Patent Publication Number: US-2018036881-A1

Title: Gripping System which is Monitored Based on Counter Electromotive Force and a Method for Controlling the Gripping System

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of priority to U.S. patent application Ser. No. 15/046,169 filed on Feb. 17, 2016, the entire specification of which is incorporated herein by reference. 
     BACKGROUND 
     Field of the Invention 
     The present invention relates to a manipulator, and more particularly to a gripping system which is monitored based on counter electromotive force, and a method for controlling the gripping system. 
     Related Prior Art 
     With the wide application of automatic production in various fields, the electric grippers with a compact size, light weight and high flexibility are becoming more and more important. To ensure the stability of a gripping operation, the electric gripper must be controlled or monitored in real time. 
     There are many ways to control or monitor the electric gripper. One of them is based on the characteristic of a stepper motor, namely, the torque of a stepper motor is inversely proportional to the rotation speed, so that the gripping force of the gripper which is driven by the stepper motor can be controlled by controlling the rotation speed of the stepper motor. However, since the torque of a stepper motor is inversely proportional to the rotation speed, when the rotation speed changes rapidly, such as rapid acceleration, the torque will decrease and cause out of step, which consequently affects grip stability. 
     Another way of control is to arrange an additional sensor to detect the occurrence of out of step of the motor. The power of the motor will be cut off immediately when the motor falls out of step, and a retaining mechanism will be used to maintain the gripping force. The disadvantage of this way of control is that the maintaining of the gripping force only occurs when the motor falls out of step. Therefore, the occurrence of out of step is unavoidable, consequently, the griping force cannot be stably controlled. Besides, this way of control further requires the use of an independent retaining mechanism to maintain the gripping force, which complicates the structure while increasing the manufacturing cost. 
     The present invention has arisen to mitigate and/or obviate the afore-described disadvantages. 
     SUMMARY 
     The present invention is aimed at providing a gripping system which is monitored based on counter electromotive force (CEMF), and a method for controlling the gripping system, which are capable of solving the problem of the conventional gripper control method and device that the gripping force can be stably controlled, and the mechanical structure is complicated. 
     Therefore, a gripping system which is monitored based on counter electromotive force in accordance with the present invention comprises: 
     a gripper; 
     a stepper motor connected to the gripper and driving the gripper to perform gripping inward or gripping outward motions; 
     a controller electrically connected to the stepper motor, and including a drive unit, a control unit, an instruction receiving unit, and an access unit which are electrically connected to one another; wherein: 
     the drive unit is electrically connected to the stepper motor to rotate the stepper motor, and provides feedback on actual counter electromotive force of the stepper motor; 
     the control unit is electrically connected to the drive unit; 
     the instruction receiving unit is electrically connected to the control unit, and able to receive a control instruction for controlling gripping status, griping speed, and gripping force of the gripper; and 
     the access unit is electrically connected to the control unit, a control parameter matrix is stored in the access unit, the control parameter matrix is a 3-D matrix defined by using the gripping status, the griping speed, and the gripping force as parameters, the gripping status includes gripping inward and gripping outward, each position in the matrix corresponds to a counter electromotive force threshold, the control unit is capable of controlling operation of the stepper motor based on the control instruction from the instruction receiving unit, the control unit is further capable of obtaining a corresponding counter electromotive force threshold by comparing the control instruction with the control parameter matrix, and then the control unit compares the actual counter electromotive force, and controls the stepper motor according to comparison result. 
     A method for controlling a gripping system based on counter electromotive force in accordance with the present invention, comprises the following steps: 
     using a controller to instruct a drive unit, via a control instruction, to rotate a stepper motor, the stepper motor during operation generates an actual counter electromotive force; 
     using the controller to compare the control instruction with a control parameter matrix stored in an access unit, the control parameter matrix is a 3-D matrix defined by using the gripping status, the griping speed, and the gripping force as parameters, the gripping status includes gripping inward and gripping outward, each position in the matrix corresponds to a counter electromotive force threshold; 
     obtaining a corresponding counter electromotive force threshold by using the controller to check position of the actual counter electromotive force in the control parameter matrix; 
     using the controller to continuously monitor the actual counter electromotive force; and 
     using the controller to compare the actual counter electromotive force with the obtained corresponding counter electromotive force threshold, stopping the stepper motor and letting the gripper maintain the gripping status when the actual counter electromotive force is smaller than or equal to the counter electromotive force threshold, and maintaining driving of the stepper motor when the actual counter electromotive force is larger than the counter electromotive force threshold. 
     The present invention measures in advance the operation parameters when the stepper motor runs well without falling out of step, which ensures that the objected can be stably gripped before abnormality occurs, thus stabilizing the gripping motion. 
     These together with other objects of the invention, along with the various features of novelty which characterize the invention, are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and the specific objects attained by its uses, reference should be had to the accompanying drawings and descriptive matter in which there are illustrated preferred embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a gripping system which is monitored based on counter electromotive force in accordance with a preferred embodiment of the present invention; 
         FIG. 2  is a flow chart of a method for controlling the gripping system which is monitored based on counter electromotive force in accordance with the preferred embodiment of the present invention; 
         FIG. 3  is an illustrative view of a control parameter matrix of the present invention; 
         FIG. 4-1  is a reference table of the counter electromotive force threshold in correspondence to the control parameter matrix, when a gripper of the present invention performs gripping inward motions; and 
         FIG. 4-2  is a reference table of the counter electromotive force threshold in correspondence to the control parameter matrix, when the gripper of the present invention performs gripping outward motions. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention will be clearer from the following description when viewed together with the accompanying drawings, which show, for purpose of illustrations only, the preferred embodiment in accordance with the present invention. 
