Motor Control IP Blocks

Figure 1 shows the block diagram of the sensorless FOC algorithm. These blocks, which are available as IP cores, are discussed in this section.

Figure 1: Block Diagram of FOC of Permanent Magnet Synchronous Motor

• PI Controller

The  proportional-integral  (PI)  controller  is  a  feedback  mechanism  that  is  used  to  control  a  system parameter. The PI controller has two tunable gain parameters which control the dynamic response of the controller—the proportional and integral gain constants. The proportional component of the PI controller is a product of the proportional gain constant and the error input, while the integral component is a product of the accumulated error and the integral gain constant. The two components are then added together. The integral stage of the PI controller can cause instability in the system because of uncontrolled increase in the data value. This uncontrolled rising of data is called wind-up. All PI controller implementations include an anti-windup mechanism which ensures that the controller output is limited. The Microsemi PI controller IP block uses a hold-on-saturation algorithm for anti-windup. The block also offers additional features to set the initial value of the output.

• FOC Transformations

FOC is an algorithm, which can supply optimal current to a motor by determining and controlling the torque and  magnetization  current  components  independently.  In  permanent  magnet  synchronous  motors (PMSMs),  the  rotor  is  already  magnetized,  and  hence  the  current  supplied  to  the  motor  contributes  to torque only. FOC is computationally intensive, but the Microsemi motor control reference design has been built to optimally use device resources. The FOC algorithm consists of the Clarke, Park, inverse Clarke, and inverse Park transforms.

• Position and Speed Estimator

FOC needs precise rotor position and speed as inputs. Determining rotor angle accurately is essential to ensure  low  power  consumption.  Adding  physical  sensors  to  determine  position  and  speed  add  to  the system cost and reduce reliability. Sensorless algorithms help in eliminating the sensor, but increase the computational complexity.

• PLL

The phase-locked loop (PLL) is used to synchronize signals. PLLs are useful in several applications, suchas angle estimation and grid-synchronization of inverters.

• Rate Limiter

The rate limiter block enables a smooth change of a variable or input to the system. For example, in a motor control system, if there is a sudden change in desired speed of the motor, the system may become unstable. To avoid such scenarios, a rate limiter block is used to transition from the initial speed to the desired speed. The rate limiter block can be configured to control the rate of change.

• Space Vector Modulation

The space vector modulation block improves DC bus utilization and eliminates short pulses to transistor switches. It improves DC bus utilization by 15% compared to using sinusoidal PWM.

• Three-Phase PWM Generation

At the end of all computations, the three phase motor voltages are available. The voltages are used to generate switching signals for transistors in the inverter. The PWM block produces switching signals for six (three high-side and three low-side) transistors, and has advanced features such as dead time and delay time  insertion.  The  programmable  dead  time  insertion  feature  helps  avoid  a  catastrophic  short  circuit condition on an inverter leg which could occur because of the transistor turn-off time. The block also has a programmable delay time insertion feature, which allows synchronization of analog-to-digital converters (ADC)  measurement  with  PWM  signal  generation.  The  block  can  be  configured  to  work  with  inverters consisting of N-MOSFETs only or both N-MOSFETs and P-MOSFETs.

• Debugging FPGA Designs in SoC

Generally,  debugging  a  design  on  a  microcontroller  is  relatively  easier  compared  to  debugging  on  an FPGA. In an SoC, the high performance of an FPGA can be utilized, while retaining the advantage of faster debugging in a microcontroller. The microcontroller subsystem and the FPGA fabric in the Microsemi SmartFusion2  SoC  FPGA  can  communicate  with  each  other  through  an  AMBA  APB  or  AXI  bus.  This allows injecting test data into the FPGA fabric, or logging debug data from the FPGA fabric, which in-turn helps in visualizing internal data at runtime for real-time debugging. The firmware code can be run in steps and breakpoints can be set in the code to analyze the FPGA register data. The SmartFusion2 SoC FPGA based multi-axis motor control solution connects to a host PC through USB, and communicates with a graphical user interface (GUI) to start and stop the motor, set motor speed value and other system parameters, and plot upto four system variables such as motor speed, motor currents,
and rotor angle.

Figure 4: Screenshot of the GUI - Plotting Internal Parameters: Rotor Angle (green), Valpha (red), Vbeta (black), 
Motor Speed (blue)

• Ecosystem

Microsemi provides a rich set of IP libraries consisting of IP blocks for several motor control functions that were  discussed  in  previous  sections.  These  blocks  are  customizable  easily  and  can  be  ported  across Microsemi  devices.  These  blocks  can  be  configured  and  connected  together  graphically  using SmartDesign tools of Libero SoC software. With the help of these IP blocks, designers can significantly reduce the time required to implement the motor control algorithm in FPGA.  These IP blocks have been tested with motors running at speeds as high as 30,000 RPM and switching frequency of 400 kHz.

• Industrial Communication Protocols

The trend in industrial networks is to migrate towards faster communication through networks instead of point-to-point communication. Implementation of such high speed communication demands for support of higher  bandwidths  which  is  not  easy  for  a  microcontroller  or  DSP  to  handle  along  with  motor  control algorithm  simultaneously.  In  many  cases,  an  additional  microcontroller  or  FPGA  is  used  to  handle  the communication  with  each  motor  controller.  Commonly  used  Ethernet  based  protocols  are  Profinet, EtherNet/IP and EtherCAT standards that are still evolving. Other protocols include CAN and Modbus. The advantage of using an SoC in this context is supporting multiple industrial Ethernet protocol standards on a single FPGA platform. Depending on the end system goals, it might be possible to optimize the system for cost by reusing IP and protocol stacks (for communication), or optimize performance by carefully partitioning of the functions into hardware (FPGA) and software (ARM Cortex-M3 processor).
Microsemi's SmartFusion2 SoC FPGA has built-in CAN, high-speed USB, and a gigabit Ethernet block as part of the MSS. A high-speed SERDES block is available for implementing protocols involving serial data transfer. 
Several  IP  blocks  are  available  through  Microsemi's  IP  partners.  For  more  information,  refer  to .