How to simplify power management solution design[Copy link]
Servo drives, also known as "servo controllers" and "servo amplifiers", are controllers used to control servo motors. Their function is similar to that of frequency converters on ordinary AC motors. They are part of servo systems and are mainly used in high-precision positioning systems. Generally, servo motors are controlled by position, speed and torque to achieve high-precision transmission system positioning. They are currently high-end products in transmission technology. Servo drives are an important part of modern motion control and are widely used in automation equipment such as industrial robots and CNC machining centers. In particular, servo drives used to control AC permanent magnet synchronous motors have become a hot topic of research at home and abroad. The current AC servo drive design generally uses a three-loop control algorithm based on vector control: current, speed and position. Whether the speed closed loop design in this algorithm is reasonable or not plays a key role in the entire servo control system, especially the speed control performance. In the servo drive speed closed loop, the real-time speed measurement accuracy of the motor rotor is crucial to improving the dynamic and static characteristics of the speed control of the speed loop. In order to seek a balance between measurement accuracy and system cost, an incremental photoelectric encoder is generally used as a speed sensor, and the corresponding commonly used speed measurement method is the M/T speed measurement method. Although the M/T speed measurement method has certain measurement accuracy and a wide measurement range, this method has its inherent defects, mainly including: 1) at least one complete encoder pulse must be detected within the speed measurement cycle, which limits the minimum measurable speed; 2) it is difficult to strictly keep the two control system timer switches used for speed measurement synchronous, and the speed measurement accuracy cannot be guaranteed in measurement occasions with large speed changes. Therefore, it is difficult to improve the speed following and control performance of the servo drive by applying the traditional speed loop design scheme of this speed measurement method. Working principle At present, the mainstream servo drives all use digital signal processors (DSP) as the control core, which can realize relatively complex control algorithms, realize digitalization, networking and intelligence. Power devices generally use drive circuits designed with intelligent power modules (IPM) as the core. The IPM integrates the drive circuit internally, and has overvoltage, overcurrent, overheating, undervoltage and other fault detection and protection circuits. A soft start circuit is also added to the main circuit to reduce the impact of the start-up process on the drive. The power drive unit first rectifies the input three-phase power or mains power through a three-phase full-bridge rectifier circuit to obtain the corresponding direct current. After the rectified three-phase electricity or mains electricity, the three-phase permanent magnet synchronous AC servo motor is driven by the three-phase sinusoidal PWM voltage inverter. The whole process of the power drive unit can be simply described as the process of AC-DC-AC. The main topological circuit of the rectifier unit (AC-DC) is the three-phase full-bridge uncontrolled rectifier circuit. With the large-scale application of servo systems, the use of servo drives, servo drive debugging, and servo drive maintenance are all important technical topics for servo drives today. More and more industrial control technology service providers have conducted in-depth technical research on servo drives. From the perspective of power supply, the industry has continued to focus on improving the performance of large-scale computing and signal processing over the past few decades. At the same time, the service demand for electronic and IT systems has grown exponentially, especially the rapid development of emerging technologies such as artificial intelligence and the Internet of Things, which has gradually penetrated the concept of intelligence into all aspects of people's lives and promoted the application of terminal products and the upgrading of electronic consumption. This trend has further triggered the industry's increasing requirements for power management. At present, the electricity consumed by Internet infrastructure alone is estimated to account for several percentage points of global power generation, and all electricity must undergo control, conversion, regulation and filtering processes before being put into use. In 1968, few engineers could have foreseen the advent of processors that, although essentially CMOS-like, operate at voltages as low as 0.9 V (+/- 2%) and can deliver tens of amps of current. This is one of the many challenges facing power system engineers. From a power perspective, whether it is a milliwatt wearable device, an energy harvesting solution, or a kilowatt-level power supply, a common