Industrial robot collision protection method

Abstract

The present invention provides an industrial robot collision protection method, belonging to the field of robot safety control technology. It solves the problem of existing technologies causing continuous squeezing of objects or human bodies. The industrial robot collision protection method includes: pre-calibrating the torque collision threshold of each joint of the robot; obtaining the position data, velocity data, and torque data of each joint of the robot; obtaining the real-time external torque of each joint based on the momentum observer model, the position data, velocity data, and torque data of each joint, comparing the real-time external torque with the torque collision threshold of the corresponding joint, and controlling the corresponding collision strategy based on the collision type when a collision occurs. The collision strategy includes an emergency stop control strategy that controls the motors of each joint axis to generate reverse motion after an emergency stop operation when a collision occurs, and a flexible motion strategy that controls the motors of the corresponding joint axis to switch from position control to torque control when a collision occurs. The present invention effectively reduces the damage caused by collision to objects and personnel.

Description

Technical Field

[0001] The invention belongs to the technical field of robot safety control and relates to a collision protection method for an industrial robot. Background Art

[0002] Industrial robots, with their high precision, high stability, and strong environmental adaptability, have been widely used in various manufacturing fields. They can efficiently complete repetitive and single tasks and replace manual operations in hazardous environments such as high temperatures, toxic substances, and radiation. However, when robots and humans work together or share a workspace, there is a potential risk of mechanical collision, which can cause equipment damage or personal injury.

[0003] Currently, there are two types of contact collision detection methods for industrial robots and external objects: sensor-based and sensorless. Sensor-based collision detection solutions rely on six-dimensional force sensors, tactile skin, or lidar to directly measure contact force or distance to determine whether a collision has occurred. Sensorless collision detection solutions use real-time monitoring of intrinsic parameters such as motor current / torque, joint angle, and end-effector force. The collision threshold is determined based on the dynamic model and expected motion trajectory, and a collision judgment is triggered when the actual torque exceeds the threshold. After a collision occurs, the current handling strategy is usually to stop immediately and resume movement after the colliding object is removed. However, this strategy can cause continued squeezing of objects or people, resulting in damage or safety accidents. Summary of the Invention

[0004] The purpose of the present invention is to address the above-mentioned problems existing in the prior art and to propose an industrial robot collision protection method, which solves the technical problem of how to reduce the damage caused by collision to objects and personnel.

[0005] The purpose of the present invention can be achieved through the following technical solutions: A method for collision protection of an industrial robot comprises the following steps:

[0006] S1. Pre-calibrate the torque collision threshold of each joint of the robot;

[0007] S2, the robot control system monitors the robot's motion process and obtains the position data, speed data and torque data of each joint of the robot;

[0008] S3. Obtaining the real-time external torque of each joint based on the momentum observer model, the position data, velocity data, and torque data of each joint, comparing the real-time external torque with the torque collision threshold of the corresponding joint based on the collision judgment formula, and determining whether a collision has occurred based on the comparison result;

[0009] S4. When a collision occurs, control is performed based on the corresponding collision strategy obtained based on the collision type. The collision strategy includes an emergency stop control strategy for controlling each joint axis motor to perform an emergency stop operation and then generate a reverse motion when a collision occurs, and a flexible motion strategy for controlling the corresponding joint axis motor to change from position control to torque control when a collision occurs. The collision types include flexible collision and rigid collision. When a flexible collision occurs, the emergency stop control strategy is adopted for control, and when a rigid collision occurs, the flexible motion strategy is adopted for control.

[0010] This industrial robot collision protection method pre-calibrates the torque collision threshold for each joint during use. While the robot is performing a task, the robot control system monitors the robot's motion in real time and obtains position, velocity, and torque data for each joint based on the servo system motor data. The real-time external torque of each joint is obtained based on a momentum observer model and compared with the torque collision threshold to determine which joint of the robot has collided. This method does not use an external torque sensor but calculates the external torque using a momentum observer. First, a threshold measurement is performed, followed by collision detection. This method can determine which joint axis has collided, effectively improving the sensitivity of collision detection. When a collision occurs, a corresponding collision strategy is selected based on the type of collision. If a collision occurs with a flexible object, an emergency stop control strategy is adopted. If a collision occurs with a rigid object, a flexible motion strategy is adopted, in which the robot actively softens to adapt to the colliding object. Selecting the appropriate collision strategy based on the type of collision can more effectively protect the safety of the colliding object or personnel and minimize damage.

