Active Magnetic and Touchdown
Bearings to Encourage Rotor ReLevitation from a Persistent Contact
Condition
Peichao Li, Necip Sahinkaya, Patrick Keogh
Department of Mechanical Engineering
University of Bath, UK
IDETC/CIE 2012, CHICAGO, ILLINOIS,
USA, August 12-15, 2012
Background
 Active Magnetic Bearings (AMBs) have
Touchdown Bearings (TDBs) for protection
 Rotor will have residual unbalance
 At certain operating speeds, continuous contact
between rotor and TDB might be induced by external
disturbances
 High contact forces could cause damage through
induced heat dissipation at high speeds
 Can AMB control and active TDB control be
used to achieve contact-free levitation?
IDETC/CIE 2012, CHICAGO, ILLINOIS,
USA, August 12-15, 2012
System Description
 Two active magnetic
bearings (AMBs)
 Two active touchdown
bearings (TDBs)
 TDBs may be moved by
piezoelectric stack
actuators pushing
hydraulic pistons
 Rotor: 800 mm long, 30
mm in diameter
 First flexural rotor mode
around 840 rad/s
Disk 1
TDB A
AMB A
Piston
IDETC/CIE 2012, CHICAGO, ILLINOIS,
USA, August 12-15, 2012
Active Bearings
 AMB force capacity
 700 N
 Radial clearance
 0.8 mm
 Active TDB force capacity
 10,000 N (piezo stack)
 Radial clearance
 0.4 mm
IDETC/CIE 2012, CHICAGO, ILLINOIS,
USA, August 12-15, 2012
Active TDB
 Positioned outboard of AMB
for better protection
Piezoelectric
 Hydraulic lines limit
actuation
bandwidth
 Two eddy current position
sensors were used to
measure the x, y position of
the TDB
Rotor f c
( xr , y r )
Hydraulic
coupling
 fc
xc
( xb , yb )

Touchdown Bearing
TDB
TDB
yc
IDETC/CIE 2012, CHICAGO, ILLINOIS,
USA, August 12-15, 2012
System Model - Rotor
 Rotor displacement equation of motion:
r  Cq r  K qr  fu  fd  Bm fm  Bt fc
Mq
Rotor displacement vector
Magnetic bearing force vector
Unbalance force vector
Contact force vector
Disturbance vector
 No base motion is considered here
IDETC/CIE 2012, CHICAGO, ILLINOIS,
USA, August 12-15, 2012
System Model - TDB
 TDB equation of motion:
t  Ct q
 t  Kt qt  ft,n  μ ft,t  fat
Mt q
Touchdown bearing displacement vector
Normal contact force vector
Tangential contact force vector
Actuation force vector
 The contact forces (normal, tangential
friction) acting on the rotor and TDB are
equal, but in opposite directions
IDETC/CIE 2012, CHICAGO, ILLINOIS,
USA, August 12-15, 2012
Rotor Radial Displacement
Without Contact
 Unbalance force acting
on Disk 1 (left)
 Predicted slow run-up
radial displacement at
TDB A (left)
 Assuming TDB removed
 Highest displacement
occurs at around 840
rad/s, which matches
the first flexible mode
IDETC/CIE 2012, CHICAGO, ILLINOIS,
USA, August 12-15, 2012
Rotor Radial Displacement With
Contact
 Same unbalance on Disk 1
 Rotor operating speed
slowly increased from 740
rad/s to 940 rad/s, then
decreased to 740 rad/s
 Rotor makes contact with
the TDB A as speed
increases
 Without TDB limit, rotor
radial displacement
decreases at speed higher
than 840 rad/s
 Different rotor dynamics
with and without contact
IDETC/CIE 2012, CHICAGO, ILLINOIS,
USA, August 12-15, 2012
Rotor Radial Displacement With and
Without Contact
 The rotor dynamics change
when contact occurs, at
speeds above the critical
operating speed

The rotor radial displacement
increases with speed when it is in
contact
Rotor radial displacement
decreases when not in contact
 Two stable orbits


With contact
Without contact
 Control is designed to bring
rotor from contact orbit to a
non-contact orbit
x 10
-4
With contact
Without contact
6
Displacement (m)

7
5
4
3
2
1
0
750
800
850
Speed (rad/s)
900
IDETC/CIE 2012, CHICAGO, ILLINOIS,
USA, August 12-15, 2012
Using an AMB to Compensate the
Unbalance Force
x 10
-4
Rotor
Touchdown Bearing
5

