Abstract:
A control method utilizing a PID controller includes detecting the position of an object and obtaining the position deviation by comparison with a predetermined position value, detecting the vibration of the object and obtaining a vibration value, adjusting the control parameters of the PID controller by analyzing the position deviation, the vibration value, and a predetermined performance of the PID controller, and the PID controller responding to the adjusted control parameters.

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure generally relates to automatic control technology, and particularly to a control method and a device utilizing a PID controller, and a robot applying the device. 
     2. Description of the Related Art 
     A proportional-integral-derivation controller (PID controller) is a generic control loop feedback device widely used in industrial control systems, such as robot manipulator controlling systems. The popularity of PID controllers can be attributed partly to their robust performance in a wide range of operating conditions and partly to their functional simplicity, which allows engineers to operate them in a simple, straightforward manner. 
     A PID controller attempts to correct an error between a measured process variable and a desired setpoint by calculating and then outputting a corrective action that can adjust the process accordingly and rapidly, to keep the error minimal. The PID controller calculation (algorithm) involves three basic coefficients: the proportional (Kp), the integral (Ki) and derivative (Kd). The proportional value determines the reaction to the current error, the integral value determines the reaction based on the sum of recent errors, and the derivative value determines the reaction based on the rate at which the error has been changing. The weighted sum of these three actions is used to adjust the process via a control element such as the position of a control valve or the power supply of a driving element. By tuning the three coefficients in the PID controller algorithm, the controller can provide control actions designed for specific process requirements. 
     A commonly used method for adjusting the three coefficients of a PID controller is manual tuning. For example, when adjusting the PID controller applied by a robot manipulator, Kp is increased to shorten response time and minimize oscillation. However, too much Kp will cause instability, and the manipulator will vibrate intensely. Therefore Kp is adjusted to an optimize PID controller performance while manually controlling the vibration. After repeated manual tunings, an optimum Kp is obtained. However, this adjustment of the Kp wastes time and is dependent upon tester intuition and experience. Additionally, in the case of the same system, the reference variables and the reference values applied to a specific operation state are not applied optimally to another state of the same system. 
     Therefore, there is room for improvement within the art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the present disclosure. 
         FIG. 1  is a plan view of an embodiment of a robot applying an embodiment of a control device. 
         FIG. 2  is a block diagram of the control device of  FIG. 1  with a PID controller therein. 
         FIG. 3  is similar to  FIG. 2 , but shows the principle of the control device of  FIG. 2 . 
         FIG. 4  is a flowchart of an embodiment of a control method of the PID controller. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a plan view of an embodiment of an industrial robot  100 . The industrial robot  100  includes a base seat  11 , a bracket  12  rotatably connected to the base seat  11 , a first support arm  13  rotatably connected to the bracket  12 , a joint portion  16  rotatably connected to the first support arm  13 , a second support arm  14  rotatably connected to the joint portion  16 , and a third support arm  15  rotatably connected to the second support arm  14 . 
     The industrial robot  100  is supported by the base seat  11  and has six rotation axes. The bracket  12  is rotatable around a first axis a. The first, second, and third support arms  13 ,  14 ,  15  are rotatable around a second, third, and fourth axes b, d, and e respectively. The industrial robot  100  further includes a fifth and sixth axes schematically indicated by c and f, respectively. An operating device (not shown), such as a clamp, a cutter, or a detector is generally positioned on an end of the third support arm  15  along the sixth axis f to realize various operations. Along each rotation axis, a stepper motor  21  is mounted thereon to drive an output shaft and the support arm connected to the output shaft to move to a desired position.  FIG. 1  shows only the stepper motor  21  mounted on the fourth axis e to drive the third support arm  15 , and the stepper motor mounted on other axes are omitted here for convenience. 
       FIG. 2  is a block diagram of a control device  200 , and  FIG. 3  shows the principle of the control device  200  applied by the industrial robot  100 . The control device  200  includes six stepper motor drivers  22  used to drive the stepper motors  21 , a position detection unit  23 , a vibration detection unit  24 , a motion control card  25  and a control center  26 . The control center  26  may be a computer including an adjustment unit  261  and a human machine interface (HMI)  262 . The vibration detection unit  24  detects vibration in the third support arm  15 , and the position detection unit  23  detects the placement of the third support arm  15 . Both the vibration and placement signals are outputted to the adjustment unit  261 . The motion control card  25  can generate pulse control signals according to the adjustment unit  261 . The stepper motor driver  22  converts the pulse control signals into angular displacement signals so that the support arm can move to a predetermined position. The stepper motor driver  22  has a PID controller  223  therein. The PID controller  223  provides a three-term control action by serially connecting a position control module  2231 , a velocity control module  2232 , and a current control module  2235 . Three coefficients Kp, Ki, Kd of the PID controller  223  may be adjustable. 
