Abstract: This paper introduces the basic criteria and setting method of gain adjustment for digital AC servo systems . The relationship between position loop gain and speed loop gain is described. The countermeasures for different rigid connections and different load inertia are also analyzed. Key words : gain; inertia; integration time constant Due to their excellent performance, all -digital AC servo systems are being used more and more widely in today's industry. However, it is not easy to adjust the AC servo and the machine to the optimal state. The most important and most difficult thing to adjust is the gain of the position loop and speed loop. The following mainly analyzes the adjustment of gain in position control. 1. Block Diagram of a Fully Digital AC Servo System As shown in the attached diagram, the servo system contains three closed-loop feedback loops: position loop, speed loop, and current loop. Users cannot adjust the current loop parameters, as the design ensures sufficient gain response. Users need to adjust parameters such as the position loop gain, speed loop gain, and speed loop integral time constant according to the machine's rigidity and load conditions. The position and speed loops must be adjusted simultaneously to achieve a balanced response. If only the position loop gain is increased, the speed reference value will fluctuate, resulting in longer positioning time and oscillations. Therefore, the parameters of the position and speed loops are mutually influential and mutually restrictive. Adjusting the servo performance according to the overall machine performance and load conditions is key to effectively using a fully digital AC servo system. 2. Basic Principles of Gain Adjustment a) Position Loop Gain K[sub]p[/sub]. The position loop gain mainly affects the servo system's response. A larger set value results in faster dynamic response, smaller tracking error, and shorter positioning time; however, excessively large values ​​may cause vibration. Therefore, under the premise of overall machine stability, a larger value should be set as much as possible. b) Speed ​​loop gain kv. This parameter determines the responsiveness of the speed loop. Set it to the largest possible value within the range where the mechanical system will not vibrate. Furthermore, the speed loop gain kv is closely related to the load inertia. Generally, the larger the load inertia, the larger kv should be. c) Speed ​​loop integration time constant Tt. Set it to the smallest possible value within the allowable range. 3. Analysis of Loop Gain Adjustment 3.1 Adjustment of Position Loop Gain Kp The position loop gain Kp is related to the overall mechanical rigidity. For high-rigidity connections, the position loop gain Kp can be set larger, but should not exceed the natural frequency of the mechanical system, resulting in a higher dynamic response. For medium-rigidity and low-rigidity connections, the Kp setting should not be too high; otherwise, oscillations will occur. Generally, mechanical rigidity can be classified as follows: a) Direct connection between the motor and ball screw. The ball screw is relatively short, which can be considered a high-rigidity connection, such as in precision machine tools and chip insertion machines, where the natural frequency of the mechanical system can typically reach 70Hz. In this case, the maximum position loop gain can be set to 70 (1ts). b) Gear or synchronous belt coupling results in a medium-rigidity connection; rack, chain, or harmonic gear reducer transmission results in a low-rigidity connection. Robot structures with gear transmissions have poor rigidity, with a natural frequency between approximately 5 and 30Hz, and their position loop gain can typically only be set between 10 and 30 (1ts). To increase mechanical rigidity, the motor load must be firmly fixed to a rigid foundation, and the coupling between the motor shaft and the load must be highly rigid. If a synchronous belt drive is used, the synchronous belt must be wide, and the clearance between the coupling gears must be minimized. 3.2 Speed ​​Loop Gain kv Adjustment When selecting AC servo motors, users often only consider the motor's power and torque, neglecting the crucial parameter of load inertia. Typically, fully digital AC servo motors are categorized into large, medium, and small inertia types to meet different user needs. Generally, a fully digital AC servo motor can operate normally when the load inertia referred to the motor shaft is within 30 times the motor's inertia. If the load inertia is too large, the performance of the transmission system will deteriorate. Following the principle that the speed loop gain should be as large as possible within the allowable range, in high-rigidity machinery such as precision machine tools, as the ratio of load inertia to motor inertia increases, the speed loop gain setting should be increased to ensure a high response of the entire system. However, when the load inertia ratio is >10, the increase in position loop gain Kp and speed loop gain should not be too large, and the speed loop integral time constant n needs to be increased to ensure the stability of the mechanical system. For medium-rigidity and low-rigidity machinery, at the same load inertia ratio, the value should be appropriately reduced, while the value of the velocity loop integral time constant should be increased. 3.3 Velocity Loop Integral Time Constant T[sub]i[/sub] The main function of the velocity loop integral element is to enable the system to respond to small inputs. Due to the delay effect of this integral element, increasing the integral time constant T[sub]i[/sub] will increase the positioning time and slow down the response. Therefore, the value of T[sub]i[/sub] should be minimized as much as possible. However, if the load inertia is very large or the mechanical system has poor rigidity, the velocity loop integral time constant T[sub]i[/sub] must be increased to prevent vibration. 4. Gain Setting Method for Position Control Due to the interrelationships between various parameters and the uncertainty of overall machine rigidity and load inertia, gain adjustment can often feel daunting. The following steps are generally recommended: a) Initially set the position loop gain K_p to a low value. Then, without producing abnormal noise or vibration, gradually increase the speed loop gain to its maximum value. Of these three parameters, only K_v has the closest relationship with load inertia. When adjusting K_v, refer to the structure of the transmission chain and the magnitude of the load inertia to predetermine the setting range. b) Gradually decrease the K_v value and increase the position loop gain K_p value. Set the K_p value to its maximum value, provided there is no overshoot or vibration in the entire response. c) The speed loop integral time constant T_i depends on the length of the positioning time. Minimize this setting value as much as possible, provided the mechanical system does not vibrate. d) After obtaining the single-step response, fine-tune the position loop gain K[sub]P[/sub], the speed loop gain island, and the integral time constant T[sub]i[/sub] to find the optimal matching point. When adjusting the performance of high-rigidity mechanical transmissions, a feedforward gain can be set to further improve dynamic response speed and reduce following error. However, if this value is too large, speed overshoot and vibration will occur. Currently, all-digital AC servo systems use automatic adjustment to set the gain. However, in many situations, such as those with poor rigidity, large clearances, or excessively long strokes, automatic adjustment cannot be completed. In such cases, it is necessary to understand the interrelationships and adjustment methods of the gains in each loop before the machine can be debugged. Analysis of Gain Adjustment in All-Digital AC Servo Systems: PDF