[0011] In the above-mentioned industrial robot collision protection method, in step S1, the operation of pre-calibrating the torque collision threshold of each joint of the robot includes:

[0012] Control the robot to enter threshold measurement mode;

[0013] Specify a work path, control the robot to run according to the specified work path, and continuously record the position data, speed data and torque data of each joint of the robot;

[0014] The position data, velocity data and torque data of each joint are input into the momentum observer model to calculate the external torque of each joint at each moment. By comparing the external torque of the same joint at each moment, the maximum external torque of each joint is obtained and calibrated as the torque collision threshold of the corresponding joint, which is recorded as , i represents a joint of the robot. A work path refers to the path a robot takes to complete an action, such as a part transfer path, welding path, or spraying path. The robot body consists of movable rotating or translating joints, or joints, each driven by a servo motor. Threshold measurement along the work path can improve the accuracy of collision detection.

[0015] In the above-mentioned industrial robot collision protection method, in step S1, the operation of pre-calibrating the torque collision threshold of each joint of the robot further includes:

[0016] Divide the specified working path into segments, then control the robot to run each working path, and continuously record the position data, speed data and torque data of each joint of the robot in each working path;

[0017] The position data, velocity data and torque data of each joint are input into the momentum observer model to calculate the external torque of each joint at each moment in each working path. By comparing the external torque of the same joint at each moment in the same working path, the maximum external torque of each joint in each working path is obtained and calibrated as the torque collision threshold of a joint in a certain working path, which is recorded as , i represents a joint of the robot, and j represents a working path. Dividing a working path into multiple segments can improve detection sensitivity and avoid problems such as long working paths and inaccurate judgments.

[0018] In the above-mentioned industrial robot collision protection method, in step S1, an operation of updating the torque collision threshold of each joint is further included, and the operation includes:

[0019] The torque collision threshold update cycle is set, and the robot operates normally according to the specified work path. During each cycle, the position, velocity, and torque data of each robot joint are continuously recorded. This joint position, velocity, and torque data are input into the momentum observer model to obtain the maximum external torque of each joint. If the maximum external torque of a joint in the current cycle is greater than the absolute value of the maximum external torque saved in the previous cycle, the current maximum external torque is updated to the torque collision threshold. Otherwise, the torque collision threshold remains unchanged. This method of adjusting the torque collision threshold can make collision judgment more sensitive and accurate, further improving safety.

[0020] In the above-mentioned industrial robot collision protection method, in step S3, the operation of obtaining the real-time external torque of each joint includes:

[0021] The momentum observer model is established, and the formula is:

[0022]

[0023] in: is the real-time external torque of the joint, is the diagonal gain matrix of the observer, p(t) is the generalized momentum of the robot at time t, p(0) is the generalized momentum of the robot at the initial time, ,speed, is the joint torque, and the matrix is ​​defined as , represents the gravity matrix, represents the transpose of the centripetal and Coriolis force matrices, yes Observed values ​​of

[0024] Joint position, velocity, and torque data are input into the momentum observer model's formula to derive the real-time external torque on the joint. The momentum observer model processes motor angle, velocity, and current data in real time, rapidly calculating and providing real-time feedback on external torque information. This allows the robot to promptly sense changes in external forces and quickly adjust its movements or take protective measures, thereby improving the safety of both the robot and personnel.

[0025] In the above-mentioned industrial robot collision protection method, in step S3, the operation of comparing the real-time external torque with the torque collision threshold of the corresponding joint based on the collision judgment formula includes:

[0026] Assume that the real-time external torque of each joint is , i=1~n, j=1~m, n and m are positive integers;

[0027] The collision judgment formula is: Collision threshold with torque For comparison, d s ≥1, is the sensitivity coefficient, Greater than When , it is determined that a collision occurs; Less than or equal to , it indicates that no collision has occurred. The subscript i can be used to determine which joint's external torque exceeded the limit, triggering the collision. The subscript j can be used to determine which path's external torque exceeded the limit, triggering the collision. This allows us to accurately determine which joint and path collided, allowing us to respond with appropriate collision strategy feedback to ensure the safety of both the robot and the operator.

