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1. Drive selection: What is it all about?
maxon Seminar at Electromate, March 2010
© by maxon motor ag, 2010
Drive Selection:
What is it all about?
The context of motor selection
Application and situation analysis
Extracting key load parameters
Seminar at Electromate
© 2010, maxon motor ag, Sachseln, Switzerland
Systematic selection process: Step 1
Step 1
overview
situation
power
communication
ambient
condition
step 2
load
step 3
drive
step 4
gearhead
step 5+6
step 7+8
motor type sensor
winding
controller
Book: Chapter 3
DC motor sizing made easy
1. Drive selection: What is it all about?
maxon Seminar at Electromate, March 2010
© by maxon motor ag, 2010
Gain an overview
mechanics
power
operating mode
motion profile
control concept
working points
control accuracy
boundary conditions
Drive system as a black box
environment
electrical power
task
current
voltage
set values
commands
temperature, atmosphere
impacts, vibration
boundary conditions
…
dimensions
service life
…
emissions
electro magnetic
heat
noise
…
mechanical power
quality, accuracy
resolution
mech. play
force, torque
velocity, speed
DC motor sizing made easy
1. Drive selection: What is it all about?
maxon Seminar at Electromate, March 2010
Mechanical drive concept
Drive design: linear – rotation
Drive elements and coupling with load
relative position of motor and load
– e.g. bridge a distance with a belt
Mechanical drive concept: Bearing
Support of all the forces
– Eccentric drive, cam, pump
– rack guidance
– pulley bearings of conveyors, cranes
and belt drives
DC motor sizing made easy
© by maxon motor ag, 2010
1. Drive selection: What is it all about?
maxon Seminar at Electromate, March 2010
© by maxon motor ag, 2010
Operating modes
continuous (S1)
load
time
short term (S2)
working cycles
intermittent (S3)
Control concept
Controlled variable
– torque, current
– speed, velocity
– position
feedback sensor
Controlled range, accuracy?
– position resolution
– speed stability
What kind of communication?
– communication with higher level host system
– set value range: 0 … 5 V or -10 … +10 V
– inputs and outputs (homing, temperature monitoring, ….
DC motor sizing made easy
1. Drive selection: What is it all about?
maxon Seminar at Electromate, March 2010
© by maxon motor ag, 2010
Task and control accuracy
command
controller
motor
gearhead,
drive
load
control loop
sensor
The accuracy of control is the combined result of all
components in a drive system!
– resolution, precision
– signal amplification, control parameter
– phase shifts, time shifts, backlash, mech. play
Particular boundary conditions
dimensions
– length, diameter
service life
– specific depending on load cycle, ambient conditions and application
– given as service hours or numbers of working cycles
– limited by the weakest component
temperature, atmosphere
– can influence the achievable power and service life
noise, vibration
– specific depending on load cycle, mounting and ambient conditions,
application
– influences on service life
DC motor sizing made easy
1. Drive selection: What is it all about?
maxon Seminar at Electromate, March 2010
© by maxon motor ag, 2010
Systematic selection process: Step 2
Step 1
overview
situation
power
communication
ambient
condition
step 2
load
step 3
drive
step 4
gearhead
step 5+6
step 7+8
motor type sensor
winding
controller
Book: Chapter 4
Working points
Pair of
– torque and speed
– force and velocity
standard representation: (x,y) = (M,n) or (F,v)
speed n
velocity v
deceleration
acceleration
torque M
force F
dwell
DC motor sizing made easy
1. Drive selection: What is it all about?
maxon Seminar at Electromate, March 2010
© by maxon motor ag, 2010
Working point and motion profile
n
1
2
3
4
1
t
n
2
3
4
extreme
working point
1
M
(M1,n1)
acceleration
friction and acceleration
(M2,n2)
const. speed
friction only
(M3,n3)
deceleration
friction helps during deceleration
(M4,n4)
