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Many electrical appliances, such as air conditioners and washer-dryers, must independently control the speed of the two motors to optimize their performance. The traditional approach used by these systems is to use a motor control with a serial link for synchronization. However, if you control two motors with one device, you can greatly simplify your hardware and system design. The recently introduced appliance control platform can simultaneously control two PM AC motors using only DC bus current feedback technology, which uses embedded FOC algorithms to reduce development time and drive appliance manufacturers to quickly adopt this technology.
Implementation of motorless control algorithm
FOC is very common in industrial drive systems, which typically use an encoder or Resolver to measure the position of the rotor. The closed-loop current control algorithm uses the angular coordinates of the rotor to correct the motor current and rotor flux to maximize torque output. The advanced rotor position estimation algorithm in the electrical control system eliminates the need for a high-resolution position Sensor. The estimation algorithm obtains the rotor flux position of the PM based on the motor model using the winding voltage and current. This method is very suitable because the magnets on the rotor determine the position of the rotor flux. The winding current measurement data is obtained from the DC link current using the correct ADC sampling timing based on the state knowledge of the power converter. The system block diagram shows that the winding current measurement data and the winding voltage drive value are inputs to the rotor flux model that calculates the rotor angular position and angular velocity. Torque and flux control loops not only achieve maximum torque output over a lower speed range, but also achieve high speed operation under field weakening.
In the first generation of FOC motor drive systems, these functions were implemented by a combination of analog and digital devices. Today, most of these motor drive systems have implemented algorithms on a single device using high speed DSP or RISC processors. Software implementation not only brings the advantages of flexibility and hardware simplicity, but also brings important software development tasks to the driver system developers. The software code that generates the control algorithm requires many steps. In the first step, the system engineer converts the control schematic into a system of differential equations representing various control functions. In the second step, the software engineer converts these differential equations into C code that represents the processor's execution instructions. This process goes wrong, which extends development time unless the code has good structure and documentation and has a long-term software maintenance team. RISC processor and DSP vendors can provide a complete set of FOC software examples to help motor drive companies accelerate development. This is very likely, because the FOC control technology is very mature, so the algorithm structure can also be defined very well. However, software implementations are currently not particularly advantageous because the flexibility of the algorithmic structure is not required.
Control system design engineers can implement FOC algorithms in hardware using digital ASIC or FPGA technology. The first step in the development process is not much different from the software approach, but instead of using C code in the second step, the hardware engineer converts the differential equation into Verilog code that represents the logic gate interconnect. This design can define and store control parameters in the control registers to provide flexibility, but hardwire the algorithm structure in a digital ASIC. This approach is very common in telecommunications systems that require high speed processing, and many motor control ASICs can implement FOC and other motor control functions. The advantage of this approach is not only the speed of execution, but also its ability to significantly reduce system development time.
The Motion Control Engine (MCE) provides an alternative approach that combines the high-speed performance of dedicated ASIC hardware with the flexibility of a programmable processor. This method is particularly effective because the FOC algorithm uses many standard functions, such as error, proportional-integral (PI) compensators, and vector rotators that appear multiple times in the control circuit. The MCE consists of a hardware motor control function library that efficiently interconnects the motion control sequencer with these functions by assigning input and output addresses to the corresponding system variables.
Control system engineers do not need to convert the control schematic into differential equations because of the fully optimized ASIC implementation in the MCE library. Instead, the control system engineer uses a schematic editing tool to graphically determine the control schematic by interconnecting standard functions in the motion control library. The graphical compiler converts the control schematic into MCE sequencer commands for interconnecting hardware control functions. The compiler assigns each address in the shared RAM area of ​​the MCE to each algorithm variable defined by the control node. The MCE sequencer command defines the memory address of each control function block as well as the input and output variables. Because MCE stores these commands in memory, it has the same flexibility as RISC processors and DSPs.
The timing of the PWM frequency setting algorithm, the ADC sample rate, and the update rate of the output voltage. The MCE library components represent space vector modulators and ADC inputs, but they appear only once in the control schematic because they correspond to physical input and output pins. On the other hand, MCE library control functions such as vector rotators or PI compensators can appear in the control algorithm multiple times because the MCE stores their inputs and outputs in the data memory. Each instantiation of a library function takes up data memory space to store variables and MCE instructions, so memory capacity limits the complexity of the algorithm. Each library function takes a certain system clock cycle each time it is executed, so the total number of clock cycles of the control loop must be less than the number of clock cycles in the PWM cycle.
The rotor angle estimator and current control loop consume approximately 1,400 system clock cycles, which is equivalent to 11 μs at 128 MHz maximum system clock frequency. Thus, control of two motors can be achieved simultaneously at a 50μs PWM period equivalent to a 20kHz switching frequency. Of course, to control two motors, the chip requires two sets of space vector PWM modulators and additional analog inputs for current sampling.
MCE library function
The key to high-speed execution control algorithms is the efficiency of MCE library functions in ASICs. Two important feedback control units (PI control compensator and vector rotation block) can be used as typical examples of library functions. ASIC implementations need to optimize the use of slices and clock cycles without sacrificing robustness and reliability. There are a variety of trigonometric identities that simplify the operation of sine and cosine terms into sine function operations ranging from 0 to 90?, but the operation of this item varies depending on the available hardware. In some micro-implementations, the lack of fast multiply functions will force software developers to rely on simple lookup tables. In a DSP or RISC processor with a single-cycle multiply instruction, the Taylor expansion can be used to calculate the sine function. A 12-bit precision ASIC implementation can be implemented in only 13 cycles based on a series of addition, subtraction, and shift functions, developing a vector rotation function called the CORDIC algorithm (Figure 4). This operation is 10 times faster than using the Taylor expansion on a 32-bit RISC processor.
Simplified motor control
Although many configurations are possible, the configuration of operating a dual motor platform with a single control IC as shown in Figure 5 is the most efficient. This configuration not only eliminates the second IC used to control the second motor (which leads to unnecessary redundant design), but also makes complex interface design for both motors possible. For example, when a motor fails (such as a short circuit or a latch), the second motor can be immediately de-energized like a reflex action, reducing the delay associated with communication with the main control system. Taking air conditioning applications as an example, the speed of the compressor motor and the evaporator fan requirements are tracked to optimize work efficiency. The control system sets the motor speed by directly writing to the MCE register and avoids complex communication between multiple ICs. 
Simplify the design of energy efficient appliances with dual motor control technology
Today, more and more appliance manufacturers are using variable speed permanent magnet (PM) synchronization to improve energy efficiency and add features. Industrial drive manufacturers have long recognized that PM motors have high energy efficiency and high power-to-weight ratios, but the latest advances in control have made PM motors widely adopted by appliance manufacturers. Directed control (FOC) with DC link current feedback technology minimizes system cost and is attractive for electrical drive applications. The sinusoidal control of the motor produces a smooth torque output with low acoustic noise. Therefore, the FOC is suitable for use in equipment that is very important for low noise and energy efficiency such as fans, pumps, washing machines and dryers.