electron beam (EB) apparatus for the semiconductor industry, a non-resonant ultrasonic motor is the most attractive device for a stage system instead of an electromagnetic motor, because the power source of the stage system is required for non-magnetic and vacuum applications. Next, we evaluated a steppingmotion and tracking motion using a synchronous piezoelectric device driver.
3.3.1 Experimental system
Fig.22 shows aphotograph of SPIDER (Synchronous Piezoelectric Device Driver)and its specifications. Fig.23 shows the experimental setup which consists of the following parts. The control system was implemented using a Pentium IV PC with a DIO board and a counter board. The control input was calculated by the controller, and its value was translated into an appropriate input for the SPIDER through the DIO board, parallel-serial transfer unit, and drive unit. The position of the positioning table was measured by a position sensor with a resolution of 100nm. The sensor's signal was provided as a feedback signal. The sampling period was 0.5ms. The table was mounted on a driving rail. The weight of the moving part of the positioning table was approximately 1.2kg. The friction tip was in contact with the side of the table. The longitudinal feed of the table was 100mm. The positioning precision of this system depends on the resolution of the position sensor, and the best precision is less than 1nm.The parallel-serial transfer unit translated the parallel data into serial data. The drive unit was a voltage generator for the piezoelectric actuator of SPIDER. Fig. 24 shows the motion of the SPIDER. The SPIDER has eight stacks and each stack consists of an extensible and shared piezoelectric element. The behavior of each stack is similar to that of a leg in ambulatory animals or human beings. Despite the limitation in the strokes of stack, the table can move endlessly. The motion sequence of the stacks is as follows (The sequence starts from the top of the left side. In this case, the table's direction of motion was to the right)
Deform stack B to the counter direction of the motion of the table;
Expand stack B to contact the stage;
Retract stack A;
Deform stack B to the forward direction of the motion of the table; then the stage move to the forward direction at one step;
Deform stack A to the counter direction of the motion of the table;
Expand stack A to contact the stage;
Retract stack B;
Deform stack A to the forward direction of the motion of the table; then the stage move in the forward direction in one step.
Continuous displacement of the positioning table can be given by repeating this sequence periodically. In addition, it is possible to be fast at speed of the motion of positioning table by increasing the amplitude of frequency and/or the voltage of this period.As we can see the positioning table is driven by the scratching and friction force via the SPIDER system. However, it is known that friction causes stick slip behavior; therefore, the frictional force is a major problem in precision positioning systems.
Next, we observe the nonlinear characteristic of the SPIDER system.To investigate the nonlinear characteristic of the SPIDER system, the open-loop control responses are measured.Fig.25 shows the control input
Next, we applied our proposed control method with the nonlinear compensator for this system and inspected the effect of the method.First, the results of stepping motion which is one pulse motion are described. Secondly, the results of tracking motion of which amplitude of position references are 1mm and 10mmare described.
3.3.2. Experimental results
To show the effectiveness of the proposed method, experiments were carried out. Fig.28 shows a block diagram of the P,PI/I-P+FF control method with proposed compensator.Digital implementation was assumed in experimental setup. The sampling time of experiments was 0.5ms. Parameters of P,PI/I-P+FF controller was chosen as in Table 3. The value of
To evaluate the proposed control method, the two position references were applied. First, the experimental results of stepping motion using conventional P,PI/I-P+FF control and proposed control are shown in Fig. 29. In the figure, ① is the position reference, ② is the table displacement and ③ is the positioning error. It is obvious that the positioning errors are greatly reduced using proposed control method compared to the conventional control
method. Note, because the resolution of the position sensor is 100nm by this experiment,the positioning resolution becomes 100nm,but can raise positioning resolution to 0.6 nm if weimprove the resolution of the position sensor.Secondly, Figs. 30, 31 showed the results of tracking motion of which amplitude of position references was 1mm or 10mm. In Figs. 30, 31, ① is the position reference, ②is the stage displacement and③is the tracking error, respectively. In these figures, the tracking errors of stroke end of the stage were expanded.Compared to each response,the results of proposed controlwere slightly improved to the results of conventional control.In order to investigate these results, the force input of the conventional control and proposed control were shown in Fig. 32. Both force inputs had a lot of vibrations to compensate undesired friction.In conventional control, the tracking error of stroke end was changed slowly in Fig. 32 (left side). On the other hand, in proposed control, the tracking error of stroke end was tracked to zero roughly in Fig. 32 (right side). Compared to these force inputs,the force input of proposed control was quickly and smoothly changed at the marked point. Therefore, we considered the tracking error of the proposed control method was improved.
In this chapter, we propose a new PID control method that includes a nonlinear compensator that it is easy to understand for a PID control designer.The algorithm of the nonlinear compensator is based on sliding mode control with chattering compensation.The effect of the proposed control method is evaluated with three kinds of single-axis slide system experimentally.The first experiment system is two slider tables comprised of an AC servo motor, a coupling and a ball-screw, and the second one is a slide table using an AC linear motor and the third one is a slide table using synchronous piezoelectric device driver. By the first experiments, it is evaluated using single-axis slide system comprised of full closed feedback via point-to-point control response and tracking control response when load characteristics of the control target change.The experimental results indicate that the proposed control method has robustness in a high-speed,high-precisionpositioning response and a low speed tracking responsewhen acceleration/deceleration of position reference change or the load characteristics of the control target change. By the second experiments, it is evaluated using a linear motor driven slider system via tracking control atlow-velocity, and the resolution of this system is 10nm. The tracking error of our proposed control methodis reduced by more than 2/3 compared to the case of the conventional PID control method at the end of a stroke.By the third experiments, it is evaluated a stepping motion and trajectory tracking motion using a synchronous piezoelectric device driver.The positioning error of our proposed control is reduced by more than 2/3 compared to the case of the conventional PID control at a stepping motion.
Future studies will address the robustness and the control parameters tuning of the proposed compensation method. Furthermore, we want to evaluate the proposed method at an industrial robot used in a car assembly line and a twin linear drive table used in a semiconductor production device.
We greatly appreciate to Dr. Kosaka’s help on the experiments of a synchronous piezoelectric device driver. This research was partially supported by Japan Grant-in-Aid for Scientific Research (C) (19560244).