dwell
depending on friction
Motion profiles
general parameters
– duration
∆ttot
– max. velocity vmax
– distance
∆s
symmetrical triangular profile
– for fast motions
– minimum acceleration
3/3 profile
– minimum power
– thermally advantageous
Book: P. 24/25
DC motor sizing made easy
1. Drive selection: What is it all about?
maxon Seminar at Electromate, March 2010
© by maxon motor ag, 2010
Operating modes
continuous operation (S1)
– acceleration and deceleration can be neglected
– constant load
– duration: longer than all thermal time constants of the system
in thermal equilibrium: constant temperatures
typically: several minutes up to hours
Tmax
temperature
load
time
Operating modes
short term operation (S2)
– very short operation time
only elements with a correspondingly short time constant can heat
up
– long dwell times
drive can cool down completely
Tmax
temperatur
load
time
DC motor sizing made easy
1. Drive selection: What is it all about?
maxon Seminar at Electromate, March 2010
© by maxon motor ag, 2010
Operating modes
cyclic operation (general case)
– load profile to be repeated permanently with relatively short dwell
times
duration of a cycle: typically up to several seconds
– thermal equilibrium is reached on average only
temperature variations
– effective load (RMS) is an important key parameter
MRMS =
Tmax
temperature
(
)
1
t1M12 + t 2M22 + t 3M32 + ...
t tot
load
time
Operating modes
intermittent operation (S3) ON - OFF M
RMS =
– special case of cyclic operation
t on
⋅ Mon
t on + t off
temperature
• winding
• housing
1s
10s
100s
1000s 10000s
10s
100s
1000s
DC motor sizing made easy
Time
1. Drive selection: What is it all about?
maxon Seminar at Electromate, March 2010
© by maxon motor ag, 2010
Friction forces
difficult to evaluate
– many influencing factors: state of
the interface layer
normal force FN
– varies upon time: ageing effects
– typical coefficients of friction from
tables (book p. 30/31)
friction force FN
FR = µ ⋅ FN
body
moving
direction
n
catio
ubri
l
,
r
laye
is
face
bas
in te r
estimation formula for ball bearings:
MR = 0.5 ⋅ µR ⋅ F ⋅ d
Acceleration of mass inertias
additional force / torque
necessary
– basic equations for
translation / rotation
∆v
∆t
∆ω
π ∆n
Mα = J ⋅ α = J ⋅
= J⋅
∆t
30 ∆t
Fa = m ⋅ a = m ⋅
mass inertias
– translation:
mass m
– rotation:
moment of mass inertia JX
with respect to axis x
Jx = r 2 ⋅ dm
– example cylinder:
Jx =
m 2
⋅r
2
DC motor sizing made easy
Book: P. 40ff
1. Drive selection: What is it all about?
maxon Seminar at Electromate, March 2010
© by maxon motor ag, 2010
Constant forces, preloading forces
Example spring preload
Example gravity
speed n
velocity v
deceleration
dwell
acceleration
torque M
force F
without gravity
with gravity
Determination of forces / torques
simple measurements
– precision needed: 5-10%
– e.g. by means of pulleys and spring balances
estimation of friction torques by means of friction
coefficients µR
using a motor for torque determination
– by means of a current measurement and the torque constant kM
– in original conditions
– Suitable current measuring tool
M = kM ⋅ I
DC motor sizing made easy
1. Drive selection: What is it all about?
maxon Seminar at Electromate, March 2010
© by maxon motor ag, 2010
Key load data for characterization
average effective load (RMS)
maximum load
duration of a load cycle
duration of the maximum load
max. velocity or speed
required position resolution
required speed accuracy
Load characterization
dominating constant load
– typically in cases of high friction
– maximum forces exceed RMS load by less than 20%
– maximum forces applied during short terms only
=> selection based on RMS load is sufficient
dominated by peak load
– typically in cases of highly dynamic drives with high acceleration
– RMS value of load very small (< 50 % of maximum load)
=> selection based on maximum load values
general case without domination of one load type
Remark: Acceleration and friction in the mechanical drive
are not taken into consideration yet!