     The position detection unit  23  is connected to the control center  26  to detect the position (rotating angular displacement) of the third support arm  15  around the fourth axis e and feed the position signals back to the adjustment unit  261 . In the illustrated embodiment, the position detection unit  23  is an optical rotary encoder sensor mounted on the output shaft of the stepper motor  21  to detect the rotating angular displacement of the third support arm  15 . 
     The vibration detection unit  24  is connected to the control center  26  to detect the vibration of the third support arm  15 . In the illustrated embodiment, the vibration detection unit  24  is an acceleration sensor mounted on the third support arm  15  to obtain both the amplitude and frequency of vibration, and feed the vibration signals back to the adjustment unit  261 . 
     The motion control card  25  may generate control signals to drive one or more stepper motors  21  to accelerate or decelerate automatically and detect the original position and the position limiting signals. The control signals include pulse control signals and direction signals. In the illustrated embodiment, the industrial robot  100  applies a six axes motion control card  25  for six axes motion control. 
     The adjustment unit  261  receives the information outputted from the position detection unit  23  and the vibration detection unit  24 , and calculates a position deviation by subtracting the detected position from a predetermined position value of the third support arm  15  set by the PID controller  223 . The adjustment unit  261  adjusts the three coefficients Kp, Ki, Kd by analyzing the predetermined dynamic performance of the control device  200 , the vibration and the position deviation. In response to the adjusted coefficients Kp, Ki, Kd, the PID controller  223  is capable of controlling the control device  200  to satisfy an optimum performance with shorter response time and less overshoot. The adjusted coefficient Kp may be transmitted by serial port from the control center  26  to the PID controller  223 . The human machine interface  262  displays the dynamic response diagram of the control device  200 , such as a time domain response, and provides an interface window to configure some of the control parameters of the control device  200 . 
     The position control module  2231  returns a position deviation from a predetermined position and minimizes the position deviation by changing the coefficient Kp. As the coefficient Kp is increased, the steady deviation may be reduced, and the response time may become shorter, but may cause more overshoot. Contrarily, as the coefficient Kp is decreased, the overshoot may be improved, but the response time may become longer. 
       FIG. 4  is a flowchart illustrating an embodiment of a control method applied by the control device  200 . Depending on the embodiment, certain of the steps described below may be removed and others may be added. 
     In step S 31 , the control parameter adjusting mode is set to a semi-automatic mode in which only the coefficient Kp of the position control module  2231  is adjustable. 
     In step S 32 , a load is applied to the controlled object. The load is set onto the third support arm  15 , and an initial condition and disturbance generated by the load is retained. 
     In step S 33 , the position detection unit  23  detects the rotating angular displacement of the third support arm  15  around the fourth axis e using an optical rotary encoder sensor. 
     In step S 34 , the vibration detection unit  24  utilizes the acceleration sensor to detect the vibration information around three coordinate axes of the coordinate space. The vibration information includes the vibration amplitude and frequency of the third support arm  15 . 
     In steps S 35  and S 36 , control parameters of the PID controller  223  are adjusted by analyzing position deviations from a predetermined position, the vibration value, and a predetermined performance of the PID controller  223 . Specifically, the range of the coefficient Kp of the PID controller  223  is set by setting a minimum vibration value and a maximum vibration value. The coefficient Kp is adjusted in the range by means of increasing the coefficient Kp when the detected vibration value is less than the minimum vibration value, and decreasing the control parameters when the detected vibration value exceeds the predetermined maximum vibration value. In the illustrated embodiment, when adjusting the coefficient Kp, the time domain response graph displayed by the human machine interface  262  is referenced, such that when the detected vibration value and the response time both reach the desired performance, the current coefficient Kp is maintained and the coefficient Kp of the PID controller  223  is set by a serial port. 
     In step S 37 , various loads are applied on the third support arm  15 , whereby the initial condition and the exterior disturbance are changed, repeating steps S 32  to S 36 . An optimized coefficient Kp is obtained corresponding to the load, and the coefficient Kp is stored in a memory of the PID controller  223 . Repeating the step S 37 , the optimized coefficient Kp corresponding to the various loads are obtained. 
     In step S 38 , a knowledge repository is established by combining the load and corresponding control parameters adjusted by the PID controller  223 . The knowledge repository may be stored in the control center  26 . When setting the coefficient Kp of other PID controllers for use by others robots, the adjusting time can be decreased by selecting the coefficient Kp in the knowledge repository to match the load, the vibration, and the response time. 
     It should be understood that although the present disclosure is, by way of example, applied to the third support arm  15  of the robot  100 , it is understood that the application of the present invention is not limited thereto. 
     It is believed that the present embodiments and their advantages will be understood from the foregoing description, and it will be apparent that various changes may be made thereto without departing from the spirit and scope of the embodiments or sacrificing all of its material advantages.