goal is to improve energy efficiency. For many years, power supply designers for consumer electronics have been working to develop power solutions that minimize power consumption both at rated power output and in standby mode. Industry regulations, including the European Union's External Power Code (CoC) Version 5 and the US Department of Energy's Level 6 efficiency requirements, have set maximum "standby power" standards for millions of plug-in power supplies that are not frequently plugged in and out. To help designers simplify the design process, this article has selected a group of new power management chips and shared some important design guidelines, allowing designers to complete their first design with the help of some online design tools. Power management is a major issue facing IoT device design. While more and more applications are being able to achieve ultra-low power designs through energy harvesting technology, this approach is not suitable for many other application designs. In this case, a battery is required to power the system. The difference between the two is that products using energy harvesting technology only need to collect small amounts of energy that is easily available in the environment to operate, while battery-powered devices will need to replace the battery at some point. The battery life of an IoT device can be determined by a simple calculation: the battery capacity divided by the average discharge rate. Therefore, minimizing the device's energy consumption or increasing the battery capacity can extend the battery life and reduce the product's total cost of ownership. Typically, the battery is the largest component in an IoT sensor system, which often limits the options for engineers. However, engineers can use a variety of processors, communication technologies, and software algorithms to design the system to provide the required battery life. Considering the high cost of battery replacement, a good system design usually requires ensuring that the original battery can support the smooth operation of the IoT sensor throughout its entire life cycle. Developing battery-powered IoT devices requires careful engineering design. Although component selection is important, the benefits of low-power processors can be eliminated by improper design. To this end, the processor design needs to achieve low-power standby mode operation as much as possible and minimize the use of wireless communication to provide good battery performance. To meet this design requirement, Texas Instruments has launched new controller chips based on the new LLC architecture, UCC256301 and UCC256303, which can provide the industry's lowest standby power consumption. Texas Instruments LLC architecture adopts a hybrid hysteresis control strategy. In order to achieve higher efficiency, the industry has continuously explored and developed a series of innovative architectures and switching modes, and the Texas Instruments LLC architecture is the latest result of the industry's continuous efforts. This type of controller not only needs to have low loss characteristics, but also needs a power supply with fast transient response capabilities to meet the sudden change requirements when the load switches quickly from low power to high power state; at the same time, its voltage regulator is actually a broadband amplifier with a fixed output level, which needs to maintain stable operation at all times. With ultra-fast transient response and simple compensation combined with powerful fault protection functions, such as avoiding zero current switching, reliable operation is ensured. 1.jpg Figure 1: Transient response of the converter using Texas Instruments hybrid hysteresis control: When switching from no-load to full-load state, the converter limits the maximum deviation of the output voltage to 1.25%, and the output voltage can return to steady state within 200μs. Wearable devices are a "hot topic" recently. Whether they are used for medical or infotainment, one of their urgent needs is to achieve low power consumption and minimize losses. Power management components can meet this unique demand. Maxim Integrated's MAX20303 is an example. It is a highly integrated programmable device with a 3.71mm x 4.The 21mm small package design provides a variety of rich functions. It has a flexible set of power-optimized regulators, including multiple buck, boost, buck/boost and linear regulators, which can output up to 220 mA. The quiescent current of each regulator is specially optimized to 1μA (typical) to extend the battery life in "always-on" applications. The complete battery management solution includes chargers, power paths, fuel gauges and battery housings. Another trend closely related to products such as wearable devices is the increasing popularity of wireless charging due to consumers' demand for convenient wireless operation. Since the birth of power-related standards, the work of increasing power levels has never stopped, even if new standard specifications have just begun to be implemented. The latest QiExtended Power standard established by