Download Motor Basic – Key terms for motor control

Motor Basic – Key terms for motor
control
Zhang Chen Jian
MAT, MDBU,
Sept 2010
DC motor behavior
Basing on DCM model, analyze potential control :
•
•
•
•
Speed
Torque
Current
Direction
Brush DC Motor Speed-Torque Curves
Increasing Voltage
Let’s assume you have a constant torque load, as shown by the red vertical
line below. As you steadily increase the voltage on the motor terminals, the
speed will go up proportionally. Conversely, if you hold the speed constant
as indicated by the blue line, and steadily increase the motor’s current, the
torque will go up. So current controls the motor’s torque, but voltage
controls the motor’s speed.
Increasing Current
Speed-torque curves for terminal voltages
of 10, 20, and 30 V.
Source: DC Motors, Speed Controls, Servo Systems, Electro-Craft Corp., 1980
PWM
• efficiency
• waveform
• filtering structure
• frequency setting
PWM Generation Simple Indication
Blue W aveform – Motor Voltage
Red W aveform – Filtered motor voltage
Center-Aligned PWMs
H-Bridge
Half-bridge – When two transistors are connected in a totem-pole arrangement as shown below, they are said to be in a halfbridge configuration. By turning each transistor on and off in a complimentary fashion, the half-bridge can drive the load
voltage alternately high and low to produce a PWM waveform at the motor terminal.
V+
V+
V+
DC Bus
A
Motor
+
_
motor
Mod
Carrier
A
B
GND
Inverter
B
(Dead-Time not shown)
H-Bridge
Dead-Time
C urrent with C orrection Disabled
With a half-bridge power structure, you must make sure
that the bottom transistor and the top transistor NEVER
get turned on at the same time.
Doing so will result in large currents flowing through the
transistor pair, as a short is created across the DC bus.
This condition is called shoot-through.
ON
ON
Top transistor
Dead-Time
Bottom transistor
ON
ON
ON
Example of Dead-time Distortion
Current Re-circulation
Inductor character and behavior (video)
Current need to be continuously maintained
What will happen if we turn off SW1 ?
Current Re-circulation
V+
V+
A
Motor
+
B
_
Mod
Carrier
A
B
(Dead-Time not shown)
H-Bridge
Solution: provide a current path for recirculation
A-sync and Sync mode recirculations
Current Recirculation: Fast Decay Mode
• Current flowing through the motor winding will be working against
the full supply voltage, plus two diode drops.
• Current decays quickly.
Current Recirculation: Slow Decay Mode
• Current re-circulates through power MOSFETs presenting a resistive
path to the current
• Current decays slower (directly proportional to the LR)
Mixed Decay Mode: The best of Both?
•
•
•
The idea behind mixed decay mode is to provide faster decay than slow, but
slower than fast.
FET switching is coordinated so that Fast Decay mode is engaged for a fixed
period of time, followed by slow decay mode
Mixed decay mode is necessary for reliable micro-stepping
Brushed DC Motors: Dynamic Braking
VM
VM
(-) BEMF (+)
(-) BEMF (+)
(+) VBB (-)
Normal Operation
VM > BEMF
Braking
BEMF Stops Motor
Critical Points:
By shorting the motor leads, you allow the Back EMF voltage to drive current in the
opposite direction of the supply current, quickly bringing the motor to a stop.
The energy of the
motor
dissipated by the “resistive load”
Current
ModeisProfiles
Stepper Winding Current
Brushed DC Motors: Coasting
VM
VM
(-) BEMF (+)
(-) BEMF (+)
(+) VBB (-)
Normal Operation
VM > BEMF
Coasting
Critical Points:
By opening the circuit, only the friction of the motor rotation slows the motor down,
bringing the motor to a slow stop.
The energy of the motor is dissipated by the friction of the motor
DC Motor BRAKE
IM
RM
RM
LM
IG
L di/dt = 0
LM
DISABLEMENT
OCCURS
VS
RDSON * 2
BEMF
BEMF  VS
Current in the winding
decays in a few micro
seconds.
Motor becomes a Generator
and pushes current in the
opposing direction.
Motor drive protection
VBB
AH
High Side
PreDriver
High Side
PreDriver
+
AL
M-
Low Side
PreDriver
Thermal
Shutdown
BH
Low Side
PreDriver
Current
Control
BL
Over Current
Protection
Protection circuits for a variety of conditions enable a
simplified design while providing protection at circuit, motor
and board levels against potentially damaging events
Over Current Protection (OCP)
VM/RDS(ON)
Without OCP:
Current rises to
destructive levels
TIME
CURRENT
CURRENT
POOF!
With OCP: Current
is limited, then
driver turns off
IOCP
TIME
TOCP
•
Designed to protect the device from damage in the event of a fault in the motor or wiring, such as a
short to ground, power, or across the winding.
•
Each power FET is protected individually, so is protected against shorts to any other signal
•
Protection must be quick enough to prevent damage but not trigger on false trips. Body diodes must
be robust enough to handle shorts.
•
Can be a latched or auto-recovery. Some devices alert the system MCU via a Fault pin on an OCP
event.
Current Regulation
Winding Current
VM
IMAX
Current Limit Kicks In
Time
RSENSE
Amp
Winding Current In Volts
IMAX
Reference Voltage
RSA
To H Bridge A Enable
VREFAB
To H Bridge A
Why is current regulation needed?
• Stepper Motors: Winding resistance is very low and there is little to no
back EMF. Typically you must have it for basic stepper operation.
• Brushed / Brushless DC: Needed to limit stall / start up currents
Configuring Itrip
VREF


