JB/T10184-2014 General Technical Conditions for AC Servo Drives specifies the test method for the bandwidth of servo drives:
The driver inputs a sinusoidal speed command with an amplitude of 0.01 times the rated speed command value . The frequency gradually increases from 1Hz, and the corresponding motor speed curve is recorded. As the frequency of the command sinusoidal wave increases, the phase lag of the motor speed waveform relative to the command sinusoidal wave gradually increases, while the amplitude gradually decreases. The frequency at which the phase lag increases to 90° is taken as the bandwidth of the 90° phase shift; the frequency at which the amplitude decreases to 1/√2 times is taken as the -3dB bandwidth, whichever condition is reached first.
The following section provides an explanation of the test method. For ease of description, this standard will be referred to as the "General Technical Conditions".
Among the control signal terminals of a servo drive, there are 1 to 2 analog voltage signal command input terminals for external input of speed or torque commands, commonly referred to as the "AD port of the servo drive". Generally, the speed command is a sine wave signal with an amplitude of ±10V, as shown in Figure 1.
Figure 1. Analog control signal input for servo driver
If the servo driver does not set a threshold (voltage dead zone) for the input command, ideally +10V corresponds to the rated speed of the motor in forward rotation, and -10V corresponds to the rated speed of the motor in reverse rotation. The motor speed changes linearly with the change in the amplitude of the command voltage, as shown in Figure 2.
Figure 2 Relationship between motor speed and input signal amplitude
The General Technical Conditions state: "The driver inputs a sinusoidal speed command, the amplitude of which is 0.01 times the rated speed command value."
Assuming the speed command amplitude is ±10V and the rated speed of the motor is 6000RPM, that is, when a sinusoidal voltage signal with an input amplitude of 0.1V is applied, the speed of the servo motor is 60RPM.
The "General Technical Conditions" state: "As the frequency gradually increases from 1 Hz, record the corresponding speed curve of the motor."
The frequency of the sine wave gradually increases from 1 Hz, as shown in Figure 3.
Figure 3 Sine wave speed command
Under the control of this command, the servo motor performs an action with four movements in one cycle: forward acceleration, forward deceleration, reverse acceleration, and reverse deceleration. When the sine wave command signal is at a certain frequency, the maximum speed of the servo motor is 60 RPM. As the frequency of the command sine wave increases, the phase lag of the motor speed waveform curve relative to the command sine wave curve gradually increases, while the amplitude gradually decreases.
The General Technical Conditions state: "The frequency at which the phase lag increases to 90° shall be taken as the bandwidth of the 90° phase shift; the frequency at which the amplitude decreases to 1/√2 times shall be taken as the -3dB bandwidth, whichever condition is met first shall prevail."
Taking the amplitude-frequency curve as an example, as shown in Figure 4. Here, the frequency f is the bandwidth of the servo driver.
