Introduction:What is position control, position control is a more common control method in the field of industrial control, usually used in the position movement control of mechanical equipment, such as machine tools and robotic arms, etc., in many high-end mechanical equipment, need to use position control. In this article, we will take the robot arm position control to carry out the introduction of the relevant content.
A robotic arm is a common type of manufacturing robot that resembles a human arm, with several parts that are very similar to the human shoulder, elbow, and wrist, and can be guided to perform various operations just like a human arm. Since the human shoulder is fixed to the body, the "shoulder" of the arm is mounted on a stationary base, while the rest of the arm can be moved in different directions.
The position control of the arm is to ensure that the arm reaches the desired position without deviation, and is an important function of the arm. The joints of a robotic arm are usually driven by motors, and the operator can control the arm by controlling the motors in the joints through a computer. Since the motors move in controlled increments, it is possible to make the robotic arm move repetitively in a very accurate way, with a high degree of accuracy and reliability. In the next section, we will briefly introduce the principle of robotic arm position control, and the method of robotic arm position control.
Principle of robotic arm position control
In order to control the position of the robotic arm, we need to use the motor to output electromagnetic torque at any time to stabilize the gravitational moment of the robotic arm. When the control arm reaches the specified position, the motor speed is zero and is in the blocking state, and the output torque is the blocking torque. Due to the different armature voltages, the blocking torque of the motor is also different, and the control of the blocking torque of the motor can be completely realized by changing the armature voltage, thus realizing the position control of the robotic arm.
Signal flow of mechanical arm position control. First of all, the input of the given value to the system needs to be completed through the button of the human-computer interaction part, which includes the two information contents of the specified position and rotation direction, and then the DSP calculates the feedforward control amount that should be added to the motor according to the specified position input, and inputs the PWM signal wave and direction signal to the motor to drive the circuit. At the same time, the code disk detects the current angle and feeds back to the DSP, which then carries out PID control according to the difference between the current position and the specified position, and adjusts the output PWM signal wave and direction signal. In addition, the liquid crystal display in the human-computer interaction part is responsible for real-time display of the information in the control process.
Mechanical arm position control methods
1,Feedforward plus three-loop control
This position control method is based on the three-loop feedback control, adding a feed-forward command value. The driver usually needs to run in CSP mode, the feedforward value generation method has two kinds, one is the controller still only sends the command position to the robotic arm, the driver generates the feedforward speed value and feedforward acceleration value through the difference; the second is the controller not only sends the command position, but also sends the feedforward speed bias value and feedforward torque bias value to the driver. This method allows the robotic arm to have dynamic response capability.
2, three-loop feedback servo control
This position control method is a familiar one. At this time, the control is all done in the driver, which generates torque according to the command position, so that the robot arm is well tracked to the command position, and the controller is just a trajectory planner responsible for sending the command position to the driver. This method has high accuracy in the steady state, and strong anti-interference ability, which can ensure the repeated positioning accuracy of the robotic arm, but its dynamic performance is relatively poor, for example, the time delay between the commanded position curve and the actual position curve is large.
3, non-linear control
The first two methods of the drive are operating under position control, while this method is located in the drive torque or current mode, applying some of the modern control theory of nonlinear control methods. The controller calculates the torque value directly according to the commanded position and sends it to the drive, which weakens the drive into a power amplifier module, while the controller can only be considered to be doing motion control. This can be used in the control method of calculated torque method, feedback linearization and some adaptive control methods.
Browsing the above article, you can learn that the position control of the robotic arm can be realized by feed-forward plus three-loop control, three-loop feedback servo control, and nonlinear control methods. The position control of the robotic arm is to ensure that the robotic arm can reach the desired position without deviation, but also the robotic arm is more commonly used as a control method, the joints of the robotic arm are usually driven by the motor, the operator can control the robotic arm through the computer control of the joints of the motor to control the robotic arm, in order to achieve a higher accuracy of the movement of the displacement. Follow Jiezhong Robotics to learn more about the knowledge and application of robotic arm position control.