User Guide

AN1292 Tuning Guide
 2010 Microchip Technology Inc. DS70640A-page 5
STEP 3 – First, tune the open loop
a) Activate Open Loop Functioning
The open loop tuning can be operated separately, by enabling a special
#define in the FOC software code; otherwise, the transition to close loop
control is automatically done. Make sure you disable closed loop transition for
the initial tuning of open loop.
userparms.h
b) Set Up Open Loop Parameters
• Current Scaling
The prescaling constant needs to be set to adapt the ADC output to correspond
to the real value in terms of sign (direction), and if necessary, to prescale it to an
intermediary value, adequate for further processing.
userparms.h
The scaling factor for currents are negative because the acquisition for shunts is
getting the reverse sense of the currents, and therefore, the value of Q15(-0.5)
represents a (-1) multiplication of the Q15 value returned by the ADC.
• Start-up Torque Current
Choose nominal current for the given motor as a starting point, as indicated
below (in this case, a value of 1.41 amperes was used):
userparms.h
If the start-up current is too low, the load will not move. If it is too high, the motor
can overheat if it runs in open loop for a long period of time.
• Lock Time
In general, a lock time of a value of a few hundred milliseconds is selected.
userparms.h
The lock time value depends on the PWM frequency. For example, at 20 kHz, the
value 4000 would represent 0.2 seconds.
• Ramp Increase Rate
The open loop acceleration should be set as small as possible at the beginning.
The smaller this value, the more capable the motor is to start with a higher
resistant torque or moment of inertia.
userparms.h