RX62T

Position Control of PMSM with Encoder

R01AN0899EU0200 Rev. 2.00

Page 11 of 22

Jan. 31, 2014

6.

Position Control Strategy

6.1 Block Diagram of Position Control

Figure 9 is block diagram of position control. The position control developed includes two loops. The outer loop is

position control to make the motor tracking and holding the given position. The inner loop is current control. Actually it

is the torque control loop. The motor currents are sampled through three shunt resistors and converted into the dq axis

currents. The control loop here is to control the q axis current for the torque.

Figure 9 Block diagram of position control

The position control scheme of the PMSM is illustrated in Figure 10. The system has an inner loop of current regulation

using vector control, and an outer loop of position regulation. This dual-loop structure ensures the fast torque response by

using the vector control, high position accuracy and fast tracking performance with the position controller.

In order to determine the d and q axis currents, the phase currents must be measured. Vector formulation uses Clarke and

Park transforms to convert the measured phase currents from the (u, v, w) frame to first transform them in the static

orthogonal (a,ß) frame (which is 90 degrees apart), and then, to the rotor frame which is also an orthogonal frame

aligned along the magnetic field axes known as the (d,q) frame. These transformations use the transcendental functions

sine and cosine of the rotor angle; thus, it is a requirement that the rotor angle is known at the time the calculation is

made. The position control requires current sensors, plus an encoder attached to the rotor shaft to measure the rotor

position.

Once the currents are transformed in the (d,q) frame, the control algorithm simply runs the PID or PI loop to calculate the

required voltages for the torque and flux. These required voltages (Vdc, Vqc) are then transformed back in the (u, v, w)

frame using the inverse Clarke and inverse Park transforms to further calculate the PWM duty cycle.

The position command is an input to the position control system. The motor has an encoder mounted on its rotor to give

the quadrature pulses A and B, as well as the zero synch pulse Z. All three of the rotor position signals are sent to the

MCU’s input-capture and timer/quadrature counter peripheral for making position and speed measurements. The

commanded position compares with the actual rotor position. The position regulator uses the traditional PID controller,

and outputs the torque control command of iq* to make the motor moving and tracking the commanded position.