maxon motion control: Control loops, Controller properties

1
maxon motion controlControl loops, Controller
properties

• Control and feedback
• Power, power stages
• Communication
• Features and demonstration of a positioning system

2
maxon motor control

• What to control position, speed, current
  (torque)?
• Which commutation type DC, EC, block,
  sensorless, sinusoidal?
• How to control open closed loop, 1Q 4Q
• How to measure the feedback value?
• What kind of Signals digital - analog?
• How much power current and voltage, voltage
  drops?
• Controller power stage linear, pulsed, chokes?
• Special features time scales, braking, measuring
  motor currents

3
Motion control servo system
electr. energy
PC, PLC
motion command
set value
controller
amplifier
energy losses
current
servo amplifier
speed signal
position signal
motor
position, speed
sensor
load
position, speed
mech. energy
4
What to control ?

• Current control torque control
• maintaining current (torque) constant
• mostly included in controller (but not always
  accessible)
• for fast motor reaction
• no special feedback device needed
• Speed control
• maintaining speed constant
• "speed 0" does not mean "position is held"
• all maxon controllers can act as speed
  controllers
• Position control
• moving from position to position, stop at and
  maintain a position
• maxon controllers EPOS, EPOS P, and MIP

5
Motor type? Commutation?

• 4-Q DC servoamplifier
• LSC (50 W), ADS (250 W, 500W)
• 1/4-Q-EC amplifier
• AECS (sensorless, 100 W)
• DEC (24 W-700 W, Hall sensor), block commutation
• 4-Q-EC servoamplifier
• DES (250 W, 700W), sinusoidal commutation
• Position control
• MIP (DC or EC, 50-500 W), block commutation
• EPOS (DC or EC, 20-700 W), sinusoidal commutation
• EPOS P (DC or EC, 120W), Sinusoidal commutation

DC motor speed controller
EC motor commutation and speed controller
DC or EC motor position controller
6
Which motor type, commutation?

• For which motor types is the controller made DC,
  EC, Stepper
• With EC motors
• What commutations system is foreseen?
• Block with Hall sensors, sensorless
• Sinusoidal commutation
• What kind of position sensors are needed for
  commutation?
• Hall sensors
• Encoder (resolution, channels, line driver)

7
How to control open vs. closed loop?

• open loop
• no feedback
• output is not measured and checked
• closed loop
• feedback loop
• output value is measured and the set value is
  adjusted , accordingly
• "Feed forward"
• system behaviour is anticipated

set value
output
set value
output
?

-
measured value
feedback
sensor
feed forward
output
?
set value

-
8
Open-loop systems examples
maxon controller LSC (Uadj), DEC (open
loop) AECS (comm. only)

• DC motor operation at fixed voltage

load ML
n

nL
U
nL
-
actuator
set value
output
M
ML

• another example stepper motor with amplifier
• set value signal pulses
• actuator amplifier and motor
• output steps/increments

9
1Q-controller, 4Q-servocontroller
speed n

• 1-Q
• only motor operation (quadrant I or quadrant III)
• direction reverse by digital signal
• braking is not controlled (friction), often slow
• 4-Q
• controlled motor operation and braking in both
  rotation directions
• mandatory for positioning

quadrant I motor drive cw
quadrant II braking cw
n
n
M
M
torque M
M
M
n
n
quadrant III motor drive ccw
quadrant IV braking ccw
10
DEC 50/5 DECV 50/5 DEC 70/10 DES sensors Hall
Sens. Hall Sens. Hall Sens. Encoder,
HS commutation Block Block Block Sinusodial n-feed
back with HS HS HS Encoder operation ranges 2x
1Q 2x 2Q "4Q" (2x 2Q) 4Q
cw
cw
cw
cw
0..5V
0..5V
10 V -10 V
10 V 5V -10 V 0V
1000 min-1
500 min-1
torque cw
torque ccw
DIR
DIR
0..5V
0..5V
ccw
ccw
ccw
ccw
open loop yes no yes with IxR (4Q) no current
mode yes no yes yes specially for EC(-max)16/22 E
C 45, EC 60 with low R with Icont gt 2A see
chapt. 4.2
11
Nested current controller
4-Q current controller e.g. ADS, DES, DEC 70/10
power amplifier
set value speed
current command
DSP
motor
set value position
current feedback
path generator
encoder
position feedback
position decoder
12
How to measure the feedback value?
motor
controller
set value
system deviation
?
?