Displacement (m)
 Rotor operating speed
840 rad/s
 Unbalance only on Disk 1 (left)
 Synchronous compensation force
was calculated* by using noncontact data.
 The synchronous force was
applied to the rotor through
AMBs at 1.5 s

AMB force is limited, hence for larger
unbalance the procedure may not be
possible
Could affect the orbit at other points
along the shaft, hence further
contacts could be induced
3
2
1
0
1
1.2
1.4
1.6
Time (s)
1.8
2
1.4
1.6
Time (s)
1.8
2
1800
1600
1400
Contact Force (N)

4
1200
1000
800
600
400
200
* Royal Society of London Paper (1983)
IDETC/CIE 2012, CHICAGO, ILLINOIS,
USA, August 12-15, 2012
0
1
1.2
Using an AMB to Compensate the
Unbalance Force
IDETC/CIE 2012, CHICAGO, ILLINOIS,
USA, August 12-15, 2012
Using TDB Motion to Release the
Entrapped Rotor
6
 Rotor operating speed
940 rad/s
 Single unbalance on Disk 1
 Disturbance force of 600 N
was applied at 1.2 s, lasting
for 0.08 s



TDB moves in the same direction
as the rotor
The radial TDB displacement
was limited to 0.1 mm
IDETC/CIE 2012, CHICAGO, ILLINOIS,
USA, August 12-15, 2012
4
3
2
0
1
Rotor makes contact with the
TDB A (left)
High contact force
 TDB A motion was applied
at 1.5 s
Rotor
Touchdown Bearing
1
1.2
1.4
1.6
Time (s)
1.8
2
1.2
1.4
1.6
Time (s)
1.8
2
4500
4000
3500
Contact Force (N)

-4
5
Displacement (m)

x 10
3000
2500
2000
1500
1000
500
0
1
Using TDB Motion to Release the
Entrapped Rotor
IDETC/CIE 2012, CHICAGO, ILLINOIS,
USA, August 12-15, 2012
TDB Control with Symmetric
Unbalance
 Rotor operating speed
 Unbalance mass on Disks
1, 2 and 3
 Disturbance was applied
on the middle of the
shaft for 0.1 s


Not able to release the
entrapped rotor
Increased rotor contact orbit
Rotor
Touchdown Bearing
5
4
3
TDB A
2
0
1
7
x 10
1.2
1.4
1.6
Time (s)
1.8
2
-4
Rotor
Touchdown Bearing
6
5
4
3
TDB B
2
1
0
1
IDETC/CIE 2012, CHICAGO, ILLINOIS,
USA, August 12-15, 2012
-4
1
Rotor makes continuous
contact after the disturbance
 The same motion was
applied at the same time
for both TDBs

Displacement (m)
940 rad/s
x 10
6
Displacement (m)

7
1.2
1.4
1.6
Time (s)
1.8
2
TDB Control with Symmetric
Unbalance
IDETC/CIE 2012, CHICAGO, ILLINOIS,
USA, August 12-15, 2012
TDB Control with Symmetric
Unbalance
7
x 10
-4
Rotor
Touchdown Bearing
 Same operating
parameters
 Motion control on TDBs
were applied at
different times
4
TDB A
3
2
0
1
TDB A at 1.7 s
TDB B at 2 s
 More effective than
applying the motion
control on TDBs at the
same time
5
1
7
1.2
x 10
1.4
1.6
Time (s)
1.8
2
-4
Rotor
Touchdown Bearing
6
Displacement (m)


Displacement (m)
6
5
4
TDB B
3
2
1
0
1
IDETC/CIE 2012, CHICAGO, ILLINOIS,
USA, August 12-15, 2012
1.2
1.4
1.6
Time (s)
1.8
2
TDB Control with Symmetric
Unbalance
IDETC/CIE 2012, CHICAGO, ILLINOIS,
USA, August 12-15, 2012
Conclusions
 Residual rotor unbalance always exists:

Disturbances could initiate rotor/TDB contact and change the rotor
dynamics
 Either synchronous AMB forces or TDB motion control can
be effective in restoring contact-free levitation when the
rotor has localized unbalance
 With distributed unbalance:

Simultaneous TDB motion may be ineffective, but sequenced TDB
motion control may restore contact-free rotor levitation
 Understanding of the contact rotor dynamics is required:

Non-linear characteristics can lead to differing rotor responses causes
by control actions that differ in sequencing only
IDETC/CIE 2012, CHICAGO, ILLINOIS,
USA, August 12-15, 2012