If you want to talk about rigidity, let's talk about rigidity first.
Stiffness refers to the ability of a material or structure to resist elastic deformation when it is stressed, and it is a characterization of the difficulty of elastic deformation of a material or structure.
The stiffness of a material is usually measured by the modulus of elasticity E. In the macro-elastic range, stiffness is a proportionality factor proportional to the part load and displacement, that is, the force required to cause a unit displacement. Its reciprocal is called compliance, which is the displacement due to unit force.
Stiffness can be divided into static stiffness and dynamic stiffness .
The stiffness (k) of a structure refers to the ability of the elastic body to resist deformation and stretching: k=P/δ
(P is the constant force acting on the structure, δ is the deformation due to the force)
The rotational stiffness (k) of the rotating structure is: k=M/θ
(M is the applied moment, θ is the rotation angle)
For example, we know that steel pipes are relatively hard and generally deform less when subjected to external forces, while rubber bands are softer and deform more when subjected to the same force. Then we say that steel pipes have strong rigidity and rubber bands have weak rigidity, or other Flexible and strong.
In the application of the servo motor, the coupling is used to connect the motor and the load, which is a typical rigid connection;
The use of synchronous belts or belts to connect the motor and the load is a typical flexible connection.
The rigidity of the motor is the ability of the motor shaft to resist external torque interference, and we can adjust the rigidity of the motor in the servo controller.
The mechanical stiffness of a servo motor is related to its response speed . Generally, the higher the rigidity, the higher the response speed, but if the adjustment is too high, it is easy to cause mechanical resonance in the motor. Therefore, in the general servo amplifier parameters, there is an option to manually adjust the response frequency. It needs time and experience to adjust according to the mechanical resonance point (in fact, it is to adjust the gain parameter).
In the position mode of the servo system, a force is applied to deflect the motor. If the force is large and the deflection angle is small, the servo system is considered to be rigid, otherwise the servo is considered to be weak.
In fact, if you do not require fast positioning, as long as it is accurate, when the resistance is not large, the rigidity is low, and accurate positioning can also be achieved, but the positioning time is long. Because the positioning is slow if the rigidity is low, there will be an illusion of inaccurate positioning when the response is required to be fast and the positioning time is short.
While inertia describes the inertia of an object's motion, moment of inertia is a measure of the object's rotational inertia about an axis. The moment of inertia is only related to the radius of rotation and the mass of the object. Generally, the load inertia is more than 10 times of the motor rotor inertia, so it can be considered that the inertia is relatively large.
The moment of inertia of the guide rail and lead screw has a great influence on the rigidity of the servo motor transmission system. Under a fixed gain, the larger the moment of inertia and the greater the rigidity, the easier it is to cause the motor to shake; the smaller the moment of inertia and the smaller the rigidity, the less likely the motor is to shake . The motor does not shake by replacing the guide rail and screw with a smaller diameter to reduce the moment of inertia and thereby reduce the load inertia.
So what exactly is "inertia matching"?
In fact, it is not difficult to understand, according to Niu's second law:
Torque required by the feed system = system moment of inertia J × angular acceleration θ
Angular acceleration θ affects the dynamic characteristics of the system. The smaller θ is, the longer it takes from the controller to issue instructions to the completion of the system execution, and the slower the system response. If θ changes, the system response will be fast and slow, which will affect the machining accuracy.
After the servo motor is selected, the maximum output value remains unchanged. If the change of θ is expected to be small, then J should be as small as possible.
In the above, the system moment of inertia J = the moment of inertia JM of the rotation of the servo motor + the moment of inertia JL of the load converted from the motor shaft.
The load inertia JL is composed of the inertia of the linear and rotary moving parts such as the workbench and the fixtures and workpieces, screws, couplings, etc., converted to the inertia of the motor shaft. JM is the rotor inertia of the servo motor. After the servo motor is selected, this value is a fixed value, while JL changes with the change of the load such as the workpiece. If the change rate of J is expected to be smaller, it is better to make the proportion of JL smaller.
This is " inertia matching " in the popular sense .
Generally speaking, the motor with small inertia has good braking performance, fast response to start, acceleration and stop, good high-speed reciprocation, and is suitable for some occasions with light load and high-speed positioning. Medium and 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. Generally, it is necessary to adjust the gain of the controller to change the system response. The inertia is too large and the inertia is insufficient. It refers to a relative comparison between the inertia change of the load and the inertia of the servo motor.