What is the purpose of this FPGA + microcontroller based motor driver design?

Question

I have used microcontrollers (Arduino, PIC, Atmel etc) so I am familiar with what they are and what they do (e.g I know about polling, interrupts and communication protocols like I2C, SPI, UART) but I have never used FPGAs before. So I don't understand why the following motor driver uses a FPGA (Altera Cyclone 2) together with a microchip (Atmel). Can someone explain the design logic? Why can't I just use a microcontroller directly with the two encoders? (Note that this item only has Japanese language info available)

http://t-frog.com/products/motor_driver/

• Marcus Müller: sorry, my Japanese isn't good enough for that. But: a motor controller can be something relatively simple, or it might do a very complex control and have very high rate ADCs for current and hall sensors. Not everything is easy or even possible at all to do within a microcontroller

Answer 1

Using Google translate, that product is meant as a development board for motor control in robotics applications. FPGA allows much quicker or tighter control loops that would be crucial in running something like a robotic joint. FPGA is also a better fit in a development board because it can be reconfigured at the logic level.

• James: I will accept the second and third sentences as the answer