-
-
current- feedback
sensor
actual value
incremental encoder
resolver
DC tacho
IxR
Hall sensor
DC or EC motor position controller
DC motor speed controller
EC motor speed controller
13
How to measure the feedback value?

• Open loop
• no feedback system
• DEC, AECS for commutation only
• Current control
• no special feedback
• Speed control
• feedback devices for DC motors Encoder, DCTacho,
  IxR
• feedback devices for EC motors Encoder,
  Hall-Sensors, sensorless commutation frequency
• Position control
• feedback devices Encoder, Hall-Sensor

14
special DC speed controller IxR
Imot
IxR compensation
Rmot . K
motor
L

set value
Umot
R
?
?


-
EMF
motor voltage
K
Umot
maxon examples LSC, ADS

• without speed sensor, low price, few cables
• feedback value motor voltage
• set value compensation for the voltage drop over
  Rmot
• compensation factor adjusted on controller (ideal
  Rmot)
• not very dynamic, not very stable (Rmot depends
  on temperature)

15
How to command? Signal processing?

• analog signal processing
• for speed and current controllers
• set values from external voltages, internal or
  external potentiometers
• very high bandwidth
• problem of temperature drifts
• digital commands and signal processing
• more sophisticated digital speed and position
  controllers
• commands from PC, PLC or microprocessors. A/D
  converted voltages
• no temperature drifts
• parameters set by software, can be recorded and
  transferred
• bandwidth limited by calculation performance of
  DSP or microcontroller

16
Analog encoder speed control loop

• speed control loop with encoder feedback
• amplification (gain) depends on parameters PID
• applies also to Hall Sensor feedback with EC
  motors (6 IMP)
• current control loop
• subordinate control loop, enhances system
  dynamics
• power amplifier (MOSFET)

maxon examples LSC, ADS, (AECS)
speed amplifier (PID)
power amplifier
R
current
current command
set value speed
E
motor
?
?


-
-
current- feedback
C
speed feedback
encoder
17
Digital control loop
maxon examples DES, DEC, PCU, MIP, EPOS

• digital parameters (profile, position,
  amplification)
• DSP digital signal processor
• Firmware software of the controller

power amplifier
set value speed
current command
DSP
motor
current feedback
set value position
position feedback
position decoder
path generator
encoder
speed feedback
18
Gain, amplification PID
amplifier (PID)
set value
current command
E
How the deviation signal E is it amplified to
produce a purposeful reaction (current command)?
?

actual value

• P Proportional (a multiplication
  "amplification")
• Problem very small deviation lead to small
  corrections only. The set value cannot be
  reached.
• Remedy Combination of P and I
• I Integration
• A persisting deviation is summed up (integrated)
  and eventually corrected.
• D Differentiation
• a sudden increasing deviation (e.g. a set value
  jump), produces a strong reaction
• for dynamic reaction
• overshoot, instability

system reaction
PI
P only
PID
set value
Zeit
19
How much power? Amplifier limits

• voltage drop over
• the power stage
• 5 -10
• LSC 5V

thermal limit of the amplifier or the motor
(adjustable)
max current different possibilities
voltage
Vcc,max
Umot,max
reserve 20
continuous operation
short term operation
Vcc,min
current
Icont
Imax
20
Amplifier limits - motor selection

• reserve 20
• variations of the supply voltage
• load variations
• varying friction
• tolerances of the components
• varying ambient conditions

speed
thermal limit of the amplifier or motor
n0,max
Vcc,max
continuous operation
max. current
Umot,max
short term operation
torque current
Mcont Icont
Mmax Imax
21
Power stage linear, pulsed? Chokes?