the Wireless Power Consortium (WPC) increases the power from 5W to 15W. To support the implementation of this new standard, STMicroelectronics has launched an advanced wireless charging transmitter chip, the STWBC-EP, which can provide the necessary foreign object detection and safety functions with the lowest standby power consumption. It contains a DC/DC boost converter and a controller, as well as Qi charging algorithm firmware. The converter and controller work together to generate input power and control signals, which are transmitted to the external half-bridge power stage to drive the charging transmitter antenna. STMicroelectronics has also launched a related evaluation kit, which includes a 15W Qi MP-A10 reference design, a 12V 2A AC/DC adapter, a USB/UART converter (for connecting a PC and a USB data cable), and pre-installed firmware. In addition, the use of USB-C continues to become more common in consumer and many other applications. With its high power transmission capability and single-point connection characteristics, the industry has launched a number of battery charger chips. Among them is the ISL95338 buck-boost regulator from Intersil, which uses a Type-C double-sided pluggable connector for various types of mobile devices, and can replace two voltage converters and provide USB PD3.0 bidirectional voltage regulation. The ISL95338 regulator can accept a wide range of DC power inputs, including AC/DC power adapters, USB PD3.0 ports, travel power adapters, power storage modules, etc., and can convert power into a stable voltage of up to 24V. The ISL95338 can convert a wide range of DC power into a 20V stable voltage at the input of the power adapter, and can operate in buck mode, boost mode, and buck-boost mode. It also uses Intersil's patented modulation technology, combining the characteristics of fixed-frequency pulse width modulation (PWM) and hysteresis PWM to achieve high-efficiency operation and provide ultra-fast transient response. Its design is fully compatible with the USB PD3.0 standard and supports programmable power supply (PPS) fast charging, providing bidirectional 5V-20V buck, boost, and buck-boost modes. Of course, not all voltage regulation is achieved through switching design. In many applications, due to extremely low noise requirements, only a linear regulator is required to achieve this function. Typical application scenarios include powering precision analog-to-digital converters or low-distortion RF amplifiers, which cannot tolerate any power supply noise. Texas Instruments' TPS7A39 meets this application requirement and provides designers with a universal component suitable for a variety of applications. The most notable feature of this 150mA regulator is that it can provide positive and negative voltage outputs for components and subsystems that require symmetrical or asymmetrical voltages on either side of a grounded digital-to-analog converter, as well as on operational/instrumentation amplifiers. The positive and negative outputs of the TPS7A39 are independently adjustable and can track each other at a constant ratio during startup. For single-supply amplifiers, its negative output can be adjusted to 0V. The TPS7A39 regulator specifies a power supply rejection ratio (PSRR) of more than 50 dB (up to 2 MHz) and 69dB (120Hz), thereby eliminating switching (and other) noise interference for linear circuits. The power supply content selected in this article only provides a preliminary introduction to the relevant information in the broad design field of "power supply". Other power supply design topics such as high-voltage integration, the latest drive technology of silicon carbide and gallium nitride power devices, and a large number of rapid prototyping rules are yet to be further explored. Designers can obtain the above manufacturers' products and other popular power application devices and related support services from element14 to achieve barrier-free design. Servo drives are an important part of modern motion control and are widely used in automation equipment such as industrial robots and CNC machining centers. In particular, servo drives used to control AC permanent magnet synchronous motors have become a hot topic of research at home and abroad. The current AC servo drive design generally adopts a three-loop control algorithm based on vector control, namely, current, speed, and position. Whether the speed closed-loop design in this algorithm is reasonable or not plays a key role in the entire servo control system, especially the speed control performance. Basic requirements Requirements for servo feed systems 1. Wide speed regulation range 2. High positioning accuracy 3. Sufficient transmission rigidity and high speed stability 4. Fast response, no overshoot In order to ensure productivity and processing quality, in addition to requiring high positioning accuracy, it also requires good fast response characteristics, that is, the response of the tracking command signal must be fast, because the CNC system requires sufficient acceleration and deceleration when starting and braking, shortening the transition process time of the feed system and reducing the contour transition error. 