UM0324 datasheet(87/105 Pages) STMICROELECTRONICS

87 / 105 page

UM0324

Library functions

87/105

Figure 46.

Time domain to discrete PID equations

In theory, the higher the sampling rate, the better the regulation. In practice, you must keep

in mind that:

●

The related CPU load will grow accordingly.

●

For speed regulation, there is absolutely no need to have a sampling time lower than

the refresh rate of the speed information fed back by the external sensors; this

becomes especially true when a tacho-generator sensor is used while driving the motor

at low to medium speed.

As discussed in Section 2.2.2 on page 19, the speed regulation loop sampling time can be

customized by editing the PID_SPEED_SAMPLING_TIME parameter in the

MC_Control_Param.h

header file. The flux and torque PID regulator sampling rates are

given by the relationship

Note:

REP_RATE

must be an odd number if currents are measured by shunt resistors (see also

Section A.2 on page 98); its value is 8-bit long.

4.8.5

Adjusting speed regulation loop Ki, Kp and Kd vs motor frequency

Depending on the motor frequency, it might be necessary to use different values of Kp, Ki

and Kd.

These values have to be input in the code to feed the regulation loop algorithm. A function

performing linear interpolation between four set-points

(PID_Speed_Coefficient_update) is provided as an example in the software library

(see MC_PID_regulators.c) and can be used in most cases, as long as the coefficient

values can be linearized. If that is not possible, a function with a larger number of set-points

or a look-up table may be necessary.

To enter the four set-points, once the data are collected, edit the MC_Control_param.h

file and fill in the field dedicated to the Ki, Kp and Kd coefficient calculation as shown below.

()

Error

sys

Error

sys

Error

sys

Error

sys

–

()

∑

()

Error

sys t

()

Error

sys

∫

()dt K

Error

sys t

()

Time domain

Discrete

domain

(sampling done at Fs = 1/Ts frequency)

⋅

RATE

REP

FREQ

PWM

rate

sampling

PIDs

torque

and

Flux

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