The MCF8315C uses motor winding resistance, motor winding inductance and Back-EMF constant to estimate motor position in closed loop operation. The MCF8315C has capability of automatically measuring motor parameters in offline state, rather than having the user enter the values themselves. The MPET routine measures motor winding resistance, inductance, back EMF constant and mechanical load inertia and frictional coefficients. Offline measurement of parameters takes place before normal motor operation. TI recommends to estimate the motor parameters before motor start-up to minimize the impact caused due to possible parameter variations.

Figure 6-41 shows the sequence of operation in the MPET routine. The MPET routine is entered when either the MPET_CMD bit is set to 1b or a non-zero target speed is set. The MPET routine consists of four steps namely, IPD, Open Loop Acceleration, Current Ramp Down and Coasting. Each one of these steps are executed if the condition shown below the step evaluates to TRUE; if the condition evaluates to FALSE, the algorithm bypasses that particular step and moves on to the next step in the sequence. Once all the 4 steps are completed (or bypassed), the algorithm exits the MPET routine. If target speed is set to a non-zero value, the algorithm begins the start-up and acceleration sequence (to target speed reference) once MPET routine is exited.

Figure 6-41 MPET Sequence

TI proprietary MPET routine includes following sequence of operation.

• IPD: The MPET routine starts with IPD, if the user enables motor winding resistance or inductance measurement by setting MPET_R = 1b and MPET_L = 1b or if the user defines MOTOR_RES = 0 or MOTOR_IND = 0. The IPD during MPET can be configured using MPET specific configuration parameters or using the normal motor operation IPD configuration parameters. The IPD configuration selection is done using MPET_IPD_SELECT. With MPET_IPD_SELECT = 1b, the IPD current limit is configured using MPET_IPD_CURRENT_LIMIT and the IPD repeat number is configured using MPET_IPD_FREQ. With MPET_IPD_SELECT = 0b, the IPD current limit and the repeat number is configured using IPD_CURR_THR and IPD_REPEAT. The IPD timer over flow or the IPD current decay time more than three times the current ramp up time can result in MPET_IPD_FAULT. TI recommends to run the MPET multiple times to observe for consistent resistance and inductance reading.
• Open loop Acceleration:

  After IPD, the MPET routine run align and then open loop acceleration if the back-EMF constant or mechanical parameter measurement are enabled by setting MPET_KE = 1b and MPET_MECH = 1b. The MPET routine incorporates the sequences for mechanical parameter measurement, if the speed loop PI constants are defined as zero, even if MPET_MECH = 0b. User can configure MPET specific open loop configuration parameters or use normal motor operation open loop configuration parameters. The open loop configuration selection is done using MPET_KE_MEAS_PARAMETER_SELECT. With MPET_KE_MEAS_PARAMETER_SELECT = 1b, the speed slew rate is defined using MPET_OPEN_LOOP_SLEW_RATE, the open loop current reference is defined using MPET_OPEN_LOOP_CURR_REF and the open loop speed reference is defined using MPET_OPEN_LOOP_SPEED_REF. With MPET_KE_MEAS_PARAMETER_SELECT = 0b, the speed slew rate is defined using OL_ACC_A1 and OL_ACC_A2, 80% of ILIMIT for current reference and 50% of MAX_SPEED for speed reference.

• Current Ramp Down: After open loop acceleration, if the mechanical parameter measurement is enabled, then the MPET routine optimizes the motor current to lower value sufficient to support the load. If mechanical parameter measurement is disabled (MPET_MECH = 0b, or non-zero speed loop PI parameters) then the MPET will not have the current ramp down sequence.
• Coasting: MPET routine completes the sequence by allowing the motor to coast by enabling Hi-Z. The motor back EMF and indicative values of mechanical parameters are measured during the motor coasting period. If the motor back EMF is lower than the threshold defined in STAT_DETECT_THR, the MPET_BEMF_FAULT is generated.

Selecting the parameters from EEPROM or MPET

The MPET estimated values are available in the MTR_PARAMS Register. Setting the MPET_WRITE_SHADOW bit to 1, writes the MPET estimated values to the shadow registers and the user-configured (from EEPROM) values in MOTOR_RES, MOTOR_IND, MOTOR_BEMF_CONST, CURR_LOOP_KP, CURR_LOOP_KI, SPD_LOOP_KP and SPD_LOOP_KI shadow registers will be overwritten by the estimated values from MPET. If any of the shadow registers are initialized to zero (from EEPROM registers), the MPET estimated values are used for those registers independent of the MPET_WRITE_SHADOW setting. The MPET calculates the current loop KP and KI by using the measured resistance and inductance. The MPET does an estimation of the mechanical parameters including the inertia and frictional coefficient at the shaft (includes both motor and shaft coupled load). These values are used to set an initial values speed loop KP and KI. The estimated speed loop KP and KI setting can be used as an initial setting only and TI recommends to tune these parameters on application by the user based on the performance requirement.