5. Low speed, high torque, strong overload capacity Generally speaking, the servo drive has an overload capacity of more than 1.5 times in a few minutes or even half an hour, and can be overloaded 4 to 6 times in a short time without damage. 6. High reliability The feed drive system of CNC machine tools is required to have high reliability and good working stability, strong adaptability to temperature, humidity, vibration and other environments, and strong anti-interference ability. Requirements for motors 1. The motor can run smoothly from the lowest speed to the highest speed, and the torque fluctuation should be small, especially at low speeds such as 0.1r/min or lower, there is still a stable speed without creeping. 2. The motor should have a large overload capacity for a long time to meet the requirements of low speed and high torque. Generally, DC servo motors are required to be overloaded 4 to 6 times within a few minutes without damage. 3. In order to meet the requirements of fast response, the motor should have a small moment of inertia and a large stall torque, and have the smallest possible time constant and starting voltage. 4. The motor should be able to withstand frequent starting, braking and reversing. What kind of pulse does the servo drive need? Positive and negative pulse control (CW+CCW); pulse plus direction control (pulse+direction); AB phase input (phase difference control, common in handwheel control) The servo drive main program is mainly used to complete the system initialization, LO interface control signal, and the setting of registers of each control module in the DSP. After all the initialization work of the servo drive is completed, the main program enters the waiting state and waits for the interrupt to occur in order to adjust the current loop and speed loop. The interrupt service program mainly includes four M timing interrupt programs, photoelectric encoder zero pulse capture interrupt program, power drive protection interrupt program, and communication interrupt program. Methods and techniques for setting important parameters of servo drives With the development of the market and the progress of domestic power electronics technology, microelectronics technology, computer technology, and control principles, domestic CNC systems, AC servo drives, and servo motors have made great progress in the past two years, breaking the monopoly of foreign countries in certain application fields. The author has been engaged in CNC technology for many years and has used many sets of digital servos from Japan's Yaskawa, Panasonic, Sanyo, etc., but recently, due to the good cost performance of domestic servos, some AC servo drives and motors produced by CNC technology companies have been used. Some simple and practical techniques are summarized for certain aspects of use. 1 Basic performance of KNDSD100 1.1 Basic functions SD100 adopts the internationally advanced digital signal processor (DSP) TM320 (S240), large-scale programmable gate array (FPGA), and Japan Mitsubishi's new generation of intelligent power module (1PM). It has high integration and small size. It has protection functions such as overspeed, overcurrent, overload, main power supply overvoltage and undervoltage, encoder abnormality and position deviation. Compared with stepper motors, AC servo motors have no step-out phenomenon. The servo motor has its own encoder, and the position signal is fed back to the servo drive, which together with the open-loop position controller constitutes a semi-closed-loop control system. The speed ratio is wide 1:5000, the torque is constant, and the torque of 1 r and 2000 r is basically the same. It has stable torque characteristics and fast response characteristics from low speed to high speed. It adopts full digital control, which is simple and flexible to control. Users can make appropriate settings for the working mode and operating characteristics of the servo by modifying parameters. The current price is only 2000 to 3000 yuan higher than that of stepper motors. 1.2 Parameter adjustment SD100 provides users with a wealth of user parameters from 0 to 59, alarm parameters from 1 to 32, and monitoring modes (motor speed, position deviation, etc.) of 22. Users can adjust parameters according to different on-site conditions to achieve the best control effect. The meanings of several commonly used parameters are: (1) "0" is the password parameter, the factory value is 315, and the user must change this password to 385 when changing the model. (2) "1" is the model code, corresponding to the same series of drives and motors of different power levels. (3) "4" is the control mode selection. Changing this parameter can set the control mode of the drive. Among them, "0" is the position control mode; "1" is the speed control mode; "2" is the trial run control mode; "3" is the JOG control mode; "4" is the encoder zeroing mode; "5" is the open-loop control mode (user test voltage and encoder); "6" is the torque control mode. (4) "5" is