• 4-Q power stage
• Linear
• MOSFETs acting as valves, driven by analog
  voltages
• Pulsed
• MOSFETs acting as switches

4 power MOSFETs
motor
Vcc
UT1
Umot
UT2
Gnd
22
Linear power stage
Umot, Imot
LSC
Vcc
time

• advantages
• simple, low priced controller
• low electromagnetic noise level
• no minimum inductance needed
• disadvantages
• high power losses at the final stage at high
  currents or low motor voltages (PV R I2)
• for small nominal power up to 100 W

R
UT
Umot
Gnd
23
Pulsed power stage (PWM)

• advantages
• low power losses
• high efficiency
• for higher nominal power
• disadvantages
• electromagnetic noise in the radio frequency
  range
• high power losses in the motor at standstill
• minimum inductance necessary

Vcc
power stage
Umot
Gnd
ADS, DEC, AECS, DES, MIP, PCU, EPOS
Umot, Imot
time
cycle time 20 - 50 ms
24
Pulsed power stage current ripple

• general measures
• reduce motor voltage
• enhance total inductance
• motor choke in controller
• additional motor choke
• enhance PWM frequency

low motor inductance
50 50
Umot, Imot
30 70
additional motor choke
25
Special features

• time scales in drive control
• names of maxon controllers
• encoder installation tips
• braking
• accuracy of speed control
• measuring motor currents

26
Time scales in control loops
frequency kHz
50 20 10 5 2 1 0.5 0.2 0.1 0.05
mechanical time constants
"slow" position controller
position controller MIP
speed controller
current controller
speed controller as "link" between fast current
controller and a slow position control (PLC)
PWM cycle time
0.02 0.05 0.1 0.2 0.5 1 2 5 10 20 ms
cycle time
27
maxon abbreviations for controllers
others LSC linear servo controller PCU position
control unit MIP mini position control EPOS easy
to use positioning system EPOS P easy to use
positioning system Programmable
signal processing A analog D digital
amplifier type C 1Q controller (2x 2Q) S 4Q -
servocontroller
max. supply voltage in V
AECS
35
3
/
max. continuous current in A
motor type D DC motor E EC motor
commutation type S sensorless V improved
28
Encoder installation tips

• use line driver
• to enhance signal quality
• with long encoder lines
• mandatory for position control
• use shielded cables
• use twisted encoder cables
• A with /A
• B with /B
• I with /I
• separate encoder and motor lines
• particularly with PWM amplifiers
• look up details in FAQ

29
Braking energy in 4-Q amplifier

• during braking energy flows back from motor
• part of this energy can be absorbed in the
  amplifier, or it is fed back to the power supply
  capacitance

C

• C "full"
• supply voltage increases
• damage to controller

30
Braking energy Solutions
1st choice
reduce acceleration rate (e.g. DES)
power supply
controller
2nd choice
add electrolyte capacitance
C
power supply
controller
3rd choice
add. shunt regulator
C
power supply
controller
R
DSR 70/30 235811 DSR 50/5 309687
31
Accuracy of speed control

• What can accuracy of speed control mean ...
• absolute accuracy speed corresponds exactly to
  the set value, e.g. 1000 rpm
• repeatability speed deviation at identical set
  values
• linearity 1 V set value 1'000 rpm
• 10 V set value 10'000 rpm
• -1 V set value -1'000 rpm
• long time stability today 1'000 rpm, and in a
  year?
• drift stability speed deviation because of
  temperature drifts (warm up)
• short time stability e.g. within one motor
  revolution (torque ripple, speed ripple)
• dynamic accuracy speed deviation after
• a perturbation (load change)
• changing the set values

32
Accuracy of speed control

• and most of the time, this is what the customer
  thinks of
• static accuracy due to load changes
• static/constant speed deviation after a certain
  time following a load change
• given as of the whole control (speed) range
• example
• 1 accuracy at maximum speed of 5000 rpm
• at 5000 rpm speed deviation of 50 rpm (4950 rpm
  1) at load change from 0 to nominal torque
• at 100 rpm speed deviation of 50 rpm (50 rpm
  50) at load change from 0 to nominal torque

33
Measuring motor currents

• PWM controller acts as an electronic transformer
• input power (from power supply) output power
  (to motor)
• motor voltage lower than supply voltage
• motor current Imot higher than supply current

power supply
PWM controller
A
A
DC motor
do not measure here
DC measure here with a true RMS Amp-meter
EC with an oscilloscope (blocked shaft at max.
phase current) use current monitor
A
PWM controller
EC motor