     Referring to  FIG. 1  to  FIG. 4-2 , a gripping system which is monitored based on counter electromotive force in accordance with the preferred embodiment of the present invention comprises: a gripper  10 , a stepper motor  20 , and a controller  30 . 
     The stepper motor  20  is connected to the gripper  10  and can drive the gripper  10  to perform gripping inward or gripping outward motions. 
     The controller  30  is electrically connected to the stepper motor  20 , and includes a drive unit  31 , a control unit  32 , an instruction receiving unit  33 , and an access unit  34  which are electrically connected to one another. 
     The drive unit  31  is electrically connected to the stepper motor  20  to rotate the stepper motor  20 , and feeds back an actual counter electromotive force of the stepper motor  20 . 
     The control unit  32  is electrically connected to the drive unit  31 . 
     The instruction receiving unit  33  is electrically connected to the control unit  32 , and able to receive control instructions for controlling the gripping status, the griping speed, and the gripping force of the gripper  10 . 
     The access unit  34  is electrically connected to the control unit  32 , and in the access unit  34  is stored a control parameter matrix which is a 3-D matrix defined by using the gripping status (gripping inward and gripping outward), the griping speed, and the gripping force as parameters. Each position in the matrix corresponds to a counter electromotive force threshold, and the above parameters are all measured when the stepper motor runs well without falling out of step. The control unit  32  is capable of controlling the operation of the stepper motor  20  based on the control instruction from the instruction receiving unit  33 . The control unit  32  is further capable of obtaining corresponding counter electromotive force threshold by comparing the control instruction with the control parameter matrix, and then the control unit  32  compares the actual counter electromotive force, and controls the stepper motor  20  according to the comparison result. 
     What mentioned above are the structure relations of the components of the gripping system which is monitored based on counter electromotive force, and the method for controlling the gripping system in accordance with the present invention comprises the following steps: 
     Step I of receiving control instructions: using the instruction receiving unit  33  to receive control instructions for controlling the gripping status, the griping speed (speed of gripping motion), and the gripping force; 
     Step II of driving the stepper motor: using the control unit  32  to access the received control instructions, controlling the operation of the stepper motor  20  based on the received control instructions, and using the stepper motor  20  to generate actual counter electromotive force; 
     Step III of comparing the control parameter matrix: using the control unit  32  of the controller  30  to compare the parameters of the received control instructions with the control parameter matrix stored in the access unit  34 , each position in the matrix corresponds to a counter electromotive force threshold; 
     Step IV of obtaining the counter electromotive force threshold: obtaining a corresponding counter electromotive force threshold by using the controller  30  to check the position of the actual counter electromotive force in the control parameter matrix. More specifically, as shown in  FIG. 3 , the control parameter matrix is a 3-D matrix defined by using the gripping status, the griping speed, and the gripping force as parameters. Of course, the elements in the respective dimensions of the control parameter matrix can be changed as desired. For example, the first dimension represents the gripping status, and the gripping status includes 2 elements which represent gripping inward (0) and gripping outward (1), respectively. The second dimension represents gripping speed, and includes 20 elements which each represent from 5 mm/s to 100 mm/s. The third dimension represents the gripping force and includes 14 elements which each represent the gripping force from 50N to 700N. When the instruction receiving unit  33  receives control instructions, the control unit  32  will access the control parameter matrix stored in the access unit  34  to obtain the corresponding counter electromotive force threshold. For example, when the instruction is “gripping outward, 20 mm/s, 300N”, the control unit  32  will check the corresponding parameters to find out the position [2] [4] [6] in the control parameter matrix, and read the counter electromotive force in that position of [2][4][6]. 
     Step V of monitoring the actual counter electromotive force: using the control unit  32  of the controller  30  to continuously monitor the actual counter electromotive force fed back from the drive unit  31 ; 
     Step VI of comparing and controlling: using the control unit  32  to compare the actual counter electromotive force with the obtained counter electromotive force threshold, stopping the stepper motor  20  and letting the gripper  10  maintain the gripping status when the actual counter electromotive force is smaller than or equal to the counter electromotive force threshold, and maintaining driving of the stepper motor  20  when the actual counter electromotive force is larger than the counter electromotive force threshold. 
     In summary, the present invention measures in advance the operation parameters when the stepper motor runs well without falling out of step, and creates the control parameter matrix by using the operation parameters. The present invention further feeds back the actual counter electromotive force in real time, and compares the control parameter matrix when the gripper  10  grips the object, so that monitoring can be performed in real time based on the comparison results, so as to achieve the purpose of monitoring in real time, and stabilizing the gripping parameters. 
     Since the counter electromotive force is used as a base for real time monitoring, and the control parameter matrix is used as a reference, the present invention can truly prevent the occurrence of out of step, ensure that every gripping motion is maintained in the best condition, and prevent loosening of workpieces, which consequently improves the yield rate. 
     Besides, with the counter electromotive force generated by the stepper motor  20 , the present invention requires the use of an independent torque sensor during the whole monitoring process. Therefore, the present invention has the advantages of simple structure and high assembly flexibility, which consequently adds value to the products. 
     Furthermore, the control parameter matrix of the present invention is a 3-D matrix defined by using the gripping status, the griping speed, and the gripping force as parameters, which means that the present invention is always capable of obtaining a corresponding counter electromotive force threshold no matter how the gripping status, the griping speed, and the gripping force change. Therefore, the present invention is very dynamic, and has high application availability. Base on this, it can further be inferred that, when the control parameter of the gripping system changes, the parameter value of the control parameter matrix also changes. The control parameter matrix is not limited to the embodiment shown in  FIG. 3 . 
     While we have shown and described various embodiments in accordance with the present invention, it is clear to those skilled in the art that further embodiments may be made without departing from the scope of the present invention.