ST AN2267 Application note

AN2267

Application note

Implementation of current regulator for BLDC

motor control with ST7FMC

Introduction

A conventional method of controlling BLDC motors is to implement an inner current loop for torque / current control. Reference to this inner loop is provided either by an outer speed loop or by some other means based on application requirement. The linearity of inner current / torque loop is greatly affected by the faithfulness of current feedback. In the first section, an outline to various approaches for obtaining current feedback is presented and analyzed with the limitations of each. In the subsequent sections, a presentation is given of a simple, linear and cost effective approach of implementing the inner current loop by sampling the DC link current at the mid-point of PWM “on time” with ST7FMC. Experimental results are also discussed.

An accompanying software file is available with this application note and can be downloaded from www.st.com/mcu

June 2006 Rev 1 1/19

www.st.com

Contents AN2267

Contents

1 Outline to various approaches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2 Obtaining the average current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3 BLDC motor control using ST7FMC . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

4 Implementation using ST7FMC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

5 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

7 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Appendix A Sampling inner current loop procedure . . . . . . . . . . . . . . . . . . . . . 14

Appendix B Event U interrupt service routine . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Appendix C ST7MC 3-phase motor control schematics . . . . . . . . . . . . . . . . . . . 16

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AN2267 Outline to various approaches

1 Outline to various approaches

A BLDC motor driven in a conventional 6-step method greatly resembles a brushed DC motor. Hence, one may choose to regulate the average DC link current. But this actually results in constant power operation for the motor because at constant DC link voltage, if the average link current is regulated at a certain value, it effectively regulates the power at that point for any variation in motor load, and the average load current / motor torque varies inversely with speed depending on the load. Any effort to compensate the average DC link current data with the duty cycle to obtain average phase current will be impaired by a filter time constant, rendering this option ineffective.

Since the DC link current does not reveal winding currents during PWM “off time”, one may choose to monitor all 3 winding currents and build a regulator. But this requires two current sensors to monitor any two phase currents, while the third phase current can be reconstructed from these two. However, the cost of these sensors makes this option expensive.

A third option would then be to regulate the peak current per PWM period. Though it is inexpensive and easy to implement, it is not exactly linear. During PWM on time, at lower duty cycles, when both speed and BEMF are small, the phase current rises much faster than at higher duty cycles when the speed and BEMF are large. The same peak currents per PWM period represent different average currents at different duty cycles. An intuitive geometric approach will reveal this as shown in Figure 1. A typical variation in average current vs duty cycle at a given peak current reference is shown in Figure 2.

Figure 1. Peak current regulation at different duty cycles with BEMF load

phase

peak

phase

peak

Figure 2. I

vs duty cycle at a given I

ave

peak

dutycycle

00.51.0

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Obtaining the average current AN2267

2 Obtaining the average current

For linear torque control, it is important that we sample the average phase current as feedback to the current regulator. It is best to get this information from the DC link current using only a shunt resistor because of its low cost and simplicity. However, the DC link current is not continuous and is present only during PWM on time. As a simple model for current control, assume a simple buck converter feeding an RL load as shown in Figure 3.

Figure 3. Buck converter feeding an RL load

PWM CONTROL

SW1

BT1

The switching frequency, PWM on time and load inductance are such that the load current is continuous. Figure 4 shows the load voltage, load current and DC link current waveforms. A close look at the load current waveform reveals that its average value is equal to its instantaneous value during the middle of PWM on time or off time. Since the load current flows through the DC link during PWM on time, sampling the DC link current during the middle of PWM on time gives the average load current.

Figure 4. Buck Converter - Waveforms

L(ave

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off

AN2267 BLDC motor control using ST7FMC

(

3 BLDC motor control using ST7FMC

The main feature of ST7FMC is its powerful motor control macro cell, capable of generating control signals to drive a sensorless or sensored 3 phase BLDC or AC motor. STMicroelectronics application notes AN1946 [1] and AN2030 [2] explain, in detail, the procedure to control a 3 phase BLDC motor using ST7FMC.