[0028] In the aforementioned industrial robot collision protection method, in step S4, the emergency stop control strategy is divided into control strategy one for reverse movement to the starting point and control strategy two for reverse movement to a designated position, based on the distance of the reverse movement. The emergency stop control strategy is further divided into control strategy one and control strategy two, which can better adapt to different robot working paths, provide more effective protection in the event of a collision, and improve safety.

[0029] In the above-mentioned industrial robot collision protection method, in step S4, the control strategy 1 includes:

[0030] When a collision is detected, the motors of each joint axis are controlled to perform an emergency stop operation;

[0031] Set the stop time when the robot comes to a complete stop;

[0032] When the stop time is reached, a reverse motion control command is generated, directing the robot's joints back to the starting point. The stop time can be set to 0.1s, but other values ​​are available depending on the situation. When the robot control system stops the motor, the motor does not stop immediately. The robot's joints will continue to move forward a small distance before coming to a complete stop. Therefore, setting a stop time ensures that reverse motion control is performed after the motor has completely stopped, directing the robot's joints back to the starting point along the original working path. This prevents collisions during retraction and improves safety.

[0033] In the above-mentioned industrial robot collision protection method, in step S4, the second control strategy includes:

[0034] When a collision is detected, the motors of each joint axis are controlled to perform an emergency stop operation;

[0035] Set the stop time when the robot comes to a complete stop, and preset the retraction distance and speed;

[0036] When the stop time is reached, a reverse motion control instruction is generated to control the robot's joints to move at a preset speed and reverse the preset retraction distance value along the straight path formed by the collision point and the stop point. The retraction distance value is generally 30-100mm, and the speed is 30%-80% of the reference speed. For example, if the reference speed is 300mm / s, 30% is 90mm / s. The retraction distance value preset by the reverse motion along the straight path formed by the collision point and the stop point can release the squeezing force between the robot and the object, preventing the flexible object from being damaged by continuous squeezing and ensuring the safety of the flexible object. Presetting a retraction distance value can effectively solve the collision problem and avoid hitting other objects, ensuring the safety of the robot or personnel.

[0037] In the above-mentioned industrial robot collision protection method, in step S4, the operation of the flexible motion strategy includes:

[0038] When a collision occurs, the control mode of the robot's collision joint axis motor and any one or a combination of related joint axis motors changes from position control to torque control; the remaining joint axis motors continue to operate in position control mode and perform an emergency stop operation;

[0039] A preset mode recovery time is set. When the mode recovery time is reached, the control mode of the robot's joint axis motors reverts from torque control to position control. The mode recovery time can be set to 1s-2s. When set, the mode recovery time is greater than the stop time. In this collision strategy, by controlling the mode switching of the corresponding joint axis motors, the robot can actively soften and deform to adapt to the rigid object when it collides with the rigid object. This protects both the robot and the rigid object and improves the safety of the robot's operation. Delaying the mode recovery time and then switching the control mode back to position control and closing the loop at the current position ensures the robot's subsequent stability.

[0040] Compared with the existing technology, this industrial robot collision protection method has the following advantages:

[0041] 1. The present invention can adopt corresponding collision strategies according to different colliding objects, can quickly release the extrusion force, reduce the damage caused by collision and extrusion, and effectively protect the safety of equipment or personnel.

[0042] 2. The present invention first allows the robot to run normally along the working path, records the torque of each joint and extracts the torque collision threshold. For longer paths, it can also be processed in sections to obtain the most appropriate torque collision threshold for each section. When the robot subsequently moves along the same path, once a collision occurs, high-sensitivity detection can be achieved with these precise torque collision thresholds, effectively improving the accuracy of collision detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 It is a control flow chart of the present invention. DETAILED DESCRIPTION

[0044] To make the objectives, technical solutions, and advantages of the present invention more apparent, embodiments of the present invention will be further described in detail below with reference to the accompanying drawings. The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit the present invention, its application, or use. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present invention without inventive effort are intended to fall within the scope of protection of the present invention.

