AC servo drives are now widely used, especially in applications requiring high precision and large torque at low speeds, such as the injection molding industry. They have proven to be highly effective, significantly outperforming conventional frequency converters. They are less prone to overload and exhibit superior dynamic characteristics. We would appreciate your insights on their application and maintenance.
I only occasionally dabble in servo motors, and I hope some industry professionals can share their insights on installation, debugging, application, and maintenance. Let me start by offering some preliminary thoughts: 1. In terms of its main circuit structure, it's completely identical to a frequency converter. 2. It operates in a speed closed-loop system, ensuring its speed accuracy. 3. In terms of control, both the software and hardware are improvements over frequency converters.
Difference between AC servo drives and frequency converters
Servo applications and motion control are extremely convenient, and the positioning accuracy is very high. I have been using servo systems for a long time.
The main circuit principle of this servo is very similar to that of a frequency converter, almost identical, except for significant differences in control. AC servos will become increasingly popular, given their price advantage.
Repairing Japanese brands like Panasonic, Fuji, and Mitsubishi is becoming increasingly difficult. Challenge 1: Testing requires a servo motor, and even within the same brand, different encoder models and interfaces may differ. Challenge 2: Problems are site-specific; for example, some machines fail to alarm and remain stationary, or their running distance deviates from the programmed parameters. Challenge 3: Imported machines often use multi-layer boards, making troubleshooting difficult.
The servo I'm referring to is a high-power AC servo, compatible with frequency converters. Setting the parameters to V/F mode puts it in open-loop operation, the same as a frequency converter. This servo is suitable for permanent magnet synchronous motors and ordinary AC motors. Encoder alarms do not need to be disabled. Regarding encoder feedback signals, I previously considered using a microcontroller to create a "simulated" feedback pulse. Alternatively, a miniature speed-regulating motor could drive the encoder to generate feedback pulses, enabling the servo to enter operating mode. This would create maintenance conditions, but I'm unsure if it's feasible. I'd like to hear your opinions.
I frequently work with servo motors, and one advantage of servo repair is that it rarely results in module failure. Repair costs are low, but the price is high, and the technical skill required is somewhat advanced. While the drive board and inverter are similar, the mainboards differ significantly.
In general applications, it's similar to a frequency converter, but there are differences when used for precision machining, such as rigid tapping. It also offers the advantages of servo motors, such as preventing the motor from overheating.
Frequency converters are mainly used for speed control, while servos are mainly used for position control, although they can also control speed. Although the main circuit principle is the same, servos have an additional position loop control.
In my opinion, the detection circuitry on a servo drive is more complex than that on a frequency converter. Also, many of the components are quite small, making inspection, replacement, and soldering a significant challenge for most repair technicians. The main circuitry and switching power supply are almost identical to those of a frequency converter. I'm a newcomer to this field and have handled quite a few servo and frequency converters, but I'm still completely baffled by the various detection circuits of servo drives. I hope to exchange ideas and learn from experienced technicians in the future.
I've repaired most vector inverters, including Yaskawa A1000 with PG card, Lenze EVS93, and Siemens G120. To do this, you need to save the user parameter set, then initialize the parameters to V/f mode, and turn off the feedback system. Then you can drive a regular motor. The G120 has a manual start function for switching.
Finding a suitable motor to test with a servo drive is very difficult, especially since some servos don't even have an operator panel. Even if there is a panel, finding an encoder with the same output method and resolution is extremely challenging. Even if the encoder issue is resolved, finding a suitable motor remains a challenge. The servo drives that come in for repair often have different power ratings and voltage levels, unlike frequency converters which can be driven by a small motor.
Because servo drives have built-in speed and current loop control algorithms, which cannot be turned off. The current loop, in particular, is a closed-loop current regulation control; the current magnitude is determined by the internal algorithm, not by the size of the motor it drives!
Based on the previously mentioned issue of not being able to debug without a servo motor, most domestically produced servos currently use Tamagawa or Nemitong encoders. Servo motors are universal, and buying a small-power servo motor is not expensive. The drive and motor are an integrated unit. Personally, I think it's more effective to test the effect by repairing the drive separately and then testing it in conjunction with the motor. As for foreign encoders, they are all in communication mode, which is indeed difficult to simulate. An AC servo drive is simply a vector inverter driving a high-performance permanent magnet motor. Its high-speed response and precise positioning are the main features that distinguish it from inverters. However, the performance of domestic servos is currently very average, and there is serious plagiarism among various manufacturers. They are basically modified versions of the Huake faction. Currently, some software is copied from Delta, and the control performance is not impressive. At present, no domestic servo manufacturer can compare with foreign brands like Lan Hai Huateng in the inverter field. Before they have developed good technology products, many brands have joined the price war.
Currently, 17-bit encoders are the mainstream in the market, and traditional 2500-line motors are likely to be phased out. Furthermore, 17-bit encoders no longer output pulse signals but rather communication signals, which is difficult to simulate for servo drives.
The development of servo drives is always related to the development of servo motors. Early high-torque servos were unthinkable due to their exorbitant prices. Servo systems were already present in practical DC speed control, with motors designed to be slender to reduce inertia. The drives were zero-steady-state-error systems, and the circuitry wasn't significantly different. While they offered better performance than modern digital servos, their functionality was less robust. Many modern servos now include a position loop and are primarily used for high-precision position control. The biggest difference between servos and frequency converters lies in their speed range. Frequency converters typically have a speed range of less than 100 rpm, with vector speeds of 100-200 rpm being quite good. Servos, on the other hand, generally exceed 4000 rpm, with some older DC servos exceeding 10000 rpm. They could output 2 to 2.5 times the rated torque at 0 Hz, a capability that frequency converters cannot achieve. Now, high-power servo motors are affordable, and the application of higher-power servos is no longer uncommon.
