1. A speed controller used in a control loop of a motor, comprising: a comparator comprising: an input configured to receive an instruction speed or a measured speed of the motor to compare with a predetermined speed value stored in the comparator; and an output configured to output a first signal in response to the received speed exceeding the predetermined speed value, and output a second signal in response to the received speed being less than the predetermined speed value; a processing device comprising: a first input configured to receive a speed difference between the instruction speed and the measured speed; a second input configured to receive the first signal or the second signal from the output of the comparator; and a first output configured to output a first proportional value in response to receiving the first signal, and output a second proportional value less than the first proportional value in response to receiving the second signal; and an arithmetic logical unit (ALU) comprising: a first input configured to receive the first proportional value or the second proportional value; and an output to output an instruction current to the control loop by calculating the received proportional value according to a predetermined calculation rule.