[0045] like Figure 1 As shown in FIG, when the industrial robot collision protection method is applied in an industrial robot, the torque collision threshold of each joint of the robot is first pre-calibrated. The operation includes: controlling the robot to enter the threshold measurement mode; specifying a working path, such as a part transfer path; after the path is selected, controlling the robot to operate normally according to the specified working path, and continuously recording the position data, velocity data, and torque data of each joint of the robot; and inputting the position data, velocity data, and torque data of each joint into the momentum observer model. The formula of the momentum observer model is:

[0046]

[0047] in: is the real-time external torque of the joint, is the diagonal gain matrix of the observer, p(t) is the generalized momentum of the robot at time t, p(0) is the generalized momentum of the robot at the initial time, ,speed, is the joint torque, and the matrix is ​​defined as , represents the gravity matrix, represents the transpose of the centripetal and Coriolis force matrices, yes The external torque of each joint at each moment is calculated according to the formula. By comparing the external torque of the same joint at each moment, the maximum external torque of each joint is obtained and calibrated as the torque collision threshold of the corresponding joint, which is recorded as , i represents a joint of the robot.

[0048] When a specified working path is long, the working path is segmented. For example, the specified working path is for the robot to transfer parts. One section of the path moves faster, reaching 80% of the speed. Next, the robot slowly puts down the parts and moves slower, at 10%. In this case, the robot is segmented according to its movement speed, which can be divided into two sections, front and back, and two thresholds are recorded respectively. The specific operation is as follows: after completing the working path segmentation, the robot is controlled to run each working path, and the position data, speed data and torque data of each joint of the robot in each working path are continuously recorded; the position data, speed data and torque data of each joint are input into the momentum observer model, and the external torque of each joint at each moment of each working path is calculated according to the momentum observer model formula. By comparing the external torque of the same joint at each moment of the same working path, the maximum external torque of each joint in each working path is obtained, and it is calibrated as the torque collision threshold of a joint under a certain working path, which is recorded as , i represents a joint of the robot, and j represents a working path.

[0049] In order to improve the sensitivity of collision detection, the torque collision threshold update cycle is set. The update cycle can be set to 2ms or other time values. The robot operates normally according to the specified working path, and the position data, velocity data and torque data of each joint of the robot are continuously recorded in each cycle; the position data, velocity data and torque data of each joint are input into the momentum observer model to obtain the maximum external torque of each joint in each working path. When the maximum external torque of the joint in the current cycle is greater than the absolute value of the maximum external torque saved in the previous cycle, the current maximum external torque is updated to the torque collision threshold. Otherwise, the torque collision threshold remains unchanged.

[0050] After completing the torque collision threshold calibration of each joint, the robot control system monitors the robot's motion process and obtains the position data, speed data and torque data of each joint of the robot. The data acquisition in this step does not require the use of an external torque sensor, but only requires the servo system motor data. Specifically, the joint position data can be obtained based on the absolute encoder of the motor; the joint position data is differentiated to obtain the joint speed data; and the motor current is calculated to obtain the joint torque data.

[0051] Enter the real-time external torque calculation step of each joint: input the position data, velocity data and torque data of each joint obtained in each working path into the momentum observer model, and calculate the torque according to the formula Obtain the real-time external torque of each joint in each working path. Based on the collision judgment formula, compare the real-time external torque with the torque collision threshold of the corresponding working path and the corresponding joint. The comparison operation includes:

[0052] Assume that the real-time external torque of each joint is , i=1~n, j=1~m, n and m are positive integers; the collision judgment formula is to convert the real-time external torque Collision threshold with torque For comparison, d s ≥1, is the sensitivity coefficient, Greater than When , it is determined that a collision occurs; Less than or equal to , it indicates that no collision has occurred. The subscript i can be used to determine which joint's external torque exceeded the limit, triggering the collision. The subscript j can be used to determine which path's external torque exceeded the limit, thereby accurately determining which joint and path collided.

[0053] When a collision occurs, the collision protection step is entered. Specifically, the corresponding collision strategy is obtained based on the collision type. The collision strategy includes an emergency stop control strategy for controlling each joint axis motor to perform an emergency stop operation and then generate reverse motion when a collision occurs, and a flexible motion strategy for controlling the corresponding joint axis motor to change from position control to torque control when a collision occurs. Among them, the emergency stop control strategy is divided into control strategy one for reverse motion to the starting point and control strategy two for reverse motion to the specified position according to the distance of the reverse motion.

