AN1905 ST Microelectronics, Inc., AN1905 Datasheet - Page 67

• Manufacturer Part Number: AN1905
• Description: St7mc Three-phase BLDC Motor Control Software Library
• Manufacturer: ST Microelectronics, Inc.

7.3 SOFTWARE SETTING CONSIDERATIONS
7.3.1 PI regulator implementation and tuning
PID regulator theory and tuning methods are subjects which have been extensively discussed
in the technical literature. Here is a basic reminder of the theory and a proposal of the empir-
ical tuning method.
■
The implemented regulator is actually a Proportional Integral one (see note below regarding
the differential term). The purpose of the regulation loop (see equation 1) is to adjust the PWM
duty cycle on the motor winding depending on the frequency.
The equation 2 corresponds to a classical PI implementation, where:
– PWM
– K
– K
The tuning and respective actions of these three parameters are discussed below.
Note: No differential correction is implemented in the current regulator. Practice shows that this term leads
■
To tune the PI regulator parameters, it is advised to proceed in the following order:
– sampling time,
– proportional coefficient,
– integral coefficient.
■
The sampling time needs to be modified to adjust the regulation bandwidth. As an accumula-
tive term (the integral term) is used in the algorithm, increasing the loop time will decrease its
effects (accumulation will be slower and the integral action on the output will be delayed). In-
versely, decreasing the loop time will increase its effects (accumulation will be faster and the
ST7MC switches from synchronous to autoswitch mode in sensorless (exit of the ramp). In
sensor mode, this constant is set to 0, but can be manually adjusted. See ‘Init_PI()’ in
‘regul.c’ file for more details.
Theoretical background
Regulation tuning procedure
Adjusting the regulation sampling Time
p
i
is the integral coefficient.
is the proportional coefficient,
to increased noise in the regulation loop (high pass function). As a result, the system may become
unstable or difficult to tune. Additional software filtering can be implemented to get proper differential
frequency error, but may result in additional response delay.
start
is a constant corresponding to the duty cycle on the motor windings when the
PWM
ST7MC THREE-PHASE BLDC MOTOR CONTROL SOFTWARE LIBRARY
Dutr
=
PWM
Start
PWM
+
Dutr
K
p
×
=
Error
f freq
(
freq
)
+
K
i
×
∑
t
Error
freq
(1)
(2)
67/77