Motor Control FOC SDK - key features for washing machines Antonino Bruno, Dino Costanzo, Gianluigi Forte ABSTRACT STMicroelectronics’ (ST) focus on three-phase motor controls enables the company to develop an unprecedented portfolio of dedicated HW and SW solutions. A new version of ST’s Motor Control Field Oriented Control Software Development Kit (MC FOC SDK) now enriches the MC Ecosystem with a new version for permanent Magnet synchronous Motors (PMSM): STM32 PMSM FOC SDK v3.4 and ST MC Workbench v3.0.2. This new version also supports the latest STM32F families -- the STM32F30x MCU series. The STM32F30x series combines a 32bit ARM Cortex-M4 core with a DSP and FPU instructions running at 72 MHz, with advanced analog peripherals, making it the right product for single and dual motor control applications. These tools, the result of deep knowledge and system expertise accrued in digital and power system solutions from over 10 years, speed up the time to market and evaluation of ST products. In fact, the company supports all kinds of motors used in home appliances thanks to a portfolio of innovative products ranging from power discretes to intelligent power modules, motor driver ICs, and the latest digital microcontrollers. The ST MC FOC SDK features and dedicated algorithms add value to the evaluation of ST products for white good applications such as our example, washing machines. Figure 1 - ST Motor Control Workbench On one hand, the firmware library provides the fundamental bricks of Field Oriented Control for PMSM (e.g. reference frame transformations, sensorless algorithm based on motor model, phase current sensing with shunt resistor(s) topology, PID phase current regulation, speed regulation, Space Vector Modulation…), and on the other, it presents very specific algorithms suit the tasks of home appliance drives: namely - for a washing machine application flux weakening, feed forward current regulation, advanced overvoltage protection, Maximum Torque per Ampere, and dual motor control. FLUX WEAKENING Flux weakening is needed to extend the speed operating range of the motor. In fact, washing machines present two different operating areas, washing and spinning; the first is slow and heavy, and the second is fast and soft. Motors are chosen to match the needs of the washing cycle, so as to bear the maximum load and optimize energy efficiency, considering that the duration of washing plus rinsing is by far the longest. Consequently, the motor nominal speed results are lower – about one fifth - than that required for spinning. The “closed loop” flux weakening algorithm implemented in the STM32 MC library enables the motor to cover the distance, paying the smallest price in terms of energy efficiency and torque capability. The control loop is based on stator voltage monitoring, in order to keep a voltage margin and prevent the saturation of current regulators. INTRODUCTION: SDK FEATURES FOR HOME APPLIANCES One scalable solution, this convenient software development kit covers all the STM32 lines right for motor control: STM32F0, STM32F100, STM32F103, STM32F2, STM32F3, and STM32F4. STM32 performances driving 3phase PMSM motors in application can be quickly evaluated, reducing development time. The SDK includes a 3-layer motor control firmware library, PC software for complete customization of pin-out assignments and drive behavior and tuning (fig. 1), as well as comprehensive documentation from API functions description to algorithms overview. FEED FORWARD The extreme drum speeds, used to shorten and maximize clothes drying times, translates in high frequency motor phase currents, thus posing a challenge in terms of distance between first harmonic and control frequency. In the STM32 MC library, this is solved by enabling the Feed Forward algorithm that improves the performance of PID current regulation by flanking a model based calculation of the control variables. In particular, the feed forward algorithm has been designed to compensate for the frequencydependent back EMFs and cross-coupled inductive voltage drops in permanent magnet motors. For instance, by enabling this control, a 16 kHz FOC frequency regulates a 1.2 kHz electrical frequency. OVERVOLTAGE PROTECTION Again, the same operating area – spinning – requires extra care from an electrical safety point of view. In fact, thanks to the flux weakening techniques available, PMSM motors can run at speeds that are multiples of their nominal; recalling that a PMSM motor at nominal speed produces a voltage near that of the DC bus, it’s easy to conclude that if something wrong happens to the control when the motor is at maximum speed (say 5x the nominal) an hazardous phase-to-phase voltage is generated in the range of 1.4 kV, which can destroy the board. Clearly, this risk is not due to the specific implementation of flux weakening, but to the motor physics that can suddenly “appear” if for any reason the control is lost. The STM32F3 has been specifically designed to cope with this issue by means of its embedded comparators and double emergency (or brake) function pins, with distinctive behavior in response to overcurrent or overvoltage: the BRK2 input works for over-current protection, opening the six switches from the power stage. the BRK input works for over-voltage protection, overriding the over-current and closing the three low-side switches to avoid current regeneration to build up bus voltage and exceed capacitors rated voltage. BRK has higher priority than BRK2. When both protections are triggered, the predefined safe state related to BRK circuitry overrides the inactive state related to the BRK2 input. These functionalities are embedded in the STM32F3 microcontroller, and the STM32 PMSM FOC SDK v3.4 supports their configuration. DUAL DRIVING A relative “new” trend in home appliances is to reduce the total bill of material cost of the solution by exploiting the ability of the last-generation microcontroller to execute simultaneous tasks. In the specific case of washing machines, this can be done using a single MCU to drive two electrical motors at the same time. This kind of control, called dual drive, has been around for a few years in the world of electronic designs. For instance in the “high-end” washing machine, in which the drum and the drain pump - both three phase PMSM motors - have to be driven at the same time during the centrifuge spinning. Other examples are dish washer main and drain pumps, multi-compressor