the speed proportional gain, the factory value is 150. The larger this setting value is, the higher the gain and the higher the stiffness. The parameter setting is determined according to the specific servo drive model and load conditions. Generally, the larger the load inertia, the larger the setting value. If the system does not produce oscillation, it should be set as large as possible. (5) "6" is the speed integral time constant, and the factory value is 20. The smaller the setting value, the faster the integral speed. Too small a value is likely to cause overshoot, and too large a value slows down the response. The parameter setting is determined according to the specific servo drive model and load. Generally, the larger the load inertia, the larger the setting value. (6) "40" and "41" are the acceleration and deceleration time constants, and the factory setting is 0. This setting value indicates the acceleration time or deceleration time required for the motor to rotate at a speed of 0 to 100 r/min. The acceleration and deceleration characteristics are linear. (7) "9" is the position proportional gain, and the factory setting is 40. The larger the setting value, the higher the gain, the higher the stiffness, and the smaller the position lag under the same frequency command pulse conditions. However, too large a value may cause oscillation or overshoot. The parameter value is determined according to the specific servo drive model and load conditions. 2 KNDSD100 parameter setting tips When the SD100 servo drive is matched with the Kanedi CNC system, only the parameters in Table 1 need to be set. The other parameters, under normal circumstances, do not need to be modified. The electronic gear ratio is set as follows: When matched with the KND-SD100 servo drive, the electronic gear ratio of the KND system should be set to CMR/CMD=1:1. The electronic gear ratio of the KND-SD100 servo drive is set to position command pulse frequency numerator (PA12)/position command pulse frequency denominator (PA13)=4×2500 (number of encoder stripes)/pulley ratio×screw pitch×1000. The numerator and denominator can be approximated to integers. For lathes, if the X axis is programmed with diameter, the denominator of the above formula should be multiplied by 2, that is: Position command pulse frequency numerator (PA12) / Position command pulse frequency denominator (PA13) = 4 × 2500 (number of encoder stripes) / pulley ratio × lead screw pitch × 1000 × 2 Example: The lead screw pitch of the X axis is 4mm, 1:1 transmission; the lead screw pitch of the Z axis is 6mm, 1:2 reduction transmission, then the electronic gear ratio of the X axis drive is PA12/PA13 = 4 × 2500/(1 × 4 × 1000 × 2) = 5/4. The electronic gear ratio of the Z axis drive is PA12/PA13 = 4 × 2500/(6 × 1000 × 1/2) (reduction transmission ratio) = 10/3 Therefore, for the X axis drive, PA/2/PA/3 should be set to 5/4, and for the Z axis drive, PA12/PA13 should be set to 10/3. 3 KNDSD100 parameter optimization tips (1) After setting the SD100 servo drive parameters as described above, start to optimize and adjust the servo performance, that is, adjust the drive gain parameters. Generally, the SD100 drive maintains the default gain parameters, which can basically meet the user's processing requirements. When the motor is running at the default gain, if the motor makes an abnormal sound, first consider whether there is a problem with the installation of the motor shaft. After checking the problem, consider using the resonance suppression method to modify parameter 7 (torque filter) and parameter 8 (speed detection low-pass filter) to suppress the vibration generated by the motor. The default parameters of parameters 7 and 8 are 100. Try to reduce parameters 7 and 8 by 10 each time and press the confirmation key. Run the motor. If it is still abnormal, reduce it by 10 again until the motor has no abnormal sound. Generally, the adjustment range of parameters 7 and 8 is between 20 and 80, which can basically achieve the effect of resonance suppression. (2) If the processing effect cannot be achieved when the factory parameters are maintained, for example, the roughness value of the inclined surface turned by the lathe is large, you can try to adjust the following parameters: ① Adjustment of speed proportional gain PA5: Confirm that the driver starts normally, and use the CNC system to manually control the motor rotation (machine tool movement). If the motor does not vibrate, increase this parameter. The larger the setting value, the greater the rigidity and the higher the positioning accuracy of the machine tool. Increase the value by 5 each time until vibration occurs. After reducing this value to a stable state, reduce this value by 10; ② Position proportional gain PA9: Within the stable range, try to set it larger so that the machine tool has good tracking characteristics and small lag error. Similar to the adjustment of speed proportional gain, this value should be increased as much as possible without generating vibration; ③ If the processing effect cannot be achieved after the above two parameters are increased, the vibration suppression parameters can be adjusted by adjusting parameters 7 and 8. After adjustment, the driver parameters 5 and 9 can be adjusted upward a little, which should meet the user's processing requirements. 