Once the motor has reached a sufficient speed for the back-EMF observer to estimate the angle and speed of the motor, the MCF8315A transitions into closed loop state. This handoff speed is automatically determined based on the measured back-EMF and motor speed. Users also have an option to manually set the handoff speed by configuring OPN_CL_HANDOFF_THR and setting AUTO_HANDOFF_EN to 0b. In order to have smooth transition and avoid speed transients, the theta_error (Ɵ_gen - Ɵ_est) is decreased linearly after transition. The ramp rate of theta_error reduction can be configured using THETA_ERROR_RAMP_RATE. If the current limit set during the open loop is high and if it is not reduced before transition to closed loop, the motor speed may momentarily rise to higher values than SPEED_REF after transition into closed loop. In order to avoid such speed variations, configure the IQ_RAMP_EN to 1b, so that i_{q_ref} decreases prior to transition into closed loop. However if the final speed reference (SPEED_REF) is more than two times the open loop to closed loop hand off speed (OPN_CL_HANDOFF_THR), then i_{q_ref} is not decreased independent of the IQ_RAMP_EN setting, to enable faster motor acceleration.

After hand off to closed loop at a sufficient speed, there could be still some theta error, as the estimators may not be fully aligned. A slow acceleration can be used after the open loop to closed loop transition, ensuring that the theta error reduces to zero. The slow acceleration can be configured using CL_SLOW_ACC.

Figure 7-52 shows the control sequence in open to closed loop transition. The current i_{q_ref} reduces to a lower value in current decay region, if IQ_RAMP_EN is set to 1b. If IQ_RAMP_EN is set to 0b, then the current decay region will not be present in the transition sequence.

Figure 7-31 Control Sequence in Open to Closed Loop Transition

Figure 7-32 Open to Closed Loop Transition Control Block Diagram