fridges, outdoor units of air-conditioners, and so on. Since 2011, STMicroelectronics has focused on this kind of solution with the introduction of STM32F103 high density product and the related STM32 FOC SDK 3.x library. Today’s offering that covers the dual drive solution includes the new STM32F3x families, and in particular, the STM32F303 product that has been designed to be the “ideal” solution for the dual drive. Figure 2 illustrates a dual drive block diagram. Figure 2 - Dual motor drive block diagram There are many new features present inside the STM32F303 conceived to address the dual drive, but two of them are illustrated in this article: the core coupled memory (CCM) and the queue of injected conversion (QIC). The CCM is a region of RAM linked to the Cortex-M4 instruction bus, and it is intended to be used to store part of the firmware code that must be executed at maximum speed. Designed to get the highest performance for any kind of time critical tasks, the CCM is suitable for use in the dual simultaneous execution of field oriented control (FOC) of permanent magnet synchronous motors (PMSM), reducing the total workload, by more than 40%, than traditional solutions. The QIC is useful when it is necessary to program an analog to digital (ADC) conversion, or a sequence of ADC conversions, that have to be enabled only after a specific event occurs (for instance an update event of an internal timer) and unloads the MCU from waiting for the occurrence of such event. This ST patented method is implemented in hardware in the STM32F3 products by means of a queue of pre-programmed sequence of conversions that are automatically activated without the intervention of the MCU. It fits the needs of a dual drive in which the motor phase currents sampling have to be programmed synchronously with the PWM control signals, and the context switching between one motor and the other is synchronized with another event (timer update). Given the generality of that method, it can also be applied for other kinds of synchronized conversions (current sensing for digital PFC and so on). CLASS B REQUIREMENTS Today, more than ever, the role of safety has become very important for many electronics applications and in particular for home appliances, subsequently raising the level of safety requirements in electronic designs. With the introduction of solutions based on a microcontroller, even for low end applications, appliance manufacturers must consider both the hardware and firmware aspects to comply with the regulations that assure user safety. ST SYSTEM EVALUATION BOARDS STMicroelectronics offers a wide set of system evaluation boards supporting diverse final applications. For example, all motor control boards have a ST standard connector in order to easily connect any control board (with different MCUs) with any power boards (with different power rates and products). In Table 1 are reported the main system evaluation boards featuring 3-phase motors for white goods. The current safety recommendations and requirements are specified by recognized international standards bodies like IEC, for instance the 60335-1, and are verified and certified by authoritative testing houses like VDE. New software techniques for improving safety are continuously being developed, and to help its customers, ST provides a set of self-check, VDE certified routines to facilitate and accelerate the software development and the certification processes. The basic principle of the safety of household electronics appliances stated in the IEC60335-1 is that the appliance must remain safe in case of concurrent component failure. This means that in the case of a “standard” electronic component, it must stay safe at least after two consecutive failures. In case of a microcontroller based solution, if the safety of the appliance relies on firmware, then that firmware has to be certified. The IEC60335 annex Q defines three safety classes for firmware: Class A when the safety does not rely on the firmware, Class B when the firmware prevents some unsafe operation, and Class C when the firmware is intended to prevent special hazards. In many home appliances relying on microcontrollers to drive electrical motors, like the drum in washing machines, pumps in dish washers, or compressors in fridges, it is possible that the safety of the appliance can be delegated to the firmware. In all of these cases, usually, the firmware is certified as Class B. In order to respect the requirements for Class B firmware, it is necessary to test several components of the system such as the MPU clock, register, memory, and peripherals. All aspects related to the microcontroller are covered by the “STM32 self-check routines” provided by ST, allowing users to focus only on what concerns their application specific tests and basically reducing the time-to-market. Moreover, as stated above, extra safety features are also embedded “in hardware” in the STM32F3 families, strengthening the already robust set of “safety-oriented” features. So on top of the “smoke-inhibit” features like the locking of sensitive registers, the CCS, two emergencies input with programmable behaviors are present in the STM32F3 products. Table 1: Main system evaluation boards for 3-phase motors Motor control starter kit for STM3210B-MCKIT STM32103 Evaluation board for STM32303C-EVAL STM32F303xx microcontrollers Evaluation board for STM32 F0 STM320518-EVAL series Control stage based on the STM32 STEVAL-IHM033V1 for motor control with serial communication user interface 150 W inverter featuring the L639x and STGD3HF60HD for 1-shunt STEVAL-IHM032V1 based sinusoidal vector control and trapezoidal scalar control 3-phase high voltage inverter power STEVAL-IHM035V2 board for FOC and scalar motor control based on the STGIPN3H60 Dual motor control and PFC STEVAL-IHM034V2 demonstration board featuring the STM32F103 and STGIPS20C60 BLDC/PMSM drive demonstration STEVAL-IHM040V1 board based on STM32 and the STGIPN3H60 1 kW 3-phase motor control STEVAL-IHM025V1 demonstration board featuring the IGBT SLLIMM™ STGIPL14K60 CONCLUSIONS We have presented the latest version of ST’s MC FOC SDK, and every year ST enriches these tools with new features supporting new technologies and products, while enlarging STMicroelectronics’ Motor Control Ecosystem in order to meet new trends in home appliances in terms of efficiency, integration, and cost optimization.
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