Figure 5 shows the simplified block diagram of the hardware motor control macro cell. The

macrocell has multiple timers performing various functions in parallel to generate control pulses for the motor. An auto scalable 8-bit timer (MTIM) monitors the time difference between successive phase back EMF zero crossings (Z events) of the motor. When a Z event occurs, the timer value is captured into MZREG and the timer restarts counting from zero, and, the previous content of MZREG is transferred to MZPRV. This timer is a part of what is called DELAY MANAGER that, based on this time difference and a delay coefficient (MWGHT), identifies the timing for next phase commutation instant (C events). All in parallel, a 12-bit free running counter generates the PWM carrier for inverter switching.

Figure 5. Simplified block diagram of Motor control Macro cell for BLDC motors

or SPEED MEASURE UNIT (not

WEIGH

DELAY = WEIGHT x

MEASUREMENT

WINDOW

GENERATOR

PWM

12-bit

Phase

Phase V

Phase W

[Z] : Back EMF Zero-crossin

: Time elapsed between two consecutive Z

[C] : Commutation

: Time delayed after Z event to generate C

(I): Current

(V): Volta

DELAY

CAPTURE

Phase

CURRENT

VOLTAGE

MOD

MTI

TIME

COMMUTE

PCN

BEMF ZERO-CROSSING

BEMF=

TACH

Encoder

INPUT DETECTION

P H A

U, V, Phase

CFAV

CHANNEL

12-bit THREE-PHASE

PWM GENERATOR

Vre

OAON

MCI MCI MCI

MCO MCO MCO MCO MCO MCO

NMCE

MCAO

MCCRE

MCPWM

MCVRE

MCAOZ/ MCCFI

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BLDC motor control using ST7FMC AN2267

Figure 6. Motor Control Macro cell - BLDC motor control configuration

ext

Board + Motor

MCPWMU / V/ W

(I)

(V)

1ext

2ext

Microcontroll er

Fcpu

ST3-0 bits

-1

MZREG Reg [ Zn]

S,H

MZP RV Reg [Z

DCB bit

MWGHT R eg [a

SWA bi t

MCOMP Reg [C

EF[2: 0]

Filte r / D

SR bit

X T16 bi t

MTIM

= FFh?

MZREG

< 55h?

MTIM [8-bit Up Counter]

]

n-1

]

n+1

A x B / 256

n+1

S,H

REF

VR2- 0

SR b it

S,H

Filt er / PWM

Reg

MPHST

CFF[2:0] bit

ISn bit

12- bit P WM ge nera tor

bits

MREF

6 6

Reg

Ch0Ch1Ch2Ch3Ch4

Dead

Time

Dead

Time

High Frequency Chopper

Dead

Time

Ch5

PCN bi t =0

DTG register

MPAR Reg

x6 x6

AO bit

CFA V bi t

MCIA

MCIB

MCIC

MCVREF

MPWME Reg

MCPWMU

MCPWMV

MCPWMW

MCO0

MCO2

MCO4

MCO1

MCO3

MCO5

NM CE S

MOE bi t

MPOL Reg

MCAOP

MCAON

MCAOZ

MCCFI

MCCREF

CP Bn bit

1 / 2

nn-1

Compare

EF[2:0] Filter / C

Rat i o

HDMn bit

1 / 4

Compare

S,H

ororor

SA3- 0 &

OT1-0 bits

SZn

bit

PZ bit

1 ¾ 1/128

Filter / C

EF[2 :0]

CP Bn bit

1 /20

REO bi t

cl r

S,H

SPLG

SWA bi t

MDREG Reg [ Dn]

Com p a re

SDMn bit

EF[2:0 ]

F ilter / C

-/+

S,H

Com p ar e U

MISR R eg

MI MR Reg

ZVD bit

]

A PWM output is generated as a result of comparison between this carrier and a compare register (MCPUH:MCPUL) that carries pulse width (duty cycle) information. This PWM signal is directed to one of the six inverter switches by a CHANNEL MANAGER that acts as a traffic diverter on the PWM output. The channel manager also selects a complementary switch, as programmed by the user, which together with the switch receiving PWM will force current into the motor windings. Based on the motor terminal voltages or Hall sensor outputs, an analog block identifies the motor phase BEMF Z events and captures the contents of MTIM timer into MZREG and the previous value of MZREG into MZPRV and this cycle repeats all over again.

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