Rigidity refers to the ability of a material or structure to resist elastic deformation under force, reflecting the ease of elastic deformation of a material or structure. The stiffness of a material is usually measured by the elastic modulus E. Stiffness is the proportional coefficient of the load and displacement of each component in the macroelastic range, that is, the force required to cause the displacement of the element. Its reciprocal is called compliance, which is the displacement per unit force. Rigidity can be divided into static stiffness and dynamic stiffness.

The stiffness (k) of a building refers to the ability of an elastomer to resist deformation and stretching.

k=P/δ

refers to the constant force acting on the structure, while delta is the deformation caused by the force.

The rotational stiffness (k) of the rotating mechanism is:

k=M/θ

Here, M is the applied torque and θ is the rotation angle.

For example, we know that the steel pipe is hard and generally deformed by external force, while the rubber band is softer, and the deformation caused by the equal force is larger, then we say that the rigidity of the steel pipe is strong, the rigidity of the rubber band is weak, or the rubber band is weak. strong rigidity.

For the application of servo motors, couplings are usually used to connect the motor and the load. This connection method usually adopts a rigid connection, while the synchronous belt or belt connecting the motor or load is a typical flexible connection.

The rigid body of the motor refers to the ability of the motor shaft to be disturbed by external torque, and the rigidity of the motor can be adjusted by the servo controller.

The mechanical stiffness of the follower motor is related to its response speed. Usually, the larger the rigid body, the higher its response speed, but if it is adjusted too high, it will cause mechanical resonance of the motor. Therefore, the general servo power amplifier parameters have the option of manually adjusting the response frequency. It needs to be adjusted according to the resonance point of the machine, which requires time and experience (in fact, adjusting the gain parameters).

When the servo system is in positioning mode, a force is applied to deflect the motor. If the torque is large and the deflection angle is small, the rigidity of the servo system is considered to be strong; otherwise, the rigidity of the servo is considered to be weak. Note that when I say rigidity, it's actually closer to the concept of responsiveness. In fact, the rigid body is a parameter on the controller, that is, the parameter composed of the velocity loop, the position loop and the time integral constant, and its size determines a response speed of the machine.

For example, Panasonic and Mitsubishi servos have automatic gain function, and generally do not need special adjustment. Some domestic servos can only be adjusted manually.

In fact, if you do not require fast positioning, as long as it is accurate, the resistance is small and the rigidity is low, and the positioning can be accurate, but the positioning time is long. Due to low rigidity and slow positioning speed, the illusion of inaccurate positioning is easy to occur when fast response speed and short positioning time are required.

Inertia describes the inertia of an object's motion, while rotational inertia is a measure of the rotational inertia of an object's axis of rotation. The rotational moment of inertia depends only on the radius of rotation and the mass of the object. Ordinary load inertia exceeds 10 times of motor rotor inertia, and can be regarded as large inertia.

In the servo motor drive system, the rotational inertia of the guide rail and the screw has a great influence on its stiffness. Under a certain gain, the greater the torque, the easier it is to cause the motor to shake; the smaller the rotational inertia, the easier the motor shakes. occur. Smaller diameter guide rails and lead screws can be replaced to reduce the moment of inertia, thereby reducing the inertia of the load, so that the motor does not vibrate.

Generally, in the selection of the servo system, in addition to considering the parameters such as the torque and rated speed of the motor, it is also necessary to first calculate the torque of the mechanical system and convert it to the inertia of the motor shaft, and then according to the actual action requirements of the machine tool and the quality of the workpiece. It is required to specifically select the inertia size of the motor.

Debugging manually and setting the inertia ratio parameters correctly is the premise to give full play to the best performance of the mechanical and servo system.

So what exactly is "habit matching"?

In fact, it is not difficult to understand, according to the second law of cattle:

The torque required by the feeding system = system moment of inertia J × angular acceleration θ.

The angular acceleration θ has an impact on the dynamic characteristics of the system. The smaller the θ, the longer the time required for the controller to send an instruction to the system to complete the operation, and the slower the system response. If θ changes, the system response will suddenly become faster or slower, affecting the machining accuracy.

After the servo motor is selected, the maximum output value of z remains unchanged. If the change of θ is expected to be small, then J should be as small as possible.

Above, the system moment of inertia J = the rotational inertia moment of the servo motor JM + the load inertia moment JL converted by the motor shaft.

The load inertia moment JL is converted into the inertia on the motor shaft from the inertia of the worktable, the upper clamping, the workpiece, the screw, the coupling and other linear and rotating moving parts. JM is the rotor inertia of the servo motor. When the servo motor is selected, this value is a fixed value, and JL changes with the change of the load such as the workpiece. If you want the rate of change of J to be smaller, then reduce the scale of JL.

It is widely known as "inertia matching".

Generally speaking, the small inertia motor has good braking performance, quick start and acceleration response, and good fast reciprocation, which is suitable for some light-load, high-speed positioning occasions. Among them, large inertia motors are suitable for occasions with large loads and high stability requirements, such as some circular motion mechanisms and some machine tool industries.

Therefore, the rigidity of the servo motor is too large and the rigidity is not enough. Usually, it is necessary to adjust the controller gain to change the system response. The inertia moment is too large and the inertia is insufficient, which refers to the relative comparison between the inertia change of the load and the inertia of the servo motor.