[0054] The choice of collision strategy is based on the robot's usage scenario. For example, if the robot is used to massage people, it means that the robot collides with a flexible object, that is, the collision type is a flexible collision, and an emergency stop collision strategy is adopted. When selecting control strategy one or control strategy two, you can further select according to the working path to ensure that people or robots can be protected more effectively. When selecting control strategy one for collision protection, control the motors of each joint axis to perform an emergency stop operation, control the motors of each joint axis to stop movement, and set the stop time when the robot stops completely, such as 0.1s; when the stop time is reached, generate a reverse motion control instruction to control the joints of the robot to return to the starting point of the movement according to the original working path.

[0055] When control strategy 2 is selected for collision protection, the motors of each joint axis are controlled to perform emergency stop operations, the motors of each joint axis are controlled to stop moving, the stop time when the robot stops completely is set, the retraction distance value and speed are preset, the retraction distance value is generally 30-100mm, and the speed is 30%-80% of the reference speed. For example, if the reference speed is 300mm / s, then 30% is 90mm / s; when the stop time is reached, a reverse motion control instruction is generated to control the joints of the robot to move at a preset speed, and reverse the preset retraction distance value according to the straight line path formed by the collision point and the stop point, thereby releasing the squeezing force between the robot and the object, avoiding damage to the flexible object due to continuous squeezing, and ensuring the safety of the flexible object.

[0056] For example, if a robot is loading or unloading material from a machine tool, which is a rigid object, the collision type is a rigid collision. The machine tool does not deform, and only the robot actively softens and deforms to adapt to the rigidity of the machine tool. This way, neither side is damaged. For this rigid collision type, the flexible motion strategy described in this method is used. The control mode of the robot's collision joint axis motor and any one or a combination of related joint axis motors is switched from position control to torque control. The remaining joint axis motors remain in position control mode and perform an emergency stop. For example, in a six-axis industrial robot, when a collision occurs, the sixth axis is the end joint axis motor. The control mode of this motor immediately switches from position control to torque control, with a torque set value of 0. The motors of the other 1-5 axes remain in position control mode and perform an emergency stop. During the emergency stop, the motors of the 1-5 axes move forward a small distance, such as 1-2 mm. At this time, the 6th axis is already in torque mode and becomes very flexible, able to passively adapt to the squeeze state and passively adjust its angle to alleviate the squeeze between the 6th axis end tooling and the collided object, thereby protecting both the robot and the collided object. In this control strategy, the axes whose control modes are switched are not limited to 6 axes; they can be any one or a combination of axes 1, 4, 5, or 6. Axes 2 and 3 do not switch modes. After completing the mode switch and emergency stop, a preset mode recovery time is set. When the mode recovery time is reached, the control mode of the robot joint axis motor is controlled to revert from torque control to position control, and the loop is closed at the current position to ensure the subsequent stability of the robot.

[0057] This method sets three collision strategies, which can adapt to different collision objects and quickly release the extrusion force when a collision occurs, reducing the damage caused by collision and extrusion, and effectively protecting the safety of equipment or personnel.

[0058] The specific embodiments described herein are merely illustrative of the spirit of the present invention. Persons skilled in the art may make various modifications, additions, or substitutions to the described specific embodiments without departing from the spirit of the present invention or exceeding the scope of the appended claims.

Claims

1. A collision protection method for an industrial robot, characterized in that: The industrial robot collision protection method comprises the following steps: S1. Pre-calibrate the torque collision threshold of each joint of the robot. The operation includes: Control the robot to enter threshold measurement mode; Specify a work path, control the robot to run according to the specified work path, and continuously record the position data, speed data and torque data of each joint of the robot; The position data, velocity data and torque data of each joint are input into the momentum observer model to calculate the external torque of each joint at each moment. By comparing the external torque of the same joint at each moment, the maximum external torque of each joint is obtained and calibrated as the torque collision threshold of the corresponding joint, which is recorded as , i represents a joint of the robot; S2, the robot control system monitors the robot's motion process and obtains the position data, speed data and torque data of each joint of the robot; S3, based on the momentum observer model, the position data, velocity data and torque data of each joint, obtain the real-time external torque of each joint, compare the real-time external torque with the torque collision threshold of the corresponding joint based on the collision judgment formula, and judge whether a collision occurs according to the comparison result; the collision judgment formula is to compare the real-time external torque with the torque collision threshold For comparison, ≥1, is the sensitivity coefficient, when the real-time external torque is greater than the torque collision threshold When the real-time external torque is less than or equal to the torque collision threshold , it means no collision occurs; S4. When a collision occurs, control is performed based on the corresponding collision strategy obtained based on the collision type. The collision strategy includes an emergency stop control strategy for controlling each joint axis motor to perform an emergency stop operation and then generate a reverse motion when a collision occurs, and a flexible motion strategy for controlling the corresponding joint axis motor to change from position control to torque control when a collision occurs. The collision types include flexible collision and rigid collision. When a flexible collision occurs, the emergency stop control strategy is adopted for control, and when a rigid collision occurs, the flexible motion strategy is adopted for control.

