When discussing the control loop of a servo drive , we know it typically includes three types: position loop, speed loop, and current loop. Although the purpose of each loop is to control the motor's output performance in several different aspects, they all share a common parameter: bandwidth.
For servo drives, the control loop bandwidth is defined as the frequency at which the closed-loop response amplitude is -3dB. At this frequency, its output proportional gain, which is the ratio of output to input, is approximately 70.7% (1/√2) of its maximum value; its output power, i.e., the power delivered to the load, is half of the input power. (We will further discuss the relationship between response amplitude, output gain, output power, and input power in future posts.)
The bandwidth of a servo system measures its response speed to changing input commands and determines how quickly it can respond to changes in position, speed, or torque parameters (such as feedback, error, etc.).
Servo drive control loops typically employ a multi-loop structure, nested layer by layer. The innermost loop is the current loop, nested within the middle speed loop, which in turn is nested within the outermost position loop. In such a nested system, the response speed of the inner control loop must be faster than that of the outer control loop; otherwise, the inner loop's influence on the outer loop would be minimal or even nonexistent. For servo control loops, the response bandwidth of the inner control loop is generally 5 to 10 times that of the outer control loop. That is, the current loop bandwidth is roughly 5 to 10 times that of the speed loop, and the speed loop bandwidth is approximately 5 to 10 times that of the position loop.
A higher frequency response bandwidth in servo drives typically provides stiffer motor output performance, helping to reduce motion errors, improve control accuracy, and enhance dynamic response characteristics. This is a key reason why frequency response bandwidth is often used to evaluate the performance of servo drive products in practical applications. For example, the speed loop response bandwidth of several Japanese servo products now generally reaches over 3kHz.
However, it should be noted that the high bandwidth of the driver may also have some negative effects on the servo system. For example, high bandwidth can make the motor very sensitive to disturbances from the load and transmission system; also, high bandwidth will cause the motor to respond to load fluctuations at a higher frequency, which places higher demands on the system's dynamic performance (such as acceleration, torque, and force).
Considering that power consumption and force (torque) have a square relationship, increasing bandwidth will significantly increase the power consumption (i.e. heat) of the motor, thereby leading to an increase in motor temperature. Since temperature rise is a key factor in ensuring the normal operation of the motor, the motor characteristics may actually limit the actual bandwidth of the servo driver to a certain extent.
In addition, other factors in a servo system, such as the resolution of the feedback element, the refresh rate of the driver, the inertia ratio between the load and the motor, and the rigidity of the coupling between the motor and the load, will also affect its maximum achievable response bandwidth.
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