DC motor sizing made easy
1. Drive selection: What is it all about?
maxon Seminar at Electromate, March 2010
© by maxon motor ag, 2010
Systematic selection process: Steps 3 & 4
Step 1
overview
situation
power
communication
ambient
condition
step 2
load
step 3
drive
step 4
gearhead
step 5+6
step 7+8
motor type sensor
winding
controller
Book: Chapters 5&6
Mechanical transformation
by means of drive
elements
– also in series
characteristic variables
– reduction i
– efficiency η
– mass inertias
– backlash
transformation equations
bearings
DC motor sizing made easy
1. Drive selection: What is it all about?
maxon Seminar at Electromate, March 2010
© by maxon motor ag, 2010
Screw drives
speed
60
nin =
⋅ v out
p
torque
Min =
p Fout
⋅
2π η
torque for acceleration
Min,α = Jin + JS +
mout + mS p 2
π ∆nin
⋅
2
η
4π 30 ∆t α
Spindle drives
spindel type: trapezoidal, metric, ball screws
spindle type has influence on
– efficiency, heating
– possible working cycles, ON time
– costs
– accuracy, play, backlash
– maximum load
– life
– application: actuators or positioning units
DC motor sizing made easy
1. Drive selection: What is it all about?
maxon Seminar at Electromate, March 2010
© by maxon motor ag, 2010
Gearhead stage
speed
nin = nout ⋅ iG
torque
Min =
Mout
iG ⋅ ηG
torque for acceleration
Min,α = Jin + J1 +
Jout + J2 π ∆nin
⋅
iG2 ⋅ ηG
30 ∆t α
maxon gear
operation at high torques and low speeds
2 standard types:
GS: spur gearhead
GP: planetary gearhead
lower power density
high power density
material options
– plastic:
lower power, lower costs
– ceramic:
higher power, longer life
bearing options
– ball bearing:
standard for larger planetary gears
– sleeve bearing:
standard for GS and small GP
DC motor sizing made easy
1. Drive selection: What is it all about?
maxon Seminar at Electromate, March 2010
© by maxon motor ag, 2010
Most important gearhead data
reduction - absolute reduction
efficiency
– as a function of torque
gearhead limits
– max. continuous torque (output)
– intermittent torque (short-term = 1s)
– max. input speed
– temperature (lubrication)
backlash
load on bearings
Operating range diagram
recommended gear input speed
defines max. output speed
output
speed
n
nmax,out = max,in
i
continuous
operation
intermittent
operation
output
torque M
Mcont
Mmax
DC motor sizing made easy
1. Drive selection: What is it all about?
maxon Seminar at Electromate, March 2010
© by maxon motor ag, 2010
Systematic selection process: Steps 5 & 6
Step 1
overview
situation
power
communication
ambient
condition
step 2
load
step 3
drive
step 4
gearhead
step 5+6
step 7+8
motor type sensor
winding
controller
Book: Chapter 7&8
DC motor designs
DC motors
BLDC motors
conventional,
with iron core
slotless
slotted
external rotor
coreless
slotted
internal rotor
DC motor sizing made easy
1. Drive selection: What is it all about?
maxon Seminar at Electromate, March 2010
© by maxon motor ag, 2010
Systematic selection process: Steps 7 & 8
Step 1
overview
situation
power
communication
ambient
condition
step 2
load
step 3
drive
step 4
gearhead
step 5+6
step 7+8
motor type sensor
winding
controller
Book: Chapter 9
Controller: The Heart of the Drive System
Communication
– Host, Master
– I/O
Power Supply
– cont. current
– max. current
– voltage
Controller
Sensor
– measured
quantity
– resolution
controlled quantity
power
digital – analogue
…
Motor
– motor type
Gearhead, mech. drive
– power
– reduction ratio
– backlash
DC motor sizing made easy
1. Drive selection: What is it all about?
maxon Seminar at Electromate, March 2010
© by maxon motor ag, 2010
Motion Control System
11001..
Application
Program
Cycle (e.g. 5 ms)
command
Path
Generator
Pos
Position Set Value
t
Actual
Position
Position/Speed
Control loop
I
Current
Control loop
t
Clock
(e.g. 1 kHz)
Motor with Encoder
and Drive
Current Set Value
Clock
(e.g. 10 kHz)
PWM
duty
cycle
Actual
Current
Driver, Power
Amplifier
PWM (e.g. 50 kHz)
Sensors
Set value
Output
Actuator
Measurement
Feedback
DC Tacho
IxR
Speed
Direction
Incremental Encoder
Resolver
Hall Sensor
∆Pos/∆t
Position
DC motor sizing made easy