2. The industrial robot collision protection method according to claim 1, characterized in that: In step S1, the operation of pre-calibrating the torque collision threshold of each joint of the robot further includes: Divide the specified working path into segments, then control the robot to run each working path, and continuously record the position data, speed data and torque data of each joint of the robot in each working path; The position data, velocity data and torque data of each joint are input into the momentum observer model to calculate the external torque of each joint at each moment in each working path. By comparing the external torque of the same joint at each moment in the same working path, the maximum external torque of each joint in each working path is obtained and calibrated as the torque collision threshold of a joint in a certain working path, which is recorded as , i represents a joint of the robot, and j represents a working path.

3. The industrial robot collision protection method according to claim 2, characterized in that: In the step S1, the torque collision threshold of each joint is also updated, which includes: Set the torque collision threshold update period; The robot operates normally according to the specified working path, and continuously records the position data, velocity data and torque data of each joint of the robot in each cycle; the position data, velocity data and torque data of each joint are input into the momentum observer model to obtain the maximum external torque of each joint. When the maximum external torque of the joint in the current cycle is greater than the absolute value of the maximum external torque saved in the previous cycle, the current maximum external torque is updated to the torque collision threshold; otherwise, the torque collision threshold remains unchanged.

4. The industrial robot collision protection method according to claim 1 or 2, characterized in that: In step S3, the operation of obtaining the real-time external torque of each joint includes: The momentum observer model is established, and the formula is: ； in: is the real-time external torque of the joint, is the diagonal gain matrix of the observer, p(t) is the generalized momentum of the robot at time t, p(0) is the generalized momentum of the robot at the initial time, Represents joint position and velocity, is the joint torque, and the matrix is ​​defined as , represents the gravity matrix, represents the transpose of the centripetal and Coriolis force matrices, yes Observed values ​​of The joint position data, velocity data and torque data are input into the formula of the momentum observer model to obtain the real-time external torque of the joint.

5. The industrial robot collision protection method according to claim 1, characterized in that: In step S4, the emergency stop control strategy is divided into a control strategy 1 for reverse movement to a starting point and a control strategy 2 for reverse movement to a designated position according to the distance of the reverse movement.

6. The industrial robot collision protection method according to claim 5, characterized in that: In step S4, the control strategy 1 includes: When a collision is detected, the motors of each joint axis are controlled to perform an emergency stop operation; Set the stop time when the robot comes to a complete stop; When the stop time is reached, a reverse motion control instruction is generated to control the robot's joints to return to the starting point of the motion.

7. The industrial robot collision protection method according to claim 1 or 5, characterized in that: In step S4, the second control strategy includes: When a collision is detected, the motors of each joint axis are controlled to perform an emergency stop operation; Set the stop time when the robot comes to a complete stop, and preset the retraction distance and speed; When the pre-stop time is reached, a reverse motion control instruction is generated to control the robot's joints to move at a preset speed and move in the reverse direction to the preset retraction distance value according to the straight line path formed by the collision point and the stop point.

8. The industrial robot collision protection method according to claim 1, characterized in that: In step S4, the operation of the flexible motion strategy includes: When a collision occurs, the control mode of the robot's collision joint axis motor and any one or a combination of related joint axis motors changes from position control to torque control; the remaining joint axis motors continue to operate in position control mode and perform an emergency stop operation; The mode recovery time is preset. When the mode recovery time is reached, the control mode of the robot joint axis motor is restored from torque control to position control.

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