ITRIP  

 GAIN  RSENSE 
M
Winding Current
In Volts (V)
RSENSE
Amp
I
MAX
Reference Voltage
VREF
Low Side Current Sensing
To H Bridge Enable
Current Regulation Example
Using current control with a DC brushed motor can allow you to limit
the high starting current, and use a motor driver IC that is rated for less
current than the stall current of the motor
Motor startup without current control
Startup (stall) current
over 14 amps!
Motor startup with current control
Current limited to less
than stall current
Motor reaches
full speed
Thermal Protection
Normal Operation
TSD
170 C
150 C
120 C
80 C
50 C
25 C
Temperature
•
Multiple thermal sensors are placed across the die, continuously monitoring
temperature. When temperature reaches over temp,
the H Bridge is tri-stated.
•
Fault can be latched or auto-recovery. Some devices alert the system MCU via a
fault pin on an over temp event.
•
Some devices have 2-stage thermal protection, providing a warning (fault flag)
prior to shutting the device down.
Undervoltage/Cross Conduction Protection
•
Undervoltage protection
– Supply voltage level is constantly monitored and the device is tri-stated when the
voltage level is too low to ensure proper control over the H-Bridge
•
Shoot-through Protection
– High side and low side are never allowed to turn on at the same time. A small
amount of delay (dead time) is inserted after turning off the high side and turning
on the low side. The longer the dead-time, the safer the operation but the worse
the linearity and efficiency.
VBB
AH
BH
AL
BL
Shoot–through!!
BIPOLAR vs UNIPOLAR Stepper
Drive Topology
Used primarily for
simple stepped mode operation
Compatible with more
sophisticated control techniques
(Commutation diodes not shown)
(Commutation diodes not shown)
Source: Airpax Stepper Motor Handbook, 1989
Source: Airpax Stepper Motor Handbook, 1989
Stepper leads
Full and Half-Step Operation
Current Vectors for Various Stepper
Excitation Modes
Source: Monolithic, Programmable, Full-Bridge Motor Driver Integrates PWM Current Control and ‘Mixed-Mode’ Microstepping,
Paul Emerald, Roger Peppiette, and Anatol Seliversto,, Allegro MicroSystems, Inc. Technical Paper STP 97-5A
Microstepped Current Waveforms
B
A
A
B
Interface Styles
ENABLE / PHASE Interface
VM
BH
ENABLE
PHASE
Logic
&
Pre
Drive
AH
OUTA
AL
OUTB
BL
ENABLE
PHASE
A
B
L
L
HIZ
HIZ
L
H
HIZ
HIZ
H
L
GND
VM
H
H
VM
GND
• Logic controls all aspects of four
FET switching.
• ENABLE signal selects whether
the entire H Bridge is turned ON
or OFF.
• PHASE signal selects whether
the H Bridge is conducting from
side A to side B or vice versa.
• A single PWM signal can
control speed and/or direction.
• Only one Inductive load can be
driven.
IN1/IN2 Interface
VM
IN1
AH
LOGIC
&
Pre Drive
BH
A
AL
IN2
LOGIC
&
Pre Drive
B
IN1
IN2
A
B
L
L
GND
GND
L
H
GND
VM
H
L
VM
GND
H
H
VM
VM
BL
• Each Half H Bridge can be controlled independently.
• Can be utilized to drive two inductive loads without current control. Per
example, two solenoids can be driven with a single H Bridge.
• Requires 2 PWM sources for speed control of a DC motor.
IN1/IN2 Possible Implementations
VM
IN1
AH
BH
IN2
M
AL
BL
VM
IN1
AH
AL
BH
BL
IN2
• Can drive a single
inductive load with current
flow in both directions.
• Can drive two inductive
loads (such as solenoids)
with unipolar current drive.
• When driving dual load,
current regulation must be
disabled
– SENSE pin can be
grounded.
Serial Interface
VM
DATA
BH
CLK
SPI
SELECT
Logic
&
Pre
Drive
AH
OUTA
AL
• Different Serial Protocols are
used to sample data which will
be transferred as a command
into the H Bridge.
OUTB
BL
• Very hard or not recommended
for PWM.
VM
SD
BH
SCLK
I2C
Logic
&
Pre
Drive
AH
OUTA
AL
OUTB
BL
• Gives the largest amount of H
Bridge devices to be controlled
with the least amount of
resources.
– E.g. A single I2C bus can
control 9 DRV8830’s