RX62T

Position Control of PMSM with Encoder

R01AN0899EU0200 Rev. 2.00

Page 9 of 22

Jan. 31, 2014

Figure 6 Speed calculation using encoder pulses A and B at control loop rate

5.2 Initial Position Identification

Incremental encoders can only give displacements from the initial position and can’t provide absolute position. For

PMSM and position control, the initial position is required. Although alignment has been calibrated, the initial starting

position before the Z pulse is still unknown.

By means of Hall sensors the rotor initial position can be identified, and further corrected when the rotor starts rotating.

Assuming the Hall sensors are located at each phase, as shown in Figure 7. The output signals of the Hall sensors are

illustrated in Figure 8. It can be seen that the resolution of the Hall sensor signals are 60° (electrical degree). Table 1

shows the possible combinations corresponding to different positions.

Figure 7 Hall sensors for initial rotor position

From Figure 8 and Table 2, given a specific Hall sensor output combination, the rotor must reside in certain section with

a range of 60°. The initial position is determined as follows. When a group of output signals are obtained, for example,

(101), we can decide which section the rotor is in (section 1 in this example). We can set the initial position at the center

of the section (30° in this example). It can be seen that the maximum error of the initial position is 30°, which occurs

when the rotor is at the edge of two regions. However, even with 30° error, the motor is still able to produce sufficient

torque to start the motor.

Once the motor starts rotating, the position can be readily corrected when the rotor moves out of the initial region and

enters the next section. This position is accurate. In the previous example, when the motor starts rotating in the positive

direction from section 1, the rotor position can be corrected when the position is 60°.