1. Why do I need force control?

1.1. Force control is the core of robot motion control

A16B-2202-0850 The purpose of robot motion control is to enable robots to complete complex action tasks and autonomous decision-making according to human instructions. The main purpose of designing and producing robots is to enable robots to perform specific tasks instead of humans, so it can be said that motion control is the core of robot research. The essence of robot motion control is to control the servo motor, and the servo system generally has three control methods, namely torque control, position control and speed control.A16B-2202-0850 Torque control: means that the servo driver only controls the torque of the motor, and the principle is to control the motor torque through the current loop. The current ring is in the innermost part, which is completely carried out inside the servo driver. Through hall installation, the output current of each phase of the driver to the motor is detected, and the PID adjustment is made by negative feedback to the setting of the current, so as to achieve the output current as close as possible to the set current.

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A16B-2202-0850 Speed control: It means that the driver only controls the speed and torque of the motor, and the principle is to control the position through the current ring and the speed ring. The PID output in the loop is directly the setting of the current loop, so the speed loop control includes the speed loop and the current loop.

Position control: It means that the driver controls the speed, Angle and torque of the motor, and the principle is to control the position through the current ring, the speed ring and the position ring. The position ring is the outermost ring and can be built between the drive and the servo motor coder or between the external controller and the motor encoder or the final load, depending on the actual situation. Because the internal output of the position control ring is the setting of the speed ring, the system performs the operation of all three rings in the position control mode, and the system has the largest amount of calculation and the slowest dynamic response speed.

A16B-2202-0850 Torque control plays an increasingly important role in robot motion control. Of the three control methods, traditional robots generally use position control, such as a robot arm moving along a pre-planned trajectory in a closed, confirmed space. However, with the diversification of robot application scenarios, the importance of torque control becomes more and more obvious. Flexible control can help the robot adapt to more scenarios. Generally speaking, position and speed control can be considered as “hard constraints”, that is, the robot can only operate in a set interval, while torque control is “soft constraints” or “flexible control”, and the robot can constantly adjust the size of the force according to the needs of the scene. For example, when a robot walks on an irregular and unknown road surface, it cannot be modeled and predicted in advance due to the unknown road surface condition, or it cannot be modeled accurately due to too many irregular road surface states. In this case, if pure position control is used, a suitable position trajectory cannot be planned. In this case, force control must be introduced to achieve real-time dynamic control.