4 KNDSD100 troubleshooting tips Once an alarm signal appears, the servo unit will prohibit the motor from running and the adjustment of user parameters until the power is turned off and then on again. The user can judge the type of fault and the cause of the fault based on the displayed alarm information. For specific troubleshooting methods, please refer to the SD100 user manual. If there is no alarm, it is naturally a driver fault. Of course, it is also possible that the servo has no fault at all, but the control signal or the host computer has a problem that causes the servo to not move. In addition to looking at the error and alarm number on the driver and checking the manual, sometimes the most direct judgment is to swap, such as the X-axis and Z-axis of the CNC lathe (only the same model can be used). Or when the power difference of the servo motor is not large, modify some characteristic parameters of the servo driver (such as the "1" model code parameter of KNDSD100), swap in a short time, and then switch back after confirming the fault. You can also modify the CNC system parameters to lock a certain axis such as the X-axis and prevent the system from detecting the X-axis to achieve the purpose of judgment. But it should be noted that when the X-axis and Z-axis are interchangeable, even if the model is the same, the machine tool may have problems due to different loads and parameters. Before confirming the inspection plan, you must consider it comprehensively to avoid unnecessary losses. In addition, because the AC servo unit usually uses the unified power supply system of the CNC system, the three-phase AC 220 V voltage comes from the servo transformer. Therefore, the operation must comply with the operating specifications during operation. For example: the three-phase outputs of U, V, and W must be connected in the correct order, otherwise the motor will not operate normally, an alarm signal will be given, and the motor will be prohibited from running. 5 Other Problem Handling Techniques for Servo Motors (1) Motor sway: When feeding, sway occurs, the speed measurement signal is unstable, such as cracks in the encoder; poor contact of the terminal, such as loose screws, etc.; when sway occurs at the moment of switching from positive direction movement to reverse direction movement, it is generally caused by the reverse clearance of the feed transmission chain or excessive servo drive gain; (2) Motor creep: Most of them occur in the starting acceleration section or low-speed feeding, generally due to factors such as poor lubrication of the feed transmission chain, low servo system gain and excessive external load. It is particularly important to note that the coupling used to connect the servo motor and the ball screw may be loose or have defects in the coupling itself, such as cracks, which may cause the ball screw and the servo motor to rotate out of sync, causing the feed motion to speed up or slow down; (3) Motor vibration: When the machine tool is running at high speed, it may vibrate, and an overcurrent alarm will be generated. Machine tool vibration problems are generally speed problems, so the speed loop problem should be looked for; (4) Motor torque reduction: When the servo motor changes from rated stall torque to high-speed operation, the torque will suddenly decrease. This is caused by heat dissipation damage to the motor windings and heating of the mechanical parts. At high speeds, the temperature rise of the motor increases. Therefore, the load of the motor must be verified before using the servo motor correctly; (5)Motor position error: When the servo axis moves beyond the position tolerance range (the factory standard setting of KNDSD100 is PA17:400, the position tolerance detection range), the servo drive will display the position tolerance alarm "4". The main reasons are: the tolerance range set by the system is small; the servo system gain is improperly set; the position detection device is contaminated; the cumulative error of the feed transmission chain is too large, etc. (6) The motor does not rotate: In addition to the pulse + direction signal connected from the CNC system to the servo drive, there is also an enable control signal, which is generally a DC + 24 V relay coil voltage. When the servo motor does not rotate, the common diagnostic methods are: check whether the CNC system has a pulse signal output; check whether the enable signal is connected; observe the system input/output status through the LCD screen to see if it meets the start-up conditions of the feed axis; confirm that the brake has been opened for the servo motor with an electromagnetic brake; the driver is faulty; the servo motor is faulty; the coupling between the servo motor and the ball screw fails or the key is disengaged, etc.
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